Concentric vs Eccentric Reducer
Identical lengths, different centrelines — and a flat-side-up rule on pump suction that is worth getting right the first time.
- End-to-end length
- Identical
- Concentric
- Common centreline
- Eccentric
- Common tangent — one flat side
- Length set by
- The larger end
Short answer. Both reducers change pipe size over the same end-to-end length; the difference is where the centreline goes. A concentric reducer keeps both ends on one centreline. An eccentric reducer keeps one side flat, offsetting the centreline by half the diameter difference. Use concentric on vertical runs, and eccentric on horizontal runs where trapped vapour or trapped liquid would cause a problem.
Same length, different centreline
ASME B16.9 gives the concentric and eccentric reducer the same end-to-end dimension H in every tabulated size, and that length is set by the larger of the two ends. Swapping one for the other therefore does not change the spool length — only the centreline elevation.
Eccentric offset = (larger ID − smaller ID) / 2
Dimensions are given in inches, with the millimetre equivalent beneath in grey.
| Larger end NPS | Concentric end-to-end H | Eccentric end-to-end H | Difference |
|---|---|---|---|
| NPS 1/2 | 1.5038.10 mm | 1.5038.10 mm | Identical |
| NPS 3/4 | 1.5038.10 mm | 1.5038.10 mm | Identical |
| NPS 1 | 2.0050.80 mm | 2.0050.80 mm | Identical |
| NPS 1 1/4 | 2.0050.80 mm | 2.0050.80 mm | Identical |
| NPS 1 1/2 | 2.5063.50 mm | 2.5063.50 mm | Identical |
| NPS 2 | 3.0076.20 mm | 3.0076.20 mm | Identical |
| NPS 2 1/2 | 3.5088.90 mm | 3.5088.90 mm | Identical |
| NPS 3 | 3.5088.90 mm | 3.5088.90 mm | Identical |
| NPS 3 1/2 | 4.00101.60 mm | 4.00101.60 mm | Identical |
| NPS 4 | 4.00101.60 mm | 4.00101.60 mm | Identical |
| NPS 5 | 5.00127.00 mm | 5.00127.00 mm | Identical |
| NPS 6 | 5.50139.70 mm | 5.50139.70 mm | Identical |
| NPS 8 | 6.00152.40 mm | 6.00152.40 mm | Identical |
| NPS 10 | 7.00177.80 mm | 7.00177.80 mm | Identical |
| NPS 12 | 8.00203.20 mm | 8.00203.20 mm | Identical |
| NPS 14 | 13.00330.20 mm | 13.00330.20 mm | Identical |
| NPS 16 | 14.00355.60 mm | 14.00355.60 mm | Identical |
| NPS 18 | 15.00381.00 mm | 15.00381.00 mm | Identical |
| NPS 20 | 20.00508.00 mm | 20.00508.00 mm | Identical |
| NPS 22 | 20.00508.00 mm | 20.00508.00 mm | Identical |
| NPS 24 | 20.00508.00 mm | 20.00508.00 mm | Identical |
H is set by the larger of the two ends, so a 6 × 4 and a 6 × 2 reducer share the same length.
Which way up the eccentric goes
This is the detail that gets built wrong, and it is worth being explicit about because the two orientations solve opposite problems.
| Location | Orientation | Why |
|---|---|---|
| Pump suction, horizontal | Flat side up (FSU) | A concentric reducer would form a high pocket at the top where vapour collects, and the pump would cavitate. Flat side up leaves no high point. |
| Horizontal run, liquid service | Flat side down (FSD) | Keeps the bottom of the line continuous so liquid drains and no low pocket forms. |
| Horizontal run, gas or steam | Flat side down (FSD) | Lets condensate run through rather than pooling in a low spot at the size change. |
| Vertical run | Concentric reducer | There is no top or bottom to trap anything, so the symmetric fitting is correct and cheaper. |
Flat side up on pump suction is the single most frequently cited reducer rule, and the one most often found installed backwards.
Concentric reducer
- Vertical runs, in any service
- Anywhere the centreline must be maintained
- Symmetric flow, so no induced swirl or asymmetric wear
- Slightly cheaper and more widely stocked
- Traps vapour at the top on a horizontal liquid line
Eccentric reducer
- Horizontal runs where a pocket would cause trouble
- Pump suction lines — flat side up, always
- Lines that must drain completely
- Steam and condensate service
- Offsets the centreline, which the pipe support design has to account for
Dimensions: concentric reducer · eccentric reducer · all B16.9 fittings · pipe sizing.
Common questions
Are concentric and eccentric reducers the same length?
Yes. ASME B16.9 gives both the same end-to-end dimension H for a given larger end size, so substituting one for the other does not change the spool length.
Which way up does an eccentric reducer go on a pump suction?
Flat side up. A flat-side-down or concentric reducer creates a high pocket where vapour collects, which leads to cavitation at the pump.
When should I use a concentric reducer?
On vertical runs, and anywhere the centreline needs to stay put. On vertical pipe there is no pocket to worry about, so the symmetric fitting is correct.
What sets a reducer's length?
The larger of the two ends. A 6 × 4 reducer and a 6 × 2 reducer have the same end-to-end dimension because both have a 6 in end.