Actuator is an assembly that is part of a control valve that provides force and movement to operate a valve based on the received control signal. Actuators are actuators that require an energy source, which can be pneumatic, hydraulic or electric. So basically, actuators convert a pneumatic, hydraulic or electrical signal into force and motion to move the valve to close or open.
A simple design of a pneumatic actuator is shown below:
Classification of actuators
Actuator classification is a vast area to explore. There are different types of actuators to operate a control valve. A broad classification is given below
Actuator classification based on movement
- Linear actuator
- Rotary actuator
Actuator classification based on design
- Manual actuator
- Diaphragm actuator
- Piston actuator
- Rack and pinion actuators
Actuator classification based on control source
- Pneumatic actuators
- Hydraulic actuators
- Electric actuators (motor)
- Electro-hydraulic actuators
- Gas actuators
Actuator classification based on active ports
- single
- Double
Actuator classification based on control measures/failure performance
- Direct actuator (air to open/close with failure)
- Reverse actuator (air to close/not open)
- The type could not be locked
Actuator classification based on movement
The actuator receives a controlled energy source and converts it into motion, which can be linear or rotary. When actuators provide straight line motion for valve operation, they are called linear actuators. And when the control signal/energy to the actuator valve in circular motion, it is called rotary actuator. However, a linear actuator can also provide rotary output with the help of mechanical gears. And linear output can be achieved using rotary actuators such as using a mechanical gear/transformer assembly.
A simple diagram for a linear and rotary actuator is shown below:
Actuator classification based on design
Manual actuator
Manual actuators are those that require manual intervention to operate a valve. No power source is needed to move manual actuators. These actuators can be used to control linear movement or rotary valves. A manual actuator uses a lever and gear mechanism to activate valve movement. Manual actuators are offered with different functional mechanisms as follows:
- Manual actuators with handles
- Manual actuator with gearbox
- Manual actuator with chain wheel / chain bar
Diaphragm actuator
Diaphragm actuators are the most common type widely used in industrial applications. A flexible diaphragm is placed between the two chambers. A spring assembly is used on one side of the diaphragm (with plate) which holds the spring against the force generated in the actuator pressure chamber to provide a fail safe position. On the other side of the diaphragm, controlled pressure (air supply) is applied, which results in the stem positioning force for valve operation.
This type of actuator is called single acting with spring return positioning force for linear movement. Actuator parts are designed so that the actuator can be assembled in two safe positions (open or failed close). The main advantage of the diaphragm actuator is its simplicity (design), which reduces maintenance and parts. The parts of a typical diaphragm actuator are as follows:
There are several configurations of diaphragm actuators as listed below:
Piston actuator
Piston actuators are another type of actuators that are widely used in industrial actuators. It uses a piston cylinder assembly to provide final movement to the rod attached to the piston. The piston actuator has either a spring return type configuration or two actuators for valve operation. In the fail-safe position return spring can be open or closed. In the spring return type piston actuator, controlled pressure is applied to the other end of the piston. When a piston actuator is used for dual action, controlled pressure is applied to both sides of the piston. When maximum pressure is applied at one end, fully open the valve. And when the maximum pressure is applied at the other end, close the valve completely. Piston actuators can work pneumatically or hydraulically. The different parts of a typical piston actuator are shown below:
Rack and pinion actuators
Another type of actuator is rack & pinion actuator, which is mostly used for on and off applications (i.e. opening and closing valves) of quarter turn valves such as butterfly valves, hubs, plugs and dampers, usually used in industrial applications. “Rack and pinion” is a general term for a pair of gears that convert linear motion into rotary motion. A linear gear rod called “rack” engages the teeth on a circular gear called “pinion”. The linear force applied to the rack causes the pinion to rotate. A simple structure of the actuator is shown below:
The Rack & Pinion mechanism uses two piston racks that move in opposite directions to ensure balanced forces on the pinion. Typically, pneumatic air pressure is used to power the actuator. By applying pressure to the piston racks, the pinion can be rotated to the desired position. The bottom of the pinion is connected to the valve shaft to open and close with the rotation of the valve pinion. These operators are available in two structures:
Spring Return – Mechanical spring return for safety applications and can be assembled for fail close or fail open safety function.
Dual Function – Dual actuators can be used for “position not locked” safety function.
The different parts of a rack and pinion actuator are shown below
Actuator classification based on control source
Pneumatic actuators
Pneumatic actuators are simple mechanical devices that use a pneumatic signal or air pressure to move a valve mechanism, that is, a controlled pneumatic signal is used to push a flexible diaphragm or a piston against a mechanical spring, resulting in the mechanical action or stroke of the valve. A key advantage of this design is that a pneumatic actuator can always reach a predefined safe condition, even after losing its primary power supply (air pressure or electrical signal to control components). Today, the most important factor is the difference between pneumatic and electric actuators. Almost all actuators are designed as single acting spring return actuators (as opposed to double acting). Pneumatic actuators are widely used in refineries, petrochemicals, gas industries, pharmaceutical industries and fertilizer factories, etc.
