Cooling tower parallel water pumps....

[Zpoint]

I'm working on a large water source heat pump installation. Several units are going down on high pressure safety. I checked the two water pumps delivering cooling tower water to the system, they are in a parallel configuration. Both pumps show 40 psig inlet pressure, one of the pumps shows 150 psig head and the other shows around 80 psig head. What's going on? I thought both pumps have to operate at the same head pressure when running parallel.
Any help or information will be appreciated.
Both pumps are running. I have not checked amp draws or anything yet.

[The Viking]

Normally you wouldn't run two pumps in parallel, I would expect them to be duty and standby.

If they are a "standard" twin pump set (=Both pumps on the same casting for their impellers) then there would also be a "flap" acting as a non return valve which could increase the head on one of the pumps.

If the above isn't the case on your site and both pumps are designed to run simultaneous I would check strainer and impeller on the pump with the lower discharge pressure.

Happy hunting.

[Zpoint]

Both pumps are running. This is the basic piping layout.Untitled.png

[Brian_UK]

A silly question maybe but are the gauges accurate?

Do you have any means of measuring the actual water flow rate?

What is the design flow rate and how does that compare with the pump design rates?

[Tayters]

A silly question maybe but are the gauges accurate?

Do you have any means of measuring the actual water flow rate?

What is the design flow rate and how does that compare with the pump design rates?

The 10,000 post milestone, well done Brian. Webram needs to get the Good Service medal to you pronto.

[RANGER1]

Lets go even more basic, are both pumps running correct direction.
Some pumps will pump a bit even running the wrong way!

Also are pumps identical with same name plate ratings?

40psi inlet pressure, is this a closed circuit? air in system?

[Magoo]

My pick would the high discharge pump has a partially or fully jammed discharge check valve. 150psig discharge is 65 ft head, and 80 psig discharge is 34 ft head, common suction, common header discharge same size pumps in parallel.
Check the pump design head by closing discharge on each pump, one at a time and read gauge pressure. And compare when operational on line together.
magoo

[Zpoint]

I think you are right, the questionable pump looks like it had a new motor installed in the not to distant past.
I bet it has been running backwards from the day it was installed. I have seen this many times on many types of equipment. Once again the simple answer is the right one, I think. I will let everyone know in a few houirs.

"Keep everything as simple as possible", "but no simpler!". Albert Einstien

[glenn1340]

Shoot me down in flames here but is the lower pressure pump definitely running ie powered up? I ask as we have three 30kw cooling water pumps on a common manfold but only one is set as the lead pump. However on several ocassions two have been running: normal sound of a motor running and fan blowing over the motor but on further investigaion the standby unit was found to be running backwards due to the check valve on that pump breaking up and the pump then acting as a turbine. A long shot I know but there again....