Pneumatic Process Valve Accessories:
(a) Reasons for Fittings:
(i) Volume Booster:
- Increases air flow rate: A volume booster amplifies the low-flow pneumatic control signal from the controller to provide a high-flow output to the actuator. This is necessary when the actuator has a large volume or requires significant force to operate, which a standard control signal might not be able to provide effectively.
- Faster valve response: By increasing the air flow rate, the booster speeds up the actuator’s response to control signals, leading to faster process adjustments and improved system dynamics.
(ii) Feedback Positioner:
- Improves accuracy and stability: A positioner measures the actual valve position and compares it to the desired position from the controller. Any deviation triggers the positioner to adjust the air pressure to the actuator, ensuring the valve reaches the exact desired position even if external forces or friction affect it. This significantly improves control accuracy and reduces steady-state errors.
- Overcomes friction and disturbances: Positioners compensate for frictional forces in the valve mechanism and external disturbances affecting the valve movement, ensuring the valve responds faithfully to the control signal despite these challenges.
(b) Actuator Types and Reasons:
(i) Fuel Supply System (Fail-Safe Operation):
- Spring-to-Close Actuator: Ideally, this actuator would close the valve (stopping fuel flow) when air pressure fails due to a control system outage. This provides a fail-safe mechanism to prevent uncontrolled fuel release, ensuring safety and system integrity.
(ii) Lubrication Oil Cooling System (Diversion Valve):
- Double-Acting Actuator: This actuator uses air pressure in both directions (open and close). In case of air pressure loss, the valve would stay in its last position, maintaining the current oil flow path (either through the cooler or bypass, depending on the valve state before failure). This avoids uncontrolled changes in oil flow that could affect lubrication and engine health.
Additional Notes:
- The specific choice of actuator and accessories depends on various factors like process requirements, safety considerations, and control performance objectives.
- Some applications might require specific valve designs or fail-safe features beyond the basic categories mentioned here.