Selection for optimum life cycle cost
It is essential to commence the pump system selection with the hydraulic requirements, and to work systematically to select the pump, motor and drive. When the pump maximum duty is known, the peak power and speed for the drive will become clear.
It is common to oversize system components (pumps, motors and drives) however this practice is not recommended since it leads to higher life cycle costs.
When selecting a rotodynamic pump in combination with a VSD, for a system with static head, the pump shall be selected such that the maximum flow rate is to the right hand side of the best efficiency point (BEP). For constant flow regulated systems, the recommendation is to select the pump to the left of BEP at maximum pressure.
All operating conditions must be considered when designing the system (see Figure 5.1). Some operating profiles may best be satisfied by installing multiple pumps, which could be fixed or variable speed. Using the specific energy calculation in Section 5.2 will verify the best solution from an energy point of view. This will also lead to the lowest maintenance costs.
Having established the working range and power required, it is possible to size the motor(s) and drive(s).