|This article needs additional citations for verification. (March 2008)|
Actuators are used for the automation of industrial valves and can be found in all kinds of technical process plants. They are used in waste water treatment plants, power plants, refineries, mining and nuclear processes, and pipelines. Valve actuators play a major part in automating process control. The valves to be automated vary both in design and dimension. The diameters of the valves range from a few inches to a few feet.
- 1 Classification of actuators according to their movement
- 2 Sample Design - Electric, Multi-Turn Actuator with Controls
- 3 Functions
- 4 Duty types
- 5 Service conditions
- 6 Additional uses
- 7 References
Classification of actuators according to their movement
Travel means the distance the closing element within the valve has to cover to completely open or close that valve. Typical closing elements include butterfly,ball, globe and gate valve discs. These three closing elements stand for the three basic movements required for covering the travel. In butterfly valve disc is operated by a 90° and the same applies to ball valve where the ball is operated by 90°swivel movement from end position OPEN to CLOSED, the globe valve disc is operated by a rather short linear movement (stroke) while the gate valve disc movement covers the full diameter of the valve. Each movement type requires a specific actuator type.
Any valve that can be operated (opened or closed) by means of turning or rotating its stem multiple times can be operated with a multi-turn actuator. A multi-turn actuator is described in the standard EN ISO 5210 as follows:
"A multi-turn actuator is an actuator which transmits to the valve a torque for at least one full revolution. It is capable of withstanding thrust."
A gate valve is an example of a valve that can be operated by turning its stem multiple times. However, a gate valve can also be operated by a Linear Actuator [see below] which attaches a piston and cylinder to the valve stem and provides an upward force on the valve. This is possible since a gate valve is a rising valve. This differs from a ball valve which must be operated by turning its stem.
A valve stem is mounted to the gate valve disc. The multi-turn actuator moves the gate valve disc from OPEN to CLOSED and vice versa via a stem nut. To cover the complete valve travel, the so-called valve stroke, the actuator has to perform – depending on the valve – a few or several hundred rotations. Due to their design, the stroke of electric actuators, contrary to that of their pneumatic counterparts, has no limits.
The multi-turn actuator has to be able to withstand the weight of the gate valve disc by means of the valve attachment, the interface to the valve. This is expressed in the second sentence of the definition.
Gate valves may have a diameter of approx. 4 inches to several meters. The torque requirement for multi-turn solutions ranges from approx. 10 N m to 30,000 N m. When a gate valve is operated by a Linear Actuator, the proper specification is not torque, but instead is thrust since it is a linear motion instead of a turning motion. The proper units of measure in this case would be force expressed in pounds or Newtons most often. Some thrusts can be as high as 200,000 lbs.
Part-turn actuators are required for the automation of part-turn valves. Major representatives of this type are butterfly valves and ball valves. The basic requirements on part-turn actuators are described in the standard EN ISO 5211 as follows:
"A part-turn actuator is an actuator which transmits a torque to the valve for less than one full revolution. It need not be capable of withstanding thrust."
Less than one full revolution usually means a swivel movement of 90°; however, there are some valve types requiring a different swing angle, such as two-way valves. The closing elements in part-turn actuators are always supported by the valve housing, i.e. the weight of the closing element does not act upon the part-turn actuator. This is expressed in the second sentence of the definition.
Part-turn valves diameters range from a few inches to several metres. The torque requirement for operating the closing element has a comparable range from approximately 10 N m to several 100,000 N m. Electric actuators are unrivalled for large-diameter valves with high torque requirements.
A Linear Actuator as applied to valve automation is an actuator that opens and closes valves that can be operated via linear force--sometimes called rising stem valves. Some examples of these types of valves include, globe valves, gate valves, rising stem ball valves, and control valves. The two main types of Linear Actuators are diaphragm and piston. Unlike a Multi-turn actuator, these types of actuators require some type of fluid to operate--most commonly air pressure.
Diaphragm actuators are made out of a round piece of rubber and squeezed around its edges between two side of a cylinder or chamber that allows air pressure to enter either side pushing the piece of rubber one direction or the other. A rod is connected to the center of the diaphragm so that it moves as the pressure is applied. The rod is then connected to a valve stem which allows the valve to experience the linear motion thereby opening or closing. A diaphragm is when the supply pressure is low, the travel of the valve is small, and the thrust required is small.
