Pressure Reducing Valve in pump suction line

[krb]

I have a situation where I need to reduce the pressure from a supply header where a pump takes its suction. With the current supply pressure and the pump inline, the discharge pressure from the pump exceeds the pressure rating on the heat exchanger in the system. I can't remove the pump because it is a safety issue, if we lose pressure on the header the pump can supply circulation to the loop to allow cooling (there is a bypass line from supply to return on the header). The pump supplies cooling water to a heat exchanger for a closed loop system, and I need to maintain the pressure in the primary loop higher than the secondary at all times. I'm thinking about installing a presssure reducing valve (CLA-VAL)in the pump suction line, but what issues are there associated with this? I need to reduce the header pressure from 100 psi to about 30 psi at the pump suction. Any other suggestions would also be welcomed. I can provide additional info as required. Thanks.

KRB

 

[KenRad]

I think you would get more stable control and be less prone to pump problems if you put the reducing valve in the pump discharge. Any problem with that?

---KenRad

 

[gregha04756]

Hi KRB,

Let me preface my comments by saying I am new on this forum, and haven't done much mechanical engineering for a while. However, with that disclaimer, here are a couple observations for what they're worth. If you put a PRV on the pump suction, and the header pressure drops to the point where the pump is needed to maintain flow, will the pump be starved? My recollection of pump installation rules of thumb is to keep the suction piping as clean (and short) as possible, no valves, elbows or other restrictions if feasible. Could you just pipe the supply header to the exchanger inlet through the PRV, and have the pump, in parallel with the PRV, on standby, with a pressure switch or whatever to kick it on when needed?

I hope some other more knowledgeable members will post, I am curious to see their suggestions.

Good Luck,

Greg Hansen

 

[imok2]

It seems to me that if 30# is high enough for the net positive suction head of the pump then it shouldn't be a problem

 

[CESSNA1]

KRB: Standard practice is to ALWAYS put the reducing or flow control device on the discharge of whatever you want to contol the flow. Putting it on the inlet can lead to cavitation, air entrainment, poor flow control and host of other undesirable things. Put it on the discharge side of the device to have the flow control

Regards
Dave

 

[krb]

thanks for all the responses.

I agree with your statements cessna1, at first I thought about the valve as a way to not waste pumping energy, but I think if the prv were in the suction or discharge the net pump energy would be the same, I'll have to check that when I get back to work tomorrow.

KRB

 

[tombmech]

Why not just put a pressure switch in the header to only turn the pump on in case of pressure drop?

 

[quark]

Like suggested already, go with the discharge control to minimize cavitation. Just take care that you are providing the safe minimum flow as suggested by the pump manufacturer. Is there any possibility that you can fit a smaller size impeller?

 

[ChasBean1]

tombmech, I agree with you unless we're both missing something... I've also seen bypass lines with checks installed parallel with the pumps, such that flow is normally supplied through the parallel line without the pump on. When pressure is lost, the pump may be started (often run on VFD to maintain pressure).

The flow control valve could be on the supply side of the HX if pressure is an issue.

Flow regulators, not advisable on the pump suction.

 

[ITSTOAST]

It sounds like improper pump curve for application. Closing the discharge on a common centrifugal pump will reduce amperage, since it is not a positive displacement pump.
I would not recommend reducing suction pressure but see the minimum requirements of manufacturer. Keep prv as far from pump as practical, if flow varies widely or distance is less than 6 feet, you will have problems mentioned by Cessna.
You don't mention you preferred discharge pressure but the pump manufacturer should be able to recommend a reduced impeller diameter. A vfd would be good for varible flow requirements but may not cure a problem with incorrect pump application and initial cost would be high.

 

[tombmech]

ChasBean1,
Yes - you are correct. I have seen that as well. Usually the pumps are multiple-horsepower size or larger, when that's done. I was thinking it might just be an inline booster pump, and free-wheeling is not much of a concern. They can burn out and freewheel much of the time anyway. Without elaborate telemetry, the only way someone knows the motor is blown is because they can't make pressure.

I disagree that it was an incorrectly sized pump. krb clearly stated the rationale.

Unless we're missing something, a pressure switch would be a whole lot cheaper and more reliable than any PRV.

 

[krb]

Thanks for all the responses. Even though I believe we could make this work as long as we avoid the isssues mentioned, it makes more sense to go with a VFD. The pump was orginally selected for a lower suction pressure than actual, and since many users on the system require the higher pressure, there is no way to change it. As for control, we plan to use a pressure switch in the discharge line to control pump speed. Unfortunately, without the pump we do not have enough pressure. This is one of those cases where many engineers have worked on the project over a number of years and the end result is not ideal, so now we must find a solution to make it operable.

Again, thanks for the responses, they are always helpful.

KRB