Composition and Classification of Control and Regulating Valves

    Composition and Classification of Regulating Valves

    A regulating valve, also known as a control valve, is a primary type of actuator. It modulates fluid flow by accepting a control signal from a controller and using an external power source to adjust the valve position. A typical regulating valve consists of two main parts: an actuator and a valve body.

    Based on the power source of the actuator, regulating valves are classified into three types: pneumatic (powered by compressed air), electric (powered by electricity), and hydraulic (powered by liquid pressure, e.g., oil). Additionally, by functionality and intelligence level, they include solenoid valves, electronic, smart, and fieldbus-enabled control valves. Valve bodies are often universal and can be paired with different actuators (pneumatic, electric, etc.).

    Valve Body Selection

    Selecting the appropriate valve body is the most critical step in control valve sizing.

    Common valve body types include: globe single-seat, globe double-seat, angle, diaphragm, low-flow, three-way, eccentric rotary, butterfly, cage-style, and ball valves. Before selection, carefully analyze the process medium, operating conditions, and system requirements. Key considerations include:

    (1) Plug Shape: Determined by required flow characteristic and unbalanced forces.
    (2) Wear Resistance: For slurries with abrasive particles, internal surfaces must be smooth and made of hardened materials.
    (3) Corrosion Resistance: Prefer simpler designs when handling corrosive media.
    (4) Temperature and Pressure: Use materials with low thermal expansion and high stability under varying T/P.
    (5) Flash and Cavitation Prevention: These phenomena (in liquids) cause noise, vibration, and erosion—select valves designed to mitigate them.

    Actuator Selection

    1. Output Force: The actuator must generate sufficient force to overcome unbalanced forces, friction, sealing loads, and gravity to ensure tight shutoff and reliable operation.
        Double-acting actuators (pneumatic/hydraulic/electric) lack springs; output force is direction-independent. Single-acting pneumatic actuators require force balance across the full stroke.

    2. Actuator Type: In explosive environments, only pneumatic actuators with explosion-proof junction boxes are permitted. Where safety allows, electric actuators are preferred for energy efficiency. Hydraulic actuators, though less common, offer high precision, fast response, and smooth motion—ideal for critical applications like turbine speed control or reactor temperature regulation in refineries.

    Fail-Safe Action (Air-to-Open / Air-to-Close)
    This applies only to pneumatic actuators and results from combining actuator action (direct/reverse) with valve action (direct/reverse). Four combinations yield two fail-safe modes: air-to-open (ATO) and air-to-close (ATC). Selection prioritizes: (a) process safety, (b) medium properties, and (c) minimizing product loss or damage.

    Flow Characteristic Selection

    The flow characteristic defines the relationship between relative flow and valve travel. Common ideal characteristics: linear, equal percentage (logarithmic), parabolic, and quick-opening. In practice, only linear, equal percentage, and quick-opening are widely used. Parabolic is intermediate and often replaced by equal percentage; quick-opening is for on/off or sequencing control. Thus, selection usually involves choosing between linear and equal percentage.

    Selection relies on empirical guidelines considering: (i) control loop performance, (ii) piping geometry, and (iii) load variability. Once selected, the flow characteristic determines plug shape—except for valves like diaphragm or butterfly, where it’s achieved via the positioner’s feedback cam profile.

    Valve Sizing (Cv Calculation)

    Sizing is based on flow coefficient Cv. Steps include:
    1) Determine Q_max and Q_min from process data.
    2) Establish pressure drop (ΔP) based on system resistance ratio (S).
    3) Calculate C_max and C_min using appropriate formulas.
    4) Select standard Cv ≥ C_max.
    5) Verify stroke: ≤90% at Q_max, ≥10% at Q_min.
    6) Ensure actual turndown ratio ≥10.
    7) Finalize seat and nominal diameter (DN) based on Cv.

(Source: Mechanical Experts Network)