Reducers and enlargers on Up/Down stream of pumps - Pump engineering (2025)

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.

"Never ever" is a bit strong though.

If you've got a small line and for some reason need a pump which just happens to have a larger inlet flange, but the numbers (pressure drop, velocity, NPSH etc) all check out then what are you going to do - Not connect it just because someone in a book decided that?

Same thing on discharge.

Sure it's not "normal" or "usual", but "never ever"?? Old Moe is going a bit far.

Oh and there is no such thing as an "enlarger", there are only reducers (as per ASME B 16.9). A bit strange sometimes when you're used to thinking in terms of fluid flow, but in the piping world they don't care so they are all reducers.

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Oh and there is no such thing as an "enlarger", there are only reducers (as per ASME B 16.9)

Click to expand...

Good to know. In op's defense, Piping and Instrumentation Diagram Development (the book referenced by op) does use the terminology enlarger throughout section 6.14.1.2

This link shows that (if the link works, not sure):

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(2B)+(2B)' ?

Yes, I've seen it used before, especially by process engineers, but in piping terms an "enlarger" is just a reducer turned the other way in terms of flow direction.

You won't see it listed as "enlarger" on a piping diagram / isometric construction drawing and would get some strange looks if you tried it on site.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Agree with LI. Our process engineers are constantly calling them expanders/enlargers. In piping, there is no such thing. They're all reducers. That book appears to be written by a process engineer/consultant, so that's probably why you see it referred to as that.

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