Electromotive drive for pipe fittings
An electromotive drive for pipeline fittings is a special type of actuator . They are preferably used in pipeline construction and plant construction.
General
The basis is a commercially available three-phase motor with left and right rotation. A gear generates the necessary torque. Because of this reduction, the travel time is high, which is usually not critical. For large nominal sizes, travel times of a few minutes are common.
The gear allows the simultaneous arrangement of a handwheel. This allows you to conveniently close or open the valve on site. There is a mechanical lock for this. If this is set to "Hand", then remote commands have no effect. It is assumed that the operator on site has to make such decisions in the event of malfunctions (leaks) or repair work (on site has priority).
In many cases, torque monitoring is also used. In pipeline operation, it is common to move large valves regularly for test reasons so that in an emergency one is not surprised that a valve is "stuck".
In addition to the frequent use as an opening and closing drive, it can also be used as a control drive, provided that the control quality is compatible with the slow travel speed.
This working principle, in which the motor is only energized for the duration of the valve movement, is completely sufficient for low or medium demands on the control quality of a process. In the case of highly dynamic positioning circles with high demands on the positioning quality , however, electrical control drives are often used.
In contrast to the operating principle mentioned at the beginning, the three-phase control motor controlled by a frequency converter is permanently under voltage and generates a torque that is in a sensitive balance of forces with the restoring forces from the process. The motor can remain in this state for an indefinite period of time without thermal overload occurring. Monitoring elements (torque switches, limit switches, etc.) to protect the drive or motor are not required. The motor develops its actuating torque gently and proportionally to the positioning deviation; and even the smallest deviations of ± 0.05% are compensated.
Classification according to the movement
The travel that the actuator in the valve must travel through in order to fully open or close the valve is referred to as the travel range. Typical actuators are butterfly discs, valve cones or slide plates. The three actuators mentioned are typical representatives of the three basic movements that are required to travel through the travel range. The flap is moved from the end position OPEN to CLOSED by a 90 ° swivel movement, the valve cone performs a relatively short stroke movement. The adjusting movement of the slide plate measures the entire diameter of the valve. An actuator type is required for each of these types of movements.
Rotary actuators
Rotary actuators are required for the automation of rotary valves. The classic representative of this type is the slide. The basic requirements for rotary actuators are described in the EN ISO 5210 standard as follows:
A rotary actuator is an actuator that transmits a torque to the valve over at least one full revolution. He can absorb shear forces.
A threaded spindle is mounted on the slide plate. The rotary actuator screws the slide plate in its guide from OPEN to CLOSED and vice versa via a threaded bushing. To travel through the entire travel range, the so-called valve stroke, the actuator must perform between a few and several hundred revolutions, depending on the valve. Due to their design, electrical rotary actuators, in contrast to pneumatic actuators, are not subject to any stroke restrictions. That is why gate valves are almost exclusively automated with electric rotary drives.
The rotary actuator must be able to absorb the weight of the gate in the valve connection, the interface to the valve. This is expressed in the second sentence of the definition.
There are sliders with a diameter of approx. 10 cm up to a few meters. The torque requirement in the area of rotary drive applications is between approx. 10 Nm and 30,000 Nm.
Rotary actuators
Part-turn actuators are required for the automation of part-turn valves. Classic representatives of this type are butterfly valves and ball valves. The basic requirements for rotary actuators are described in the EN ISO 5211 standard as follows:
A quarter turn actuator is an actuator that transmits a torque to the valve over less than one full revolution. It does not have to be able to absorb any thrust.
Over less than a full turn usually means a swivel movement of 90 °, but there are types of valves that require different swivel angles, e.g. B. two-way valves. The actuators in swivel valves are always stored in the valve body, i. H. the weight of the actuator does not affect the rotary actuator. This is expressed in the last sentence of the definition.
Swivel fittings are available with diameters from a few centimeters to several meters. The torque range for actuating the actuator is comparably extensive. It ranges from approx. 10 Nm to several 100,000 Nm. For valves with large diameters and high torque requirements, electric actuators are unrivaled.
Linear actuators
Linear actuators, also called linear actuators outside of standardization, are used wherever the shut-off element in a valve is to be pushed in front of a seat opening, i.e. the movement must be linear. A typical representative of the fittings to be automated is the control valve . Similar to how the plug is pressed into the sink in the bathtub, the cone is pushed into the cone seat by a lifting movement. The pressure of the medium acts against the cone. The thrust drive applies the appropriate thrust to move and hold the cone against this pressure. Electromechanical linear drives are available today in a wide range of stroke sizes and actuating forces. In connection with intelligent controls, they form an indispensable part of automated processes in heating, air conditioning and ventilation technology, in process technology and in power and water supplies.
The majority of non-electric linear actuators are pneumatic diaphragm actuators. They are characterized by a simple design principle and are therefore inexpensive. However, a requirement for their use is the availability of a compressed air supply. Outside of this condition, the use of electric thrust drives is possible, the energy supply of which can be implemented more easily.
