Thermal Relief Valve Failure Cause

[jgar1]

I am evaluating a failure of a relief valve for liquid application to lift at the required set pressure during bench testing. The valve lifted 6% high (636 psi vs. 600 psi set pressure). 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?

It's not really that relevant, but the valve protects the high head safety injection system in a pressurized water reactor and discharges to the pressurizer relief tank.

 

[hacksaw]

when was the last time the valve was adjusted and are you including the cold set correction

 

[jgar1]

It was adjusted in March of '06. We are not including the cold set correction. But had we included it, the cold set correction would be 1% I think, which means it still would've failed.

Was the adjustment too recent for this failure cause to be credible?

 

[desertfox]

Hijgar1

I have never heard of a spring being work hardened due to periodic adjustment, usually you need a high stress level
to work harden a material for example if you put several
permanent bends in a piece of mild steel it will work harden.
I can however understand the spring becoming weaker over a
period of time as you describe.

Regards

desertfox

 

[Ashereng]

Please also do not double post.

If you need to, you can include a link to the primary posting.



"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[jgar1]

OK, did not know that, I am new to this.

 

[JimCasey]

I do not think that the theory about work-hardening the spring is credible. It is possible to compress the spring to binding without causing plastic deformation.

Mineral deposits around the disc and nozzle are a more plausible explanation for a high pop. That's why valves are required to have a lever. Burp them periodically to flush the dried crust from the trim. Your model number was incomplete: you probably have a JB-36-D, with the D indicating a packed lever ( corresponding to the need for a bellows.)