A simple spring return pneumatic actuator is shown in figure.
Positive features of pneumatic actuators
- Pneumatic systems work with air pressure, which is a safe fluid medium.
- All types of safe positions are available and can be configured with pneumatic actuators.
- Pneumatic control equipment is widely available and relatively inexpensive.
- Pneumatic control systems can be configured to achieve a wide range of functions.
- Heavy duty pneumatic actuators can be used to adjust applications.
- Pneumatic actuators enable very high speed operation.
- Pneumatic actuators are preferred for small to medium size valve applications
Negative features of pneumatic actuators
- Pneumatic actuators are typically more expensive than equivalent torque electric actuators due to the use of control components such as solenoid valves, air filter regulators, and other pneumatic instrumentation.
- Currently, pneumatic actuators are not easily integrated into electronic database management.
- By increasing the size of the valve, the size of the pneumatic actuator also increases, which leads to the slow operation of the valve.
- For larger actuator sizes, larger pneumatic cylinders are used that store very high volumes of air, resulting in higher energy costs for compressed air and high weight on the pipework support.
Hydraulic actuators
Hydraulic actuators are actuators that use a hydraulic signal or fluid pressure (mostly oil) to move a valve mechanism, i.e. controlled hydraulic oil pressure is used to push a piston instead of a diaphragm against a mechanical spring or balancing pressure that leading to mechanical action or valve impact. Therefore, high pressure hydraulic oil is applied to the piston to convert fluid pressure into mechanical force. Therefore, it is necessary to design the piston (pressure rating) to apply high pressure. Here, the hydraulic fluid (oil) used for the actuators is incompressible in nature and its lubricating property helps to overcome the friction problem of piston actuators. Hydraulic oil pressure is generated by an electric motor pump connected through an oil tank. Oil pressure up to 400 bar (6000 PSI) can be used. Hydraulic actuators are used to apply very large forces.
- oil tank
- Oil temperature monitoring system
- heater
- Electric motor pump
- Pressure monitoring system in different stages
- Relief valve
- Check valves
- Particle filter and moisture absorbing filter
- Manual valves, electric valves
- I/H converter
- High pressure hose/pipe
- piston cylinder set (actuator)
A simple block diagram of the hydraulic and actuator system is given below:
A few things to remember
Hydraulic actuators are mainly designed for the fail safe position to lock the actuator and the closed or fail position is achieved by the ESD opening function.
It should be noted that releasing the fluid pressure in the open atmosphere is not possible, so the return of the fluid to the tank path is additionally provided.
Hydraulic actuators are designed for both linear and rotary operation.
As the fluid pressure is very high, periodic inspection of pipes, hoses at regular intervals is highly recommended.
Electric actuators (motor) MOV
Electric actuators or motorized valves (MOV) are used for large valve applications such as dampers. An electric motor is arranged to control valve position or for on and off application through a motor control circuit that receives the control signal and produces the required rotation. Advances in motor design and motor control circuits have brought motorized valvetrain (MOV) technology to the point where it now competes with older actuator technologies such as pneumatics in actuating gas valves.
A simple diagram of a motorized valve is shown in Fig.
Negative features
- Electric actuators provide exceptional positioning accuracy for controlling or modulating valve operation.
- Since the electric actuator uses electrical power to operate, it is relatively cheap, easy to manage, and commonly available in most industrial sites.
- Electric actuators provide a high degree of process monitoring, data logging and information feedback.
- The capital cost of electric actuators is usually cheaper per equivalent unit of output torque/thrust.
- All necessary control functions such as open, closed and throttle position monitoring are part of electric actuators.
- Electric actuators significantly reduce control wiring costs by enabling distributed control. They simplify control logic by integrating control commands and feedback into the customer’s SCADA or DCS systems.
- With increased torque and thrust requirements, electric actuators weigh less and have a smaller footprint compared to pneumatic actuators.
- Electric actuators may be combined with external gearboxes to produce very high thrust and torque values.
Electro-hydraulic actuators ( EHA )
Electro-hydraulic actuators are basically a type of hydraulic actuators with their unique compression. These operators are independent actuators with only electric power. Electro-hydraulic actuators eliminate the need for separate hydraulic pumps and pipes. In their basic form, electrohydraulic actuators combine an electric actuator motor and pump assembly to pressurize a reservoir of hydraulic fluid, a hydraulic control group to control the opening and/or closing action, and a hydraulic cylinder with an actuator piston and shaft. . valve. This basic design can be extended to include a wide range of electrical or electronic control components, providing command and feedback capabilities equivalent to electric actuators. The position control is often integrated, i.e. a force balance element with an electrical input signal generally (4 to 20 mA) and corresponding feedback via a spring board.
