Baghouse Differential Pressure – Why Important?

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Diagram of how to install a magnehelic differential pressure gauge on a baghouse dust collector.

Accurate differential pressure readings are essential for efficient baghouse operation. This article outlines the importace of baghouse differential pressure and what baghouse problems it can be used to diagnose. 

By Dominick DalSanto
Baghouse Technology Expert and Sales Director
Baghouse.com

Baghouse Differential Pressure – Why Important?

Within a baghouse a number of factors must be controlled to ensure the efficient operation of the system at all times. Of these, the most important variable to control is the system differential pressure. This measurement is the key indicator of how the baghouse is operating and the most important factor to consider when diagnosing and troubleshooting issues with the baghouse system.

Over time more and more dirt will penetrate deep into the fibers of a bag being harder to remove. When this happens the bags become blinded or are unable to be adequately cleaned. This causes massive differential pressure increases.

Over time more and more dirt will penetrate deep into the fibers of a bag being harder to remove. When this happens the bags become blinded or are unable to be adequately cleaned. This causes massive differential pressure increases.

Differential pressure (also known as pressure drop or Delta-P) is the difference in pressure between the dirty-air side of the baghouse and the clean-air side. As in the incoming air is pulled through the filter media (i.e. filter bags) vacuum is lost, resulting in the air entering the baghouse having a weaker vacuum than the air exiting the baghouse. For example, let’s say that the system fan is pulling 10″ w.g* of vacuum pressure. When the dirty air comes into the baghouse the pressure is at 3.5″ w.g.* of vacuum, but after entering into the baghouse and passing through the filters the pressure rises to 10″ w.g.. This means the pressure drop across the baghouse is 6.5″ w.g. (Note: This example assumes a clean-air side fan or negative pressure system)

Differential pressure readings are used to determine a number of things about the operation of a baghouse system, such as filter bag condition,  and structural problems with the unit, (airlock and conveying system condition and door seals condition among other things). Furthermore, a high system differential pressure usually indicates that the system is not running efficiently and therefore is incurring higher operating costs than it should under optimal circumstances. The following problems can be diagnosed from observing the system differential pressure:

Filter Bag Condition

  1. Bags are blinded off
    • Dirty bags will become more resistant to airflow thus causing the force to push the air through them to rise.
  2. Bags have holes in them
    • This will create a path of less resistance for the air to flow through leading to lower pressure drop
  3. Bags are not installed properly
    • See above

Structural and Sealing Issues

  1. Leaks within the structure
    • Common leak areas include airlocks, welds, joints (especially panelized construction units), and door seals
  2. Airlock leaks
    • Common around flaps,rotars, gaskets and connection points.
  3. Conveying system leaks
    • Common at connection point to hopper/airlock, holes in ductwork structure, etc.
  4. Doors and hatch sealing
    • Gaskets on all doors and hatches, including viewports. Also includes making sure all doors have sufficient fasteners (i.e. bolts) to secure the door/hatch securely to form a tight seal.
Diagram of how to install a magnehelic differential pressure gauge on a baghouse dust collector.

Diagram of how to install a magnehelic differential pressure gauge on a baghouse dust collector.

Why High Differential Pressure Means Higher Operating Costs

Controlling your baghouse differential pressure is required to get the maximum performance and efficiency from your system. If your system is running at a high differential pressure it will inevitably cost more to operate, have lower performance and experience more downtime. High DP means the system fan needs to work harder to pull the same amount of airflow throughout the system. IF DP starts to rise above the recommended levels,  maintaining the same level of draft (i.e. suction) at all of the systems pickup points will prove difficult. This will lead to much higher energy costs to run the system fan at high speed and can if over taxed lead to premature fan/motor failure. If the system fan is not adjusted to compensate for the higher differential pressure, the system will lose draft at all of its pickup points. This will mean less performance from your system and inescapably cause problems for your facility process whatever it maybe, especially so for certain industries that are more dependent on the dust collection system as part of the process such as cement, powdered metals, chemical processing, etc. In the end, this will result in the process being shutdown or even a shutdown of parts or the entire facility until the system is running again.

