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Excess line pressure when pump is shut off

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!

RE: Excess line pressure when pump is shut off

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

RE: Excess line pressure when pump is shut off

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

rmw

RE: Excess line pressure when pump is shut off

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

RE: Excess line pressure when pump is shut off

(OP)
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.

RE: Excess line pressure when pump is shut off

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

  

RE: Excess line pressure when pump is shut off

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.

RE: Excess line pressure when pump is shut off

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

RE: Excess line pressure when pump is shut off

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

RE: Excess line pressure when pump is shut off

(OP)
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.

RE: Excess line pressure when pump is shut off

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

RE: Excess line pressure when pump is shut off

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

Ted

RE: Excess line pressure when pump is shut off

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.

http://virtualpipeline.spaces.msn.com

RE: Excess line pressure when pump is shut off

(OP)
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?

RE: Excess line pressure when pump is shut off

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.

http://virtualpipeline.spaces.msn.com

RE: Excess line pressure when pump is shut off

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

RE: Excess line pressure when pump is shut off

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

RE: Excess line pressure when pump is shut off

(OP)
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.

RE: Excess line pressure when pump is shut off

Can you eliminate the blocked in volumes by removing the check valve or car sealing the butterfly valves open.  What you are after is the fluid upstream of the solenoid can expand back to the supply tank and the fluid downstream of the solenoid can expand forward to the discharge tank.  The closed solenoid (fail closed, right?) may make the check valve redundant.

Good luck,
Latexman

RE: Excess line pressure when pump is shut off

Can you do away with the solenoid valve and just have the float switch turn the pump off? Perhaps you could lower the float switch to compensate for additional fuel that could drain into the discharge tank from any high points in the piping.

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

(OP)
Latexman, my understanding is that fuel is being routed from the relief valve into a drip pan rather than the suction side of the pump so the fuel would not make it back to the supply tank.  Changing that would be an option, but my understanding is that an internal relief valve on a pump is only meant as a temporary solution to overpressure - I believe it's better to have an external relief valve for this section of pipe.      

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

Add another solenoid valve, normally open, to connect the trapped fluid back to the pump suction when the delivery tank float switch closes the feed solenoid valve and shuts off the pump.

Ted

RE: Excess line pressure when pump is shut off

(OP)
Hi Ted.  I'm not sure that this would work well in this case - there are multiple tanks that require fuel and if one tank is off, but another is on, the line with the shut-off tank is going to be sending fluid back to the pump and basically recirculating the fluid.  

RE: Excess line pressure when pump is shut off

Wire the bypass solenoid valve to each demand system.  When any one tank requires fuel, the bypass valve will close.

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

Most gear pumps with internal relief I've seen discharge back to suction.  That's a first for me.

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

Continuing on, you can solve this by adding a hydraulic relief PSV, or by eliminating the check valve and eliminating or administratively controlling the manual valves.  Your choice may be influenced by past practice, policy, the unit’s layer of protection analysis (LOPA), your boss, your co-workers, etc.  I am an advocate of the KISS approach (Keep It Safe and Simple), so I’d recommend working towards eliminating both the check valve and manual valves.  I really cannot think of a good reason to keep the check valve.  Automation has made it redundant, thus creating the thermal expansion hazard (no good deed goes unpunished).  Your operations and maintenance folks may have good reasons to keep the manual valves, so have Plan B (administratively controlling the manual valves) in your back pocket.

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

(OP)
Hi Latexman, thanks for your thoughts.  There are 1" solenoid valves and 3" butterfly valves at this project (3" header, 1" branches to each day tank).  There are only electric & manual actuators, no pnematics, and there is a back-up power supply.  I appreciate that you've related your experiences - it's good to know what has and hasn't worked for others.  I'll keep it in mind if I ever have to work on a similar design!

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