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At a recent valve selection seminar, Crane Engineering’s Valve Manager, Scott Nicholas asked the group what the arrow on a knife gate valve represents. The audience resoundingly replied, “Direction of flow.” This is a common misconception.
It’s true, for many control valves, the arrow does indicate direction of flow. But for the knife gate valve, it’s different.
A knife gate valve has a gate that moves up and down, and a seat on one side. In order for the valve to seat properly and seal without leaking, pressure must be placed on the correct side of the valve. Therefore, the arrow indicates which side the valve seats.
Let’s use an example. Let’s say that we have a pump that needs to be repaired. Valves are closed on the suction and discharge side of the pump. The knife gate valve installed on the suction side has the arrow pointing in the direction of flow. This is correct. Pressure against the gate will help the valve seat properly.
On the discharge side, the arrow on the knife gate valve is pointed against the direction of flow. When this valve is closed, pressure from the fluid in the pipe on the discharge side will allow the valve to properly seat and stop leakage.
Now of course this doesn’t apply to every knife gate valve. When does it? As I’ve learned over my tenure here at Crane Engineering, the answer you’ll always get is “It depends.”
So you’re probably wondering, why don’t I skip the confusion, and just install all bidirectional knife gates? You can. The biggest difference is price. You can expect to pay about 30% more for a bi-directional knife gate valve versus a uni-directional valve.
Just like pumps, proper installation is at the core of a valve’s long term performance. Be sure to read the installation manual before (instead of after!) installing new valves.
Leaking valves? Ask us about it! We gladly provide technical assistance to businesses and municipalities in Wisconsin and Upper Michigan.