Motor's Performance on Throttled Pump

The motor won't care and no damage will result to it. Efficiency may go down. It will still run at 3000 rpm but it won't be doing as much work. So lower amp draw.

The pump may overheat and fail if sufficient flow through the volute is not maintained.

That unused flow is turned into heat inside the pump.

If this is the permanent pumping duty you are killing head with the valve and the pump is running at a lower efficiency.

you should consider skimming the impeller or select a smaller pump.

You could save 5 - 7 kW (guess) which could pay for the changes in a short time.

The guy didn't ask about efficiency. Lyn gave the correct answer except that at 50% flow, there would be no overheating of the pump.

The important thing is to set one's mind further open than 50%.

No answer so far approaches being a complete one.

On the assumption that the pump is a standard radial or mixed flow design:

If the electric motor you refer to is is an induction motor (as is likely), the pump speed will increase only slightly with reduced output volume, and if your initial output is the same as the design (pressure volume) optimum for the pump there should be a decease in efficiency. Whether this is the case will depend if the pump is being used for its designed purpose. For example, a "med to high pressure" pump for fire fighting will not be efficient if it is used as a "low-pressure-high-volume" bilge pump, and it was used in the second role it may well be more efficient if its volume output was reduced.

Reduced flow in the pump will cause a very small temperature increase but there is no way that reducing the flow by only 50% will cause any overheating problem in the pump. The normal problem in this area is where the flow has completely stopped (so no cooling inflow to the pump contents) for a period of time.

Reduced output volume could however, with the higher consequent internal pressures that follow, could cause earlier bearing and seal failure.

If the motor is (say) a series motor of some sort, then the speed would increase significantly with reduced output put (as the input torque requirement will drop) and this will cause an increase in output pressure, but with less heating in the motor and once again more of less motor efficiency as before. An increase in impeller speed would increase the possibility of impeller cavitation and the serious matter of possible metal erosion in the pump.

Pump design is not difficult but there any many factors to consider and balance.

The motor question has been answered.

But some people have missed the point that the OP says "discharge valve 50 % opening", not pump at 50% flow or pump at 50% flow reduction. We have no way of knowing where on the curve the 100% and 50% valve opening points are. 50% valve opening could result in an operating point close to closed-valve, or equally close to BEP.

May, or may not be a valid point.

You are right about not knowing what OP is really doing, or where the valve is really set.

We suffer with this lack of, or no meaningful, information from most posters.

Part of my point is that this is a very general question on the effects of running a motor at reduced load, and it is quite easy to give a general answer that answers the question quite accurately.

The other part of my point is that this is not a question about pump performance. We know where the valve is set (50% open), but we have absolutely no idea where this puts us on the pump performance curve. If the pump is operating in a high head system, then closing the valve to 50% could have a proportionally minimal effect on overall head, and you could find, for example, that it moves the operating point from 120% of BEP to 100% of BEP. In this case the pump efficiency is actually increasing, and pump performance is better. In this instance this scenario seems unlikely since the absorbed power at 50% valve opening is only 20kw, but my point is that we just don't know.

So why is there talk of pump performance when it is totally irrelevant and potentially inaccurate, and the question is about motor performance?

I don't know. My answer was about the motor, with an aside as to why you will see extra heat.

It's a rare discussion here when the correct answer is given, and comments stop because of it. <Edit: I'm probably the biggest offender here.>

Typical engineers, we can't leave well enough alone.

True.

OP says the pump is operating on its performance curve, but not where. As long as the pump is not operating below minimum flow, there should be no harm. Yes, the higher pressure at the lower flow may have a effect on seal (due to possible higher PSI at the seal) and bearing life (due to possible higher thrust loading), but the main thing to consider is the loss of efficiency, both in the pump and the motor (the pump is not at its BEP (Best Efficiency Point) and the motor is operating at less than half load).

Here, I want to Conclude One thing:

You are Controlling the Discharge Valve to 50 % that means Presently the pump which is running is of over rated Capacity .

First , calculate Your Requirement then how much the existing Pump is exceeding . Based on the Pump Design ( RPM ,TORQUE etc ) Motor Should be Selected.

As Suggested, You can go for Small Pump or variable Speed Drive, to select between these 2 options ( either Small pump or drive ),first compare the PUMP cost and DRIVE Cost and Choose Which ever is less.

Also if such Centrifugal Pumps are more number in use with Similar application in the Plant then choose for the DRIVE . Based on the capacity of the Drive, 1 Drive can be looped to number of Motors.

Nithin