A parallel edge results in the two pipes

          A reducer is the pipe fitting component in a pipe line that reduces the pipe size from a larger to smaller diameter.          Generally the length of reduction is the average of larger and smaller pipe diameters. There are two types of Reducers are used in pipe lines, they are (1) Concentric reducer   (2) Eccentric reducer.          Depending on the Mach Number of the flow reducer can be used as Diffuser or as Nozzle.           Design guidelines and operating manuals mostly prescribe the selection of reducer. A reducer permits for a modification in pipe size to satisfy hydraulic flow requirements of the system, or to adapt to existing piping of a different size. Usually we use concentric reducer but eccentric reducer are used when there is requirement to maintain the same-top or bottom of pipe level. These are manufactured in inch and metric units too.You can refer to this link for check the reducer size based on ANSI B 16.9.          Generally in pumps the suction line is larger than inlet nozzle and also the delivery nozzle is smaller or same size as delivery line. Therefore, a reducer is required in both suction and delivery lines. But some people generally confuse that what type of reducer is used? Concentric or eccentric? and in which position?          In this topic we will share our experience about what type of reducer is used. we hope it will help you a lot.Concentric Reducer or Conical Reducer :          Concentric reducer is also called as Conical reducer. Concentric reducers are used to join pipe or tube sections whose ends are on same axis. It maintain the center line elevation. When larger pipe and smaller pipe center lines to be maintained on same axis in that case we use Concentric reducers.Uses of Concentric reducer:1. Concentric reducers facilitate in transporting slurries and abrasive liquids.2. They are useful in the service of cavitation.3. In the discharge lines of pumps Concentric reducers are used.Eccentric Reducer :          In Eccentric reducers fittings the center of both ends is not on the axis. Unlike concentric reducers, these have an edge which is parallel to the connecting line. This parallel edge results in the two pipes having offset center lines. For eccentric expander or increaser   same fitting is used in reverse.           Horizantal liquid reducers are always eccentric, having top flat (unless on control set, same as PV, TV, HV, LV) or pipe rack, which prevents the build up of air bubbles in the system. Eccentric reducers are used at the suction side of the pumps to ensure air does not accumulate in the pipe. The gradual accumulation of air in a concentric reducer could result in a large bubble that could eventually cause the pump to stall or cause cavitation when drawn into the pump.          Horizontal gas reducers are always eccentric, bottom flat set, which allows condensed water or oil to drain at low points.          Reducers in vertical lines are generally concentric unless the layout dictates otherwise.Uses of Concentric reducer:1. In this systems less turbulence or material entrapment2. Absorbs pipe wall and fluid borne noise and vibrations3. These are used with flat side up in pump suction to avoid cavitation.IMAGE SOURCE : GOOGLEEccentric reducers installation instructions:Case 1: When the source of supply is present below the pump, the eccentric reducers must be placed with the flat side at the top and vice versa. Eccentric reducers installation when source of supply is below or above the pump suction nozzle — Source: http://www.enggcyclopedia.comCase 2: In the case of horizontal pipe runs, air pockets are avoided by installing the eccentric reducer with the flat side up and vice versa. Eccentric reducers installation in case of long horizontal pipe runs — Source: http://www.enggcyclopedia.com