Valve Accessories

Accessories are components within a valve automation system. The components are required to operate, override, and support the actuation assembly. Select accessories based on the valve, actuator, and control system requirements.

1. Solenoid Valvesuse electromagnetic force to switch pressure ports. A spring holds the valve plug in position (usually closing a port). When electric current is applied to the coil of the solenoid valve, the spring opposed plug opens or switches ports within the valve. Pneumatic pressure is then allowed to load or exhaust the actuator cylinder. 3-way valves are used on spring-return actuators and 4-way valves are used on double acting actuators. Solenoid valves can be normally open (de-energized to load actuator) or normally closed (de-energized to vent actuator). Solenoid valves typically come in aluminum, brass and stainless steel. Select the appropriate coil voltage and area classification.

2 .Limit Switchesare activated by cams connected to the actuator shaft. When activated, they send discrete electrical signals to indicate actual valve position, usually confirming that a valve fully opened or closed operated remotely. In addition, these signals can be used as interlocks in a control system. For example, locking out a pump until its supply valve has been opened. Mechanical switches should be housed in an enclosure (Switchpak), which will also provide visual position indication. Proximity (magnetically latching switches) are hermetically sealed which makes them less susceptible to moisture intrusion. Select the appropriate switch type and area classification.

3. Positionersare used to position a valve based on a control signal. The signal can be pneumatic or electric. The positioner is mechanically attached to the valve stem. It compares the valve position to the input signal and sends the required output to the actuator to bring the valve to the correct position. The positioner can be analog or digital. Analog positioners use mechanical or electro-mechanical methods to position the valve. Digital (Smart) positioners use microprocessor technology. Smart positioners can operate on digital networks as well as traditional analog signals. They provide 2-way communication with the microprocessor which allows remote configuration and auto calibration, operating information such as actual valve position, and diagnostics information such as cycle count, deviation, friction band, and valve signature. Digital positioners should be used whenever possible.

4. I/P Transducersare used to convert a current signal to a pneumatic signal. Typically a 4-20mA control signal to an existing 3-15 psi pneumatic positioner. I/P's are very economical and can be used to directly control a valve, however an I/P positioner will usually provide much better performance.

5. Filter Regulatorsshould be used whenever possible. They are used to filter and regulate the supply pressure for pneumatic instruments. Instruments have small orifices that can become restricted and plugged by dirty air, moisture can corrode and short electrical components, and pneumatic instrument performance is optimum with regulated supply pressures. Set regulators to 5 psi greater than the maximum supply pressure required by the actuator. Solenoid valves with built in timers are ideal for blowing off regulator drain cocks.

6. Speed Control Valvesare used to control the rate at which automated pneumatic valves operate. Metering valves reduce valve speeds by restricting the flow in and/or out of the actuator. Volume boosters increase the actuator loading time and exhaust valves increase the actuator unloading time.

7. Fail Safe Accumulatorsare used to provide a fail safe means for double acting pneumatic actuators. This system consists of a volume tank, inline check valve and trip valve. Upon loss of supply pressure the trip valve causes the stored gas to drive the actuator to its fail safe position. Stand alone trip valves can be used to lock a double acting actuator in last position upon loss of supply pressure. Solenoid valves can be added to provide electric override or fail safe action.

8. Manual Overrides can be used to manually operate a valve in an emergency or to bypass automatic operation. A handwheel gear operator is mounted in between the valve and the actuator. The handwheel operator is engaged by locking the gear. Manual overrides should be considered on critical valve applications and for emergency shutdown valves.