An Actuator is an assembly/ component of a control valve which delivers force and motion to operate a valve based on control signal received. Actuators are termed as mover that requires an energy source which can be pneumatic, hydraulic or electric type. So basically actuators converts pneumatic, hydraulic or electric signal into force and motion to move a valve towards closing or opening.
A simple design of pneumatic actuator is shown as follows-
Classification of actuator is a wide area to explore. There are several type of actuators exist for operation of a control valve. A broad classification is given below-
Actuator classification based on Motion
Actuator receives a controlled energy source and convert into motion the motion can be either linear or rotary type. When actuators provide straight line motion to operate valve, are called as linear actuators. And when control signal/ energy to actuator drive valve in circular motion, it is called as Rotary Actuator. However linear actuator can also deliver rotary output with the help of mechanical gears. And Linear output can be achieved using rotary actuators same as by using mechanical gear/ conversion assembly.
A simple diagram for Linear and rotary actuator is shown as follows-
Actuator Classification based on design
Manual actuators are those in which manual intervention is required to operate a valve. There is no requirement of any type of energy source to move Manual actuators. These actuators can be used either linear motion control or rotary valves applications. Manual actuator utilizes lever and gear mechanism to enable movement of valve. Manual actuators with different operating mechanism are given as follows-
- Manual Actuators with handles
- Manual Actuator with hand wheel
- Manual Actuator with chain wheel/ chain bar
Diaphragm actuators are most common type widely used in industrial application. A flexible diaphragm is placed in between two casings. At one side of diaphragm (with plate) spring assembly is used which holds a spring opposing the force generated within the pressure chamber of the actuator to provide fail safe position. Another side of diaphragm, controlled pressure (air supply) is applied that results in a positioning force of the stem to operate valve.
This type of actuator is called single acting with spring return positioning force for linear motion. The actuator parts are such type designed that the actuator can be assembled in two fail safe positions (Fail to close or open). Diaphragm actuator’s main advantage is its Simplicity (design) that reduces maintenance and parts. Parts of a conventional diaphragm actuator is as follows-
There are several configuration of diaphragm actuator as mentioned follows-
Piston actuators are another type of actuator widely used in industrial actuator. It uses a piston cylinder assembly to provide final stem movement attached directly to the piston. The piston actuator have configuration either spring return or double acting type to operate valve. In spring return to fail safe position can be open or close. In spring return type piston actuator, controlled pressure is applied to another end of piston. When a piston actuator is used for double acting, controlled pressure is applied at both the side of piston. When pressure applied max at one end, valve stroke to full open. And when pressure applied max at another end, valve stroke to full close. Piston actuators can be operated either pneumatic, and or hydraulic pressure. Different parts of a typical piston actuator is shown as follows-
There are various types of design in piston actuator as shown below-
Rack and Pinion Actuators
Another type of actuator is Rack & pinion actuator which is used for mostly on-off application (i.e. to open and close valves) of quarter-turn valves like Butterfly, Ball, Plug valves & dampers usually in industrial applications. “Rack and pinion” is a generic term for a pair of gears which convert linear motion into rotational motion. A linear gear bar called “the rack” engages teeth on a circular gear called “the pinion”. Linear force exerted on the rack will cause a rotational motion of the pinion. A simple construction of actuator is shown as follows-
Rack & pinion mechanism uses two piston-type racks moving 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 turned to the desired position. The pinion bottom connects to the valve stem to open and close the valve as the pinion turns. These actuators are available in two constructions-
Spring Return- Mechanical Spring Return is for fail-safe applications and can be assembled for “Fail Close” or “Fail Open” safety function, and
Double Acting- Double Acting actuators can be used for a “Fail to lock Position” safety function.
Different parts of a rack & pinion actuator is shown as follows-
Actuator Classification based on controlling source
Pneumatic actuators are simple mechanical devices, that uses pneumatic signal or air pressure to move a valve mechanism i.e. controlled pneumatic signal is used to push either a flexible diaphragm or a piston against mechanical spring which results mechanical action or valve stroke. A key advantage of this design is that a pneumatic actuator can always reach a predefined safe condition, even after the loss of its primary power supply (air pressure or electric signal to the control components). it is the most important differentiator between pneumatic and electric actuators today. Almost all actuators are designed spring return single acting actuator (as opposed to double-acting). Pneumatic actuators are widely used in refineries, petrochemical plants, gas industries, pharma industries, & fertilizer plants etc.
A simple spring return pneumatic actuator is shown in figure-
- Pneumatic systems run on air pressure, a safe fluid medium.
- All types of fail-safe positions are available and can be configured with pneumatic actuator.
