A control valve actuator is used not only for controlling valves but also for precisely regulating their position. By providing compressed air to the piston actuator or diaphragm positioner, an adjustable valve is then adjusted in response to the control signal. In order to ensure the valve does adjust according to the requirements of the control system, the valve positioner is connected to the valve/actuator stem and monitors its position throughout the control process. It is possible to control the valve very precisely based on the feedback of the valve stem position.
Automatic control systems have the following components: a fluid to be controlled, a sensor to determine the process variable, an actuator to modulate the valve, a controller to modulate the actuator, and a control valve to regulate flow.
As the flow of fluids in pipes is controlled by the interplay of these elements, any control system can, in principle, regulate all fluids flowing through pipes using valve automation such as a valve actuator. According to the needs of the process, several different systems can be used depending on how the system is implemented.
Through the use of valve automation, a valve actuator regulates the rate of flow through valves. Consequently, it needs to be designed so that it can withstand all the challenges that may arise while working in the field, and even the most difficult or unusual ones. All valves may not be appropriate for every single conceivable fluid flow condition, and this cannot be stated explicitly. It is imperative that the valves are installed to handle the conditions of operation specified. Over the years, many types of control valves have been developed for use in various applications. There are a number of these types of systems which cover a range of processes in high numbers whilst others have been developed for special requirements.
It is a general rule that ball valves are used for processes involving hydrocarbon oil and gas, as well as aggressive services where chemicals are present. Ball valves using actuators are usually used to control the flow of steam, rather than to throttle (control flow) using automation. As a result, use of the spherical ball can determine whether the flow is blocked by the inside diameter of the valve. When a hole is drilled into the ball by means of a lever, the hole in the ball will rotate 90°, and the flow will be allowed when the hole through the ball is drilled along the axis of the pipe.
With regard to high-pressure classes of reservoirs, injection lines leading to seawater injections are mixed in with the hydrocarbon services that provide enhanced oil recovery. A ball valve is by far the best valve to use in this type of application since it allows the valve to be switched on/off. In process pumping applications, there are other kinds of valves such as butterfly valves and wedge gate valves that are less robust than ball valves.
A big advantage of ball valves over through conduit gate (TCG) valves is that ball valves are more compact vertically than TCG valves. TCG valves fill up a lot of space vertically. However, their actuation occurs through a vertical mounted actuator that occupies a lot of space horizontally.
Fast opening applications are not suitable for ball valves since the valves open slowly. Generally, it is possible to reduce the opening time of a fail-open actuated valve through installation of a quick exhaust valve on the control panel to release the instrument air from the pneumatic actuator in the fail mode quickly. However, ball-valve seats and disks are in contact during the opening and closing, which can jeopardize the fast opening characteristic.
A smaller stem torque and a larger actuator are also necessary to move the relatively large and heavy ball. Moreover, moving the relatively large and heavy ball may take longer. Ball-valve manufacturers were consulted about the use of soft-seat balls for this application, but the manufacturers felt that the rapid opening of a soft seat ball valve in 2 seconds could lead to the soft seat being damaged due to the repeated contact with the ball.