Pressure variation in a vent pipe
Pressure variation in a vent pipe
(OP)
Hi everybody, how does pressure varies in the vent pipe connected to a PSV? I think it should go from the opening pressure of the valve to 1 ATM or 100 KPa (atmospheric pressure). If I am right is this variation linear during the lenght of the pipe? Does it happen suddenly at the open side of the pipe??
Many thanks
Many thanks





RE: Pressure variation in a vent pipe
RE: Pressure variation in a vent pipe
To find out where most of the pressure drop is, you need to calculate the pressure drop in the outlet pipe and then see which segments have the highest pressure drop.
One thing to keep in mind, the pressure on the outlet of a PSV is the constant back pressure, in your case it atmospheric, plus the pressure needed for the fluid to get through the pipe, the pressure drop.
RE: Pressure variation in a vent pipe
Is it a correct description??
RE: Pressure variation in a vent pipe
For a conventional valve, the safety valve takes greater than 90% of the pressure drop.
In your example, the exit flange pressure would be about 0.5 atm.
From there, line loss is approximately linear with pipe length and normal amounts of elbows.
The typical velocity is between Mach 0.3 to Mach 0.8.
Expansions really cause significant line loss due to localized velocity increase.
The Mach numbers are locally high at expansions.
Of course the remaining pressure dissipates at very end of pipe with exit velocity approaching 1.0.
The software I use routinely calculates Mach numbers higher than 1.0 at end of pipe.
All my co-workers say this is impossible.
But I have Goggled the topic and find references with greater than Mach 1.0 inside pipe.
Using a balanced bellows safety valve allows greater than 10% exit pipe loss.
Some designers limit the loss to 30% of set -- although 50% of set is not uncommon.
RE: Pressure variation in a vent pipe
RE: Pressure variation in a vent pipe
You have probably not discovered the perpetual motion machine or some equally new phenomena. Of course, if you have you should patent it pdq.
RE: Pressure variation in a vent pipe
RE: Pressure variation in a vent pipe
I early on told you NO more than 10%.
RE: Pressure variation in a vent pipe
You can think of the relief valve as an orifice in a pipe. It will cause most of the pressure drop but not all. Therefore, the pressure at the outlet of your valve would equal the pressure needed push the flow through the rest of the outlet piping.
RE: Pressure variation in a vent pipe
Yitbos: What are you references for > Mach one for safety valve tail pipes and what software do you use?