- Pneumatic control equipment is widely available and relatively inexpensive.
- Pneumatic control systems can be configured to achieve a vast range of functions.
- Heavy-duty pneumatic actuators can be used for modulating applications.
- Pneumatic actuators allow very high-speed operation.
- Pneumatic actuators are preferred for small to medium size valve application.
- Pneumatic actuators are typically more costly than equivalent torque electric actuators considering uses of control components such as solenoid valves, air filter regulators and other pneumatic instrumentation.
- At present, pneumatic actuators are not easily integrated into electronic database management.
- As valve size increases, pneumatic actuator size also increases, which leads slow operation of valve.
- For a larger size of actuator, larger pneumatic cylinders are used, storing very high air volumes, leading to more energy costs for compressed air and high weight impacting on piping construction support.
Hydraulic actuators are those actuators which uses hydraulic signal or fluid (mostly oil) pressure to move a valve mechanism i.e. controlled hydraulic oil pressure is used to force a piston rather than a diaphragm against mechanical spring or balancing pressure which results mechanical action or valve stroke. Thus high pressure hydraulic oil is applied to piston to convert fluid pressure into mechanical force. So it is necessary that piston (pressure rating) must be designed for high pressure application. Here hydraulic fluid (oil) used for actuators are non-compressible in nature & its lubricating property helps to overcome the friction problem of piston-type actuators. Hydraulic oil pressure is generated by a electric motor driven pump connected through a oil reservoir. The oil pressure can be used up to 400 bar (6000 PSI). Hydraulic actuators are used for extremely large force application. A basic requirement of hydraulic system including hydraulic actuator assembly is given below-
- Oil reservoir
- Oil temperature monitoring system
- Electric Motor driven pump
- Pressure monitoring system at different stages
- Relief valves
- Check Valves
- Particle Filter & moisture absorbing filter
- Manual valves, solenoid valve
- I/H convertor
- High pressure hoses/ tubing
- Piston cylinder assembly (Actuator)
A simple block diagram of hydraulic system & actuator is given below-
Some points to remember-
- Mostly Hydraulic actuators are designed for Fail safe position to Lock actuator & fail close or fail open position is achieved by ESD trip function.
- It should be noted that fluid pressure release cannot be done in open atmosphere so fluid return to reservoir path is provided additionally.
- Hydraulic actuators are designed for both linear action & as well as rotary action too.
- Since fluid pressure is very high so periodic inspection of tubing, hoses is highly recommended at regular interval.
Electric Actuators (motor operated) MOV
Electric actuators or motor operated valves (MOV) are used for large valve application such as damper. An electric motor is arranged for valve position control or for on- off application via motor control circuitry which receives control signal and produces required rotation. Advancement in motor design and motor control circuitry have brought motor operated valve (MOV) technology to the point so that it now competes with legacy actuator technologies such as pneumatic in actuating throttling valves as well.
A simple diagram of motor operated valve is shown in figure-
- Electric actuators provide superior positioning accuracy for control or modulating valve functions.
- Since electric actuator uses electric power to operate which is relatively inexpensive, easy to manage, and normally available to most industrial sites.
- Electric Actuators provide high degree of process monitoring, data logging and information feedback.
- The capital cost of electric actuators is typically cheaper per equivalent unit of torque/thrust output.
- All necessary control functions such as open, close & throttle position monitoring are integral to electric actuators.
- Electric actuators significantly reduce control wiring costs by enabling distributed control. They simplify control logic by integrating control commands and feedback into customer SCADA or DCS systems.
- As torque and thrust requirements increase, electric actuators weigh less and have smaller footprints compared to pneumatic actuators.
- Electric actuators may be combined with external gearboxes to produce extremely high output thrust and torque values.
Electro-hydraulic Actuators (EHA)
Electro-hydraulic Actuators are basically a type of hydraulic actuator with its unique compactness. These actuators are electrical power-only self-contained actuators. Electro-hydraulic Actuators eliminate the need for separate hydraulic pumps and tubing. In its basic form, electro-hydraulic actuators combine an electric driven motor and pump assembly to pressurize a reserve of hydraulic fluid, a hydraulic control group to handle open and/or close functionality, and a hydraulic cylinder with plunger and actuator stem to operate the valve. This basic design can be expanded to include a wide range of electrical or electronic control components, allowing command and feedback capabilities equivalent to those of electric actuators. Position control is often integrated, i.e. a force balance element with a generally (4 to 20)mA electric input signal and corresponding feedback via a range spring.
