Valve actuator

In an electric motor drive for pipe fittings is a special type of actuators. They are preferably used in pipeline and plant construction.

• 3.1 Motor ( 1)
• 3.2 and torque sensor ( 2)
• 3.3 gearbox ( 3)
• 3.4 Connection of fittings ( 4)
• 3.5 manual override ( 5)
• 3.6 Actuator controls (6 )
• 3.7 Electrical connection ( 7)
• 3.8 fieldbus connection (8)

• 4.1 Automatic switch-off in the end positions
• 4.2 Safety functions
• 4.3 Process Engineering Functions
• 4.4 Diagnostics

• 6.1 Control Operation
• 6.2 positioning
• 6.3 Control mode

• 7.1 Degree of protection
• 7.2 ambient temperatures
• 7.3 Explosion protection

Generally

Base is a commercially available AC motor with Links-/Rechtslauf. A transmission produces the necessary torque. Due to this reduction, the travel time is high, which is not usually important. For large diameters traversing are common of a few minutes.

The transmission allows the simultaneous placement of a hand wheel. With this you can close the valve are available on site or open. There are a mechanical interlock. Is this to "Manual ", then remote commands have no effect. It is assumed that the operator has to take place before such decisions disorders (leakage ) or repairs (on site takes precedence).

In many cases, a torque control is added. In the pipeline operation, it is common for large valves on a regular basis to proceed for test reasons, so you will not be surprised in case of emergency, that " caked " is a fitting.

In addition to the frequent use as Auf-/Zu-Antrieb also used as a control drive is eligible, provided that the control performance is consistent with the slow travel speed.

This principle of operation, in which only the motor is energized for the duration of movement of the valve is sufficient for small and medium demands on the quality of control of a process of completely. In highly dynamic Positionierkreisen with high demand on the positioning quality but find continuous electrical actuators widespread use.

In contrast to the above-mentioned principle of operation of a frequency controlled three-phase motor control here is under constant voltage and generates a torque which is in a sensitive balance of power with the restoring forces from the process. In this state, the motor can remain without any time limit, without a thermal overload occurs. Any monitoring elements ( torque switches, limit switches, etc.) to protect the drive / motor are not required. The engine develops its restoring torque proportional to gentle and positioning error and even the smallest deviations of / - 0.05 % to compensate.

Classification according to the movement

As the actuator travel distance is referred to, which must pass through the regulating body in the valve to fully open the valve, or close. Typical regulating body are valve discs, valve plug or slide plates. The three adjusting body above are typical representatives for the three basic movements that are needed to travel along the adjustment path. The flap is moved through a 90 ° rotary movement of the end position OPEN to CLOSED, the valve cone performs for a relatively short stroke. The actuating movement of the slide plate by measuring the full diameter of the valve. For each of these types of motion, an actuator type is needed.

Rotary actuators

Rotary actuators are required for the automation of quarter turn valves. The classic representative of this type is the pusher. The basic requirements for rotary actuators are 5210 described as follows in the standard EN ISO:

A rotary drive is an actuator which transmits a torque of at least one full revolution of the armature. It may take up shear forces.

On the slide plate has a threaded spindle is mounted. About a threaded bushing of the rotary drive screws the slide plate in their leadership from OPEN to CLOSED and vice versa. Of passing through the entire adjustment path, the so-called Armaturenhubes, the actuator must be depending on fitting between a few and run several hundred revolutions. By design, electric rotary actuators, as opposed to pneumatic actuators, no stroke limits are subject. Therefore sliders are almost entirely automated with electric rotary actuators.

The rotary drive must be in the nut connection, the interface to the valve, can support the weight of the slide plate. This takes place in the second sentence of the definition expression.

Slide there diameters of about 10 cm to several meters. The torque requirement in the field of rotary drive applications is between about 10 Nm up to 30 000 Nm.

