Excess line pressure when pump is shut off

[mechem03]

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!

 

[budt]

Could be the trapped oil is expanding from heat and since it has no place to expand into, until you expel some liquid, pressure increases.

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

 

[rmw]

Is the discharge line between the pump and the shut off valve subjected to any kind of heat, or say direct sunlight?

rmw

 

[MikeHalloran]

I've seen a 3000psi hydraulic system do something similar. Given a leaktight closed center directional valve or pilot operated checks in the cylinder lines, the energy stored in metal strain and compressed fluid can cause a typical cylinder to work as an intensifier, pressurizing the rod end to ~4500 psi when the valve is centered.

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

 

[mechem03]

The pipes are not exposed to direct sunlight, but they are located within a few feet of diesel generators which likely does add some heat to the pipes. I am starting to believe that maybe it is thermal expansion that's causing such a large pressure increase. I will recommend the temperature gauge and it if turns out that the fluid temperature is rising significantly, a small expansion tank may be a good solution.

Thanks to all three of you who responded, it was very helpful.

 

[MJCronin]

As I see it, the pressure must be from one of two sources.

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

 

[vzeos]

I agree with MJCronin. It sounds like the solenoid valve seat may be leaking fluid from the tank back into the pipe. It may be time to inspect thr solenoid valve.

 

[danielthb]

It sounds very much like thermal loading on the product in a closed line. A single degree increase in deg F can raise the pressure scores of psi.

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

 

[BigInch]

I'd guess You're pumping diesel that is colder than the ambient air surrounding the discharge pipe. Does it happen that way at night too (or anytime the air is colder than the diesel)?

As Danial said, Shut off the pump before shutting down the line.

http://virtualpipeline.spaces.msn.com

 

[mechem03]

Daniel, I agree that it may be beneficial to shut the pump off before the solenoid valve - we are currently looking into whether this can be done with the existing controls. I also agree that pressure increase is not caused by the downstream tank since the pipe discharges into the top of the tank and the tank itself is vented.

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.

 

[zerosum]

Sounds like you need to add thermal relief valve(s) to the system.

 

[hydtools]

Change the solenoid valve to a three-port valve that dumps the pump output to its suction when the valve is 'off'.

Ted

 

[BigInch]

You are shrinking the pipe and expanding the diesel. The pressure should go up quickly until the pipe reaches the diesel temp, then go up more at a slightly lesser rate (not the pipe is expanding again, but the diesel expanding faster) until both the pipe and the diesel reach ambient building temperature at which time it should stabilize, if it hasn't blown a gasket by then. I'll guess that the system is code piping under either B31.3, or B31.4 both of which would require, as zerosum has already said, a thermal relief valve for closed in pipe sections. You could run it from a connection downstream of the meter to an entry point into the tank.

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[mechem03]

The issue I have is that the supply tank is outside and at a higher elevation than the pump, so to get the excess fluid back to the supply tank, the pipe would have to travel a couple hundred feet at an upward incline and be routed underground. An easier way to deal with the overflow might be to install a new storage tank inside the plant (which would need a float switch and alarms to make sure it doesn't overfill)... But I was hoping that installing a small expansion chamber on the line would account for the pressure increase without needing to spill fluid into a new tank. Do you think this is a code violation or that it would not satisfy the pressure reduction that is needed?

 

[BigInch]

IMO its a code violation, because you can't guarantee that the expansion space has air in it. You also need it for thermal protection from a fire in the building, where any air pressure inside the pipe would also build. A small tank inside the building might be the solution in that case, especially if the tank can be vented to an outside location. Check the fire regs in your area on reqirements for inside (low volume) tanks. It won't have to hold much.

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[Latexman]

What pressure class is the piping and pump?

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

 

[BigInch]

He doesn't say its draining through a pump relief.

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[Latexman]

Is "internal pressure relief valve set to 50 psi" and "it creeps up until it reaches 50 psi" a coincident? I suspect not.

Good luck,
Latexman

 

[mechem03]

FYI, he = she

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.

 

[BigInch]

Sorry couldn't tell. FYII "mechem03" = "it"

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