How to determine if the filter bag of a large offline bag filter needs to be replaced?

To determine whether the filter bag of a large offline bag filter needs to be replaced, a comprehensive judgment should be made based on three dimensions: abnormal operating parameters, appearance inspection, and functional failure. Through dual verification of "data monitoring+on-site inspection", it is necessary to avoid a decrease in dust removal efficiency or equipment damage caused by filter bag failure. The specific judgment method is as follows:

1、 Core basis: Abnormal warning of operating parameters (early signal of filter bag failure)

During system operation, if the following parameters show persistent abnormalities, it usually indicates a decrease or damage in the performance of the filter bag, and priority should be given to troubleshooting:

1. Excessive dust concentration at the air outlet (direct "evidence of failure")

Judgment logic: Under normal circumstances, the dust concentration at the outlet of the offline bag filter should be ≤ 10mg/m ³ (in accordance with GB 16297-1996 standard). If the concentration continues to exceed the standard value or suddenly rises (such as from 5mg/m ³ to over 50mg/m ³) through online monitoring or portable detectors, it is highly likely that the filter bag is damaged (dust directly penetrates) or the filter bag fibers are aging (pores become larger, filtration accuracy decreases).

Verification method: If there is no monitoring device on site, observe whether there is "obvious dust dispersion" at the air outlet (such as "fog" appearing at the exhaust outlet or rapid accumulation of dust from surrounding equipment), or place white filter paper at the air outlet. If the filter paper turns significantly gray after 5 minutes, it indicates that the filter bag has failed.

2. The negative pressure of the system remains high and cannot be adjusted (core signal of filter bag blockage/bag sticking)

Judgment logic: The normal operating negative pressure is -2000~-4000 Pa. If the negative pressure continues to exceed -4500 Pa and there is still no downward trend in negative pressure after "shortening the cleaning interval and increasing the cleaning gas source pressure" (such as adjusting from 0.5 MPa to 0.7 MPa), it indicates that the dust on the surface of the filter bag has "solidified and blocked" (sticking the bag) or the fiber aperture of the filter bag is blocked by dust (unable to be removed by cleaning). At this time, the filter bag has too much filtering resistance and has lost its excellent filtering ability.

Exception exclusion: Non filter bag problems such as "intake valve not fully open, excessive air volume of induced draft fan, and blockage of ash hopper" need to be ruled out before determining as filter bag blockage.

3. Abnormal differential pressure (filtration resistance) of filter bag

Judgment logic: The normal filtration resistance of the filter bag (pressure difference before and after the filter bag) is 800~1500Pa. If the pressure difference remains below 500Pa, it may be due to extensive damage to the filter bag (direct airflow passing through without resistance); If the pressure difference continues to be higher than 2000Pa, it indicates that the filter bag is clogged (synchronous with high negative pressure), and the filter bag needs to be replaced in both cases.

2、 Key means: on-site visual inspection (direct evidence of filter bag status)

After stopping the machine, enter the filter bag room (perform the "power-off, pressure relief, ventilation" safe process), and visually and tactilely inspect the appearance of the filter bag, which is the final basis for determining whether to replace it:

1. Damaged filter bag (needs to be replaced immediately)

Typical manifestations:

There are obvious holes and tears on the surface of the filter bag (mostly located at the top of the filter bag (where it is connected to the flower board), the bottom (where it is in contact with the filter cage), or the middle (where dust cleaning impacts are frequent);

There is "hanging ash" on the filter cage (damaged filter bag fibers wrapped around the filter cage), or "abnormal coarse particle dust" appears in the ash hopper (if graded dust is being processed, coarse particles should have been intercepted by the front-end equipment, and the damaged filter bag will enter the ash hopper).

Positioning method: Use the "offline room one by one inspection method" - close the "offline valve" of a certain filter bag room, observe whether the system negative pressure and air outlet concentration have returned to normal. If they have, there are damaged filter bags in the room that need to be inspected and replaced one by one.

