Condensate pump installed for cooling tower

[sunny1230]

there is chilled water system having cooling towers.the suction pipe from the cooling tower outlet to the inlet of the condensate pump is sized having a flow velocity of 9 feet/second(it is quite high).when calculated NPSHa for the same system and compared it with NPSHr then it was found that the difference between NPSHa & NPSHr is about 7 feet.NSPHa is more than required.

my question is that will there be cavitation in system because of smaller suction pipe even though NPSHa is more than NPSHr. if there will be cavitation then how to get rid of it?

 

[sunny1230]

please reply fast

 

[Artisi]

If NPSHa is greater than NPSHr by 7ft, then there shouldn't be any major problem, however a sketch of the system and your calculations wouldn't hurt to make sure.

Fast enough reply?
P.S.
A "thank you" is never missed.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[LittleInch]

Agree with artisi. Seven feet above the NPSHr should be enough to avoid cavitation - remember the NPSH curve is NOT the cavitation curve, this can be several metres above the NPSH curve depending on your pump and where it is on the curve (further to the right is usually where you get problems), but is so variable we can't advise remotely. Turn it on and see what happens....

If it cavitates then either increase the static head, lower the friction losses (bigger pipe) or install a bigger slower speed pump or a pump with an inducer on.

PS, A 5 hour delay in responding on a Sunday afternoon isn't something to get upset at....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[GPRnD]

You didn't give details of the pump NPSHr, but assuming this is a flooded system, that suggests a pump NPSHr of around 15 to 20 ft.

So to answer your question, no that won't be sufficient to completely suppress cavitation. To do that you'd need a NPSHa of > 80ft.

However what is more important is that you operate with the NPSH margins recommended by HI 9.6.1 (2012). For cooling tower pumps the ratios recommended are 1.1 in the POR and 1.3 in the AOR.

 

[ProcessHVAC]

You also haven't provided any details in regard to the suction layout, you may have enough NPSH but if the suction has a poor design you may have some problems. Check Hydraulic Institute standards.

 

[1gibson]

Is the 9 ft/sec at the rated flow or at end of curve? Same question with NPSHA/NPSHR margin.

 

[sunny1230]

Thank You to every body for their kind feed back and suggestions. i will upload the drawings and calculations along with the photos from the site to understand better.
In mean time may i know what is recommended maximum velocity limit for the suction line?

let me explain a bit about the project. there are 3 cooling towers(counter flow)having capacity of each 1500 TR.cooling towers are installed on a steel structure that is 2.5 meter high from floor level. Condensate pumps(3D+1S)having a flow rate of 4500 USGPM each are located on same floor level. Total water flow from cooling towers is 1500 TR x 3gPM/TR x 3 cooing tower= 13500 GPM.Suction header pipe from cooling towers is sized to be 600 mm having velocity of about 3 M/s.
As per Manufacture curve,NPSHr is 4 meter where as NPSHa is 6 meter.

Today pumps are started again but unable to get design flow & head when run at 50 HZ. all pumps are equipped with VFD.we are getting 70% of design flow rate.Thanks

 

[LittleInch]

Will be good to see the information as it sounds rather more complex than first appeared.

Below about 1m/second the friction losses start to be come nearly negligible, especially for short distance lines, but other than that the maximum velocity is what your system needs to perform in the lowest possible cost

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way