I have written about this many times in some of my previous posts about what the problem is.  A few years ago, I had an 80 ton cooling tower, which was manufactured by Baltimore Air Coil (BAC), installed on one of my design projects in Toronto.  It sounds like you are having the same problem.  The cooling tower pumps were located at least 20 ft below the cooling tower so there was plenty of NPSHa but my pumps were not performing correctly.  Assuming your theoretical calculations say you have enough NPSHa I will reintegrate my story and how I solved it:

?The local BAC rep set the working water levels in the cooling tower "as they have always been doing it"

?When the system was put into operation, I had flow meters on my pump discharges and the readings were fluctuating wildly.
  
?I put my ear to the on the pump discharge and it did not sound like it was flowing full.  It sounded like water was dripping.

?The motor shaft/pump shaft couplings experienced short life

?I read the BAC O&M manual and it clearly states that the working water level in the tower is critical and if it is set too low you can get entrained air sucked into the pump.(From expererience I knew that you will get cavitation sounds and it can cause the pump to vibrate if is the air bubbles are very severe...you can also experience a very shortened pump/motor coupling life...which I experienced)

?I checked the BAC shop drawings but could not determine what the working water levels should be.

?The local BAC rep gave me the working water levels that they always set their cooling towers at.  Let's call this working water "level A".  It was confirmed by tape measurements.

?I phoned the BAC manufacturing plant in the US and asked them to send information on what the working water level should be.  They sent me two published documents.  I had to do some arithmetic and found that there was a discrepancy in both documents (the discrepancy was not apparent until one does the arithmetic!!!).  One document said the cooling tower working water level should be "level A".  The other document showed a higher working water level which we shall call working water "level B".  I discussed this with BAC and they were not aware of the two discrepancies.  They later advised "level B" was the correct setting.
 
In summary, once the cooling tower was set to "level B" the pipes sounded full, the flow meter readings are stable (except for one which slightly bounces) and the pump couplings are okay.
 
In order to give you a feel on the importance of the working water levels the following is an excerpt from my email to the contractor telling him of the water level discrepancies

1.The START TO FILL elevation in the cooling tower was found to be 10 ?" measured from the bottom of the pan.  From BAC's cut sheet that was emailed from the factory today it should be 13 7/8".

2.The STOP FILL elevation in the cooling tower was found to be 12 3/4" measured from the bottom of the pan.  From BAC's cut sheet that was emailed from the factory today it should be 14 15/16".

This small change in water level settings made all the difference in the world.  Check this lead out and tell us what you found out!  Send me your address....I'll send you my bill later.....  Sweage pumps &Sludge pumps

2011-06-07

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