In many kinds of processing equipment it is necessary to have one part move in relation to another part without excessive leakage of a fluid around the moving member. This is true in packed expansion joints and in valves where the stem must enter the valve body and be free to turn without allowing the fluid in the valve to escape. It is also necessary where the shaft of a pump or compressor enters the casing, where an agitator shaft passes through the wall of a pressure vessel, and in other similar places Common devices for minimizing leakage while permitting relative motion are stuffing boxes and mechanical seals. Neither completely stops leakage, but if no leakage whatever of the process fluid can be tolerated, it is possible to modify the to that only innocuous fluids leak into or escape from the equipment The motion of the moving part may be reciprocating or rotational or both together it may be small and occasional, as in a packed expansion joint, or virtually continuous, as in a process pump