When comparing the differences in diameter on the suction and discharge pump nozzles, a number of considerations are taken into account.

The discharge nozzle size of a pump is sized to relate to the line velocity of the flow for which the pump is designed.

The suction nozzle size is usually sized one size larger than the discharge nozzle. Some exceptions are noted in the case of solids-handling pumps where the suction nozzle size is made the same size as the discharge nozzle so that particles that enter the pump can also exit the pump. This assumes that the impeller design in that pump will also pass the same particle size.

The reducer on the inlet side of a pump ensures that the velocity of liquid in the suction line is slowed sufficiently to provide a smooth flow of liquid with minimized friction losses in the line to ensure as high as possible an NPSH available from the system.

The increaser on the discharge side of the pump is designed to increase the pipe diameter from the pump nozzle to reduce the velocity of the liquid in the discharge line in order to reduce the total head the pump is required to develop to operate the system.