The filter bag is the core filtration element of bag filter, the quality of bag decides the quality of dust removal function. The main reasons for the bag failure include the improper filtering material selection, faulty manufacturing, mechanical wear, chemical corrosion, too high flue gas temperature, condensation and bonding. The usual bag failure phenomenon includes bag wear, bag burn-up, bag embrittlement and bag clogging,etc.

The usual damage is stitches breakage at the bag mouth, bag body and bottom part, see Graph 1. Due to different air suction method , filtration method and dust removal method, the bag will wear out and be fail in the end. The bag wear can be divided into two types: mechanical wear and airflow scour.

Graph 1  Filter bag mechanical wear

1. Filter bag mechanical wear

When the bag and bag cage are installed in the project, the disorderly stacking often causes the deformation of the cage and the mechanical damage of the bag. The deformation of the bag perforated plate and perforated plate hole deviation will accelerate the wear progress of the bag mouth.

At the same time, the non-vertical bag cage will cause the mechanical wear between the bag and another bag (or the shell), the lower part of the bag will have the wear failure, and it is can be observed that the wear occurs on the outside of the bag.

When the installation of bag and bag cage is too tight or too loose, the mechanical wear will be caused by the bag intense expansion,contraction,distortion and stretching of the bag. Especially the rust,welded scar, burr and other surface roughness caused by the rough processing will accelerate the wear and failure of bag. Such wear occurs on the inside of the bag body.

2. Filter bag airflow scour

The damage and failure caused by the airflow scour mainly caused by the high pressure airflow directly inject into the upper part of the bag during dust removal. The way of flue gas inlet ,excessive concentration of flue gas, excessive filtration speed of flue gas and the excessive rise speed of flue gas.

For example, the tilt of bag cage caused by the installation and deformation of the perforated plate, will not only bring the center line of bag cage offset, resulting in airflow scouring. The excessive pressure of dust cleaning gas, nozzle inclination and uneven airflow distribution among nozzles are the common reasons for upper airflow scouring.

The airflow scour caused by the flue gas inlet method is mainly caused by the the airflow uneven distribution in the bag chamber, the airflow in local parts is excessive, which causes the dust impact the bag and accelerate the wear and failure of the bag. Especially for the abrasive gas, the local flue gas velocity is too large, the damage and failure of the bag is more frequent. See Graph 2

 Graph 2  Filter bag airflow scour

When the selection of filtration material isn’t considered comprehensively, the filter bag will quickly fail. The failure of filter bag caused by chemical action is chemical failure, mainly for hydrolysis reaction, oxidation corrosion and acid and alkali corrosion.

The common used filter material polyester and polypropylene is easy to have hydrolysis reaction. After hydrolysis reaction, the tensile strength of filter material decreases and is prone to wear. The leaked water from the bag filter top, the water vapor produced by the flue gas concentration will cause the filter material hydrolysis reaction.The higher moister content in the gas, the higher the temperature, the faster the hydrolysis.

3. Filter bag damaged by high temperature flue gas

The excessive high flue gas temperature includes instantaneous high temperature and continuous high temperature.The excessive high temperature will cause the irreversible failure of filter bag, for example, bag burn-up,oxidation corrosion, detonation and high temperature shrinkage and other problems. According to the influence factor of hydrolysis and oxidation corrosion process, the process of hydrolysis and oxidation corrosion will accelerate when the flue gas continuously run at high temperature, therefore it is necessary to cool down the flue gas in order to prolong the service life of filter bag.

3.1 Filter bag burning

Each kind of filter bag has its own nominal temperature upper limit, when the actual working temperature is higher than the upper limit, it will cause damage to the base cloth, laminated membrane and stitches of the filter bag.

Therefore, for the flue gas with the nature of detonation, measures such as dust collector,cooling spray tower and sparkle capture device should be adopted; at the same time anti static filter bag or metal mesh should be adopted. For the work condition of flue gas temperature fluctuation caused by the technological reason, attention should be paid to the flue gas temperature at the bag filter inlet, and a control system of high temperature alarm should be set up.

When the flue gas temperature exceeds the set value, the temperature can be reduced by means of spray cooling and cold air valve suction cooling. It is worth mentioning that, it is an effective and energy-saving way to recover the waste heat of flue gas and reduce the flue gas temperature by means of heat ex-changer, waste heat boiler.

3.2 Filter bag shrinkage at high temperature

When the actual flue gas continuous or instantaneous temperature exceeds the normal use temperature of filter bag, due to the temperature characteristics of filtration fiber, the longitudinal and latitudinal thermal shrinkage rate will become larger. When the thermal shrinkage rate is too large, the filter bag will become smaller upwards and tightly wrap on the filter cage, which can easily affect the dust removal effect and the cage can not be extracted from the bag. See Graph 3. At the same time, in the vertical direction, the filter bag will become shorter, the bottom of bag is easily broken.

Graph 3  Filter bag shrinkage at high temperature

4. Filter bag damaged by moisture condensation

For the flue gas under the fixed pressure, when the temperature is lower than the dew point temperature, the vapor in the flue gas will produce condensation phenomenon. If the design or operation is not good, the condensation and caking phenomenon will occur. In order to ensure the bag filter runs above dew point and in stable condition, measures such as the membrane laminated fiber, shell insulation and mixture of hot air should be adopted.

In cold condition, the cold startup and cold shutdown of bag filter equipment will cause the vapor in the high temperature flue gas to produce condensation and the bag will fail.

Therefore according to use environment and nature of flue gas, when the bag filter is not running meanwhile the necessary auxiliary heat equipment shall be running. For the bag filter needs to be restarted, the preheating work should be done before the bag filter is put into operation.

In addition, when spray cooling and cold air valve suction cooling are used to reduce the flue gas temperature, the condensation may occur at the bag filter flue gas inlet. So in the design stage, the characteristics of flue gas and the workplace environmental feature should be fully understood, according to the actual condition, a reasonable solution should be worked out .

Moreover, the condensation occurs at the bag inlet of bag filter, due to the compressed air temperature is too low or humidity is too large, or both. When the compressed air is used for clean the dust, the condensation is also created at the bag inlet. To avoid the condensation phenomenon, some measures can be taken, such as heating the compressed air, the dehumidutifying it or venturi tube added to increase the drainage gas and reduce the air volume.

5. Filter bag blockage

The dust layer will form on the filter bag when the flue gas with dust pass through the bag, the existence of dust layer will increase the filtration efficiency, with the thickness of layer increases, the filtration rate will higher and higher, but the running resistance will also be higher. When the running resistance exceeds the set value, the dust can be removed by means of mechanical vibrating, back purging or pulse impacting. However, the tar,vapor and ultra-fine powder inside the dust will go into the filtration material, they will block the bag and cause the excessive running resistance and the filter bag will fail.

When the bag filter is used to treat the high temperature flue gas, high humidity flue gas or the operating environment temperature is low, the condensation should be avoid , by which the deliquescent dust will adhere to the surface of filter bag and the bag blockage will occur. Even the pasting problem will cause the bag to fail.

If the processed flue gas dust particle is fine, the dust particle can easily go into the filtration material. In general, under such condition, the membrane laminated filtration material or the pre-coating layer should be made before the bag filter is put into operation. The pre-coating layer can protect the filtration material,improve the efficiency of bag filter and reduce the running resistance.

Bag filter is the indispensable equipment to treat atmospheric dust pollution, frequent breakage of filter bag will not only bring economic losses to enterprises, but also bring difficulties to enterprise management. Correct installation, reasonable operation and maintenance can significantly improve the efficiency of bag filter.