A new article from Baghouse.com has been featured on the well-known environmental blog Triplepundit.com. This article highlights the plight of a group of farmers and ranchers in Texas whose livelihoods have been devastated by acid rain, as they struggle to gain compensation, and recognition from the near by coal-fired power plants that they say are causing the problem.

 

You can read the full article here online: http://www.triplepundit.com/2011/02/pecan-growers-blame-coal-fired-plant-killing-crops/

 

 
About the Author

| Dominick DalSanto is an Author & Environmental Technologies Expert, specializing in Dust Collection Systems. With nearly a decade of hands-on working experience in the industry, Dominick’s knowledge of the industry goes beyond a mere classroom education. He is currently serving as Online Marketing Director & Content Manager at Baghouse.com. His articles have been published not only on Baghouse.com , but also on other industry related blogs and sites. In his spare time, Dominick writes about travel and life abroad for various travel sites and blogs.

By Gilda Martinez
Environmental Author
Baghouse.com

February 10, 2011, Cemex, the largest producer of cement in United States has agreed to pay $1.4 million for Clean Air Act violations at its cement plant in Fairborn, Ohio, to the environmental protection agency (EPA) and to the Justice Department. An additional 2 million will need to be spent by Cemex on system improvements including the installation of pollution control technology in order to achieve EPA environmental requirements.

The plant located in Ohio was affecting the health of the local population. One assistant administrator for EPA’s Office of Enforcement and Compliance Assurance said that the emissions of sulfur dioxide and nitrogen oxides can lead to grave health and environmental problems such as premature death and heart disease.

Increasingly, many environmental activists and politicians are taking are coming to believe that the only way to combat pollution is by levying heavy fines for companies that are habitually found to be in violation of current environmental regulations. This action they feel will force them to invest in pollution control technology to reduce harmful emissions.

This is a very important step taken by EPA, since it will mean not only an environmental improvement for the Fairborn populace and the surrounding region today, but also reduce childhood asthma, acid rain and smog caused by pollution, in the future.

According to the agency, Cemex annual emissions of NO2 and SO2 are expected to be reduced by approximately 2.300 tons and 288 tons.

Interestingly among the violations listed in the citation issued by the EPA, is the charge that also Cemex made substantial changes to the plant without first obtaining the proper permit. The largest polluters are required to apply for permits before beginning any work that may increase (even temporarily) air emissions.

The heavy fine for Cemex is part of an overall strategy by the EPA to mentioned push the cement industry to install the latest pollution control. For this reason the dust collection industry is expanding at a rapid pace since the demand for this equipment grows every year.

Tougher Enforcement Part of EPA Plan for 2011 – 2013

The imposing fines on the largest sources of emissions, such as Cemex and other cement manufacturers, is part of the EPA’s National Enforcement Initiatives for 2011-2013 in reducing air pollution. The efforts put by the EPA in this respect are noted in the following figures:

During 2010 due to tougher enforcement of emissions regulations in cement manufacturing, coal-fired power generation, glass and acid industries the EPA achieved the following results:

  • Prevented the release of 370 million pounds of pollution across all industries.
  • 1.4 in pollution controls due to installation of Baghouses, dust collectors, and other air filtration equipments.
  • $14 million in civil penalties
Dust Explosion at the Imperial Sugar plant in Georgia

By Dominick DalSanto
Environmental Expert & Author
Baghouse.com

The dangers of combustible dust explosions are among the most overlooked of industrial workplace safety issues. However the price for negligence in this area is often payed not only with millions of dollars, but with workers very lives.

But only recently has the this issue began to attract mainstream attention outside of the industrial world. Recent incidents such as the one that occurred at the Imperial Sugar Plant in Port Wentworth, Georgia on February 8th 2007 that claimed the lives of 14 works, and injured 38 others, have brought this safety issue to the forefront of industrial safety activists.

This segment from the CBS new program “60 Minutes” entitled: The Danger of Combustible Dust – examines the efforts of the victims families and others to force OSHA to create a national combustible dust standard.  Scott Pelley reports on the deaths and property damage caused by dust explosions at American factories, a problem critics say the government needs to do more to prevent.

This report which is of great interest to all in the dust collection industry can be viewed at the link below.

http://www.cbsnews.com/video/watch/?id=4162555n&tag=related;photovideo

 
About the Author

| Dominick DalSanto is an Author & Environmental Technologies Expert, specializing in Dust Collection Systems. With nearly a decade of hands-on working experience in the industry, Dominick’s knowledge of the industry goes beyond a mere classroom education. He is currently serving as Online Marketing Director & Content Manager at Baghouse.com. His articles have been published not only on Baghouse.com , but also on other industry related blogs and sites. In his spare time, Dominick writes about travel and life abroad for various travel sites and blogs.

Introduction to Dust Collector Troubleshooting

Operating and maintaining an equipment system as complex as an Industrial Dust Collector can be a challenge. Here at Baghouse.com we pride ourselves on being experts in our field, with decades of experience designing, installing, maintaining and servicing every kind of Dust Collect design available. We have prepared this short troubleshooting guide in order to help you solve some of the more commonly encountered issues involving Dust Collectors.

Table of Contents

  1. Blower (Fan) & Ductwork Issues
  2. Common Baghouse Issues (All Designs: Shaker, Reverse Air, & Pulse Jet)
  3. Baghouse Design Specific Problems
    1. Shaker
    2. Reverse-Air
    3. Plenum Pulse-Jet
    4. Pulse-Jet

Blower (Fan) & Ductwork Issues

Many Baghouse difficulties originate as problems with the main Blower, or Fan and the supply and exhaust Ductwork.

Problem: Insufficient Airflow Rate coming from the Blower, or Ductwork System

  • Is your Blower (System Fan) powering on and operating properly?
  • Action: Check electrical connections and turn on the Blower.

Addition Questions

  • Is the motor pulling the specified proper amount of Amps?
  • Action: Check wiring
  • Is the fan turning in the right direction?
  • Action: Make sure that motor leads are attached to the proper terminals.
  • Is there excessive vibration?
  • Action: Ensure that there is no excess build up of material on the fan blade, or Blower housing.

Are you getting the proper amount of Air Flow (Cubic Feet per Min) from the Blower?

  • Is the Fan Dampener Open?
  • Action: Close Dampener.
  • Is the air volume at fan rated capacity?
  • Action: See Below.
  • (If your Blower output is normal) Has the Ductwork System been inspected for obstructions, leaks or design flaws that increase static resistance?
  • Action: Redesign Ductwork System to have lower resistance.

Addition Questions

  • Are there elbows, or other directional changing Ductwork immediately preceding the Blower Inlet?
  • Action: Redesign Ductwork to remove any Elbows, or similar configurations near the Blower.
  • Is there an obstruction near the outlet of the Ductwork?
  • Action: Removed any obstruction and try again.

Problem: Excessive Airflow Rate

  • Is the Blower set to the proper speed?
  • Action: Check setting and adjust.
  • Is the Ductwork System oversized?
  • Action: Evaluate the Ductwork System and consider redesigning if needed.
  • Are there any access ports on the Ductwork that are open?
  • Action: Close all ports, and ensure they are sealed properly.

Problem: You have High Static Pressure and a low Airflow Rate

  • Are there any obstructions in the Ductwork System?
  • Action: See above
  • Is the Ductwork System to restrictive?
  • Action: See above

Common Baghouse (All Designs: Shaker, Reverse Air, & Pulse Jet) Issues

Many of these issues can be resolved with a simple maintenance procedure; others may require a qualified service technician to implement a solution a particular problem.

Problem: There is a higher than anticipated Pressure Drop in the Baghouse

  • Have all gauges and pressures sensors been checked for accuracy?
  • Action: Clean all pressure taps, check houses for leaks, for proper fluid level in Manometer, and diaphragm in gauge.
  • Is the Baghouse the undersized for the application?
  • Action: Consider upgrading to a larger unit.
  • Is the cleaning mechanism adjusted to the proper settings?

Addition Questions

  • Is the cleaning timer working properly?
  • Action: Reset the timer. Check wiring, and replace if needed.
  • Is the dust not able to be removed from the Filter Bags by the cleaning mechanism?
  • Action: Check for condensation on Bags. Dry clean bags, or replace them. Take dust samples and send them to the manufacturer for analysis.
  • Is there excessive reentrainment of dust on the Filter Bags?
  • Action: Empty Hopper continuously.

Problem: Dirty discharge at stack

  • Are the Bags leaking from either the clamps, or are from being too porous?
  • Action: Replace Bags, isolate leaking compartment or module. Allow sufficient filter cake to form. Check and tighten clamps. Change to a different Filter Bag; smooth out Bag before clamping.
  • Are the seals between the different compartments  (Dirty Air, and Clean Air Compartments) of the Baghouse leaking?
  • Action: Repair by caulking or welding seams.

Problem: Moisture in the Baghouse

  • Is the Baghouse temperature below the dew point?
  • Action: Raise gas temperature; insulate unit.

Additional Questions

  • Are there any cold spots where pipes or other components connect?
  • Action: Eliminate direct metal line through insulation.
  • Has the Baghouse been sufficiently preheated (Certain applications only)?
  • Action: Run system with hot air only before process gas is introduced.
  • Is the system purged properly after each shutdown?
  • Action: Run fan for an additional 10 min after processing is shut down.

Problem: Material is bridging in the Hopper, thus preventing proper operating of the Baghouse

  • Is there excess moisture in the Baghouse?
  • Action: (See previous solutions)
  • Does the Hopper retain too much material, or is it cleaned on a regular basis?
  • Action: Clean Hopper on a regular schedule.
  • Is the Hopper slope sufficient to allow for the collected material to fall?
  • Action: Redesign and replace.
  • Is the opening for the Screw Conveyor (Or similar device) of adequate size?
  • Action: Redesign and replace.

Problem: The Bags fail prematurely, or wear or faster than they should

  • Is the Baffle Plate worn out?
  • Action: Replace with a new Baffle Plate; Determine whether the Gas stream is striking the Baffle Plate correctly, if it is not, consult with the manufacturer, redesign and replace.
  • Is the dust load to high for the particular Baghouse, or Bags?
  • Action: Install a Primary Dust Collector (Pre-Filter) to reduce dust loads to the Baghouse.
  • Are the Bags being cleaned at the proper intervals?
  • Action: Clean less often.

Baghouse Design Specific Problems

The most common variations in Baghouse design regard the cleaning mechanism.  The three most common are Shaker, Reverse Air, & Pulse Jet. While the proceeding information applies to all Baghouse designs, the following covers specific design related problems.

Shaker Baghouse Type Specific Issues

Problem: Cleaning Mechanism Does Not Function Properly

  • Does Shaking action take place, as it should?
  • Action: Check pins, Keys, Bearings, Etc and repair if necessary.
  • Is the Shaking action strong enough?
  • Action: Increase Shaking rate.
  • Have the Filter Bags been checked to have proper tension?
  • Action: Tension Bags to proper rate.
  • Are any other Baghouse functions affected when Shaking process begins (Fan, or Isolation Dampener, etc)?
  • Action: Repair Isolation Damper, or stop Fan.
  • Are the different compartmental isolation dampener valves functioning properly?
  • Action: Check linkage, Valve Seals, and Air supply of the Pneumatic Operators.
  • Is the cleaning cycle set to the proper interval?
  • Action: Set to the shortest interval possible between compartments.
  • Is the Air to Cloth Ratio at least 3:1?
  • Action: Add Bags; Consider installing a larger unit.

Problem: Filter Bags fail prematurely

  • Is the shaking mechanism set too high?
  • Action: Slow down shaking mechanism.

Reverse Air Baghouse Type Specific Issues

Problem: Cleaning Mechanism Does Not Function Properly

  • Are the different compartmental Isolation Dampener valves functioning properly?
  • Action: Repair if necessary.
  • Do the Bags have the proper amount of tension?
  • Action: See above.
  • Is the Reverse Air Fan powering up/running properly?
  • Action: Run Fan and check differential pressure.
  • Does the Reverse Air Fan spin in the correct direction?
  • Action: See section:  Blower (Fan) & Ductwork Issues
  • Is the Air to Cloth Ratio at least 3:1?
  • Action: Consider acquiring a larger Baghouse.

Plenum Pulse Jet Baghouse Type Specific Issues

Problem: Cleaning Mechanism Does Not Function Properly

  • Is the air pressure at the Pulse Valves within the recommended levels and are all Solenoids and Diaphragms operating properly?
  • Action: Check for leaking solenoids and pulse valves; check compressed air source and check differential pressure.
  • Are the cleaning pulses at set to the correct duration (0.1 sec)?
  • Action: Reset to 0.1 sec.
  • Is cleaning interval at the lowest setting the will allow air manifold pressure to rebuild?
  • Action: Change setting, and check the differential pressure.
  • Do all poppet valves seal properly?
  • Action: Adjust and/or repair all valves and check differential pressure.
  • Is the Air to Cloth Ratio at least 4:1?
  • Action: Switch to pleated media; Consider installing a larger unit.

Pulse Jet Baghouse Type Specific Issues

Problem: Cleaning Mechanism Does Not Function Properly.

  • Is the manifold pressure within the manufacturer’s suggested range?
  • Action: Check for leaks at the solenoids and pulse valves; Check compressed air source and then check differential pressure.
  • Are the cleaning pulses at set to the correct duration (0.1 – .015 sec)?
  • Action: Set to 0.1 – 0.15 duration.
  • Is cleaning interval at the lowest setting the will allow air manifold pressure to rebuild?
  • Action: Change setting and check differential pressure.
  • Is the compressed air pressure at the proper level?
  • Action: Check for leaks; Increase pressure.
  • Is the Air to Cloth Ratio at least 6:1?
  • Action: Switch to pleated media; Consider installing a larger unit.