Piston actuators operate by the same basic principle as diaphragm actuators, but instead of applying fluid pressure to a piece of restrained rubber, pressure is applied to a steel piston which moves along the length of a steel cylinder. Again, there is a rod (piston rod) with one side attached to the piston and the other side attached to the valve stem which allows the valve to experience the linear motion. Piston actuators typically allow for higher pressures, allow for longer travel ranges for larger valves, and can provide higher thrusts than diaphragm actuators.
A spring is used to provide a Failure Mode in the case of loss of power. This is important in safety related incidents and is sometimes the driving factor in specifications. An example of loss of power is when the air compressor (the main source of compressed air that provides the fluid for the actuator to move) shuts down. If there is a spring inside of the actuator, it will force the valve open or closed and will keep it in that position while power is restored. In the case of an electric actuator, losing power will keep the valve stationary unless there is a back up power supply.
Currently there is no international standard describing linear actuators or linear thrust units. A typical representative of the valves to be automated is the control valve. Just like the plug in the bathtub is pressed into the drain, the plug is pressed into the plug seat by a stroke movement. The pressure of the medium acts upon the plug while the thrust unit has to provide the same amount of thrust to be able to hold and move the plug against this pressure.
Sample Design - Electric, Multi-Turn Actuator with Controls
Robust asynchronous three-phase AC motors are mostly used as the driving force, for some applications also single-phase AC or DC motors are used. These motors are specially adapted for valve automation as they provide higher torques from standstill than comparable conventional motors, a necessary requirement to unseat sticky valves. The actuators are expected to operate under extreme ambient conditions, however they are generally not used for continuous operation since the motor heat buildup can be excessive.
Limit and torque sensors (2)
The limit switches signal when an end position has been reached. The torque switching measures the torque present in the valve. When exceeding a set limit, this is signalled in the same way. Actuators are often equipped with a remote position transmitter which indicates the valve position as continuous current or voltage signal.
Often a worm gearing is used to reduce the high output speed of the electric motor. This enables a high reduction ratio within the gear stage, leading to a low efficiency which is desired for the actuators. The gearing is therefore self-locking i.e. it prevents accidental and undesired changes of the valve position by acting upon the valve’s closing element. This is of major importance for multi-turn actuators which are axially loaded with the weight of the gate valve disc.
Valve attachment (4)
The valve attachment consists of two elements. First: The flange used to firmly connect the actuator to the counterpart on the valve side. The higher the torque to be transmitted, the larger the flange required.
Second: The output drive type used to transmit the torque or the thrust from the actuator to the valve shaft. Just like there is a multitude of valves there is also a multitude of valve attachments.
Dimensions and design of valve mounting flange and valve attachments are stipulated in the standards EN ISO 5210 for multi-turn actuators or EN ISO 5211 for part-turn actuators. The design of valve attachments for linear actuators is generally based on DIN 3358.
Manual operation (5)
In their basic version most electric actuators are equipped with a handwheel for operating the actuators during commissioning or power failure. The handwheel does not move during motor operation.
The electronic torque limiting switches are not functional during manual operation. Mechanical torque-limiting devices are commonly used to prevent torque overload during manual operation.
Actuator controls (6)
Both actuator signals and operation commands of the DCS are processed within the actuator controls. This task can in principle be assumed by external controls, e.g. a PLC. Modern actuators include integral controls which process signals locally without any delay. The controls also include the switchgear required to control the electric motor. This can either be reversing contactors or thyristors which, being an electric component, are not subject to mechanic wear. Controls use the switchgear to switch the electric motor on or off depending on the signals or commands present. Another task of the actuator controls is to provide the DCS with feedback signals, e.g. when reaching a valve end position.
Electrical connection (7)
The supply cables of the motor and the signal cables for transmitting the commands to the actuator and sending feedback signals on the actuator status are connected to the electrical connection. The electrical connection can be designed as a separately sealed terminal bung or plug/socket connector. For maintenance purposes, the wiring should be easily disconnected and reconnected.
Fieldbus connection (8)
Fieldbus technology is increasingly used for data transmission in process automation applications. Electric actuators can therefore be equipped with all common fieldbus interfaces used in process automation. Special connections are required for the connection of fieldbus data cables..
Automatic switching off in the end positions
After receiving an operation command, the actuator moves the valve in direction OPEN or CLOSE. When reaching the end position, an automatic switch-off procedure is started. Two fundamentally different switch-off mechanisms can be used. The controls switch off the actuator as soon as the set tripping point has been reached. This is called limit seating. However there are valve types for which the closing element has to be moved in the end position at a defined force or a defined torque to ensure that the valve seals tightly. This is called torque seating. The controls are programmed as to ensure that the actuator is switched off when exceeding the set torque limit. The end position is signalled by a limit switch.
