The schedule rating of a pipe is a standardized system that indicates the wall thickness of the pipe, which correlates to its ability to withstand internal pressure. A higher schedule number signifies a thicker pipe wall and, consequently, a greater ability to handle higher pressures.
Here's a more detailed breakdown:
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Definition: A pipe schedule is a dimensionless number that designates the approximate relationship between allowable stress for the material at its design temperature and the required pressure. It essentially represents the pressure-bearing capability of the pipe.
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ANSI/ASME Standard: Pipe schedules are defined by standards like ANSI/ASME B36.10M (Welded and Seamless Wrought Steel Pipe) and B36.19M (Stainless Steel Pipe). These standards provide specific dimensions (outside diameter and wall thickness) for various pipe sizes and schedules.
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Common Schedules: Common pipe schedules include:
- SCH 5
- SCH 10
- SCH 20
- SCH 30
- SCH 40
- SCH 60
- SCH 80
- SCH 100
- SCH 120
- SCH 140
- SCH 160
- STD (Standard)
- XS (Extra Strong)
- XXS (Double Extra Strong)
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Relationship to Wall Thickness: As the schedule number increases, the wall thickness of the pipe increases for a given nominal pipe size (NPS). For example, a Schedule 80 pipe will have a thicker wall than a Schedule 40 pipe of the same NPS. This increased thickness allows the pipe to withstand higher internal pressures.
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Calculation Approximation: While not directly equal, the schedule number is related to the ratio of internal design pressure (P) to allowable stress (S) for the pipe material. A simplified approximation is:
Schedule ≈ 1000 x (P/S)
Where:
- P = Internal Design Pressure (psi)
- S = Allowable Stress of the material (psi)
Important Note: This is a simplified approximation. Actual schedule selection must consider various factors, including temperature, safety factors, and specific code requirements. Consulting the relevant standards is crucial for proper pipe selection.
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Applications: Pipe schedules are used in a wide variety of applications, including:
- Oil and gas pipelines
- Chemical processing plants
- Power generation facilities
- Water distribution systems
- HVAC systems
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Example: Consider a 2-inch nominal pipe size. A Schedule 40 pipe will have a different wall thickness than a Schedule 80 pipe of the same size. The Schedule 80 pipe will have a thicker wall, making it suitable for higher pressure applications.
In summary, the schedule rating provides a standardized way to define the wall thickness of a pipe relative to its pressure-bearing capacity, crucial for selecting the right pipe for a given application.
