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Steam Ejector Psv Set Pressure

steam ejector psv set pressure

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#1 ChemEng01

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Posted 31 August 2015 - 01:14 AM

Hi

 

I currently have a PSV on a steam ejector suction chamber which has a set pressure above the test pressure. 

 

Design Pressure = 15 PSIG

Test Pressure = 22.5 PSIG

PSV set pressure = 25 PSIG

 

The PSV is protecting the suction chamber of the ejector. The ejector has a discharge valve. If this is closed during operation, or not opened during start-up then the suction chamber (which will usually be at vacuum condition) could see the full motive steam pressure of 399 PSI. The motive steam piping is all rated, and the surface condenser from which steam is being pulled from has its own overpressure protection. 

 

HEI standard for steam jet vacuum systems says that the set pressure should be 15 PSIG (or no higher than design pressure).

 

The vendor literature also says the set pressure should be 15 PSIG (or no higher than design pressure). I've been in touch with vendor however cannot come to a resolution of why this PSV was set this high. (It was done back in 1982, however the HEI standards haven't changed regarding PSV set pressure. The vendor is now saying that the PSV set pressure can be higher than set pressure but no higher than the test pressure which is still not OK at my PSV is set above the test pressure. 

 

I'm recommending reducing the set pressure to 15 PSIG and checking PSV capacity is OK.(which it isn't!!) However  I am really interested in why this set pressure was chosen in the first place. The ejector mechanical drawings and documentation specifically say hydrotest pressure 22.5 PSIG, and PSV set pressure 25PSIG so I cant see this being a mistake as someone would have surely seen it. (Am I missing something or misinterpreting the HEI standard?)

 

Anyone clued up on steam ejector PSVs???

 

Appreciate any help

 



#2 fallah

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Posted 31 August 2015 - 03:26 AM

ChemEng01,

 

Your understanding is correct and PSV should be set at 15 psig; but in 1982 this PSV had wrongly been set at 25 psig most probable in order to compensate the capacity reduction due to lowering the size of PSV which had been very big for relevant steam relief load at 15 psig set pressure...


Edited by fallah, 31 August 2015 - 03:32 AM.


#3 ChemEng01

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Posted 31 August 2015 - 03:39 AM

Thanks Fallah. 

 

I agree it seems the set pressure was selected so a smaller PSV could flow the governing relief rate. When I do the calculation at this set pressure the PSV is pretty much at its rated capacity. 

 

Thanks for the help






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