EXCESSIVE PRESSURE DROP ACROSS FILTER BAG

EXTREMELY LOW PRESSURE DROP

PRIMARY (CONTINUOUS) DUSTING

SECONDARY (INTERMITTENT) DUSTING

SHORT FILTER BAG LIFE

EXCESSIVE PRESSURE DROP ACROSS FILTER BAGS

The differential pressure gauge or manometer on your dust collector should read 6″ w.g. or less. Higher readings and/or steadily increasing readings will result in a reduction of the main air flow through the dust collector which may produce a potential process problem such as poor suction through the evacuation control system. In extreme cases (over 10″ w.g.) filter elements will be damaged. Check the following:

1. Differential Pressure Gauge Operating – check the differential pressure gauge and the tubing leading to the dust collector. Disconnect the lines at the gauge and clear with compressed air. Look for loose fittings, cracked, broken or pinched tubing. Make sure the gauge is zeroed.
2. Pulsing System– inspect the compressed air cleaning system as follows, to make sure that all of the filter bags are being pulsed clean:
   1. If none of the solenoid valves are operating, check the timer.
   2. Check the air pressure at the header; it should recover to 90-100 psig before each pulse. If not, check to make sure that the compressed air supply system is in good operating condition, correctly sized, and supply lines are not too small or restricted. Listen for the sound of compressed air flowing continuously through one or more rows of bags; an indication of a valve or valves “stuck” in the pulsing position. The usual causes for this condition are leaks in tubing to solenoid pilot valves, and dirt in the solenoid or diaphragm valves.
   3. Check that all solenoid pilot valves are firing in sequence by holding a finger over each solenoid exhaust port as described earlier.
3. Water or Oil in Compressed Air – inspect upper portions of the filter bags for dust caking, dampness, or oil. Any or all of these symptoms are indications of moisture or oil in the compressed air supply. Install equipment that will ensure a continuous supply of clean, dry, oil-free compressed air. See your compressor supplier for recommendations.
4. Bags Loaded with Dust– a condition known as blinding. If the dust is dry, see Paragraphs A thru D; if the dust is wet, see Paragraphs E and F.
   1. Dust Not Discharging from the Hopper – check hopper for overloading or bridging across the dust discharge. Correct by repairing dust discharge equipment, replacing with higher capacity equipment, or installing hopper vibrators, etc., as required to keep the hopper clear. Never store material in the hopper.
   2. Air Flow Too High – if the main air flow is too high to allow dust to drop off of the filter bags, an excessive pressure drop across the dust collector will result and dust will build up in the system. In many cases, this high pressure drop in turn leads to a reduction in the main air flow, so that it is necessary to remove the dust accumulation from the bags (and the rest of the system) before measuring the main air flow volume.
      Visually inspect the bags for heavy caking. If caking is evident, see the note below and take the necessary action to clean the bags. Next, measure the main air flow with a pitot tube or equivalent device and compare with the original volume for which the unit was designed. If the flow is too high, cut back the main fan to prevent a recurrence of the problem.
   3. Particle Size and Dust Load – if possible, compare the dust particle size and loading with the original design specifications. Finer dust may cause a higher pressure drop. Do not hesitate to call us if you need to discuss further this potential problem.
   4. Filter Bag Dimensions Changed – Chemical cleaning or washing the filter bags may alter the dimensions which could cause the bags to not properly fit on the support cages. If the bags are not correct dimensionally, insufficient flexure, during the cleaning cycle, can result, which will reduce the force required to loosen the caked dust.
   5. Water Leaks – inspect the dust collector housing and ductwork for holes, cracks, or loose gasketing where water could enter the collector.
   6. Condensation – if moisture has been condensing inside the collector, check the dew point temperature of the incoming air stream. It may be necessary to insulate the collector and/or the ductwork leading to the collector to keep surface temperatures above the dew point and prevent condensation on the filter bags.

Note – Collectors that have had blinded or caked bags can possibly be put back into service by first running the pulsing air system for 15 to 30 minutes with a 3 second “off time” setting and without the main fan or blower operating. If the pressure drop is not lower when the main fan is started again, the bags must be changed.

EXTREMELY LOW PRESSURE DROP

In addition to inspecting the filter bags for proper installation or holes or tears, the following items should also be considered:

1. Pressure Gauge Working – check the differential pressure gauge and the tubing leading to the dust collector.
2. Ductwork, Dampers – inspect the ductwork to and from the dust collector for air leaks or blockage. Make sure that any dampers in the system are correctly positioned to allow air to flow through the dust collector.
3. Leaks in the Housing – check the tubesheet and the dust collector housing for holes, cracks or loose gasketing that would permit air to bypass the dust collector or filter bags.

PRIMARY (CONTINUOUS) DUSTING

A continuous flow of dust out of the exhaust of the clean air plenum is considered a primary dusting problem. Other causes of this condition are as follows:

1. Holes in the Filter Bags or Bags Incorrectly Installed – inspect the filter bag assemblies for holes, rips, tears, or excessive wear. Make sure that the filter bags were installed correctly and that no bag/cage assembly has dropped off.
2. Check the tubesheet for holes, cracks or loose bolts that would permit dusty air to bypass the filter bags.
3. If primary dusting is noted, the problem should be investigated immediately. Dust in the clean air plenum can be blown back into the bags during the pulse cleaning. The dust can be extremely abrasive and could seriously abrade holes in an entire set of bags in a very short time.

SECONDARY (INTERMITTENT) DUSTING

Puffs of dust that are noted in the exhaust of the clean air plenum after each cleaning pulse is a secondary dusting problem. This condition is normal with new filter bags, and should stop after the first several hours of operation. Other causes of this condition are as follows:

1. Air Header Pressure Too High – check the air header pressure gauge. If the pulsing air pressure is over 100 psig, filter bags may be flexing excessively and allowing fine dust to pass through the bag material.
2. Worn Filter Bags – inspect the filter bags for wear. Thin bags may not stop fine dust when flexed by a compressed air pulse.
3. Residual Dust – if dust has gotten into the clean air plenum because of a dropped or torn bag, hole in tubesheet, etc., the pulsing air may stir up the dust and allow it to escape into the clean air exhaust after each pulse. Residual dust may also be driven down inside the filter bags by the pulsing air. If the filter bags are filled with several inches of dust, clean both the clean air plenum and the bag/cage assemblies to avoid further problems.

SHORT FILTER BAG LIFE

This is often a complicated problem to diagnose, and we recommend that you call us for advice. The following list may be helpful in performing some preliminary checks:

1. Temperature – operating the gas temperature above the recommended limit of the filter bag material.
2. Chemical Attack – the filter media can degrade due to attack from certain chemicals in the dust or gases in the air stream.
3. High Moisture – a high moisture content in the collector may cause certain filter bag materials to shrink or degrade. The process will be accelerated at elevated temperatures.
4. Localized Abrasion – premature bag wear can occur near the tops of the filter bags if the bag and cage assemblies have been installed improperly or if an accumulation of dust on the clean side of the tubesheet has developed which may abrade the elements during the pulsing cycle. Localized abrasion can also occur near the bottom of the filter bags in the vicinity of the dust gas inlet. If this situation exists, an inlet baffle may be required to deflect the incoming gas stream if one does not already exist.
5. Cage Deterioration – corroded, rusted or broken filter bag cage assemblies can cause excessive bag wear. Stainless steel or coated cages are available.
6. Filter Bags Rubbing – premature wear can occur if the bags are found rubbing against each other or against one of the interior walls of the unit.