Pilot operarted PSVs and rated flow.

[PLar]

Hi
I am working with a new installation where we will install Pilot Operated PSVs with a maximum back pressure that is close to the set pressure of the PSVs. After contacting our normal PSV supplier, I got the information that the max capacity for the PSVs will be reduced to 80% when we have the maximum back pressure. But my question now is when calculating the rated flow (it will be popping PSVs) should I use the full capacity for the valve (without the 80% factor) due to the risk that I don’t have full back pressure in the system when I get a relief from my new valve.
In the end we will have the sizes calculated supplier but in this early stage we need some data for sizing of piping.

 

[mk3223]

The proper PSV valve sizing should be considered with the back pressure resulted from the maximum relief flow in the discharge piping. In order for PSV to reach the required relief capacity, any undersized piping, as a result from an engineering error in the early design stage, should be replaced.

 

[PLar]

Hi mk3223
Thanks for your response.
I will clarify my question a bit.
I am working with a new installation and we have will be connected to a large flare system with a maximum specified backpressure of 7 barg from the client, my question is when calculating the maximum capacity for the valve should I use the flow with low backpressure (the capacity will be 20% larger than with max capacity with 7 barg in back pressure). This max pressure will be used for the piping calculations.

 

[Latexman]

Sounds like a good question for the technical team of the flare.

Why not report both. Both are valid depending on what else is going to the flare.

Good Luck,
Latexman
Pats' Pub's Proprietor

 

[PLar]

Thanks for responses.
I decided to go for the most conservative alternative and use the flow with low pack pressure to calculate the rated flow to size my piping. after finished the calculations and documents the client informed that they have recalculated the flare system and ended up with a lower back pressure so I have redo the calculations again.

 

[142846]

you are saying if you use pilot psv the piping flare system shall have been larger? approximately 20 percent? what is important for you?

 

[PLar]

Hi Daria
Th e important thing for me is to get a correct estimate of the maximum flow in the piping as input to the pipe sizing. It is important to get the sizing accepted by the client because changes in the pipe sizing will afect many othe rdocuments.

 

[142846]

PLar,
in my experience finally we should trust to manufacturer/vendor/supplier data.

 

[georgeverghese]

When reading API 520 part II on this topic, it does not stipulate that the rated capacity shall always be used to size the discharge piping. It allows the designer to select either the rated capacity or the actual process design relief rate. In the latter case, the actual calculated max superimposed backpressure on the common header is used to estimate the backpressure on the PSV. This would always < this 7barg upper limit specified by the Plant Owner.
But of course, the exit piping dia should be no less than the PSV exit nozzle dia, and the max velocity in the exit lateral should not exceed 0.7Mach at the max calculated backpressure in the common discharge header at this PSV.
So in your case, use a derated flow (which should = to the process design required flow) corresponding to the lower, actual value of the max backpressure quoted by your client.

 

[Sawsan311]

If your PSV is a pop up type then rated capacity to be considered as a conservative approach an the same is inline with API 520 PII.

For modulating type pilot operatded you may use the required capacity for calculating the relief piping (inlet and tail pipe) However, so many consultants dont tend to favor that and use rated capacity instead in order to account for any future expansion and increase in the required relieving rate.

 

[MortenA]

At the end of the day the rated capacity will only occur as a transient phenomena. After a while the flow will drop but you may enter a cycle where the valve will pop and the close again after a while, and then repeat: Thats why a modulating pilot operated PSV may be your best choice.

I take it that your set pressure is rather low and close to the max back-pressure and that why:

A) You want a pilot operated valve (its a really big and expensive valve and with the pilot you may go closer to your operating pressure in the protected vessel)?
B) Thats why the downstream pressure impacts the capacity - because its no longer choked flow?

Best regards, Morten