 

 
About the Author

| Dominick DalSanto is an Author & Environmental Technologies Expert, specializing in Dust Collection Systems. With nearly a decade of hands-on working experience in the industry, Dominick’s knowledge of the industry goes beyond a mere classroom education. He is currently serving as Online Marketing Director & Content Manager at Baghouse.com. His articles have been published not only on Baghouse.com , but also on other industry related blogs and sites. In his spare time, Dominick writes about travel and life abroad for various travel sites and blogs.

A few years ago the state of Maryland enacted legislation to require the installation of specialized emissions control technology to capture excess mercury emissions from power plants. And according to the latest published reports it appears that the initiative has been a success. Coal-burning power plants in Maryland are now required to install new pollution controls that reduce mercury emissions by 80 percent.

But appearently that is not enough to keep the air quality in the state safe enough for all. Why?

A serious health threat still exists because neighboring states have yet to crack down on the toxic pollutant, an environmental group’s report says. The neighboring states, particularly Pennsylvania, Ohio and West Virginia are among the worst in the nation for mercury emissions, ranking second, third and fourth highest, respectively, in the country. All are within Maryland’s “airshed,” where pollutants put into the air in one state are carried by prevailing winds into neighboring states.

Robert M. Summers, acting secretary of the environment, noted in a news release that 73 percent of the mercuy air pollution measured in Maryland is coming from outside the state’s borders.

He and others called on the Environmental Protection Agency to follow through with an air-quality standard it is set to propose in March that would curb mercury and other toxic air pollution from power plants.  The federal standard, if proposed as drafted, would reduce mercury emission by more than 90 percent, advocates say.

The report – and a recent press conference – are meant to put public pressure on EPA to go through with the regulation in the face of pushback from industry and its supporters in Congress, where legislation to block new EPA rules is said to be in the works.

 

 
About the Author

| Dominick DalSanto is an Author & Environmental Technologies Expert, specializing in Dust Collection Systems. With nearly a decade of hands-on working experience in the industry, Dominick’s knowledge of the industry goes beyond a mere classroom education. He is currently serving as Online Marketing Director & Content Manager at Baghouse.com. His articles have been published not only on Baghouse.com , but also on other industry related blogs and sites. In his spare time, Dominick writes about travel and life abroad for various travel sites and blogs.

Often our customers find it difficult to determine exactly what kind of Filter Media they require for their specific Dust collection system. Other times they know the particular type of filter media they need, but are unable to determine the exact size they need for their Baghouse.

To assist our customers, we at Baghouse.com have prepared this article to help you select the right filter media to match your specific needs.

If you would like to speak directly with one of our dust collection experts for additional help in selecting your Filter Media, or if you would like to receive a free Baghouse Filters quote, please call us at  800 351 6200 or Contact Us using our online form.

 

Step 1 – Filter Media Selection

Choose the media from which your filter bags will be constructed of based on the type of application they will be used for. Take the following things into consideration prior to selecting your filter media:

•    Temperature – Do your bags need to withstand extreme temperatures?
•    Material – What is the filter being used for?
•    Chemistry – Can your filter withstand the chemical makeup of the dust particles
•    Resistance- Is the filter media able to resist the abrasion of the dust particles

Choosing the correct filter media is an important and sometimes difficult process. To assist you in the identification of the right media for your bags, keep the following in mind: Filter bag performance is directly related to how well it can tolerate the environment in which it is being used. How efficiently it can remove the dust particles from its fabric and its ability to be cleaned by the dust collector is also important. You must first learn to identify the type of filter media currently used in your application. Below you will find a list of typical construction methods:

•    Woven felts
•    Non-woven felts
•    Natural fibers
•    Synthetics (Thermoset or Thermoplastics such as Polypropylene “PPRO” – Polyphenylene sulfide “PPS” – Polyester “PE”)

For additional information on media types please examine our Filter Fabrics Chart below. A simple test to determine if a material is a thermoplastic is to take a small swatch and put a flame to it. A thermoplastic material will begin to melt when exposed to direct heat. The selection criterion eliminates materials based on temperature and chemical characteristics. The first cut is usually made based on temperature. Then the chemical characteristics of the gas stream are considered to further refine the search. Next, the efficiency of the material further dictates the construction of the material such as the weight, oz/sq. ft., fiber and surface treatments/membranes. Last but not least, if there are still two or more candidates it comes down to a price versus performance trade off.

Dust Collector Filter Fabrics

 

Popular Materials

 

Polyester FeltPolyester Felt - Baghouse Filter Fabric

Recommended continuous operation temperature: 275°F
Maximum (short time) operation temperature: 300°F
Supports combustion: Yes
Biological resistance (bacteria, mildew): No Effect
Resistance to alkalis: Fair
Resistance to mineral acids: Fair+
Resistance to organic acids: Fair
Resistance to oxidizing agents: Good
Resistance to organic solvents: Good
Available weights: 10 oz. – 22 oz.

Polypropylene Felt - Dust Collector Filter Fabric

Polypropylene Felt

Polypropylene Felt

Recommended continuous operation temperature: 190°F
Maximum (short time) operation temperature: 210°F
Supports combustion: Yes
Biological resistance (bacteria, mildew): Excellent
Resistance to alkalis: Excellent
Resistance to mineral acids: Excellent
Resistance to organic acids: Excellent
Resistance to oxidizing agents: Good
Resistance to organic solvents: Excellent
Available weights: 12 oz. – 18 oz

 

High Temperature Materials

 

Conex® / Nomex® Felt (Aramid) - Dust Collector Filter Fabric

Conex® / Nomex® Felt (Aramid)

Conex® / Nomex® Felt (Aramid)

Recommended continuous operation temperature: 400°F
Maximum (short time) operation temperature: 425°F
Supports combustion: No
Biological resistance (bacteria, mildew): No Effect
Resistance to alkalis: Good
Resistance to mineral acids: Fair
Resistance to organic acids: Fair+
Resistance to oxidizing agents: Poor
Resistance to organic solvents: Good
Available weights: 10 oz. – 22 oz.

P84® Felt Polyimide - Dust Collector Filter Fabric

P84® Felt Polyimide

P84® Felt Polyimide

Recommended continuous operation temperature: 475°F
Maximum (short time) operation temperature:500°F
Supports combustion: No
Biological resistance (bacteria, mildew): No Effect
Resistance to alkalis: Fair
Resistance to mineral acids: Good+
Resistance to organic acids: Good+
Resistance to oxidizing agents: Good+
Resistance to organic solvents: Excellent
Available weights: 14 oz. – 18 oz.

Ryton® Felt / PPS - Dust Collector Filter Fabric

Ryton® Felt / PPS

Ryton® Felt / PPS

Recommended continuous operation temperature: 375°F
Maximum (short time) operation temperature: 400°F
Supports combustion: No
Biological resistance (bacteria, mildew): No Effect
Resistance to alkalis: Excellent
Resistance to mineral acids: Excellent
Resistance to organic acids: Excellent
Resistance to oxidizing agents: Fair
Resistance to organic solvents: Excellent
Available weights: 16 oz. – 18 oz.

Dust Collector Filter Specialty Materials

 

Homopolymer Acrylic Felt - Dust Collector Filter Fabric

Homopolymer Acrylic Felt

Homopolymer Acrylic Felt

Recommended continuous operation temperature: 250°F
Maximum (short time) operation temperature: 275°F
Supports combustion: Yes
Biological resistance (bacteria, mildew): Good+
Resistance to alkalis: Fair
Resistance to mineral acids: Good+
Resistance to organic acids: Excellent
Resistance to oxidizing agents: Good
Resistance to organic solvents: Good+
Available weights: 15 oz. – 18 oz.

Epitropic Felt Antistatic - Dust Collector Filter Fabric

Epitropic Felt Antistatic

Epitropic Felt Antistatic

Recommended continuous operation temperature: 275°F
Maximum (short time) operation temperature: 300°F
Supports combustion: Yes
Biological resistance (bacteria, mildew): No Effect
Resistance to alkalis: Fair
Resistance to mineral acids: Fair+
Resistance to organic acids: Fair
Resistance to oxidizing agents: Good
Resistance to organic solvents: Good
Available weights: 14 oz. – 16 oz.

Step 2 – Dust Collector Filter Measurements

Accurate measurements lead to the best fit. It’s likely that your dust collector has been modified over the years due to permitting issues or changes in your process which called for a reconfiguration of the Baghouse. In this case OEM configurations will not fit and you will need to obtain accurate measurements for your filters before ordering replacement filter bags. If you currently have filter bags installed that are functioning properly, you can remove one of these bags to get the proper measurements for your replacement order. A spare bag that has not been used yet can also be measured if available. However, be sure to verify the bag measured is the same as the bags currently being used in the dust collector. If you are removing a used bag to measure, please be sure to use all necessary precautionary measures set in place prior to removal i.e. gloves, protective garments and respiratory equipment if needed. It is best not to rely only on the numbers off the unit of OEM filter specifications because of possible changes to the configurations. Of course the best solution is to mail the manufacturer a new or used bag that can be used a guide sample.

Flat Width: Place the filter on a flat surface such as a large table or cement floor. With the filter stretched out, press down on the side. Using a measuring tape, very accurately record the width. Be sure to hold the filter down firmly on an even surface when taking this measurement.

Diameter: When measuring the tube sheet hole of a pulse jet style dust collector, first make sure the hole has not been damaged or warped in any way. Clean the surface thoroughly with a wire brush then using a micrometer, measure the hole in both directions. If the measurements are at all different locate another hold and repeat this process.

Length: Remove the filter from the unit. Preferably with the assistance of another person, stretch the filter out. While maintaining tension on the filter record the length from the longest point at each end using a measuring tape. Do not include and straps, metal caps or other hanging hardware in the measurement, just the length of the filter itself.

Step 3 – Top & Bottom Construction

The top and bottom construction of a filter bag involves a variety of possible configurations. Identifying the type of cleaning process used by your dust collector will help to determine which configuration is needed. The most common types of dust collectors are “Pulse-Jet” “Shaker” “Reverse Air”. The chart below can help you identify which type of dust collector filter you are using.

Filter Configuration Chart

Pulse-Jet Dust Collectors (Reverse jet) – Found in almost every industrial environment. They are the most popular design and are seen in nearly all industry segments. Pulse-Jet Units can be divided into two major groups Top load or bottom load units sometime called top entry (walk-in plenum) or bottom entry (common in bin vents) because of the point of entry used to change out the filters.

Typical filter configuration for a top load unit:
Snap Band Top (double-beaded ring)
Disk Bottom (w/o wear strip)

Typical filter configuration for a bottom load unit:
Raw End Top
Disk Bottom (w/o wear strip)

Shaker Dust Collectors (Mechanical Cleaning) – Usually found in older applications where unscheduled down time is not a major concern.

Typical filter Top Configurations
Loop Top
Grommet Top
Strap or Tail Top
Metal Hanger or Cap

Typical Filter Bottom Configurations
Corded Cuff with Clamp
Snap Band
(Double-Beaded Ring)

Reverse-Air Dust Collectors – Usually found in very large air handling environments such as power generation and cement plants although they do have uses in a variety of industries. Sometimes called a structural bag, these filters usually have a series of support rings spaced every few feet throughout the length of the bag.

Typical Top Configurations
Compression band w/Metal Cap & Hook

Typical Bottom Configurations
Compression band
Snap Band
Cord w/Metal Clamp

Snap Band - Dust Collector Filter Configuration

Snap Band

Raw Edge - Dust Collector Filter Configuration

Raw Edge

Cord - Dust Collector Filter Configuration

Cord

Hanger - Dust Collector Filter Configuration

Hanger

Grommet - Dust Collector Filter Configuration

Grommet

Loop - Dust Collector Filter Configuration

Loop

Strap - Dust Collector Filter Configuration

Strap

Support Ring - Dust Collector Filter Configuration

Support Ring

Rubber O-Ring - Dust Collector Filter Configuration

Rubber O-Ring

Disk - Dust Collector Filter Configuration

Disk

Disk With Wear Strip - Dust Collector Filter Configuration

Disk With Wear Strip

Flange - Dust Collector Filter Configuration

Flange

Hem - Dust Collector Filter Configuration

Hem

Sewn Flat - Dust Collector Filter Configuration

Sewn Flat

Envelope - Dust Collector Filter Configuration

Envelope

Step 4 – Additional Options

Ground Wires – Use to comply with Factory Mutual requirements for static dissipation. Ground wire can be made from stainless steel or copper however this technique only works on a localized area of the filter. For optimal static dissipation look at conductive fiber filter made with Epitropic or Stainless Steel fibers.

Wear Cuffs – Used to combat abrasion at the bottom of the bag either from a sandblasting effect or from bag-to-bag abrasion due to turbulence in the bag house. Usually 2 to 4 inches in length and made of a material similar to that of the body of the filter bag.

Special Finishes – There are many finish options that can be added to the filter media at the time it is manufactured. Please refer to the materials selection area for further details. If you want to order a specific brand or special type of finish please add that request into the additional comments section when ordering.

 

 
About the Author

| Dominick DalSanto is an Author & Environmental Technologies Expert, specializing in Dust Collection Systems. With nearly a decade of hands-on working experience in the industry, Dominick’s knowledge of the industry goes beyond a mere classroom education. He is currently serving as Online Marketing Director & Content Manager at Baghouse.com. His articles have been published not only on Baghouse.com , but also on other industry related blogs and sites. In his spare time, Dominick writes about travel and life abroad for various travel sites and blogs.

This article has been designed to educate customers by giving a brief overview of all the Dust Collection Systems available today. A detailed explanation of the benefits and drawbacks of each type of system can be found in further articles on Baghouse.com

What is a Dust Collector?

