Concentric vs Eccentric Reducer
Pipe reducers transition between two different pipe sizes in a piping system. The two standard types are concentric (cone-shaped, centerlines aligned) and eccentric (offset, one flat side). Both are manufactured per ASME B16.9 for buttweld ends, in sizes NPS 1/2 to NPS 48, and the choice between them depends on service conditions, orientation, and piping layout.
Comparison Table
| Feature | Concentric Reducer | Eccentric Reducer |
|---|---|---|
| Centerline alignment | Coaxial (centered) | Offset (one flat side) |
| Cross-section | Symmetrical cone | Asymmetric transition |
| Air/vapor pockets | Possible at top | Eliminated (flat on top) |
| Liquid drainage | May trap liquid at bottom | Full drainage (flat on bottom) |
| Pump suction | Rarely used | Preferred (flat on top) |
| Vertical piping | Standard choice | Not typically used |
| Pipe rack routing | Centerline shifts | Maintains BOP or TOP |
| ASME B16.9 coverage | Yes | Yes |
| Cost | Baseline | 5-10% premium |
| Stress analysis | Simpler | Slightly more complex |
Key Differences
Centerline Behavior
A concentric reducer keeps the pipe centerline aligned on both ends. This simplifies stress analysis and is the default for vertical piping runs, symmetrical headers, and applications where gravity drainage and air accumulation are not concerns.
An eccentric reducer offsets the transition so that one side remains flat. This flat side can be installed at the top (flat-on-top, or “FOT”) or at the bottom (flat-on-bottom, or “FOB”) depending on service requirements.
Pump Suction Lines
Eccentric reducers with the flat side on top are standard practice on horizontal pump suction lines. This configuration prevents air or vapor pockets from forming at the reducer, which would cause cavitation and damage the pump impeller. Concentric reducers in this service create a high point where gas can accumulate, leading to unstable pump operation.
Pipe Rack and Gravity Drainage
On pipe racks, eccentric reducers with the flat side on the bottom maintain a consistent bottom-of-pipe (BOP) elevation. This keeps pipe supports at a uniform height and simplifies structural steel design.
For gravity drain lines and slurry services, a flat-on-bottom orientation ensures complete drainage and prevents solids from settling in the transition.
Vertical Piping
In vertical runs, the size change is symmetrical around the pipe axis, so concentric reducers are the standard choice. Eccentric reducers serve no purpose in vertical orientation unless a specific offset is required to align with equipment nozzles.
Specification
Both types are covered by ASME B16.9 for buttweld fittings and are available in all standard carbon steel, alloy, and stainless steel materials. Forged reducers per ASME B16.11 are available for socket weld and threaded connections in smaller sizes, though forged reducers are typically concentric only.
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