A reducer joins two pipe sizes. The choice between its two shapes is not cosmetic — using the wrong one at a pump can cause real operational problems.
The Two Shapes
Concentric reducer (CON). Symmetrical cone; both pipe centerlines stay aligned. The default choice for vertical lines and general service.
Eccentric reducer (ECC). One side is flat, so the centerline shifts by half the size difference. The flat side can face up or down depending on what must stay level.
The Pump Suction Rule
The classic application of the eccentric reducer is horizontal pump suction lines, installed flat side up (FOT — flat on top). Why: a concentric reducer in a horizontal suction line creates a high point where air collects; that trapped air reaches the pump and causes cavitation. Flat-on-top keeps the top of the pipe level, letting air move on to the pump where it is vented, and avoiding pockets.
Conversely, flat side down (FOB — flat on bottom) is used where complete drainage matters or where pipes rest on racks and the bottom must stay level.
| Situation | Use |
|---|---|
| Vertical line | Concentric |
| Horizontal pump suction | Eccentric, flat on top |
| Pipe rack, drainage lines | Eccentric, flat on bottom |
| General horizontal runs | Either per project spec |
Ordering Tips
- Specify both ends: “Concentric Reducer 6” × 4”, SCH40 × SCH40, A234 WPB, ASME B16.9” — wall schedule can differ per end if the design requires it
- Eccentric reducers cost slightly more than concentric in most sizes
- FOT/FOB is an installation orientation, not a product difference — the same eccentric reducer serves both
- Reducing a line by more than two sizes at once? Factories can make non-standard combinations to drawing, but two standard reducers in series is sometimes cheaper — ask for both prices
Full size combinations and lengths are in ASME B16.9; send us your list for a same-day quotation.
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