Constructive structure
Engine (1)
Robust asynchronous motors for three-phase current are predominantly used as electric motors . However, AC and DC motors are also used. The motors are specially adapted to the requirements of valve automation. Due to their design, they provide a significantly higher torque from standstill than comparable conventional motors. This property is required in order to be able to remove stuck fittings from their seat. Electric actuators are used under extreme environmental conditions. Fan motors do not offer the required degree of protection and cannot be used. Therefore, actuators cannot be used for continuous operation, as the motors need a cooling phase after a certain operating time. This corresponds to the application, because fittings are not operated permanently.
Position and torque sensors (2)
The travel traveled through is measured via a limit switch and the reaching of an end position is signaled; a torque switch records the torque present in the valve. If a set limit value is exceeded, this is also indicated. The actuators often have a remote position transmitter that outputs the valve position as a continuous current or voltage signal.
Transmission (3)
A worm gear is often used to reduce the high speed of the electric motor. This enables a high reduction in one gear stage and it has a low degree of efficiency, which is desired in the case of actuators. This makes the gearbox self-locking. d. H. it counteracts undesired changes in the valve position caused by forces acting on the valve body. This is particularly important for rotary drives that are axially loaded with the weight of slide plates.
Fitting connection (4)
The fitting connection consists of two elements. First of all, the flange with which the actuator is firmly screwed to the corresponding counterpart of the valve. The greater the torque to be transmitted, the larger the flange must be.
Secondly, there is the type of connection via which the torque or thrust is transmitted from the actuator to the valve shaft. According to the multitude of valve designs, there is also a multitude of connection types.
The dimensions and shape of the connection flange and connection types for rotary and part-turn actuators are specified in the standards EN ISO 5210 and EN ISO 5211. In the case of linear drives, DIN 3358 is generally used as a guide.
Manual operation (5)
Most electric actuators have a handwheel in the basic version with which the actuator can be operated manually during commissioning or if the power supply fails . Depending on the version, the handwheel can either stand still or run while the motor is running.
Actuator controls (6)
The signal processing of the drive signals on the one hand and the movement commands of the process line on the other hand takes place in an actuator control. In principle, this task can be performed by an external control, e.g. B. a PLC ( programmable logic controller ) can be taken over. Modern actuators contain an integrated control that carries out the signal processing on site and without delay times. The switching devices required to control the electric motor are also part of the control system. These can be reversing contactors or thyristors, which as electronic components are not subject to mechanical wear. The controller uses these switching devices to switch the electric motor on and off in accordance with the signals and commands present. Another task of the actuator controls is to provide the process line with the necessary feedback, e.g. B. when a valve end position is reached.
Electrical connection (7)
The supply lines of the motor and the signal lines for transmitting the commands to the drive and for reporting back the drive status are connected to the electrical connection. Ideally, the electrical connection is designed as a plug connection, so that, for. B. in the case of maintenance work not the complete wiring has to be solved individually.
Fieldbus connection (8)
In the field of process automation, fieldbus technology is becoming more and more popular for data transmission . Electric actuators are therefore available with all fieldbus interfaces commonly used in process automation. A special connection technology is required to connect the fieldbus data lines.
Functions
Automatic switch-off in the end positions
After receiving a travel command, the actuator moves the valve in the OPEN or CLOSE direction. When the end position is reached, an automatic shutdown process is initiated. Two fundamentally different shutdown mechanisms are possible. The control switches off the drive as soon as the set switching point is reached. One speaks of travel-dependent shutdown. However, there are valve types in which the control element must be moved into the end position with a defined force or a defined torque so that the valve closes tightly. This type of shutdown is called torque-dependent shutdown. The control is then parameterized so that the drive is switched off when the set torque limit is exceeded. The end position signal of the limit switching is used to signal the end position.
Security functions
The torque switching is not only effective for the torque-dependent shutdown in the end position, but also serves as overload protection for the valve against excessive torque over the entire travel range. If an excessive torque is set on the actuator in an intermediate position , z. B. by a jammed object, the torque switching responds when the set shut-off torque is reached. In this situation there is no end position signaling due to the limit switching. The control is thus able to differentiate between an operationally appropriate torque-dependent shutdown in an end position and an operationally inappropriate one in an intermediate position.
Temperature sensors are required to protect the motor against overheating. With some makes, the increase in the motor current is monitored; the most reliable have proven to be thermal switches or PTC thermistors, which are embedded in the motor. They respond when the limit temperature is exceeded and the control then switches off the motor.
Process engineering 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 transferred to be reduced. This trend was promoted in particular by the introduction of fieldbus technology. This development also affects electric actuators, the scope of which has increased considerably. The simplest example of this is position control. Modern posture regulations have a self-adaptation, i. H. the control behavior is monitored and the controller parameters are permanently optimized.
Electric actuators are now also available with full-fledged process controllers (PID controllers). Especially with remote installations, e.g. B. the flow control to an elevated tank, the actuator can take over the tasks of an otherwise additionally installed PLC.
diagnosis
Diagnosis has two aspects. Modern actuators have extensive diagnostic functions that make it easier to identify the cause in the event of a fault. The second point is the production data acquisition. By evaluating the data, conclusions can be drawn about the past operating history. This is the basis for optimizing operation by changing the parameters and reducing wear on the actuator and valve.
