A valve system includes a spring-biased locking slide plate which extends in an axial direction parallel to and offset from the axis of the valve stem and selectively extends through a base plate mounted to the top of a valve to selectively engage stop tabs on the valve body. The locking slide plate also extends through the valve handle and through a locking member to selectively lock the valve in open or closed positions. In one embodiment, the locking slide plate comprises two sections which include a lower section having tines which selectively engage tabs on the valve body and a second upper section which is spring-biased to the lower section to float to allow the lower section to accommodate different diameter valve bodies.