I have had several ball valves fail because the seat "extrudes" when we close the valve on 150# saturated steam service.  When closing the valve, the ball lip graps part of the seat and deforms seat, not letting valve close completly.  Is this a condensate problem, P/T rating of seat material or system design conern?

I sounds like the seat is swelling and/or shifting.  In steam service I'd check the temperature rating on the valve and make sure you got what you should have gotten.  If the seat is "extruding" it sounds like maybe the material got soft with heat.  As long as you have designed your system/proceedures to deal with condensation within a shut ball (e.g., specifiy that the cavity on a trunion ball valve must be drained prior to a cold start-up, or using drilled balls on single-direction flow lines) there is nothing inherently wrong with specifying ball valves in steam service.

Worcester has a technical paper TP12D "Failure analysis of Ball Valves".  Your local distributor should have it on a CD-ROM and he can zap you a copy. It has photos of various seat failure modes and it pretty useful.

Basically if the seat is extruding your seat material becomes too soft for the pressure at the working temperature.  If your operators are throttling with the ball valve in a partially open condition the assymetrically-supported seat and local high velocity will aggravate the problem.  Ball valves for throttling are not the same as off-the-shelf commodity ball valves.  

Just guessing, but you are probably using virgin PTFE seats.  Loaded or glass-filled seats will be better, or you could go to PEEK or even metal seats. Some tradenames for reinforced PTFE seats are Polyfill and High-Per-Fill. The  Metal seats should not be needed for 150 psi saturated steam, but it's an option.  

You might also want to look at whether the line upstream of the valve is appropriately steam-trapped, and if the trap is functioning.  If the valve is getting liquid condensate, the liquid could be flashing in the valve and causing 2-phase flow with velocity as high as sonic.  That's not good for the valve, either, and can cause additional damage besides just sucking the seats out. 

The root cause of the failure depends upon the type of ball valve and which, if not both seats are extruding.  If it is a trunnion ball, then the seats are not able to handle the pressure & temperature applied.  If floating ball, could be a seat design problem, ball port radius manufacturing problem, assembly problem, or product not designed to withstand the application.  Knowing ball type and which seat failed will ensure that advice given is accurate and appplicable to your situation.

MORE NEWS