Concentric Vs. Eccentric Reducer

In piping systems, reducers seamlessly connect pipes of different diameters, ensuring smooth fluid flow. Reducers are broadly categorized into two types based on their geometry: concentric reducers and eccentric reducers. Knowing how these two pipe fittings are different and what's good about each one can help make your pipes work better and last longer.

This article explains two types of pipe connectors that change size: concentric reducers and eccentric reducers. You'll learn what each one does, their good points and differences, when to use them, how to put them in, and how to pick the right one.

What Is Concentric Reducers?

A concentric reducer is shaped like a funnel. The big end and the small end are centered, so a straight line runs perfectly through the middle of both openings. Visually, it resembles a truncated cone placed centrally between the two pipes.

The walls of the concentric reducer get thinner evenly from the big end to the small end, all the way around the center. This symmetry ensures that the wall thickness transition is even around the entire circumference.

Fluid entering the concentric reducer experiences a gradual reduction in cross-sectional area as it progresses towards the outlet. This design aims to promote a relatively smooth, symmetrical flow path. But when you lay this funnel-shaped reducer flat (horizontally), the slanted walls naturally make the top of the reducer a "high point" and the bottom a "low point".

The concentric reducer's purpose is to effect a change in pipe diameter while maintaining the centerline elevation of the pipeline. Its even design works well where the liquid needs to flow straight down the middle of the pipe, or where it isn't a big problem if some liquid might gather inside the pipe part itself (this often happens in pipes going straight up and down).

What Is Eccentric Reducers?

Unlike other types, an eccentric pipe reducer has an uneven shape. The centers of the large pipe and small pipe are side-by-side but not in a straight line.This offset results in one side of the reducer being straight while the opposite side tapers.

The main feature is the parallel offset of the centerlines. This results in two different configurations depending on the installation direction:

• Flat on Top: The straight side is positioned at the top of the pipe.

• Flat on Bottom: The straight side is positioned at the bottom of the pipe.

This uneven shape prevents a bump inside the pipe fitting when it's laid flat. This lets the bottom (or top) of the big pipe line up perfectly with the bottom (or top) of the small pipe.

An eccentric pipe reducer changes the pipe size. Its uneven shape prevents trapped air or pockets where liquids or solids could collect. It also creates extra space for pipes or structures underneath. It achieves a constant bottom-of-pipe or constant top-of-pipe elevation in horizontal lines.

Concentric and Eccentric Reducers Advantages

Concentric and eccentric reducers offer distinct advantages derived from their shape:

Concentric Reducer Advantages:

• Symmetrical Flow: Provides the most uniform flow path reduction in vertical piping, minimizing asymmetric turbulence.

• Simplified Support: Symmetry often simplifies pipe support design and installation alignment.

• Uniform Stress Distribution: Under ideal conditions, a symmetrical shape can lead to more uniform stress distribution under internal pressure.

• Standard Choice for Verticals: It is universally accepted and optimal for vertical pipe runs.

Eccentric Reducer Advantages:

• Eliminating air and vapor pockets is important for pump suction lines. This helps prevent vapor lock and cavitation. It ensures that gas bubbles flow toward the pump instead of building up.

• Preventing sediment buildup is important for slurry lines or systems that tend to collect solids. This keeps the bottom of the pipe flat and allows it to drain itself.

• Maintaining BOP/TOP Elevation: This helps horizontal pipelines stay level at the bottom or top. It makes drainage, pigging, and avoiding interference with structures easier.

• The offset can create space for pipe racks, instruments, or other items. This space can be above or below the pipeline.

Concentric vs. Eccentric Reducers: Comparison

Feature	Concentric Reducer	Eccentric Reducer
Geometry	Symmetrical cone. Centerlines aligned.	Asymmetrical. Centerlines offset parallel. One side flat.
Horizontal Flow Path	Creates high point (air trap) and low point (sediment trap) within the fitting.	Allows constant BOP (FOB) or constant TOP (FOT). Eliminates internal trap.
Primary Function	Change diameter, maintain centerline.	Change diameter, maintain BOP/TOP elevation, eliminate traps, provide clearance.
Ideal Orientation	Vertical piping.	Horizontal piping (FOT for pump suction/liquids, FOB for drain/slurry/clearance).
Flow Symmetry	Symmetrical (vertical), Asymmetrical (horizontal - creates trap)	Asymmetrical flow path but avoids trap creation.
Installation Complexity	Generally simpler (radial symmetry).	More critical; must be oriented correctly (FOT vs FOB).
Key Application Drivers	Vertical runs, aesthetics, symmetry, simplicity.	Preventing vapor lock (suction), preventing sedimentation (drain/slurry), maintaining grade, providing clearance.
Pressure Drop	Generally slightly lower than eccentric in vertical flow.	Can be marginally higher than concentric due to asymmetry, but often negligible compared to trap elimination benefits.
Stress Concentration	Potential at weld junctions due to thickness change, but symmetrical.	Potential at weld junctions, and potentially slightly higher at the toe of the offset taper due to asymmetry. Careful design mitigates.

When to Use Eccentric vs. Concentric Reducers

Use Concentric Reducers When:

The pipeline is vertical. Maintaining the centerline elevation is explicitly required. The fluid is a clean gas or liquid in a vertical run where trap creation isn't an issue. Simplicity of support and alignment is a major factor.

