Maintain & Operate Dust Collectors with Differential Pressure

By Dominick DalSanto, Baghouse.com

Differential pressure is the key data metric used to determine how a dust collector is operating. System operators and maintenance technicians must keep a close watch on the differential pressure at all times to ensure proper operation of the system. They must also monitor the differential pressure in order to plan maintenance as well as any modification to the system such as to increase capacity or improve efficiency.

What Is Differential Pressure in a Baghouse and Why Is It Important?

Differential pressure (also known as pressure drop, or Delta P) is a measurement of vacuum on two different sides on the dust collector (clean-air side and dirty-air side). On a negative pressure dust collector, dirty air is pulled by the system fan along the length of the ductwork into the dust collector and then through the filters before being exhausted out. The vacuum pressure or suction of the fan is greatest near the fan and decreases as you get farther away from the fan down through the ductwork. Every turn, every bend, and every split adds resistance to airflow that the fan needs to overcome to pull the air all the way back to the collector. This is referred to as the static resistance of the system. This more or less stays consistent for the life of the system.
    
However, the greatest resistance comes from the baghouse filters. As the system runs these filters get dirtier and dirtier, and the more dust that builds up, the harder it is for the air to pass through the filters. As the resistance rises, the vacuum pressure on the dirty side of the unit is lower than on the clean side. A differential pressure reading is taken between these two points to provide an idea of exactly how much resistance the filters are adding to the system at any given moment in time.
     
The primary use for differential pressure readings is to determine how the loaded with dust the filters are inside the unit. In addition to this, DP can also be used to diagnose a number of different issues within the system. As it is one of the only sources of operational data coming from a baghouse it is vital to pay close attention to baghouse DP to ensure proper operation.
    
Let’s review a few things that differential pressure can tell us about how a baghouse is running.

Dust Collector Filters

High Differential Pressure
- Consistently high DP is a sign that your filters are blinded (i.e. fabric fully saturated with dust particles). Once blinded, filters can no longer be cleaned by the cleaning systems and must be replaced.
- Blinded filters or even filters with large temporary dust cake buildup will have higher emissions (i.e. leaking). Check opacity meters or broken bag detectors to verify emissions levels are within acceptable limits.
- Dust may be too difficult to remove from existing filters. Switching to different fabric and/or treatment may be required (e.g. Oleophobic coating, PTFE membrane, etc.)
- Consistently high DP could be an indicator of insufficient air to cloth ratio (ratio of how much airflow for the total amount of filter media within the baghouse) for the application will result in the bags being overloaded with dust and the cleaning system will be unable to effectively clean the bags. This could be due to temporary upset conditions in the process or due to the dust collection system being undersized for the application. Solutions include switching to pleated filters to increase filter cloth ratio

Low Differential Pressure
- Abnormally low DP (below 3 in.) can indicate the presence of holes, tears, or loose seams in the bags, or improperly installed filters (misaligned snap band on top load models, loose bag clamps on bottom load units).
    
A sudden drop in DP could be the result of massive bag failure such as due to upset conditions, chemical attack (e.g. acid flash), exceeding recommended maximum temperature, fires/explosions, and foreign objects in the airstream can all lead to catastrophic failures that would destroy the bags and drop the DP very quickly.

Baghouse Cleaning System

High Differential Pressure
- Inadequate bag cleaning will result in higher DP since the dust cake will build up faster than it can be cleaned off
- In a pulse jet dust collector this could be caused by low compressed air pressure, contaminated air (dirt, water, or oil in air reservoir), misaligned installed blow pipes or misaligned/missing cage venturi.
- If the cleaning cycles (i.e. air pulses) are not set correctly the bags will not be cleaned properly.
- “On time" is how long the pulse valve is opened. If set too long the air burst will be too weak to clean the bag. If set too short it will not release enough air to clean the entire length of the bag.
- The order the bags are cleaned (called "firing sequence") should be staggered (1, 4, 2, 5, 3, 6) so that successive rows are not cleaned one after another (e.g. 1, 2, 3, 4, 5, 6,). Failure to do so will result in dust merely being transferred from one row of bags to another rather than dropping down into the hopper.

Low Differential Pressure
- Over-cleaning of bags places undue wear and tear on the filters and leads to early bag failure.
- While higher compressed air pressure cleans the filters more thoroughly, if the pressure is too high it also causes early bag failure by creating abrasion problems or even creating holes in the fabric.
- Since emissions are at their highest when the filters are pulsed, over pulsing leads to increased emissions. Additionally, sufficient dust cake must be maintained in order to achieve maximum filtration efficiency for fabric filters (excluding for membrane bags). By over-cleaning the filters the DP will stay lower, but the emissions will increase proportionally.

Problems with Dust Collection System

High Differential Pressure
- Sudden changes in DP can be caused by upset conditions or changes in the process the system is venting. Any problems in the dust collector could be sign that something potentially far more serious is taking place further upstream from the unit.
- Consistently high DP might be a sign that the system is undersized and not able to keep up with the load placed on it.
- Worn out baffle plates, dropout boxes, and poor airflow design contribute to overloading of filters and high DP.
- Excessive dust build up in the hopper can lead to dust re-entrainment and overload the filters.
- Wet or sticky dust -- which is difficult to clean -- can be a sign of leaks near the hatches or cracks in the baghouse structure that allow moisture or cold air to enter causing condensation on the filters.

Low Differential Pressure
- Cracks or holes in the structure, or the tube sheet of the dust collector can cause significant reductions in DP and lead to an enormous surge in emissions.
- Dust accumulation upstream from the baghouse is preventing dust from reaching the baghouse. This could be a sign that dust is collecting in the ductwork and blocking off airflow to the baghouse. This could potentially lead to collapse of the portions of the ductwork or present a fire/explosion hazard in applications involving combustible dusts.

Beware of False Readings
Since differential pressure readings are so vital to understanding what is going on within a dust collection system it goes without saying that it is of the upmost importance to ensure accurate DP readings are taken from the baghouse.
    
We have seen how vital it is to monitor differential pressure in a baghouse. However, it is quite common for operators to receive false DP readings from their instruments. All too often this incorrect information is used to make poor decisions regarding maintenance and operation. Therefore, maintenance personnel need to ensure that differential pressure readings coming from the unit are accurate. To this end, it is best to make maintenance on the airlines, DP gauges, and baghouse controllers a regular part of the dust collector's preventative maintenance program.

Do Not Neglect Your DP Readings - It Could Cost You!
Many facilities are required to operate in accordance with air quality permits issued by local, state, or federal agencies. In many cases, differential pressure is used by these agencies to determine whether or not a baghouse dust collector is operating at required efficiency. For this reason, rather than citing specific particulate emissions levels many air permits simply require that a baghouse’s DP be recorded and submitted to the permitting agency to demonstrate compliance. In such cases, managing differential pressure is an ever=present priority, as even temporary surges must be recorded and could result in large fines.  Permitting agencies take this matter very seriously. In some cases, plants have even been fined for submitting DP figures from a DP gauge later found to be broken. In the end, the plant was fined $325,000.

Conclusion
How many maintenance and operation decisions regarding your baghouse dust collector are impacted by differential pressure? All of them. As we have seen, differential pressure deserves the full attention not only of maintenance technicians, but also system operators, maintenance planners, and managers. So next time you find yourself struggling to diagnose some problem in your baghouse, remember that little gauge on the side and how it can lead you in the right direction.

Dominick DalSanto is dust collection expert and author at Baghouse.com.

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