How to choose the filter bag leakage prevention process?

As we mentioned before, there are 3 processes to avoid filter bag leakage. Hot melt treatment is the best method, with low cost and good leak prevention effect. When hot melt process cannot be used, how should we choose the remaining two processes? Taking the flue gas of coal-fired power plants as an example, the following evaluates the coating process and PTFE tape process from the two aspects of heat resistance and acid corrosion resistance, providing a reference for the selection of filter bag leak prevention measures.

1 PTFE tape heat resistance

The flue gas temperature of coal-fired power plants is usually above 100°C, and in some special working conditions it can reach 170°C, and the instantaneous operating temperature can even reach above 200°C. Filter bags used in thermal power plants need to be able to work in high temperature environments. In order to simulate the high temperature environment of actual working conditions, the test samples with a specification of 5×5 cm were placed in a high-temperature oven, and their appearance changes were observed after heat treatment at 200°C for 24 hours. As shown in Figure 2.1, from the comparison of the samples treated by the glue coating and PTFE tape process before and after high-temperature treatment, it can be seen that the appearance color of the glue-coated sample slightly turned light yellow, but the sealant was firmly bonded to the filter material substrate; while the PTFE tape shrank significantly, and obvious dark yellow substances oozed out of the edge of the PTFE tape. Therefore, it can be explained that the fusion of the PTFE tape and the seam is not based on the thermal fusion of PTFE and the substrate, but on the adhesion of the adhesive, and this type of adhesive is not suitable for high-temperature environments.

Figure 1 Sample after high temperature treatment (the upper picture shows coated with glue, and the lower picture shows PTFE tape)

2 Acid corrosion resistance

Sulfur is produced when coal is burned, and then sulfuric acid with strong corrosive properties is formed after oxidation and contact with water, which will have a certain effect on the sealant and PTFE tape used to seal pinholes. In order to simulate the acid corrosive environment under actual working conditions, a specimen with a specification of 5 x 5 cm was placed in a 35% sulphuric acid solution and observed for significant changes after 24 hours of immersion. As shown in Figure 2.3, the specimen treated with adhesive has no obvious colour change in appearance after contact with the sulphuric acid solution, and the colloid is slightly sticky, but the sealant can be firmly adhered to the substrate of the filter material; the specimen treated with PTFE tape is detached after contact with the sulphuric acid solution, and is almost separated from the substrate of the filter material. The reason may be that the adhesive of PTFE tape is not resistant to acid corrosion, leading to the peeling off of PTFE tape. Therefore, it is more appropriate to use the adhesive coating process in engineering applications where PTFE tapes are prone to peeling off in strong acid corrosive environments, leading to the risk of pinhole seal failure and dust leakage.

Figure 2 Samples after sulfuric acid treatment (the upper picture shows coated with glue, and the lower picture shows PTFE tape)

In conclusion, experimental comparisons show that the heat and acid resistance of the adhesive coating process is superior to the PTFE tape process.

3. Typical case analysis

After a year of use, the customer's filter bag with PTFE tape had a lot of issues.

We noticed that the filter bag had a number of PTFE tape bulges and shedding on the outside. This was on the pinhole, bag head, bag body and the bottom of the bag. Figure 3.1 shows the PTFE tape bulging in the bag body. The tape is bulging, falling off and leaving a lot of dust inside. When we looked at it under a microscope, we could see that the dust had spread to the edge of the pinhole and was getting into the local pinhole.

Figure 3.1 PTFE tape bulging in a part of the filter bag (the upper picture is the overall effect picture, the lower picture is a partial microscope magnified picture)

4 Conclusion

Filter bag as the core component of bag filter, filter bag stitching at the pinhole may appear dust leakage, in order to reduce the risk of dust leakage caused by excessive emissions, must be grasped from the source of the filter bag leakage production to meet the requirements of the use of the filter bag stitching preferred hot melt process, when it is not possible to use the hot melt process for stitching, you can choose to use adhesive coating process and PTFE tape process. Experimental results show that the adhesive coating process has better heat resistance and acid corrosion resistance than the PTFE tape process. In addition, there is a risk of PTFE tape peeling off and dust penetration through the pinholes in practical applications of PTFE tape. Therefore, when it is not possible to use the hot melt process, you must choose a reliable, strong adhesive coating process, the choice of PTFE tape process must be careful.