Recently I was invited to investigate a case of frequent burning of baghouse filter bags.
There were five such baghouses connected to five furnaces of a steel plant.
The client reasoned that the material of the bags was not suitable for the temperature of the gas it handled. However, with change of material the frequency of bag burning did not change. So it needed a different approach to home onto the reasons for the failures.
Hence, this is how I went about solving the case:
First I did a Weibull analysis of the failures. Engineers use Weibull distribution to quickly find out the failure pattern of a system. Once such a pattern is obtained an engineer can then go deeper in studying the probability distribution function (pdf). Such a pdf provides an engineer with many important clues. The most important clue it provides is the reason for such repeated failures, which are broadly classified as follows:
- Design related causes
- Operation and Maintenance related causes
- Age related causes.
In this case it turned out to be a combination of Design and Age related causes.
It was a vital clue that then guided me to look deeper to isolate the design and age related factors affecting the system.
I then did a modified FMEA (Failure Mode and Effect Analysis) for the two causes.
The FMEA revealed many inherent imperfections that were related to either design or aging.
Broadly, the causes were:
- Inability of the FD cooler (Forced Draft cooler) to take out excess heat up to the design limit before allowing the hot gas to enter the bag house.
- Inappropriate sequence of cleaning of the bag filters. It was out of sync with the operational sequence thus allowing relatively hot dust to build up on the surface of the bags.
Next, the maintenance plan was reviewed. The method used was Review of Equipment Maintenance (REM). The goal of such a review is to find maintenance tasks that are either missing or redundant for which new tasks are either added/deleted or modified. With such modification of the maintenance plan the aim is to achieve a balance between tasks that help find out incipient signals of deterioration and tasks that would help maintain longevity and stability of the system for a desired period of time.
Finally the investigation was wrapped up by formulating the Task Implementation Plan (TIP). It comprised of 13 broad tasks that were then broken up into more than 100 sub-tasks with scheduled dates for completion and accountability.