Users do not have an idea on what should be the minimum pump efficiency. So they are less likely to determine what is the best achievable efficiency for the pump. This defeats the purpose of determining the most efficient pump for the given duty point.

However, references point out that capacity / throughput can be used as basis in predicting acceptable efficiency as presented in Figure 12-17 of the Peters and Timmerhaus book or Figure 20.14 of the Sinnott book.

Moreover, Figure 2.42 of Pump Characteristics and Applications by Volk 3rd edition show the theoretically maximum efficiency obtainable by centrifugal pumps according to capacity and specific speed.

I strongly suggest to either remove the minimum efficiency as criteria and have the optimizer guess according to the data presented by P &T or Sinnott.

OR

Replace the minimum efficiency field with specific speed and utilize Figure 2.42 of the Volk book.