PNEUMATICALLY OPERATED CONTROL VALVE - SKETCH, OPERATION, FAILURE MODE, MAINTENANCE

Sketch and description: A pneumatically operated control valve consists of:

• A valve body with a valve plug/ stem and seat, controlling the flow of the process fluid.
• An actuator (a diaphragm or piston actuator) mounted on the valve, which positions the valve stem.
• A pneumatic signal (from the controller, e.g. 3-15 psi or 4-20 mA converted to a pneumatic signal) applied to the actuator diaphragm/ piston.
• A spring in the actuator that opposes the air pressure (spring-return).
• A positioner (optional) that ensures the valve stem reaches the position corresponding to the signal.
• Air supply (instrument air) and the signal line.

Working: The controller sends a pneumatic signal to the actuator. The air pressure acts on the diaphragm/ piston, moving the valve stem against the spring. The valve plug opens/ closes to regulate the process flow. The positioner compares the actual stem position with the signal and adjusts the air to the actuator to position the valve accurately. The valve is "air-to-open" or "air-to-close" depending on the arrangement (fail-open or fail-closed).

How the valve is operated if the controller fails:

• If the controller fails (loss of signal), the valve is designed to fail in a safe position:
• Fail-closed (air-to-open): On loss of air/ signal, the spring closes the valve (safe for processes that must shut off on failure).
• Fail-open (air-to-close): On loss of air/ signal, the spring opens the valve (safe for processes that must remain open on failure).
• The valve can also be operated manually: a handwheel/ manual override on the actuator allows the valve to be positioned by hand if the air/ signal fails.
• The valve may be operated in manual/ local mode by adjusting the air supply or by using the bypass valve.

Routine maintenance required:

• Regular inspection of the valve, actuator and positioner for leaks, wear and correct operation.
• Checking and cleaning the air supply (filter/ regulator) and the signal line.
• Lubricating the valve stem/ packing and checking the packing for leaks.
• Checking the valve travel/ stroke and the positioner calibration.
• Testing the fail-safe action (fail-open/ fail-closed) and the manual override.
• Checking the valve seat/ plug for wear and the diaphragm for damage.
• Replacing worn parts (packing, diaphragm, seat) as required.
• Calibrating the positioner and checking the valve responds correctly to the signal.