External Fire and Two Phase Flow
External Fire and Two Phase Flow
(OP)
I have been struggling with external Fire and two-phase flow.
ASME Code says 21% vessel overpressure is allowed for external Fire.
The rules were written when only all vapor venting was generally considered.
So I have been using 10% overpressure for two phase flow based mainly on the assumption that ASME Code has not has not caught up with the modern world.
My client disagrees - the Code makes no distinction between all vapor venting and two phase flow.
Of course he is correct.
But as a PE, it is my "professional opinion" the 10% overpressure is a correct evaluation.
This is based mainly on the uncertainity in two phase flow engineering science.
Any comments would be appreciated.
ASME Code says 21% vessel overpressure is allowed for external Fire.
The rules were written when only all vapor venting was generally considered.
So I have been using 10% overpressure for two phase flow based mainly on the assumption that ASME Code has not has not caught up with the modern world.
My client disagrees - the Code makes no distinction between all vapor venting and two phase flow.
Of course he is correct.
But as a PE, it is my "professional opinion" the 10% overpressure is a correct evaluation.
This is based mainly on the uncertainity in two phase flow engineering science.
Any comments would be appreciated.





RE: External Fire and Two Phase Flow
otherwise, wasn't it an accepted practice to look at what % of the flow was vapor and what was liquid and effectively do two calculations.
one for the in^2 needed for the vapor and one for the in^2 for the liquid and sum for a total in^2?
i guess that, as a PE, my opinion would be that you use current software from the modern world that accounts for two phase flow at a 21% overpressure and make those recommendations to your client.
RE: External Fire and Two Phase Flow
What is it that you do not trust in the current two-phase flow sizing approaches?
RE: External Fire and Two Phase Flow
If it's the former, and you're serious about that based on an actual parametric evaluation of the uncertainty of the two-phase flow assumptions and calculations (i.e. out of concern justified with calculations rather than merely fear of doing the necessary engineering), then it's a safety issue- a safety issue which will also involve a great many other vessels designed for 21% accumulation during fire.
If it's the latter, then it IS the owner's call rather than yours. I doubt an owner is going to be too unhappy if a vessel is trashed but not ruptured during a major protracted fire. Their insurance company might be, but are THEY going to pay for the extra cost of the larger relief valve and associated equipment?
RE: External Fire and Two Phase Flow
I also agree with the other repondees in that two-phase calculations for the most part are pretty well defined. The procedure outlined by API 520, Part 1, 8th edition, Annex C can be used. Use the Direct Integration method and you'll be fine. And being a PE, if you're not comfortable in doing the calculation, you can sub-contract it out.
RE: External Fire and Two Phase Flow
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