The required capacity of the pump is normally dictated by the requirements of the system in which the pump is located. A process system is designed for a particular throughput. A vessel must be filled (or emptied) in a certain amount of time. An air conditioning system requires a particular flow of chilled water to do the job it is designed to perform.
Regardless of the pump system being designed, it is usually possible to arrive at a design flow rate for the pump. Sometimes there will be a planned duty cycle for the pump that will require it to operate at only a fraction of the full design capacity during certain periods and at higher or lower capacities at other times. Examples of this include a process plant with a variable capacity, or a chilled water system designed to meet a variable air conditioning load. The operating duty cycle of the pump should be estimated, using the best available process and operations estimates, as this helps in selecting the best type of pump and control system.