Actuators serve the general purpose of controlling movements within machines. However, there are various kinds of actuators that produce varying motions and use different power sources. And this article covers five common types of actuators and their applications with examples.
Types of Actuators
A manual actuator employs levers, gears, or wheels to move the valve stem with a certain action. Manual actuators are powered by hand.
The main advantages of manual actuators are inexpensive, typically self-contained, and easy to operate by humans.
The main disadvantages is that some large size ball valves are impossible to operate manually and some valves may be located in remote, toxic, or hostile environments that prevent manual operations in some conditions. As a safety feature, certain types of situations may require quicker operation than manual actuators can provide to close the valve.
Air (or other gas) pressure is the power source for pneumatic valve actuators. They are used on linear or quarter-turn valves. Air pressure acts on a piston or bellows diaphragm creating linear force on a valve stem. Alternatively, a quarter-turn vane-type actuator produces torque to provide rotary motion to operate a quarter-turn valve. A pneumatic actuator may be arranged to be spring-closed or spring-opened, with air pressure overcoming the spring to provide movement.
(1) Most of the actuators used in industrial control occasions are pneumatic actuators, because it is more economical to use air source as power than electric and hydraulic, and the structure is simple, easy to master and maintain. From the point of view of maintenance, pneumatic actuators are easier to operate and calibrate than other types of actuators, and the front and back can be easily interchanged on the spot.
(2) From the point of view of maintenance, pneumatic actuators are easier to operate and calibrate than other types of actuators, and the front and back can be easily interchanged on site.
(3) Its biggest advantage is safety. When using the positioner, it is ideal for flammable and explosive environments, and if the electrical signal is not explosion-proof or intrinsically safe, there is a potential fire hazard due to ignition.
Slow response, poor control accuracy, and poor anti-deviation ability, which is due to the compressibility of gas, which is the need for air to fill the cylinder and empty it when using a large pneumatic actuator.
Hydraulic actuators convert fluid pressure into motion. Similar to pneumatic actuators, they are used on linear or quarter-turn valves. Fluid pressure acting on a piston provides linear thrust for gate or globe valves. A quarter-turn actuator produces torque to provide rotary motion to operate a quarter-turn valve.
The main advantages of Most types of hydraulic actuators can be supplied with fail-safe features to close or open a valve under emergency circumstances. Hydraulic pressure can be supplied by a self-contained hydraulic pressure pump. In some applications, such as water pumping stations, the process fluid can provide hydraulic pressure, although the actuators must use materials compatible with the fluid.
The main disadvantages is that Hydraulic actuators are expensive, bulky, complex and requires special engineering, so most of them are used in some special occasions such as power plants and petrochemicals.
Electric valve actuator mounted on a pressure reducing needle valve. The electric actuator uses an electric motor to provide torque to operate a valve. They are quiet, non-toxic and energy efficient. However, electricity must be available, which is not always the case, they can also operate on batteries.
Main applications of electric actuators:
(1) Power Plant.
(2) Process control.
(3) Industrial automation.
The main advantages of electric actuators are a high degree of stability and the output thrust or torque is basically constant. It can well overcome the unbalanced force of the medium and achieve accurate control of the process parameters, so the control accuracy is higher than that of pneumatic actuators.
The main disadvantage is that the structure of the electric actuators are more complex and more prone to failure. Due to its complexity, the technical requirements for field maintenance personnel are relatively higher.
Spring-based actuators hold back a spring. Once any anomaly is detected, or power is lost, the spring is released, operating the valve.
The main advantages of they don’t require a powerful electric supply to move the valve, so they can operate from restricted battery power, or automatically when all power has been lost.
The main disadvantage is that they can only operate once, without resetting, and so are used for one-use purposes such as emergencies.
Discs lie in between the body and the ball. Seats provide a seal between the two and support the ball.
For ball valves, suitable ball valve seat materials can be selected according to different working environments.
(1) Peek Ball Valve Seat
It is a high temperature resistant thermoplastic and can be used for a long time at 250°C. Compared with other high temperature resistant plastics such as ZJCLV, etc., the upper limit of the operating temperature is nearly 50°C higher, and PEEK resin is not heat resistant. Compared with other high temperature resistant plastics, the cloth mesh has high strength, high modulus, high fracture toughness and excellent dimensional stability.
(2) PTFE Ball Valve Seat
a. PTFE is an engineering plastic with excellent comprehensive properties, known as the king of plastics.
With excellent chemical stability, even at high temperature and concentrated acid, concentrated alkali or strong oxidant, it does not work. And boiled in concentrated sulfuric acid, concentrated nitric acid, hydrochloric acid or even in aqua regia, it’s weight and performance also are unchanged.
b. It has high heat resistance and cold resistance.
c. It has a very low coefficient of friction and is a good self-lubricating material.
d. It has excellent water resistance, does not absorb water at all, and has good weather resistance and anti-aging properties.
The ball is designed with a center bore (hole)where the media flows through. The direction of the ball is controlled by turning the stem.They are usually made of stainless steel, PVC, chrome plated steel or chrome plated brass. The ball design is categorized as floating or trunnion based on their design and support.
The bore size of these valves is smaller than the diameter of the inlet/outlet connections. Thus, there is a friction loss in the system. However, the losses are still small in comparison to other valves. Most ball valves use a reduced bore design.
(2) Full Bore
A full bore design is also called a full port design. The bore size of these valves is the same size as the diameter of the pipe. Thus, there is no extra friction loss and they are also easy to clean. However, due to its larger diameter, the cost is slightly higher and not required for most application, for which standard reduced bore is sufficient.
(3) V-shaped Bore
The bore in these valves have a V-shaped profile. Thus, a precise flow rate can be achieved by rotating the ball. A linear flow can be achieved by optimizing the profile of the V-shaped bore.
Although most small ball valves can use handwheels, large ball valves or some other dangerous working environments are not suitable for this method. So there are also other ball valve actuators.
(1) Manual Control
These are designed with handles or levers that must be controlled by an operator. This option is not intended for high-cycle applications.