Determing a set point for a Liquid Pressure Relief Valve
Determing a set point for a Liquid Pressure Relief Valve
(OP)
Hi all, I have been a long time lurker and I finally have a question of my own. I am having problems with a relief system designed for a positive displacement pump. I have a proportional relief valve that is hooked to my supply tank from the discharge of my external gear pump. We set the relief valve with N2 on our valve shop bench at 130 psig and installed it in the unit. We then ran the test to see if the valve was relieving and our pressure indicator showed a pressure of 175 psig and building. We had to shut it off because we were approaching the maximum DP across the PEEK gears. We then field adjusted the relief valve to the pressure we desired. My question is, what is the difference if any between setting a relief valve for liquid service with a compressible gas? I was under the impression that a gas such as N2 would lift at a higher pressure due to its compressible nature. I have listed all of the flow data needed for the system below. Please let me know if you need any clarification.
1/2" Inlet
1/2" Outlet
Flow: 5.5 GPM
Relief Valve diameter: 5/16"
Setting: 130 psig
Back Pressure: 10 psig
Pipe length before RV: 4ft of 3/4" tubing (Elevation change ~4ft)
Pipe length after RV: 20ft of 3/4" tubing (Elevation change ~8ft)
SG of fluid: 1.94
Viscosity: .67 cp
Temperature is Ambient.
N2 was used at ambient conditions as well.
Thanks for the help!
1/2" Inlet
1/2" Outlet
Flow: 5.5 GPM
Relief Valve diameter: 5/16"
Setting: 130 psig
Back Pressure: 10 psig
Pipe length before RV: 4ft of 3/4" tubing (Elevation change ~4ft)
Pipe length after RV: 20ft of 3/4" tubing (Elevation change ~8ft)
SG of fluid: 1.94
Viscosity: .67 cp
Temperature is Ambient.
N2 was used at ambient conditions as well.
Thanks for the help!





RE: Determing a set point for a Liquid Pressure Relief Valve
RE: Determing a set point for a Liquid Pressure Relief Valve
RE: Determing a set point for a Liquid Pressure Relief Valve
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Determing a set point for a Liquid Pressure Relief Valve
Based on my reading it looks like the elevation change was only 8' i.e. only 7 psi at SG 2. Also, if the pipe wasn't already filled with liquid, the valve should open at 140 psig. This means the backpressure would be 13.5 psi and it should begin opening at 143 psig if you had calibrated it to open at 130 with nitrogen.
My only other guess would be that the valve was beginning to open, but that it's open area was too small for the liquid flowrate - I doubt this is the case though.
RE: Determing a set point for a Liquid Pressure Relief Valve
RE: Determing a set point for a Liquid Pressure Relief Valve
The speed of pressure rise may have something to do with it as well and it is not clear what the opening process is for the valve. Gas will quickly show flow as soon as the valve lifts a little bit, but liquid needs a bigger opening of the valve before appreciable amounts of flow occurs. Therefore setting it with gas could result in a higher pressure occurring with liquid by 10% or more.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way