Condenser water pumps - strainers 7

[Geby]

I am attempting to solve a pump cavitation problem in a condenser water system with an open cooling tower. The pumps are base mounted single suction with inlet strainers installed. I am thinking that the additional pressure drop caused by the inlet strainers may be causing the pumps to cavitate, but have not tested the theory yet. I wanted to throw out this general question to the audience: Since cooling towers are normally equipped with sump strainers, is it really necessary to keep the pump inlet strainers installed beyond initial startup? Yes, some minor junk like pine needles will get past the sump strainer and into the water loop, but I do not think that this kind of thing would cause any major pumping problems, and we do have a solids seperator in the loop. I am thinking that pump inlet strainers installed in an on an open condenser water system could easily reduce the available NPSH below that required.

 

[gepman]

I am a big believer in the adage that "90% of pump problems are caused at the suction". Removing them should help your situation.

What kind of condenser do you have? Can it readily by cleaned if by chance some solids did get by are are stuck? Will your sump strainer remove solid particles larger than the maximum particle size of the pump?

Of the cooling towers that I am most familiar with, none had inlet strainers so I believe you should be OK.

 

[RossABQ]

Yes, this can be a problem and I have seen it a couple of times. The typical pump in ths range will handle 3" solids, put the strainer on the downstream side if you need to. Usually the CT just has an anti-vortex cross in the sump, you might want to consider a truly coarse (chainlink fence like) "filter" across the drain to keep plastic bags and other trash out of the system.

 

[willard3]

You could also clean the strainer regularly and the pressure drop and boiling would be minimized.

Also, if this is a variable-speed pump, the tangential solids separator will not work well.

 

[bbohsali]

I would suggest finding out if you have enough head on the suction side of the pumps. Run a pressure loss calculation to figure out the pump's NPSHR compared to your available NPSHA. Remember that you are typically dealing with absolute (vs. gage) pressures here.

Yes, a dirty strainer will choke the flow on the inlet side of the pump to the point where it can cavitate, especially if you have marginal elevation of the tower sump over the inlet of th e pump. There are many other situations that lead to pump performance problems and each can be solved once you have diagnosed the problem.

I agree that a centrifugal separator can under perform in an installation with variable flow, whether it is a pump with VFD, a system with 2-way valves or if you have multiple chillers. I would definitely leave the strainer in the piping. You can move it to the discharge side if your inlet pressure is marginal. You can add differential pressure sensors around the strainer to alarm a high pressure loss to the BMS and initialize service.

 

[Artisi]

As you have inline strainers advised in your posting together with sump inlet strainers, I would check the inline strainers for clogging and or excessive head loss.

 

[SAK9]

Geby,

Not sure if you are on the right path.Have a look at:

http://www.pressure-drop.com/Online-Calculator/pda/1107.html

Determine the pressure drop through the Y strainer.I am sure it would not be high enough to cause cavitation.If in doubt,take out the basket(w/o basket pressure drop~0) and see if there is a difference.

I would recommend focussing on other installation issues such as piping config,correct reducers,height of tower basin above pumps etc

 

[Artisi]

Seems the gremlins got at my earlier posting and deleted some of it.


As you have inline strainers advised in your posting together with sump inlet strainers,I think it would be appropriate to remove them and rely on the sump strainers. Although I would check the inline strainers for clogging and or excessive head loss as a check if this is the problem.

 

[sterl]

1) Open the Strainers

2) If the baskets are full of something organic or compounded, find out what it is and where it came from...Then pursue what can be done to eliminate it.

3) If the baskets are simply full of rust, slime and what have you; Throw these away and get something a little finer but about as solid as a rock sieve for a sump strainer...

4) If there's nothing in the baskets: Throw them away and investigate your pump levels...

5) If there's something in the baskets that you can't remove and can't avoid, but would not collect in an idle pump, move the strainers to the pump discharge...but buy them new baskets anyways.

Note that the baskets only survive in Case 5.