IGE REC 3 – Medium Pressure

Introduction

The IGE Rec 3 – Medium Pressure calculator computes the volumetric flow rate, pressure drop, pipe diameter, or pipe length for a gas pipe segment using the Institution of Gas Engineers Recommendation 3 Medium Pressure flow equation (IGE3-MP). Developed by the British Gas Engineers Institute and published in IGE/TD/3, this Reynolds Number dependent equation is designed for medium-pressure gas distribution systems operating between 1.1 psig (75 mBar) and 29 psig (2 bar). Like all Pipe Flow calculators, the unknown parameter — flow rate, pressure, diameter, or length — is selected by the user and computed from the remaining known values.

Important: The IGE3-MP equation is intended for medium-pressure distribution systems at pressures greater than 1.1 psig (75 mBar) and less than or equal to 29 psig (2 bar). It is Reynolds Number dependent and uses a smooth pipe law friction factor. Applying the equation outside its intended pressure and Reynolds Number range may yield inaccurate results. For pressures at or below 1.1 psig (75 mBar), use IGE Rec 3 – Low Pressure. For general distribution applications up to 101.5 psig (7 bar), use IGE Rec 3 – General.

Background

The Institution of Gas Engineers Recommendation 3 (IGE/TD/3) is a family of gas pipe flow equations developed by the British Gas Engineers Institute for use in UK gas distribution networks. The family comprises three variants tailored to different operating pressure ranges: Low Pressure (LP), Medium Pressure (MP), and General. All three are Reynolds Number dependent and are based on smooth pipe law friction factor behaviour.

The Medium Pressure variant was developed for systems operating above the low-pressure threshold (75 mBar / 1.1 psig) and up to 2 bar (29 psig). At this pressure range, the squared-pressure form of the pressure difference term (ΔP = P₁² − P₂²) governs the flow relationship, and the smooth pipe law friction factor is expressed as a polynomial function of the Reynolds Number. The General variant uses the same mathematical structure as the Medium Pressure equation and is applicable to pressures up to 7 bar (101.5 psig).

Smooth Pipe Law Friction Factor

The IGE3-MP equation uses a friction factor derived from the smooth pipe law. Rather than being expressed as a direct function of pipe roughness (as in the Colebrook-White or AGA Fully Turbulent equations), the smooth pipe law friction factor is expressed as a function of the Reynolds Number alone. This reflects the assumption that, for the pipe sizes and operating pressures typical of medium-pressure gas distribution, the effect of pipe wall roughness is negligible relative to the viscous and turbulent flow effects captured by the Reynolds Number.

Pipe Efficiency

All pipe flow equations include a hydraulic efficiency term (E) that allows the user to calibrate the equation against field measurements or apply a design safety margin. An efficiency below 1.0 produces conservative results; an efficiency above 1.0 increases the predicted capacity. Efficiency values outside the range 0.80–1.20 may indicate either incorrect input data or a poor fit between the equation and the application.

Elevation Effects

When inlet and outlet elevations differ, the ΔP term in the flow equation is replaced by an elevation-compensated squared-pressure difference that accounts for the hydrostatic head of the gas column between the two endpoints. The atmospheric pressure at each end is also adjusted for elevation, affecting the absolute pressures used in the calculation. Both corrections are applied automatically when inlet and outlet elevations are entered.

Equations

IGE3-MP Flow Equation

As implemented in GASCalc, the Institution of Gas Engineers Recommendation 3 — Medium Pressure flow equation is:

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

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 (ft³)
T_B − Base temperature (°R)
P_B − Base pressure (psia)
f_spl − Smooth pipe law friction factor
ΔP − Pressure difference term (psia²)
SG − Specific gravity of the gas
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)

Smooth Pipe Law Friction Factor

The smooth pipe law friction factor term is computed from a polynomial expression in the base-10 logarithm of the Reynolds Number:

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

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

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

X = \log_{10}(Re) – 5

Where:
f_spl − Smooth pipe law friction factor
X − Intermediate friction factor variable (dimensionless)
Re − Reynolds Number (dimensionless)

Elevation Adjustment

When an elevation difference exists between the pipe inlet and outlet, the ΔP term is replaced by:

\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

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 (°R)
Z − Compressibility factor

IGE Rec 3 Family — Pressure Range Summary

Case Guide

Part 1: Create Case

Input Parameters

Part 2: Outputs/Reports

Results

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.
• Industrial Press — Gas Engineers Handbook, 1965.
• Gas Processors Suppliers Association — Engineering Data Book, Eleventh Edition (FPS), 1998.

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