Control valves

A control valve, and control valve is often the actuator in a closed loop. A throttle device can thus be set in a pipeline, the desired flow rate. The valve stem is connected to the valve plug. In the closed position of the valve cone closes the so-called seat ring. Driving the valve stem upward, the annular flow opening becomes larger and the flow rate becomes larger. In process engineering, is spoken of " control valves ".

The geometric shape of the plug determines the flow characteristic. This is linear, equal percentage or in rare cases differently. The flow ratio, which can be used for the scheme is, in simple valves 1: 10 For elaborately constructed valves ranges from 1 are: 50 possible.

Control valves have a typical appearance. Hence valves that are part of a control loop are not but have the same design, often referred to as a control valve are.

All pressure devices in a pipeline locally constrict the flow cross section. This reduces the pressure of the medium and the speed increases. After the cross-sectional narrowing roughly the velocity returns back to its previous value, if the pipe diameter is the same before and after the constriction. There is a permanent pressure loss.

Pinhole are also among the primary devices. But you are not adjustable and can not therefore be a actuator. Butterfly valves are adjustable and are particularly useful in gases or in large pipe diameters an affordable alternative.

The drive can be done via an electric actuator, pneumatically or hydraulically via a membrane via a piston.

In order to standardize the control of the control valve actuator, often additional positioners are used.

With the variable flow can control the pressure, temperature, level, or flow, even in a process plant.

Valve design

For cost reasons and because of the valve authority necessary for the control capability to Choose the valve size is often one or two size degrees smaller than the pipe itself is often trying to get along with only a few nominal diameters in the entire system. Here are advantages in the spare parts inventory.

The necessary adjustment to the flow conditions can be achieved by the selection of seat and plug. In a valve housing fit different seat - cone combinations.

Authoritative figure is the valve flow value. It must also be examined whether or not the maximum speed located in the extreme case, below the speed of sound, or usually less than a few meters per second, so as to avoid cavitation or excessive noise.

The corrosion resistance determines the material. In building (heating and air conditioning) are used in the control valves made ​​of cast iron and brass, in special cases, gunmetal and ductile cast iron, while also cast steel, stainless steel or plastics are used in the industrial sector to use.

Control valves usually have a certain leak rate. In view of the need to save energy but takes in building the importance of tightly closing valves continues to be, and this applies in the case of 3 -way valves for all three ways.

The installation conditions ( welded, clamped ( sandwich construction ) or flanged) arising from the requirement profile of the associated pipe class.

The choice of actuator is dependent on the application and the availability of the required power supply. In most process plants, there is a compressed air system ( typically 6 bar). Therefore, the pneumatically operated control valves are widely used. In most cases, mechanical springs produce the necessary counter-force. Upon failure of the compressed air valve then closes automatically. In a few cases, this must be reversed and the spring force the valve will open. Pneumatic actuators have the advantage of high positioning speed. Electric drives on the other hand have the advantage of requiring no compressed air supply. Next tilt control valves with electric actuators far less undesired swing, as pressure changes in the medium due to the self- locking of the gear practical can not exert any influence on the position of the valve plug. In the case of building services applications (heating, air conditioning) almost exclusively electric valve actuators are used.

• Valve