The 3% rule
The 3% rule
(OP)
The company where I work has several relief valves identified as having inlet line losses greater than 3%.
- What kind of engineering analysis can be done to show that the installation will not chatter?
- What guidelines can be used to ensure the installations will not chatter?
Thank You!
- What kind of engineering analysis can be done to show that the installation will not chatter?
- What guidelines can be used to ensure the installations will not chatter?
Thank You!





RE: The 3% rule
Did you look at this one. thread1203-383741: Relief Device Inlet Piping - Beyond the 3% Rule
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: The 3% rule
http://oilandgasprocessing.blogspot.com/2014/01/ps...
RE: The 3% rule
Notice that API 520 doesn't prescriptively define a particular engineering analysis methodology. But this is a complex topic, so I don't suggest going off on your own and creating an engineering analysis. Over the last decade a lot of research has been done on PRV stability analysis, and there are process safety engineering companies that are practicing analysis methodologies based on that research. ioMosaic's website advertises a seminar in which they teach how to do these engineering analysis calculations. Alternatively, you can pay one of these process safety companies to do this work for you.
This is still an evolving technology. There's no guaranteed way to be certain that a PRV installation will not cause the valve to chatter. However, what is known based on research and testing is that the legacy 3% rule isn't sufficient for predicting chatter. This research has shown that a PRV with 2% inlet loss may chatter, and a PRV with 10% inlet loss may not chatter.
RE: The 3% rule
RE: The 3% rule
Per ISO, only the term Safety Valve is used regardless of application or design.
RE: The 3% rule
RE: The 3% rule
RE: The 3% rule
Many thanks!
RE: The 3% rule
From the title of this RAGAPEP, I gather these are also recommended good practices, and do not carry the same mandatory / regulatory compliance obligations as with the ASME BPVC.
To summarise, if you wish to exceed 3% inlet dp loss by following some route stated in the API, the risk is all yours - if anything goes wrong, you may not be able to take umbrage by the API at an audit / inquiry (depending on who runs the audit) since it is a ASME Code violation.
RE: The 3% rule
OK, specifically what is that risk? For US facilities it is the risk that OSHA can successfully claim that API 520 Pt II is not a RAGAGEP standard. Can they claim such a thing? Yes - anything can be claimed. Would such a claim be upheld in a court of law? Again, anything is possible, but it's very unlikely that a globally used standard, such as API 520, could be judged to be contrary to RAGAGEP.