2. Filter bag aging (recommended to replace the entire room)

Typical manifestations:

The material of the filter bag becomes brittle and hardened: by gently pinching the surface of the filter bag with your hand, the fibers are prone to detachment (normal filter bags have a certain elasticity and are not prone to fiber detachment);

Abnormal color of filter bags: such as polyester filter bags changing from "white/light gray" to "dark brown, burnt yellow" (aging caused by long-term overheating), or showing "local fading, fibrosis" (aging caused by chemical corrosion, such as when dealing with acid/alkali containing dust);

Filter bag deformation: The filter bag cannot remain upright (due to deformation of the filter cage causing compression of the filter bag, or loss of support after aging of the filter bag), resulting in wrinkles and stacking (affecting the filtration area).

Replacement principle: Aging filter bags need to be replaced throughout the room to avoid mixing old and new filter bags (old filter bags have high resistance, while new filter bags have low resistance, and airflow will preferentially pass through the new filter bags, causing the new filter bags to block at a suitable speed and shorten their overall lifespan).

3. Filter bag paste bag (clean first, replace if ineffective)

Typical manifestations:

Dust hardening on the surface of the filter bag: When touched by hand, the dust layer on the surface of the filter bag does not have a "fluffy feeling" and cannot be blown off by compressed air (after normal dust cleaning, only a "thin and uniform dust layer" remains on the surface of the filter bag, which is the "initial filter layer");

Filter bag moisture absorption and agglomeration: If dealing with high humidity dust (such as flour, lime powder), the surface of the filter bag will show "moisture agglomeration", and even adhesion between the filter bags.

Processing logic: First, try using "low-pressure compressed air" (pressure ≤ 0.3MPa) to blow the filter bag in reverse. If the compacted dust still cannot be removed, or if the breathability of the filter bag is ineffective after blowing (the pressure difference is still high), the filter bag needs to be replaced (the fiber pores of the pasted filter bag have been blocked, and the filtering ability cannot be restored).

4. Filter bag service life expires (preventive replacement)

Judgment logic: The filter bag has a clear "design service life" (such as 1-2 years for polyester filter bags and 2-3 years for Flumes filter bags). If the equipment operates in harsh environments (such as high temperature, high concentration, and highly corrosive dust), the service life will be shortened. Even if there is no obvious damage to the appearance of the filter bag, if it has been used for more than 80% of its design life and there is a slow decrease in filtration efficiency (such as the concentration at the air outlet increasing from 3mg/m ³ to 8mg/m ³), it is recommended to replace it preventively (to avoid sudden large-scale failure and affect production).

3、 Auxiliary verification: offline sampling and functional testing

For large dust collectors (such as those with more than 10 filter bag chambers), the status of the filter bags can be further confirmed through "offline sampling":

Randomly select filter bags: Randomly select 2-3 filter bags from different filter bag chambers and check their appearance, weight (clogged filter bags will have a significant increase in weight due to dust accumulation), and breathability (use a hair dryer to blow air into the filter bags and feel the air volume at the outlet. If the air volume is small, the breathability is poor);

Simulated filtration test: Install the sampled filter bags in a "small filtration device", introduce dust with a concentration similar to the on-site concentration, and detect the filtered dust concentration. If it exceeds the standard, it proves that the batch of filter bags has failed and needs to be replaced as a whole.

Summary: The "Core Judgment Criteria" for Filter Bag Replacement

Core conditions that need to be replaced for determining dimensions

The dust concentration at the air outlet of the operating parameters continues to exceed the standard; The system negative pressure/filter bag pressure difference continues to be abnormal (exceeding the upper or lower limit) and cannot be adjusted through dust cleaning

The on-site appearance of the filter bag shows obvious damage and tearing; Brittle, aging, and fiber shedding of filter bags; The filter bag is stuck and cannot be cleaned

The actual usage time exceeds the design life, and the filtration efficiency shows a downward trend

By using the above methods, it is possible to carefully determine whether the filter bag needs to be replaced, which not only avoids cost waste caused by "premature replacement", but also prevents "late replacement" from exceeding environmental standards or equipment failures.