using relief valve to maintain minimum pump flow

[MPKG]

This is probably on another thread somewhere, but I could not readily find the discussion I was looking for with this site's search feature.

Any thoughts on using a relief valve for maintaining minimum pump flow? I am thinking of using a straight spring or pilot operated relief valve -- not an overpressure bypass valve that maintains a constant upstream pressure -- but one that would normally be used as a safety relief. This would be for water from a submersible well pump. The existing system is as follows: submersible motor and pump, air relief at ground surface, a manual bypass valve, and then a PRV/check combination before distribution. The pump's BEP is 250 ft. @ 1200 GPM. I think my best alternative is to use a modulating control valve and a flow meter to maintain minimum pump flow at 50-60% or so of BEP -- but we can't afford all that. Swapping the manual bypass valve with a relief valve is way more economical.

Thanks in advance.

 

[MortenA]

There is no problem in using a PSV for min flow.

If you constantly need to have a flow in the min flow line then i think you should look for another solution.
Best Regards

Morten

 

[CB2]

Suggest you consider the cost of wasted energy and water if the bypass system will be operating for any more than a very short time.
I am estimating about 65 BHP at your bypass condition,$$$$.

 

[electricpete]

Remember to design to avoid chattering of releif valve under all conditions of system demand.

 

[Scipio]

Have you looked at proper autorecirculation valves? From the outside they just look like a flanged tee in the discharge piping, and operate off of a fixed flowrate rather than pressure. There are a few manufacturers out there, one is HBE Engineering (

http://www.hbe-engineering.com)

- there's another one that's more of a standard in the industry, but I can't remember the name of the company for the life of me, HBE was the first one that popped up doing a websearch.

 

[dadfap]

YoungGuy,

If CB2's calculations are correct then the wasted energy is going to pay for a VFD pretty quickly.

dadfap

 

[Kawartha]

YoungGuy,

You have not provided a complete review of your planned operating procedure, so I thought you may be interested in a couple of points that you may want to consider.

1. Preventing waterhammer in this installation is of major importance. This can be accomplished by starting and stopping the pump against the shut-off flow condition (0 flow). Use a slow opening PRV/Check valve - this must also be slow closing. The shut off condition is the most damaging, if you are operating at high flow. Therefore you need to have some means of closing the valve prior to pump shut down. I assume that you will have a check valve on the pump discharge, but this will not prevent waterhammer.
I'm not sure how you prevent waterhammer if power failure occurs, but if you cover for powered shutdown, you should provide protection for 95% of cases.

2. What is your normal condition for shut down of the pump (is it based on pressure, flow rate?)? What storage to you have? You may consider periodic starts/stops rather than minimum flow control. If there is storage in the pressurized system, you may be able to shut down when pressure reaches required point. Submersible motors can survive many more starts/hour than air cooled motors, because the motor is water cooled and able to remove heat of start faster than air cooled motors. Granted, mechanical wear and tear is higher, but have a look at this option. I assume you will be using approx. 100 hp motor for this application - as I recall you should be able to start this size motor about 10 x / hour, maybe 15 (depending on manufacurer).

3. Have you checked minimum flow with the manufacturer. Often the minimum flows for these low specific speed pumps, and diffuser type discharge, can be relatively low (probably 25%, or less). Keep in mind that the manufacurers rating for minimum flow is for continuous service at this flow. So you can in fact operate the pump at lower flow rates for reasonable periods of time without affecting pump life. The manufacturer should be prepared to comment on this.

 

[KernOily]

In my opinion a pressure relief valve is not a good solution for this problem. Direct-acting, spring-operated PSV's are not designed for this type of service. In continuous operation I am sure you will see erosion and cut-out of the valve trim after a short time, perhaps weeks. Also, these valves are not terribly repeatable after having gone through a few operating cycles - the blowdown starts to increase (blowdown = difference between opening pressure and reseating pressure). I am sure you will not be satisfied with the results and will be looking for something else after awhile.


Thanks!
Pete