Condenser Water Pump Issues

Hi

Multiple pumps sharing a common suction line is not good practice, in my opinion. Sounds like your problem could be associated with air trapped in the offending pump, this could be caused by cavitation, leaks in the suction pipework, pump gland etc.or vortexing at the cooling tower exit point/s.

Is it always the same pump that malfunctions? Does the problem occur immediately the pumps are switched on, or does it take time for it to happen - if so roughly how long? Are all the pumps switched on simultaneously or seqentially - if sequentially does this cause different pumps to misbehave depending on the starting order?

My choice would be to run individual suction lines to each pump and, thus, making problem solving so much easier.

Hope this sheds a little light.

Mike G

Hi Mike

Thanks for your comments. It was interesting for me to note that the problem could happen to any of the pumps when I started them sequentially. The last pump started would exhibit this problem. That was why I thought it is a sizing problem.

Cavitation and air trapped is highly possible, which I thought is linked to the 4 hungry pumps sharing limted supply of water. Regarding the common suction header, we had no choice due to the limited plant room space and I believed this is a fairly common setup elsewhere.

Do you know of any rule of thumb or formulas/web page I could refer to for proper sizing of pipes and pumps for water flow? I would have to look at all areas, including those areas that you pointed out. Thanks mate!

This could be caused by several things besides air. Cavitation is one - check to be sure that the pressure drop through the suction pipe does not put you net positve suction head below that required for the pump. Another could be that the pumps cannot produce enough head. When you put 4 pumps in parallel, they will each try to pump 25% of the flow, but all will see the same head. Problem is that your head will increase with the square of the flow. So, as you add each pump and it tries to move water, each one needs to have significantly more presssure, or its flow will drop off. Look at the pump curves, measure the pressure in and out of each pump and also chack the power it is drawing (amp meter or kW meter). You might find that the last pump on cannot move anymore water at the head that is produced. Are all of the chillers valved? How do they control the flow of water? Many variables here.

You have recieved quality advise from each of the responders and what they are telling you is highly likely to be true. I have seen what you are describing when one of the pumps was not like the others. An Impeller that was .125" smaller which did two things:

1. Reduced the head capability

2 Placed the cutwater further from the impeller and increased recirculation.

This caused the flow to be less the heat to be more and for all observation seemed to be cavitating while it was not. You didn't mention gauge pressure on the discharge and suction side of each pump. This will help you describe the error.

h2om@hotmail.com

Some questions to consider:

Are these being fed from the ceiling, and do they have properly sized and installed suction diffusers?

Are there reducers involved on the suction side?

Is there that grinding-marbles kind of "cavitation noise" that would lead you to believe it is cavitation? (btw, if it's just a noise you're trying to eliminate, look here for a discussion: bellgossett.com/Press/BG-mystery.asp)

I put vertical inline pumps in this application now, as it eliminates a lot of related issues...but usually size only two pumps to share the load, with a third piped in as a backup. Never used four at once...yet.

The new pumps are likely under warranty, and I'd think you could likely get the pump manufacturer's rep (who sold them to your installation contractor) to come out and take a look at the installation itself. He can help you evaluate the design, the piping, the accessories, etc. to make sure there isn't something onsite that we can't see from your description but is affecting the performance.

Look at the system curve for the 4 pumps and make sure the individual pump curve does crosses this system curve. If it does not, that would indicate you have incorrect type of pumps for your application.

Sam

Way back when I was employed by York, the chiller folks, we had a similar problem. The pumps are of the same type with the same pump curves. When you start up the first few pumps they will develop head until they reach the given head for the given GPM being forced through the condensers. when you start the last pump it is trying to open it's discharge check valve however the pressure on the discharge header is at or near it's maximun head. So the check stays closed and the impeller just churns the existing water in it's body and overheats. Try increasing the flow through the condensers to decrease the pressure on the discharge header. Or may be bypass some of the water to get the check opened. Sounds like you have design problem not a pump problem. we ran into the a similar problem with VSDs. One guy thought he would run one or two pumps across the line and control his head pressure by using a VSD on the last pump in the system. The problem was the VSD operated pump's check would not open at anything but near full speed because the pressure on the discharge header feeding the condenser water side was greater than the head developed at low speed.

Good luck, if I can be of any help contact me at wary@aol.com

Thanks folks! The comments are constructive and professional...you guys worked for Nasa or something?

Jokes aside, figuring it out, I think it is the system curve which needs to be modified to improve flow for the same pumps. Assuming all possible flow restrictions are eliminated, there is only so much water the pump can push as the pump curve cuts the system curve. Running a set of new pipes from the cooling tower may not solve the problem as I still need to overcome the system resistance at the chiller side.

I guess I might need to upgrade to larger pumps with higher heads to "ride" the system curve at the expense of higher losses and lower efficiencies.WDYT?