Notes on Pressure Drop Calculations in Natural Gas Systems

Introduction

• Focus on pressure drop calculations in natural gas pipelines.
• Importance of understanding various types of connections in natural gas plants.

Types of Piping Systems

• Includes transportation of natural gas and hydrocarbon gases (e.g., LPG).
• Examples:
  • Natural gas gathering systems
  • Gas distribution piping
  • Gas transmission piping

Losses in Piping Systems

• Frictional Losses: Caused by the fluid flowing through the pipeline (skin friction).
• Form Drag: Loss of pressure due to changes in flow direction.
• Total Loss: Sum of major and minor losses.
  • Major Losses: Losses in straight pipes.
  • Minor Losses: Losses through fittings, valves, and changes in cross-sectional area.

Bernoulli's Equation

• Used to understand pressure losses:
  • Considers potential energy, kinetic energy, and pressure energy.
  • Modifications for frictional losses.
• Key components:
  • Elevation: Measured by the central axis of the pipeline.
  • Velocity and pressure at the inlet and outlet.

Pressure Drop Calculations

• Various formulas derived from Bernoulli's equation to estimate pressure drop:
  • Flow Rate: Related to gas properties, pipe size, temperature, and pressure drop.
  • Friction Factor: Calculated using formulas like Colebrook-White and modified Colebrook-White equations.

Fundamental Flow Equation

• Connects gas flow rate, properties, pipe size, temperature, and pressure drop.
• Assumes constant fluid temperature, varying pressure.

Average Pressure Calculation

• Average pressure is critical for calculations:
  • Better representation than simple arithmetic average.
  • Expression for average pressure: ( P_{avg} = \frac{P_1 + P_2 - \frac{P_1 P_2}{P_1 + P_2}}{2} )_

Elevation Effect

• Consideration of elevation differences in calculation:
  • Equivalent length of pipeline (due to elevation changes).
  • Various equations for different unit systems (FPS and SI).

Compressibility Factor (Z)

• Determined using various methods; important for non-ideal gases.
• Common equations include Yarbrough and CNG equations for estimating Z.

Velocity and Flow Rate

• Gas Velocity: Determined from volumetric flow rate and cross-sectional area.
• Maximum Allowable Gas Velocity: Critical for preventing erosion and noise.
• Operational velocity is typically set at 50% of maximum allowable.

Darcy's Equation

• Major loss due to friction represented by Darcy's equation.

• Friction Factor: Varies based on flow regime (laminar vs. turbulent).

  • Laminar flow: ( f = \frac{64}{Re} )
  • Turbulent flow: Use Moody's chart or empirical formulas.

Reynolds Number and Flow Regime

• Reynolds Number (Re): Determines flow regime (laminar or turbulent).
• Affects friction factor calculations based on flow characteristics.

References

• Suggested standard fluid mechanics books for further reading on equations and derivations.