IGT – Improved

Introduction

The IGT – Improved calculator computes the volumetric flow rate, pressure drop, pipe diameter, or pipe length for a gas pipe segment using the Institute of Gas Technology Improved flow equation. Developed in the 1960s, the IGT-Improved equation is one of the most widely applied pipe flow equations in the natural gas industry, suitable for distribution, transmission, gathering, and plant piping across a broad range of operating pressures (1–500 psig). The equation accounts for gas-specific gravity, viscosity, compressibility, pipe diameter, pipe length, and a user-defined pipe efficiency.

Important: The IGT-Improved equation is Reynolds Number dependent. It applies to distribution systems where Reynolds Numbers range from approximately 16,000 to 3,000,000 with 2% deviation from the smooth pipe law. Applicable diameter and pressure ranges, as documented in the AGA GEOP text, are: 3–30 inch pipe at inches water column, 1.5–20 inch pipe at 2–20 psig, and 0.75–120 inch pipe at 20–100 psig. Using the equation outside these ranges may yield inaccurate results.

Background

Gas pipe flow equations relate the volumetric flow rate through a pipe segment to the pressure drop across it, accounting for the resistance imposed by pipe geometry and gas properties. All such equations share the general principle that flow increases with increasing pressure drop and decreases with increasing resistance. However, each equation was developed empirically for a particular flow regime or application, and no single equation accurately reflects all possible conditions in all types of gas piping systems.

The IGT-Improved equation was published in 1967 in Gas Age Magazine and is derived from smooth-pipe law friction factor behaviour. It was developed specifically for distribution systems but has become the de facto general-purpose equation in the software for medium and high pressure distribution, lower-pressure transmission, gathering, and plant piping applications. The Gas Engineers Handbook notes that it is widely used for ten-inch water column to sixty psig systems for two-inch and larger diameter pipe.

Pipe Efficiency

All pipe flow equations include a hydraulic efficiency term (E). This parameter allows the user to tune the equation to better match actual field measurements. An efficiency of less than 1.0 provides a conservative safety factor; an efficiency greater than 1.0 increases the predicted capacity. For the IGT-Improved equation, efficiencies between 0.80 and 1.20 are typical for well-matched applications — values outside this range may indicate either incorrect input data or a poor equation-to-application fit.

Compressibility

The IGT-Improved equation includes the compressibility factor Z in its pressure drop term. Ignoring compressibility (setting Z = 1.0) produces conservative results — lower flow rates, higher pressure drops, or larger calculated pipe sizes than if compressibility were accounted for. For design calculations this is generally acceptable at pressures below 100 psig; above 400 psig, the gas begins to deviate significantly from ideal behavior, and an accurate compressibility constant becomes necessary. For measurement calculations a compressibility factor method should always be applied.

Elevation Effects

When inlet and outlet elevations differ, two corrections are applied. First, the atmospheric pressure at each end is adjusted for elevation, affecting the absolute pressure used in the calculation. Second, when the pressure difference term is expressed as a squared-pressure difference (ΔP = P₁² − P₂²), the hydrostatic head of the gas column is accounted for by replacing ΔP with an elevation-compensated form (described in the Equations section). Both effects are computed automatically when inlet and outlet elevations are entered.

Equations

IGT-Improved Flow Equation

As implemented in GASCalc, the Institute of Gas Technology Improved flow equation is:

Q = 664.3 \times \frac{T_B}{P_B} \times \frac{1}{\mu^{0.111}} \times \left(\frac{\Delta P}{SG^{0.8} \times T_F \times L \times Z}\right)^{0.556} \times D^{2.667} \times E

Q = 664.3 \times \frac{T_B}{P_B} \times \frac{1}{\mu^{0.111}} \times \left(\frac{\Delta P}{SG^{0.8} \times T_F \times L \times Z}\right)^{0.556} \times D^{2.667} \times E

Where:
Q − Volumetric flow rate at the specified base pressure and temperature (ft³)
T_B − Base temperature (°R)
P_B − Base pressure (psia)
μ − Absolute (dynamic) viscosity of the gas (lbm/ft·sec)
ΔP − Pressure difference term (psia²)
SG − Specific gravity of the gas (dimensionless)
T_F − Average gas flowing temperature (°R)
L − Pipe length (ft)
Z − Compressibility factor
D − Inside pipe diameter (in)
E − Pipe hydraulic efficiency (0-1)

Where for standard (non-elevation-adjusted) calculations:

\Delta P = P_1^2 - P_2^2

\Delta P = P_1^2 – P_2^2

Where:
ΔP − Pressure difference term (psia²)
P₁ − Pipe inlet (upstream) absolute pressure (psia)
P₂ − Pipe outlet (downstream) absolute pressure (psia)

Elevation Adjustment

When an elevation difference exists between the pipe inlet and outlet, the ΔP term is replaced by an elevation-compensated pressure difference to account for the hydrostatic head of the gas column:

\Delta P = P_1^2 - e^S \times P_2^2

\Delta P = P_1^2 – e^S \times P_2^2

Where:
ΔP − Pressure difference term (psia²)
P₁ − Pipe inlet (upstream) absolute pressure (psia)
P₂ − Pipe outlet (downstream) absolute pressure (psia)
e − Napier’s constant (2.71828)
S − Elevation compensation factor (dimensionless)

S = \frac{0.0375 \, SG \, (E_1 - E_2)}{T_F \times Z}

S = \frac{0.0375 \, SG \, (E_1 – E_2)}{T_F \times Z}

Where:
S − Elevation compensation factor
SG − Specific gravity of the gas
E₁ − Pipe inlet elevation (ft)
E₂ − Pipe outlet elevation (ft)
T_F − Average gas flowing temperature (Napier’s constant)
Z − Compressibility factor

Recommended Applications

The following table summarises the applications for which the IGT-Improved equation is recommended by B3PE, alongside competing equations for comparison:

Case Guide

Part 1: Create Case

Input Parameters

Part 2: Outputs/Reports

Results

References

• Industrial Press — Gas Engineers Handbook, 1965.
• American Gas Association — Gas Engineering and Operating Practices: System Design, GEOP Series Book D-1, 1990.
• Gas Processors Suppliers Association — Engineering Data Book, Eleventh Edition (FPS), 1998.
• Gas Age Magazine — “Gas Behavior in Distribution Systems,” May 1967. (Primary source for the IGT-Improved equation.)

FAQ