Q & A > Question Details
We have one PSV Set pr.:-16.5Kg/cm2, Cold Set Pr:-17Kg/cm2 & back pressure:-1.7 Kg/cm2. However, after dismantling it was observed the bellow is in cracked condition. We have withdrawn the bellow against the psv code but it doesn’t match with original bellow. We have also looked for other matching probability but we have failed to find any matching bellow. we have to install PSV without bellow. What will be new pressure set for the PSV without bellow?

Further question:
Do we need to raise the CDSP (cold set pressure) of PSV? if not, then why install bellow type PSV?
 
Answers
30/07/2010 A: Bob Kalaria, KBR, bhavdip.kalaria@kbr.com
Taking the bellows out from the PSV will not affect its set pressure. The set pressure will remain the same. I believe the purpose for installing the bellows type relief valve in the first place was to meet the API total back pressure design guideline since it looks like you have a high back pressure. In this case if you take the bellows out of the existing relief valve, it will be like a conventional relief valve and your API allowable back pressure will be 10% of the set pressure instead of 30% allowed for the bellows type relief valve. Having said that, it becomes clear that you will be violating this criteria and if that's the case you may want to replace the relief valve with another bellows type relief valve as there is no other way to meet API total back pressure criteria.
29/07/2010 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Disclaimer: My comments on matters of safety are not to be accepted for design or operation. No liability is assume or accepted on my part. Having said that: If the rupture disc is installed below the PSV for the purpose of keeping the insides of the PSV clean, no change in the PSV set pressure is required.