Selection of Standard Wall Thickness Series Numbers for Pipe Fittings

Literature Overview

The paper by Du Baojun, Huang Li, and Liu Zhiguang, published in Pipeline Technology and Equipment (Issue 2, 2003), addresses a frequently encountered problem in pressure pipeline design: the selection of standard wall thickness series numbers (Schedule Numbers or SCH) for pipe fittings. The authors trace the historical development of the Schedule system, analyze its technical basis, and provide guidance on its proper application in engineering design.

Historical Context and Technical Basis

The Schedule Number system originated from the American National Standards Institute (ANSI) and was formalized in ASME B36.10 for wrought and seamless steel pipe. The system was designed to provide a convenient reference for ordering pipes of different wall thicknesses, with the Schedule Number serving as an index rather than a direct measure of wall thickness. The fundamental formula underlying the Schedule system is the Barlow formula for hoop stress in a thin-walled cylinder:

S = P × D / (2 × t)

Where S is the allowable stress, P is the internal pressure, D is the outside diameter, and t is the wall thickness. Rearranging for wall thickness:

t = P × D / (2 × S)

The Schedule Number was originally intended to represent a ratio of the pipe's outside diameter to its wall thickness, scaled by a factor of 1000. However, as the paper notes, this relationship has been modified over time due to changes in manufacturing practices, material standards, and code requirements. The result is that the Schedule Number no longer maintains a simple mathematical relationship with the actual wall thickness, and the deviation between the Barlow formula calculation and the actual Schedule wall thickness can be significant.

Schedule Number Classification

The Schedule system is organized into several categories, each applicable to different pipe sizes and pressure ranges:

The paper emphasizes that Schedule Numbers are merely a convenient indexing system for ordering purposes and should not be confused with pressure rating designations. The actual pressure rating of a pipe or fitting must be calculated using the appropriate formula from the governing code (such as ASME B31.3 for process piping or GB/T 20801 for Chinese pressure piping).

Selection Criteria for Engineering Design

The selection of an appropriate Schedule Number for a given application requires consideration of several factors:

1. Design pressure: The internal pressure at which the pipe or fitting must operate safely, including surge and transient pressures.
2. Design temperature: The maximum operating temperature, which affects the allowable stress of the material.
3. Material allowable stress: Determined from ASME Section II Part D or equivalent national standards, with temperature-dependent adjustments.
4. Corrosion allowance: An additional wall thickness to account for material loss over the design life of the system.
5. Minimum wall thickness requirements: Code-mandated minimums for specific applications (e.g., cryogenic service, high-velocity flow).
6. External loads: Wind, seismic, and other external forces that may require increased wall thickness.

The authors provide a practical approach to Schedule selection:

• Calculate the minimum required wall thickness using the Barlow formula or the more comprehensive formula from the applicable code.
• Add the corrosion allowance to obtain the total required wall thickness.
• Select the smallest Schedule Number that provides a wall thickness equal to or greater than the total required thickness.
• Verify that the selected Schedule meets any minimum wall thickness requirements for the specific application.

Application to Pipe Fittings

A critical point emphasized in the paper is that the pressure rating of pipe fittings should be calculated based on the same material and formula as the straight pipe, not simply assumed to match the Schedule designation. Fittings such as elbows, tees, and reducers have different wall thicknesses and geometric configurations than straight pipe, and their pressure ratings must be independently verified. The paper notes that the Schedule designation on a fitting indicates the wall thickness series from which the fitting was manufactured, but does not guarantee that the fitting's pressure rating matches that of a straight pipe of the same Schedule.

For example, a 90-degree long-radius elbow manufactured from SCH 40 pipe will have a wall thickness at the centerline equal to the SCH 40 thickness, but the wall thickness at the inside bend radius will be greater due to the forming process. The pressure rating of the fitting must therefore be calculated using the minimum wall thickness (at the centerline) and the appropriate formula for the fitting geometry.

Engineering Practice and Common Pitfalls

In practice, several common errors arise in Schedule selection:

• Confusing Schedule with pressure rating: Assuming that a SCH 40 fitting can handle the same pressure as a SCH 40 pipe without independent verification.
• Ignoring corrosion allowance: Selecting a Schedule based on the bare minimum calculated thickness without adding the required corrosion allowance.
• Using wrong material allowable stress: Failing to adjust the allowable stress for the design temperature, leading to undersized wall thickness.
• Overlooking minimum wall thickness requirements: Selecting a thin-wall Schedule that does not meet code-mandated minimums for specific applications.
• Mismatching pipe and fitting Schedules: Using a fitting of a different Schedule than the connected pipe, which can create a weak point in the system.

Study Insights and Implications

This paper, though published over two decades ago, remains highly relevant to practicing engineers. The fundamental principles of Schedule selection and pressure rating verification are unchanged, and the common pitfalls identified by the authors continue to appear in engineering practice. The paper's emphasis on the distinction between Schedule as an indexing system and Schedule as a pressure rating is particularly valuable, as this confusion remains prevalent. For engineers involved in pipeline design, the key takeaway is that Schedule selection must be a calculated process based on the specific design parameters of the application, not a default choice based on convention or habit.