Alternative constructive structure
This drive essentially consists of a motor, a planetary gear with a sliding worm arranged as a torque support, a handwheel without switching, built-in transducers and the drive control module.
All parts of the planetary gear are arranged around the hollow shaft. Since with this planetary gear - in contrast to normal spur gears, several teeth are always in surface engagement, a very compact gear with a long service life can be realized.
Gear principle
Lifetime lubricant filling. No mechanical handwheel switching required. No start-up problems at low temperatures. Longest service life even in normal operation, due to the low surface pressure together with the low relative movement of the teeth in engagement and due to optimal lubricant distribution. Any installation position.
How it works when the engine is running
The motor (1) drives the eccentric (3) via the intermediate gear (2). The planet gear (4), which rolls in the internal toothing of the sun gear (16), is rotatably mounted on the eccentric (3). Due to the different number of teeth on the two wheels, a relative speed is created which is transmitted via drive pins (17) to the drive plate (5) attached to the planetary gear (4). The drive plate (5) is positively connected to the hollow shaft (15) by serrations.
Torque-dependent switching
In addition to the internal teeth, the sun gear (16) also has external teeth which mesh with the axially displaceable worm (13). The sliding screw (13) is held in the middle position by pretensioned Md springs (12). If a higher load torque acts on the drive than the torque specified by the spring preload, the circumferential force on the sun gear (16) pushes the sliding worm (13) out of its central position and actuates the Md lever (14). At the Md tap (11), torque limit values are recorded using adjustable cam disks and microswitches, or the shutdown torques are measured analogously using a connected electronic sensor.
How it works in manual mode It is not necessary to switch from motor mode to manual mode. When operated manually, the forces are transmitted via the worm (13), the sun gear (16) and the planet gear (4) to the drive plate (5) and thus to the output.
Operating modes
Control mode
If valves are used as shut-off devices, the valve is open or closed. Intermediate positions are not approached. The valve is operated relatively seldom, the time interval can be a few minutes or even several months.
A characteristic of drives that are suitable for these applications is the short-term operation mode S2 of the electric motor according to the IEC 34-1 standard. The operating mode is also identified by specifying a maximum permissible running time without interruption. Typical for actuators here is 15 min.
Positioning mode
To set a static flow through a pipeline, predefined intermediate positions are approached. There are runtime restrictions as in control mode.
Regular operation
The frequent adjustment of the actuator, due to changing conditions, for example to set a certain flow rate, is characteristic of control applications. In sensitive control applications, this is necessary every few seconds. The demands on the actuator are higher than in control or positioning mode. The mechanics and motor must be designed accordingly in order to withstand the high number of switching operations over long periods of time without the control accuracy deteriorating.
The operating mode of the electric motors suitable for these applications is called intermittent operation S4 or intermittent operation S5 . The limitation of the running time is regulated by the relative switch-on time, usually 25% for actuators for control operation.
Conditions of use
Electric actuators are used worldwide, in all climatic zones, in all types of industrial plants under special local environmental conditions. The fields of application are often safety-relevant, which is why the system operators place high demands on reliability. The failure of an actuator can lead to accidents in process engineering systems or to the release of toxic substances into the environment.
Process engineering systems are often in operation for several decades, which is why high demands are placed on the service life of the devices.
This is why electric actuators are always designed with a high degree of protection. The manufacturers of the devices go to great lengths to protect against corrosion.
Protection class
The types of protection of the actuators are specified according to the so-called IP codes of EN 60529. Most electric actuators in their basic version already correspond to the second highest protection class IP 67. This means that they are dust-tight and waterproof against temporary submersion (30 min with a water column of 1 m). Most providers offer the devices in protection class IP 68. This offers protection against permanent flooding, usually up to a water column of 6 m.
Ambient temperatures
Temperatures down to - 60 ° C prevail in Siberia, and the + 100 ° C mark can be exceeded in process engineering systems. The use of the right lubricant is crucial for the functionality of the drives under these conditions. Fats that work well at room temperature become much too solid at low temperatures, so that the drive can no longer overcome this resistance in the device. Conversely, these greases become thin at high temperatures and lose their lubricating effect. When designing the actuator, 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 can occur. These include refineries, pipelines, oil and gas exploration and mining. If an explosive gas-air mixture or gas-dust mixture occurs in such a system, the actuator must not act as an ignition source. Essentially, it is a matter of preventing surfaces that are too hot on the device and of avoiding the device from emitting ignitable sparks. This can e.g. B. can be achieved by a flameproof enclosure, d. H. the device housing is designed in such a way that even an explosion inside the device does not allow ignitable sparks to escape.
Actuators in such applications must be qualified as explosion-proof devices by a named test center. There is no globally uniform standard, but depending on the country in which the devices are used, various guidelines must be taken into account by the manufacturer. ATEX 94/9 / EG applies in Europe, the NEC in the USA or the CEC in Canada. Explosion-protected devices must have the design features specified in these guidelines.
See also
literature
- The Library of Technology No. 148, Actuators, ISBN 3-478-93102-9
- ABB Intelligent Drives for Process Automation, 30 / 68-104-EN