The torque switching is not only used for torque seating in the end position, but it also serves as overload protection over the whole travel and protects the valve against excessive torque. If excessive torque acts upon the closing element in an intermediate position, e.g. due to a trapped object, the torque switching will trip when reaching the set tripping torque. In this situation the end position is not signalled by the limit switch. The controls can therefore distinguish between normal operation torque switch tripping in one of the end positions and switching off in an intermediate position due to excessive torque.
Temperature sensors are required to protect the motor against overheating. For some applications by other manufacturers, the increase of the motor current is also monitored. Thermoswitches or PTC thermistors which are embedded in the motor windings mostly reliably fulfil this task. They trip when the temperature limit has been exceeded and the controls switch off the motor.
Process control functions
Due to increasing decentralisation in automation technology and the introduction of micro processors, more and more functions have been transferred from the DCS to the field devices. The data volume to be transmitted was reduced accordingly, in particular by the introduction of fieldbus technology. Electric actuators whose functions have been considerably expanded are also affected by this development. The simplest example is the position control. Modern positioners are equipped with self-adaptation i.e. the positioning behaviour is monitored and continuously optimised via controller parameters.
Meanwhile, electric actuators are equipped with fully-fledged process controllers (PID controllers). Especially for remote installations, e.g. the flow control to an elevated tank, the actuator can assume the tasks of a PLC which otherwise would have to be additionally installed.
Modern actuators have extensive diagnostic functions which can help identify the cause of a failure. They also log the operating data. Study of the logged data allows the operation to be optimised by changing the parameters and the wear of both actuator and valve to be reduced.
If a valve is used as a shut-off valve, then it will be either open or closed and intermediate positions are not held.
Defined intermediate positions are approached for setting a static flow through a pipeline. The same running time limits as in open-close duty apply.
The most distinctive feature of a closed-loop application is that changing conditions require frequent adjustment of the actuator, for example, to set a certain flow rate. Sensitive closed-loop applications require adjustments within intervals of a few seconds. The demands on the actuator are higher than in open-close or positioning duty. Actuator design must be able to withstand the high number of starts without any deterioration in control accuracy.
Actuators are used worldwide, in all climate zones, in all kinds of industrial plants under special local ambient conditions. The applications are often safety related, therefore the plant operators put high demands on the reliability of the devices. Failure of an actuator may cause accidents in process-controlled plants and toxic substances may leak into the environment.
Process-controlled plants are often operated for several decades which justifies the higher demands put on the lifetime of the devices.
For this reason, actuators are always designed in high enclosure protection. The manufacturers put a lot of work and knowledge into corrosion protection.
The enclosure protection types are defined according to the IP codes of EN 60529. The basic versions of most electric actuators are designed to the second highest enclosure protection IP 67. This means they are protected against the ingress of dust and water during immersion (30 min at a max. head of water of 1 m). Most actuator manufacturers also supply devices to enclosure protection IP 68 which provides protection against submersion up to a max. head of water of 6 m.
In Siberia, temperatures down to – 60 °C may occur, and in technical process plants + 100 °C may be exceeded. Using the proper lubricant is crucial for full operation under these conditions. Greases which may be used at room temperature can become too solid at low temperatures for the actuator to overcome the resistance within the device. At high temperatures, these greases can liquify and lose their lubricating power. When sizing the actuator, the ambient temperature and the selection of the correct lubricant are of major importance.
Actuators are used in applications where potentially explosive atmospheres may occur. This includes among others refineries, pipelines, oil and gas exploration or even mining. When a potentially explosive gas-air-mixture or gas-dust-mixture occurs, the actuator must not act as ignition source. Hot surfaces on the actuator as well as ignition sparks created by the actuator have to be avoided. This can be achieved by a flameproof enclosure, where the housing is designed to prevent ignition sparks from leaving the housing even if there is an explosion inside.
Actuators designed for these applications, being explosion-proof devices, have to be qualified by a test authority (notified body). There is no worldwide standard: depending on the country where the actuators are used, different directives and regulations have to be observed. Within the European Union, ATEX 94/9/EC applies, in US, the NEC (approval by FM) or the CEC in Canada (approval by the CSA). Explosion-proof actuators have to meet the design requirements of these directives and regulations.
Small electric actuators can be used in a wide variety of assembly, packaging and testing applications. Such actuators can be linear, rotary, or a combination of the two, and can be combined to perform work in three dimensions. Such actuators are often used to replace pneumatic cylinders. 
|Wikimedia Commons has media related to Valve actuators.|