IGE Rec 3 – General

Introduction

The Institution of Gas Engineers Recommendation 3 — General (IGE Rec 3 — General) flow equation calculates the various values associated with gas flow through a pipe segment — outlet pressure, flow rate, hydraulic diameter, pressure drop, and velocity — for general gas distribution systems. The method was developed by the British Institution of Gas Engineers and is recommended for pressures less than or equal to 101.5 psig (7 bar).

The IGE Rec 3 — General equation is Reynolds-Number dependent: the friction term is evaluated from a smooth-pipe law that takes the Reynolds Number as its input, so a representative gas viscosity and flow rate are required. The equation responds to changes in gas viscosity, flow rate, and pipe diameter rather than to an entered wall-roughness value.

Background

There are various equations available for calculating the volumetric flow through a pipe segment, and nearly all of them are derived from a single general pipe flow equation. In general terms, the flow in a pipe segment is proportional to the pressure drop across the segment, divided by a combination of the pipe and gas resistance to flow: the larger the pressure drop, the higher the flow rate; the higher the resistance, the lower the flow rate. Beyond this general relationship, each individual equation has been modified to reflect a particular flow regime or set of application-specific conditions.

The IGE Rec 3 — General equation was developed by the British Institution of Gas Engineers for general distribution systems. As implemented in Technical Toolboxes, it expresses flow as a function of the base conditions, the smooth-pipe-law friction factor, the squared pressure difference, the inside diameter raised to a fixed exponent, and a hydraulic efficiency factor. Because the friction term is a function of the Reynolds Number, the equation responds to changes in gas viscosity, flow rate, and pipe diameter rather than to an entered wall-roughness value.

Equations

IGE Rec 3 — General

Q = 117.3 \times \frac{T_B}{P_B} \times \sqrt\frac{1}{f_{spl}} \times  \sqrt{ \frac{\Delta P}{SG \times T_F \times L \times Z}} \times D^{2.5} \times E

Q = 117.3 \times \frac{T_B}{P_B} \times \left(\frac{1}{f_{spl}}\right)^{0.5} \times \left(\frac{\Delta P}{SG \times T_F \times L \times Z}\right)^{0.5} \times D^{2.5} \times E

Where:
Q − Volumetric Flow Rate at the specified base pressure and temperature, standard ft³/hr
T_B − Base Temperature, °R
P_B − Base Pressure, psia
f_spl − Smooth Pipe Law Friction Factor
ΔP − Pressure Drop term across the segment, psia²
SG − Specific Gravity, dimensionless
T_F − Average Gas Flowing Temperature, °R
L − Pipe Length, feet
Z − Compressibility Factor, dimensionless
D − Inside Pipe Diameter, inches
E − Pipe Efficiency, decimal fraction

\Delta P = P_1^2 - P_2^2

\Delta P = P_1^2 – P_2^2

Where:
ΔP − Pressure Drop term, psia²
P₁ − Pipe Inlet (Upstream) Pressure, psia
P₂ − Pipe Outlet (Downstream) Pressure, psia

Smooth Pipe Law Friction Factor

The friction term in the IGE Rec 3 — General equation is evaluated from a smooth-pipe law expressed as a polynomial in X, where X is a function of the Reynolds Number:

\sqrt{\frac{1}{f_{spl}}} = 14.7519 + 3.5657\,X + 0.0362\,X^2

\left(\frac{1}{f_{spl}}\right)^{0.5} = 14.7519 + 3.5657\,X + 0.0362\,X^2

Where:
f_spl − Smooth Pipe Law Friction Factor, dimensionless
X − Reynolds-Number-dependent intermediate term, dimensionless

X = \log_{10}(Re) - 5

X = \log_{10}(Re) – 5

Where:
X − Reynolds-Number-dependent intermediate term, dimensionless
Re − Reynolds Number, dimensionless

Reynolds Number

Because the IGE Rec 3 — General friction term depends on the Reynolds Number, the Reynolds Number must be evaluated for the flowing conditions. As implemented in Technical Toolboxes, the Reynolds Number is calculated using the following equation:

Re = 0.01146 \times \frac{P_B}{T_B} \times \frac{Q \times SG}{D \times \mu}

Re = 0.01146 \times \frac{P_B}{T_B} \times \frac{Q \times SG}{D \times \mu}

Where:
Re − Reynolds Number, dimensionless
P_B − Base Pressure, psia
T_B − Base Temperature, °R
Q − Volumetric Flow Rate at the specified base pressure and temperature, standard ft³/hr
SG − Specific Gravity, dimensionless
D − Inside Diameter of Pipe, inches
μ − Absolute Viscosity, lbm/ft-sec

Unless otherwise noted, all values are in consistent dimensional units.

Case Guide

Part 1: Create Case

Input Parameters

Part 2: Outputs/Reports

Results

Note: The red colored labels indicate which items may be calculated. Only one red item may be selected as the “unknown”; the remaining items must be “known.” Pressure values are entered and displayed as gauge pressures, and flow values represent standard volume adjusted to the base conditions. The number of decimal places shown for any calculated item can be set under File > Preferences > Decimals.

References

• The Institution of Gas Engineers and Managers, Steel and PE Pipelines For Gas Distribution, Recommendations on Transmission and Distribution Practice, Standard IGE/TD/3 Edition 4, 2003.
• American Gas Association, Gas Engineering and Operating Practices — System Design, GEOP Series Book D-1, 1990.
• Industrial Press, Gas Engineers Handbook, 1965.
• Gas Processors Suppliers Association, Engineering Data Book, Eleventh Edition — FPS, 1998.

FAQ