Low current consumption of pump working in negative suction

[rgtech]

We have a centrifugal pump installed working in Negative pressure at suction side.
Rated Shaft power, P2 = 18.5Kw. Full-load current of motor = 32A. Motor has star-delta starter.
Please find the pump curve below:

http://files.engineering.com/getfil...9e-43e3-bb5f-00c27c84bbfe&file=Pump_curve.jpg

The pressure gauge at the discharge of pump indicates pressure of 50m.
With an NPSH of 4m, this means that the pump head is at (50 + (10-4))=56m.

At 56m, the flow should be 67m3/h and P2= 15.4 KW.
At 15.4Kw, Current should be about 28A.
However when measuring the current on each phase, we get 9.5A.

Can you please advise what could be causing the motor to be running at low current?


thanks

 

[bimr]

NPSH is not part of the system head calculation.

System head = total discharge head - total suction head

H = hd - hs

The total discharge head is made from three separate heads:

hd = hsd + hpd + hfd
•hd = total discharge head
•hsd = discharge static head
•hpd = discharge surface pressure head
•hfd = discharge friction head

The total suction head also consists of three separate heads

hs = hss + hps - hfs
•hs = total suction head
•hss = suction static head
•hps = suction surface pressure head
•hfs = suction friction head


If the pump is not operating on the pump curve, the pump suction may be partially blocked.

 

[LittleInch]

Ok, give us the full story and we might be able to guess a bit more.

Motor:
Voltage, no phases (3?), power factor, rated size

Pump
Inlet pressure, outlet pressure, density of what you're pumping,

Is this a single pump or a parallel or series unit.

How are you measuring flow? If you pump less fluid then you use less energy. How are you measuring this?

Your curve is a little odd, is your flow capable of getting over the "hump"



My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[BigInch]

As BIMR notes, probably suction blockage. As LittleInch says, curve is not a progressive fall, actually rises for awhile and it looks like that same 56 m head could also occur at 8 m3/h. 8 m3/h flow coupled with bad efficiency at that flowrate might actually result in a power consumption of near 15 kW. What's going on with the temperatures?




I hate Windowz 8!!!!

 

[Artisi]

Pump is running at lower than expected current due to operating at a low flow rate.

You will need to accurately measure the flow / head and get back to us.

As BigInch has pointed out the pump could very be running left on the curve where the head at 56m is the same as it is at something less than 10m.
To check if this is the problem you need to start the pump against an open discharge so that it runs out until it hits curve and then runs up the H/Q curve - it should settle at the higher flow rate, if you then throttle the discharge and increase the head you can then check if it moves left to the lower flow rate, if it does it will stay there.
If the pump is started against a close valve or a fully charged discharge line, once it hits 56m then it will run there at the lower flow and won't run out on its curve to the higher flowrate.

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.)

 

[Artisi]

Just to add a bit / correction;

"If the pump is started against a close valve or a fully charged discharge line, once it hits 56m then it will run there at the lower flow and won't run out on its curve to the higher flowrate."
I should say "it may run at the higher flow as it runs up the curve or it may well move further left to the lower flow."
No way of predicting either - only the onsite result will confirm where it will run - which in your case looks like the lower flow.

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.)

 

[goutam_freelance]

Suction blockage of such magnitude should manifest itself in head loss and cavitation.

I feel that blockage or partial closure of some valves in the discharge side needs to be checked as this will also reduce flow and motor load.