Hydraulics

Introduction

Hydraulic analysis evaluates the relationship between flow rate, pressure and pipe geometry across a pipe segment. Nearly every gas flow equation is derived from a single “general pipe flow equation,” which states that the flow through a segment is proportional to the pressure drop across it, divided by the combined resistance of the pipe and the gas. In practical terms, a larger pressure drop produces a higher flow rate, while greater resistance produces a lower one.

Beyond that general relationship, each equation has been modified to reflect a specific flow regime or application. Some were developed to predict laminar or partially turbulent flow in very low-pressure distribution systems, while others target partially or fully turbulent flow in high-pressure, large-diameter, long-length transmission lines. Many older equations also fold in simplifying assumptions that may or may not hold for a given application, so understanding an equation’s origin and intended use matters when selecting one.

There is no single flow equation that accurately reflects every condition that can occur in all types of gas piping systems. The User must ultimately compare their specific application to each equation’s intended use and select the most appropriate one. The table below summarizes when to apply each equation; the two general-purpose recommendations – IGT – Improved and Colebrook-White – are highlighted at the top. The remaining equations are suited to more specific flow regimes, pressure ranges or applications, and several belong to the wider software equation set rather than this module.

Equation	Best Suited For	Pressure Range / Flow Regime
IGT – Improved (Recommended)	General-purpose; distribution, low-pressure transmission and gathering – the recommended default for most applications	1 to 500 psig; Reynolds Numbers 16,000 to 3,000,000; balanced, moderate results
Colebrook-White (Recommended)	Many general gas flow applications; the “Fundamental” equation	Applies the Moody friction factor with the Colebrook-White approximation for partial and fully turbulent flow; results vary with roughness
AGA – Turbulent	Transmission lines in fully turbulent flow	Fully turbulent; rough pipe law with a fixed relative roughness; moderately conservative
AGA – Partially Turbulent	Transmission lines in partially turbulent flow	Partially turbulent; smooth pipe law; moderately conservative
Panhandle – A	High-pressure transmission	300 psig and greater; large-diameter lines; efficiency typically 0.90 to 0.92
Weymouth	Transmission and high-pressure gathering; short lines	Conservative; commonly used for 3 to 20 psig systems
IGE Rec 3 – General	General distribution systems	≤ 101.5 psig (7 bar)
IGE Rec 3 – Medium Pressure	Medium-pressure distribution systems	> 1.1 psig (75 mBar) and ≤ 29 psig (2 bar)
IGE Rec 3 – Low Pressure	Low-pressure distribution systems	≤ 1.1 psig (75 mBar)
Spitzglass – Low Pressure	Low-pressure distribution	< 3 psig; conservative; recommended for sub-3 psig distribution
Oliphant	Vacuum gathering and plant piping	Vacuum to 100 psig; conservative; diameter dependent
International Fuel Gas Code	Fuel and plant piping	Low- and High-Pressure variants (below / at and above 1.5 psig)
IAPMO	Fuel and plant piping	Low- and High-Pressure variants (below / at and above 1.5 psig)
International Mechanical Code	Fuel and plant piping	Low- and High-Pressure variants (below / at and above 1.5 psig)

Module/Application

• AGA – Turbulent
  • Use Case: Transmission systems operating under fully turbulent flow conditions
  • Special Considerations:
    • Applies the rough pipe law with a fixed relative pipe roughness
    • Moderately conservative compared with other transmission-style equations
  • Differentiator: Uses a fixed roughness rather than a user-entered value
• AGA – Partially Turbulent
  • Use Case: Transmission systems operating under partially turbulent flow conditions
  • Special Considerations:
    • Applies the smooth pipe law to account for relative pipe roughness
    • Moderately conservative compared with other transmission-style equations
  • Differentiator: Models the partially turbulent regime rather than the fully turbulent one
• IGT – Improved
  • Use Case: Distribution systems, though often used for all system types operating between 1 and 500 psig; the recommended default for medium- and high-pressure distribution, low-pressure transmission and gathering
  • Limitations:
    • Reynolds Number dependent
    • Per the AGA GEOP text, valid where Reynolds Numbers range between 16,000 and 3,000,000 with a 2% deviation from the smooth pipe law
    • Widely used from ten-inch water column to sixty psig for two-inch and larger diameter pipe
  • Differentiator: Balanced, moderate results; developed by the Institute of Gas Technology in the 1960s
• IGE Rec 3 – General
  • Use Case: General distribution systems
  • Limitations: Recommended for pressures less than or equal to 101.5 psig (7 bar)
  • Differentiator: Developed by the British Institution of Gas Engineers; the base equation from which the Medium- and Low-Pressure variants are derived
• IGE Rec 3 – Medium Pressure
  • Use Case: Medium-pressure distribution systems
  • Limitations: For pressures greater than 1.1 psig (75 mBar) and less than or equal to 29 psig (2 bar)
  • Differentiator: A modified version of the IGE Rec 3 – General equation tuned to the medium-pressure range
• IGE Rec 3 – Low Pressure
  • Use Case: Low-pressure distribution systems
  • Limitations: For pressures less than or equal to 1.1 psig (75 mBar)
  • Differentiator: A modified version of the IGE Rec 3 – General equation tuned to the low-pressure range

Reference

• Gas Processors Suppliers Association, Engineering Data Book, Eleventh Edition – FPS, 1998.
• 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 (GEOP) Series.
• Gas Engineers Handbook, Industrial Press.
• Gulf Publishing Company, Pipeline Rules of Thumb Handbook.

FAQ

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