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Eccentric reducers and straight runs of pipe at pump suction

09.01.2010  |  Bloch, H. P.,  Hydrocarbon Processing Staff, 

Keywords: [pumps] [piping] [maintenance]

Questions relating to proper reducer application in centrifugal pump suction lines date back many decades. Until his death (at age 84, in 1995), world-renowned pump expert Igor Karassik frequently corresponded with the writer and other pump users on pump-related subjects. We rarely pass up an opportunity to highlight some of his experience-based comments.

Once, a pump user referred to Fig. 1 and noted that this was quite typical of illustrations found in many textbooks. In essence, Fig. 1 indicates that, with a suction line entering the pump in the horizontal plane, the eccentric reducer is placed with the flat at the top. Available texts often give no indication as to whether the pumpage came from above or below the pump.


  Fig. 1. Illustration of eccentric reducer mounting from
  Hydraulic Institute Standards.  

Igor Karassik agreed that, if the supply source was from above the pump, the eccentric reducer should be installed with the flat (horizontal) surface at the bottom. Entrained vapor bubbles could then migrate back into the source instead of staying near the pump suction. If the pump suction piping entered after a long horizontal run or from below the pump, the flat of the eccentric reducer should be at the top.1

Still, in many older texts it has been assumed that the pumpage source originated at a level below the pump suction nozzle. Karassik reminded us that older Hydraulic Institute Standards commented on the suction pipe slope:

“...Any high point in the suction pipe will become filled with air and thus prevent proper operation of the pump. A straight taper reducer should not be used in a horizontal suction line as an air pocket is formed in the top of the reducer and the pipe. An eccentric reducer should be used instead.”

This instruction applies regardless of where the pumpage originates. Depending on the particulars of an installation, trapped vapors can reduce the effective suction line cross-sectional area. Should that be the case, flow velocities would tend to be higher than anticipated. Higher friction losses would occur and pump performance would be adversely affected.

In the case of a liquid source above the pump suction, and particularly where the suction line consists of an eccentric reducer followed by an elbow turned vertically upward and a vertical pipe length—all assembled in that sequence from the pump suction flange upstream—it will be mandatory for the eccentric reducer flat side to be at the bottom. That said, Fig. 2 should clarify what reliability-focused users need to implement.


  Fig. 2.  Suggested modifications for eccentric reducer

Also, whenever vapors must be vented against the flow direction, the line size upstream of any low point must be governed by an important criterion. The line must be a diameter that will limit the pumpage velocity to values below those where bubbles will rise through the liquid.

In general, it can be stated that wherever a low point exists in a suction line, the horizontal piping run at that point should be kept as short as possible. In a proper installation, the reducer flange will thus be located at the pump suction nozzle and there is usually no straight piping between reducer outlet and pump nozzle. Straight pipe lengths are, however, connected to the eccentric reducer inlet flange. On most pumps, one usually gets away with five diameters of straight length next to the reducer. In the case of certain unspecified velocities and other interacting variables (e.g., viscosity, NPSH margin, pump style, etc.), it might be wise to install as many as 10 diameters of straight length next to the reducer inlet flange. The two different rules-of-thumb explain seeming inconsistencies in the literature, where both the 5 and 10-D rules can be found.  HP


1 Karassik, Igor J., Centrifugal Pump Clinic, 2nd Ed., Marcel Dekker, Inc., 1989.

The author 

Heinz P. Bloch  is HP’s Equipment/Reliability Editor. The author of 17 textbooks and over 470 papers or articles, he advises process plants worldwide on reliability improvement and maintenance cost reduction opportunities. His coauthored Bloch/Budris text, Pump User’s Handbook, is comprehensive and very widely used. Find the 2nd edition under ISBN 0-88173-517-5.  He can be contacted at HB@HydrocarbonProcessing.com.

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ASME B16.9 buttwelded eccentric reducers are not oblique cone-shaped as shown in the illustration.


How to use eccentric reducer?

Dharmendra Patel

As per literature cited, position of flat line of eccentric reducer depends on nature of fluid handled! The vapors handle near to dew point by Centri.MCs equiped with flat (horizontal) surface at TOP where as fluid handles near to bubble point equiped with flat surface at BOTTOM! The Centri.MCs to be interpreted as BLOWER or PUMP as per fluid application!


what is the clearance of eccentric & concentric for suction and discharge pipe routing?

rheno maestrado

i want to see the images of a reducer like eccentric

rheno maestrado

why use eccentric reducer in pump suction line?

Lalit Kumar

Want to know how to manufacture eccentric reducer or production process .And formula to calculate layout of material required


This article is really nice and it clears some myth.
For more detail about pump piping at


why use eccentric reducer in pump suction line

Manish Joshi

Some of the EPCs using a standard where they use ecentric reducer at Pump suction with flat side always at the top. Very valuable reference to ask them to be flexible with their standard if the pumpage is from above the pump.
Thanks for bringing this clarity !!


Eccentric are placed flat side down in line .


Great article ... I have argued this point with engineers who INSIST that a reducer shall be placed flat side up, regardless of the source of the pumpage.

Consider this article BOOKMARKED!

I know I'll be pulling it out for reference.


Can we use eccentric reducer instead of concentric reducer if we do what will be the effect; is there any disadvantage?... Coz one of the fitter had used eccentric instead of concentric in PSV connection. Upper side Big Dia Lower side Small Dia

Please suggest


i want to know that the use of concentric and eccentric reducer.

Pradip Sinha

we have 8 nos high pressure pump (capacity 156m3/hr,max.pressure 15kg/cm2) with a single suction header.
So, every month one of them pump shaft is being damaged.
I want solution.

Abid Bhatti

Thanks for very help full article.

Mohamed metawa

thanks a lot .


As to why the eccentric reducer should be mounted with flat bottom at bottom for pumpage Source above pump suction is still very unclear to me..Kindly elaborate the same


what benefits of eccentric reducer with 1",2" pipe at suction of C F pump? also which type of material service & parameter required in pump (Viscocity,spe.gravity) are more suitable for eccentric reducer?


is there any possibility that concentric reducers be used on horizontal line piping?

mojtaba javan

tanks for this paper.


Thanx for your info. Now i know.


Why should provide the eccentric reducer in the pump suction and why not provide concentric reducer????......if any special purpose of providing the eccentric????...... pls give suggestion.....


As to why the eccentric reducer should be mounted with flat bottom at bottom for pumpage Source above pump suction is still very unclear to me..Kindly elaborate the same .




Dear Sir,

Please also include some guide line for
(1) straight length requirement for various types of Pumps i.e. OH2 / BB1 / BB2 / Multistage Pumps and for type of Suction i.e. single / double suction

(2) If due to Lay out constrain can we go for 3D stratght length for suction

Waitng for your reply



Thank God you clarified. There are innumerable references & books propagating the incorrect reducer connection And there must be millions of incorrect installations. I as a project man (non Mech.) and going by basic principals had argued with Pump specialists over this anomaly when a pump was misbehaving but was shown the text book.


I had the priviledge to attend a pumping problems class led by Igor Karrasik. I still have the course notes and have used them for over 30+ years.

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