CDTP & Stamped SP help

[Cobb20]

I have a question about CDTP and “stamped” set pressure.

If I have a conventional PRV that is protecting the TS of an exchanger (MAWP = 150 psig) and the PRV is located on the discharge of a pump and the PRV discharges back to the suction of the pump (max suction pressure = 65 psig = constant back pressure for PRV), how exactly is the CDTP and set pressure supposed to be set up? Is the CDTP supposed to be MAWP - Constant Backpressure and the “stamped” set pressure supposed to be the MAWP? Or is the CDTP = MAWP and the “stamped” set pressure supposed = MAWP + constant backpressure? I’m just a bit confused and could use some clarification.

Thank you

 

[ChemE08]

Note that CDTP considers both temperature and pressure components in the adjustment. I like to think of the CDTP as the corrections made to validate performance of the PSV on the test stand. Remember, you can't easily replicate process conditions on the PSV test stand.

For a conventional valve, the backpressure impacts the set pressure. Essentially adding to the original set pressure. For example:
* A conventional PSV set at 100psig routed to atmosphere (i.e. no backpressure) will essentially lift at 100psig.

*Take that same relief valve and instead of going to the atmosphere it is routed to process and has 20psig of backpressure. Now that relief valve has 20psig fighting to keep it closed on the outlet. So now that PSV will have to reach 120psig to mimic the situation above. SO, what we do is mechanically set the valve on the test stand to 80psig so that when the 20psig backpressure is observed, the PSV will behave as if it was set at 100psig.

NOTE: your relief sizing and design would follow the 100psig set but your physical test stand setting would be 80psig with the idea that the PSV will see a constant 20psig backpressure. CAUTION: if you have variable backpressure, then you may have to make an engineering judgment on how best to set the PSV or consider maybe a Bellows design (if acceptable).

 

[Latexman]

If I have a conventional PRV that is protecting the TS of an exchanger (MAWP = 150 psig) and the PRV is located on the discharge of a pump and the PRV discharges back to the suction of the pump (max suction pressure = 65 psig = constant back pressure for PRV), how exactly is the CDTP and set pressure supposed to be set up?

Can you post the P&ID, because that PRV is probably not protecting the TS of the exchanger? In my mind, there’s several questions that the P&ID should help answer.