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Eccentric reducer at pump suction

Eccentric reducer at pump suction

Eccentric reducer at pump suction

Hello to all

I have noticed using eccentric reducer at the pump suction for horizontal centrifugal pumps "Double Suction Split Case" for water pumping in HVAC application. Is this eccentric reducer required for all types of pumps like vertical centrifugal pumps and the end suction centrifugal pumps. Advices and comments are highly appreciated..


RE: Eccentric reducer at pump suction

The thinking is that with an eccentric reducer (flat side down)you won't tend to stream a gas pocket into the pump.  Some CFD work I saw a few years ago predicted that it would not work (i.e., if there is enough gas to stream, it has to go somewhere and trying to trap it upstream of a reducer will allow the pockets of gas to be larger than they would be with a concentric reducer or no reducer).

There are many companies that absolutely require an eccentric reducer upstream of any sort of pump--I have not personally seen any data that supports this configuration, but culture is often a stronger force than science.  

David Simpson, PE
MuleShoe Engineering
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The harder I work, the luckier I seem

RE: Eccentric reducer at pump suction

slight correction - flat side up, not down.

goto the McNally Institute website for sufficiently related articles on typical pump installations, etc.

good luck!

RE: Eccentric reducer at pump suction

For vertical centrifugal pump, you do not install a suction pipe, do you?

The eccentric reducer is only required on horizontal suction pipe. Reason as zdas04 said.

RE: Eccentric reducer at pump suction

Mr bulkhandling

By vertical pumps, I did not mean the vertical turbine pumps but the split case centrifugal pump mounted vertically, do we need to install the eccentric reducer for this type as well? thanks

RE: Eccentric reducer at pump suction

Mechya, this subject was discussed in thread378-145212

My personal belief is that eccentric reducers are not important, except in the most extreme cases.

Katmar Software
Engineering & Risk Analysis Software

RE: Eccentric reducer at pump suction

If the pump manufacturer's installation instructions call for something, then just do it. Particularly when the cost isn't much of a factor. (The installed cost of an eccentric won't be much more than a concentric, and will be the square root of nothing in the cost of the job). If you follow the installation instructions, then you're in a much better postion if there are operational problems. I've been involved in a couple of cases where I was very glad that the instructions had been followed to the letter. It's nice to be able to sit in a meeting with several very unhappy parties, and ask the manufacturers rep if these are the published installation instructions for his equipment. Then ask him to point out which ones were missed or botched. The few pages that are the installation instructions can make a very nice shield - but only if you've followed them.

RE: Eccentric reducer at pump suction

TBP - very good advice. The answers to technical questions are not always technical!

Katmar Software
Engineering & Risk Analysis Software

RE: Eccentric reducer at pump suction

Using eccentric reducers on the horizontal run of suction piping is technical and very important in most all pump installations. How can you calculate line loses in the suction pipe if the liquid is forced through only part of the pipe because the rest of the pipe is occupied by a large air pocket? What happens if a large part of the air pocket becomes entrained and goes into the pump suction? If you don’t like eccentric reducers use a concentric reducer in the vertical section of pipe if you have a vertical run and the piping loses allow. I have corrected many so called pump problems over the years by replacing concentric reducers on horizontal runs with eccentric reducers. Also you do see on occasion where they are installed up side down. Most of the time this is a mistake but the thought can be that solids that settle out will migrate better through the piping and if the piping is sloped back towards the supply air will vent back to the supply. Normally you are better off trying to maintain a velocity that will carry the solids through the system.

RE: Eccentric reducer at pump suction

checman, please do not think I am trying to start an argument here. I am very keen to learn from your experiences and I am not challenging your statements. I have posted here before stating that I have never seen a pump problem cured by replacing a concentric reducer with an eccentric one. So I am very interested to know what sort of problems you solved, and what sort of flow regimes were involved. I would also be interested to hear from others who have solved specific problems by replacing concentrics with eccentrics, or by swinging the eccentric from flat-bottom to flat-top.

What were the pump symptoms? eg Was it cavitation, decreased capacity, vibration?? Was the flow known to be two-phase? In two-phase flow I would agree that you should use flat-top eccentrics. Was it a start-up problem, where the flow was never able to reach a steady state?  Did the pump run OK for a period and then gradually develop the symptoms?

I would be very grateful if you would write a paragraph or two describing these experiences so that I can watch out for these problems in future.


Katmar Software
Engineering & Risk Analysis Software

RE: Eccentric reducer at pump suction


I agree on you first half but when you mentioned about solids I think you just mean some minor solids in fluid.

When the fluid contains certain amount of solids, we call it slurry. For slurry pumping, solids settling is a major consideration far more important than air pocket. Most of the time the suction pipe is sized lower than the critical velocity that means solids will settle and slide at the bottom of the pipe. So bottom flush reducer is always required.

By the way, slurry pumps are designed solid enough to handle some air.

RE: Eccentric reducer at pump suction

bulkhandling (Mechanical) 9 Mar 06 14:08  
"By the way, slurry pumps are designed solid enough to handle some air."
Just a small point - I don't really see any connection between "designed solid enough" and "handle some air". Air-handling is not a function how solid a pump is designed but a function of impeller geometry and volute design. However, I would agree that a slurry pump is more capable of handling "nuisanse air" entrained in the pumped product than a close-clearance clean product pump.   

Naresuan University

RE: Eccentric reducer at pump suction

I agree with Checman RE eccentric reducers flat side on bottom for slurry pumps. Decreased potential for solids build up and wear, easier to flash.

For water pumps- flat side top or concentric- not as critical though.



RE: Eccentric reducer at pump suction

Thanks to all for providing a good information and sharing their experiences, still if anybody else can comment on the problems if any? asociated with using concentric reducer on a Double Suction Split Case (DSSC) vertical Centrifugal pump.

Thanks once again....

RE: Eccentric reducer at pump suction

I did not read all the posts word-for-word and I am not commenting on the merits of eccentric reducers...

But when they are recommended (right, wrong, or otherwise), from what I recall, the position of the flat side (top or bottom) depends on whether you have a suction lift or an elevated supply.

RE: Eccentric reducer at pump suction

Mechya - If the pump suction is vertical use the concentric reducer.

Another thing I do remember with double suction pumps is if you have an elbow on the suction side close to the pump, you should arrange the elbow so the flow does not cause an imabalance on the impeller.

This is not easy to explain in words but just think of the flow profile on the inside radius of the elbow compared to the outside radius. This is a known phenomenon and you should be able to find information on it.

RE: Eccentric reducer at pump suction

If I understand your pump description correctly, the suction and discharge lines are both vertical, in which case you should use a concentric reducer as opposed to an eccentric.  In systems like cooling towers where the fluid can have alot of entrained air, concentrics are not recommended in horizontal runs for the same reason eccentrics should be installed with the flat side on top, they can trap air leading to a reduced effective area and cavitation.  In vertical piping, this is not an issue and a concnetric has lower friction losses.  The only issues I can see are maintaining the recommended flow velocity after the reducer and checking the NPSHa with the reducer in the line.  If NPSHa is insufficient, you will need to look at the system and find a way to correct it.


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