A simple structure of an electro-hydraulic actuator with its control function is shown below:
A flapper nozzle system provides controlled pressure inside the double-acting hydraulic cylinder through a special hydraulic high-pressure valve until the desired position of the valve stem is reached. This results in a proportional relationship between the input signal and the valve movement (stroke). Like an electromechanical actuator, it is possible to reverse the direction of movement and force. If a power plant has multiple electro-hydraulic actuators, a central hydraulic unit is often used.
Positive features
- Very high actuator thrust is achievable.
- A relatively high control rate (stroking speed) is possible.
- High stiffness (travel stiffness) reduces valve stem oscillations.
- Automatic failover position in case of loss of auxiliary power.
Negative features
- Very heavy, extensive construction, expensive.
- In most cases, not every installation position is suitable.
- Explosion protection can only be achieved at high cost.
- Not suitable for very high and low temperatures.
- Sensitive to harsh service conditions, frequent maintenance is required
Gas actuators
Gas actuators are specifically designed to operate with high pressure natural gas or any high pressure pneumatic or hydraulic fluid. Ideal for pipeline applications where there is no external source of actuator force, these actuators offer a robust heavy-duty design and long life with industry-leading maintenance intervals. This makes them the actuator of choice for installation in remote or unmanned installations. There are two types of gas actuators:
- Direct gas
In this configuration, pipeline gas is directed directly to the actuator power cylinder.
- Gas over oil
This design pumps pipeline gas into a reservoir used to pressurize hydraulic fluid, which is subsequently piped to the actuator power cylinder.
A simple diagram of gas actuators is given below:
Advantages of gas actuators
- Gas actuators do not need an external power source. Actuator power is supplied by the pipeline product and is always available for use.
- Line pressure supports the use of large actuators in any environment and allows isolation or safe operation via a mechanical spring (on direct gas versions) or via stored hydraulic pressure (on gas over oil models).
- Direct gas actuators weigh less than oil models and have a lower initial capital cost because they do not require a reservoir or hydraulic control. They also have simpler control circuits because they only operate on a gas source.
- Gas actuators have the advantage of using clean hydraulic fluid in the actuator power cylinder over oil. This is particularly noticeable in the case of linear gases with heavy particles or corrosive contents.
- Despite the durability advantages of gas over oil, markets using gas-fired actuators are moving toward direct gas due to improved anti-corrosion treatments.
Disadvantages of gas actuators
- The main drawback of gas actuators is tied to their main advantage. Using the pipeline product leads to a relative “waste” of the product.
- More importantly, each blow of the valve discharges pipeline gas into the atmosphere with negative environmental effects. In these cases, an efficient torque mechanism and a lower cylinder volume per torque unit are important to reduce the amount of exhaust gas.
Actuator classification based on active ports
Single action
In a single-action actuator, control pressure is applied to an actuator inlet port to operate/open the valve. And the closing of the valve is determined by the stored energy of the spring. For the valve to close, the applied pressure is proportionally reduced and guided by the return/release of the spring. There are two possible types of operation (without using the air lock relay) –
- Direct Actuator (press to open/close on failure)
- Reverse actuator (press to close/not open)
When the control pressure applied to the actuator is used to open the valve and the energy in the spring return is used to safely close the valve, it is called a direct acting actuator. In direct actuator control, signal failure results in valve closing as a fail safe action. When the control pressure applied to the actuator is used to close the valve and the energy in the spring return is used to safely open the valve, it is called a reverse acting actuator. In reverse actuator control, signal failure causes the valve to open as a fail safe action.
However, the Fail to Lock position can be achieved by installing an air lock relay, so that when the control signal fails, the pressure applied to the actuator must be constant to hold/lock the valve in the same position.
Double action
In the double action actuator, there are two ports to apply the control pressure, in which the controlled pressure in one port is used to open the valve and the controlled pressure in the other port is used to move the valve towards closing. Mainly, double-acting actuators are non-locking type. However, failure opening and closing conditions can also be obtained by using control valve accessories such as valve positioners and solenoid valves.
Actuator classification based on control measures/failure performance
- Direct actuator (air to open/close with failure)
- Reverse actuator (air to close/not open)
- The type of lock was fail lock
When the control pressure applied to the actuator is used to open the valve and the energy in the spring return is used to safely close the valve, it is called a direct acting actuator. In direct actuator control, signal failure results in valve closing as a fail safe action.
When the control pressure applied to the actuator is used to close the valve and the energy in the spring return is used to safely open the valve, it is called a reverse acting actuator. In reverse actuator control, signal failure causes the valve to open as a fail safe action.
Fail to Lock actuators are those that hold/lock the valve position in the same position when a signal failure occurs. In the single actuator, it is possible to install the air lock relay, and in the dual actuator, non-locking can be achieved by keeping the constant control pressure in both ports of the actuator or by releasing the pressure in both ports.