Rotary actuators

Turn actuators are required for the automation of quarter turn valves. Classical representatives of this type are butterfly valves and ball valves. The basic requirements to rotary actuators are described in standard EN ISO 5211 as follows:

A pivot drive is an actuator (actuator), which transmits torque to less than one full revolution of the armature. He must accept no shear forces. "

For less than one full revolution is generally a pivoting movement of 90 °, but there are types of valves, which require different tilt angle, for example, two-way valves. The regulating body in turn valves are always stored in the valve body, ie the weight of the actuator body has no effect on the swing drive. This takes place in the last sentence of the definition expression.

Turn valves are available with diameters of a few centimeters to several meters. The torque range for operating the actuator body is comparable extensive. It ranges from 10 Nm up to several 100,000 Nm. For valves with large diameters and high torque requirements of electric actuators are unrivaled.

Linear actuators

Linear actuators, outside of standardization also called linear drives are used wherever in the shut-off valve to be pushed in front of a seat opening, so the movement must be linear. A typical representative of the valves to be automated is the control valve. Similar to the plug is pushed into the bath in the spout, the cone is pressed by a reciprocating movement in the conical seat. Cone acts against the pressure of the medium. The thrust actuator moves to the corresponding thrust to move and hold the cone against this pressure. Electromechanical linear actuators are available today in a wide range of Hubgrößen and actuating forces available. In conjunction with intelligent control systems form an essential part of automated processes in the heating, ventilation and air conditioning technology, in process and in power and water supplies.

The vast majority of the non-electric linear actuators are pneumatic diaphragm actuators. They are characterized by a simple design principle and are consequently inexpensive. However, a prerequisite for their use is the presence of a compressed air supply. Outside this condition, offers the use of electric linear actuators, whose power can be realized easier.

Mechanical construction

Engine (1)

As electric motors mostly robust asynchronous motors for AC use. But there are also AC and DC motors. The motors are specially adapted for the requirements of valve automation. Depending on the design they make from a standstill, a substantially higher torque willing than comparable conventional engines. This property is needed in order to solve stuck valves from their seats can. Electric actuators are used under extreme environmental conditions. Fan motors do not provide the necessary protection and can not be used. Therefore actuators can be used for continuous operation because the engines need a cool down after a certain time. This corresponds to the application, because valves are not permanently activated.

And torque sensor ( 2)

About a limit switching by the extended travel range is measured and signals the reaching of an end, a torque circuit detects the pending in the valve torque. When the set limit value is exceeded, this is equally signaled. Frequently, the actuators have a position transmitter which outputs the valve position as a continuous current or voltage signal.

Transmission (3 )

The reduction of the high speed of the electric motor, a worm gear is commonly used. This allows a high reduction in a transmission stage and has a low efficiency which is desired in the case of the actuators. The reduction gear is self-locking. ie it acts against unwanted changes in the valve position by force act upon the valve actuator body. This is particularly important in multi-turn actuators, which are axially loaded with the weight of slide plates.

Valve connection ( 4)

The valve attachment consists of two elements. First, the flange to which the drive is firmly bolted to the corresponding counterpart of the fitting. The greater the torque to be transmitted, the greater must be the flange.

Second, this is the connection form of the torque and the thrust force is transmitted from the actuator to the valve shaft. According to the variety of valve designs, there are also a variety of connection types.

Dimensions and shape of flange and connection types are rotary and rotary actuators in the standards EN ISO 5210 or EN ISO 5211 set. For linear actuators are based generally on the DIN 3358th

Manual override ( 5)

Most electric actuators have the basic version of a handwheel, with which the drive can be operated manually during commissioning or in case of power failure. The handwheel can, depending on the version, stand still or run with the engine operating.

Actuator controls (6 )

The signal processing of the driving signals on the one hand and the movement commands of the process line on the other hand, takes place in an actuator controls. This task can be taken over (Programmable Logic Controller ) by an external controller, eg a PLC in principle. Modern actuators include an integrated controller, which performs the signal processing on the spot and without delays. Component of the control is also necessary for the control switching devices of the electric motor. This may be reversing contactors or thyristors, which are subject to mechanical wear as an electronic component. About this switching devices, the control switches the electric motor according to the incoming signals and commands on or off. Another object of the actuator controls is to provide the process line with necessary feedback, such as when a valve end position is reached.

