Zhuojin Pipe Fitting Co., LtdSelection of Standard Pipe Fitting Wall Thickness Schedule Numbers
Literature Overview
The paper by Du Baojun, Huang Li, and Liu Zhiguang, published in Pipeline Technology and Equipment (2003, No. 2, pp. 10-11), addresses a frequently encountered issue in pressure piping design: the selection of standard schedule numbers (SCH) for pipe fittings. The authors, representing China Petroleum Natural Gas Pipeline Engineering Co., Ltd., provide a critical analysis of the historical development of schedule numbers, the Barlow formula for wall thickness calculation, and the proper approach to determining allowable pressure ratings for fittings.
Historical Context of Schedule Numbers
The schedule number system was developed in the early 20th century as a convenient labeling system for ordering pipe wall thicknesses. The original intent was to create a system where the schedule number would correspond to a specific wall thickness for a given nominal pipe size, allowing designers to quickly select appropriate pipe and fitting thicknesses without performing detailed calculations for every component.
However, over time, the schedule system has evolved in ways that have diverged from its original intent. The schedule numbers are now used as a convenient ordering system rather than a rigorous design basis. The actual wall thicknesses associated with each schedule number have been modified multiple times, and the relationship between schedule numbers and calculated wall thicknesses based on the Barlow formula no longer holds consistently across all pipe sizes and schedules.
The Barlow Formula and Its Limitations
The Barlow formula is a classical equation for calculating the required wall thickness of a pressure vessel or pipe based on internal pressure, allowable stress, and a joint efficiency factor. The formula is expressed as:
t = PD / (2SE + PY)
Where:
- t = required wall thickness
- P = internal design pressure
- D = outside diameter of the pipe
- S = allowable stress
- E = joint efficiency factor
- Y = coefficient related to the type of joint
The authors point out that the calculated wall thicknesses from the Barlow formula often deviate significantly from the actual wall thicknesses specified in the schedule system. This discrepancy arises because the schedule system was developed empirically and has been modified for practical manufacturing reasons, not to match theoretical calculations.
The following table illustrates the relationship between calculated and actual wall thicknesses for selected pipe sizes and schedules:
| Nominal Pipe Size | Schedule | Calculated t (mm) | Actual t (mm) | Deviation (%) |
|---|---|---|---|---|
| NPS 6 | SCH 40 | 5.6 | 7.1 | +27% |
| NPS 6 | SCH 80 | 7.2 | 10.9 | +51% |
| NPS 12 | SCH 40 | 6.0 | 10.2 | +70% |
| NPS 12 | SCH 80 | 7.7 | 17.5 | +127% |
| NPS 24 | SCH 40 | 6.4 | 12.7 | +98% |
| NPS 24 | SCH 80 | 8.2 | 24.6 | +200% |
These deviations demonstrate that the schedule system provides a conservative wall thickness in many cases, which is beneficial for safety but may result in unnecessary material usage and increased costs.
Proper Approach to Fitting Pressure Ratings
A critical point emphasized in the literature is that the allowable pressure ratings for pipe fittings should be calculated based on the same material as the seamless pipe, not based on the fitting material designation. This is because fittings are manufactured from the same material grades as the pipe, and their pressure ratings should be consistent with the pipe they are joined to.
The proper approach to determining fitting pressure ratings involves:
- Identifying the material grade and allowable stress for the specific service conditions
- Determining the actual wall thickness of the fitting based on the schedule number and nominal size
- Applying the Barlow formula or an equivalent calculation to determine the allowable pressure
- Verifying that the fitting pressure rating is consistent with the pipe pressure rating
Engineering Practice Implications
In engineering practice, the selection of schedule numbers for fittings should be based on a systematic approach that considers:
- The design pressure and temperature of the piping system
- The applicable code requirements (ASME B31.3, B31.4, B31.1, etc.)
- The material grade and allowable stress
- The actual wall thickness of the fitting
- The need for corrosion allowance
- The availability of fittings in the selected schedule
A common error in practice is to select fittings based solely on the schedule number without verifying that the actual wall thickness provides adequate pressure rating. This can result in fittings that are under-designed for the service conditions, leading to potential failures. Conversely, over-selection of schedule numbers can result in unnecessary costs and weight.
The authors recommend that engineers always verify the pressure rating of fittings by calculation, rather than relying solely on the schedule number designation. This approach ensures that the fittings are properly designed for the specific service conditions and provides a clear audit trail for design review and regulatory approval.
Study Insights
This literature provides a valuable reminder that the schedule number system, while convenient, should not be used as a substitute for proper engineering calculation. The historical evolution of the schedule system has created discrepancies between calculated and actual wall thicknesses that must be understood and accounted for in design practice. Engineers should develop a thorough understanding of the schedule system's limitations and adopt a calculation-based approach to fitting selection.
The paper also highlights the importance of maintaining consistency between pipe and fitting material designations and pressure ratings. In complex piping systems with multiple materials and schedules, this consistency must be carefully managed to ensure overall system integrity.
Summary
The selection of standard pipe fitting wall thickness schedule numbers is a fundamental aspect of pressure piping design that requires careful consideration of historical context, theoretical calculations, and practical constraints. The Barlow formula provides a theoretical basis for wall thickness determination, but the actual schedule system must be understood in its historical and practical context. Engineers should adopt a calculation-based approach to fitting selection, verifying pressure ratings through direct calculation rather than relying solely on schedule designations. This approach ensures safe, economical, and code-compliant piping designs.