The fail-safe mode of a pneumatic

The fail-safe mode of a pneumatic/spring valve is a function of both the actuator’s action and the valve body’s action.

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.

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.

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.

The most common combinations mix a direct-acting valve body with either a reverse – or direct acting valve actuator.

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 tend to be more complex in construction than direct-acting valve bodies, and so they are less common in control valve applications.