What changes between concentric and eccentric reducers?
Centerline alignment versus a continuous upper profile
A concentric reducer keeps the large and small pipe centerlines aligned. Both the upper and lower surfaces taper toward the smaller opening. An eccentric reducer offsets the centerlines and keeps one side of the transition straight. Rotating that fitting changes the position of the straight side.
In a horizontal suction-lift layout, a concentric reduction can leave the crown of the larger pipe above the smaller outlet. Gas collecting there may interrupt liquid flow into the pump. A suitable flat-on-top eccentric transition removes that particular local pocket; it does not remove air already entering through leaks or a poorly submerged intake.
When should the eccentric reducer be flat on top?
Suction lift: avoid a high point ahead of the pump
For suction lift, the liquid source is below the pump. A continuous rise toward the pump helps prevent trapped gas along the suction line. KSB’s Centrifugal Pump Lexicon, “Formation of air pockets,” calls for eccentric reducers in horizontal suction-lift lines. Xylem’s HL-series manual, section 4.2.2, specifically shows the horizontal side at the top and the sloping side below.
Flooded suction: follow the complete air path
Positive suction head means the source liquid level is above the pump inlet. That fact alone does not settle the reducer orientation. Trace the full pipe profile back to the source and identify the intended escape path for air. A flat-on-top detail copied from a suction-lift drawing may be wrong for a different approach.
Mahaffey and van Vuuren’s 2014 study, “Review of pump suction reducer selection: Eccentric or concentric reducers,” Table 1, distinguishes horizontal or rising approaches from a supply entering from above. Its CFD comparison also found more uniform outlet flow with appropriately designed concentric reducers in the modeled conditions. This supports a conditional choice between air transport and inlet-flow quality; it does not establish one fitting or angle for every pump.
Compare the reducer options for the actual inlet layout
First confirm that the pump manufacturer and piping design accept the proposed fitting, material and inlet arrangement. When both shapes are permitted, the tradeoff is between an offset profile that manages a potential gas pocket and a symmetric contraction that preserves the pipe centerline. Neither advantage proves the whole suction system is satisfactory.
Swipe to view the complete table.
| Condition to resolve | Eccentric reducer | Concentric reducer | Selection consequence |
|---|---|---|---|
| Horizontal suction lift with possible gas collection | A suitable flat-on-top profile avoids a local crown pocket | The reducing crown can create a gas-collection point | Use the eccentric arrangement required by the pump installation detail |
| Approved air-free horizontal inlet | Offset contraction changes the approach geometry | Symmetric contraction maintains the common centerline | Consider the concentric option when the manufacturer accepts that inlet layout |
| Source above pump; downward approach | Orientation can preserve the intended return path for gas | May suit the verified air-free arrangement | Check the whole elevation profile; positive head alone does not choose the fitting |
| Inlet centerline and elevation | Offset changes the relationship between the two pipe axes | Keeps the two axes aligned | Check the nozzle elevation and spool drawing before selecting either pattern |
| Nearby elbow, valve or strainer | Does not cancel an upstream disturbance | Does not cancel an upstream disturbance | Keep the required approach length and review the complete inlet geometry |
Reducer orientation does not establish NPSH margin
Check suction head at the limiting condition
Available net positive suction head, NPSHA, describes the suction energy available above the liquid’s vapor-pressure head at the pump reference location. It depends on source pressure, liquid level, temperature and suction losses. The pump’s required NPSH and the application margin must also be checked. The Hydraulic Institute’s “The Basics of NPSH & Pump Operating Regions” explains why a published NPSH3 value is not a promise of cavitation-free operation.
A correct reducer cannot compensate for a clogged strainer, excessive suction losses or inadequate liquid level. Check the limiting operating condition, including the lowest source level and the relevant maximum liquid temperature. Keep gas accumulation and cavitation as separate checks: they can both disturb pump operation, but they are not the same mechanism.
Use the pump-specific approach length
Do not apply one universal straight-pipe length. The HL-series checklist uses at least four pipe diameters between the inlet flange and the closest elbow; other pump instructions can differ. Record the distance, its reference points and the pump model on the layout. Count a reducer as straight pipe only if the governing installation detail permits it.