Electrical connection ( 7)

At the electrical connection to supply lines of the motor, and the signal lines are connected for transmitting the instructions for driving and feeding back the driving state. Ideally, the electrical connection is designed as a plug-in connection, so that for example in case of maintenance work is not complete, the wiring needs to be resolved separately.

Field bus connection (8)

In the field of process automation is becoming stronger for data transmission, the fieldbus technology through. Electric actuators is therefore available with all standard in process automation fieldbus interfaces. For connecting the field bus data lines, a special connection system is required.

Functions

Auto power off in the end positions

After receiving a run command, the actuator moves the valve in direction OPEN or CLOSE. When reaching the end position, an automatic shutdown is initiated. There are two fundamentally different protection circuits are possible. The control unit switches off the drive as soon as the set switching point is reached. One speaks of limit seating. There are valves types where must the actuator body with a defined force or a torque defined to be driven into the Endlagensitz ensure that the valve closes tightly. This type of seating is called torque seating. Control is then parameterized so that the drive is switched off when the set torque limit is exceeded. The end position of the limit switching is used to signal the end position.

Security features

The torque switching acts not only in the torque dependent switch-off in the final position, but it is used for the whole travel as overload protection of the valve against excessive torque. If located in an intermediate position excessive torque builds up at the obturator, eg by a trapped object, speaks at reaching the set tripping torque to the circuit. In this situation, the end position signaling is absent by the switching. The controller is thus able to distinguish between a good and serviceable torque dependent switch-off in an end position and an inoperative fair to distinguish in an intermediate position.

To protect the motor from overheating, temperature sensors are required. With some products, the increase of the motor current is monitored as the most reliable because thermal or PTC thermistors have proven that are embedded in the motor. You respond when the temperature limit is exceeded and the control then switches off the engine.

Process-related functions

Due to the trend towards decentralization in automation technology and favored by the introduction of microprocessors more and more functions have been relocated from the control system to the field devices in recent years. This enabled the amount of data to be transmitted can be reduced. This trend was particularly encouraged by the introduction of the fieldbus technology. This development also relates to electric actuators whose functionality has increased significantly. The simplest example of this is the position control. Modern position rules, while enjoying the self- adaptation, ie the control behavior is monitored and constantly optimize the controller parameters.

Electric actuators are now available with full process controllers (PID controller ). Especially in remote installations, such as ramp metering at a high container, the actuator can take over the tasks of an otherwise additionally be installed with PLC.

Diagnosis

Diagnosis includes two aspects. Modern actuators have extensive diagnostic functions to help in case of failure to identify the cause. The second point is the data acquisition. By analyzing the data, conclusions can be drawn on the past operating history. This is the basis to optimize by changing the parameters of the operation and reduce wear on the actuator and valve.

Alternative structural design

This drive consists essentially of motor, planetary gear with a arranged as a torque arm sliding screw handwheel without switching, built- measuring sensors and the drive control assembly.

All parts of the planetary gear are arranged around the hollow shaft. Since in this planetary gear - unlike normal spur gears always more teeth are extensively engaged, a very compact gearbox long life can be realized.

Transmission principle

Life -time lubricant filling. No mechanical Handradumschaltung required. No start-up problems at low temperatures. Highest life even in normal operation, due to low surface pressure together with low relative movement of the teeth in engagement and optimum lubricant distribution. Mounting position.

Operation in motor mode

The motor ( 1) drives on the countershaft (2 ) to the eccentric (3). On the eccentric (3) is rotatably supported, the planetary gear ( 4), which rolls off the inner teeth of the sun gear (16). Due to the different number of teeth of the two wheels arises a relative speed, which driver pin (17) attached to the planet gear ( 4) drive plate (5) is transmitted. The driving plate (5) is connected by a spline form-locking with the hollow shaft (15).