A simple construction of Electro-Hydraulic Actuator with its control functionality is shown as follows-
A flapper-nozzle system provides controlled pressure within 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 valve travel (valve stroke). Like an electro-mechanical actuator, it is possible to reverse the direction of movement and force. If a plant has numerous electro-hydraulic actuators, a central hydraulic unit is often used.
- Very high actuator thrusts attainable.
- Relatively high control rate (stroking speed) possible.
- High rigidity (travel stiffness) dampens oscillations of valve stem.
- Automatic failure position in case of auxiliary energy loss.
- Very heavy, extensive construction, expensive.
- In most cases, not every mounting position suitable.
- Protection against explosion attainable only with high cost.
- Unsuitable for extremely high and low temperatures.
- Sensitive to rough service conditions, frequent maintenance required
Gas powered actuators are specifically designed to be operated by high pressure natural gas, or any other high pressure pneumatic or hydraulic fluid. These actuators are ideal for pipeline applications where no external sources of motive power are present and provide a robust heavy duty design and long service life with industry leading maintenance intervals. This makes them the actuator of choice for installation in remote or unmanned facilities. There are two types of gas-powered actuators:
- Direct gas-
In this configuration, pipeline gas is piped directly into the actuator power cylinder.
This design pumps pipeline gas into a tank used to pressurize hydraulic fluid, which is subsequently piped to the actuator power cylinder.
A simple diagram of gas powered actuators is given below-
- Gas-powered actuators require no external power supply. Motive power is provided by the pipeline product and is always available for use.
- Pipeline pressure supports the use of large actuators in any environment, allowing isolation or fail-safe action either through a mechanical spring (in direct gas versions) or through stored hydraulic pressure (in gas-over-oil models).
- Direct gas actuators weigh less than gas-over-oil models and have lower initial capital cost because they require no tanks or hydraulic controls. They also have simpler control circuitry because they operate only on the gas supply.
- Gas-over-oil actuators have the advantage of using clean hydraulic fluid in the actuator power cylinder. This is especially significant in case of line gases with heavy particulate or corrosive contents.
- Despite the longevity benefits of gas-over-oil, markets that use gas-powered actuators are moving toward direct gas because of improved anti-corrosion treatments.
- The primary drawback of gas-powered actuators is tied to their main advantage. Using the pipeline product results in a relative “waste” of the product.
- More importantly, every stroke of the valve exhausts pipeline gas into the atmosphere with negative environmental effects. In these cases, an efficient torque mechanism and a smaller cylinder volume per unit of torque are important to reduce the amount of exhaust gas.
Actuator Classification based on acting ports
In single acting actuator, control pressure is applied at one inlet port of actuator to operate/ open the valve. And valve closing is determined by stored energy of spring. For closing of valve, the applied pressure proportionally is decreased and it is guided by spring return/ release. There are two type of possible action (without use of air lock relay)-
- Direct Acting Actuator (Apply pressure to open/ Fail close)
- Reverse Acting Actuator (Apply pressure to close/ Fail open)
When applied control pressure to actuator is used to open valve and energy at spring return used for safe valve closing, it is called as direct acting actuator. In direct acting actuator control signal failure results to close valve as fail safe action. When applied control pressure to actuator is used to close valve and energy at spring return used for safe valve opening, it is called as reverse acting actuator. In reverse acting actuator control signal failure results to open valve as fail safe action.
However, Fail to Lock position can be achieved by installing Air lock relay so that when control signal fails, pressure applied to actuator should be constant to hold/ lock the valve at same position.
In double acting actuator, there are two ports to apply control pressure in which controlled pressure at one port is used to move valve open and controlled pressure at another port is used to move valve towards closing. Mostly double acting actuator are fail to lock type. However fail open & fail close condition can also be achieved by using control valve accessories such as valve positioner and solenoid valve.
Actuator Classification based on control Actions/ Fail action
- Direct Acting Actuator ( Air to open/ Fail close)
- Reverse Acting Actuator (Air to close/ Fail open)
- Fail to Lock type
When applied control pressure to actuator is used to open valve and energy at spring return used for safe valve closing, it is called as direct acting actuator. In direct acting actuator control signal failure results to close valve as fail safe action.
When applied control pressure to actuator is used to close valve and energy at spring return used for safe valve opening, it is called as reverse acting actuator. In reverse acting actuator control signal failure results to open valve as fail safe action.
Fail to Lock actuators are those which hold/ lock the valve position at same position on signal failure occurs. In single actuator, it can be achieved by installing air lock relay and in double acting actuator, fail to lock can be achieved either by holding constant control pressure at both the ports of actuator or by releasing pressure at both the ports.