I'm currently in the process of troubleshooting a valve leakage problem in a 400 psia 700 F steam system. The problem area is consists of a 14" block and bleed setup using tricentric butterfly valves. leakage through the upstream block valve has persisted since the system was commissioned in 2002. this valve oriented vertically about 1/4 of the way up a 25 ft. riser. a one inch bleed valve cannot remove enough steam to keep up with the leak. the client believes that thermal growth in the piping system may be contributing to the leakage problem. system stresses are at 70% of 31.1 allowables. the valve is lugged and no gasket leaks have been recorded. i would think that if loading due to thermal growth was causing valve leakage that gasket leakage would have been noticed first. the original valve was replaced with a spare and no improvement in leakage was evident. the valve manufacturer has given us the run-a-round and doesn't seem to be too concerned about losing a customer. has anyone encountered similar problems with tricentric butterfly valves in a high temp service?

Has the vendor offered any guidance on what kind of bending moments the valve body can take?

I wouldn't be surprised if the butterfly valve leaks before the flange gaskets. I could see it taking relatively little distortion of the valve body from a bending moment to cause the disc to not seat properly.

With regard to the 70% of B31.1 allowable - its important to remember that thos allowable are with regard to the pipe wall. In many cases, flanges, pumps, vessels, etc. are the weak links in the system and the loads that they can take are so low that your piping system may only be 30-40% of allowable. So, if youa are seeing 70% of allowable, particularly near the valve, would cause me concern.