In brief
A control valve is a final control element that continuously influences flow, pressure or temperature in a plant by means of a variable opening area.
Contents
What is Control valve?
Unlike isolating valves, which know only open or closed, a control valve can take up any intermediate position. It consists of the valve body with seat and plug or rotary element, a valve actuator and usually a positioner, which converts the signal from the process controller into a precise stroke position. Control valves are found in steam, water, gas and process lines as well as in heating and cooling circuits.
Globe valves in straight-through or three-way form are common, alongside rotary plug valves, segmented ball valves and control butterfly valves. The central parameter is the Kvs value, which describes the flow when the valve is fully open at a defined differential pressure. Further sizing parameters are the valve characteristic (for example linear or equal percentage), rangeability, nominal size, nominal pressure, seat leakage, materials and the type of stem sealing.
Typical damage includes seats and plugs washed out by cavitation or erosion, leaking stuffing boxes and faults in the positioner. Many control valves can be repaired with trim sets consisting of seat, plug and seals, provided the Kvs value and characteristic of the original trim are known.
In practice: maintenance and procurement
Replacement trims or a replacement valve require the Kvs value, characteristic, nominal size, nominal pressure, materials, medium, pressures and temperature as well as the data of the actuator and positioner. Without the Kvs value and characteristic, it cannot be ensured that the control loop will work in the same way after the replacement.
Synonyms and abbreviations: regulating valve · modulating valve · final control element · globe control valve
Frequently asked questions
What is the Kvs value of a control valve?
The Kvs value indicates how much water in cubic metres per hour flows through the valve when it is fully open at a differential pressure of one bar. It is used for sizing and comparing valves. An excessively large Kvs value means that the valve works almost exclusively in the lower stroke range and controls poorly.
What is the difference between a control valve and an isolating valve?
An isolating valve is intended to open a line fully or close it tightly. A control valve changes the flow continuously and works permanently in intermediate positions. For this purpose, it is equipped with a defined characteristic, a sensitive actuator and usually a positioner. Some control valves also perform an isolating function, but do not always achieve the same tightness.
What information is needed for a replacement control valve?
Required are the Kvs value, valve characteristic, nominal size, nominal pressure, connection type and face-to-face dimension, materials of the body and trim, medium, inlet and outlet pressure and temperature. In addition, the actuator type, fail-safe position on loss of power and the positioner signal are needed. The nameplates of the valve and actuator contain most of this data.
What does equal percentage characteristic mean?
With an equal percentage characteristic, each equal change in stroke produces the same percentage change in flow relative to the current value. In the lower stroke range, the flow therefore changes little, and in the upper range it changes a lot. This characteristic is often chosen when the pressure drop across the valve varies considerably with the load.