Bag House Operation Problems

Dust Collection

Bag House Operation Problems

Failure Patterns, Problem Identification and Solutions
More stringent air-pollution laws are drawing more attention to the off gas cleaning facilities of industrial and power plants. Nowadays, bag filters are gaining more and more attention also in medium and high temperature applications. Typical problems arising from physical function and chemical impact will be featured in this article.
(ed. WoMaMarcel - 03/9/2015)
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The Pressure Drop of the Filter Bags

Influences on the pressure drop of the filter fabric are:

  • Air to cloth ratio: Similar as the effect on the dust layer an appr. linear dependency of the pressure drop of the felt from the a/c-ratio can be considered. Above critical limits the stable behaviour (a more or less constant pressure drop over the bag life) ceases and penetration of dust into the felt results in clogging (penetration of dust into the felt or densification of dust close to the surface of the felt).
  • Dust properties and operating conditions: Changing dust properties, which may also be related to changing operating conditions (e.g. humidity, grinding aids, SOx/HCl ratio, phospates,...), influence the amount of dust which remains on the felt after cleaning (Fig. 4). Surface coatings can improve dust release and prevent from damage due to sticky dust to a certain degree.
  • Filtration efficiency of the felt: High dust loads and fine dust is often above the limits for standard filter media. Fine fibres and fibres with non-round shaped cross sections are a common approach to increase the filtration efficiency.

Fig. 4: Contamination from the clean gas side.

Typical pressure drop of standard filter media:

  • membrane materials:   1 to 2 mbar
  • needle felts in stable operation     : <1 mbar
  • membranes in operation (after cleaning): in best case close to values of new membranes

Depending on the construction of the filter media and dust properties different typical failure patterns can be observed which result in either an increased pressure drop, increased emissions or both. This topic is discussed later on (penetration of dust through the bag material without appearance of macroscopic holes).

Reasons for high Emissions

Well performing needle felts and membrane materials offer emissions not exceeding single digits of mg/m³. Higher emissions are indication for penetration due to

  • leakages of the filter (welding/corrosion) improper sealing between bags and cell plate,
  • faulty seams of the bags,
  • holes in the bags, and
  • penetration through the bag material without macroscopic holes.

Leakages/improper Sealing

Leakage tests with fluorescent powder are useful for identification of macroscopic holes. Seams are a critical point if not properly made, welded seams as well as stitched seams. Experienced manufacturer ensure adequate processing into bags.

Holes and Cracks of Bags

The listing below presents different causes for holes and cracks which are explained and illustrated afterwards:

  • Chemical aging (loss of tensile strength)
    - thermo-oxidative ageing
    - hydrolysis/condensing acids
  • Shrinkage/elongation (excessive temperature)
  • Mechanical wear
    - abrasion from locally high can velocities
    - abrasion from pulsing (improper adjusted cleaning nozzles, rust or dust from the clean gas plenum entering during cleaning)
    - mechanical wear along folds (oversized bags)
    - abrasion along cage wires (corroded/damaged cages, improper cage alignment ––> bag-bag or bag-wall contact)
    - burden from incorporated dust (penetration)

Laboratory analyses of bags can help to identify specific damage patterns if bags are failing, chemical damage can be detected to differentiate between rather mechanical or chemical causes.

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