PSV set point

[JWilson73]

Hi, I have a question, in one application have a three-phase separator (gas-oil-water), the max operation pressure is near to 72 psig, but my client need to design the separator in 300 psig, I need to put a pressure relief valve on it, but how to consider the set point? the sizing is for fire or blocked discharge?

Thanks for your comments.

 

[tickle]

All of them and more. The sizing should be for all possible causes of over pressure. It is your job to determine the over pressure sources.

 

[Chance17]

The set point can be any value that does not exceed the MAWP, irregardless of the overpressure contingency.
Most of the time (90%), the PSV activation is nearly equal to the vessel MAWP.
I do routinely come across facilities with safety valve set below the vessel MAWP.
The reasons for doing this are diverse.
Typically the vessel is connected to lower MAWP equipment.
Sometimes the set pressure is reduced due to known events which increases Flare header pressure.
For unstable materials, the set pressure may be reduced to prevent high temperatures.
Some organizations will install rupture disks at MAWP in series with the PSV at 95% of MAWP.
This insures the safety valve is fully open when the disk burst.

 

[zdas04]

To try to directly answer your question:
1. Set the system MAWP based on the least capable component in the system (so if you have an otherwise 600 psig system with an ANSI 150 flange on the inlet, you just derated the system to 280 psig).
2. Evaluate all of the scenarios that could cause overpressure and determine the flow at the system MAWP for that scenario. Pick the scenario with the largest flow rate as your controlling scenario and use its flow rate. This is an important analysis since your source might be something like a gas well that has an AOF of 3 MMCF/d, flow rate at normal operating pressure of 200 MCF/d, and a flow at MAWP of 50 MCF/d, the only one of those that matters for PSV sizing is the 50 MCF/d
3. Pick your set point and valve size such that during the control scenario you have less than 110% of MAWP. You can set the valve wherever you want as long as credible maximum pressure is less than 100% of MAWP.

David

 

[sheiko]

Set pressure = design pressure (or MAWP if you already have it from the manufacturer) i.e 300 psig

As for the scenario, i have attached an article FYI.

"We don't believe things because they are true, things are true because we believe them."