Relief Valve response to Pulsation from Positive Displacement Pump 3

[JAlton]

A relief valve set for 1250 psi on a system with a positive displacement pump (2500 rpm) sees pressure spikes of 1275 psi every 1.5 seconds. However, the RV does not lift. Does anyone know why it does not respond to the pressure spikes?

J. Alton Cox

http://www.delucatest.com

 

[unclesyd]

Comeback with a little more information as to the type pump, the fluid being pumped, the distance between the disharge valve and RV, lines sizes, niminal pressure, and is there a pulsation dampner in line.

 

[budt]

Also, is it a DIrect Acting or a Pilot Operated Relief Valve?

A Direct Acting Relief Valve will respond faster that a Pilot Operted type but both have to move a mechanical part that is spring loaded and that requires time. A Pilot operated Relief valve can have as much as twice the pressure setting override each time it is forced to open.

You must be reading the 1,275 PSI peaks with a digital gauge so you can see what the pressure spike is if you block pump outlet suddenly.

I always use a small bladder or diaphragm type accumulator as a pulsation dampener and locate it as close as possible to the pump discharge port. Set the pre-charge pressure at or slightly under the relief valve setting and there will be very little pressure spikes in the line to the circuit.


Bud Trinkel CFPE
HYDRA-PNEU CONSULTING, INC.
fluidpower1 @ hotmail.com

http://www.fluidpower1.us

 

[stanier]

How are you measuring the 1250 psi? If you were using a fast response highly accurate calibrated transducer with it sensor head directly on the pipeline with a data logger with high capture frequency you may be assured that it is 1250 psi. However if you are looking at a bourdon tube pressure gauge on the end of an imoulse line the data may be highly inaccurate. The impulse line could be creating local surges that peak at the gauge.

 

[xcrosby]

A valve set at 1250psig may actually be set as high as 1287.5 psig if you consider the 3% tolerance on set pressure. Also the valve may not lift until 1375 psig which is the 10% overpressure. If you provide more info on the type of valve I may be able to help more.

 

[TD2K]

The 10% overpressure is the maximum overpressure allowed by ASME section 8, Div 1 in sizing a single PSV for a process upset (that is, a vessel designed for 1000 psig must have a PSV with sufficient capacity to prevent the pressure from exceeding 1100 psig).

10% overpressure does not imply anything about the pressure the PSV will start to open.

 

[JAlton]

TD2K is correct. ASME Sec. VIII, DIv. 1, requires the PRV to open within +/- 3% of Set Pressure and be in a full lift by 110% of Set Pressure or 10% Overpressure. My concern is the failure of the PRV to respond to rapid pressure pulsations which spike above Set Pressure. I have been working with PSVs for years, and I have read some articles about pulsation, but I am not sure how to address this issue. Budt had some useful info above. I know the system has a bladder installed, which I suspect is absorbing the pulsations. I do not know the location of the pressure measurement equipment which is sensing the pulsations in relation to the Bladder or the Relief Valve. I will try to get more details and hopefully get a definitive answer to this problem. Thanks to everyone.

J. Alton Cox

http://www.delucatest.com

 

[xcrosby]

Many liquid pressure relief valves do not lift until the pressure is almost at the 10% overpressure value. At the pressure that the valve is set to open there is a very small amount of flow. The flow does not significantly increase until the high lift pressure is reached, which can be as high as 10% above the open pressure. At the high lift pressure the valve suddenly opens fully.

 

[TD2K]

"At the high lift pressure the valve suddenly opens fully"

Is this similar to the 'pop' effect for valves in vapor service. I haven't heard of this though I'm not saying it's not true. My impression was that liquid valves basically open in proportion to the overpressure.

ASME certified liquid valves are fully open at 10% overpressure. The older overpressure used to be 25% overpressure to get full flow and the PSV manuals had derating factors in them for their capacity at 10% overpressure.

 

[xcrosby]

In the mid 80's ASME Section VIII added rules for the capacity certification of liquid relief valves. The required overpressure was 10%. To achieve a full lift at 10% overpressure valve manufacturers redesigned their valves. One feature that most of these valves have is a "pop" action similar to vapor valves. Some of these valves "pop" into a full lift at or near 10% overpressure. Prior to reaching the high lift pressure the flow rate is very low and sometimes confused with leakage. I designed several pressure relief valve product lines in one of my previous lives. The code definitions describing valve operation is outdated. There are few if any modern code certified spring loaded pressure relief valves that are propotional opening valves.