Dust leakage on bag filters : problem solving (breakage detection, prevention)

The use of pressure differential sensors but also more original options, such as triboelectricity measurement, can be used to detect damages of a filter baghouse that could lead to leakage of dust. This page explores those different possibilities but also how to prevent and mitigate damages to filter bags.

1. Bag filter breakage

The damage of a bag filter can have different levels of consequences depending on the filter type, size, position and process on which it is operated. It can be a simple dust emission within a process area, quickly identified and repaired. But leaking dust collectors can also lead to environmental problems if the dust is toxic, or to immediate process safety risks in case the leakage dust concentration is such that there are risks of explosions.

Bag filter damages can be due to different causes :

• An improper assembly of the bag
• A lack of maintenance (bag not changed in time)
• Some wear due to the particles flow hitting too directly to the filters
• A too high pressure difference applied to the filter
• A too high cleaning pressure for pulse jet filters (pressure at which air in pulse backwards to the filter)
• Too high temperature, or presence of chemical damaging the filters

Once damage, the filter baghouse is not able anymore to retain particles that can then leak to the "clean" side of the filter and be released to the environment, sometimes through other process equipment (duct, fan...).

For most critical filters, companies must consider the risk of filter breakage and put in place measure to prevent, detect, mitigate those failure of the filtration in their process. Some elements are given in this page.

2. Detecting a leaky filter

How to detect that a filter bag is damaged and let powder leak ? How to detect a broken bag filter ?

In case of damage of a filter, it is critical to be able to detect a leakage as soon as possible in order to correct it and avoid potential quality, safety or environmental consequences. The following possibilities can be explored to detect a dust collector leak :

• Measure the pressure difference (differential pressure) through the baghouse

Filter bags are creating a pressure drop when the air is crossing the filter material. It is possible to monitor this pressure and detect in case there is a sudden decrease of the pressure drop which could indicate that a filter is broken and let the air flow without resistance.

• Detect the presence of dust flowing behind the filter thanks to a triboelectric sensor

Bulk solids, when in movement, are generating charges. The charging process is called triboelectrification [1]. This phenomena can be applied to detect a bag failure. In normal operation, there is no dust flowing behind the filter, so no static electric charges created by the clean air, but if dust is suddenly entrained through the filter, charges will be created and it will be possible to detect them, indicating that the filter baghouse is not anymore tight.

3. Mitigating the consequences of a filter bag failure

It is important, in case of filter leakage, to be able to manage the effects. The following solutions can be applied to mitigate the consequences of having a dust collector leak.

• A quick detection of the leakage : the detection of a leakage should be coupled to an alarm, or even to an interlock to stop the process and make sure that the uncontrolled emission is minimized as much as possible
• In some cases, it is possible to install a safety filter after the main filter to stop possible leakages of dust. It is applied in the case of vacuum conveying to protect the blower or vacuum pump from the ingress of powder. The safety filter should also be equipped with a pressure differential sensor which can detect increase in the pressure drop, showing that the filter starts clogging due to dust leaking from the main filter.

4. Preventing filter bag failures

It is possible to prevent damages to filter baghouse thanks to the following practices :

• Make sure the bag unclogging system is operational and well set. Especially if compressed air is used to clean the filters : a too high cleaning pressure can damage the filters. Follow supplier's recommendations or try to set the pressure lower and verify if it can still achieve to clean the bags for your particular application. The bag cleaning frequency should also be tuned as low as possible to get just the desired cleaning effect.
• Do not operate the bag filters through a too high pressure drop. Install and monitor a pressure differential sensor to detect if the pressure drop accross the filters become too high. It is also important to design and size properly the filter according to the dust load foreseen (very fine dust can cause a high clogging of the filter) and the air flow foreseen (the pressure drop depends on the air velocity through the filter).
• Do not set the air pulse duration for too long, try to set the minimum pulse duration that still allows to clean properly the filters
• Make sure the environment in the filter remains dry (humidity can cause clogging of the filters, with some food problem even lead to growth or fungi and bacteria)
• Select the filters according to the temperature service or to resist to chemicals that may come into contact with the filter
• Have a level sensor in the hopper to ensure the material do not reach too high, especially not up to the filter, which would reduce the air exchange area available, lead to overpressure and damage of the filters.
• At design stage, make sure there is enough clearance in between the inlet of materials and the baghouse to avoid that powder directly hit the bags and create wear.
• Inspect, clean, change the filter regularly according to the manufacturer recommendations. Use the right specification for the filter media when replacing a bag.
• Train the maintenance team in order to make sure the bags are correctly installed. This can also be especially important if the bag filters need to be individually grounded (dust explosion risk - see this page)

Source

[1] Triboelectricity : a parameter for solids flow measurement, Dechene and Averdieck, PBE, 1987