After the contaminated air is captured by a Dry Dust Collection system, either by means of a Central Collection System, or in a unit Collector. The dust filled air then needs to be treated and the contaminates removed, before the air can be recirculated back into the facility or dispersed into the atmosphere. The Dust Collector separates the particles from the airstream and sends it on to its final destination.

Why are Dust Collectors Needed?

There are many reasons why having a proper Dust Collection System installed in your facility is needed, a few important reasons are:

•    To protect employees and society from exposure to pollution,
•    To recover valuable products from the dust filled air
•    To facilitate compliance with health and air emission standards.

Types of Dust Collectors

The five principal types of industrial dust collectors that will be discussed in this article are:

•    Cyclone Collectors (Inertial separators)
•    Baghouses (Fabric collectors)
•    Wet scrubbers
•    Electrostatic precipitators
•    Unit collectors

Cyclone Collectors (Inertial Separators)


Inertial separators
work by making use of one or more of the following forces centrifugal, gravitational, and inertial in order to separate dust from the airstream. Once separated, the dust is removed to a hopper by gravity for temporary storage. While this type of collect can be used in applications where particle sizes are large and only a “rough” air filtration is desired, the main usage for this type of collector is as a precleaner, to remove larger particles and debris and avoid overloading and damaging more efficient dust collectors.

The three types of Inertial Separators are:

•    Settling Chambers
•    Baffle Chambers
•    Centrifugal Collectors

A Settling Camber is a large box installed into the ductwork.  The sudden larger area for the airstream to pass through causes the air stream to slow down, which in turns causes the larger particles to settle to the bottom of the chamber. This type of collector is rarely used as the primary dust collector due to it’s large space requirements, and low efficiency. However, the fact that it can be fashioned from almost any material and its simple design, which requires little maintenance, leads it to being a wise choice as a precleaner for a more efficient Dust Collector.

A Baffle Chamber has a fixed baffle plate that causes the airflow to rapidly change its direction, first turning downward, and then making a 180 degree turn back up. In the process, the larger particles fall to the bottom of the chamber and can be collected from there. As with Settling Chambers this type of collector is best used as a precleaner for another more efficient collector further in the collection system. Also like a Settling Chamber its relatively simple design and low maintenance needs make it an excellent choice for the beginning of any large scale collection system.

Centrifugal Collectors create a vortex in the airstream within an enclosure, similar to water going down a drain. Normally this is done by having the airstream enter the collect at an angle, which causes it to spin. As the airstream is spun around the collector, the particles strike the wall and fall into the hopper below.

Within this category there are two main types of systems in use:

•    Single Cyclone systems
•    Multiple Cyclone systems

A Single Cyclone Collector creates a dual vortex, a main downward vortex to disperse the coarser matter, and a secondary upward vortex to remove the finer particles on the return to the outlet to the duct system.

A Multiple Cyclone Collector works in the same manner as the Single Cyclone variety albeit with several small dynamiter cyclones instead of just one. The multiple cyclones work in parallel and share the same air input and output.

Between the two, the Multiple Cyclone Collector will operate more efficiently because of being longer in length and smaller in dynamiter. The smaller dynamiter cause the centrifugal force generated to be greater, and the longer length allows for more contact with the surface of the collector by the particles thereby causing more particles to be removed from the airstream. However, a greater loss of pressure is found in Multiple Cyclone Collectors than in Single Cyclone Collectors.

Again as with the other kinds of Inertial Separators, this systems main advantage is the lack of moving parts thus requiring less maintenance and repair. While it can be designed to remove a specific size range of particles, it still remains best used as a precleaner to eliminate coarse particles and ease the load on more efficient Dust Collectors further along in the system.

Advantages & Disadvantages – Centrifugal Collectors

Types Advantages Disadvantages
Cyclones Have no moving parts Have low collection efficiency for respirable particulates
Can be used as precleaners to remove coarser particulates and reduce load on more efficient dust collectors Suffer decreased efficiency if gas viscosity or gas density increases
Can be designed to remove a specific size range of particles Are susceptible to erosion
Have drastically reduced efficiency due to reduction in airflow rate
Cannot process sticky dust
Multiple Cyclones Have no moving parts Have low collection efficiency for respirable particulates
Are more efficient than single-cyclone separators Are prone to plugging due to smaller diameter tubes
Have low pressure drop when used as a precleaner Improper gas distribution may result in dirty gas bypassing several tubes
Cannot process sticky dust
For a given gas volume, occupy more space than single-cyclone separators

Common Operating Problems & Solutions – Cyclone Collectors

Symptom Cause Solution
Erosion High concentrations of heavy, hard, sharp-edged particles Install large-diameter “roughing” cyclone upstream of high-efficiency, small-diameter cyclone.
Line high-efficiency cyclone with refractor or erosion-resistant material.
Corrosion Moisture and condensation in cyclone Keep gas stream temperature above dewpoint.
Insulate cyclone.
Use corrosion-resistant material such as stainless steel or nickel alloy.
Dust Buildup Gas stream below dewpoint Maintain gas temperature above dewpoint.
Very sticky material Install vibrator to dislodge material.
Reduced Efficiency or Dirty Discharge
Leakage in ductwork of cyclone Clean cyclone routinely.
Check for pluggage and leakage and unplug or seal the ductwork.
Close all inspection ports and openings.
Reduced gas velocity in cyclone Check the direction of fan rotation; if rotation is wrong, reverse two of the tree leads on motor.

Common Operating Problems and Solutions – Multiclones

Symptom Cause Solution
Erosion High concentrations of heavy, hard, sharp-edged particles Install cast iron tubes.
Install a wear shield to protect tubes
Overloaded tubes Uneven gas flow and dust distribution Install turning vanes in elbow, if elbow precedes inlet vane.
Loss of volume in tubes
Uneven pressure drop across tubes
Plugging in inlet vanes, clean gas outlet tubes, and discharge hopper Low gas velocity Install turning vanes in elbow inlet
Uneven flow distribution Insulate multiclone
Moisture condensation Install bin-level indicator in collection hopper.
Overfilling in discharge hopper Empty hopper more frequently.
Reduced efficiency or dirty gas stack Leakage in ductwork Seal all sections of ductwork and multiclone to prevent leaks
Leakage in multiclone

Startup/Shutdown Procedures – Centrifugal Collectors

Type Startup Shutdown
Cyclones 1. Check fan rotation. 1. Allow exhaust fan to operate for a few minutes after process shutdown until cyclone is empty.
2. Close inspection doors, connections, and cyclone discharge. 2. If combustion process is used, allow hot, dry air to pass through cyclone for a few minutes after process shutdown to avoid condensation.
3. Turn on fan. 3. Turn off exhaust fan.
4. Check fan motor current. 4. Clean discharge hopper.
5. Check pressure drop across cyclone.
Multiclones 1. Conduct same startup procedures as cyclones. 1. Conduct same shutdown procedures as cyclones.
2. At least once a month, measure airflow by conducting a pitot traverse across inlet to determine quantity and distribution of airflow.
3. Record pressure drop across multiclone.
4. If flow is significantly less than desired, block off rows of cyclone to maintain the necessary flow per cyclone.

Preventative Maintenance Procedures – Centrifugal Collectors

Type Frequency Procedure
Cyclones Daily Record cyclone pressure drops.
Check stack (if cyclone is only collector).
Record fan motor amperage.
Inspect dust discharge hopper to assure dust is removed.
Weekly Check fan bearings.
Check gaskets, valves, and other openings for leakage.
Monthly Check cyclone interior for erosion, wear, corrosion, and other visible signs of deterioration.
Multiclones Daily Same as cyclones.
Weekly Same as cyclones.
Monthly Check multiclone interior for erosion, wear, corrosion, and improper gas and dust distribution.
Inspect individual cyclones and ducts for cracks caused by thermal expansion or normal wear.

Fabric Dust Collectors

Fabric Collectors (commonly known as a Baghouse) are among the most widely used dust collection systems. They benefit from having the potential to be one of the most efficient (up to 99% of very fine particles) and cost effective dust collection systems you can choose.

The way they work

The Gas stream enters into the Baghouse via the location’s duct system. Once inside the dust filled gases come into contact with the filter bags within. As the gases pass through the filters the dust particles are trapped on the filter media. Over time a layer of cake dust is built up on the surface of the filter bags. This is the secret to this filter medium’s high efficiency potential. Once the cake dust has formed, it further impedes the passage of dust through the filters in four different ways:

•    Inertial Collection: The incoming Gas stream strikes the filter media, which is located perpendicular to the Gas flow before changing direction causing the dust particles to remain on the filter.
•    Interception: Particles that do not cross the fluid streamlines come in contact with fibers because of the fiber size.
•    Brownian movement: By means of diffusion, there is an increased chance of contact between the filter and the dust particles due to their molecular motion.
•    Electrostatic Forces: An increased attraction can occur between the dust particles and the filter media when an electrostatic charge is found on the dust particles.

Air to Cloth Ratio

An understanding of the term Air to Cloth Ratio is vital to understand the mechanics of any Baghouse system regardless of the exact type used. This ratio is defined as the amount of air or process gas entering the Baghouse divided by the sq. ft of cloth in the Baghouse. An example of an Air to Cloth Ratio is provided below courtesy of http://www.usairfiltration.com

(Bag diameter in inches x pi x bag length in inches)
Total Cloth area = 144 x total number of bags
A standard 6” bag has a 5-7/8” diameter
This bag is 12’ long
There are a total of 132 bags in the Baghouse
= (5-7/8” x 3.1416 x 144”) ÷ 144 x 132
= (5.875” x 3.1416 x 144”) ÷ 144 x 132
= (2657.79) ÷ 144 x 132
= 18.46 SF of cloth per bag x 132 bags
Total cloth area = 2,436 sq. ft.
Assume the Baghouse is handling 13,000 ACFM of air
Air to cloth ratio = ACFM ÷ total cloth area
= 13,000 ÷ 2,436
= 5.34 : 1

Different Baghouse designs

There are three main types of Baghouse systems currently in use today. The same basic mechanics are present in all of them, the main difference being how filter bags are cleaned.

•    Mechanical Shaker
•    Reverse Air
•    Reverse Jet (Or Pulse Jet)

A Mechanical Shaker is a design where the filter bags are suspended from the top of the Baghouse by horizontal beams and fastened to a cell plate on the bottom. When the Gas stream enters at the bottom of the Baghouse it is then forced up through the inside of the tubular filter bags, thereafter passing unto the airflow outlet at the top. The cleaning of this type of Baghouse is done by a shaking of the top horizontal bar that the filter bags are attached to.  This is caused by a motor driven shaft and cam system that sends waves down the surface of the filter bags causing the dust to fall off the interior of them into the hopper below.  This Baghouse has a relatively low Air to Cloth Ratio requiring large amounts of space. Despite this draw back, the simple design remains a noted advantage, leading to this system being widely used in the mineral processing industry.

In a Reverse Air Baghouse, filter bags are connected to a cell plate on the bottom of the Baghouse and are suspended from an adjustable hanger frame on top. The Gas stream, as in the Mechanical Shaker design enters into the Baghouse and passes through the filter bags from the bottom leading to the dust collecting again on the interior of the filter bags, thereafter leaving through the outlet port at the top.  Again the main difference in this style of Baghouse system when compared to others is the cleaning mechanism. In this system, a cleaning cycle starts with injecting clean air into the Collector in the reverse direction of the normal flow. This causes the compartment to become pressurized. The pressure causes the bags to collapse slightly releasing the cake dust to crack and fall off to be collected by the hopper below.  Since it is necessary to shut down normal airflow to the Baghouse during the cleaning cycle, this type of Baghouse is normally compartmentalized so as to allow for only a partial shutdown of the system.

With a Reverse Jet or Pulse Jet Baghouse, the same basic design is found as in the other types of Baghouse design, however, with a few very important differences. In a Pulse Jet Baghouse, the baghouse filter bags are individually overlaid on a metal cage, which is then attached to a cell plate at the top of the compartment. The Gas stream enters the Baghouse at the bottom and is forced through the outside to the inside of the filter bags after which the Gas stream exits the compartment from the outlet port at the top. The main advantage of this Baghouse is that it does not require a shutdown of any kind to run a cleaning cycle. A digital sequential timer is attached to the one of the filter bags inside the Baghouse. This timer signals a solenoid valve to start the cleaning cycle when it detects a certain amount of build up on the bag. It consists of a small burst of compressed air being fired down through the filter bags. Which cause the excess cake dust to fall off into hopper at the bottom of the Baghouse where it can be collected. The cleaning cycle of the Pulse Jet collectors provides a more complete cleaning and reconditioning of the filter bags than in the Shaker, and Reverse Air designs. Also the short nature of the cleaning cycle also leads to a reduction in the recirculation and redeposit of dust. Finally, enabled by the continuous cleaning feature of the design, this kind of collection system has a higher Air to Cloth Ratio so the space requirements are much lower than in other systems.

Cartridge Collectors

Unlike Baghouse collectors which feature the use of woven or felt filter bags, Cartridge Collectors use perforated metal cartridges that are cylindrical shaped and open on one or both ends lined with a pleated nonwoven filtering media. Once installed, one end of the cartridge is sealed off and the open end is used for the clean exhaust. Similar to a Baghouse, the Gas stream is forced through the outside of the cartridge to the inside where it then exits back into the system. Cartridge Collectors are also compatible with Reverse or Pulse Jet cleaning. Large numbers of these Collectors can be installed and used for continuous filtration for a location’s dust collection system.

