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?