Control Valves Calculator

Overview

Engineering reference data for Control Valves Calculator 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

Understanding SCFH (Standard Cubic Feet per Hour)

SCFH is a standard unit of measurement for gas flow rate. It represents the volume of gas, in cubic feet, that flows per hour under standardized conditions of temperature and pressure (typically 70°F and 1 atm). This standardization allows for consistent comparison and calculation of gas flows regardless of actual operating conditions.

Cv Formula for Gas Flow

The Flow Coefficient ($C_v$) for a control valve in gas service can be calculated using the standard flow rate in SCFH. The general formula relates $C_v$ to the gas flow, upstream pressure, downstream pressure, gas specific gravity, and temperature.

$$C_v = \frac{Q_g}{N \cdot P_1 \sqrt{\frac{G_g \cdot T}{Z} \cdot \left[ \left( \frac{P_1 - P_2}{P_1} \right) \left( 1 - \left( \frac{P_1 - P_2}{P_1} \right)^{2/7} \right) \right] }}$$

Where:

• $Q_g$ = Gas flow rate (SCFH)
• $P_1$ = Upstream pressure (psia)
• $P_2$ = Downstream pressure (psia)
• $G_g$ = Gas specific gravity (Air = 1)
• $T$ = Absolute temperature (°R = °F + 460)
• $Z$ = Gas compressibility factor (dimensionless)
• $N$ = A numerical constant (typically 1360 for the units shown)

This formula is used for gases where the pressure drop ratio is not critical (i.e., non-choked flow).

Key Variables for Gas Cv Calculators

• $C_v$ (Flow Coefficient): A valve's capacity. It is defined as the number of US gallons per minute (GPM) of water at 60°F that will flow through the valve with a pressure differential of 1 psi.
• SCFH (Standard Cubic Feet per Hour): The standardized volumetric flow rate for the gas entering the valve.
• $P_1$ & $P_2$ (Upstream/Downstream Pressure): The absolute pressures immediately before and after the valve. The difference ($\Delta P = P_1 - P_2$) is the pressure drop.
• $G_g$ (Specific Gravity): The ratio of the gas's density to the density of dry air at standard conditions.
• $T$ (Temperature): The absolute temperature of the flowing gas, crucial for correcting volume to standard conditions.

References

• Original Source