Why is the discharge pipe diameter of a centrifugal pump smaller than the suction pipe?
The suction pipe of a centrifugal pump is not necessarily a larger bore (diameter) than the discharge pipe. They can be the same size.However, the design philosophy differs slightly between the suction and discharge lines:Liquid lines are sized primarily on the basis of velocity. Typically, velocities are kept within the range 3 to 15 ft/s (approx. 1 to 5 m/s) as per API 14E.Suction lines are generally designed to keep friction losses to a minimum. This is in order to ensure sufficient available Net Positive Suction Head (NPSHa) to meet the NPSH required (NPSHr) by the pump. This is, in part, accomplished by using as large a diameter line as you can for the given service/conditions. (Other factors include minimising the number of bends and fittings, the use of long radius elbows, full bore valves, etc.)For discharge piping, sizing is determined by the available head and economic considerations.This often means that in practice the discharge line is one or two standard pipe sizes smaller than the suction line (but not necessarily).Edited to Add:Other standards that cover line sizing, such as NORSOK P-001, for example, specify different recommended maximum pressure drops for centrifugal pump suction and discharge lines.For a sub-cooled liquid P-001 recommends a maximum pressure drop of 0.25 bar / 100 m in the suction piping, but up to 0.9 bar / 100 m in the discharge piping. Depending on the exact configuration, this could allow for the discharge piping to be a smaller bore than the suction piping and still comply with the standard.