Why Does Reducing the output flow also reduce the Motor current??

[andycarmi]

Hi sorry if this seems like a stupid qustion but could someone please explain to me why when using a water pump system if you reduce the size of the output pipe i.e by closing a valve the current drawn on the motor turning the water pump is reduced?

We have a problem at the company where i work we have a water scrubber system used for cleaning dust from air.
The system uses a water shear pump powered by an 11.5kw motor. if you try and run the motor with no water passing though it for a few seconds the draw on the motor is 8amps does this seem excessive???
when the system is filled with water the motor pulls 21.5amps which is 1.5amps above the maximum full load current on the motor name plate.

The Company that service the pump say there is nothing wrong with pump as it turns freely enough. and problem must be with are pipework, they are saying that because the other unit that we have is running at 1.5 bar and this problem unit is only running at 0.5bar that we need to slow the water down leaving the pump to increase the pressure which in turn will reduce the current/amps being drwn on the motor. is this correct

Thank you for you time.

A carmichael

manchester england.

 

[jet1749]

For a radial flow centrifugal pump your advice is correct. The work done by the pump is the volume moved and not the pressure developed, although the relationship between pressure (head)capacity (volume)and power are all interdependant. For a fuller understanding search for "affinity laws" Basically for a given set of circumstances:
Reduce flow by increasing discharge pressure = lower power
Increase flow by reducing discharge pressure = higher power
Reduce speed = lower discharge pressure, lower flow and lower power
Increase speed = higher discharge pressure, higher flow,and higher power
Different rules apply for axial flow centrifugal pumps and constant displacement pumps

 

[katmar]

The power used is basically the volume pumped multiplied by the pressure developed. The answer is in the shape of the pump curve. As you throttle the delivery valve you follow the pump curve, and this will generally mean that the volume decreases by a larger fraction than the pressure increases. So the product of volume and pressure decreases as you throttle the valve (or increase the head in some other way) and you use less power.

At the bottom of the pump curve there will generally be a power curve which makes this obvious.

 

[JJPellin]

It could be thought of by using the definition of work: force applied over distance. If I push against the wall, but the wall does not move, I didn't do any work. But if I push a piece of heavy furniture across the room, I did a lot of work. The motor amp draw is an indication of how much work it is doing. If I run the pump against a blocked valve, it is building pressure but not moving any volume and thus it is not doing much work. When I open the valve and begin to move a volume of fluid the work done by the motor increases as noted in the excellent replies above. It sounds to me as if you are being given good advice about increasing the downstream pressure in order to keep your motor from overloading.

 

[bulkhandling]

This thread actually brings out another question: Shall the motor be sized for the run-out condition? Normally for water pumps, I would require the motor size be big enough to cover the whole pump curve. That is, when the operating pressure is smaller than the calculated, the motor still take the bigger flowrate and not overloaded.

 

[Artisi]

It is not a law that the motor has to be sized for the run out condition, but a word of warning, you have to know how the pump / system will interact with each other and you need to know what YOU are doing in making these decisions.

Naresuan University
Phitsanulok
Thailand

 

[katmar]

The run-out spec is basically an economic decision. For a small 1 or 2 HP motor on a duty where it is possible to get the run-out condition you can get a run-out safe motor for a few extra bucks. So I would be tempted in this case to do it. However, with a 150 HP motor on a cooling tower duty where it is hard to imagine how a run-out situation could develop I would most probably not specify a run-out safe motor - because of the extra cost.

 

[d23]

Pumps in general:

Generally I would agree with most of the statements in this thread; however I would recommend that you check the specific speed of the impeller before making any “matter of fact” type statement that the “shut-in HP will be less than open flow HP.”

D23