Advantages and Disadvantages – Baghouses

Types Advantages Disadvantages
Shaker Baghouses Have high collection efficiency for respirable dust Have low air-to-cloth ratio (1.5 to 2 ft/min)
Can use strong woven bags, which can withstand intensified cleaning cycle to reduce residual dust buildup Cannot be used in high temperatures
Simple to operate Require large amounts of space
Have low pressure drop for equivalent collection efficiencies Need large numbers of filter bags
Consist of many moving parts and require frequent maintenance
Personnel must enter Baghouse to replace bags, creating potential for exposure to toxic dust
Can result in reduced cleaning efficiency if even a slight positive pressure exists inside bags
Reverse Air Baghouses Have high collection efficiency for respirable dust Have low air-to-cloth ratio (1 to 2ft/min)
Are preferred for high temperatures due to gentle cleaning action Require frequent cleaning because of gentle cleaning action
Have low pressure drop for equivalent collection efficiencies Have no effective way to remove residual dust buildup
Cleaning air must be filtered
Require personnel to enter baghouse to replace bags, which creates potential for toxic dust exposure
Pulse Jet (Reverse Jet) Baghouses Have a high collection efficiency for respirable dust Require use of dry compresses air
Can have high air-to-cloth ratio (6 to 10ft/min) May not be used readily in high temperatures unless special fabrics are used
Have increased efficiency and minimal residual dust buildup due to aggressive cleaning action Cannot be used if high moisture content or humidity levels are present in the exhaust gases
Can clean continuously
Can use strong woven bags
Have lower bag wear
Have small size and fewer bags because of high air-to-cloth ratio
Some designs allow bag changing without entering Baghouse
Have low pressure drop for equivalent collection efficiencies

Common Operating Problems and Solutions – Baghouses*

Symptom Cause Solution
High Baghouse pressure drop Baghouse undersized consult vendor
Install double bags
Add more compartments or modules
Bag cleaning mechanism not properly adjusted Increase cleaning frequency
Clean for longer duration
Clean more vigorously
Shaking not strong enough (S) Increase shaker speed
Compartment isolation damper valves not operating properly (S, RA) Check linkage
Check valve seals
Check air supply of pneumatic operators
Compressed air pressure too low (PJ) Increase pressure
Decrease duration and frequency
Check compressed-air dryer and clean it if necessary
Check for obstructions in piping
Repressurizing pressure too low (RA) Speed up repressurizing fan.
Check for leaks
Check damper valve seals
Pulsing valves failed (PJ) Check diaphragm
Check pilot valves
Bag tension too tight (RA) Loosen bag tension
Bag tension too loose (S) Tighten bags
Cleaning timer failure Check to see if timer is indexing to all contacts
Check output on all terminals
Not capable of removing dust from bags Check for condensation on bags
Send dust sample and bags to manufacturer for analysis
Dryclean or replace bags
Reduce airflow
Excessive reentrainment of dust Empty hopper continuously
Clean rows of bags randomly instead of sequentially (PJ)
Incorrect pressure-drop reading Clean out pressure taps
Check hoses for leaks
Check for proper fluid level in manometer
Check diaphragm in gauge
Dirty Discharge at stack Bags leaking Replace bags
Isolate leaking compartment or module
Tie off leaking bags and replace them later
Bag clamps not sealing Smooth out cloth under clamp and re-clamp
Check and tighten clamps
Failure of seals in joints at clean/dirty air connection Caulk or weld seams
Insufficient filter cake Allow more dust buildup on bags by cleaning less frequently.
Use precoating on bags (S, RA).
Bags too porous Send bag in for permeability test and review with manufacturer
High compressed-air consumption (PJ) Cleaning cycle too frequent Reduce cleaning cycle, if possible
Pulse too long Reduce pulsing duration
Pressure too high Reduce supply pressure, if possible
Diaphragm valve failure Check diaphragm and springs
Check pilot valve
Reduced compressed-air pressure (PJ) Compressed-air consumption too high See previous solutions
Restrictions in compressed-air piping Check compressed-air piping
Compressed-air dryer plugged Replace dessicant in the dryer
Bypass dryer temporarily, if possible
Replace dryer
Compressed-air supply line too small Consult design
Compressor worn out Replace rings
Check for worn components
Rebuild compressor or consult manufacturer
Pulsing valves not working Check pilot valves, springs, and diaphragms
Timer failed Check terminal outputs
Moisture in Baghouse Insufficient preheating Run the system with hot air only before process gas flow is introduced
System not purged after shutdown Keep fan running for 5 to 10 min after process is shut down
Wall temperature below dewpoint Raise gas temperature
Insulate unit
Lower dewpoint by keeping moisture out of system
Cold spots through insulation Eliminate direct metal line through insulation
Water/moisture in compressed air (PJ) Check automatic drains
Install aftercooler
Install dryer
Repressurizing air causing condensation (PJ) Preheat repressurizing air
Use process gas as source of repressurizing air
Material bridging in hopper Moisture in Baghouse See previous solutions
Dust stored in hoppers Remove dust continuously
Hopper slope insufficient Rework or replace hoppers
Screw conveyor opening too small Use a wide, flare trough
High rate of bag failure, bags wearing out Baffle plate worn out Replace baffle plate
Too much dust Install primary collector
Cleaning cycle too frequent Slow down cleaning
Inlet air not properly baffled from bags Consult vendor
Shaking too violent (S) Slow down shaking mechanism
Repressurizing pressure too high (RA) Reduce pressure
Pulsing pressure too high (PJ) Reduce pressure

* S =  Shaker
RA = Reverse Air
PJ  = Pulse Jet

Startup/Shutdown Procedures – Baghouses

Startup Shutdown
1. For processes generating hot, moist gases, preheat Baghouse to prevent moisture condensation, even if Baghouse is insulated. (Ensure that all compartments of shaker or reverse-air Baghouses are open.) 1. Continue operation of dust-removal conveyor and cleaning of bags for 10 to 20 minutes to ensure good removal of collected dust.
2. Activate Baghouse fan and dust-removal conveyor.
3. Measure Baghouse temperature and check that it is high enough to prevent moisture condensation.

Preventive Maintenance Procedures – Baghouses

Frequency    Procedure

Daily

•    Check pressure drop.
•    Observe stack (visually or with opacity meter).
•    Walk through system, listening for proper operation.
•    Check for unusual occurrences in process.
•    Observe control panel indicators.
•    Check compressed-air pressure.
•    Assure that dust is being removed from system.

Weekly

•    Inspect screw-conveyor bearings for lubrication.
•    Check packing glands.
•    Operate damper valves.
•    Check compressed-air lines, including line filters and dryers.
•    Check that valves are opening and closing properly in bag-cleaning sequence.
•    Spot-check bag tension.
•    Verify accuracy of temperature-indicating equipment.
•    Check pressure-drop-indicating equipment for plugged lines.

Monthly

•    Check all moving parts in shaker mechanism.
•    Inspect fans for corrosion and material buildup.
•    Check drive belts for wear and tension.
•    Inspect and lubricate appropriate items.
•    Spot check for bag leaks.
•    Check hoses and clamps.
•    Check accuracy of indicating equipment.
•    Inspect housing for corrosion.

Quarterly

•    Inspect baffle plate for wear.
•    Inspect bags thoroughly.
•    Check duct for dust buildup.
•    Observe damper valves for proper seating.
•    Check gaskets on doors.
•    Inspect paint, insulation, etc.
•    Check screw conveyor for wear or abrasion.

Annually

•    Check fan belts.
•    Check welds.
•    Inspect hopper for wear

Wet Scrubbers

Another effective method of dust collection is the use of Wet Scrubbers (Air Washers). These systems use a scrubbing liquid (usually water) to filter out finer dust particles.  After being filtered the Gas Stream is then sent through a mist eliminator (demister pads) to remove the excess moisture from the Gas stream. Afterward the Gas stream exits the collector through the outlet port and returns back into the system. Wet Scrubbers are ideal:

•    For the collection of explosive material
•    Where “slurry” produced could be reused (either in other parts of process or sold)
•    Where chemical reactions could be generated with other collection methods
•    To absorb excess air

Wet scrubbers have the advantage of low start up costs and low space requirements. They are well suited for treating high temperature and high humidity Gas streams. They also are able to process both air and “sticky” particulates.  The main disadvantages are that they are costly to operate, require a precleaner for any heavy dust loads, cause water pollution that then needs to be addressed, and can erode with high air velocities.

There are a vast variety of different designs and applications of this type of filtration system but all of them have three basic operations they perform:

•    Gas-humidification: The gas-humidification process conditions fine particles to increase their size so they can be collected more easily.
•    Gas-liquid contact: This is the entire basis for the operation of this type of system. The method of contact between the liquid is done in four main ways:

•    Inertial impaction takes place when the Gas stream is forced to flow around the droplets in its path. The stream separates and flows around the droplet. However the larger particles continue to be carried by inertial force in a straight path coming in direct contact with the liquid.
•    Interception: Finer particles while not directly coming in contact with the droplets, do however brush up against the side of them causing them to be absorbed into the liquid.
•    Diffusion occurs when a fine mist is created from the liquid being used. As the particles pass through the mist they make contact with the surfaces of the droplets by means of the Brownian effect, or diffusion.
•    Condensation nucleation is the effect of a gas being cooled below its dew point while within a moisture rich environment, causing the vapor to condense of the surface of the particles thereby encapsulating them.

•    Liquid separation: After going through the cleaning phase the remaining liquid and contaminates must be removed before the Gas stream can be sent back into the system. This is accomplished by means of a Mist Eliminator (Demister Pads). Which remove the liquid and dust mixture from the Gas stream and send it to a collector. Once in the collector, the solid waste settles to the bottom where it is removed by means of a drag chain system to be deposited in a dumpster or another collection area.
Wet Scrubbers are further categorized by pressure drop (in inches water gauge) as follows:

•    Low-energy scrubbers (0.5 to 2.5)
•    Low- to medium-energy scrubbers (2.5 to 6)
•    Medium- to high-energy scrubbers (6 to 15)
•    High-energy scrubbers (greater than 15)
The large amount of different Wet Scrubbers in use makes it impossible to comment on every single design in this article. However a brief overview of the most common types will enable you to understand the basic operational procedures present in all of them.

Low Energy Scrubbers:

•    The most basic design is that of a Gravity Spray Tower Scrubber. In this system the contaminated air enters at the bottom of the cylindrically shaped scrubber and rises through a mist of water sprayed from nozzles at the top. The dirty water collects at the bottom of the tank and the clean air (mist) exits from the top of the collector. This collector has a relatively low efficiency compared to other kinds of Wet Scrubbers. However it’s main advantage is it can handle very heavy dust loads without getting backed up.

•    Dynamic wet precipitators also called Wet Fan Scrubbers are a popular design used for medium energy scrubbing applications. In this system the Gas stream passes through a larger fan that is constantly kept wet with the cleaning liquid. The particles are trapped in the liquid and are then by means of centrifugal force thrown off the spinning fan blades unto the sides of the collector where they eventually settle at the bottom enabling them to be collected.

•    Orifice Scrubbers work in a very similar way to inertial separators but with one important difference, Orifice Scrubbers use a water surface to capture the dust particles. When the Gas stream enters the collector it is rapidly redirected when it comes in contact with the water surface. Causing the dust particles to be removed from the Gas stream. A greater efficiency can be obtained by the addition of liquid spray nozzles to further separate the contaminates from the Gas stream. While these are an effective filtration system one should note that they tend to be ineffective against fine particles as these tend to be redirected off of the water surface by the high surface tension.

Low to Medium Energy Scrubbers:

•    Wet Cyclone Scrubbers are nearly identical to their normal cyclone collector counterparts. In a Wet Cyclone Scrubber the Gas stream enters the collector and is then forced into a cyclone movement by the strategic placement of stationary scrubbing vanes. Liquid is introduced at the top of the collector allowing the dust particles to stick to the wet walls of the collector when they are thrown off by the vortex. As with dry Cyclone Collectors, this type of system has the benefit of few to no moving parts and it is efficient for particles up to 5um and above.

Medium to High Energy Scrubbers:

•    Packed Bed Scrubbers consist of a bed of packing media, which is then sprayed with water. The packing media allows for a very wide distribution of the water, which in turn allows the Gas stream to have the maximum contact with the water during its passage though the collector. Air enters at the bottom of the collector where it first makes contact with the water in the recirculation tank. Then it is forced up through the various layers of the filtering media, and after being sent through a Mist Eliminator is sent back into the system via the exit port at the top.

Within the category of Packed Bed Scrubbers there are three different variations on the implementation of this filtering mechanism they are:

•    Cross-flow scrubbers are designed to minimize height for low-profile applications. In this design the packed media is laid as sheets perpendicular to the Gas stream. The Gas stream enters in one side of the Scrubber and flows horizontally through it passing though the packing media and then exiting out the opposite side
•    Co-current flow scrubbers
•    Counter-current flow scrubbers

High energy Scrubbers:

•    Venturi Scrubbers make use of the Venturi effect to accelerate the Gas stream to speeds of 12,000 to 36,000 ft/min. The Gas stream enters into the Scrubber through a Venturi shaped inlet where it is sprayed with water. The water hitting the extremely high speed air causes it to instantly atomize. The very fine water droplets attach to the dust particles and form a slurry, which then falls to the bottom of the collector. After passing through a Mist eliminator the Gas stream is sent back into the system.

Advantages and Disadvantages – Wet Scrubbers

Advantages Disadvantages
Have low capital costs and small space requirements Have high operating and maintenance costs
Have low capital costs and small space requirements Require corrosion-resistant materials if used with acidic gases
Are able to collect gases as well as particulates (especially “sticky” particulates) Require a precleaner for heavy dust loadings
Have no secondary dust sources Cause water pollution; require further water treatment
Are susceptible to erosion at high velocities
Collect wet products
Require freeze protection

Common Operating Problems and Solutions – Wet Scrubbers

Problem Solution
Wet/dry buildup Keep all areas dry or all areas flooded.
Use inclined ducts to a liquid drain vessel.
Ensure that scrubber is installed vertically.
Maintain liquid seal.
Dust buildup in fan Install clean water spray at fan inlet.
Excessive fan vibration Clean fan housing and blades regularly.
Liquid pump failure Divert some of the recycle slurry to a thickener, settling pond, or waste disposal area and supply clean water as makeup.
Increase the water bleed rate.
Worn valves Use wear-resistant orifice plates to reduce erosion on valve components.
Jammed valves Provide continuous purge between valves and operating manifold to prevent material buildup.
Erosion of slurry piping Maintain pumping velocity of 4 to 6 ft/s to minimize abrasion and prevent sedimentation and settling.
Plugged nozzles Replace nozzles or rebuild heads.
Change source of scrubbing liquid.
Supply filtered scrubbing liquid.
Buildup on mist eliminators For vane-type demisters, spray the center and periphery intermittently to clean components.
For chevron-type demisters, spray the water from above to clean the buildup.

