Introduction
Reducers are used in piping systems to transition from one pipe diameter to another. They come in two configurations: concentric and eccentric. Choosing the correct type and installing it properly is critical for system performance, especially in pump suction lines. This FAQ explains the differences and proper applications.
Concentric Reducer
Design: The inlet and outlet centers are aligned - the reducer is symmetric around the pipe centerline. It looks like a cone centered on the pipe axis.
Best for: Vertical piping runs where the flow direction changes diameter but the centerline must remain vertical. Used in pump discharge lines, vertical process piping, and applications where the pipe centerline alignment is maintained.
Flow characteristics: Provides smooth, centered flow transition. No tendency to trap air or liquid.
Eccentric Reducer
Design: One side of the reducer is flat (parallel to the pipe centerline), while the other side tapers. The inlet and outlet centers are offset - the reducer is "eccentric" relative to the pipe axis.
Best for: Horizontal piping runs, pump suction lines, and applications where you want to maintain a flat bottom or flat top on the pipe.
Why Orientation Matters on Pump Suction
This is the most critical application for eccentric reducers. On pump suction lines, the eccentric reducer must be installed with the flat side facing UP. This prevents air pockets from forming at the top of the reducer, which could cause vapor binding, cavitation, and pump damage.
If the flat side faces down, the top of the reducer becomes a high point where air can accumulate, creating an air pocket that disrupts flow and damages the pump impeller.
On pump discharge lines, the eccentric reducer is typically installed with the flat side facing DOWN to prevent sediment or debris from settling in a low pocket.
Key Differences Summary
Shape: Concentric is symmetric; eccentric has one flat side.
Centerline: Concentric maintains the same centerline; eccentric shifts the centerline.
Vertical lines: Use concentric reducers.
Horizontal pump suction: Use eccentric reducers with flat side up.
Horizontal pump discharge: Use eccentric reducers with flat side down, or concentric if space allows.
Common Mistakes
Using a concentric reducer on a horizontal pump suction line, creating an air pocket.
Installing an eccentric reducer with the flat side down on pump suction, trapping air at the top.
Using an eccentric reducer where centerline alignment is required, causing pipe misalignment.
Forgetting that the wall thickness must match the mating pipes for pressure-containing integrity.
Conclusion
Both concentric and eccentric reducers serve essential but distinct purposes. Concentric reducers maintain centerline alignment for vertical or symmetric transitions. Eccentric reducers are essential for horizontal piping, especially pump suction lines where preventing air pockets is critical for pump reliability. Always follow the recommended orientation for the specific application.
References
1. ASME B16.9, Factory-Made Wrought Buttwelding Fittings.
2. HI 9.6.7, Rotodynamic Pump Installation Guidelines.
