Characteristics of Regulating Valves

    A control valve regulates flow, pressure, or liquid level of a process medium. It automatically adjusts valve opening based on signals from a controller to maintain desired process conditions. Types include electric, pneumatic, and hydraulic control valves.

    A control valve comprises an actuator (electric or pneumatic) and a valve body. Common body types are globe single-seat and globe double-seat. The double-seat design offers higher flow capacity, lower unbalanced forces, and stable operation—making it ideal for high-flow, high-pressure-drop, and low-leakage applications.

    Flow coefficient Cv is a key sizing parameter. Cv is defined as the flow rate (in m³/h or US gpm) of water at 60°F (density ≈1 g/cm³) that passes through the valve at full open with a pressure drop of 1 psi (≈0.1 MPa). Valve nominal diameter (DN) is selected based on calculated Cv.

    Flow characteristic describes the relationship between relative flow and valve travel under constant ΔP. Three primary types exist:

    (1) Equal Percentage (Logarithmic)
    Equal percentage of flow change per unit stroke. Provides fine control at low flows and coarse control at high flows—offering consistent resolution across the stroke. Optimal for most throttling applications.

    (2) Linear
    Flow change per unit stroke is constant. Relative flow changes are large at low flows and small at high flows—less ideal for wide-range control.

    (3) Parabolic
    Flow varies with the square of travel—intermediate between linear and equal percentage.

    In terms of control performance, equal percentage is generally superior, followed by parabolic, then linear. Selection depends on specific process requirements.

(Source: Mechanical Experts Network)