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this is an existing application. Four double suction centrifugal pumps most of the time the operators run three in parallel in order to provide enough pressure in the system for a certain demand. According to the pump manufacturer's curve they should be pumping the same flow at the same pressure easily with only two pumps running. The question is why. According to some documentation made available to me two of the four pumps were refurbished two years ago, impellers replaced claiming damage caused by cavitation.
The pumps are fed off of a 36 suction pipe, plenty of NPSH available. Three of the four pumps have inadequate suction piping or at least appeared to be. Flow branches off of a 36"x16" Tee; goes through 16" valve, followed immediatly with 16"x12" ecc. reducer and 12"x 10" concentric reducer connected to the pump flange. Flow is between 1,500 and2,300 gpm per pump (if divided by three (total flow 4,500 to 6,000 gpm. I am suppose to recommend the improvements to the system so they should utilize two instead of three pumps. Is it possible that all this happens due to cavitation cause bu assumed imbalanced flow on double suction pump. Would the flex vane or similar thing help? Coul final concentric reducer have an impact? I would appreciate any comments.

Hi Water,

Quote "most of the time the operators run three in parallel in order to provide enough pressure in the system for a certain demand

Please clarify, are you just increasing the normal/rated flow or you will use the same pumps to supply other facility in a certain demand or condition if it so your TDH will increase hence you will not meet enough pressure.

Quote "According to the pump manufacturer's curve they should be pumping the same flow at the same pressure easily with only two pumps running. The question is why. According to some documentation made available to me two of the four pumps were refurbished two years ago, impellers replaced claiming damage caused by cavitation."

This is confusing on the design, if two pumps can. why you have 4 pumps in parallel?

Quote "The pumps are fed off of a 36 suction pipe, plenty of NPSH available"

Mostly the NPSHr given by the pump manufacturer are only intended at rated flow (for a one pump only). Please check the NPSHr at end of the curve for a 4 pumps in parallel.

Lowering the number of pumps is a remedy two pumps one standby will be fine plus proper piping system for a parallel pump configuration.

Please reconfirm your duty condition first(i.e. flow, head, suction pres., etc.) and check pump capability vs these. Then we can go for proper piping system.

gud luck.. give me update.

Maybe I wasn't clear enough. What I am trying to figure out is why the pumps are currently pumping way below their design points? Fourth pump should be a spare pump. Three pumps were originally designed for much higher demand than today. The current demand should be easily handled by two pumps, unfortunately the operator is forced to run three pumps in order to maintain the required pressure in the system. So basically the pump curves look like as if the impellers are trimmed to a smaller diameter. New impellers are installed about two years ago, but the flow/pressure characteristics appear to remain the same.

Hi,

Yes you should check the correct impeller trim required. If problem is same. You may check the following:

a. Check the impeller wear rings and impeller setting, it may be worn out or the impeller is not properly set.

b. You may also check the flow meter and the check valve if still ok.

c. Check the driver as well. I mean the rotation if correct, motor speed if you are using VFD, voltage condition also.

d. Check the suction pipes etc., there maybe a leak or damage, check air or gas introduction in suction and in stuffing box as well.

You may also check your system head (actual TDH). Review all losses.

Worst is check the pump capability to a parallel operation.

Hope this helps.

give me update.

Otherwise you can provide me a copy of pump curve size and motor rating for me to evaluate the pump.

Thanks for your response. My problem unfortunately is to figure out why the pumps are pumping LESS than they should. Each pump is putting out half gpms as they should based on their certified curves. I know that two of them have been completely rehabed, with visible signs of cavitation. The question is why cavitation occurs and what to do to prevent. Again NPSH required is 23 feet. The pressure gauge at the suction manifold shows 1.5 psi. My main concern is inadequate length of a straight suction pipe. From a common suction header there is only 52" to pump's suction. That includes 36"x16" "T", 16" valve and 16"x12" and 12" x10" eccentric reducers.

the flow to each pump is between 1,500 and 2,500 gpm.

,

first why is cavitation occurs?

Main is you have inadequate NPSH available. It seams that the NPSH require of 23 feet is quite high. Please recalculate your NPSH available.

For the suction pipes and "T" and elbows.

How far is your "T" from the suction of the pump?

Please note that elbows and "T" at pump suction can cause trouble specially on double suction pumps. It must be at least 2D away from your eccentric reducer. A straight run of pipe will do. This will eliminate noise and unstisfactory operation.

It seems theres a lot of jobs. Just I said first you must calculate your NPSH avaible. then we can go on the next stage. If your having problem in the computation let me know.

Regards