Relief Valve Failure Cause
Relief Valve Failure Cause
(OP)
I am evaluating a failure of a relief valve for liquid application to lift at the required set pressure during surveillance testing. The valve lifted 6% high. It is an obsolete Crosby JB-36 valve.
I read some operating experience where an engineer attributed a failure to lift (lifting high) to spring relaxation and cold working. Spring relaxation has the effect of reducing the set pressure of the spring. The theory is that adjusting the spring over time because of spring relaxation can have an opposite effect - the effect of work hardening the spring, leading to a higher required pressure to lift the valve.
Is this a well-accepted phenomena / failure cause?
I read some operating experience where an engineer attributed a failure to lift (lifting high) to spring relaxation and cold working. Spring relaxation has the effect of reducing the set pressure of the spring. The theory is that adjusting the spring over time because of spring relaxation can have an opposite effect - the effect of work hardening the spring, leading to a higher required pressure to lift the valve.
Is this a well-accepted phenomena / failure cause?





RE: Relief Valve Failure Cause
How do you know it only moved 6%, that would be a few hundreds of an inch? The valve will start opening at set pressure and will not go full open until between 105% and and 110% of set pressure.
RE: Relief Valve Failure Cause
Just because something is well accepted and is true, it doesn't mean that it is the cause of your specific problem.
Have you also considered back pressure?
We usually use the Crosby JOS line, so I am assuming that the JB 36 is a conventional PSV.
It is entirely possible that the springs go weak over time (heat/cool cycles, fatigue, etc). However, you need proof of this (from testing, investigations).
I am also curious as to how you knew the valve only lifted 6% high.
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376: Eng-Tips.com Forum Policies to make the best use of Eng-Tips Forums?
RE: Relief Valve Failure Cause
Unfortunately the valve was readjusted and placed back in service before we could do any internal inspections. So for now I'm forced to come up with a "best" explanation, based on valve history and age of course.
RE: Relief Valve Failure Cause
RE: Relief Valve Failure Cause
RE: Relief Valve Failure Cause
OK. You are describing testing an object (the PSV) with two tests only 636 and 627 psi. The two tests are not more than 1,5% apart. This is indicating that the accuracy of repetitive action is fairly good, perhaps within acceptable variation for the PSV. (You need more tests to prove this to search for failure course - but should of course anyway change the spring before putting the valve back in service)
Then you have the 'systematic' variation, claiming that the valve is systematically about 6% off target. To prove this you would have to prove that all conditions are exactly equal when the valve was first set originally compared to the second tests, exactly same testrig, and that the valve was sealed and untampered with inbetween.
In a closed pipeline system (pressurized tank for outlet) and a manual operated test procedure, and with an 'unproven' (?) original set point for the valve, I am more leaning to believe that 'outside influence' rather than a suddenly occuring mechanical variation in spring material is the cause.
As given from others above, you could list a lot of possible explanations, the simplest being variatons between tests (if any?) when first setting the original set-point and the second pressure test. Even human error and calibrations of pressure reading instruments will contribute.
RE: Relief Valve Failure Cause
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376: Eng-Tips.com Forum Policies to make the best use of Eng-Tips Forums?
RE: Relief Valve Failure Cause
RE: Relief Valve Failure Cause
We've already considered a lot of possibilities, test rig, pressure reading calibration, human error, procedural guidance, etc. The unfortunate truth is we may never know for sure because the valve was never taken apart for an inspection.
And I'm already convinced that my original curiosity has been satisfied, work-hardening of the spring is not what has happened.
Having said that, I am content with the outcome of this thread and I am greatful for everybody's input.
RE: Relief Valve Failure Cause
RE: Relief Valve Failure Cause
The 36 indicates high temperature >450F to 800F. There may have been a temperature correction applied during the previous test/adjustment. If the bonnet vent was plugged, it could have caused pressure traped in the bonnet toprevent the bellows from flexing, thereby rendering the set pressure higher. Also, if you got 636 psi the first time and 627 psi the second, you should have made a third lift to determine if the Set Pressure was trending downward. ASME PTC 25 requires Set Pressure to be established and stabilized with no significant trend either upward or downward. This means that three (3) pops is the minimum in order to establish a trend. If the spring is trending, you need a new spring. I know you don't have a definitive answer, but those are just some things to consider.
JAC