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Safety Relief Valves - flashing
2

Safety Relief Valves - flashing

Safety Relief Valves - flashing

(OP)
Hi Forum, my 1st post.
For flows where there is flashing of liquid to vapor when a safety relief valve opens, what is the best and recommended method of sizing> I've seen terms like the omega and diers methods discussed, but don't really know which to use and when/why. I need to size for some vessels containing LNG. I can use and follow vendors programs but want to understand more independently. Any guidance will be helpful. Thanks.

RE: Safety Relief Valves - flashing

Your best start is to review the methods in API-520 Part 1. Over the years more information has been added on 2 phase flow sizing with the more acceptable methods. The API-520 Part 1 is currently in edition 10 (issued October 2020). I don't have this revision yet, but do know that it now includes information on what PRV trims to use - this being the various liquid and/or gas trims that manufacturers have developed over the years and recommend for multi phase flows.

*** Per ISO-4126, the generic term 'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

RE: Safety Relief Valves - flashing

2
Deciding which method to use can be confusing. For 2-phase sizing there are three methods that are explained in API 520 Pt I. By default, I use the Direct integration Method. It's easy, assuming you're using a sizing software that does the calculations for this method. Alternatively, in your case, you could use the second option (Omega for Flashing Liquid). All three of these methods are related. The two Omega methods are less rigorous versions of the Direct Integration method.

1. Direct Integration (or HEM - Homogeneous Equilibrium Model)
- General purpose method that is conservative and applicable in all cases.

2. Omega for Flashing Liquid
- Applicable for saturated flashing liquid
- Applicable for combination of non-flashing liquid and non-condensable gas

3. Omega for Sub-Cooled Liquid
- Applicable for a subcooled liquid that flashes as it enters the PSV inlet
- Not suitable for streams containing condensable vapor or non-condensable gas

RE: Safety Relief Valves - flashing

(OP)
Thanks for the replies. A bit more clearer now. Will try to perform these. May come back!

RE: Safety Relief Valves - flashing

Have you checked out the FAQs section? It’s not exactly what you need for your flashing liquid, but FAQ1203-1293: A Simple Numerical Method for Gas/Vapor Flow in a Safety Valve does show a simple integration method for vapor/gas. HEM is missing, but it may be a good starting point before getting immersed in the two phase methodology you need in API 521 Part I.

Good Luck,
Latexman

RE: Safety Relief Valves - flashing

(OP)
pierreick - Thanks but the information in that document is well out of date and refers to API-520 7th edition 2000. We are now edition 10 2020.

Latexman - Thank you. I'm new here so picking up, Thanks for the pointers.

RE: Safety Relief Valves - flashing

Hi,
My thoughts were that you needed materials to support your work , the reason why I shared this document. Good to you if you have the updated API-520 edition to perform your work . As long as the document shared is supporting the software recommended by Leser I believe the formula are valid.
my 2 cents .
Pierre

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