Avoiding design problems in Steam Extraction Expansion Joints
Avoid Bad Design Features In The LP Turbine Expansion Joints
Lessons learned in steam extraction piping – Part 1
The Bellows Bottom Line is a publication of
Most expansion joints inside the LP turbine last over 30 years. I’ve seen a few failures that could have been
easily avoided. And when expansion joints fail in this
application; they fail really badly.
The problem is not the bellows
What’s really bad? When you walk into a room to help diagnose an expansion joint failure and most of the fragments can fit inside a bucket; that’s bad. The worst part about this failure was that the expansion joint supplier had gone to great lengths to talk the utility into upgrading the bellows from stainless steel to 2-plies of Alloy 625 ($$$), only to have it fail because of a bad liner design. The previous single ply stainless steel bellows had lasted 32 years.
The liner attachment weld failed due to poor penetration, resulting in high steam flow vibrating the bellows until it resonated itself into bucket size pieces. The faulty liner attachment design was intended to improve fabrication by eliminating the need for a tight tolerance fit. Instead of slipping inside the pipe, the liner butts up to the pipe edge. Unless the liner edge is beveled, this design makes it difficult to complete a full thickness weld. In this case the weld only penetrated 1/8 of an inch on the 3/8 inch thick liner.
In addition, this weld is nearly impossible to inspect
A typical V-groove weld has an angle opening of 75
degrees (two times 37.5). The angle opening of this notuncommon liner design is around 25 degrees, way too small to get electrode penetration.
The Bottom Line
For steam extraction applications, expansion joint liner attachment welds should have either a full thickness fillet weld, or a full thickness V groove large enough to ensure proper weld penetration.