Startup/Shutdown Procedures – Wet Scrubbers

Prestart Checklist Shutdown
1. Start fans and pumps to check their rotation. 1. Shut down fan and fan spray. Insulate scrubber from operation.
2. Disconnect pump suction piping and flush it with water from an external source. 2. Allow liquid system to operate as long as possible to cool and reduce liquid slurry concentrations.
3. Install temporary strainers in pump suction line and begin liquid recycle. 3. Shut off makeup water and allow to bleed normally.
4. With recycle flow on, set valves to determine operating conditions for desired flow rates. Record the valve positions as a future baseline. 4. When pump cavitation noise is heard, turn off pump and pump gland water.
5. Record all system pressure drops under clean conditions. 5. Open system manholes, bleeds, and other drains.
6. Perform all recommended lubrications.
7. Shut down fan, drain the system, and remove temporary strainers.
Startup
1. Allow vessels to fill with liquid through normal level controls. Fill large-volume basins from external sources.
2. Start liquid flow to all pump glands and fan sprays.
3. Start recycle pumps with liquid bleed closed.
4. Check insulation dampers and place scrubber in series with primary operation.
5. Start fan and fan inlet spray. Leave inlet control damper closed for 2 min to allow fan to reach speed.
6. Check gas saturation, liquid flows, liquid levels, fan pressure drop, duct pressure drops, and scrubber pressure drop.
7. Open bleed to pond, thickener, or other drain systems so slurry concentration can build slowly. Check final concentration as cross-check on bleed rate.

Preventative Maintenance Procedures – Wet Scrubbers

Frequency Procedure

Daily

•    Check recycle flow.
•    Check bleed flow.
•    Measure temperature rise across motor.
•    Check fan and pump bearings every 8 hours for oil level, oil color, oil temperature, and vibration.
•    Check scrubber pressure drop.
•    Check pump discharge pressure.
•    Check fan inlet and outlet pressure.
•    Check slurry bleed concentration.
•    Check vibration of fan for buildup or bleeds.
•    Record inlet and saturation temperature of gas stream.
•    Use motor current readings to detect flow decreases. Use fan current to indicate gas flow.
•    Check pressure drop across mesh and baffle mist eliminators. Clean by high-pressure spraying, if necessary.
Weekly

•    Check wet/dry line areas for material buildup. Clean, if necessary.
•    Check liquid spray quantity and manifold pressure on mist eliminator automatic washdown.
•    Inspect fans on dirty applications for corrosion, abrasion, and particulate buildup.
•    Check bearings, drive mechanisms, temperature rise, sprocket alignment, sprocket wear, chain tension, oil level, and clarifier rakes.
•    Check ductwork for leakage and excessive flexing, Line or replace as necessary.
•    Clean and dry pneumatic lines associated with monitoring instrumentation.
Semiannually

•    Verify accuracy of instruments and calibrate.
•    Inspect orifice plates.
•    Clean electrical equipment, including contacts, transformer insulation, and cooling fans.
•    Check and repair wear zones in scrubbers, valves, piping, and ductwork.
•    Lubricate damper drive mechanisms and bearings. Verify proper operation of dampers and inspect for leakage.

Electrostatic Precipitators

Electrostatic Precipitators use electrostatic forces to collect dust from the Gas stream. Several high power Direct Current Discharge Electrodes are places inside the collector. The incoming Gases pass by the first set of Discharging Electrodes (ionizing section) that give the particles a negative charge (ionization). The now ionized particles travel pass the next set of electrodes (the collection section) that carry a positive charge. The positively charged plates attract the negatively charged particles causing them to collect on the plates. Cleaning is accomplished by vibrating the electrodes either continuously, or at a timed interval, causing the captured dust to fall off into a hopper below. All of this can be done while the system is operating normally.

Electrostatic Precipitators are best used in an ambient capture type system with low particle loads. Without an automated self-cleaning feature, this type of collector can very easily reach its maximum particle retention limit, which will result in a system failure. Further, for a high dust load system a great amount of dust storage is needed. Media Filtration (Baghouse) or Pleated Filtering Media (Cartridge Collectors) provide a much great surface area for dust storage than Electrostatic Precipitator systems do.  However the advantages of this system are great for their intended applications. They have the ability to be extremely efficient (in excess of 99.9% in some cases), can function within vary large temperature ranges (between 700 °F and -1300°F), and can have large flow rates with minimal pressure and temperature changes. They are also very well suited for the collection of fine dust particles as well as materials like acids and tars which other system may have difficulty with.

All electrostatic Precipitators have four main components:

•    A Power supply to provide the system with electricity
•    An Ionizing section to negatively charge the incoming particles
•    A cleaning system designed to remove collected particles from the Electrode collection plates
•    A housing to enclose the Precipitator section

Within the category of Electrostatic Precipitator Collectors, there are two main types of systems:

•    High Voltage Single State Precipitators (Cottrell type)
•    Low Voltage Dual State Precipitators (Penny type)
High Voltage Single State Precipitators are further divided between two main designs:

Plate Precipitators are made up of several flat parallel plate collectors that are usually between 8 and 12in apart. Placed directly in the middle of each set of directly adjacent plates are a series of high voltage (40,000-70,000 volts) DC Discharge Electrodes. As the Gas stream passes through the plates it is ionized by the Discharge Electrodes and then immediately deposited unto the collection plates. The plates are then cleaned by vibrating the plates causing the debris to fall into a hopper or collection bin below. The majority of Single State Precipitators in use today are of the plate variety.

Tubular Precipitators operate in the same manner as Plate Precipitators however in a different configuration. This design uses a tubular shaped collection device with the Discharge Electrodes placed in the middle of the tube. As the Gas stream flows through the tube it is first ionized by the Discharge Electrode in the center, and then the charged particles are attracted to the inside of the positively charged tube. The cleaning mechanism can be one nearly identical to that of Plate Precipitators or it can be used as part of a Wet Static Precipitator system, wherein the sides of the Precipitator are flushed with water thereby cleaning them.
Tubular Precipitators are widely used in the mineral processing industry. They are highly valuable for use in high temperature Gas streams (boiler exhaust gas on power plants) because of their ability to adjust to the expansion and contraction of metal parts in the system. In addition this type of collector is also able to handle vapor collection, containing adhesive, “sticky”, radioactive, and extremely toxic compounds.

Low Voltage Dual Stage Precipitators contain several grounded plates about one inch from each other with another intermediate plate that also contains a charge. This system uses a much lower voltage than the High Voltage type (a 13,000-15,000 volt DC supply with intermediate supply of 7,500 compared to 40,000 to 70,000). This type of system is widely used to collect fumes and particles generated by welding, grinding or burning operations. They are also used in hooded and ducted welding machines and welding booths.
Low Voltage Dual Stage Precipitators have the advantages of being highly efficient, the possibility of a self contained washing system, and a longer service life since cleaning is only required on a monthly basis. However because maintenance requires removing the Precipitator Frames and the manual cleaning of the cleaning assemblies which are quite delicate, this type of Precipitator requires a great amount more care and caution to be used when performing maintenance.

Advantages and Disadvantages – Electrostatic Precipitators

Advantages Disadvantages
Have collection efficiencies in excess of 99% for all particulates, including sub-micron-sized particles Have high initial investment costs
Usually collect dust by dry methods Do not respond well to process changes such as changes in gas temperature, gas pressure, gas flow rate, gaseous or chemical composition, dust loading, particulate size distribution, or electrical conductivity of the dust
Have lower pressure drop and therefore lower operating costs Have a risk of explosion when gas stream contains combustibles
Can operate at high temperatures (up to 1200º F) and in colder climates Product ozone during gas ionization
Can remove acids and tars (sticky dust) as well as corrosive materials Require large space for high efficiency, and even larger space for dust with low or high resistivity characteristics
Allow increase in collection efficiency by increasing precipitator size Require special precautions to protect personnel from exposure to high-voltage
Require little power Require highly skilled maintenance personnel

Unit Collectors

For certain applications, Unit Collectors are a better choice for a facilities needs than a conventional Central Collection System. These collectors control contamination at their source. They benefit from low initial cost, direct return of captured material to the main material flow, and very low space requirements. These collectors are best used when the dust source is isolated, portable or changes position often. Some examples of instances where this type of collector might be useful are dust-producing operations, such as bins and silos or remote belt-conveyor transfer points.

Depending on the particular desired application is there are a number of different designs available to choose from with a capacity of 200 to 2,000 ft3/min. The two main types are:

•    Fabric Collectors
•    Cyclone Collectors

Unit Fabric Collectors are very similar to their bigger relatives used in a Central Collection System. They usually employ either a Mechanical Shaker, or a Pulse Jet system for cleaning. This type is well suited for the collection of fine particles such as in the mineral processing industry.
Unit Cyclone Collectors also operate on the same principles are the kind used in Central Collection Systems. Dust is collected and deposited into a hopper, which then can be removed later for cleaning. This type of collector is best used in the collection of coarse of larger particles.

Central Collection System

Every Dust Collection System must have a Central Collection System in place in order to send the contaminated air to the filtration system. A Central Collection System consists of a series of collection inlets, and the necessary duct work to transport the dust laden Gas stream to the collector and afterward on to be either recirculated back into the facility or dispersed into the atmosphere. The pressure in this duct system is supplied by the Fan and Motor System.

Fan and Motor

Choosing the right Fan and Motor System requires a number of different factors to be taken into consideration including but not limited to:

•    Volume required
•    Fan static pressure
•    Type of material to be handled through the fan (For example, a Radial Blade Fan should be used with fibrous material or heavy dust loads, and a nonsparking construction must be used with explosive or flammable materials.)
•    Limitations in space
•    Acceptable levels of noise caused by the fan
•    Required operational temperature (For example, sleeve bearings are suitable to 250º F; ball bearings to 550º F.)
•    Adequate size to handle pressure and volume requirements with minimum horsepower usage
•    Whether any corrosive materials are going to be handled and what protective coatings may be needed
•    Ability of fan to accommodate small changes in total pressure while maintaining the necessary air volume
•    Need for an outlet damper to control airflow during cold starts (If necessary, the damper may be interlocked with the fan for a gradual start until steady-state conditions are reached.)

Also to be considered is what type of drive system for the fan you plan to use. A Direct Drive fan is run directly off of a drive shaft from the motor, this provides for lower space needs, but places the fan at a constant unchangeable speed. While Belt Driven fan, which uses a belt to flywheel configuration needs more space, it can allow for the fan speed to be easily changed which is vital for some applications.

There are two main types of fan designs that are used in industrial applications:

•    Centrifugal fans
•    Axial-flow fans

A Centrifugal Fan (also called a Squirrel-cage fan for its resemblance rodent exercise devices) is a fan build with blades (or ribs) surrounding a central hub.  The air enters into the side of the fan and then turns 90° and is accelerated and thrown out of the fan by means of centrifugal force.  The diverging shape of the scroll also converts a portion of the velocity pressure into static pressure. The fan is driven by means of a drive shaft that extends out from the center hub of the fan.

There are three main types of Centrifugal fan blades that can be used:

•    Forward Curved Blades
•    Backward Curved Blades
•    Straight Radial Blades

Forward Curved Bladed Fans have blades that are curved in the direction of the rotation of the fan. These fans are highly sensitive to particulate buildup and are used for high airflow, low pressure applications.

Backward Curved Bladed Fans contain blades that are positioned away from the fans rotation direction. These fans will provide an efficient operation, and can be used in Gas streams with light to medium particle concentration. While they can be fitted with wear protection, this type of blade can still become backed up if the particle load gets to be too heavy. This fan type is most often employed in medium speed, high pressure, and medium airflow applications.

Straight Radial Bladed Fans provide the best choice for heavy particle loads. This design features a series of blades that extend straight out from the center hub. This design is used for high pressure, high speed and low volume applications.

Fan dampeners

Fan dampeners are metal plates that can be adjusted to reduce the energy usage of the fan. Placed on the Outlet port of a fan, they are used to impose a flow resistance to control the Gas stream. They also can be placed on the Inlet port, which can perform the same function, as well as redirect how the Gas stream enters into the fan.

Axial Flow Fans

Axial Flow Fans have blades that are mounted unto a center drive shaft. They induce the air to move parallel to the shaft the blades are mounted on by the screw-like action of the propellers. The air is blown across the axis of the fan hence the name Axial Flow Fans. This type of fan is commonly used in systems with low resistance levels.

The three main different designs of Axial Flow Fans are:

•    Propeller
•    Tube Axial
•    Vane Axial

Propeller Fans is the most simple fan design. It is used to move large an amount of air against very low static pressure from the rest of the system. General and Dilution ventilation are two common uses for this type of axial fan.

The Tube Axial design is very similar to a normal propeller type fan, except that the propeller is enclosed in an open ended cylinder. This design is more efficient than simple propeller types and is often used in moving Gas streams filled with condensable fumes or pigments.

Vane Axial Fans are nearly identical to Tube Axial Fans. But these contain specially attached vanes that are designed to straighten the Gas stream as is passes through the fan. These can produce high static pressures relative to this type of fan. However these fans are in most applications used only for clean air.

