a dust valve for the discharge line of a dust collection system includes a removable closet with two wear surfaces and a coacting removable valve door with two wear surfaces mounted within the valve body . once the coacting wear surfaces on the closet and the door are sufficiently worn that they need replacement , each element can be disconnected from its supporting structure , inverted , and then reconnected to the valve body so as to expose a new wear surface . the useful lives of these elements is thus doubled over conventional single wear surface closets and doors .