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The 3% rule
2

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!

RE: The 3% rule

Engineering analysis (alternative to the 3% rule) is explained in API 520 Pt. II, 6th ed., section 7.3.6. Included is a high-level explanation of the suggested steps and suggested analyses. There are two analyses. One is a force balance and other is an acoustic analysis. I suggest you get a copy of the ioMosaic papers in references 31 and 32.

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

Also note that while the API publishes obligatory recommended practices, the ASME BPVC design codes are mandatory in jurisdictions where these are enforced, and this 3% inlet dp loss rule is stated here too - so you wont be able to wriggle your way out of the ASME BPVC, no matter what your engineering analyses may conclude.

RE: The 3% rule

Your PSV solution is to install a pilot operated SRV, but with remote sensing line. Instead of the sensing line pick up being located on the inlet neck of the SRV. engineer it such that it will pick up directly from the vessel. This works and is an option from all SRV manufacturers.

Per ISO, only the term Safety Valve is used regardless of application or design.

RE: The 3% rule

George - Possibly that is true in Australia - I'm not knowledgeable about Australia's legal requirements. But in the US, the legal requirement (OSHA) is that relief design practices must be consistent with "RAGAGEP" (Recognized And Generally Accepted Good Engineering Practices), and the practices in API 520 Pt. II are cited by OSHA as RAGAGEP. API standards and ASME Sec VIII are complementary rather than competing standards. Engineering analysis, as an alternative to the 3% rule, is an accepted and legal practice in the US. The practices published by API are also accepted (and legal) in most other countries too.

RE: The 3% rule

Thank you, don1980 and georgeveghese for sharing.

RE: The 3% rule

(OP)
I want to thank everybody for your comments. I have since looked at articles in this database and taken various notes based on your feedback.
Many thanks!

RE: The 3% rule

Don1980, The last time I saw this clause in one of the appendices in ASME Sec 8 Div 1, there is nothing stated about an engineering analysis or otherwise to allow for inlet dp to exceed 3%.

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

Quote (georgeverghese)


To summarise, if you wish to exceed 3% inlet dp loss by following some route stated in the API, the risk is all yours....

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.

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