psv inlet and discharge lines sizing 1

[gelsi]

Hi,
this is my question:
sizing of the in/outlet lines must be done with the PSV required flowrate (minimum area required) or with the maximum flowrate (without kd=0,9 and with orifice area selected)?
i think the second but need a confirm, tx

 

[BenThayer]

maximum flow.

a standard PSV (e.g.,a non-modulating) will pop and pass its rated capacity until it drops the pressure in the system low enough to reseat.

 

[don1980]

The requirement is to use the rated capacity, which is the value that is calculated by using the 0.9 factor and the actual manufacturer's orifice. The exception, as mentioned by Ben, is when you have a modulating valve (non-pop valve).

 

[zdas04]

A quick answer really doesn't cover the issues. The more fluid flowing in the inlet piping, the more friction. In extreme cases, a dead leg can open the PSV, then the friction drop in the suction piping can cause pressure at the valve to fall below the setpoint and slam the valve shut. The dead leg can then pressure back up. I've seen that scenario slam the valve so hard that it broke nozzles off the vessel and then life got exciting.

On the tail pipe, too much friction can take a gas PSV out of critical flow (significantly reducing flow rate).

Neither situation is good. I generally find a max required relieving rate (from the critical scenarios), double it, size the inlet piping for 0.5 psid in the inflated flow rate. For the tail pipe I look for dP less than 1 atm (i.e., I allow no more than twice the design exhaust pressure) using 2X max credible scenario.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[moltenmetal]

Review API 520/521- things are pretty well spelled out there.

 

[VAL82]

Hi,

In order to calculate the dP of the inlet line to a liquid certified (trim) relief valve, shall I still use the rated flow? or is this type of valve considered a 'modulating' one and thus, the required flow is ok?
P.S: general info about liquid trim valves is appreciated.

Thanks!