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Outside Screw and Yoke (os&y) Gate Valve (pump suction)-

The OS&Y gate valve in the suction piping of a fire pump serves two purposes. As liquid flows into a fire pump, it needs to be as free of turbulence and to avoid unbalanced loads on the impeller. When a gate valve is in the fully open position, the wedge is retracted into the body of the valve, leaving the liquid passageway clear of any obstruction and effectively enabling laminar flow. The OS&Y valve also provides a way to isolate the fire pump from the liquid supply so repairs can be made to the fire pump. A listed outside screw and yoke (OS&Y) gate valve shall be installed in the suction pipe.


The required placement of the valve in the suction line is intended to limit turbulence. While the gate in an outside screw and yoke (OS&Y) valve is outside the waterway when fully open and does not interfere with flow, the “butterfly” in a butterfly valve is always suspended in the pipe and cause turbulent flow. The use of a butterfly is permitted if the valve is located at a sufficient distance from the suction flange of the pump. By locating the butterfly valve 50ft (15.3m) or more away from the suction flange of the pump, the turbulence can be eliminated before the water reaches the suction flange.



1. Where the suction supply is from public water mains, the gate valve is located as far as is practical from the suction flange on the pump.

2. Where it comes from a stored water container, the gate valve should be located at the outlet of the container.