Introduction to Valve Sizes and Pressure Ratings

6 min read

Valves are critical components in fluid control systems, and their performance depends significantly on two key parameters: nominal pipe size (NPS) and pressure rating. This guide explains these technical parameters in detail, focusing on how they are standardized in both the European and American systems.

Nominal Pipe Size (NPS)

Nominal Pipe Size

  • Nominal Pipe Size (NPS) refers to the approximate diameter of the valve's internal flow path.
  • It does not necessarily match the exact diameter but serves as a standard size reference.
  • The size selection is based on the required Cv value, which indicates the valve's flow capacity. Manufacturers provide guidelines on determining the appropriate NPS using Cv values.

NPS Conversion: Metric (DN) and Imperial (Inches)

SequenceImperial (in)Metric (mm)
11/84
21/48
33/810
41/215
53/420
6125
71-1/432
81-1/240
9250
102-1/265
11380
123-1/290
134100
145125
156150
168200
1710250
1812300
1914350
2016400
2118450
2220500
2322550
2424600
2526650
2628700
2730750
2832800
2934850
3036900
3138950
32401000
.........

This table illustrates the conversion between the Imperial and Metric systems, commonly used for valve sizing.

Pressure Ratings / Pressure Classes

  • Pressure rating defines the maximum allowable working pressure for a valve at a specified temperature.
  • The European system uses PN values to indicate the nominal pressure, such as PN 2.5 or PN 25.
  • In the American system, the ANSI standard uses pressure classes (e.g., ANSI 150#).

ANSI Pressure Rating Conversion Table

ANSI Pressure Class (bf/in²)Nominal Pressure (PN)MPaBarKgf/cm²
150PN 202.02020.4
300PN 505.05051.0
400PN 686.86869.4
600PN 10010.0100102
900PN 15015.0150153
1500PN 25025.0250255
2500PN 42042.0420428.4
3500PN 59059.0590601.8

This table shows the relationship between ANSI pressure classes and their corresponding nominal pressures in various units.

Conclusion

Understanding Nominal Pipe Size (NPS) and pressure ratings is crucial for selecting the right valve for specific fluid control applications. The use of standardized units (NPS and PN) allows for compatibility and proper specification across different systems, ensuring optimal performance and safety in industrial processes.

FAQs

  1. What is the difference between NPS and DN in valve sizing?
    • NPS (Nominal Pipe Size) is used in the American system, while DN (Diameter Nominal) is used in the European system. They are roughly equivalent but use different measurement units.
  2. How do I determine the correct valve size for my application?
    • Valve size is determined by calculating the required Cv value, which relates to the valve's flow capacity. This value can then be used to select the appropriate NPS from manufacturer guidelines.
  3. Why are pressure ratings important for valves?
    • Pressure ratings ensure that a valve can safely handle the fluid pressure within a system at a specific temperature, preventing leaks and potential system failures.
  4. What units are used for pressure ratings in different regions?
    • In the European system, pressure ratings are represented in PN (e.g., PN 25), while the American system uses ANSI standards with values like 150#, 300#, etc.
  5. Can I use a valve with a different pressure rating than specified?
    • It is not recommended to use a valve with a lower pressure rating than required, as this can compromise safety and system integrity. Always choose a valve that matches or exceeds the specified pressure requirements.

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