Conclusion

Clearly, it is of vital importance for maintenance staff and operators to keep close watch on dust collection system differential pressure. If system DP is higher than recommended it can be a indicator of several potentially serious issues, ranging from blinded bags, to holes in the structure to poor seals, etc. Accordingly, obtaining accurate differential pressure readings is vital to have an accurate picture of what is going on within your baghouse system. But what can you do if your equipment is giving you suspicious or even false readings? How can you determine when your DP gauges and controls are sending false readings? These questions will be in the following article in the series: Baghouse Differential Pressure – How To Troubleshoot False Readings

Footnotes:

* “w.g. stands for inches of water gauge, that is vacuum pressure as measured in inches of water (sometimes mercury) as in a magnehelic gauge.

 

| 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 Sales Director 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.
Baghouse.com Corp

51 replies
  1. Ken Kelly
    Ken Kelly says:

    Hi Dominick
    Im sure you were trying to say the correct thing in your footnote, however it may be confusing to some.
    The Magnahelic uses a magnet attached to the indicator needle and a helix attached to a diaphragm to measure the suction pressure.Hence its name.
    More confusing is a photohelic.

    Reply
  2. John H
    John H says:

    Just a comment, the power increase of the fan is only true if variable speed, you will infact use less power with higher d.p. if it is not.

    Reply
  3. Peter Stone
    Peter Stone says:

    Very interesting.
    I have expirienced all these facts with filters.
    I would like to be added to your mailing list.
    Peter

    Reply
  4. paul
    paul says:

    Very informative article. Gives me ideas on a few mysterious pressure spikes ive experienced. Run a hot mix plant with a Dusteater.

    Reply
    • dominickdalsanto
      dominickdalsanto says:

      We find that a lot of operators are surprised that their DP readings can tell them so much about what is going on with their system. What exactly is going on with your system?

      Reply
  5. G Rajarao
    G Rajarao says:

    We have around 150 bag filters in the plant. The purging is continuous. We want to operate with DP. what shall be the DP to be maintained in a good Bag filter.

    Normally, i feel 110mmwc to start purging and 80 mmWC for stopping is OK.
    Your comments pl.

    Reply
    • dominickdalsanto
      dominickdalsanto says:

      In answer to your question, in general a good range to run your baghouse is between 3.5″ and 5″ of differential pressure. Sometimes you can tweak that a bit, but in general that is a good range.

      Reply
      • Kanchan
        Kanchan says:

        Hi Dominic, Thank you for this article. It was intuitive and I learnt a lot from it. However I could not understand why the differential should be maintained at 3.5″ to 5″? But logically shouldn’t it be that an equipment is operating in perfect condition when the differential pressure is at 0?

        Any light on this would help me, thank you vey much again.

        Reply
        • dominickdalsanto
          dominickdalsanto says:

          Kanchan,

          There will always need to be a difference in pressure, or resistance to flow, otherwise nothing will be captured on the filters. Brand new filter bags with no dust on them will by themselves add a resistance of about 1″ to 2″. Once you add dust to them they should rise up to about 3″. 3.5″ – 5″ (or 6″) is the standard for bags that are working properly. Once they bags cannot be cleaned down past that they are old and blinded and need to be replaced.

          Reply
  6. Mostafa
    Mostafa says:

    Hi dominick

    Thank you for the fruitful article. I work in a cement plant. I would like if this definition of dp is correct: It is the difference between the suction pressure before and after the filter. I would like to know if this is correct. For bypass filter the dp from 6 to 7 and for clinker coooler filter is always 8

    Reply
    • dominickdalsanto
      dominickdalsanto says:

      Mostafa,

      YEs, that is essentially correct. But a dp of 6-8 is quite high. That high of dp usually means your baghouse filters are blinded. It should be running between 3″ – 6″ roughly. Any higher than 6″ and you have a problem.

      Reply
  7. mostafa
    mostafa says:

    Thanks alot

    I would like to know what is the difference between on cleaning and post cleaning or in other way i would like to know what is post cleaning.?

