Control valves

Efficient flow control in industry

Whether in the chemical and petrochemical industry, in food production or paper manufacturing - in industry, a lot revolves around steam, liquids and gases. This is where the control valve comes into play. It is like a kind of master conductor that controls the flow of these media in a process system. Among other things, the valve regulates pressure, temperature and fill level in order to keep the process variable as close as possible to the target value.

But what exactly is a control valve, how does it work and what tasks does it perform in a control loop?

What is a control valve?

A control valve serves as an actuating element in a control circuit. It is an essential component for shutting off and precisely regulating the flow of process media such as gas, steam, water or chemical compounds. However, these versatile valves can also be found in everyday applications, e.g. in water taps, beer taps or radiator thermostats.

Definition of control valves according to DIN EN 60534-1

According to DIN EN 60534-1, a control valve is defined as a power-operated component that regulates the flow in a process system. It consists of a fitting that is connected to an actuator. The actuator changes the position of the throttle element in the valve as soon as a signal is triggered. The task of the actuator is ultimately to convert the signal into a movement, thereby regulating the position of the throttle element. The signal can be pneumatic, electric, hydraulic or a combination of these.

Other components of a control valve are

Valve body (housing)
Valve bonnet (attachment)
Inner trim
Actuator (pneumatic, electric, hydraulic)
Yoke or pillars (intermediate piece)

All these components work together to ensure a precise and reliable flow.

Further information in the FAQ

The different types of control valves

Different types of control valves are used in process automation. They differ in their design and mode of operation. The selection of the right valve depends on the specific application and the requirements of the system. Common types include ball valves, cone valves and butterfly valves.

A. Hock offers, for example, control valves with linear movement, two-way globe valves (2-way valves) or three-way valves (3-way valves).

Two-way globe valves control the flow of a liquid or gas in two directions.
Main feature: The flow medium can flow through the valve in both directions.

Two way globe valves (S2000)

Three-way valves have three connections and are used for mixing and dividing fluids. For example, to mix two fluids before they are processed further in the process or to divert a fluid between two different paths.
Example: In heating systems, three-way valves are used to direct the flow of hot water from a boiler either towards a radiator or towards a hot water tank.

Three-way valves (S2003/13)

How a control valve works

The flow rate

In control valves with a linear stroke movement, the flow of a medium is controlled by the vertical movement of a plug stem or a plug. The cross-sectional area of the flow changes.

In detail, this means that when the plug moves upwards, it increases the space between itself and the seat ring of the control valve. This leads to an increase in the cross-sectional area through which the liquid or gas can flow. An increase in the cross-section leads to a larger flow rate. If, on the other hand, the plug moves downwards, it reduces the space between itself and the seat ring, which reduces the cross-section and therefore the flow rate.

The contour

The contour of the plug plays an important role. Depending on the shape of the plug, a specific valve characteristic can be mapped, which describes the relationship between the position of the plug (H/H100) and the flow rate (KV/KV100).

The most common contours are:

linear
equal percentage
fast opening

The throttling process

In the valve, pressure energy is converted into velocity energy by reducing the cross-section between the seat and plug, i.e. throttling. The following applies: As the flow velocity of a liquid or gas increases, the static pressure decreases.

The flow rate, the service life of the control valve and the noise emissions that can occur during operation are significantly influenced by the pressure and speed curve. For example, too high a flow rate can lead to more wear or a high flow velocity can lead to noise. It is therefore important to design the control valve correctly to ensure efficient and safe operation.

Design of a control valve using the example of a globe valve

Globe valves (2-way valves) or globe valves are the most widely used type of control valve and can be found in all industries. They offer an extremely wide range of variants, can be used flexibly and are therefore suitable for numerous applications.

Especially when high demands are placed on control quality, high differential pressures, reliable sealing against the environment and variable flow rates.

The individual components of a globe valve have different tasks in a control process:

Pneumatic diaphragm actuator: A diaphragm connected to the valve spindle is activated by means of externally supplied compressed air. The force causes the closing element to move.
Positioner: It regulates the compressed air supply to the actuator and thus sets the desired position of the valve plug.

Coupling / clamp connection: The coupling ensures a secure connection between the actuator and valve stem. In the picture, designed as a clamp connection, the mechanical stroke movement of the actuator is transferred to the valve.
Valve stem: This component is located in the valve bonnet, it is connected to the actuator and positions the closing element.
Valve bonnet: The valve stem moves through the valve bonnet. A device for sealing against leakage is located along the stem. The valve bonnet consists of a traverse for mounting the actuator. It is also used to exert pressure on the packing unit.
Packing: The packing describes the part of the valve bonnet that seals the movable valve spindle against leakage to the outside.
Guide bushing: The valve bonnet usually contains a guide bushing to align the movement of the valve stem or plug and ensure correct guidance. In some cases, a seat ring is also used for alignment.
Valve plug: A movable body that is connected to the valve stem and acts as a variable flow restrictor in the valve passage.
Seat ring: The seat ring is screwed into the valve body in this valve. It forms the valve passage and can also be used to guide the valve cone. The sealing surface of the seat ring is called the valve seat. The valve cone moves into the valve seat and is fully seated there when the valve is closed.
Housing: The housing of the control valve has an inlet and an outlet and at least one opening for mounting the internals

The role of the control valve in the control loop

Control valves play a decisive role in control loops for controlling and regulating processes. They fulfill two main functions:

regulating the flow rate in a control loop and
maintaining a certain pressure in a system despite malfunctions.

A control valve is therefore always involved in the distribution of energy within a system.

The control loop - from control variable to measuring device

The primary goal of a control system is to keep the controlled variable constant at a specified value (e.g. fill level, temperature or pressure) in accordance with the reference variable. At the same time, the influences of disturbance variables (e.g. existing processes, cold air) must be compensated for. The entire system is referred to as a controlled system.

The closed control loop consists of the controlled system and the control device. The control equipment includes devices such as sensors, transmitters, controllers and actuators.

Other components of the control loop::

Actuating device: This is located between the controller and the controlled system and consists of an actuator and a final control element.
Measuring device: It records the value of the controlled variable and converts it into a feedback variable that is evaluated by the controller.
Controller: The controller (e.g. PID controller) compares the feedback variable with the reference variable, whereby the control difference is read out. If there is a difference, the controller automatically adjusts the controller output variable.
Controller output variable: The actuator converts the controller output variable into a manipulated variable that acts on the controlled system and influences the controlled variable.

A stable control loop is crucial for the proper functioning of the control system. To achieve this, the controller must be adapted to the specific controlled system, as this can have different response times and effects on the controlled variable.

If a control loop is stable, the controlled variable is within the defined tolerance range around the reference variable. If the control loop is unstable, a change in the reference variable or the disturbance variables will cause the controlled variable to overshoot with increasing amplitudes.

Control valves from A. Hock

The two-way and three-way valves from A. Hock can be flexibly configured. What is important here is an exact specification. This is the only way to guarantee optimum control behavior and a long service life. Whether with pneumatic or electric actuator - we have the right high-quality product in our portfolio for every request.

Andreas Schalwig

Technical Sales

I’m happy to help you with projects and enquiries in the field of valve technology as well as measurement and control technology. I have more than 15 years of experience in the dimensioning of control valves and actuators. What’s more, I help customers from all over the world to modernise their industrial plants.

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