Valve Actuator Accessories - Principles, Uses, Advantages
Valve actuators are most commonly used to regulate fluid flow in a process.
The International Society of Automation defines a valve actuator as a power-operated device that operates fluid flow in a process control system.
If the valve actuator is equipped with the appropriate accessories, it can become more useful for process control and can also provide smooth and efficient operation. Pneumatic valve actuator accessories help ensure optimal performance and are important components in process control systems.
Purpose of Valve Actuator Accessories
There are five basic reasons for adding accessories to your valve actuator:
- Improve process control
- Improve the safety of processes or people
- Improve valve performance or response speed
- Monitor or verify valve responsiveness
- Diagnose potential valve problems
Advantages of Valve Positioners Include
1. Minimize the effects of friction, hysteresis, and dead space on the valve stem.
2. Enable signal range changes.
3. Allow valve actuator signal reversal: The positioner can operate in direct-acting or reverse-acting mode. In direct-acting mode, an increase in control signal pressure will cause an increase in positioner output air pressure. In reverse-acting mode, an increase in valve actuator signal pressure will cause a decrease in positioner output air pressure.
4. The positioner improves the response speed of the actuator.
5. Allows changing the flow characteristics of the valve.
6. Allow split-range operation. In a split-range control loop, one actuator output is used to drive two valve actuators.
Current to Pressure Converts a 4 to 20 mA current input to a proportional 3 to 15 psi pressure output using a converter module.
In some applications, a high level of positioning accuracy from the positioner is not required. In these applications, only electro-pneumatic (I/P) sensors can be used.
When the actuator is large, the positioning response speed of the valve actuator becomes slower.
A volumetric booster is used to provide additional pneumatic thrust output capability to the valve assembly.
Single-acting actuators usually use a booster. Double-acting actuators require at least two boosters.
The purpose of a position transmitter is to provide independent, individual valve position feedback to the control system. Position feedback from a valve actuator is often necessary as it is used by the operator in the control room for process monitoring, troubleshooting, or start-up/shutdown verification.
Position transmitters are mounted directly on the valve yoke and measure the position of the valve stem or valve shaft.
The Electromagnetic Valve
In some applications, solenoid valves trap air in the actuator to lock the valve in its current position.
In some applications, the solenoid valve will bleed air from the actuator to allow the valve to move to an air-free position.
The purpose of a limit switch is to provide a discrete open or discrete closed input signal to the control system when the valve reaches a specific position within its travel range.
Limit switches mounted on valve actuators and yokes are also used by operators in the control room for process monitoring, troubleshooting, or start-up/shutdown verification.
These switches only provide specific valve position feedback. These can be mounted on the valve actuator to obtain additional feedback signals to a DCS or PLC system depending on our requirements.
Trip System - Airbrake Relay
Trip systems such as air lock relays are used in control applications that require specific actuator action when air supply pressure is lost. They are used with double-acting actuators that do not have inherent air or fault conditions, or with single- or double-acting actuators to provide pneumatic locking.
When the instrument supply pressure falls below the trip point, the trip valve can cause the actuator to fail, lock the valve actuator in the final position, or fail to drop.
For double-acting applications, the volume tank provides reserve air capacity to operate the valve in the event of an air supply failure until supply pressure is restored. When the air supply pressure recovers and rises above the trip point, the trip valve automatically resets and returns to normal, allowing the system to resume normal operation.
The handwheel of the valve actuator diaphragm actuator is often used as an adjustable travel limiter. They also provide a manual method of locating the valve actuator in an emergency.
They are arranged on the valve actuator in two ways.
1. Side-mounted handwheel
2. Install the hand wheel on the top
3. The side-mounted handwheel on the valve actuator can be used to open or close the valve in any direction at any point in the actuator stem travel.
The side-mounted handwheel can limit travel in any direction, but not in two directions at the same time. By holding the valve actuator's handwheel in the neutral position, automatic operation is achieved throughout the valve's travel.
The top-mounted handwheel is used for "infrequent maintenance" to manually stroke the valve.
The above briefly introduces the principles, uses, and advantages of valve actuator accessories. If you want to know more or want to purchase valve actuators and accessories, please contact us.
UG is a professional custom valve actuators and accessories manufacturer, we provide solutions across all industries. We leverage our engineering expertise and industry experience to continuously improve our products and strive to provide efficient solutions and competitive pricing. UG is also a global supplier of highly engineered actuators and accessories serving the oil and gas, mining, chemical, pharmaceutical, water and power, food and beverage, and general industrial markets.
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