Cooling Tower Make-up Pumps

[macmet]

I'm having an issue at my plant with our cooling tower's make-up water pumps.

What we're seeing is our cooling tower calls for make-up water, solenoids valves open downstream of the pump, five seconds later our make-up pump turns on, and then instead of pushing water up to the tower, the pump appears to just spin, eventhough a pressure gauge near the outlet reads 160psi. The expansion tank is always full as designed and the fill control loop seems to work well on it.

Some times the system will work right away, other times it takes an hour or two of manually overriding the control system to get the fluid up to the tower.

Other system details,
- cooling tower is 250 ft above the pump outlets
- expansion tank's surface is about 6 ft above the pumps
- fluid is water at common city supply temperatures
- the system worked last summer without issue.

I've read in our troubleshooting manual that the pumps may need to be primed. An online search seemed to point in that direction too, but my operator is adament that they do not need to be primed and that it is not possible for air to be causing the problem. I'm not sure I agree, but I'm not a pump expert and would like to hear what other people think may be causing the problem.

As always, appreciate any feedback or thoughts.

 

[DubMac]

Looks like a flat curve from where I am in the world....

350' TDH design point
----
380' Shutoff

30 / 380 = about 8% head rise to shutoff

 

[DubMac]

Curve is absolutely flat from 0 - 250 gpm; if this ain't a flat curve, there ain't a cow in Texas.

 

[Pumpsonly]

You mentioned there is no check valve in the discharge piping,and the pump is programmed to start 5 sec after the solenoid valve open.
During the 5 secs, the water in the vertical pipe will be rushing down to the suction tank and back spinning pump and motor. If the motor is energized during this time, you have been very lucky not to had a broken shaft or damaged motor.

Hearing a clicking sound at the solenoid valve does not necessary meant it is open. I would suggest you open the valve to check.

 

[Artisi]

Seems as though my by-line should be applied at this point in time.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[BigInch]

The less data you have, the easier the answer, as long as you know when to stop.

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

 

[Artisi]

A bit like buying wooden dowling at our local hardware, 1/2", 3/4" or 1" diameter in 2 or 3 metre lengths.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[QualityTime]

"It sounds like you have a decently flooded suction (no suction lift), so really the only way for air to get into the pumps would be for the suction inlet to vortex from the free surface of the tank. However since you appear to be generating pressure at the pump outlet, I don't believe that is happening. "


This comment is not necessarily true. If the water level is low in the cooling tower you will be drawing air into the pump suction. Put your ear to the pump piping and listen to hear that it is flowing full

 

[macmet]

Just wanted to let everyone know that we're going to go ahead and check out the solenoid valves. We are just awaiting approval on a new unit and a rebuild kit in case they need to be swapped out.

With the way this place moves, it'll probably be next summer before we can look at it, but I'll update this thread as I learn more.

I'll be following the thread in the meantime in case anyone else has any points to add.

Thanks for your help,

Cheers