As the names suggest, a wedge gate valve has non-parallel seat faces. The wedge gate valve is much more common, with many advantages. The advantage of a wedge gate valve is that it is “self seating”, i.e. it will shut off flow as tight as you can turn the handle. The flaw with that design is what is solved by the parallel gate valve, such as what if there’s some small debris stuck in the seal somewhere? The parallel gate valve in design “pushes the debris” off the parallel seals as the valve is shut. But this works only if the valve is manufactured with high precision, which is why such valves find use in bigger, more costly applications, where reliability of true shut-off is mandated.
Wedge gate valves, if free of debris, are more capable of shutting off flows at higher pressures than parallel gate valves. The latter is limited by the machined precision of its parallel seat faces. As a rule, high pressure valves rely on the mechanical pressure that can be exerted on the seal faces by valve operation, and parallel gate valves cannot exert any beyond a certain level by design.
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