Definition
- Nominal Pipe Size (NPS): The standard designation for pipe sizes in the imperial system, typically related to the approximate inside diameter of the pipe.
- Fractional Inches: The pipe size represented as a fraction of an inch.
- Decimal Inches: The pipe size in decimal inches.
- Metric (mm): The pipe size converted to millimeters.
- Diameter Nominal (DN): The international metric designation, which roughly corresponds to the pipe’s inner diameter in millimeters but is more of a nominal reference number.
Nominal Pipe to Fraction to Decimal to Metric to DN Conversion Chart
You can download pipe size chart in mm and inches pdf here：
Why do Nominal Pipe Size (NPS) and Fractional Inches differ?
Nominal pipe size (NPS) differs from fractional inches because NPS does not directly represent the actual diameter of the pipe, but rather is a legacy size marking system for standardizing pipe specifications. Here are some key reasons for the difference:
1. NPS is a nominal size
NPS is a nominal size, meaning it does not necessarily represent the pipe’s actual inside or outside diameter.
2. Historical evolution
Early pipe standards were developed based on the engineering needs and practices of the time, mainly defining pipe sizes based on flow capacity rather than precise physical dimensions.
3. The impact of pipe wall thickness
Changes in the pipe’s wall thickness (i.e., Schedule) will also vary the actual inside and outside diameters. NPS only represents the nominal inside diameter; the exact size will vary depending on the wall thickness.
4. Manufacturing standards
Different manufacturers and standards organizations may produce according to their own standards, leading to differences between nominal and actual sizes.
5. Metric to Imperial Conversion
Nominal sizes in the imperial system often cause dimensional inconsistencies when converted to the metric system.
Example
For instance, the actual outside diameter of an NPS 1/2″ is 0.840 inches (21.34 mm) instead of 0.5 inches (12.7 mm). This is a typical difference between nominal and actual sizes.
In summary, NPS is primarily a traditional size designation instead of reflecting an actual physical size, making it different from actual fractional or decimal inch values.
Why is it important to know how to convert nominal pipe size to fraction, decimal, metric, and nominal diameter (DN)?
Understanding how to convert between nominal pipe size (NPS) to fraction, decimal, metric, and nominal diameter (DN) is crucial for engineering, purchasing, and installing piping systems for the following reasons:
1. Standardization and Consistency
International standardization: In global engineering projects, pipes and related components may come from different countries and regions. NPS is a standard used mainly in North America and some other areas, while DN is an internationally used metric standard. Understanding the relationship between them helps ensure consistent pipe specifications in the project and avoid size mismatches.
2. Accurate Calculation and Design
Accurate measurement and matching: In piping system design, knowing how to convert between fractional inches, decimal inches, and metric can help engineers and designers accurately calculate the pipe’s flow capacity, pressure drop, and other performance parameters.
Adapting other components: Accessories (such as valves, flanges, joints, etc.) in different piping systems must accurately match the pipe size. Understanding these conversion relationships helps select the correct accessories and ensure the system’s safety and functionality.
3. Procurement and Inventory Management
Accuracy of procurement: Suppliers and customers may use different units and standards when purchasing pipes and related components. Mastering these conversion relationships can ensure that the purchased materials meet the design requirements and reduce rework or delays caused by size discrepancies.
Inventory Management: Companies need to know the actual size of the pipe when managing inventory to ensure that the correct materials can be quickly obtained when required.
4. Installation and Maintenance
Correct installation: During the installation process, construction personnel need to know the actual pipe size to ensure that the pipe and equipment are properly connected to prevent water, air, and other problems.
Maintenance and replacement: Understanding these size conversion relationships helps identify and accurately replace the required pipe components in the maintenance and replacement of the piping system.
5. Cross-standard work
Cross-standard projects: When engineering projects involve multiple standards (such as ANSI, DIN, JIS, etc.), mastering the conversion of NPS to DN is essential. This helps to coordinate the compatibility between different standards in the project.
Summary
Understanding the conversion between nominal pipe sizes and fractional, decimal, metric, and nominal diameters is the key to ensuring accuracy and consistency in the design, procurement, installation, and maintenance of piping systems. This not only helps avoid size mismatch issues but also improves engineering efficiency and system reliability.
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