Calculating Piping Diameter and Thickness According to ASME B31.8

ASME B31.8, "Gas Transmission and Distribution Piping Systems," provides guidance on determining the necessary diameter and thickness for gas piping systems. The exact calculation methods depend on factors like:

• Operating Pressure: The pressure at which the gas will be transported.
• Operating Temperature: The temperature of the gas during transport.
• Material: The type of pipe material being used (e.g., steel, polyethylene).
• Fluid Properties: Properties of the transported gas, such as density and viscosity.
• Design Conditions: Whether the piping system is subject to external loads like wind, seismic activity, or soil pressure.

Here's a breakdown of the key steps involved in calculating pipe diameter and thickness:

1. Determine the Flow Rate (Q): This is the volume of gas to be transported per unit time, typically measured in cubic feet per hour (cfh).

2. Calculate the Design Pressure (P): This is the maximum pressure the piping system will be subjected to, including safety factors. It's usually determined by considering the maximum operating pressure, pressure surges due to transients, and potential overpressure scenarios.

3. Determine the Design Temperature (T): This is the maximum temperature the piping system will be exposed to, taking into account ambient temperatures and potential temperature variations during operation.

4. Select the Material and its Allowable Stress (S): Based on the intended application, select a suitable pipe material. ASME B31.8 provides allowable stress values for different materials at various temperatures. The allowable stress (S) is the maximum stress the material can safely withstand under given conditions.

5. Calculate the Required Pipe Diameter (D): This is where the flow rate, operating pressure, and fluid properties come into play. There are various methods for calculating pipe diameter, including the following:

• Velocity Method: This method aims to maintain a desired gas velocity within the pipe to ensure efficient flow and minimize pressure drops.
• Pressure Drop Method: This method calculates the pipe diameter based on the desired pressure drop along a specific pipe length.
• Friction Factor Method: This method uses the Darcy-Weisbach equation to calculate the pressure drop based on the pipe diameter, friction factor, and flow rate.
• Nomogram Methods: ASME B31.8 provides nomograms for quick estimations of pipe diameter based on flow rate, pressure, and material.

6. Calculate the Required Pipe Wall Thickness (t): This step involves considering the design pressure, allowable stress, and the selected pipe material. Here's the formula for calculating wall thickness according to ASME B31.8:

t = (P * D) / (2 * S * E) + C

Where:

• t: Minimum required pipe wall thickness (inches)
• P: Design pressure (psi)
• D: Pipe outside diameter (inches)
• S: Allowable stress of the material (psi)
• E: Efficiency factor for the pipe joint (typically 0.85 for welded joints)
• C: Corrosion allowance (inches) - This is added to account for potential material loss due to corrosion over time.

7. Verify the Calculated Thickness: The calculated wall thickness must meet the minimum wall thickness requirements specified in ASME B31.8 for the chosen pipe material and operating conditions.

8. Consider Additional Design Factors: Besides the above calculations, ASME B31.8 also addresses other critical aspects of pipe design, including:

• Stress Intensification Factors: These are used to account for stress concentrations at areas like pipe bends, welded joints, and nozzle connections.
• External Load Considerations: If the piping system is subjected to external loads, additional design calculations are required to ensure its structural integrity.
• Hydrostatic Test Pressure: The required hydrostatic test pressure for the piping system needs to be determined to verify its strength and leak tightness before commissioning.
• Expansion and Contraction: For long sections of piping, expansion and contraction due to temperature changes need to be accounted for by incorporating expansion loops or other mitigation strategies.

Note: It's highly recommended to consult a qualified engineer with expertise in ASME B31.8 to ensure accurate and safe design of your gas piping system. The provided formulas are simplified representations and may not encompass all the intricacies of the code.