    Reply
  8. Neel
    Neel says:

    Hi there, we have an application where air is already being forced out of a vent naturally (no fan). If we attach a baghouse unit to this vent, what happens if the forced air volume exceeds the baghouse fan volume? i.e. 3,000 cfm of vent air to a baghouse unit with 1,000 cfm fan

    Reply
    • dominickdalsanto
      dominickdalsanto says:

      Neel,

      Thanks for the question.

      The amount of CFM a baghouse is rated for has to do more with the filter area within the baghouse than the fan. The same baghouse with the same amount of filters will have a different airflow depending on air to cloth ratio demanded by the application.

      It sounds like you are going to use the baghouse like a bin vent (no specific baghouse fan). If that is the case, what is the CFM you expect to push into the baghouse?

      If you are planning on hooking up a baghouse with a fan rated at 1,000 CFM to an airstream of 3,000 CFM it might cause some problems. But if you have an air volume of 3,000 CFM already why do you need the fan connected anyways? You can always use a damper to block off the fan when you have the external air pushing into the system and then you can open up to the fan when you need to use the fan to generate the airflow.

      Reply
  9. Douglas
    Douglas says:

    We are using a Bagfilter house without Manometer. Now, we want to install a U-tube manometer. How to decide correct sizing(mm) of U-tube manometer for Bagfilter house and how to fit manometer to the Bagfilter house.

    Reply
  10. ATUL
    ATUL says:

    Hi Dominick,

    We are operating a Air Jet Mill to which 10 kg of compressed air is supplied.
    Now to check DP across filter bags we are facing some issues.
    1. What type & Range of DP?
    2. There are two different location for outlet (Vent)air, so do we require to install two DP
    gauges?

    Awaiting your valuable response!

    Reply
    • dominickdalsanto
      dominickdalsanto says:

      The two gauges are to make sure you always have an accurate reading. You really only need one gauge, but you tap it on both sides of the collector.
      The range for a normal pulse jet should be between 3″ – 6″ +/-

      Reply
  11. Jeff
    Jeff says:

    Hi recently we had a filter change on a 60,000 cfm DC with a 250 hp with 1700 filters and we got sold on a company that said their bags where more efficient but after these new bags where installed they turned the collectors off right after pre coating them and we normally run them 24hrs strait for a pre coat not to mention the new filters are 5 times thicker cloth ever since we experience nothing but high readings and the cleaning cycle barley can keep them down Durring operation and it takes all day to get the readings down on an offline shakedown cycle but the readings climb fast back up I looked over the system many times checking dampers and the shake system and can’t find a issue I’m thinking we need to return to our old style filter but just curious if such change can be thisvmuch of an issue

    Reply
    • dominickdalsanto
      dominickdalsanto says:

      Sounds like you got sold on something without testing it first! Brand new filters should only pull 1″ – 1.5″ DP when they are first installed. After precoating they should be in normal operating range of 3.5″ – 6″ +/-. My question is why are you still using a shaker system? If you want to update your system a better investment would be to convert to a newer pulse jet style collector rather than on investing in “fancy” new fabric choices.

      Either way, you should not be having problems unless they sold you the wrong kind of fabric/treatment.

      What kind of process do you have? what kind of fabric are you using now and what were you using before?

      Reply
    • hari singh
      hari singh says:

      in himenviro bag house any time seen to increased a dp now we checked hopper condition or any leakage in a system . there fore i requist you to solved this problem as soon as possible

      Reply
      • dominickdalsanto
        dominickdalsanto says:

        IF your DP if rising it means the filters are not being cleaned sufficiently. There are many causes for this, not just hopper levels and leakage. Check the cleaning system or consider replacing the filters.

        Reply
  12. hitesh
    hitesh says:

    In our plant reverse air bag house hence how to set Cleaning cycle in DP rang and also get me how to operation in constant DP mode.

    Reply
    • dominickdalsanto
      dominickdalsanto says:

      You simply set the reverse air cleaning cycle to trigger only once the DP has risen above a certain point, such as 5″ or 6″. IT works exactly the same as a pulse jet. A simple Photohelic gauge should do just fine.