Use Eccentric Reducers (FOT) When:

The pipeline is horizontal and feeds into a pump suction. This is the most critical application for eccentric reducers. FOT prevents air/vapor accumulation at the pump inlet, avoiding cavitation and potential pump damage.

When moving liquids through level pipes, sometimes we need to stop air pockets from forming to keep the liquid flowing. This is especially important for liquids that turn into gas easily or have tiny bubbles trapped in them. Maintaining a constant top-of-pipe elevation is necessary. Use eccentric reducers to provide clearance above the pipe.

Use Eccentric Reducers (FOB) When:

The pipeline is horizontal and serves as a drain line or carries slurries or liquids with solids. FOB ensures the bottom is flush, preventing sediment build-up and facilitating complete drainage. Maintaining a constant bottom-of-pipe (BOP) elevation is required for grading, drainage, or pigging operations. Leaving clearance below the pipe, like over pipe racks, foundations, and instrumentation.

How to Use and Install Concentric and Eccentric Reducers

Concentric Reducers:

• Primarily used in vertical orientations. When the liquid flows UP, put the BIGGER end DOWN. When the liquid flows DOWN, put the BIGGER end UP. Centerline alignment is automatic due to symmetry.

• You can rotate a horizontal pipe to any angle, but add strong supports near the reducer to keep it from bending.

Eccentric Reducers:

• Pump Suction (Horizontal): ALWAYS install FLAT SIDE ON TOP (FOT). The straight edge must be at the highest point to allow gas to flow through, not accumulate.

• Drain Lines/Slurries (Horizontal): ALWAYS install FLAT SIDE ON BOTTOM (FOB). The straight edge must be at the lowest point to allow solids to be swept through and ensure complete drainage.

• Maintaining BOP/TOP: Install FOB for constant bottom and FOT for constant top.

• Clearance: Install FOB to maximize clearance below, and FOT to maximize clearance above.

• Support: Place supports close to the reducer, especially on the larger diameter side, to manage weight and potential moments. Ensure the reducer is properly aligned with adjacent piping.

Concentric vs. Eccentric Reducers: How to Choose

Selecting concentric or eccentric reducers depends on your piping system's requirements:

Pipe Direction: This is the primary filter.

• Vertical: Concentric is standard and preferred.

• Horizontal: Eccentric is almost always required.

Fluid Characteristics and Trap Prevention:

• Pump Suction Line/Liquids with Gas: The eccentric FOT is mandatory to prevent vapor lock/cavitation.

• Drain Line / Slurry / Solids-Bearing Liquid: Eccentric FOB is mandatory to prevent sedimentation and ensure drainage.

• Clean Gas: Both positions (centered and off-center) work, but off-center is often chosen to stay in place or allow more space. Concentric creates a small liquid trap, which might be undesirable.

Elevation and Clearance Requirements:

• Must Maintain Constant Bottom Elevation (BOP): Eccentric FOB.

• Must Maintain Constant Top Elevation (TOP): Eccentric FOT.

• Require Maximum Clearance Below Pipe: Eccentric FOB.

• Require Maximum Clearance Above Pipe: Eccentric FOT.

Diameter Ratio (Both Types): Significant reductions (>2 sizes) can increase turbulence and pressure drop. Consider stepped reductions or consult hydraulic calculations if the change is extreme. Ensure availability of standard reducing dimensions.

Material Selection: Choosing the right material is very important. This is especially true in tough places like chemical plants, oil fields, power stations, and drug factories.

• 304/304L, 316/316L, 317L, 904L, Duplex 2205, Super Duplex 2507: Excellent corrosion resistance, hygiene, and strength. Choose a grade based on the specific service environment. Austenitic grades offer good general resistance; duplex/super duplex offer superior strength and chloride resistance.

• Nickel Alloys: Essential for severe corrosive environments, extreme temperatures, or where exceptional pitting/crevice corrosion resistance is required.

Manufacturing Method and Quality:

• Seamless vs. Welded: Seamless reducers for smaller sizes and high pressures. Welded reducers are common and cost-effective for larger sizes and moderate conditions.

• Production Standards: Following recognized standards is essential for performance, safety, and interchangeability. These include ASME B16.9 for dimensions, tolerances, and pressure ratings. We also follow ASTM/ASME material specs like A403 and A182. Additionally, we adhere to NACE MR0175 and ISO 15156 for sour service.

• Wall Thickness: Must be specified to match the connected pipe schedule or pressure class requirements. Consider internal pressure, external loads, and corrosion allowance. Ensure smooth internal bore transition.

Conclusion

Concentric reducers excel in vertical pipe runs. In pump suction, drain lines, and slurry transport, the eccentric reducer is very important. It must be installed in the right direction (FOT or FOB) when elevation control or clearance is needed. Its ability to eliminate vapor and sediment traps is critical for preventing operational failures.

As a top manufacturer of stainless steel and nickel alloy, we have strong metallurgical knowledge. We also have strict manufacturing skills and a commitment to high standards. This allows us to create reducers for the toughest applications.

Rigorously evaluate your system's orientation, fluid characteristics, elevation needs, clearance constraints, and material demands. This careful method helps you choose the best concentric or eccentric reducer. It is made from the right alloy, ensuring top performance and long life for your important piping system.