Trying to Figure out Flow from a pump. HELP! 5

[Chris21E]

Hey guys,

I had a question and it might be a no brainer but I would appreciate any help. I have a pump that was reused in a project. We are using a Cornell 8NHPP pump. We upgraded the motor on it to a 25HP motor. I only have a static head height of about 14' and looking at the pump curve the lowest RPM head/flow curve starts at a head higher than 14'. I have tach'd the motor and found that the pump is spinning at 770RPMs so there is not a area that really the required head meets the operation curve on the pump curve for that RPM range. I cannot imagine that the friction head would add enough head to get to a point that I can meet on the 770RPM operation curve. Anyone have experience with this situation and how can I move forward to get a flow out of the pump. The system has 5 90degree sweeps which are 3' sweeps and one 6" gate valve. Any help would be greatly appreciated. Thanks!!

 

[bimr]

You have not posted enough information.

Unless there is something wrong with the pump, the pump is probably pumping on the curve, but at the extreme right side. The peak flow through the pump is most likely limited by the friction loss in the piping or the NPSH. Is the pump cavitating?

The maximum flow in the piping is likely in the 15-20 ft/sec velocity range.

Items that you should obtain to estimate the flow include:

Inlet and discharge pressure readings.
Fluid properties: s.g., viscosity, vapor properties
Piping diameters and length of piping.
Suction head

If you have a pressure reading on the discharge side of the pump and the pump is not cavitation, you should be able to get an idea where you are operating.

Is this the correct pump curve?

 

[LittleInch]

Chris21E,

From the data supplied, it would seem there is some sort of blockage in the piping or other item at fault, not the pump.

I looked up the pump and yes, at 770 rpm, the DIFFERENTIAL head is realistically between 20 and 35 feet. Hence if your vertical lift is no more than 14 feet, then you should have flow. However you don't give us any indication of what the inlet / suction pressure/head is when running.

Has anything else changed from the previous pump arrangement?

We have no sight of what the inlet piping or discharge piping configuration is or what you are actually pumping so it's very difficult to tell what might be the cause.

Some things worth looking at - make sure the pump is going round the right way (there is an arrow on the pump casing and with 3 phase motors its very easy to have it spinning backwards) - you will still get some flow / head, but not as much. Is the gate valve stem still connected to the slab? Is there a blockage somewhere?

Other things worth looking at - if the flow is actually very high, you might be exceeding the NPSHR of the pump and pulling a vacuum - Is the pump fed with liquid at positive pressure? If not how is the inlet pipework charged / filled up with liquid. Centrifugal pumps generally won't self prime. If this is a foot valve arrangement, check the non return valve is working / sealing.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Artisi]

If you upgraded the motor, have you checked for correct rotation, yes yes I know it's correct - but have YOU physically checked.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[MikeHalloran]

Another star for Artisi.

I have lost count of the number of times I have found three-phase motors running backwards, after having been assured that everything was fine. The last one was a hydraulic pump that 'stopped working for no reason', months after startup, when someone repaired a phase sequence error at the building service without going inside to look for motors that had been connected 'wrong' in response to the original installation error.

In that case, it was a positive displacement pump, so produced zero flow, and didn't run for long before someone noticed there was a problem. A centrifugal pump can run backwards forever and still produce usable flow and head; it just won't be on the curve that the manufacturer provided.

Safety note:
Don't put your fingers anywhere near a rotating shaft or coupling.
Instead, use the eraser end of a pencil that you don't value much, or a random wooden stick.
Feed it slowly, radially toward the shaft until the drag tells you which way the shaft is turning.
Splinters suggest that the pencil/stick is being struck by a setscrew or other protruding feature that would have removed your finger.





Mike Halloran
Pembroke Pines, FL, USA