Eccentric Reducer
General / GeneralDefinition
An eccentric reducer transitions between two pipe diameters with the centerlines offset (eccentric) so that one side (the 'flat side') remains on the same plane. The other side tapers at an angle to reach the smaller diameter. This offset design is essential in horizontal piping where maintaining a consistent top or bottom elevation prevents air pockets (flat side up) or liquid drainage (flat side down). Eccentric reducers are one of the most important fittings for proper pump installation and horizontal piping layout.
Working Principle
Unlike a concentric reducer where both centerlines align, an eccentric reducer offsets the small end centerline from the large end centerline. The offset equals half the difference between the two diameters. When installed flat side up (FSU), the top of the pipe remains at the same elevation through the reducer, and any air or gas pockets can vent downstream along the flat top surface. When installed flat side down (FSD), the bottom stays at the same elevation, allowing liquid to drain through the reducer in a gas or vapor line. The internal flow transition is the same smooth taper as a concentric reducer, just offset from the centerline.
Key Components
| Large end | Connection to the larger pipe diameter |
| Small end | Offset connection to the smaller diameter |
| Flat side | The side maintaining constant elevation (top or bottom) |
| Taper side | The angled side transitioning between diameters |
| End preparation | Beveled (butt-weld), socket, or threaded |
Types & Variants
| Orientation | Flat Side Position | Prevents | Used On |
|---|---|---|---|
| Flat side up (FSU) | Top of pipe is level | Air pockets in liquid lines | Horizontal pump suction, liquid pipelines |
| Flat side down (FSD) | Bottom of pipe is level | Liquid accumulation in gas lines | Horizontal gas/vapor headers, compressor suction |
| Flat side left/right | Side of pipe is level | Used for pipe rack elevation matching | Specific routing requirements |
Materials
Same materials as concentric reducers: ASTM A234 WPB (carbon steel), A234 WP11/WP22/WP91 (alloy steel for high-temperature), A403 WP304/316/321 (stainless steel), A402 WPL3/WPL6 (low-temperature), A860 WPHY (high-yield pipeline). Socket-weld and threaded eccentric reducers in A105 (carbon), A182 F304/F316 (stainless) per ASME B16.11.
Applicable Standards
ASME B16.9 defines dimensions for butt-weld eccentric reducers (end-to-end length, offset, out-of-roundness tolerances). ASME B16.11 covers socket-weld and threaded versions. ASME B31.3 specifies the orientation rules (flat side up for pump suction, flat side down for gas lines) as standard piping design practice. API 610 (Centrifugal Pumps) requires eccentric reducers on horizontal pump suction lines with the flat side up to prevent air accumulation.
Applications
Pump suction piping (horizontal): eccentric reducer flat side up is mandatory per API 610 and good engineering practice to prevent air pockets from entering the pump. Horizontal gas and vapor pipelines: eccentric reducer flat side down allows condensate to drain and prevents liquid holdup that could cause slugging. Compressor suction headers: flat side down to keep liquid from entering the compressor. Expansion loops on horizontal pipe racks: eccentric reducers maintain the bottom-of-pipe elevation to keep consistent support heights. Any horizontal piping where air or liquid accumulation must be avoided.
Advantages
- Flat side up prevents air pockets in horizontal liquid lines
- Flat side down drains liquid from horizontal gas lines
- Maintains pipe elevation on one side through the size transition
- Mandatory for proper pump suction piping per API 610
- Same pressure and temperature ratings as concentric reducers
Disadvantages
- More expensive than concentric reducers (sli