Control Valve Authority

Overview

Engineering reference data for Control Valve Authority 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

Symbol	Description	Unit
$Re$	Reynolds number	—
$\rho$	Fluid density	kg/m³
$v$	Flow velocity	m/s
$D$	Characteristic dimension	m
$\mu$	Dynamic viscosity	Pa·s
$P$	Pressure	Pa
$f$	Darcy friction factor	—

Example: Valve Pressure Drop in a Heating System

The valve pressure drop dpv can be calculated by rearranging the authority formula:

$dpv = \frac{N \cdot dpc}{1 - N}$

Given a system with:

• Total circuit pressure drop dpc = 15 kPa
• Desired valve authority N = 0.8

Calculation: $dpv = \frac{0.8 \cdot 15 \text{ kPa}}{1 - 0.8} = 60 \text{ kPa}$

For a flow rate of 1.4 l/s, this pressure drop corresponds to a flow factor (Kv) of approximately *7, which typically selects a 15 mm Straight Through Diaphragm Valve.

Valve Authority and Control Quality

The valve authority N directly influences the controllability of the system. The following ranges indicate the expected control performance:

Interactive Charts

References

• Original Source