The motor doesn't know anything about the valve. It only cares about how hard it is to turn the pump shaft. If the pump torque requirements go up, the current goes up, and vice versa.
Ask yourself if the pump needs more power or less power with the valve closed. It has nothing to do with the motor.
"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg
In general:
* for a radial flow centrifugal pump, opening up on a system throttle valve will increase BHP demand and current draw.
* for an axial flow centrifugal pump, opening up on a system throttle valve will decrease BHP demand and current draw.
* for mixed flow centrifugal pump, the curve is non-monotonic. As a very rough thumrule peak BHP is near BEP so current increases when adjusting flow in a direction toward BEP.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
If I had to ask myself if the pump need more power (as dps sugested) I will say that the pressure on outgoing pipe will increse, and the pump will need more power to defeat this preasure.
You need to look at the pump specs for the pump in question!
The most common type of centrifugal pump in my experience would be radial flow and as electricpete points out above, closing a valve on the discharge would decrease both flow and amps drawn by the motor.
This may not seem correct, since as you say, pressure increases inside of the pump, but power requirements are closely related to fluid flow through a pump of this type and a throttling valve will decrease flow if it is partially closed.
You'll see for radial flow, BHP increases with flow and so current increases with flow.
You'll see for axial flow, BHP increases with flow and so current increases with flow.
Again mixed flow is trickier.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
Your belief is not correct for centrifugal pumps (or fans). If the flow is restricted for centrifugal pump outlet, there is lesser flow, the head remains constant so net power decreases.
This is not true for a positive displacement pumps.
Look up some basic text books on centrifugal machines.
If you have a specific pump in mind just test it. Close the valve a little more see what happens.
Here, lets try it!
Here is the simple straight forward setup. It's a magnetic drive pump. An 1100gal/hr Little Giant.
A standard schedule 80 bulkhead was used and all piping and the ball valve are 1 inch which is the same as the pump inlet and outlet.
Close up of the pump.
A close up of the excellent wattmeter that was used.
The unrestricted bulkhead fitting.
Here we go. The system running with the ball valve wide open.
The pump is consuming about 100W of power doing maximum work, which is
moving about 1100gals/hr of water. Note the solid column of water returning to the tank. 103.8W
Now for some throttling. The ball valve is about half closed.
Notice the flow has dropped substantially. Notice also the power has not risen. The pump is not working harder.. It is working LESS. The power has decreased. 72.7W
Now the valve has been closed about 25%. Power has dropped still further.
57.9W
Here I have closed completely, blocked, the flow. Notice the power has dropped further yet. Of course it is not zero as there is a fan being run, bearing friction is present. Windage is occurring in the motor. There is resistance in the windings and there is some recirculation occurring in the pump head. And magnetization current.
Keith,
You get the award for going the farthest to prove a point in this forum!
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376
Electricpete's post is more precise. It depends upon the type of impeller the centrifugal pump has. What itsmoked tested may be a radial impeller centrifugal pump (which is the most common impeller type).
![[medal]](/data/assets/smilies/medal.gif "[medal] [medal]")
