pump taking high current

[pkg80]

hello

we have three similar vertical pump in parallel operation with full load current of 75amps. One pump is taking current more than full load current 75amps. It takes 76-80amps. Others pumps takes 65-69amps @same time for same parameter.generally two pumps always in operation.

I have checked suction/discharge pressure which are same for both pumps. Flow meter is common for all pumps so we cant take individually but i think is same as pressure are same.

There is all parameter (vibration,temperature etc.) are within limit.

can any one suggest ,what may be the probable causes of high amps.

reagrds
pradip

 

[LittleInch]

Yes,

Not all "identical" pumps are actually identical.

Your one pump drawing more power will be pumping more flow than either of the other two.

Unless each actual pump has been individually tested to create a unique pump curve you won't find put why. It also makes a big difference on the shape of the pump curve and where on the curve the duty point is.

Less than 10% from duty point is not unusual for parallel pumps.

Can you post the pump curves and mark or note the duty point?

Other issues can be that the inlet to the three pumps is not equal and hence one pump has a lower inlet head than the other. Can you sketch out the physical inlet (it there is one) and outlet headers.

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

 

[pkg80]

thanks littleinch for ur reply...

As I said these pumps are identical and generating same head (same discharge pressure)with same suction pressure.They have common suction and discharge piing.
usually two pumps run parallel.

 

[LittleInch]

Yes, but one is pumping more fluid for the same differential head, therefore more power is required. A variance of 3 or 4% is "acceptable" as far as the vendor is concerned.

One could also be slightly less efficient as well.

If you have the pump curves then post them along with the inlet and outlet headers and then we can see if they really are "identical".

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

 

[Artisi]

Has this rogue pump always been overcurrent?
Has the running clearance been checked?
What pump type are they? -- vertical pump means nothing without configuration description.
Is o/current reading on all phases, have all electrics been confirmed as Ok?

quoting LittleInch "Remember - More details = better answers"


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.)

 

[georgeverghese]

This may be due in part to a pump curve which is more or less flat at around the operating point. Try throttling the discharge valve on the high amps pump a little - that should even out the amps consumption between the 3 pumps ?

 

[pkg80]

hello all
thanks for your interest and reply

I have seen the all pump curves supplied by the vendor and it is similar for all three pumps .I will provide to you pump curve. Here difference in ampere is by 8-10 amps in both pumps which delivering similar head in parallel operation. As I said the pump is delivering same head, it mean flow must be same if it is identical.

layout is like pumps suction from common header first is A then B and then C and in discharge its reverse .
C is taking high current.

 

[LittleInch]

Look forward to seeing them. Check to see if each pump was actually performance tested or whether you simply have a "typical" curve for each pump.

"similar" and "identical" are not the same thing.

also we don't have any process conditions for these units. If the head developed is small, the it won't take much to increase the head of one pump versus another.

Also check Artisis' post and confirm the motor is actually good.

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

 

[JJPellin]

Unless you have individual flow meters, you cannot assume that all pumps are producing the same flow just because they are operating at the same head conditions. You have still not provided the pump configurations. If they are vertical turbine pumps with semi-open impellers or lateral wear rings, the lift setting can dramatically change the performance. Some vertical turbine pumps have a horsepower curve that increases at low flow. As noted above, a phase imbalance in the electrical supply or drive motor could confuse the situation. Normally, the ammeter is only on one phase.

Are there minimum flow spill-back lines? Does each pump have an individual spill-back or is there one common spill-back for the group? Are there suction strainers or screens? Is this a service prone to fouling, lime formation, trash or solids?


Johnny Pellin

 

[pkg80]

i have attached here pump curve.

 

[bimr]

Another great post by JJPellin.

Metering in larger pumping stations, particularly at the highest voltage level tends to be more complex and comprehensive than in smaller pumping stations on lower voltage level. Service technicians may need additional instruments to measure and record, for example, circuit and insulation resistance

 

[pkg80]

Hello jjpellin /LITTLEINCH

Thanks for reply

This pump has closed impeller and we have already checked all 3 phase current and t is like 76 78 & 76 for one pump and 66 67 & 68 for another pump.

These pumps have not any minimum flow lines individually.

is it good idea to check motor by solo run test,before jumping on the pump?

what may be the probable causes for unhealthy motor taking high amps ,I mean, what parameter I should check?

 

[LittleInch]

Interesting pump curve. I still think they only actually tested one pump. When you look at the curve it has the same test number and the same deviation from a straight line. In fact with this pump, the one taking more power might actually be seeing less flow, as the flow decreases there us a small hump as the efficiency drips off faster than the effect of flow.

If possible can you dead head each pump separately for a few seconds and see if you get the same pressure. That will start to tell you if there is actually something different with the basics of each pump.

I don't know enough about motors to know what the tests could be, but try the deadhead thing first. If they are all the same it might be flow related, but you're gradually knocking things off one by one

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

 

[JJPellin]

Are you sure that the problem is the pump with high amps and not the pumps with low amps. I assume that the motor is rated for 500 HP. If so, I would expect all motors to be running close to full load. There could be a problem with the pumps running low amps.

Johnny Pellin

 

[EdStainless]

Do you have tap points to check pressure at the pump inlets and outlets?
A slight variation in plumbing could cause a difference in load.
I would not assume that the motors are identical either, unless you have motor test data.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Artisi]

Good to see the pump curve, at the moment the only useful data supplied so far, we now understand we are not dealing with a few small process pumps but reasonably high flow high energy units -- makes a big difference to the thinking of the guys in this forum.

We are still waiting on pump type / configuration

An installation drawing etc. would assist.

How about an answer to my earlier question - has this pump always run at higher current yes / no.

Is the o/current pump running over maximum allowed current?

It's looks like developing into one of those enquiries where we need to squeeze every bit of info. from the OP step by step.


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.)

 

[georgeverghese]

Pump curve not flat, so would agree that flow may be the same at the same differential head.
These amp reading indicates 3.3kV 3phase line voltage
Vertical turbine pump ? Long shaft ?

A check on design case power for this rated flow of 8925usgpm and h = 424ft at e = 0.81, assuming liquid density is 1000kg/m3 for water tells me design case shaft power is 1185HP and not 519HP ?? What are we pumping here - LPG or LNG?

 

[Artisi]

georgeverghese: case in point

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.)

 

[pkg80]

hello Artisi
yes ,this is running with overcurrent 76-80 amps (More than full load current 75 amps ) always and sister pump with 65-69amps ,it not like spike.This is vertically suspended (V3) centrifugal pump. What do you mean by running clearance ?
phase current already checked and its nearly same for all phase.

 

[pkg80]

hello EdStainless

suction / discharge tapping on the same distance from the piping for all pumps and it is just before and after the pump.
Motor is identical in all pumps.