Worked choice: an 8 × 6 in. horizontal suction transition
This hypothetical comparison uses an NPS 8 suction line and an NPS 6 pump inlet. The material, wall schedule and pressure duty have already been accepted. Size labels identify the transition; no flow rate, NPSH margin or reducer angle is being inferred from them.
Case A: the installation requirement decides
Case A has a source below the pump, a continuously rising suction line and a pump manual requiring an eccentric reducer flat on top. Choose that orientation and show the straight side on the spool drawing. A concentric reducer is excluded by the installation requirement before price comparison.
Case B: compare two accepted inlet geometries
Case B has an air-free, flooded horizontal inlet to an axially split pump. Assume the manufacturer has reviewed the complete layout and accepts both reducer shapes, including the approach length and nozzle loads. Choose the concentric reducer when maintaining the common centerline and the symmetric inlet profile is the project’s preference. The eccentric option offers no needed gas-pocket benefit under those assumptions and would require a different elevation detail.
If Case B can ingest air at low tank level, its air-free premise no longer holds. Reassess the intake submergence, venting and pipe elevations before choosing a revised reducer arrangement. Simply changing the fitting to eccentric does not prove that air ingestion has been solved.
Specify both reducer sizes and the installed orientation
Separate fitting supply from spool orientation
For a butt-weld reducer, give the large and small nominal sizes, material specification, wall schedule or required end thickness at each end, fitting standard and quantity. “8 × 6 reducer” does not identify concentric versus eccentric geometry. Put the required pattern on the purchase line.
For an eccentric fitting in a fabricated spool, add the flat-side orientation, pipe centerline elevations, flow direction and drawing revision. Check the actual reducer length against the approved layout. These details belong with the butt-weld fitting schedule; the flange and pump-nozzle checks remain separate.
Frequently asked questions
Is an eccentric reducer always installed flat on top?
No. Flat on top is common for horizontal suction-lift arrangements. Source elevation, the complete gas escape path and the pump manufacturer’s layout determine the orientation; some suction-head arrangements use a different detail.
Can I use a concentric reducer on a horizontal suction line?
Yes, where the pump manufacturer and piping design accept the complete inlet arrangement and the air-pocket concern has been resolved. Horizontal orientation alone is not enough to approve or reject it.
Does a larger suction pipe eliminate cavitation?
No. It can reduce friction loss at a given flow, but source pressure, liquid level, vapor pressure, other losses and the pump’s required margin still matter. Reducer selection is only one part of the suction-system check.
What happens if the eccentric reducer is rotated 90 degrees?
The offset moves sideways and the top profile no longer matches a flat-on-top detail. Treat that as a layout change, not an equivalent installation, and verify the intended orientation before welding.
Do both ends of a reducer use the same wall schedule?
The purchase specification must state the required wall or schedule at both ends. Matching nominal schedule names does not necessarily mean matching wall thickness across two different pipe sizes; the weld-end requirements must suit the connected pipes.
Can this selection rule be used for slurry or gas service?
Do not transfer it unchanged. This article addresses liquid pump suction and gas-pocket control. Solids transport, drainage and gas service can introduce different requirements that need a separate piping and equipment review.
Technical Basis and Source Scope
KSB, Centrifugal Pump Lexicon, “Formation of air pockets”: suction-lift slope and eccentric fittings. R. M. Mahaffey and S. J. van Vuuren, “Review of pump suction reducer selection: Eccentric or concentric reducers,” Journal of the South African Institution of Civil Engineering, Vol. 56 No. 3 (October 2014), pp. 65–76, Table 1 and CFD discussion: conditional reducer selection and approach-flow differences. The modeled geometries do not establish a universal installation rule.
Xylem, HL Series Installation, Operation, and Maintenance Manual, section 4.2.2, printed p. 22: eccentric reducer orientation and the model-specific elbow-distance check. Hydraulic Institute, “The Basics of NPSH & Pump Operating Regions,” December 7, 2022: NPSHA, NPSH3 and application margin.
The 8 × 6 in. cases are hypothetical layout decisions with stated approvals. They do not calculate hydraulic performance or approve an installation.