Fan Rating Table

Once all of the preceding material has been examined, the final step in the selection of the proper fan for your system is to consult a Fan Rating Table. This is used to list all of the specifications for the various fans produced by a certain manufacturer. When reviewing a Fan Rating Table one must keep these few points in mind:

•    The rating tables show all of the possible pressures and speeds that can be achieved within the limits of the fan’s normal operation range.
•    A fan that operates at a single or fixed speed and has a fixed blade setting will only have one possible rating. The only way to gain multiple ratings is by varying the speed and the blade setting.
•    It may be possible to obtain the same fan in different construction classes
•    Increasing the exhaust volume will in turn increase the static and total pressure in the system

Fan installation

Once a system has been installed in the field, inevitably certain differences between design and field installation will require a field test to be done to find the exact measurements of static pressure and volume. This step is crucial in order for a proper fan system to be selected. A few brief points are good to keep in mind during the installation process that can cause your real world values to change from the original design specifications:

•    Elbows and bends near the fan’s discharge will increase the systems resistance thereby lowering your fan’s performance
•    Make sure to install the impeller in the proper direction desired.
•    Certain types of fittings such as elbows, mitered elbows and square ducts, can cause a nonuniform airflow which in turn will again lower performance
•    Build up of debris in the inlets, blades, passages as well as obstructions should be checked and remedied
•    In a belt driven system one must check the motor sheave and fan sheave are properly aligned and that proper belt tension is present

Electric Motors

An Electric Motor is what supplies the power necessary to operate the fan (Blower) in the Dust Collection System.  Electric Motors are usually grouped as either Induction, or Synchronous designs. Induction designs are the only ones that are used in Dust Collection Systems today.
Induction Motors normally operate on three phase AC current. The two most common types used in Dust Collection Systems are:

•    Squirrel Cage Motors are generally used where a constant speed is desired
•    Slip Ring Motors by contrast are general purpose or continuous rated motors that are used in applications where there is a need for an adjustable speed in the motor.

Another important design consideration is whether the Motor is one of these two enclosure designs:

•    Drip Proof and Splash Proof Motor are types of Open Enclosed Motors, which use a kind of electric motor enclosure that has vents to allow airflow but to prevent liquids and solids from entering the motor. This design is not suitable for application where particles that can damage the interior of a motor are found in the ambient atmosphere around the motor.
•    Totally Enclosed Motors have an exterior fan mounted unto the backside of the motor drive end. The fan blows air over the motor enclosure to provide additional cooling for the motor. Since the actual motor is totally enclosed this design provides the best protection against dust and other contaminates that might damage the motor if allowed inside.
Both Types can also be constructed in explosion and dust ignition proof models to protect against accidental ignition of dust particles.
The following factors need to be considered when choosing which motor meets your needs:

•    Horsepower and RPMs
•    Power supply needs such as voltage, single or three phase AC and frequency
•    The environment in which the motor will have to operate (humidity, temperature, open flames or corrosive elements
•    What kind of load is going to be placed on the motor (fan and other drive mechanisms) and power company restrictions on cold starts.
•    Sufficient power supply for cold starts
•    Overload protection needed for the particular motor

Fan & Motor Troubleshooting Chart

Symtom Probable Cause Solution
Insufficient airflow, low ft3/min Fan
Forward curved impeller installed backwards Reinstall impeller
Fan running backwards Change fan rotation by reversing two of the three leads on the motor
Impeller not centered with inlet collar(s) Make impeller and inlet collar(s) concentric
Fan speed too low Increase fan speed by installing smaller diameter pulley
Elbows or other obstructions restricting airflow Redesign ductwork
Install turning vanes in elbow
Remove obstruction in ductwork
No straight duct at fan inlet Install straight length of ductwork, at least 4 to 6 duct diameters long, where possible
Increase fan speed to overcome this pressure loss
Obstruction near fan outlet Remove obstruction or redesign ductwork near fan outlet
Sharp elbows near fan outlet Install a long radius elbow, if possible
Install turning vanes in elbow
Improperly designed turning vanes Redesign turning vanes
Projections, dampers, or other obstructions near fan outlet Remove all obstructions
Duct System
Actual system more restrictive (more resistant to flow) than expected Decrease system’s resistance by redesigning ductwork
Dampers closed Open or adjust all dampers according to the design
Leaks in supply ducts Repair all leaks in supply duct
Too much airflow, high ft3/min Fan
Backward inclined impeller installed backwards (high horsepower) Install impeller as recommended by manufacturer
Fan speed too fast Reduce fan speed
Install larger diameter pulley on fan
Duct System
Oversized ductwork; less resistance Redesign ductwork or add restrictions to increase resistance
Access door open Close all access and inspection doors
Low static pressure, high ft3/min Fan
Backward inclined impeller installed backwards (high horsepower) Install impeller as recommended by manufacturer
Fan speed too high Reduce fan speed
Install larger diameter pulley on fan
Duct System
System has less resistance to flow than expected Reduce fan speed to obtain desired flow rate
Gas Density
Gas Density lower than anticipated (due to high temperature gases or high altitudes) Calculate gas flow rate at desired operating conditions by applying appropriate correction factors for high temperature or altitude conditions
Low static pressure, low ft3/min Duct System
Fan inlet and/or outlet conditions not same as tested Increase fan speed
Install smaller diameter pulley on fan
Redesign ductwork
High static pressure, low ft3/min Duct System
Obstructions in system Remove obstructions
Duct system too restricted Redesign ductwork
Install larger diameter ducts
High horsepower Fan
Backward inclined impeller installed backwards Install impeller as recommended by manufacturer
Fan speed too high Reduce fan speed
Install larger diameter pulley on fan
Duct System
Oversized ductwork Redesign ductwork
Access door open Close all access/inspection doors
Gas Density
Calculated horsepower requirements based on light gas (e.g., high temperature or high altitude) but actual gas is heavy (eg.,cold startup) Replace motor
Install outlet damper, which will open gradually until fan comes to its operating speed
Fan Selection
Fan not operating at efficient point of rating Redesign system
Change fan
Change motor
Fan does not operate Electrical
Blown Fuses Replace Fuses
Electricity turned off Turn on Electricity
Wrong voltage Check for proper voltage on fan
Motor too small and overload protector has broken circuit Change motor to a larger size
Mechanical
Broken belts Replace belts
Loose pulleys Tighten or reinstall pulleys
Impeller touching scroll Reinstall impeller properly

Dust Disposal

After the Airstream has been cleaned, the dust that has been collected must be disposed of in a proper way to ensure that recontamination is avoided.  In many cases where the collected material is of value, it can be returned to the product stream and reused. However this is not practical in all applications. Minimizing secondary dust problems is also a key component in an effective dust disposal system. Operations such as loading and unloading of the collected material, or the transportation of wet slurry can present further contamination problems that need to be addressed.
All Disposal Systems have to accomplish these four objectives without further contaminating the environment, in order to be effective in their role in the Dust Collection System:

•    Collected material from the hopper must be removed
•    Transportation to storage
•    Storage of the collected material
•    Treatment necessary before final disposal

Removal Of Dust From The Hopper

The hopper must be emptied of the collected dust on a regular basis to prevent overfilling. Often this process is done while the collector is still operating. If this is the case, rotary air locks, or tipping valves need to be used in order to maintain a positive air seal and thus avoid massive pressure loss that would be detrimental to the normal operation of the system. Some materials display what is called a bridging tendency, which is a tendency to stick together and form long strands that can over time build up into bridge like formations that can impede the normal operations within a hopper. If material of this kind is present in the system, special equipment such as bin vibrators, rappers, or air jets should be used to ensure that the material that has a bridging tendency does not interfere with normal operation of the hopper.

Dust Transportation

Once the dust has been removed from the collector, it must be transported to a storage area where is can be given any final treatments needed before it is disposed of.  There are four main systems that can be used to transport the collected material to holding there are:

•    Screw conveyers
•    Air conveyers
•    Air Slides
•    Pressurized piping system for wet material (Slurry)

Screw conveyers use rotating shaft to move material to the desired location. These systems are very effective methods of dust transportation.  However several areas of concern in this type of system are that they tend to have a noted lack of easy access for maintenance purposes, the castings and bearings can wear out easily when used with abrasive materials with air leaks being the end result.

Air Conveyers are used mainly for dry dust applications. Making use of a high velocity low air volume principle, these collectors are a great choice because of their few moving parts and their ability to move dust both vertically and horizontally. The main concerns with this system are that the piping can over time suffer from excessive wear from abrasive compounds. They also require large initial investments of capital and have higher operating costs.

Air Slides are widely used for light dust loads with nonabrasive materials. Air fluidization of the dust is the main operating principle behind this system. This system while able to transport great amounts of material has the downside of only being able to do so in a horizontal direction. Areas of concern are the need to maintain a constant down pitch in the ductwork, and greater maintenance costs.

Pressurized piping systems are needed when transporting the slurry made from using a Wet Scrubber design. This system is used to send the slurry to a settling pond for further treatment. Great care must be taken by the operators of this system to ensure that no leakage occurs which would result in an environmental hazard caused by water pollution.

Dust Storage

Storage tanks and Silos are the most common storage locations for dry dust compounds after their collection.  These sites are then fitted to allow loading of the material into enclosed trucks or rail cars below.
When using a wet collection system often times a settling pond is needed.  In a settling pond the captured particles are separated by means of the process of decantation.  The slurry from the Wet Scrubbers is left to sit in a large pond or basin, allowing the captured particles to over time slowly settle to the bottom of the pond; afterwards the clean water is discharged. Again certain factors to consider in the use of a settling pond are that the water holding area can only be decanted in the warmer, dryer part of the year, and in most instances two settling ponds are needed to operate efficiently.

Final Disposal

When deciding on a final disposal method, one must remember that great care needs to exercised in order to avoid recirculation of the dust by the wind. Sometimes in because of this concern, and for easier transportation, the captured material is processed into pellets before final disposal.  Generally four different options are available for the final disposal of the collected material:
•    Placement in a landfill
•    Recycling
•    Byproduct utilization
•    Collected material may be suitable for backfilling land fills and quarries

Selection of a Dust Collector

The differences in design, operation, efficiency, space requirement, construction, and maintenance needs, as well as the initial start up, operating, and maintenance costs differ greatly between various products and systems. However in choosing which system will meet your needs the best, the following point should be considered:

Dust concentration and Particle size – Within any kind application the specific sizes and dust concentrations can vary enormously. Therefore, knowing the exact range of particle size and concentration levels that will be present will be vital in your choosing the proper Collection System.

Degree of collection required – How intensive of a filtration action is needed is determined by several factors. The exact dangers and hazards of the contaminates to be captured, its potential as a public health risk or nuisance, site location, the allowable emission rate by the regulatory body for the given substance, characteristics of the dust, and any recyclable value.

Characteristics of the Gas stream – Differences in Gas stream temperatures and humidity levels can great affect certain types of collectors. For example Gas temperatures above 180° F (82°C) will destroy many types of filter media (Filter Bags) used in Fabric Collectors (Baghouses). Water vapor or steam can blind certain types of Filter Media. Corrosive and other chemicals can erode certain metals and other materials used in the construction of many Collectors.

Types of Dust – Certain types of Collectors have a great deal of physical contact between the particles and the Collector itself. A number of different materials such as silica or metal ore are quite abrasive and can cause erosion through prolonged contact with the Collector. Other “sticky” compounds can attach themselves to the interior surfaces of the collector and cause blockages. The size and distinct shape of some types of dust render certain collection methods useless. When certain types of materials are fluidized into the air they become highly combustible. Under these circumstances Electrostatic Precipitators are instantly ruled out, along with most Inertial Separators.

Disposal Methods – Differences in disposal methods betweens different locations. Collectors can be arranged to unload their collected matter either in a continuous mode or at a predetermined time interval. Removal of collected matter from dry systems can also result in secondary causes of dust pollution and contamination. While using a Wet Scrubber System will eliminate this concern, proper handling of slurry created during the cleaning cycle will involve an entirely different set of problems, such as precautions against water pollution, and proper care and maintenance of the retention ponds.

 

About the Author

| Dominick DalSanto is an Author & Environmental Technologies Expert, specializing in Dust Collection Systems. With nearly a decade of hands-on working experience in the industry, Dominick’s knowledge of the industry goes beyond a mere classroom education. He is currently serving as Online Marketing Director & Content Manager at Baghouse.com. His articles have been published not only on Baghouse.com , but also on other industry related blogs and sites. In his spare time, Dominick writes about travel and life abroad for various travel sites and blogs.

By Gilda Martinez
Environmental Expert & Baghouse.com Staff Writer

Beingin, China, Sunday 10th of October 2010 –
32 people are killed in traffic accidents along roads that have become almost invisible due mainly to the heavy smog and fog in urban China’s overly polluted air. The largest contributor to that polluted air is by far fly ash, a residue generated by the combustion of coal, which is China’s single biggest source of solid industrial waste and, one of its gravest problems.

The purpose of this article is to draw attention to how much is being done in the development and implementation of Clean Coal technologies. With these emerging technologies it may be possible to prevent more situations like the one mention above from happening again elsewhere.  An examination of how the use of Coal as a fuel affects the environment, what the term Clean Coal technology really means, what Clean Coal techniques are being developed, and put into use today, and why it is so important for the health of both our planet, and the general population.

Why Is Coal So Highly Sought-After?

The use of coal is an integral part of almost every industry on Earth. For instance, 54% of the electricity generated in USA comes from burning Coal. Electric companies and businesses with power plants burn coal to make the steam that turns turbines and generates electricity. Not only The U.S but also China as it was mentioned before also produces a great amount of its electric power from coal, an even larger percentage than the US. A report states China meets 70% of its energy needs through this precious mineral, with electricity generation accounting for half of all coal consumption. The simple fact is that there isn’t a cheaper and sufficiently plentiful mineral that could replace this great power source.

