If i change my pump motor from 18.5 kw to 11kw is there a way to calculte the new flow and pressure the pump will deliver( assuming the impellers etc remain the same)
If one has a performance curve for the pump, and you know where on that curve the pump normally operates, then one can calculate the BHP to determine if the hydraulic horsepower is less than or equal to the motor horsepower.
In English units: BHP = [head(FT) x flow(GPM) x SG] / [3960 x eff.] Convert the resulting BHP to kw units and if it is less than 11 kw, then there should be no change in the pumps operating point.
To answer the question, changing the HP of the electric motor driving the pump has no effect whatsoever on the pump's operating point, unless the RPM's also change.
Its possible that the smaller motor will run more efficiently, if it is loaded within the optimal range. For example, an 11 kw motor delivering 10 kw (90.9% load) should actually run at a higher efficiency than an 18.5 kw rated motor delivering 10 kw (54% load).
It will depend on the kW needed to drive the pump at the duty - do you know how many amps the 18.5 kW used when operating? Knowing this, there is a chance that we can make an estimate if you can use the 11kW - also can the flow be reduced - if you can afford to reduce flow by throttling the discharge via a valve this will further reduce the amount of power input which might allow the use of the 11kW motor.