Pump performance at different frequency

[kloroform]

hi all,

i want to utilize an existing 60 hz centrifugal pump in the 50 hz electrical system in the plant. i need to evaluate the discharge pressure of this. pump performance curve available from the vendor is only available at 60 hz - 3560 RPM. I have tried to contact the vendor but somehow they did not respond.

is there any equation to relate frequency to head/discharge pressure of centrifugal pump ? i want to evaluate the discharge pressure of a 60 hz 3560 RPM centrifugal pump if it is operated at 50 hz and 290 gpm

thanks in advance

 

[DubMac]

Easy. Flow varies directly with the speed increase. Head varies as the square of the % increase.

These are the basic "laws of affinity" for centrifugal pumps. Easily Googleable.

 

[Artisi]

Or decrease in speed. 50/60 x flow -- (50/60)^2 x head.
As pointed out by DubMac - first 5 minute of basic 101 pump lessons.

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.)

 

[kloroform]

ok thanks all for your very informative responds



If the wheel diameter is constant, change in pump wheel velocity can simplify the affinity laws to

dp1 / dp2 = (n1 / n2)2

n = wheel velocity - revolution per minute - (rpm)
dp = head or pressure


but...how do i calculate the rpm ? if i change the frequency ?

(50/60) x 3560 = 2967 RPM

so, if the discharge pressure of the pump at 60 hz is 193 psig. it means the discharge pressure at 50hz is :

193/dp2 = (3560/2967)^2
dp2 = 134 psig

am i wrong ?




and how about the NPSHr ? does it remain constant ?
at 60 hz, the NPSHr is 12.5 ft

 

[Artisi]

You really need to check with the motor manufacturer for motor speed if you need an exact speed - but using the speed you calculated will suffice for normal discussion.

Your 50Hz pressure of 134 psi based on above is OK.

NPSHr, again - unless accuracy is needed NPSH can be assumed to remain fairly constant, but again, if accuracy is needed ask the pump manufacturer for 50Hz NPSHr curve.

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.)