The reverse-acting gate valve body shown in the left-hand illustration is open, with fluid flowing around the stem while the wide plug sits well below the seat area. Reverse-acting valve bodies may also be used, with opposite results: The most common combinations mix a direct-acting valve body with either a reverse- or direct-acting valve actuator, as shown in this illustration: Thus, a sliding-stem, pneumatically actuated control valve may be made air-to-open or air-to-close simply by matching the appropriate actuator and body types. Sliding-stem valve bodies are classified as direct-acting if they open up when the stem is lifted, and classified as reverse-acting if they shut off (close) when the stem is lifted. For sliding-stem valves, a direct-acting actuator pushes down on the stem with increasing pressure while a reverse-acting actuator pulls up on the stem with increasing pressure. The fail-safe mode of a pneumatic/spring valve is a function of both the actuatorвЂ™s action and the valve bodyвЂ™s action.

For pneumatic and hydraulic actuated valves, the option exists of having a large spring provide a known вЂњfail-safeвЂќ position (either open or closed) in the event of fluid pressure (pneumatic air pressure or hydraulic oil pressure) loss. For electrically actuated valves, this is typically the last position the valve was in before loss of electric power. An important design parameter of a control valve is the position it will вЂњfailвЂќ to if it loses motive power.

Gamepad controls burnout drift