      Reply
  13. SAJU SIMON
    SAJU SIMON says:

    Hi, Domanik, Thanks for your sharing…

    I want to know the reason for very low D.P in Kiln bag house in cement plant…

    In a particular date the temperature has reduced 50C (for 8 hrs) and the SO2 reached to 580 mg/Nm3 for 2 hrs.And in the same date the D.P measured as 15 mm Wc average for 8 hrs.
    I couldn’t understand the reason for this very low D.P .. Can you explain please…

    Reply
    • dominickdalsanto
      dominickdalsanto says:

      Abnormally low DP is usually caused by one of two things:
      – Faulty readings (broken gauge, blocked air lines, operator errors, etc.)
      – Leaking filters (displaced filters, torn filters, improperly installed filters, etc.)

      If can also happen if the baghouse is over sized (unlikely) or if the wrong fabrics are used and the bags are over cleaned. If you over clean the bags and they are not PTFE membrane bags you can see a low DP.

      Reply
  14. Atul Agarwal
    Atul Agarwal says:

    Dear Dominick,

    We have facing high DP problem in our Bag house. We have replaced the all bags by new one. We have five compartment in our Bag house . We measure DP across each compartment and getting 4.5 to 5″ but measure total Bag house DP it is approx 10″ wg. Could you please help us to find out reason.

    Reply
    • dominickdalsanto
      dominickdalsanto says:

      I hope you don’t mean you literally changed each bag one at a time as this would cause a different set of problems. Bags need to be changed as an entire set or else only a few at a time if there are leaks otherwise you will wear out the bags around the new ones.

      First thing to do when faced with inconsistent readings is to check the instruments to make sure they are working properly. Then you can look at problems with operating levels, dust loading, poor bag selection and other issues that could be causing the bags to plug up.

      Reply
    • dominickdalsanto
      dominickdalsanto says:

      Most bags do not hold up well when washed. Baghouse filters are not designed to be washed and reused like this so any attempt to do so usually results in early bag failure and increased emissions. Only certain specialty fabrics meant for a limited number of applications like food processing are purposefully designed to be washed and reused. But even these do not hold up well to multiple washings.

      Reply
  15. ekrem turan
    ekrem turan says:

    We are operating coal fired power plant (hard coal, 2×600 MW) and utilize baghouse filtration for dust handling.

    For bag cleaning pulse air system is used. (cleaning pressure 4-5 bar). Each flue gas channel includes 6 compartments.

    We are a new power plant and have been operating for 2 years and our bag filter DP is increasing. It was 800 Pa at commissioning and now it reached up to 1750 Pa after 2 years. Manufacturer set value for alarm is 2000 Pa.

    My question is that how long can we run baghouse even manufacturer set the value of 2000 Pa for changing bag filters. I mean, can we go up to 2500 Pa with arranging ID fan operation conditions.

    What is disadvantages going up to 2500 Pa, ıs it possible

    Reply
  16. paras
    paras says:

    Hello dominick , ee are unable to have sesuction at the end, though blower is working properly. Bag filters are new.. we have blower of 2800 rpm with the motor of 12.5 hp. Ducting size of is 15.5″. Ducting length is approx 20 foot long inclusive of bag filters unit..what may be the cause of loosing suction ..

    Reply
    • dominickdalsanto
      dominickdalsanto says:

      You need to check if the ductwork is full of holes that could be losing suction. More likely, the fan is wired backwards and thus spinning backwards. Or else the fan blades are not oriented correctly, they are not sized properly (happens when people try to repair fan blades on their own and they make them too small) or its the wrong kind of fan.

      Did the fan come from an OEM that specializes in dust collector fans? You cannot just take any fan and put it on the dust collector…our fans are designed to generate lots of vacuum pressure…many other fans create lots of airflow but little to no suction pressure. Check also for any repairs done to the fan…we recently had a case where the mechanics had done homemade repairs to the fan blades and made them too small…so the gap between the fan blades and the housing was so large it was only generating 20% of the vacuum it originally did…

      Reply
  17. Birbal Ram Bishnoi
    Birbal Ram Bishnoi says:

