Back-pressure
Back-pressure
(OP)
Hi,
I have a safety valve including inlet and outlet piping and can say that the system is ok (<3% inlet loss, <10% outlet loss) with a superimposed back-pressure equal to atmosphere.
If I lower the superimposed backpressure, e.g. the pressure in the tank in which the safety valve outlet pipe ends - is it possible that the total back-pressure at the outlet of the valve could increase?
Is there any easy way to prove this by using appropriate reference litterature?
More specifically I'm wondering about this because of flashing water which reach choked flow in outlet piping at lower back-pressures.
Drex
I have a safety valve including inlet and outlet piping and can say that the system is ok (<3% inlet loss, <10% outlet loss) with a superimposed back-pressure equal to atmosphere.
If I lower the superimposed backpressure, e.g. the pressure in the tank in which the safety valve outlet pipe ends - is it possible that the total back-pressure at the outlet of the valve could increase?
Is there any easy way to prove this by using appropriate reference litterature?
More specifically I'm wondering about this because of flashing water which reach choked flow in outlet piping at lower back-pressures.
Drex





RE: Back-pressure
Good luck,
Latexman
RE: Back-pressure
To correct the model you can divide it in smaller parts. Start iterating from the tank using two-phase at sonic velocities. At this point I stop trusting my excel/fluid dynamics skills. That's why I would rather be able to simplify.
RE: Back-pressure
With the tailpipe going to atmospheric, is it liquid flow or two phase?
Good luck,
Latexman
RE: Back-pressure
RE: Back-pressure
Some distance downstream of the start of the transonic region you'll start to see flow that kind of lends itself to incompressible flow analysis (but you need to be really careful choosing the closed-form equation that you use, most have problems with the transition to vacuum). Finding that point analytically is nearly impossible.
I would assume that the end of transonic is an insignificant distance downstream and use the Spitzglass formula (see GPSA Engineering Field Data book), restructured to calculate upstream pressure. Since you know your flow rate and downstream pressure this should give you a direct answer to your question.
Just for assurance, I'd then assume that the end of transonic flow is in the middle of the length of pipe. That should give you some confidence in the answer to your original question "can lowering the downstream pressure raise upstream pressure?".
David
RE: Back-pressure
Good luck,
Latexman
RE: Back-pressure
David
RE: Back-pressure
Good luck,
Latexman
RE: Back-pressure
David