Excess line pressure when pump is shut off
Excess line pressure when pump is shut off
(OP)
Hi everyone,
I am wondering if anyone has had any experience with a piping system becoming overpressurized when the pump is shut off. Here is my scenario:
Fluid: No. 2 diesel fuel
Pump: 2 hp, 1800 rpm gear pump
Flow: 30 gpm
Design pressure: 30 psi
There is a 12 ft negative suction lift at the pump. Downstream of the pump we have: internal pressure relief valve set to 50 psi, pressure gage, butterfly & check valves, 420 micron strainer, oscillating piston flow meter (which requires the 420 micron strainer), and an automatic solenoid valve. After this point, the fuel is discharged into a storage tank. A float switch in the tank shuts off the solenoid valve when fuel is no longer needed which then sends a signal to the PLC to shut off the pump.
When the pump is operating at steady state, the discharge pressure gage reads at 15-20 psi. When the pump is shut off, the pressure gage initially reads 5 psi, but after several minutes, it creeps up until it reaches 50 psi. After draining some of the fluid out of the line, the pressure falls back down to 5 psi, but then creeps back up to 50 psi. After draining the system again, the gage stays around 5 psi.
I've been puzzled by this all afternoon. Anyone have any opinions or advice? I'd appreciate any thoughts on this. I don't think it's due to thermal expansion, but if anyone disagrees, let me know!
I am wondering if anyone has had any experience with a piping system becoming overpressurized when the pump is shut off. Here is my scenario:
Fluid: No. 2 diesel fuel
Pump: 2 hp, 1800 rpm gear pump
Flow: 30 gpm
Design pressure: 30 psi
There is a 12 ft negative suction lift at the pump. Downstream of the pump we have: internal pressure relief valve set to 50 psi, pressure gage, butterfly & check valves, 420 micron strainer, oscillating piston flow meter (which requires the 420 micron strainer), and an automatic solenoid valve. After this point, the fuel is discharged into a storage tank. A float switch in the tank shuts off the solenoid valve when fuel is no longer needed which then sends a signal to the PLC to shut off the pump.
When the pump is operating at steady state, the discharge pressure gage reads at 15-20 psi. When the pump is shut off, the pressure gage initially reads 5 psi, but after several minutes, it creeps up until it reaches 50 psi. After draining some of the fluid out of the line, the pressure falls back down to 5 psi, but then creeps back up to 50 psi. After draining the system again, the gage stays around 5 psi.
I've been puzzled by this all afternoon. Anyone have any opinions or advice? I'd appreciate any thoughts on this. I don't think it's due to thermal expansion, but if anyone disagrees, let me know!





RE: Excess line pressure when pump is shut off
Use a temperature gauge and measure liquid temperature at pump shutoff and monitor it as pressure climbs.
After the temperature stops increasing pressure also stops increasing.
One possibility.
Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING
RE: Excess line pressure when pump is shut off
rmw
RE: Excess line pressure when pump is shut off
I.e., both ports of the flowmeter are at about the same pressure when it's running, but when you trap liquid at both ends, the differential area works to supercharge one end.
Mike Halloran
Pembroke Pines, FL, USA
RE: Excess line pressure when pump is shut off
Thanks to all three of you who responded, it was very helpful.
RE: Excess line pressure when pump is shut off
1) Heating of the fluid and expansion- the primary suspect
2) Pressurization from another source (?)
Could there be leakage from what ever is downstream of the solenoid valve back into your system ?
Is your system new ?
MJC
RE: Excess line pressure when pump is shut off
RE: Excess line pressure when pump is shut off
Doesn't sound like backpressure from the tank. With diesel you'd have to have about a 140 ft head.
Also, pardon my limited mechanical experience, but doesn't that gear pump want to be shut down before you close a valve on it?
DB
RE: Excess line pressure when pump is shut off
As Danial said, Shut off the pump before shutting down the line.
http://virtualpipeline.spaces.msn.com
RE: Excess line pressure when pump is shut off
BigInch, the fuel is being pumped from an outdoor storage tank and going to the inside of the building, so there is a significant temperature increase, which continues to build when the pump is shut off and the fuel is allowed to sit in the pipes. I'd originally discounted the effect on pressure from a small volume increase, but maybe it's because I am more used to thinking about expansion and compression of gases.
RE: Excess line pressure when pump is shut off
RE: Excess line pressure when pump is shut off
Ted
RE: Excess line pressure when pump is shut off
http://virtualpipeline.spaces.msn.com
RE: Excess line pressure when pump is shut off
RE: Excess line pressure when pump is shut off
http://virtualpipeline.spaces.msn.com
RE: Excess line pressure when pump is shut off
Sounds like the pump's internal relief valve is relieving this thermal expansion scenario back to suction quite nicely (I assume there is no spillage). If your pipe and pump is good for 125 or 150 psi, what's the problem? Let it sit there with 50 psi on it.
Good luck,
Latexman
RE: Excess line pressure when pump is shut off
http://virtualpipeline.spaces.msn.com
RE: Excess line pressure when pump is shut off
Good luck,
Latexman
RE: Excess line pressure when pump is shut off
The 50 psi developed in the pipe line is a coincidence. The fluid is trapped between the discharge side of a check valve and a solenoid valve, sorry if my description of the pump's pressure relief valve confused the issue. The pressure gauge on the line in question has a range of up to 60 psi, but people at the site say the pressure only gets up to 50 psi. This might be due to the fact that they are periodically draining the system to relieve some of the pressure - otherwise, it might get even higher. The pipe is rated for 150 psi, but the pressure gauges are not.
RE: Excess line pressure when pump is shut off
http://virtualpipeline.spaces.msn.com
RE: Excess line pressure when pump is shut off
Good luck,
Latexman
RE: Excess line pressure when pump is shut off
I am not familiar with an alternating-piston flow meter, but it almost sounds like it could act as a line block at some point.
RE: Excess line pressure when pump is shut off
Alanator, the fuel forwarding system serves multiple tanks and the tanks do not all call for fuel at the same time so we aren't able to remove the solenoid valve. It would defintitely help if we could, though!
RE: Excess line pressure when pump is shut off
Ted
RE: Excess line pressure when pump is shut off
RE: Excess line pressure when pump is shut off
Wire it to the pump so that anytime the pump is running the bypass valve is closed.
Ted
RE: Excess line pressure when pump is shut off
In my previous post I tried to described how to eliminate the hydraulic relief scenario. It'll work for multiple destinations too. No check valve and only one closed solenoid valve (we use automated block valves) at each destination.
Good luck,
Latexman
RE: Excess line pressure when pump is shut off
During this entire post there has been something nagging at me in the back of my mind, and it may be the difference in the industry you work in and the chemical industry I work in, or it could be a company-thing, region-thing, or culture-thing. Anyway, is this small bore piping you are dealing with? I think of small bore as < 1 inch nominal diameter. Using a solenoid valve directly in line (which is what I have interpreted you have) suggests small bore piping, but using butterfly valves suggests large bore piping. I don’t believe my company would use a solenoid valve directly in the fuel line, especially if it is large bore piping. We would use a quarter-turn valve with spring loaded actuator for fail safe positioning in the fuel line. Usually a 3-way solenoid valve in air service would be used to drive the actuator. Anyway, I was just wondering what you were dealing with and compare that to my experience. That’s all.
Good luck,
Latexman
RE: Excess line pressure when pump is shut off