Low Cooling Tower Sump, CW Pump NPSH Issues

[ChasBean1]

I have a chiller plant application where the surface of the water in the open cell cooling towers is only about five feet higher than the condenser water pumps' inlets.

There's about an 80 foot run of condenser water supply piping leading to the pumps. During operation, pump inlet pressure is just below atmospheric. Auto air vents installed at the pump inlets suck in air instead of bleeding it out. Needless to say, there is some vapor entrainment and cavitation issues...

Have you faced this before and how did you resolve it? Raising the cooling towers would be very expensive and have substantial schedule impact at this point.

Can a priming line from each pump's discharge to its suction help the cause?

Thanks for any input!

CB

 

[ClicketyClack]

What is the NPSHa and NPSHr? Depending on how close they are to each other, there are a few things you can do.

What is the flow? Is it possible that you are vortexing?

 

[Artisi]

First up - entrained air and cavitation are 2 seperate issues and are not interrelated.

Secondly - get rid of the air vents - if they are pulling air into the system and if the inlet pressure at the pump is below atmospheric the entrained air is expanding even further making the problem worse.

As asked by ClickityClack - let us know the NPSHa/r so NPSH can be checked.

 

[BigInch]

You must have NPSH before you can think about recycling from discharge to suction, otherwise you have nothing to recycle.

http://virtualpipeline.spaces.msn.com

 

[krb]

I strongly recommend calculating NPSHa and NPSHr as already mentioned, but when I have faced this problem before I have looked to see how I can minimize pressure drop in the suction piping. Tees, valves, strainers all add to pressure drop and reduce NPSHa. BAC has some good info regarding correct CT piping that was adapted from Bell and Gossett. THe link is here:

http://www.baltimoreaircoil.com/english/info_center/techresources.html

 

[ChasBean1]

Thanks for your feedback! I'll try to get pump info (didn't have the submittal or nameplate info handy).

The air vent was installed after the problems were encountered as an attempted resolution. I believe they've been removed or isolated since.

The CW supply pipe serving each pump takes about a five foot rise from a below grade pit, into a duplex strainer, and back down about two feet into the pump suction. So it's a little bit of a torturous path just before the pump inlet.

As far as velocity in the main (~80 ft) run, the condition was observed with one of four condenser water pumps running (very low velocity relative to design). I climbed a ladder and looked into the CT sumps and didn't notice any vortexing with the pump on. One pump was distributing water through five towers operating in parallel.

All in all it appears this was the best case condition: minimum flow with only one pump and largest net sump size (all in parallel). It will be interesting to see what 2 or 3 CW pumps in operation do.

Tx again, and I'll post NPSH info if I can get it.

CB

 

[krb]

Look close at the need for the strainer, this is usually the highest pressure drop in the system. Typically these are better installed on the downstream side of the pump as mentioned in the literature in the the link I mentioned earlier. Unless you have some specific reason to put them in the suction piping, I would remove it or place it downstream of the pump. There is probably a grate or large mesh screen in the CT sump to keep out the big stuff anyway. I had the same problem on another system, the majority of the pressure loss was in the strainer (which was also undersized for the flow) we took them out and problem was solved. Also look at the suction piping at the pump, are there eccentric reducers? If so, they should be installed with the float side on top. If not, this is another place air can be trapped.

 

[rmw]

Since your fluid is not near its saturation temperature I'd be surprised if NPSH is your problem. You would have to have a awful lot of system loss to overcome the margin the temperature gives you.

That said, your water has air entrained and it can be coming out in the suction piping at the pressure loss points especially the strainer. You need to vent it, but not with the type of vents that you were using.

Also, any pipe leaks in the portion of the suction that is below atmospheric will leak air in instead of water out. Check the pipe for water leaks when the system is down.

rmw

 

[Artisi]

"During operation, pump inlet pressure is just below atmospheric."

This seems to indicate that you could have excessive losses on the inlet side of the system which results in a slightly negative pressure at the pump inlet, this in itself is not a problem provided the NPHa/r margin is adequate. Look at the comments from other posters re strainers, valves, bends etc.

"Needless to say, there is some vapor entrainment and cavitation issues..."

Check if the air release valves have been removed, if not, check yourself (don't believe others or old records)that they are isolated from the system.

Low inlet pressure is not a problem that leads to air entrainment, however, air entrainment can result in a problem if the air is expanding in the inlet or impeller eye, this can put the pump off performance or even put it "off prime". As for cavitation, this is a function of NPSHa/r and not a function of air entrainment unless entrainment is causing restricted inlet flow.

 

[quark]

I agree with rmw on NPSH issues unless your suction pipe size is too small. If you have different suction pipes for different pumps then the condition for vortexing is same whether you observe it for one pump running or many. If it is a common header then better watch it.

Check the strainer if your vent valves are installed after the strainer.

Is your suction pipe length 80feet? What is the individual pump flowrate?

I fully agree with Artisi that you should remove your vent valves.