Why Does it Cause Pollution?

Coal is the “dirtiest” of all the fossil fuels currently in use. Why is it so dangerous?  Coal is composed mainly out of carbons and hydrocarbons. When it is burned it releases large amounts of carbon dioxide CO2. This oft mentioned Greenhouse gas is one that while allowing sunlight to reach the Earth, also prevents some of the sun’s heat from radiating back into space, thus warming the planet. Additionally, when it is burned it releases fly ash (coal ash) a residue generated due to combustion.
According to a report dated on August 25th, 2008 by The Union of Concerned Scientists, a group of scientists that combine scientific research and citizen action to develop practical environmental solutions, a coal fire plant generates:

•    3,700,000 tons of carbon dioxide (CO2), the primary human cause of global warming, which is as much carbon dioxide as cutting down 161 million trees.
•    10,000 tons of sulfur dioxide (SO2), which causes acid rain that damages forests, lakes, and buildings, and forms small harmful airborne particles that can penetrate deep into lungs.
•    500 tons of small airborne particles, which can cause chronic bronchitis, aggravated asthma, and premature death, as well as haze obstructing visibility.
•    10,200 tons of nitrogen oxide NOx, as much as would be emitted by half a million late-model cars. NOx leads to formation of ozone smog, which inflames the lungs, burning through the lung tissue making people more susceptible to respiratory illness.
•    720 tons of carbon monoxide CO, which causes headaches, and places additional stress on people with heart disease.
•    220 tons of hydrocarbons, volatile organic compounds VOC, which form ozone.
•    170 pounds of mercury, where just 1/70th of a teaspoon deposited on a 25-acre lake can make the fish unsafe to eat.
•    225 pounds of arsenic, which will cause cancer in one out of 100 people who drink water containing 50 parts per billion.
•    114 pounds of lead, 4 pounds of cadmium, other toxic heavy metals, and trace amounts of uranium.

Due to all the contaminants coal burning comes along with, there is an increasing need for technology development in the Clean Coal field. From here an analysis of what technologies are being used the most to remove pollution from coal burning residues.

Carbon Capture & Storage

Among all the existing Clean Coal technologies, the one that is the most popular and efficient is Carbon Capture and Storage. It consists of a process that captures carbon dioxide CO2 emissions from industrial sources and stores them in geological formations miles deep inside in the earth.
CCS Carbon Capture and Storage is an integrated concept consisting of three distinct components: CO2 capture, transport and storage including measurement, monitoring and verification. All three components are currently found in industrial operation today, although mostly not for the purpose of CO2 storage.
Depending on the process or power station in question, three approaches to Carbon Capture exist- pre-, post- and oxy-fuel combustion:

•    Pre-combustion capture systems remove CO2 prior to combustion. This is accomplished via gasification. The gasification of a fossil fuel produces a “synthesis gas” syn-gas, which is primarily a mixture of carbon monoxide, methane and hydrogen. Before combustion, the syn-gas is reacted with steam to produce CO2 that is subsequently scrubbed from the gas stream, usually by a physical or chemical absorption process. The result is a hydrogen-rich fuel that can be used in a wide range of applications. Pre-combustion systems are not a mature market technology but are intended for deployment in conjunction with Integrated Gasification and Combined Cycle (IGCC) technology. The use of IGCC for coal-based electricity production is limited with only four coal-based IGCC demonstration plants in operation globally. Reliability, availability and cost of technology have hindered wider deployment of IGCC.
•    Post-combustion techniques are the standard practice for removing pollutants, such as sulfur, from the flue gas of coal-fired power stations. Flue gas typically contains up to 14% CO2, which must be separated- either through absorption chemical or physical, cryogenics and membrane technologies. For CO2 capture, chemical absorption with amines, such as Monoethanolamine MEA, is currently the process of choice. Once recovered, the CO2 is cooled, dried and compressed for transport. Post-combustion systems are posited as a carbon mitigation solution for the existing fleet of coal-fired power plants around the globe. However, retrofitting a capture system to a power station requires major technical modifications. These alterations are quite costly and are accompanied by substantial decreases in generating efficiency. For example, an MEA retrofit of an existing 500 MWe subcritical pulverized coal PC power plant cuts efficiency by 14.5 %. Net electrical output is diminished by over 40% to 294 MWe. Such a retrofit is expected to impose capital costs of USD 1600/kWe
•    Oxy-fuel combustion burns fossil fuels in 95% pure oxygen instead of air. This results in a flue gas with high CO2 concentrations greater than 80% that can be condensed and compressed for transport and storage. This method of CO2 capture is still in the demonstration phase.

Other Clean Coal Technologies

The Sulfur gas produced by burning coal can be partially removed with scrubbers or filters. In conventional coal plants, the most common form of sulfur dioxide control is through the use of scrubbers. To remove the SO2, the exhaust from a coal-fired power plant is passed through a mixture of lime or limestone and water, which absorbs the SO2 before the exhaust gas is released through the smokestack. Scrubbers can reduce sulfur emissions by up to 90 percent, but smaller particulates are less likely to be absorbed by the limestone and can pass out the smokestack into the atmosphere.  In addition, scrubbers require more energy to operate, thus increasing the amount of coal that must be burned to power their operation.

Other coal plants use “fluidized bed combustion” instead of a standard furnace. Fluidized bed technology was developed in an effort to find a combustion process that could limit emissions without the need for external emission controls such as scrubbers. A fluidized bed consists of small particles of ash, limestone and other non-flammable materials, which are suspended in an upward flow of hot air. Powderized coal and limestone are blown into the bed at high temperature to create a tumbling action, which spurs more effective chemical reactions and heat transfer. During this burning process, the limestone binds with sulfur released from the coal and prevents it from being released into the atmosphere.

Fluidized bed combustion plants generate lower sulfur emissions than standard coal plants, but they are also more complex and expensive to maintain. According to the Union of Concerned Scientists, sulfur emissions decreased by 33 percent between 1975 and 1990 through the use of scrubbers and fluidized bed combustors, as well as switching to low-sulfur coal.

Another technology used to clean coal is gasification which means to burn coal in oxygen to produce a cleaner gaseous fuel known as syngas mixture of hydrogen and carbon monoxide. This process reduces the emissions of Sulphur, nitrogen oxides and mercury, which results in a cleaner fuel. The resulting hydrogen gas can be used for electricity generation or as a transport fuel. The gasification process also facilitates capture of CO2 emissions from the combustion effluent (see discussion of carbon capture and storage below).

Integrated gasification combined cycle IGCC systems combine gasification with a heat recovery system that feeds a secondary steam-powered generator, thereby increasing the power generated from a given amount of coal. These systems are currently being employed in many new coal-fired power plants worldwide.

Why Clean Coal Technologies Are So Important

Discussion has shown how airborne pollution is affecting the planet to such a degree that scientists believe that there is a urgent need to take action, otherwise humankind will begin to suffer the consequences in short order if not now currently.

Government and environment advocates are doing their best to implement all the available strategies and to create new ones. Carbon Capture and Storage is being approved for use by many industries but, as with all that is new, this technology is very expensive and it consumes much more energy than others.

Therefore there is still a large demand for conventional Scrubbers and Filters for companies that cannot afford the latest to implement the latest technological advances.

We hope this article has provided a better understanding of this polluting mineral, the latest methods of reducing the environmental impact of coal, and raised awareness that the need for these environmentally friendly technologies is increasing every year.

In Texas, the EPA has informed a sizable number of oil refiners, chemical and plastics manufacturers that they need to bring their air pollution permits in line with federal and not Texas state levels.

All but three of the 74 companies have informed the Environmental Protection Agency that they will bring the state-issued permits into compliance with federal law within the next year.

This action is in response to the on-going conflict between federal air quality standards, and Texas state legislation which allows for so-called flexible permits. The permits in question require refineries, chemical plants and other facilities to meet an overall emissions cap but allows them to choose how to do so. Federal rules, however, require plants to limit emissions of certain pollutants from each source within a facility.

With this turn of events, it appears that even while Texas is still battling the EPA on this issue, industry in general in Texas is bringing itself in line with federal standards.

Commenting on Texas industries Al Armendariz, the EPA’s administrator based in Dallas stated “They understand these permits are an anomaly,” adding that none of the companies has indicated that it will sue to keep its flexible permit. “It’s now a question of how do they fix them instead of whether they should”.

Texas has filed suit to block the EPA’s disapproval of flex permits, asserting that there is no legal or technical justification for the federal agency’s action.

The EPA rejected the state’s use of the permits in June, saying they fall short of the federal Clean Air Act’s requirements. But those with the permits reacted slower than Armendariz liked, so he threatened fines and other penalties if they did not move by Dec. 22 to resolve their permits.

The EPA would not say which companies failed to meet the deadline because of the possibility of taking enforcement action against them. But Armendariz said his staff had heard from the 30 largest permit-holders, which account for roughly 90 percent of emissions released under flexible permits.

Pam Giblin, an Austin-based Baker Botts attorney who represents many large flexible permit holders, said industry waited to respond until the EPA had explained how to make their permits comply — and not in protest.

Pollutants and permits

The single overall cap, the EPA argues, makes the Texas permits nearly unenforceable and allow plants to emit more than similar facilities in other states. But state officials say the system cuts red tape and pollution without violating federal law.

TCEQ spokesman Andy Saenz said companies “must decide for themselves how to deal with EPA’s overreaching and unnecessary regulatory demands. The TCEQ is not requiring any ‘fix’ but the agency is accommodating any legally viable transition option that flex permit holders may choose to exercise.”

The EPA has encouraged companies to follow the leads of Flint Hills Resources and INEOS Olefins & Polymers USA – both of which agreed to apply for new state-issued permits after negotiating some terms and conditions with the federal agency.

That’s important because the agreements ensure that the state permits will meet federal requirements, said Ilan Levin, an attorney with the Environmental Integrity Project, which has filed legal challenges to some of the flexible permits.

Request for transparency

“The devil is in the details, and the TCEQ hasn’t been willing to guarantee that these be done in a transparent way,” Levin said.

Valero Energy Corp., for example, has asked TCEQ to set limits for each emissions source at its plants, but to leave the rest of the permit alone. The state has issued new permits for five of the San Antonio company’s six Texas facilities, although the EPA has raised concerns about whether the revisions are federally compliant.

Valero spokesman Bill Day said the company is talking with the EPA to resolve their differences.

 

 
About the Author

| Dominick DalSanto is an Author & Environmental Technologies Expert, specializing in Dust Collection Systems. With nearly a decade of hands-on working experience in the industry, Dominick’s knowledge of the industry goes beyond a mere classroom education. He is currently serving as Online Marketing Director & Content Manager at Baghouse.com. His articles have been published not only on Baghouse.com , but also on other industry related blogs and sites. In his spare time, Dominick writes about travel and life abroad for various travel sites and blogs.

At least 281 combustible dust fires and explosions occurred in general industry between 1980 and 2005 in the United States, which caused at least 119 fatalities and 718 injuries; including seven catastrophic dust explosions in the past decade, involving multiple fatalities and significant community economic impact; and occurred in a wide range of industries and involved many types of combustible dusts.

According to a report by the US Chemical Safety Board (CSB) a major factor that lead to the high amount of incidents was the overall lack of education regarding the danger of Dust Explosions. Without this information, plant operators are not able to implement proper safety precautions, and adequately train their personal about the precautions they need to take to minimize the chance of a Dust Explosion occurring in their facility.  This article has been prepared to help bring awareness to the dangers of Dust Explosions, and what precautions can be taken to avoid them.

What Is A Dust Explosion?

A factory that has been destroyed by a Dust Explosion

A Dust Explosion that begins in the Dust Collection System can lead to the destruction of an entire facility.

Most solid organic materials, in addition to many metals and inorganic nonmetallic materials, when reduced to a finely divided size, and sufficiently dispersed into the atmosphere will explode under the right conditions. Many combustible dusts are intentionally manufactured for a wide range of applications such as Metallic Powder Coatings, or certain foodstuffs such as Corn Starch, Flour, and Granulated Sugar. Others are produced during the manufacture and transport of materials such as wood processing, and stone quarrying. Additionally, during the manufacturing process for many materials, actions such as milling, polishing, and transportation may create substantial amounts of dust that can later accumulate on a wide range of surfaces.

Any industry that produces materials of a fine particle size that are combustible, and many that simply though their day-to-day operation create large amounts of secondary dust are at risk for Dust Explosions. Industries such as Metal, Food, Plastic, and Wood Processing are just a few that are at risk for this kind of industrial accident.

The Anatomy of a Dust Explosion

The Beginnings

The basics of combustion deal with the so called “Fire Triangle” that illustrates the importance of the three main factors that need to be present for combustion to take place. These three are Fire, Heat (Ignition Source) and Oxygen. With regards to Dust Explosions, we need to add another two ingredients to create what has been termed the “Dust Explosion Pentagon” Dispersion and Confinement. When all five of these factors are present in the right balance, a dust explosion will occur. The more of these factors that can be controlled or be kept below the combustion threshold, the less likely there will be an incident.

When a material is finely divided into a dust or power form it in most cases it becomes much more likely to combust than it would in a solid state. The reason for this is because when a material is smaller in size, and is dispersed into the air, it creates a much larger surface area to ignite. For example, a 1 kg sphere of a material with a density of 1g/cm3 would be about 27 cm across and have a surface area of 0.3 m3. However, if it was broken up into spherical dust particles 50µm in diameter (about the size of flour particles) it would have a surface area of 60 m² This greatly increased surface area allows the material to burn much faster, and the extremely small mass of each particle allows it to catch on fire with much less energy than the bulk material, as there is no heat loss to conduction within the material.

