RELIEF VALVE LOADING FOR LIQUID SERVICE

To the extent I know API 520 do not address the issue of Relief valve thrust force due to liquid.The paper by Brandmaier et al " Steam Flow through safety valve vent pipes" on which the CAESAR II Relief Load Synthesizer is based upon also does not address the issue of liquid.

Besides computing based on Bernoulli's eqn, is there any reference to paper/books where the issue with liquids has been discussed? For example I would like to know the role of the different input parameters in the computation of thrust force under "liquid" in the CII Relief load sysnthesizer.

Looking forward to tips from forum members.Also if someone can post any paper/link, his/her kind help will be highly appreciated.

Regards

I have not seen any paper about this. Gas flow may have chocking condition at orifice and vent inlet/exit but Liquid flow is not because of imcompressible flow. So, Bernoulli’s equation is enough to calculate pressure drop, then calculate momentum force and thrust.
In my opinion, generally liquid PSV is used for low pressure and utility system as pressure relief device rather than safety valve. So, do we really need to calculate this flow seriously

Dear Anindya,

You will get one calculation in page 6/9 below:

http://www.cheresources.com/reliefvalve.pdf

It is simply F = rho * V^2 ( DLF = 2.0 considered). But, I attached this document as it explains the above with an example.

regards,

sam

Sam,

Thanks a lot. This is exactly what I wanted.

Best regards

Anindya

To anindya,

Be careful about considering the 'incompressible flow' assumption when evaluating liquid relief valve discharge. A small amount of liquid mass could flash to vapor. The vapor's greater specific volume could result in much higher flow velocities. The forces from V^2 will be much different in the event of two phase flow.