Calculating Piping Diameter and Thickness According to ASME B31.3

ASME B31.3, Process Piping, provides comprehensive guidelines for designing and constructing process piping systems. Here's a breakdown of the process for calculating pipe diameter and thickness, along with the relevant formulas:

1. Determine Fluid Properties & Operating Conditions:

• Fluid Type: Identify the fluid flowing through the pipe. Its properties (density, viscosity, etc.) are critical for pressure drop calculations.
• Flow Rate: Specify the required volume flow rate (e.g., gallons per minute, cubic meters per hour).
• Operating Pressure: Determine the maximum internal pressure the pipe will experience during operation.
• Operating Temperature: Identify the maximum and minimum temperatures the pipe will encounter.
• Corrosion Allowance: Consider the potential for corrosion and add an appropriate allowance to the required wall thickness.

2. Calculate Pipe Diameter:

• Fluid Velocity: Choose a suitable fluid velocity based on factors like:
  • Fluid Type: Different fluids have different acceptable velocities.
  • Erosion: High velocities can cause erosion, especially for abrasive fluids.
  • Noise: Excessive velocity can lead to noise and vibration.
• Flow Area: Use the flow rate and desired velocity to calculate the required flow area (A) using:
  • A = Flow Rate / Velocity
• Pipe Diameter: Calculate the inner diameter (ID) of the pipe using the flow area and the formula for the area of a circle:
  • A = π * (ID/2)^2

3. Determine Pipe Wall Thickness:

• Pressure Design Basis: ASME B31.3 provides several design basis options:
  • Design Pressure (P): The maximum allowable pressure for the pipe.
  • Internal Pressure (Pi): The pressure inside the pipe.
• Stress Factors:
  • Stress Intensity (S): The allowable stress for the pipe material at the operating temperature. You'll need to refer to material specifications and ASME B31.3 tables for these values.
  • Joint Efficiency (E): A factor that accounts for the strength of the welded joint (typically 0.85).
  • Design Factor (F): A safety factor to account for uncertainties (generally 0.72 for normal service, 0.60 for high-pressure service, and 0.40 for severe service).
• Corrosion Allowance (CA): Add this to the calculated wall thickness to account for potential corrosion over the pipe's lifespan.
• Thickness Calculation: Use the following formula to calculate the minimum required wall thickness (t):
  • t = (P * D) / (2 * S * E * F - P) + CA
  • Where:
    • t: Minimum required wall thickness
    • P: Design Pressure (internal pressure)
    • D: Outside diameter of the pipe
    • S: Allowable stress
    • E: Joint efficiency
    • F: Design factor
    • CA: Corrosion allowance

4. Select Standard Pipe Size:

• Schedule: Pipe wall thickness is standardized in schedules (e.g., Schedule 40, Schedule 80).
• Select: Choose the nearest standard pipe size and schedule that meets or exceeds the calculated wall thickness.

5. Consider Other Factors:

• Pipe Material: Choose a suitable material based on:
  • Corrosion Resistance: Consider the fluid's corrosivity and select a material that can withstand it.
  • Temperature Rating: Choose a material with a suitable temperature rating for the operating conditions.
• Pipe Supports: Design appropriate pipe supports to withstand the weight of the pipe and its contents, including thermal expansion and contraction.
• Insulation: Consider the need for insulation to prevent heat loss or gain and maintain operating temperatures.

Example:

Let's say you have a process piping system carrying a fluid with the following characteristics:

• Fluid: Water
• Flow Rate: 100 gallons per minute (gpm)
• Operating Pressure: 150 psi
• Operating Temperature: 150°F
• Corrosion Allowance: 0.125 inches

1. Calculate Pipe Diameter:

• Velocity: Assume a velocity of 5 ft/s for water in a piping system.
• Flow Area: A = 100 gpm / (5 ft/s) = 20 gpm/ft/s = 0.154 ft^2
• Pipe ID: 0.154 ft^2 = π * (ID/2)^2 -> ID = 0.44 ft = 5.28 inches

2. Determine Pipe Wall Thickness:

• Design Pressure: Assume a design pressure (P) of 175 psi (higher than the operating pressure for safety).
• Material: Assume carbon steel pipe with an allowable stress (S) of 17,000 psi at 150°F.
• Joint Efficiency: Assume E = 0.85.
• Design Factor: Assume F = 0.72 for normal service.
• Thickness: t = (175 psi * 5.28 inches) / (2 * 17,000 psi * 0.85 * 0.72 - 175 psi) + 0.125 inches = 0.25 inches.

3. Select Standard Pipe Size:

• Schedule 40: This schedule has a wall thickness of 0.218 inches, which is less than the required 0.25 inches.
• Schedule 80: This schedule has a wall thickness of 0.312 inches, which meets the requirement.

Therefore, based on these calculations, you would choose a 5.28-inch OD pipe with a Schedule 80 wall thickness for this process piping system.

Remember: This is a simplified example. The actual calculations will vary depending on the specific project and fluid characteristics. Consult ASME B31.3 and other relevant engineering codes and standards for complete design and construction requirements.

answer July 28, 2024, 12:56 p.m.

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