The source of ignition in a Dust Explosion is often times very difficult if not impossible to determine with absolute certainty. This is because in an industrial setting there is such a larger amount of possible ignition sources that after an incident, it cannot always be pinpointed with absolute certainty.  Some possible sources include, Open Flames, Electrostatic Discharge, Friction, Chemical Reactions, Arcing (From machinery or other equipment) and Hot Surfaces.

Primary and Secondary Explosions

Primary Dust Explosions, in an industrial setting, usually involve a dust cloud (Dispersed Dust) that is ignited by an ignition source. This explosion while possibly involving a substantial amount of dust is often not the most devastating. That is because this initial explosion can cause a pressure wave that can dislodge settled dust from other areas within a facility (Such as on the top of structural elements like beams and columns, high shelving, and machinery, or other areas that dust and debris may collect) causing it to disperse and then cause a much larger explosion that is termed a Secondary Dust Explosion. The majority of fatalities, and damage caused by dust explosion incidents, are actually caused by Secondary Dust Explosions.

Conditions That Lead To A Dust Explosion

The same CSB report cited earlier, after having discussed several different Industrial Dust Explosion Incidents, concluded that while all had many different factors that contributed to the respective incidents, all had the following circumstances in common:

* Facility management failed to conform to NFPA (National Fire Protection Agency) standards that would have prevented or reduced the effects of the explosions.
* Company personnel, government standards enforcement officials, insurance underwriters, and health and safety professionals inspecting the facilities failed to identify dust explosion hazards or recommend protective measures.
* The facilities contained unsafe accumulations of combustible dust and housekeeping to remove such accumulations was inadequate.
* Workers and managers were often unaware of dust explosion hazards.
* Procedures and training to eliminate or control combustible dust hazards were inadequate.
* Previous fires and other warning events were accepted as normal, and their causes were not identified and resolved.
* Dust collectors were inadequately designed or maintained to minimize explosions.
* Process changes were made without adequately reviewing them for potential hazards.

Listed here are a few of the summery reports published by the CSB. As you will see the above-mentioned factors all played a role in the eventual incidents.

Organic Dust Fire and Explosion: Massachusetts (3 killed, 9 injured)

In February 1999, a deadly fire and explosion occurred in a foundry in Massachusetts. The Occupational Safety Health Administration (OSHA) and state and local officials conducted a joint investigation of this incident. The joint investigation report1 indicated that a fire initiated in a shell molding machine from an unknown source and then extended into the ventilation system ducts by feeding on heavy deposits of phenol formaldehyde resin dust. A small primary deflagration occurred within the ductwork, dislodging dust that had settled on the exterior of the ducts. The ensuing dust cloud provided fuel for a secondary explosion, which was powerful enough to lift the roof and cause wall failures. Causal factors listed in the joint investigation report included inadequacies in the following areas:

* Housekeeping to control dust accumulations;
* Ventilation system design;
* Maintenance of ovens; and,
* Equipment safety devices.

Organic Dust Fire and Explosion: North Carolina (6 killed, 38 injured)

In January 2003, devastating fires and explosions destroyed a North Carolina pharmaceutical plant that manufactured rubber drug-delivery components. Six employees were killed and 38 people, including two firefighters, were injured. The U.S. Chemical Safety and Hazard Investigation Board (CSB), an independent Federal agency charged with investigating chemical incidents, issued a final report2 concluding that an accumulation of a combustible polyethylene dust above the suspended ceilings fueled the explosion. The CSB was unable to determine what ignited the initial fire or how the dust was dispersed to create the explosive cloud in the hidden ceiling space. The explosion severely damaged the plant and caused minor damage to nearby businesses, a home, and a school. The causes of the incident cited by CSB included inadequacies in:

* Hazard assessment;
* Hazard communication; and
* Engineering management.

The CSB recommended the application of provisions in National Fire Protection Association standard NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, as well as the formal adoption of this standard by the State of North Carolina.

Organic Dust Fire and Explosion: Kentucky (7 killed, 37 injured)

In February 2003, a Kentucky acoustics insulation manufacturing plant was the site of another fatal dust explosion. The CSB also investigated this incident. Their report3 cited the likely ignition scenario as a small fire extending from an unattended oven, which ignited a dust cloud created by nearby line cleaning. This was followed by a deadly cascade of dust explosions throughout the plant. The CSB identified several causes of ineffective dust control and explosion prevention/mitigation involving inadequacies in:

* Hazard assessment;
* Hazard communication;
* Maintenance procedures;
* Building design; and,
* Investigation of previous fires.

Metal Dust Fire and Explosion: Indiana (1 killed, 1 injured)

Finely dispersed airborne metallic dust can also be explosive when confined in a vessel or building. In October 2003, an Indiana plant where auto wheels were machined experienced an incident, which was also investigated by the CSB. A report has not yet been issued, however, a CSB news release told a story similar to the previously discussed organic dust incidents: aluminum dust was involved in a primary explosion near a chip melting furnace, followed by a secondary blast in dust collection equipment.

Prevention, Safety and Mitigation

Now that we have discussed many of the contributing factors that can lead to a Dust Explosion, we are going to highlight several areas that if given the proper attention, will lead to a safer working environment, and lessen the potential for property damage bodily harm.

Hazard Analysis

We have discussed the great danger that Dust Explosions can pose to life and property. Now we have listed several areas that if given the proper attention will greatly reduce the probability of a dust explosion occurring, and should one occur, lessen the severity of said explosion, possibly saving lives and lessening the damage to the facility in the process.

Facility Dust Hazard Assessment

Being aware that the possibility of a Dust Explosion exists is the first step to avoiding one. As mentioned previously, most dusts or powders will burn and if dispersed in the air in the right proportions and may explode. The same CSB study quoted earlier found that despite the long history of Dust Explosions in industry, in many cases the hazards involved with explosive dusts were largely ignored by plant operators, as well as by outside insurance auditors and government inspectors. Therefore, recognizing the great potential for this kind of accident during the initial design of the facility and while doing regular hazard analysis, are crucial

Here are some of the items to look for when conducting a facility hazard analysis with regard to the potential for Dust Explosions.

Dust Combustibility

Above all else, it must be determined whether or not that various types of dust produced in the facility are indeed combustible. As stated before, most materials in dust or powder form will burn when dispersed into the air in the right proportions. However, those proportions vary with each material. Therefore, it is vital for those responsible to gather as much data as possible about the particular materials present in the facility. One potential source of said data is the particular material’s MSDS or Material Safety Data Sheet. In some cases, additional information such as combustibility test results will be available from chemical manufacturers. However as noted before, many times a manufacturer MSDS may be lacking sufficient data regarding the combustibility of the material in dust or powder form. Therefore additional testing may be necessary to determine this information.

Electrical Considerations

Areas that require a special electrical equipment classification due to the presence (or potential presence) of dubitable dust need to be identified during a facility hazard analysis. There are several published sources of guidelines and/or regulations regarding special electrical equipment classification. These include: The OSHA Electrical standard (29 CFR Part 1910
Subpart S), NFPA 70, the National Electrical Code®, and NFPA 499, Recommended Practice for the Classification of Combustible Dusts and of Hazardous (classified) Locations for Electrical Installations in Chemical Process Areas.

Several of these guidelines identify three different groups of combustible dusts, (Metal, Carbonaceous and Other) and the different safety considerations that are needed for each. For example Metal dusts are considered electrically conductive; therefore special care needs to be taken to ensure that no electrical current can pass through layers of the dusts causing short circuits and arcs, which could then lead to an ignition. Additionally, in certain industrial settings, other high-energy ignition sources such as welding arcs may be present and need to be accounted for.

Potential For Dust Accumulation

The exact amount of dust accumulation necessary for an explosion to occur can vary greatly. As discussed earlier variables such as particle size, methods of dispersion, ventilation system models, air currents, physical barriers and volume of the area where the airborne dust exists can all vary in each different type of dust. With the site-specific data at hand, potential areas of concern can be identified. And the hazard analysis can then be tailored to the specific circumstances in each area and the full range of variables affecting the hazard.

Even seemingly small amounts of accumulated dust can cause catastrophic damage. The CSB estimated, for example, that the explosion that devastated a pharmaceutical plant in 2003 and killed six employees was caused by dust accumulations mainly under 0.25 inches deep. The NFPA warns that more than 1/32 of an inch of dust over 5 percent of a room’s surface area presents a significant explosion hazard.

Many different locations throughout a facility can be a potential starting point for a conflagration. An area where dust is concentrated is an obvious place to start. In Dust Collectors for example, a combustible mixture of diffused dust and air can be found whenever the Collector is operating. Additionally, locations where dust can settle whether occupied, or concealed spaces (such as in ceiling rafters, the tops of shelving, etc). When conducting the Hazard Analysis, careful consideration needs to be given to all possible scenarios in which any previously identified settle dust can be dispersed into the air, either though normal operations, or potential failure modes.

Precautionary Measures

After hazards have been assessed and hazardous locations are identified, one or more of the following prevention, protection and/or mitigation methods may be applied.

Dust Control

Controlling the amount of dust generated, where it is generated, and the dispersion of it throughout the facility, is key to reducing the likelihood of an explosion from occurring. The following steps should be taken in this regard:

* Minimize the amount of dust that escapes from processing equipment and ventilation systems.
* Install a Dust Collection System and monitor it closely to ensure it is operating properly.
* Where possible, install materials (Surfaces) that collect dust poorly and facilitate easy cleaning.
* Inspect and note all hidden or concealed spaces where dust accumulation might occur.
* Maintain a set schedule for cleaning all dust prone areas, and follow it closely.
* Use cleaning methods that do not themselves generate dust clouds when ignition sources are present.
* Locate Relief Valves away from dust hazard zones.
* Maintain a comprehensive dust control program, with hazard dust inspections, testing, housekeeping, and control initiatives.

In several of the cases highlighted earlier, the initial explosion spread by means of ductwork that connected various equipment (usually the Dust Collection System, and/or different parts of the ventilation system) throughout the plant. It is therefore vital that these ductwork systems be fitted with isolation values and inspected regularly to remove excess sitting dust accumulations.

Additionally, certain dust generating operations (such as the use of abrasives, blasting, grinding, or buffing) fall under OSHA  (or similar governmental agencies) ventilation requirements.

Ignition Control

Along with Dust Control, controlling all possible ignition sources also plays a major role in any comprehensive Dust Control Program. Along with Electrical Considerations, there are many other areas that merit attention with regard to ignition potential. Here are several key recommendations for controlling possible Dust Ignition sources.

* Proper Installation, Classification, Operation, and Maintenance of all Electrical Equipment and Wiring (Class II wiring methods and equipment such as “dust ignition-proof” and “dust-tight” should be employed)
* Employ adequate Static Electricity control methods such as Grounding Wires/Rods, etc.
* Limit Smoking, Open Flames, and Sparks in work area.
* Limit or isolate sources of mechanical sparks and friction
* Separate foreign materials that may ignite combustibles from process materials.
* Limit contact between heated surfaces and heating system from combustible dusts.
* Install spark arrestors/spark traps in all dust collector ductwork.

Further resources including US regulation, guidelines, and recommendations can be found in the following sources:

* NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids
* OSHA’s Powered Industrial Trucks standard (29 CFR 1910.178).

Damage Control

Despite the best efforts of all parties involved, incidents may still occur. It is therefore, the wise course of action is to prepare for the worst, and implement a strategy that will reduce the severity of such an incident should it occur. The following is a list of recommended steps to take to minimize the impact of a Dust Explosion:

* Separate, and Segregate the Hazard to the extent possible. Place distance between the hazard and the work area, and isolate the hazard with barriers where possible.
* Install deflagration venting.
* Install pressure relief valves on applicable equipment.
* Employ Spark/Ember detection systems, and extinguishing equipment.
* To the extent possible, install explosion protection system, including sprinkler systems, and other assorted specialized suppression techniques.

Proper Employee & Management Training

Even with all of the aforementioned precautions, without a workforce, both employees and management, that have been properly educated about the dangers of Dust Explosions, and safety procedures to reduce the likelihood of their occurrence, and control, and limit the damage should they occur, there still remains high degree of probability for a Dust Explosion occurring.

Employees

Workers that are trained in preventing, and proper incident response techniques are integral to the safe operation of any facility. They are the people closest to the hazard, if these ones are trained to recognize and prevent these types of occurrences from taking place, they can accomplish much in this regard. These ones should also be encouraged to feel free to report unsafe working conditions, or areas where there could be an improvement in safety standards. Therefore all employees, whether they are working directly in hazard areas or not, should be adequately trained in safe work practices applicable to their job tasks, as well as on the overall plant programs for dust control and ignition source control. Periodic refresher courses should also be arranged to keep these safety issues fresh in their minds, and up to date with any possible changes to the hazard conditions themselves.

Management

A qualified team of managers should be responsible for conducting a facility analysis (or for having one done by qualified outside persons) prior to the introduction of a hazard and for developing a prevention and protection scheme tailored to their operation. Supervisors and managers should be aware of and support the plant dust and ignition control programs. Their training should include identifying how they can encourage the reporting of unsafe practices and facilitate abatement actions.

Conclusion

The dangers of Dust Explosion are quite real; they have caused great amounts of damage to property, and have cost many lives. The importance of implementing a comprehensive dust control program, including hazard analysis, implementation of proven dust control and ignition control techniques, damage mitigation, and employee and management training cannot be overstated.

View this video report by the CSB

 
About the Author

| Dominick DalSanto is an Author & Environmental Technologies Expert, specializing in Dust Collection Systems. With nearly a decade of hands-on working experience in the industry, Dominick’s knowledge of the industry goes beyond a mere classroom education. He is currently serving as Online Marketing Director & Content Manager at Baghouse.com. His articles have been published not only on Baghouse.com , but also on other industry related blogs and sites. In his spare time, Dominick writes about travel and life abroad for various travel sites and blogs.