Influence of the regulation mode in the selection of the number of Fixed Speed Drives (FSD) and Variable Speed Drives (VSD) pumps in water pumping stations.

Proper design of a pumping system requires the use of the pump curve and the system setpoint curve. Both have to be as close as possible so that the energy used is optimal. This is achieved by means of regulation systems, in which the type of control to be used (flow or pressure) and the combination between Fixed Speed Drives (FSD) pumps and Variable Speed Drive (VSD) pumps are involved.
 

The objective of this work is to determine the optimal number of FSD and VSD pumps for each flow rate range in order to discuss the classic design of pumping stations and their regulation modes. For this, a methodology consisting in defining a parametric form of the pump curve, the efficiency curve and the set-point curve in relation of the best efficient point is applied. In this way, dimensionless expressions are obtained and the influence of the set point parameters in the design of regulation modes can be analyzed. Additionally, the method includes to get an expression to estimate the performance of the frequency inverter based on the load and pump speed rotation.

The application of the methodology to different studie cases allows questioning many classic methods procedures for pumping stations. In summary, it can be concluded that the appropriate number of fixed speed pumps for each regulation mode cannot be established in advance but requires an in-depth study of the different options available.