Vertical Pump question 2

[impeller1]

Good day,

A question here..I have 5 vertical pumpsets in parallel and they all discharge into a common header. If 3 or more pumps are online, the system functions well, however, if only one pump is in operation, that one pump becomes very noisy as if it's cavitating and its discharge pressure becomes almost 0 psi. However, the sump water level is up fairly high..what could be causing this?? Any ideas?

Thank You.

 

[SNORGY]

My immediate reaction is that it sounds like the piping system head curve (with piping providing the resistance to flow) is gradual so that a single pump is operating at run-out. In other words, the piping is oversized relative to the flow / capacity performance of a single pump - or conversely, one pump is undersized relative to the piping system.

If you have inverter duty rated motors and MCC + IO space, maybe try VFD(s), otherwise, install some means of discharge throttling. Alternatively, if your process allows it, raise the downstream delivery pressure when you expect to have one pump running.

Regards,

SNORGY.

 

[JJPellin]

Cavitation in a vertical pump such as this depends on more than just adequate submergence. It is likely that when one pump is running alone, you are running out to the end of the curve (run-out as SNORGY pointed out). At a flow rate this high the Net Positive Suction Head required could very easily be greater than the NPSH available, even with a full sump. Do the math. Calculate your NPSH available at the end of the curve using a realistic value for the vapor pressure of your water. Compare that to the NSHR required as shown on the test curve (or book curves) for this pump.

The simplest solution would be to pinch back on the discharge to limit the flow to a point where you would not require more suction head than you have available.


Johnny Pellin

 

[hydtools]

What is on the intake side of the pump(s)? Enlarge the suction plumbing to reduce intake velocity at the high flow, low head operating point.

Ted

 

[JoeySoap]

I think the only way to run this short term at all is to throttle the discharge valve of the pump. It is a less than statisfactory situation that you have here, you are running off or right at the end of the pump curve. Running for any length of time with this degree of cavitation will wreck your pump.

 

[Artisi]

Single pump operation- pump is too far out on its curve and cavitating. You need to impose more head on the pump to shift the operating point to the left - an orifice plate between the pump discharge and the common pipeline is one way to cure your problem.

 

[SNORGY]

If I might assume that each pump has a discharge check valve and block valve which may or may not be flanged and bolted to one another...if so, separate them...

I might be inclined to suggest consideration of a bypass globe valve around the discharge block valve, with the globe valve being one size smaller, on the downstream side of the check valve in the discharge line from each pump. When operating a single pump, close the primary block valve and throttle using the bypass globe valve.

Regards,

SNORGY.

 

[MartinSr00]

I'm taking a couple of guesses here. Let's do a mind experiment.

Let's assume the pumps are sized correctly as a system. Let's also assume that your system curve is mostly friction (not a high static head rise). What would happen if the flow were reduced significantly.

The system resistance would decrease by the square of the system flow -- that is, the head the pump(s) would see would be reduced.

Let's say one pump is operating. What would it see? It would see the reduced system head. What would it do? It would move way out on the curve. Head would reduce to near zero (your resistance is down from the reduced to 1/9th it's value -- remember resistance goes as the square of the flow).

What happens as it goes out on the curve? You could look at your NPSH requirements, but that may not tell the story. What you would really like to see is the incipient cavitation curve. Such curves (NPSH vs. Flow) for incipient cavitation, are often "U" shaped, where the NPSH head falloff curve is flatter. The bottom of the "U" is normally around the best efficiency point.

What would the pump do, then as it's NPSH requirements suddenly got higher? It would make a lot of noise, head may fall off, and flow drop because the suction was starved.

Could this be your problem? Take a pump curve and estimate a system curve, and find out.

If you wanted to fix this problem, you would need to put flow restriction in the discharge line. This would fool the pump into thinking that the system resistance just got higher. Yes, flow would be reduced, and you'd need to turn on a second or third pump sooner, but you'd be better off.

Run a test. Scrunch down on a system discharge valve. See if the noise goes away.

 

[impeller1]

Thanks to all for your valuable comments and insight..what you all are saying is what i suspected as well as subsequent to these comments, we did throttle the discharge valve more and it does seem to alleviate the problem..thanks again!!

 

[Artisi]

Now look to fitting orifice plates to each pump so they cannot run out on their curves.

 

[BigInch]

Whoa. Let's think about it first. Its possible that runout only occurs at low flowrates and adding orifice plates to all pumps, or possibly even one, may make attaining the current full flowrate of more or all pumps impossible. Hold off on orifice plates until you check it out.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[dabluffrat]

Question; are all 5 pumps identical (same make, model, rated conditions, etc)? Ideally you want all pumps operating in parallel to be identical, but with older systems this often isn't the case. If one pump has a higher rated flow/head then the others, consider using that one when the system requires single pump operation.

Did you know that 76.4% of all statistics are made up...

 

[impeller1]

Yes all 5 pumps are identical in terms of make, model and rated conditions. The set up is there is a check valve and butterfly valve on each pump discharge before they discharge into the header. We have found that throttling seems to eliminate the problem somewhat as MartinSR00 was saying...