When will it Fail 1

[zdas04]

I was doing a compressor-station turn-around last week to get ready to tie in an additional compresor. I was standing around waiting on x-ray and noticed a 150 ANSI plug valve on the bottom of a dehy. Not being a fan of plug valves or 150 ANSI components on high-pressure equipment I traced the line and found that it was on a nominal 450 psig line. It had been there for 5 years, and the flanges were holding fine. I changed it out for a valve more appropriate to the service.

My question (yes there is a question here) is "does anyone have a feeling for how many times you can take a 150 ANSI flange to basically hydro pressure before it should be expected to fail?"


David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[jte]

David-

I'll take a shot at this one. I'd say there are two approaches depending on how you define failure.

If you are concerned about stress in the flange, then I'll assume your design stress is 17.5 ksi. This 1998 value puts me into using the 1.5 multiplier for the "hydro". Now, assuming the flange was actually designed to an allowable stress (many standard flanges won't pass a standard design) then the "hydro" stress is roughly 27 ksi. The alternating stress is half that or 13.5 ksi. From the VIII Div. 2 Appendix 5 fatigue table I'd say you have a design life of 50,000 cycles.

If you define failure with a strain (flange rotation) based criteria which would predict a gasket leak then I'd say the system will last indefinitely. Presuming creep is not an issue, I would not expect the strain to ratchet. You would not expect any more rotation the next time you pressure up the system than you had the last time, and the flanges held sucessfully then.

Having said that, I'd be very wary of recommending continued operation. The flange is routinely being operated beyond its rating and B31.3 allowable overpressures. If something - anything - goes wrong and you find yourself in court the opposition's expert witness will have to do little besides point out that this component was knowingly and willfully operated beyond its design envelope. This is a situation where your recommendation as an engineer pretty much has to be "replace it at the next opportunity with an (appropriate class) ASME B16.5 flange."

jt

 

[zdas04]

jte,
I was more than wary, I refused to allow the station to be restarted with the flange in the system. I saw it for the first time at about 11:15 am and it was gone by 12:30 pm. That whole "knowingly allowed it to be operated outside its design envelope" thing scares the heck out of me. I've been deposed a bunch of times and never got caught in that trap and I'm not about to start now.

David

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem