Eccentric Suction Reducer and Concentric Discharge Reducer

By the way, the pump operating point (total dynamic head) with the friction losses that may be created due to reducers, match the rated point as per the pump curve.

There's usually no choice. Typical velocities based on pump suction and discharge sizes are quite high, and could cause excessive headloss if the same sizes were used for the pipe runs, specially if if long lengths are involved. So you have to use reducers, and include the additional friction loss caused in the pump duty.

Review this recent thread.

That (recent thread) explains it nicely.

But that deals mainly with where to use concentric and eccentric reducers. Only #7 mentions velocities. I believe current query is mainly about whether reducers are needed at all.

I thought you had already addressed that in post #2. How else to connect one size pipe to another size nozzle?

Sir,

1. I know of two types of reducers--threaded ones often called bushings where the change in pipe diameter is abrupt, and tapered ones (usually with flanges). For the first, there can be substantial turbulence and friction loss, but with the second, the turbulence and friction loss is fairly low (quite low if the taper is gentle). I suspect your question involves the use of tapered reducers.

2. Cavitation occurs if the available suction head pressure at the operating flow rate in the impellers is below the boiling point of the liquid being pumped at the liquid's temperature. A pretty good approximate measurement of the risk for cavitation is to measure the suction head pressure.

3. An oversized suction line will reduce the friction losses on the suction side of the pump, resulting in a higher suction pressure and a lower risk of cavitation.

4. An oversized discharge line will reduce the friction loss component of net head pressure and can thus increase the flow the pump is producing. This results in a higher risk of cavitation because the higher flow rate through the impeller has the effect of reducing the pressure inside the impeller.

5. You say the pump is operating within its designed pump curve. As long as the suction head pressure is above the minimum specified by the pump manufacturer, then you should have no problems with cavitation. The engineer's concern with turbulence is minor.

--JMM