ryn376 and SawsanAli311:
Bobby Strain is absolutely correct in his evaluation and advice given on this topic. His comments can be proven by inspecting a standard Mollier or T-S chart on the liquid fluid involved. What he simply states is also common sense.
Ryn376 states that his exchanger is having “a hydraulic expansion case where water can be heated above it's normal boiling point by steam when blocked in”. He then asks: “do we need to size the relief valve orifice for two phase flow or consider a two phase flow discharge line pressure drop?”
Both of you are dwelling in a mix up of what API 521 is covering and discussing. You fail to understand the actions of the relief taking place due to an over-pressure. As Bobby has pointed out, the system under hydraulic pressure is 100% filled (as defined). If you attempt to “boil” this water to steam, you must have a space to accept the steam as the hydraulic pressure increases. Since you don’t have that expansion space, your system will remain as high pressure liquid at the developed pressure.
The result of the hydraulic over-pressure will be that your thermal relief valve will open and high pressure LIQUID will flow through the valve’s trim and instantly expand into a 2-phase mixture AFTER exiting the valve’s orifice. The valve’s outlet (whether atmosphere or exit piping) will be what has to withstand the expanded 2-phase flow - NOT THE VALVE PROPER. Consequently, any relief valve outlet piping has to take the 2-phase flow into consideration - not the relief valve itself. Please read API 521 carefully and understand what is being explained. I am furnishing you with a copy of my 5th Edition as attached. Read it carefully and you will not be surprised to find that when you submit your relief valve datasheet to your valve supplier you will probably be told that a conventional ¾” -1” valve size will do because of the small liquid quantity exerting the hydraulic expansion.
SawsanAli311:
When you quote any Engineering Standard or code to an engineer, always cite the Edition or version number. As Bobby has correctly pointed out to you, the API is dictated by human beings - and they inevitably make mistakes, omissions, and errors and for that purpose always upgrade their standards. So please be specific.
Ryn376:
The answer to your question is that you will have mixed flow exiting the valve and only high pressure liquid entering it. Handle the exit flow appropriately. And study up on Phase Equilibria.
Hydraulic Pressure Relief - API 521 5th Ed..docx 101.44KB
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