The control valves can be operated pneumatically, electrically, hydraulically or by a combination of these The first is mostly used.
The forces that the actuators must overcome are caused by the pressure drop through the valve, the friction between the fluid and moving parts, the weight of these parts and the imbalance of the rod that becomes significant for large pressure drops. Mention the basic characteristics of the actuators used for modulation of service and therefore exclude the solenoid valves and other mechanical and electrical operators used in on-off service.
Let's start ...
TIRES
They fall into two basic types of spring and the diaphragm and the cylinder or piston (not spring). The first is the most used and can be "off air" (air-to-lower) or "air to rise" (air-to-raise). Here, an interest which side enters the pneumatic signal from the controller. If the top of the diagram, requires that the deformation of it, causes a downward displacement of the rod, however if air enters the chart below, the motion is upward.
If the valve action is push-to-close, lower the rod due to the action of the pneumatic signal will go the same closing is controlled allowing fluid flow continuously. If the valve action is push-to-open, the drop will cause the rod itself, will open as the pneumatic signal is applied.
Therefore, the combination of actuator and valve action depends Sa wálvuia control. If the pneumatic signal originates at the final closing of the valve (air-to-close) will talk about direct action on the contrary, the valve opens (air-to-open), an action shall be immersed.
The variation of the action of the valve can be easily accomplished with a reversible actuator, however this can be costly. Fortunately you can also find a non-reversible actuator, not necessarily changing the valve, but with the help of a valve positioner as discussed below.
Figure 7 Bridges pneumatic actuator
Diagram actuators are reversible, in which the air causes a movement of the rod down or up according to the method of attachment of the actuator.
Figure 8 Actuator Actions
Actuators without spring, piston or cylinder type, are used when a great power or require faster action. The first results from the ability to handle higher feed pressures.
ELECTRICAL
Generally used in areas where there is no supply air supply or when you want without pneumatic systems. They are classified by type of energy used to propel them and the type of movement required. Use single or three phase electric motors and gears, both in those so-called multi-turn as in a quarter turn. Most of them have a steering wheel clutch mechanism for manual operation in case of power failure.
Modern actuators have special components such as relays for monitoring, protection stuck valve, thermostat, etc.. Current technology allows them to be controlled remotely via a two wire connection. This will be connected to a network served by many industrial protocol.
Figure 10 Actuator
ELECTRO
They combine the action of the electrical control signal (0 / 4 ~ 20mA), with the force that can be achieved by hydraulic pressure, coupling these with a system of balance of forces. Typically, these systems operate at pressures up to 3,000 psi, providing power and speed to control requirements of high accuracy.
Figure 11 Electrohydraulic Actuator (courtesy of Samson)
