Normally the pump nozzles are smaller than inlet / outlet piping. That is why we need to use reducers and expanders at the pump suction and discharge piping. We have a choice of using concentric or eccentric reducers.
Using eccentric reducers for centrifugal pump suction piping
Eccentric reducers are typically installed at the centrifugal process pump suction nozzles in order to facilitate proper transition from the the larger diameter (low flow velocity, moderate friction loss) suction piping to the pump suction nozzle.
You must ensure proper installation of eccentric reducer in pump suction piping to minimize accumulation of any air or gas bubbles in the suction. If concentric reducers are used, their upper part can allow accumulation of trace amounts of air or vapors at the pump suction. This can lead to creation of bigger gas bubbles near the pump suction.
If these gas bubbles manage to escape from the reducer and get into the pump suction, resulting cavitation is damaging for the pump. Otherwise, if they stay trapped in the reducer, they reduce the available cross sectional area of the pipe for liquid flow. This results in higher liquid velocity in the pump suction, increasing frictional losses. It also affects the overall performance and long term reliability of the pump.
Orientation of eccentric reducers
When the source of supply is above the pump, then the eccentric reducers must be placed with the flat side at the bottom.
Picture 1 - Eccentric reducers installation when source of supply is below or above the pump suction nozzle
In case of long horizontal pipe runs, air pockets are avoided by installing the eccentric reducer with the flat side up.
Picture 2 - Eccentric reducers installation in case of long horizontal pipe runs
Straight length requirement for pump suction piping
Higher liquid velocities and higher turbulence in the pump suction can sometimes cause cavitation in the pump. However you can minimize this by installing a sufficient length of straight run pipe before the pump suction.
Whenever a low point exists at the pump's suction line and a concentric reducer is used at pump suction nozzle, it is possible to have vapor accumulation close to the pump suction nozzle (Picture-2). In such cases, straight horizontal pipe between reducer and pump suction should be kept to a minimum. In such installations, the reducer flange is often directly connected at the pump's suction nozzle. There is no straight length of piping between the reducer outlet and the pump nozzle.
Straight pipe lengths are however connected to the inlet flange of an eccentric reducer.
Usually, a length equivalent to 5 times the pipe size is considered appropriate for the suction piping of a centrifugal pump. In some cases it may even be considered to install 5D length of a straight pipe run in pump suction next to the reducer flange.
In general it is acceptable to have a straight length of 5D to 10D between the pump suction and reducer, where D is the pipe size.