- It is used for manual operation of control valve during start up or event failure.
- They are frequently used when lockup or bypass valves are not provided.
2.Limit switches and solenoid
- Limit switches are mounted on top to operate signal lights for valve position, valve operation or alarm.
- Solenoid valve may be provided for remote loading and unloading of valve.
- They are used to reduce lag time resulting from long transmission lines or to actuate large diaphragms.
4. Lockup Valve
- Certain applications require valve to remain in it’s last controlled position in event of air supply failure.
- The valve position tends to drift depending on plug forces in event of air supply failure.
- Thus understanding necessity an airlock utilizing capacity tank is used.
- This stored pressure in cushion provides positive or negative valve opening.
- Electronic control loops require I/P converter when control valve is pneumatically operated.
- If control valve is subjected to vibrations they must be mounted on rigid support.
7. Jacketed bodies
- They are required when valve body needs to be heated.
- They provide efficient and uniform heating using copper tubing tracing.
- Most common accessory is air filter regulator.
- Filter removes air-particulates.
9.Ratio & reversing relays:
- A ratio relay is used to multiply or divide inlet pressure
- Reversing realy is used when control action needs to be reversed
E.g. Split range valve may be operated where one is air to open and other is air to close.
- A reversing relay might be used with one of the valves to achieve desired action.