Cooling water pump

[TinusE]

A pump was installed which takes suction out of the existing cooling tower sump. A civil channel was created to supply water to the pump sump from where the vertically installed centrifugal pump takes suction. The pump is cavitating. Flow rate 1800 m³/h.

How do I calculate the friction loss for the civil channel?
How do I know if the sump was correctly designed?
How do I calculate the friction loss over the vertical screen installed to prevent garbage from entering the pump suction? I thought I would treat it as a orifice in the suction is that correct?

Thanks Tinus

 

[rmw]

I notice no one has taken a shot at this one yet. For my part, I need you to help me with some terminology. I don't know the meaning of civil channel, although it implies to me an added channel to a new sump pump.

If that is the case, vertical pumps have several requirements. One is minimum submergence. You have to check with your pump manufacturer to find out that.

Another is that there are specific requirements for vertical pumps with respect to the velocity of the liquid flowing in the channel towards the pump, and specific requirements for distances from adjacent walls, and/or other pumps in the same sump.

And, yes, if there is a level change across the inlet screen, it could be affecting your minimum submergence, and/or your velocity of the flow approaching the suction.

It sounds like you are getting vortexing due to one or more of the above problems. Do you see the vortex eddies and swirls in the surface of the fluid in the vicinity of the pump. If so, then that is your problem.

rmw

 

[dave2109]

Since the pump is already operating, you can determine the friction loss through your system by comparing the channel inlet water level with the level at the pump. Same with the friction loss across the screen.

rmw makes a good point about checking the minimum submergence of the pump. If you have too large a drop through the system, it should be reflected in a low water level at the pump. For a pump of this size, the minimum submergence is likely around 4' to 5' but the manufacturer should be consulted to get the exact value.

The Hydraulic Institute gives some guidelines for designing pump channels. Things to look at include channel velocities, transition angles, how close the pump is to the side and back walls of the channel, bottom clearance, etc. I'd check your channel design against these guidelines and see how they compare.
You could also have a hydraulic analysis of your channel design to determine if it is the problem.