Control valves for fluids, steam and gas

Control valves are designed for reliable continuous throttling of a medium with a minimum of dead band and hysteresis. All valves suitable for throttling a mass flow have one thing in common: they can be considered as a variable restriction, something like a variable resistor in an electrical circuit.

A control valve can be thought of as a device consisting of a body with an interface to the adjacent piping system and an externally powered actuator. Typically, a control valve receives a continuously varying (non-binary) signal (typically 4-20mA) from an automatic control system. The signal can be either electrical or pneumatic.

The most common function of a control valve is to control a specific flow (e.g. liquids or gases) in a control loop. Another task is to maintain a certain pressure in a system (either inlet or outlet pressure) - despite disturbances in the system. This means that a control valve always has to do with the distribution of energy.

The valve characteristic describes the relationship between the relative valve stroke (h/h100) and the relative flow coefficient (KV/KV100) at constant pressure drop across the control valve.

For more information on the flow coefficient, see the article on the KVS value

There are a number of conditions that must be considered to satisfy not only the process conditions, but also the laws of fluid dynamics.

• Limited Turndown Ratio: A control valve controls the flow of the medium within a certain working/flow rate range. Usually, these limits are between 5% to 95%. Outside this control ratio, the valve characteristic deviates from the desired control response.
• Pressure drop: The valve must have a minimum pressure drop to fulfill its control function (operating range, disturbances, energy conversion).
• Leakage: Control valves are usually not designed for tight shut-off. A minimum leakage rate must therefore be accepted. How high this may be is precisely defined. Shut-off valves, on the other hand, have no leakage or a significantly lower leakage.