Something that might be considered -- trim down the impeller diameter.

Pump Affinity Laws for a Specific Centrifugal Pump

Volume Capacity

The volume capacity of a centrifugal pump can be expressed like

q1 / q2 = (n1 / n2) (d1 / d2) (1)

where

q = volume flow capacity (m3/s, gpm, cfm, ..)

n = wheel velocity - revolution per minute - (rpm)

d = wheel diameter (m, ft)

Head or Pressure

The head or pressure of a centrifugal pump can be expressed like

dp1 / dp2 = (n1 / n2)^2 (d1 / d2)^2 (2)

where

dp = head or pressure (m, ft, Pa, psi, ..)

Power

The power consumption of a centrifugal pump can be expressed as

P1 / P2 = (n1 / n2)^3 (d1 / d2)^3 (3)

where

P = power (W, bhp, ..)

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Assuming the desire for slowing the pump rotation rate is to minimize power loss into the pump, the impeller diameter could be reduced to the point of requiring the same power input as standard but at the increased rpm.

For instance, using the above relations, and the assumptions of n1= 5000, n2 = 8000, d1 = 2.5, d2 = ?, we find that d2 = 1.56 would demand the same power at 8000, and produce the same flow rate and pressure.

If this impeller is then run at 5000, the power would be .24 of that at 8000, with flow rate fraction of .63 and pressure of .39.

This modification does not, however, treat any tendency the pump may have to cavitate at the higher speed.