Inlet pressure drop - lower set pressure?

[Bill3752]

I think I know the answer to this, but it is worth a try..
Is it acceptable to lower the set point to solve an inlet dP problem. Consider the following: MAWP = set = 75, oper pressure = 45, inlet dP = 5% (3.75 psi). Can I lower the set pressure to say 71 psig and meet the 3% code value?

 

[don1980]

For vapor cases, increasing the set pressure is an effective way to lower the inlet loss, but lowering the set pressure makes it worse. The pressure drop is calculated at 10% overpressure (1.1 x set pressure). Lowering the set pressure lowers the gas density which results in higher inlet pressure drop.

 

[TD2K]

At a lower set pressure, assuming you keep the same 10% overpressure, the capacity of the PSV is reduced which is what the inlet line loss calculations are done at under API 520 offset as Don points out by the lower density. That doesn't seem to be enough of a change the inlet line losses that much but if your numbers support it, it's an option.

 

[Bill3752]

Don and TD, thanks for the input. My question really is "can I compensate for a high inlet dP by setting the valve to open earlier?". So while I would have ~ the same inlet dP, the valve would be opening to keep the MAWP from being exceeded. I think by the nature of your responses, that the 3% issue still remains, meaning I would still get chattering, etc. So, as I suspected, this won't solve my problem. Thanks, Bill

 

[Drexl]

No you can not compensate that way - it doesn't help the valve. Re-designing the inlet piping will