External Fire and Two Phase Flow

[Chance17]

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.

 

[BenThayer]

aren't you using current software that allows you to actually calculate for two phase flow? if so, why not use the 21% allowed?

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.

 

[CMA010]

API has dropped the vapour + liquid flow approach.

What is it that you do not trust in the current two-phase flow sizing approaches?

 

[moltenmetal]

Yitbos, is it likely that the difference between your 10% and 21% overpressure assumption will mean the difference between safe relief and catastrophic vessel failure? Or is it more likely that what it will amount to is the difference between safe vessel relief leading to a high certainty of being able to re-use the vessel after it is engulphed in a huge fire, or in the alternative, needing to scrap a vessel which yielded but did not rupture during a huge fire?

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?

 

[pleckner]

There is nothing in current thinking and practices that would preclude you from using 21% overpressure for a fire case, all vapor or otherwise. The only exception to this would be if the fire caused a chemical reaction, then use 10% overpressure, otherwise 21% is OK.

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.

 

[Chance17]

Thanks to everyone for your comments.
Happy Holidays