    We having bag filter capacity 50000/60000/100000 cubicmeter flow/hour and motor rating is 110 kw for 50000 capacity bag filter.
    It is sufficient when we run the motor on 100% damper position and RPM 725,running load KVA 60 of VFD.
    As our motor rating is 110 kw, full RPM 970.
    Can we replace above motor in lower ratings?
    Please suggest what ratings are appropriate for above capacity bag filters.
    Our bag inlet side of suction is from bag and discharge side is in air.
    Please best suggestion is guide us for power saving as well optimum motor rating selections.
    With best regards.
    B.R.Bishnoi
    bishnoibr@shreecementltd.com

    Reply
    • dominickdalsanto
      dominickdalsanto says:

      B.R.,

      Thanks for posting your comment. Thats quite a complicated engineering problem to answer per a comment box. IF you would like more help on that please send us an email at info@baghouse.com with all the system details and fan information and we can recommend a few options for you.

      Reply
    • dominickdalsanto
      dominickdalsanto says:

      Srikanth,

      Im afraid I do not understand exactly what you are asking…if you want to know when to clean the bags, a good range is between 3″ and 5″ of w.g. for the differential pressure.

      Reply
    • dominickdalsanto
      dominickdalsanto says:

      It depends…in most cases you don’t want to make any modifications to the amount of filters as this will affect the air to cloth ratio and thus performance. However, at times you can make adjustments for certain reasons. For example, you might take out a handful of filters right in front of the inlet to reduce excess wear in that area. The most common reason to change the number of filters is when converting to pleated filters. Since you have more fabric area per filter you can sometimes get more with less total filters. Or you might want to bump it up to increase airflow.

      Send us the details of your system via the contact form and we can make some recommendations for you.

      Reply
    • dominickdalsanto
      dominickdalsanto says:

      Sandeep,

      You can learn a lot by reading the various articles in our blog. You can also inquire about our training courses. Contact us using the contact form for more information.

      Reply
  18. michal gromadzki
    michal gromadzki says:

    HI Dominic

    We use PTFE filter bags in our baghouse with pulse jet cleaning system. When DP starts to go above 6.2″ bag purging sequence starts until PD falls below 5.8″. Currently, we use 7bar compressed air to clean filter bags.
    What compressed air pressure do you recommend to use for bag cleaning process and what is maximum pressure PTFE can withstand?

    Reply
    • dominickdalsanto
      dominickdalsanto says:

      Hi Michal,

      First, it sounds like the DP range you operate in is too small…normally we like to see bags run between 2.5″ – 6″…maybe slightly tighter depending on the exact design of the system. If you pulse it that much you are probably over cleaning the unit.

      As for pressure, you want to start at about 60 PSI and work your way up from there. You want to use only as much pressure as required to clean the bag…nothing more. So only go higher if you need to. Some designs call for up to 100 PSI. The bags can take it but obviously they will wear out quicker than if you use 60 PSI.

      If you want more tips on how to improve the operation besides just increasing the PSI you can email us using the contact form below and we will be happy to review the application and make some recommendations for you.

      Reply
      • Shazana
        Shazana says:

        Hi Mr. Dominick,
        I work in cement plant. This DP of 2.5-6 inch is a must have DP for bag filter or is it depending on the size of bag filter?

        Reply
        • dominickdalsanto
          dominickdalsanto says:

          Hi Shazana,

          The DP doesnt really change with the size of the filter…not as long as you have a proper industrial dust collector. Small little lab testing rigs or homemade units might give you other ranges.

          Reply
  19. Roshan vali
    Roshan vali says:

    Hi this is Roshan,
    Recently we have installed filter bags in our RABH i.e.fiber glass acid resistance bags with PTFE membrane, now we are cleaning chambers in DP mode taking refrence DP across the bag house, cleaning start at 130mmwc and cleaning stop at 100mmwc, after cleaning it was observed that DP across tube sheet is around 20mmwc, above cleaning set points are OK, and I want to operate bag house at lower DP i.e.cleaning start at 100mmwc and cleaning stop at 70mmwc across the bag house, pls send your valuable inputs,
    Thanks Roshan

    Reply
    • dominickdalsanto
      dominickdalsanto says:

      Roshan,

      Its hard to recommend cleaning high and low points without knowing more about your application. But running between 100mm and 70mm is a very tight range. If you need to run it that tight it tells me you do not have that much capacity to space on the fan and that could mean your system is undersized.

      Reply

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