Pipe Reducers: Concentric Reducers, Eccentric Reducers

A pipe reducer is a type of pipe fitting that reduces the nominal bore from a bigger inner diameter to a smaller inner diameter. Piping and Pipeline Systems are not of uniform size and there is a requirement of reducing or expanding the lines depending on process requirement, hydraulic criteria, or availability of material. Here comes the importance of a special pipe fitting called Pipe Reducers or Pipe Expanders.

Pipe reducers are manufactured following the ASME B16.9, DIN2615, JIS B2312, ASME B16.11 standards and are highly reliable and compact components. Pipe reducers are usually made by a forging method known as “outer dia method”.

Types of Pipe Reducers

Pipe Reducers are one of the most extensively used fittings in the piping industry to reduce or expand the size of the straight part of the run pipe. Basically, there are two types of pipe reducers:

  • Concentric reducers and
  • Eccentric reducers.

Concentric Reducers

As shown in Fig. 1, In concentric reducers the area reduction is concentric and the centerline of the pipe on a bigger end and smaller end remains the same.  These styles are normally used for vertical lines. So in concentric reducers, the pipe axis remains the same. Reducers are reversible and can be used in any direction.

Concentric Reducers
Fig.1: Concentric Reducers

In Concentric Reducer, the size reduction from larger to the smaller size is uniform a constant rate over a specified length. A symmetry is maintaining around the fitting keeping the same centreline. Joining of pipe or tube sections of different diameters on the same centreline axis is made possible by using the concentric reducers.

Eccentric Reducers

As shown in Fig. 2, in eccentric reducer there is an offset in between the center lines of the bigger end and the centerline of smaller end. This offset or eccentricity will maintain a flat side either on top or on the bottom side.

In Eccentric Reducer, the reduction in the pipe size is achieved at a constant rate but maintaining one side of the fitting horizontally. The use of eccentric reducers are also reversible and can be used as eccentric expanders. Eccentric reducers are not symmetrical about their centerlines.

Eccentric Reducer
Fig.2: Eccentric Reducer

This offset or eccentricity can easily be found out by the following equation:

Eccentricity=(Bigger end ID-Smaller end ID)/2

Eccentric Reducer Installation

While using eccentric reducer, the user has the option of orienting the flat side. Usually, for horizontal lines, eccentric reducers are oriented with either the flat side up or down, and the same with deviation is mentioned in isometric.

Normally eccentric reducers with the flat side down are preferred for following cases on horizontal lines:

  • On Sleepers and Piperacks: To maintain same BOP for supporting.
  • On lines requiring gravity flow
  • On the pump suction line which handles slurry.
  • Control valve station: An eccentric reducer with flat side down is required to get a constant flow through the control valve with less flow disruptions.

In horizontal gas, steam, or vapor piping, eccentric reducer are required to be installed with flat side down which allows condensed water or fluid to drain at low points.

Eccentric reducers with the flat side up are used for all pump suction lines (excluding pumps handling slurry) on horizontal lines. This way one can avoid air getting trapped inside the pipeline during initial venting through pump casing and will help in avoiding Cavitation.

Types of Piping Reducers based on End Connection

Depending on end connections of this fitting with a straight pipe, reducers are grouped as follows:

Butt-welded pipe reducers

The applicable pressure rating, dimensional and material standards for butt welding reducers are the same as those applicable to butt welding elbows.

Socket welded pipe reducers

As shown in Fig.3. such reducers are available in concentric type only &  in the form of coupling with one end socket to fit larger diameter pipe and another end socket to fit smaller diameter pipe.  Standards are the same as those applicable to socket welding elbows.

Socket Welded Reducers
Fig.3: Socket Welded Reducers

Screwed piping reducers

Available only in the concentric type and are in the form of coupling having one end to fit bigger pipe and another end to fit smaller pipe.  ASME B16.11 is an applicable dimensional standard. Material standards including pressure ratings are the same as of screwed elbows.

Flanged and Screwed Reducer
Fig. 4: Flanged and Screwed Reducer

Flanged pipe reducers

Their pressure rating, use, material, and dimensional standards are the same as those applicable to flanged elbows. Regardless of reduction, their face to face dimensions is governed by the larger pipe size.

Materials of Pipe Reducers

Pipe reducers are made of various material like Carbon Steel, Alloy, or Stainless steel and much more like non-ferrous materials. As compared to the Stainless Steel Reducer, Carbon Steel Reducer possesses higher strength, high-pressure resistance, and wear resistant but this can be easily corroded.

  • Carbon Steel material standards and grades used for pipe reducers are: A234 WPB, A420 WPL6, MSS-SP-75 WPHY 42, 46, 52, 56, 60, 65 and 70.
  • Stainless Steel Pipe Reducer material grades are: ASTM A403 WP 304, 304L, A403, 316, 316L, 317, 317L, 321, 310 and 904L, etc.
  • Alloy Steel Pipe Reducer material grades include: A234 WP1, WP5, WP9, WP11, WP22, WP91 etc.
  • Duplex Steel: ASTM A 815 UNS NO S31803, S32205
  • Nickel Alloy: ASTM/ASME SB 336 UNS 2200 (NICKEL 200), UNS 8020 (ALLOY 20 / 20 CB 3, UNS 2201 (NICKEL 201), UNS 4400 (MONEL 400), UNS 8825 INCONEL (825), UNS 6625 (INCONEL 625), UNS 10276 (HASTELLOY C 276), UNS 6600 (INCONEL 600), UNS 6601 (INCONEL 601).

Pipe Reducer Fitting Dimension

The dimensions for butt welded piping reducers are provided by ASME B 16.9. In pipe Data Pro software, you can easily get the required pipe reducers dimensions. The following image provides part of the pipe reducer size chart as a reference.

Pipe Reducer Dimensions
Fig. 5: Pipe Reducer Dimensions

Piping Reducer Ordering Information

The following information should be supplied to the vendor while placing an order for pipe reducers.

  • Pipe Size and Outer diameter
  • Governing standard (ASME B16.9, GB, HG, HGJ, SH, SY, DL, AN, SI, JIS, or DIN).
  • End connection (Welded/Flanged, etc)
  • Distance from end to end.
  • Material of construction.
  • Seamless or welded construction.
  • Fluid service and Corrosion allowance
  • Pressure class

Few more related resources for you.

Piping Elbows vs Bends: A useful literature for piping engineers
“Pipe Coupling”-A short Introduction for the piping professionals
Different Types of Hose Couplings
A Literature on Piping Nipples for piping and plumbing industry

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Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

3 thoughts on “Pipe Reducers: Concentric Reducers, Eccentric Reducers

  1. Thanks for the very good article.

    I wanted to know about the basic difference between Swage nipple and Reducers.

  2. Thanks for the article

    In addition, it must say that standard ASME B16.9 used for butt welding reducers and B16.11 for forged steel, socket welding and threaded ones.
    In selection of the materials, reducers and other components must made of material having at least the similar allowance stress of the pipe and must have a pressure retention strength of at least 87.5% of the wall thickness of the matching piping.


  3. Furthermore, when the reducer size dropped less that 2″, it will be called Swage nipple and it is covered by MSS SP 95. connections are socket welding and threaded.

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