I’m reading “Piping and Instrumentation Diagram Development” by Moe Toghraei. In the centrifugal pump section, it says:
“On the suction side of centrifugal pump we never ever use and enlarger, and on the discharge side of a centrifugal pump we never ever use a reducer.”
Can anyone help me understand this?
You want the suction piping to be as big as possible (within reason, there are minimum velocity guidelines as well) to reduce NPSH loss for centrifugal pumps. This reduces the risk of cavitation occurring which is damaging to the pump. The author is saying, for example, if you had a 3" suction connection you would not supply 2" suction piping and enlarge to 3" at the pump.
The rule of thumb about avoiding reducing on the discharge side of the pump is primarily to avoid pressure drop issues. If your pump has a 4" discharge and you immediately reduce down to 3", your velocity will be higher which correlates to higher losses; additionally valves and other components will experience significantly larger pressure drop as well.
Above is a snippet.