Determining the appropriate pipe schedule primarily involves calculating it based on the service pressure and the allowable stress of the pipe material. Here's a breakdown of the process:
Understanding Pipe Schedule
Pipe schedule refers to the wall thickness of a pipe. A higher schedule number indicates a thicker wall. Selecting the correct schedule is crucial for ensuring the pipe can safely withstand the internal pressure of the fluid or gas it's carrying.
The Primary Formula
The most common formula used to approximate pipe schedule is:
SCH = 1000 (P / S)
Where:
- SCH = Pipe Schedule (a dimensionless number, but related to wall thickness)
- P = Service Pressure (internal pressure the pipe will experience, in psi)
- S = Allowable Stress (maximum stress the pipe material can handle safely, in psi)
Example:
Let's say you have a pipe where:
- P = 500 psi (Service Pressure)
- S = 10,000 psi (Allowable Stress)
Then:
SCH = 1000 (500 / 10,000) = 1000 0.05 = 50
This suggests a schedule near 50 would be appropriate. However, standard schedules are typically 40, 80, 160, etc. You would then consult pipe schedule charts to determine the actual wall thickness corresponding to a given schedule for a specific pipe size. You'd likely select Schedule 40 in this example, verifying it provides adequate pressure rating for 500 psi for your specific pipe diameter.
Steps to Determine Pipe Schedule:
- Determine Service Pressure (P): Calculate or determine the maximum internal pressure the pipe will experience during operation. This should include any surge or water hammer effects.
- Determine Allowable Stress (S): This value depends on the pipe material and the operating temperature. Consult material specifications and engineering codes (e.g., ASME B31.3 for process piping) to find the allowable stress for your chosen material at the operating temperature. This allowable stress includes safety factors.
- Calculate the Preliminary Schedule: Use the formula SCH = 1000 (P / S) to calculate an initial estimate of the required schedule.
- Consult Pipe Schedule Charts: Pipe schedule numbers don't directly translate into wall thickness in inches or millimeters. You must refer to pipe schedule charts or tables (easily found online or in engineering handbooks) that list the wall thickness for each schedule number for a given nominal pipe size (NPS). These charts will show the relationship between NPS, Schedule, and actual wall thickness.
- Select the Appropriate Schedule: Choose the schedule that provides a wall thickness sufficient to withstand the service pressure with a safety margin. Often, the calculated schedule will fall between standard schedules, and you'll need to select the next higher schedule to ensure adequate strength. You must verify the chosen schedule's pressure rating exceeds the service pressure, including any safety factors.
- Consider Other Factors:
- Corrosion: If the fluid or gas being transported is corrosive, you may need to select a thicker wall (higher schedule) to account for material loss over time.
- Erosion: High-velocity fluids or fluids containing abrasive particles can erode the pipe wall.
- External Loads: Consider any external loads on the pipe, such as weight, wind, or seismic forces. These may require a higher schedule for structural integrity.
- Manufacturing Tolerances: Pipe wall thickness has manufacturing tolerances.
Important Considerations:
- The formula SCH = 1000(P/S) is an approximation. It's a good starting point but should not be the only factor in determining pipe schedule. Always consult relevant codes and standards.
- Nominal Pipe Size (NPS): The pipe schedule needed depends on the size of the pipe (NPS). A Schedule 40 pipe will have a different wall thickness depending on whether it's NPS 1", NPS 2", or NPS 4".
- Engineering Codes and Standards: Always adhere to applicable engineering codes and standards (e.g., ASME, ASTM, API) when selecting pipe schedule. These codes provide detailed guidelines and safety factors for pressure piping design.
- Safety Factors: Allowable stress (S) already incorporates safety factors.
- Material Properties: The allowable stress (S) varies greatly between different materials. Steel, stainless steel, PVC, and copper will each have different allowable stresses and different temperature limitations.
By following these steps and considering all relevant factors, you can determine the appropriate pipe schedule for your application, ensuring the safe and reliable operation of your piping system. Always consult with a qualified engineer for critical applications.
