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My house's main water shutoff is a gate valve. My plumber has told me several times to remember always to make sure the valve is either completely open or completely closed, never in between.

I'm wondering why this is. The water pressure does not appear to be any different if the valve is halfway open or fully open, so why does it matter?

I'm not doubting the plumber's advice. I'm just curious about why this is the case.

Secondarily, if gate valves should never be only partly open, why don't they just make them so that they have to be fully open or fully closed, like a ball valve? Are ball valves a newer invention?

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    i can see why a ball valve needs to be fully open,,But a gate valve i was taught ,to open all the way then come back a little so if it gets stuck you can move it back and forth to free it up. – user101687 May 30 at 0:16
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If a gate valve is partially open, the stem that holds and moves the gate will be in the water stream. If left in this condition for a long period of time, the water stream will wear away the stem, especially the end that attaches to the gate. The usual failure mode is that the valve is closed (stem pushes on the gate), but then as one tries to open it the stem pulls out of the gate and the valve will not reopen.

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painter48179,

The reason is because the type of metal that the stem is derived from will not hold up if water continually passes against it. The only way to keep it from happening is either fully extend it or fully retract it. The best thing to do is replace it with a ball valve giving you the guarantee of a 100% shutoff during emergencies. He is simply trying to save you money in my opinion. Hope this helps you understand. Take care.

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