Service report; blinded filter bags
The blinded filter bags occurred in an Ultra-Flow 60 bag advanced technology baghouse dust collector, collecting rubber, fiberglass fiber and metal slivers from two hammer mills, in a tire recycling plant.
Problem: Dust filter bags started out at 1-1.5”wg pressure drop, then, rose to 7-7.5” in a short space of time. Cleaning off-line only reduces the pressure drop to 5-5.5”wc.
Then a third set of bags, installed a year later, started at 4”wc and clogged (7-8”wc) in 2-3 hours.
Observation (first set of bags, June 2, 2009):
- There was no indication of leakage from the dirty side to the clean side of the collector.
- A bag was sent to our testing lab for analysis. The findings were;
- Confirmed no leakage of the filter bags.
- The bag was totally blinded on the dirty side. It was a paste-like dust cake. It is also an indication of moisture in the process getting onto the bags.
- The bag was cleaned well but permeability was still low at 2-4 CFM. This indicates chemical attack of the media, likely from some kind of solvent. That renders the bags not recoverable.
III. Further very deep laundering of the bag with a surfactant started recovering the bag and increased the permeability to almost the “as new” state. This revealed that instead of chemical attack, the pores of the media were being “painted” making the bag coat with a sticky substance. No dust collector cleaning system can handle paint, blinding the filter bags media. There is likely rain, snow, road tar and solvent coming in with the tires. This moisture, when heated by the action of the hammer mill, would create latex and/or solvent based paint that got onto the bags. This situation would be intermittent. Once the moisture got through and the process dried up, the paste/paint on the bags would dry out also, leading to the false conclusion that the dust collected was dry.
- A subsequent squeeze test of the dust collected, alone, revealed no presence of oily substance, leading us to conclude that the painting is caused by moisture (water or snow remaining in the tires when put through the hammer mill.
Solution:
- Teflon bags would resist solvents. We would advise Teflon impregnated bags, not membrane (i.e. Gortex), to keep the permeability at a manageable level. The use of Teflon membrane would require de-rating the dust collector performance by 50%.
III. Too much heat may be generated at the hammer mill which can be corrected by using a current controller (instead of a voltage limiting speed limiting controller) on the drive, producing a powder instead of paint. Also an inert dust could be fed in as pre-coat to make it a powder cake instead of a gooey coating.
- Same as III.
Observation (third set of bags, April 2010):
- These filter bags no longer displayed the wetting and hydrocarbons of the first set, due to corrections in the process. The polypropylene bags, from a new supplier, appeared heavier and glazed on both sides of the bag material.
- The snap band collar was uniformly dirty, not just the part below the tube-sheet groove, and there was dirt on the inside of the bag. This was an indication that the bags were installed wrong. Instead of snapping the band’s groove into the tube-sheet edge, they laid the snap band on top of the tube-sheet. That created a loose fit and a large leak of air from the dirty chamber to the clean air chamber. Each pulse would blow dirt back down into the bag, coating the clean side with dirt. This negated the cleaning system by blinding the bags.
- The hopper was being discharged into a porous bag instead of the original sealed bin. This results in the air being drawn up through the discharge into the collector. The upward “can velocity” hangs up the dust into the collector and causes the dust to be re-entrained onto the bags during a cleaning pulse and will not drop to the hopper and bag below. When we opened the access door on the hopper a large amount of dust would drop to the bag when the suction was released at the discharge.
- A replacement set of polyester bags had a high 4-5”wc initial pressure drop when installed.
Solution:
- Our testing laboratory confirmed that the bags, when clean, were 5-10 cfm permeability instead of 25-35 as specified for those bags. That accounted for the high initial pressure drop. The test also confirmed that we now have normal dry rubber dust and none of the sticky stuff from before. A new set of bags was supplied, under warranty, switching back to the standard singed polyester material since moisture was no longer an issue. Polypropylene was supplied for the second set to account for the moisture getting in the process, at the time.
- The new set of bags was supplied with our standard o-ring seal. In this way, the bags could not be installed improperly, creating a reliable seal at the tube-sheet.
- The hopper discharged was sealed by a rotary air-lock which prevented dust hanging up in the hopper and filter bags.
- The new set worked well. The initial high pressure drop reading was the result of a clogged air line filter to the magnehelic gauge. After cleaning out the filter and air line, the false reading came back to the normal 1.5”wc.
For assistance… troubleshooting dust collector problems