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.
Flat side is on top to prevent an air pocket that could cause the pump to cavtate.
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.
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:
Could not a concentric then be used>
Flat on bottom is typically used on steam systems.
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).
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.
Wayne, I was only asking in reference to a vertical supply feeding the pump.
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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