Determing a set point for a Liquid Pressure Relief Valve

[brs766]

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!

 

[micalbrch]

Did you simulate the back pressure on the test stand? I think one of the problems is the fact that it is a proportional relief valve, not a full stroke valve.

 

[brs766]

No, we did not simulate it. I feel this is a contributor but I am still confused as to why the pressure kept raising. It was climbing fast to the point where we would have been well above 200 psig in a couple of seconds. My understanding is that the back pressure would just be an additional resistance that the system would need to overcome. Thanks for the answer though and please let me know if I have said anything incorrectly.

 

[LittleInch]

If I read this right a proportional relief valve works by applying the pursue setting between inlet and out let. Hence if you take your 10psi back pressure and add on the elevation effect of 20ft pipe with your sg of 1.9, then the setting of the fluid pressure becomes 130 + 10 +20 = 160 psi. I can only surmise that your valve was about to lift when you shut it off. If you want it to lift at 130 regardless of back pressure you needs a pilot operated valve with a sensing point on the line and also allow for your elevation change and back pressure from the pipe during flow.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[BlackSmoke]

This seems like an odd (interesting) result. Double check all your meter calibrations of course.

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.

 

[brs766]

Thank you all for your responses, all in all I was just curious to see if there was a large difference (due to flow coefficients and the equations that govern valve systems) between setting a valve with N2 and a valve with your process. I will let you know if I find out anything more.

 

[LittleInch]

I got a few of the figure s wrong, but the point still applies that you need to take into account the head difference. AS he said the discharge was hooked to the supply tank it is a reasonable supposition that it is flooded.

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