When is Eccentric Reducer's Flat-Side on Bottom?

11/18/2020

When/why would an eccentric reducer ever be installed on a fire pump's suction side with the flat side on bottom?

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13 Comments

Wayne Ammons

11/18/2020 08:23:00 am

The purpose of an eccentric reducer on the suction side of a fire pump (when a size change is necessary) is to create a horizontal transfer of air/water into the pump. The orientation of the flat side depends on the direction of incoming water. If the water is incoming from below the pump, the flat side should be on top, if from above the pump, the flat side should be on the bottom, if there is a long horizontal run of water entering the pump the flat side should be on the top. You want as laminar a flow as possible entering the pump to help prevent future wear and tear.

Paul gowland

10/13/2021 02:38:28 pm

Hello Wayne I understand the eccentric reducer with flat end up under normal circumstances , however when supply comes from above main point of supply & back flow preventer is a floor above. So u are saying the eccentric reducer should be installed with the flat end down ? I did not see this in nfpa 20. Where is this noted per intall requirements? Thx!

gary lagnese

11/18/2020 10:10:30 am

Flat side is on top to prevent an air pocket that could cause the pump to cavtate.

Derek Gaston

11/18/2020 10:14:45 am

Hi Gary!

gary lagnese

11/18/2020 01:07:51 pm

Hi Derek!!!

Craig Vesely

11/18/2020 10:21:06 am

The flat side of the eccentric reducer is to have the flat side of the fitting on the top to prevent an air pocket from forming. Refer to Figure A.6.3.1(a) in the 2019 edition of NFPA 20.

Franck

11/18/2020 10:35:13 am

Wayne is right
This is a grey area within NFPA which could lead to wrong installation as the flat is not always on top!
The sketch on NFPA only shows the flat at the top, because the drawing is with an horizontal pipe and water coming from below the pump. It is to illustrate that we don't want concentric reductions.

But if the water is coming from above with a vertical pipe to the suction flange, then the flat should be at the bottom.

For more information, have a look at:
https://www.enggcyclopedia.com/2011/12/pumps-suction-piping-eccentric-reducers-straight-lengths-piping/

gary lagnese

11/18/2020 10:51:20 am

Could not a concentric then be used>

Glenn Berger

11/18/2020 11:01:00 am

Flat on bottom is typically used on steam systems.

Franck

11/18/2020 11:04:26 am

Not if you want the flow to be as laminar as possible.

Note that is is a pure reliability topic.
Your pump will work well, with a concentric reduction.
But, because of the small perturbation, you may have an early ageing of your impeller.
This would save you some maintenance cost and longer lifetime for your pump.
But it doesn't have an impact on its actual performance for your sprinkler system.

It is more or less the same for the provision of a gate valve on the suction side of your pump.
A butterfly valve will work the same and the small perturbation will not cause your sprinkler system to fail.
But again, you will have an early ageing of your pump... Reliability, reliability...

Plus the fact that you may broke your valve inside the pipe and damage the impeller inside the pump casing (very rare event, but could happen).

Wayne Ammons

11/18/2020 11:18:05 am

Replying to Gary Lagnese
11/18/2020 10:51:20 am
Could not a concentric then be used>

Concentric reducers are only permitted on the discharge side of the pump. It is the sloping quality of the reducer that has the potential for an air pocket to form if used on the suction side and oriented with the sloping side facing up.

gary lagnese

11/18/2020 01:12:17 pm

Wayne, I was only asking in reference to a vertical supply feeding the pump.

Franck

11/18/2020 11:12:30 am

You can also add to this list of reliability items the elbow at the intake on the horizontal plane...

I have seen fire pumps installations with all possible mistakes at the intake (elbow + butterfly valve + concentric reduction), and the results (flow curve) were quite good.
In case of fire, it would have done its duty.
But you may need to replace them earlier because of characteristics deviations over the years.

Unless you don't test them (as you create cavitation and degradation only if the pump is running).
But I would not recommend not to test a pump, especially a diesel engine driven fire pump, as the rate of failure increases when the time between 2 tests increases.

But again, the testing frequency of fire pumps is reliability.
And the good balance between testing too often (ageing of material) and not enough (possible failure when you need it) is considered as a weekly test.

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When is Eccentric Reducer's Flat-Side on Bottom?

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