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Required motor power for centrifugal pump

Required motor power for centrifugal pump

(OP)
Hi all,

Need your help. We have bought bare shaft centrifugal pump (without motor)which requires 20kW motor however we have motor with power rating of 27kW on site from old pump and we are planning use this motor. Please note there is no change in operating point (flowrate and head) of the pump and impeller speeds. The difference in motor kW is because the new pump is bit more efficient than the old pump.

We are having a bit of discussion whether we can install the high power motor (27kW) to the pump that only requires 20kW and whether it would have adverse effect of pump shaft, bearing, seals and hence pump performance?

Someone says, as long as the speed required matches, high power motor should not affect the performance.

I am confused. Thanks for your help in advance.

RE: Required motor power for centrifugal pump

The "someone" is correct. IMHO.

A motor power rating is the maximum it can produce, but the pump will only consume what it needs to do the duty.

The only other downside is that the cables and motor starter need to be rated based on motor maximum power, but if this is an existing motor than you will have no issues providing the motor speed and the pump speed are the same.

You also need to carefully align the pump and motor via a coupling, but I assume you know that. Do that wrong and you break your coupling and pump seals very quickly.

Clearly the motor shaft an pump shaft need to be connected somehow, but this is a simple mechanical thing.

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

RE: Required motor power for centrifugal pump

Oversizing motors may have adverse effects.
1. Reduced efficiency
Motors are intended to operate at a certain speed and torque point depending on their designed load. An oversized motor run at a lower torque point will operate less efficiently and therefore draw more power than is needed. This means electricity consumption will be greater than if the motor was correctly rated for the application.
2. Poor performance
If a motor is oversized then it may not run close to full load. This increases the risk of overheating which in turn can melt the winding insulation and lead to premature failure.
3. Increased equipment cost
The larger a motor is, the more steel and copper it contains. This is why electric motor cost tends to rise in relation to frame size, so for every size above specification you’re needlessly wasting more money.

In your case, the motor is not very big and the adverse effects should not matter too much.

RE: Required motor power for centrifugal pump

(OP)
@LittleInch,

Thanks for your reply and clarification.

In fact, we are experiencing very temperature at the bearings and bearings are failing prematurely and it's such a high temperature that causes the bearing to pull away and the lubricant is turning in to powder. Yes, we are aware of alignment issues but we don't think that's the case here.

Pump is very noisy and it seems it's cavitating but NPSHa is ok. However, discharge pressure drifts quite a lot I.e. It drifts from BEP to nearly shut off head.

@bmir

Thanks for your reply. You mentioned that high power motor may lead to the poor performance but would you think above stated (premature failure of bearings) would be possible?

Thanks

RE: Required motor power for centrifugal pump

The pump has no idea what the motor is. All it cares about is the speed of the shaft. Yes, a larger motor will have slightly less efficiency running at part load than a smaller motor running at the same, but full, load.

A motor will not overheat when running at part load. It will run cooler. bmir, perhaps you meant a motor running at low speed, rather than low load. At low speed the cooling fan does not work as well.

RE: Required motor power for centrifugal pump

Motor bearing high temp tells me you've got an alignment issue or some mismatch on the motor - pumpshaft coupling type. Sounds like a setup by inhouse mech maintenance mob, with no specialist rotating machinery engineering input.

RE: Required motor power for centrifugal pump

Check the pump has a thrust bearing and is bearing against a shoulder on its shaft.Sounds like the pump shaft is floating, so giving variable pressures.
The thrust bearing should be positioned to take the back thrust of the impeller while allowing heat expansion along the shaft.

Offshore Engineering&Design

RE: Required motor power for centrifugal pump

The catalog motor speed is often shown at the motor synchronous speed rather than the actual speed. The actual speed is a result of the slip in the motor needed for it to produce torque. Generally, as efficiency increases, the slip decreases, resulting in a higher speed motor. There are no strict rules on how much speed a motor might gain with an efficiency increase because it depends on the manufacturers design.

Have you checked the rpm and pumping rate?

When replacing a motor on an older pump, be aware of the higher speed for newer efficient motors that may result in the pump producing more flow and operating the motor above its rated load point. The pump impeller may be trimmed or replaced to reduce the load.

RE: Required motor power for centrifugal pump

How does the new pump curves compare to the old one. You indicated that BEP efficiency has improved, how does the absorbed power at the right of the curve compare?
Most likely you have a good fit with the old motor, inquiry this might be pushing but am just curious..

RE: Required motor power for centrifugal pump

Is this pump in parallel with another?

Is the curve shape different from before?

What type of coupling are you using?

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

RE: Required motor power for centrifugal pump

(OP)
All,

Thanks everyone for your input.

Just to answers your questions:

1)Absobed power for new pump is 14kW while it was 20kW for the old pumps
2)The shapes of not pumps are quite same/similar.
3)Pump position is no different to the previous one and maintenance people are confident that alignment issue is not be possible.
4)RPM on motor and pump are the same

@chief

I am not sure about bearing type.

RE: Required motor power for centrifugal pump

Very noisy pump - could impeller be rubbing on casing, or some other interference happening?
As others have indicated, use of a higher power motor would not lead to higher temperatures. At the conditions stated, a 20kW motor running at 75% load will only have a marginally different efficiency than a 27kW motor running at 50% load.

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