Valve drive
The valve drive is the drive with which a control valve can be mechanically operated. A distinction is made between electric and pneumatic drives. The valve drive changes the position of the valve cone and thus the flow through the valve. With the electric 3-point drive and the electric continuous drive, the movement of the valve spindle is generated by an electric motor , which converts the rotary movement of the motor into a lifting movement via a screw gear. The valve drive is mounted directly on the upper part of the valve. The valve spindle, which is connected to the drive, is also located there.
Types
3-point drive
The valve is controlled via two electrical connections that drive the motor directly. If the OPEN connection is activated, the valve is opened by the motor. With the CLOSED connection, the valve is closed with the motor. If none of the connections is activated, the valve rests in its current position. The position of the valve is determined via the running time of the valve, via a potentiometer or via the distance pulses from a Hall sensor . Running time is the time that the valve drive needs to move the valve from the CLOSED end position to the OPEN end position. The running time must be entered in the respective controller ( DDC-GA component) so that it can work with the valve. The running time for valves in heating technology is usually approx. 60 to 180 s.
Possibly susceptible to failure, as after a while the current position can no longer be determined exactly. Potentiometers are often used for position feedback, which naturally wear out and are therefore frequent sources of error. Contactless, incremental and therefore wear-free Hall sensors, such as those used in some modern drives, are more advantageous.
Constant drive
The valve is controlled via the operating voltage and a continuous analog control signal 0 ... 10 V, 0 ... 20 mA or 4 ... 20 mA ("life zero"). The analog value (applied voltage or current) corresponds to the respective valve opening. The valve is opened continuously.
| tension | electricity | Valve opening | |
| 0 V | 4.0 mA | 0% | |
| 1 V | 5.6 mA | 10% | |
| 2 V | 7.2 mA | 20% | |
| .... | .... | .... | |
| 10 V | 20 mA | 100% |
The analog signal in the drive is converted into the exact position by a motor. This in turn is controlled by the electronics integrated in the drive.
Current signals 4 ... 20 mA are usually used in industrial applications. The lower limit of 4 mA for the valve position 0% enables the rapid detection of a possible interruption of the control signal in the event of a system defect. In building technology, voltage signals 0 ... 10 V are mostly used.
- Advantage: easily adjustable
- Disadvantage: higher price due to the integrated electronics
Modern microcontroller-controlled drives are designed in such a way that they are equally suitable for 3-point and continuous input signals and offer additional features such as adjustable speed, hysteresis, blockage detection of the valve cone, etc.
Electrothermal drive (pseudo-continuous)
An electrically heated expansion element acts directly on the valve stem. The valve opening is determined by the duty cycle of the control ( pulse width modulation ).
- Advantage: very low price, noiseless
- Disadvantage: possibly restricted permissible ambient temperature range
magnetic valve
In addition to the two states OPEN and CLOSED, intermediate states can also be implemented for solenoid valves with built-in springs with defined spring constants. Current control enables variable positioning of the armature , which leads to a variable position and thus regulation of the flow.
pneumatic drive
Pneumatic membrane actuators are based on a completely different functionality than the electrical actuators described above. A membrane coupled to the valve spindle is moved by externally supplied compressed air. The exact position is achieved via an electrical positioner that regulates the compressed air supply. The actuating force of pneumatic diaphragm actuators depends on their diaphragm area and the pressure of the compressed air used. Membrane areas of approx. 80 cm² up to approx. 2,800 cm² are common.
Pneumatic drives have the advantage of being able to be actuated very quickly and of being able to achieve high actuating forces. Furthermore, they are mostly uncritical in potentially explosive environments, in outdoor use and at low temperatures. Disadvantages are the large size and the need for an external compressed air supply, which requires an appropriate infrastructure. Accordingly, they are used almost exclusively in the industrial sector and in power plants, but not in the field of building technology.
Portable drive (pneumatic, battery operated, with petrol engine)
There are also portable valve drives with a torque of up to 800 Nm for handwheel valves or underfloor valves with square for slide keys. With handwheels, depending on the shape and number of spokes of the handwheel, the valve drive is adapted with a self-centering safety adapter or with spoke grippers for handwheels in a conical design. Fixed handwheel adapters are also possible. Different options such as angle drive, revolution counter or torque limiter are possible. Torque supports with safety lines secure the operating personnel. Various extensions and adapters enable operation from a greater distance.
- Petrol engine driven: With 4-stroke engine up to 700 Nm depending on the gear ratio
- Driven by a pneumatic motor: ATEX-compliant designs for potentially explosive environments up to 700 Nm
- With battery operation
Advantages:
- Safe operation without physical effort, the operator can e.g. B. watch the traffic.
- Increase in the working speed (sometimes only 1/10 of the valve operation only with muscle power)
- Reduction of the dwell time at the valve (road traffic, chemical plants)
- Torque limitation possible so that a valve cannot be damaged.
- Exact counting of the revolutions for partial openings
- Optional extensions increase the safety distance e.g. B. with dangerous substances.
- Compressed air drives according to ATEX for potentially explosive environments
disadvantage
- Investment costs for the valve drive
Drive with safety function
To ensure a defined valve position in the event of a failure of the control energy (especially in the case of electrical drives), emergency actuators have a mechanical energy store, e.g. B. a spring that ensures safe closing or opening of the valve. These drives are type-tested in connection with suitable valves in accordance with DIN EN 14597 (replacement for the old standard DIN 32730).
application
Valve drives are used to operate valves mechanically. The signal is generated by the control system (compact controller, DDC-GA component or process control system) of the system. With the flow of liquids changed by the valve, the amount of heat or cold is regulated at the same time. In practice, the choice of valve drives depends on the field of application and the built-in valve, since the mounting, the stroke and the necessary force of the valve drive vary. The actuating forces of electric valve drives for common heating and air conditioning applications as well as in the industrial sector range from a few hundred N to over 25 kN, pneumatic drives reach approx. 40 kN, hydraulic drives well over 500 kN.