ARI® control valves – precise control for industrial applications

A control valve is a device that specifically regulates the flow of liquids, gases or steam. As a component of a control loop, it enables process variables such as temperature, pressure, flow rate or level to be maintained at a desired value or to be adjusted to a specified setpoint.

To achieve this, the control valve changes the size of the flow passage inside the valve body. In a globe valve, for example, a valve plug moves relative to the valve seat. Depending on its position, it opens up a larger or smaller cross-section/flow passage. The actual volume of fluid flowing through also depends on the pressure conditions and the properties of the fluid.

In a control valve with an electric or pneumatic actuator, a sensor measures the process variable to be controlled. A controller compares this measured value with the setpoint and outputs a corresponding control signal. The actuator converts this signal – with the aid of a positioner where necessary – into a valve movement. This allows the flow rate to be precisely adjusted between the ‘open’ and ‘closed’ end positions.

A typical example is temperature control in a heat exchanger. The control valve regulates the flow of steam supplied so that the medium to be heated reaches the desired temperature. If the heat demand changes, the control loop adjusts the valve opening accordingly.

Control valves can be classified according to their design, flow pattern and method of operation. These characteristics can be combined.

• In terms of design, a distinction is made between, amongst others, linear valves and rotary valves. In linear valves, the throttling element – for example, a valve cone – moves in a straight line. The control behaviour can be adapted to the application by the design of the cone and seat. Rotary valves alter the flow cross-section through a rotary movement. These include, for example, segment ball valves, control dampers and rotary cone valves. In each case, it is crucial that the specific design is suitable for the intended control task.
• Based on the flow path, a distinction is primarily made between 2-way and 3-way control valves. A 2-way valve regulates the flow between an inlet and an outlet. A 3-way valve can mix two partial flows or distribute a volume flow across two paths. To do this, it must be designed as a mixing or diverting valve.
• Control valves are classified according to their actuation method as those with or without auxiliary power. Electric, pneumatic or hydraulic actuators use supplied energy to change the valve position. Self-acting regulators without auxiliary power, such as certain pressure reducers or temperature controllers, do not require an external power or compressed air supply for this purpose.

Control valves are used in industrial plants, process engineering and building services. They are employed wherever the flow of a medium needs to be adjusted to meet varying requirements, for example to regulate temperatures, pressures or liquid levels.

Typical applications include:

• Heating and cooling circuits: Control valves regulate the flow of water, steam or heat transfer oil to provide the required heating or cooling capacity. When fitted to a heat exchanger, they thus influence the temperature of the medium to be heated or cooled.
• Steam and gas supply: Control valves adjust the flow rate to match consumption and enable, for example, the regulation of pressure on the downstream side of the system.
• Mixing and distribution circuits: 3-way control valves mix partial flows or divert a medium across different flow paths, such as through a heat exchanger or a bypass.
• Tanks and process plants: A desired fill level can be maintained by controlling the inflow or outflow of a medium.
• Production processes: Control valves supply the required quantities of medium and adapt them to different operating conditions or production rates.

In practice, the terms ‘control valve’ and ‘actuated valve’ are often interchangeable. A useful technical distinction is as follows: ‘control valve’ describes the role of the valve in the control process. ‘Actuated valve’ refers to the unit comprising the valve and the actuator, which uses supplied energy to convert a control signal into a change in flow rate.

The umbrella term ‘control valve’ therefore covers both valves requiring external power and self-acting regulators that do not require external power. According to DIN EN 60534, a control valve operates with an electric or pneumatic actuator. This actuator moves the throttling element in accordance with the specified signal.