Torque -dependent switching

The sun gear ( 16) out of the internal teeth or external teeth, which is axially displaceable with the screw (13) engaged. The displacement screw (13) is held by prestressed Md- springs (12) to the middle position. Acts on the drive a higher load torque than the predetermined by the spring preload torque, the circumferential force pushes the sun gear ( 16) the displacement screw (13) from its central position and actuates the Md- lever (14). On the MD -tap (11 ) are detected torque limits of adjustable cam disks and microswitch or the measured analog via a coupled electronic sensor, the tripping torque.

Operation in manual mode A switch from motor to manual mode is not required. For manual operation, the forces on the screw ( 13), the sun gear ( 16) and the planetary gear (4) on the driving disc (5) and thence to the output.

Modes

Control operation

If used as shut-off valves, the valve is opened or closed. Intermediate positions are not approached. The fitting is relatively seldom, the time interval may be a few minutes or even several months.

A hallmark of drives that are suitable for these applications, the short-time operation mode S2 of the electric motor in accordance with IEC 34-1. The mode is additionally marked by specifying a maximum duration without interruption. Typical actuators are here 15 min.

Positioning

To set a static flow through a pipeline predefined intermediate positions to be approached. There are maturity restrictions as in the control mode.

Control mode

Characteristic of rule applications is the frequent tracking of the actuator, due to varying conditions in order, for example, set a specific flow. In sensitive applications this is usually required every few seconds. The request to the actuator are higher than in the control or positioning. Mechanics and motor must be designed appropriately to withstand the high number of operations over long periods of time without the modulating accuracy.

The operating mode of the suitable for these applications, electric motors is called intermittent duty S4 or S5 intermittent. The limitation of the duration is controlled by the duty cycle for actuators for modulating duty is usually 25%.

Conditions of use

Electric actuators are used worldwide in all climate zones, industrial plants of all kinds under special local ambient conditions. The fields of application are often safety- relevant, therefore high demands on the reliability provided by the plant operator. The failure of an actuator can lead to accidents in process plants or release of toxic substances into the environment.

Processing plants are often several decades in operation, therefore high demands on the equipment life are.

Therefore, electric actuators are always executed in high protection. For corrosion protection for the equipment manufacturer to great lengths.

Protection

The degree of protection of the actuators are determined by the so-called IP codes of EN 60529. Most electric actuators correspond in their basic version, already the second highest degree of protection IP 67 That is, they are dustproof and waterproof against temporary immersion (30 min at a water column of 1 m). Most providers offer the equipment in protection class IP 68. This provides protection against permanent flooding, usually up to a water column of 6 m.

Ambient temperatures

In Siberia, temperatures are up to - 60 ° C, in process plants can also 100 ° C mark will be exceeded. A crucial point for the functioning of the drives under these conditions is to use the correct lubricant. Fats that work well at room temperature, are much too solid at low temperatures, so that the drive can no longer overcome this resistance in the device. Conversely, these fats are low viscosity at high temperatures and lose their lubricating effect. In the design of the actuator is the question of the ambient temperature and thus the selection of the correct lubricant is of considerable importance.

Explosion protection

Electric actuators are also used in areas where explosive atmospheres may occur. These include refineries, pipelines, oil and gas exploration or mining. If in such a system, an explosive gas -air mixture or gas-dust mixture on, the actuator must not act as an ignition source. Essentially, it is important that to the device may not occur to hot surfaces and avoid that the device emits Sparks. This can be achieved by a pressure-resistant encapsulation, for example, that the appliance housing is so constructed that even penetrate through an explosion inside the device does not spark ignitable outward.

Actuators in applications of this kind must be qualified as explosion-proof equipment by a Notified Body. In this case, there is no uniform global standard, but depending on the country in which the equipment is used, different policies by the manufacturer must be observed. In Europe, the ATEX 94/9/EC, in USA the NEC or CEC in Canada. Explosion-proof equipment must have prescribed in this policy design features.