Control Valve - Overview

Calculation and Selection Methods of Industrial Control Valves

1. Introduction

In industrial processes, control valves are essential elements that enable the precise regulation of variables such as pressure, temperature, flow, and level. An improperly selected control valve can disrupt system stability, create energy loss, and increase maintenance costs. Therefore, the selection process must be meticulously executed not only from a mechanical perspective but also from a fluid dynamics, control theory, and process safety perspective.

Kontrol Vanası - Schubert & Salzer

2. Basic Definitions and Concepts

Flow Coefficient (Cv or Kv): This is the most important characteristic of the control valve. Cv is the flow rate in gallons per minute (GPM) of water at 60°F under a 1 psi pressure difference. Kv is the flow rate in m³/hour of a fluid with a density of 1 kg/dm³ under a 1 bar pressure difference. Relationship: Kv = 0.865 × Cv. Flow Characteristics: Linear, Equal Percentage, and Quick-Opening types. Valve Pressure Drop (ΔP): The control valve should handle approximately 10–30% of the total pressure loss in the system (not always applicable).

3. Calculation Steps

Correct collection of process data is essential: flow rates, pressures, temperatures and fluid properties. Flow coefficient calculation for water: Cv = Q / (N₁ × √(ΔP / G)). For gases, the correction coefficient (Y) and the compressibility factor (Z) must be taken into account.

The critical flow condition is determined by the formula: r_c = (2/(γ+1))^(γ/(γ−1)).

4. Selection Criteria

Valve body types:

• Sliding Gate Control Valve: Very high precision, energy saving, low carbon emission value

• Globe: General applications with good precision.

• Butterfly: Economical solution for large flow rates.

• Ball (Sector - Segment): Abrasive fluids, high Cv.

• Diaphragm: Fluids with high corrosion risk.

• Pinch: Fluids with solid particles. Actuator Selection: Pneumatic (fast), Electric (sensitive), Hydraulic (powerful).

5. Determining the Valve's Operating Point

The ideal operating range is 20–80% open. At the normal flow rate, the valve should be selected to operate at 60–70% open.

6. Sample Application

Given: Water (ρ = 1000 kg/m³), Q = 25 m³/h, ΔP = 1 bar. Calculation: Kv = Q × √(G / ΔP) = 25 × √(1 / 1) = 25.

Suitable valve: DN40 equal-percentage Sliding Gate Control valve, opening ≈ 65%.

7. Special Situations and Matters to Be Considered

• Multi-stage trim should be used to prevent cavitation and flashing risks.

• Expansion allowance at high temperatures must be taken into account.

• In viscous fluids, correction must be made according to the Reynolds number.

• Actuator stroke time must be compatible with process dynamics.

8. Conclusion

Control valve selection should be based not only on catalog values but also on a holistic assessment of process data, flow characteristics, control strategy, and safety requirements. A correctly calculated valve makes a significant difference in energy efficiency and system stability.