Motor Running above Overload point

[rota1]

Friends,
Please advise!
We have here a two stage centrifugal pump, 157gpm capacity, at 995 ft head.This pump is in a water injection service supplying three streams, the flow is controlled via valves which are throttled to meet requirement.
The motor driver is 100HP.According to the pump curve this HP is sufficient for the pump, in fact the actual HP required is 85HP.
The problem is the motor is running above the overload (24.3 amps).This is baffling to us , the ammeter was checked,we did a shut off head test ....this was ok,so we ruled out the possibility of pump inefficiency, however on further opening of the valves ,the ammeter reading went as far up as 30 amps, flow went off scale.
What next...what are we not seeing??!!

 

[ccor]

Sounds like the pump may be running in reverse. Common problem at start-up, and easily fixed by reversing the electrical leeds. Oddly enough, many centrifugal pumps running in reverse perform close to the published head curve but they do it at much higher power consumption (less efficiency). Worth checking into.

 

[rota1]

Thanks CCOR for the quick response, the motor rotation was checked and verified to be correct.

 

[alexit]

If flow at pressure and operating speed is per pump spec I guess pump is operating correctly.
If motor power is correct and motor current is high I would check line voltage. Maybe dyno motor to determine if is operating correctly?

 

[rota1]

Thank you Alexit....some additional information...
On our shut off head test we measured the rpm of the motor,this equalled to 3581 rpm, on opening the control valves we observed the motor speed to be 3561 rpm, bear in mind that the rated speed on the nameplate is 3565rpm.
What we know is the pump is not operating at full capacity.

 

[Zapster]

… reading went as far up as 30 amps, flow went off scale. What next...what are we not seeing


My guess is that you are not seeing the relationship between flow, pressure, and required power. You stated, “We have here a two stage centrifugal pump, 157gpm capacity, at 995 ft head.” What is the flow and pressure when the pump is running in the overload condition you discussed? Have you calibrated your flow meter and pressure indicator? The solution may be as simple as increasing the head with a downstream valve. This might drop the flow down and the associated power.

 

[JJPellin]

A few possibilities occur to me. Another post already mentioned verification of voltage. At lower voltage you will get high amps. There is also a chance that you lost one of the three phases to the motor. A three phase motor with one dead phase will draw very high amps on the other two. The ammeter is probably only reading one leg. Check the amps on all three legs. They should be equal. I assume you are indicating that you are still getting high amps at rated flow with all three flow paths regulated to the correct flow rate. I would check the calibration on those three flow meters. If the flow is actually much higher than you think, this would require higher amps from the motor. And in water wash service, there may not be any other indication of excessive wash water rate. You might be able to verify total wash water if you can measure sour water flows collected downstream.

 

[waross]

From the electrical side,
Check your voltages.
Low voltage has been mentioned. A motor should function at 10% below rated voltage. That would be 414 volts for a 460 volt motor. The voltage is seldom that low, but you may have a long feeder that is not sized correctly for voltage drop.
A voltage unbalance will cause a disproportionate current unbalance and overloading and overheating.
Check your flow. Many pump-motor combimations have the potential to exceed the rated horsepower of the motor if the flow is excessive.
My first check would be voltage unbalance.
My second check would be low voltage (at the motor, not at the panel if the feeder is long).
My third check would be flow.
respectfully

 

[rota1]

Thanks a lot guys for such valuable input.
I forgot to mention that during our shut off head test(motor on no load) the ammeter reading was 17Amps, is this indicative of some problem? it seems to me that there is a little difference of Amps between no load and load(24.3A)
This motor is 100 HP and 2300 Volts, frequency 120Hz
For this motor, what should be the no load Amps?

 

[waross]

Hello rota1
There is a difference between shut-in and no load.
At shut-in the load is the sheer in the pumped fluid.
The current is a combination of magnetizing current and load current. These two currents are out of phase with each other and cannot be added directly. This is related to power factor.
The 17 amps indicates to me;
Probably moderate to poor power factor and
moderate to high sheer losses.
I can't say without a lot more information but it could be a reasonable figure.
respectfully

 

[rota1]

Thank you Waross..... in the shut off mode,from the pump curve, we see the power required is 50HP..with this new information can you say the shut in current of 17Amps is ok for this motor.

 

[stanier]

Are you maeasuring the current at the motor or at the switchboard. If at the switchboard you may need to check the cable sizing as this may be causing the problem.

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng

http://www.waterhammer.bigblog.com.au

 

[waross]

Hello rota1
It depends. It looks like the overall efficiency of the motor is below 80%. The current may be reasonable.
If you have a voltage unbalance problem causing excess current under load, it will also be causing increased current at shut-in condition.
In response to stanier's suggestion, #10 cable is almost adequate for 24.3 amps, but is 1.2% below code. Code size is #8 AWG and this should be adequate for extremely long runs at 2300 volts.
Check the current on all three legs and check the voltages on all three legs. Low and/or unequal voltages may well be the problem and should be investigated.
respectfully

 

[bingopin]

Check for a hidden flow path, such as backflowing through a check valve on the spare pump (if it exists).

 

[Enkidu]

adding to bingopin, perhaps the hidden flow path could even be a missing or damaged wear ring.
I'm not on sure footing here, however, I would expect some very serious motor issues if one of the phases shut down - probably leading quickly to a motor failure - any thoughts?
I would start the investigation by quantifying the flow rate and differential pressure you are operating at and consolidate that with the pump curve and hp requirments.
good luck.

 

[d23]

One question missed is the fluid specific gravity. Being an injection pump what and why are you injecting? My guess is that the pump curve you have is based on a specific gravity of 1.00. Are you pumping brine or some other heavy liquid?

D23