Discharge head

[Chiefkocak]

Gentlemen, I should remember this from school but its been 30 years. I have a 3000 gpm centrifugal pump that the only way to get the head needed is to reduce flow. But this causes many thrust problems with broken shafts. Slowing down the speed of the motor will reduce flow but how much will it reduce discharge head.

 

[BigInch]

Approximately by H1/rpm₁^2 * rpm₂^2

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[CH5OH]

Volume Capacity
q1 / q2 = (n1 / n2)

Head or Pressure
dp1 / dp2 = (n1 / n2)^2

Power
P1 / P2 = (n1 / n2)^3

 

[Chiefkocak]

Answered the question.
Ship originally installed the 3000 gpm sea water pump with load of close to rated gpm. About a year later AC chiller condenser tube went so they change it to FW cooled. This left
SW pump with little load but close to shutoff head was still needed to cool another AC condenser very high up in the ship. So throttling to get discharge pressure caused shafts to break. 20 years later Port Engineer wants to put a variable speed drive controller on pump. I told him slowing down the pump wouldn't do the trick.

 

[Artisi]

How will you "slow" the speed of the motor? If you are running a fixed speed motor the option will be to reduce the impeller diameter.