Pump discharge lower than suction

JimGPE_3

Please get back to the Wall when you figure this one out. I'd be real interested in hearing the details.

Thanks.

Jeff B.

pump discharge lower than suction

I have 3 out of 5 secondary coil pumps on 5 respective AHUs which have a lower discharge pressure than the suction pressure. The primary chilled water is 72 psig entering the mech room and 54 psig leaving. The pump suctions are 70 psig entering and 50 psig leaving(to the coil). The design is for 85 psig leaving. There is a 2-way control valve on the return line to the chilled water return main. The system is 20 years old and very poorly maintained. There are no suction diffusers but there are wye strainers on the suction side of the pumps. What would cause the discharge to be lower?

Unknown

Is ther a change in elevation on the piping?

Does the discharge line drop down through a building from there?

PJO

Pressure Checks...

Make sure your gauges are right - sounds like you have multiple ones, and if they all are close that's not the problem. If you are looking at just one (or two) double check them by opening/closing valves.

Are the pumps turning the right way? Is there a by-pass open somewhere (pushing against the discharge)? Is there a pump(s) outside the area you are that's affecting things? Does the discharge line open up real big (with a smaller suction...would seem crazy, but...)? Just thinking out loud here...

Hope this helps, PJO

Jeff B.

elev change - pressure checks

Thanks for responding.

I'm convinced that the pumps are piped correctly and that the gauges are reading accurately (within reason).
The fact that 3 out of 5 have the same problem seems to imply that it is not just a misapplication. Also since they have been in place a long time, you would think that originally they would have fixed any application problems.

The piping does not go down from there. The primary S&R mains are at the ceiling.

My thought is that the strainers are so plugged that there is a 100% blockage and zero flow, thus the return pressure is equalized with the primary return pressure and no head at all is added by the pump. Does this sound plausible?

PJO

Jeff

I was assuming the pumps themselves where fine. If you have a complete blockage of the strainer, the related pump would be complaining about it, me thinks :-) If it's a partial blockage, you would still have a good possibility of cavitation...and complaining.

For the piping, I was just wondering if there was a (wrong) valve open somewhere. Is there another area of pumps that this area feeds? If so, maybe there is a lot of "pull" from there (all forward pumps running, but usually just one or two)...I saw this once many moons ago.

Like the Wallies say often, tough to troubleshoot it from here, but maybe something will help.

Take Care, PJO

JimGPE_3

This is a good one!

First, I'd see if there wasn't some way to hook a manometer or a single differential pressure gauge across the pump so that gauge calibration errors are eliminated. You might not have a problem beyond bad gauges.

Second, pump rotation is not your answer. I assume these are common or garden variety centrifugal pumps. If so, they will develop (positive) head across the pump when rotating in either direction. If they are rotating backwards, their performance degrades, but does not reverse, centrifugal force being what it is. You should check for proper rotation just because it is the right thing to do, but it is not your problem.

Third, do the pumps have check valves at the discharge? Are the air handlers cooling at all? Which pipe is warmer - the one on the pump side of the coil or the one on the "return" side of the coil? If you truly do have inverted pressures, you either have no flow (the check valve is closed) and the AHU's are not cooling at all, or you have inverted flow, in which case the air handlers cool, but not great (you'd probably see high humidity problems in those zones) and pipe at the pump will be warmer.

Fourth, the strainers could be causing bad operation, but they cannot be reversing the pressure gradiant.

JMHO, I'm guessing you either have bad gauges; or you have no check valves, the secondary systems are not properly decoupled, and you have two pumps pumping into three other pumps that as a consequence have reverse flow.

Like the man said before me, its tough to diagnose from here, but I hope some of this helps.

[Deleted User]

Don't forget to make sure...

that you are pumping away from the point of no presure change. If not, the pumps will deduct their pressure differentials from the system fill pressure. Way out for sure, but a real possibility.

ME

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JimGPE_3

True, pumping to the point of no pressure change will cause a much lower suction pressure and no change in the discharge pressure, but it won't cause the discharge pressure to be lower than the suction pressure.