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Limitation For The Velocity At The Outlet Of The Psv

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#1 bp77


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Posted 16 October 2020 - 01:32 PM

Do we have any limitation for the velocity at the outlet of the PSV, not at the tailpipe?
We have a PSV 4"X6".At the outlet of the PSV, we have to change the pipe size to 16" with two reducers.
The gas's velocity is 969 m/s at 6" outlet, but it will decrease for 16" tailpipe.
The mass flow is about 160000 kg/hr, and the reaction force is about 40 kN; it seems this force is too high for this PSV.

#2 latexman


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Posted 16 October 2020 - 02:05 PM

Is it a 4P6?  If so, beware of extra high inlet and outlet pressure drops and:


Body Flow Limitations:
Reference: Guidelines For Pressure Relief and Effluent Handling Systems, Center For Chemical Process Safety
of the American Institute Of Chemical Engineers, 1998, pages 38-39.  "On certain occasions, the designer can encounter a dilemma when sizing a tail pipe to meet the built-up back pressure limitation for conventional pressure relief valves. Calculations for a tail pipe of the same size as the outlet flange can say that the built-up back pressure exceeds the allowable limit even if the length is reduced to zero."
The 4P6 simply has a flow nozzle (orifice) that is too big for the API 526 body.
Solutions may be:
  1. Raise or lower the set pressure (there is a lower practical limit and MAWP sets upper limit)
  2. Change body size keeping same orifice size (limited by availability)
  3. Use a smaller orifice in the same body (limited by availability)
  4. Use a restricted lift valve
  5. Use two or more smaller valves
  6. Use a balanced valve

Edited by latexman, 16 October 2020 - 02:12 PM.

#3 bp77


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Posted 16 October 2020 - 02:56 PM

Thanks Latexman for your reply

yes this is 4P6

Edited by bp77, 16 October 2020 - 02:56 PM.

#4 breizh


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Posted 16 October 2020 - 09:23 PM

Hi ,

To add , consider this extract of LESER'S  handbook , chap 6 part 6-4 .


Hope this is helping you


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