Control Valves Flow Characteristics

Overview

Engineering reference data for Control Valves Flow Characteristics in fluid mechanics.

Key Formulas

Reynolds Number

$Re = \frac{\rho v D}{\mu}$

Ratio of inertial to viscous forces — determines flow regime.

Bernoulli's Equation

$P + \frac{1}{2}\rho v^2 + \rho g h = \text{const}$

Conservation of energy for steady, inviscid, incompressible flow.

Continuity Equation

$A_1 v_1 = A_2 v_2$

Conservation of mass for incompressible flow.

Darcy-Weisbach

$\Delta P = f \frac{L}{D} \frac{\rho v^2}{2}$

Pressure drop due to friction in a pipe.

Variables

Inherent Flow Characteristics

The inherent flow characteristics describe how the control valve capacity changes with stem travel under constant pressure drop. Common types include:

• Linear: Flow capacity increases linearly with valve travel.
• Equal Percentage: Flow capacity increases exponentially with valve travel. Equal increments of travel produce equal percentage changes in the existing Cv.
• Modified Parabolic: Approximately midway between linear and equal-percentage characteristics. It provides fine throttling at low flow capacity and approximately linear characteristics at higher flow capacity.
• Quick Opening: Provides large changes in flow for very small changes in lift. It usually has too high a valve gain for use in modulating control, so it is limited to on-off service, such as sequential operation in batch or semi-continuous processes.
• Hyperbolic: A flow characteristic type, though specific details are not elaborated in this context.

Installed Flow Characteristics

When valves are installed in a system with pumps, piping, and fittings, the pressure drop across the valve varies as the plug moves through its travel, resulting in installed flow characteristics that differ from inherent ones. This variation occurs because the pressure drop decreases as the valve opens. For example:

• A linear inherent curve will generally resemble a quick opening characteristic when installed.
• An equal percentage inherent curve will generally resemble a linear characteristic when installed.

Understanding this shift is essential for accurate control system design and valve selection in real-world applications.

References

• Original Source