Control Valves

A control valve modulates a process variable such as pressure, flow, level or temperature by continuously adjusting its opening in response to a signal or to the pipeline conditions themselves. It is the final control element in most process loops. Two broad families exist: powered control valves, driven by a pneumatic, electric or hydraulic actuator following a controller signal, and self-acting or automatic control valves, driven by the energy of the fluid through a pilot circuit and needing no external power.

Powered control valves are usually globe or rotary bodies fitted with a positioner that matches valve travel to the demand signal. The trim, the disc, seat and cage or plug, is engineered for a defined flow characteristic (linear, equal-percentage or quick-opening) and for low noise and cavitation resistance at the design conditions. Rotary control valves, based on segmented ball or eccentric disc designs, offer high capacity and rangeability in a compact body.

Self-acting automatic control valves are hydraulically operated diaphragm or piston valves in which a pilot circuit senses the controlled variable and modulates the main valve. They are widely used in water distribution for pressure reduction, flow limiting, level control and pump protection, where their independence from external power is a practical advantage at remote sites.

Relevant standards include IEC 60534 for industrial-process control valve sizing and testing, with flanges to EN 1092 or ASME B16.5 and pressure testing to EN 12266 or API 598. Bodies are made in cast and forged steel, stainless steel and ductile iron, with trim selected for the service. Accurate sizing is essential: an oversized control valve operates near its seat and controls poorly, while an undersized valve cannot pass the required flow.

To specify, provide the flow range, inlet and outlet pressures, medium and temperature, the required control function and the available actuation or signal type. We can then propose a valve, trim and actuator suited to the loop.