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 (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)
Sequence | Imperial (in) | Metric (mm) |
---|---|---|
1 | 1/8 | 4 |
2 | 1/4 | 8 |
3 | 3/8 | 10 |
4 | 1/2 | 15 |
5 | 3/4 | 20 |
6 | 1 | 25 |
7 | 1-1/4 | 32 |
8 | 1-1/2 | 40 |
9 | 2 | 50 |
10 | 2-1/2 | 65 |
11 | 3 | 80 |
12 | 3-1/2 | 90 |
13 | 4 | 100 |
14 | 5 | 125 |
15 | 6 | 150 |
16 | 8 | 200 |
17 | 10 | 250 |
18 | 12 | 300 |
19 | 14 | 350 |
20 | 16 | 400 |
21 | 18 | 450 |
22 | 20 | 500 |
23 | 22 | 550 |
24 | 24 | 600 |
25 | 26 | 650 |
26 | 28 | 700 |
27 | 30 | 750 |
28 | 32 | 800 |
29 | 34 | 850 |
30 | 36 | 900 |
31 | 38 | 950 |
32 | 40 | 1000 |
... | ... | ... |
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) | MPa | Bar | Kgf/cm² |
---|---|---|---|---|
150 | PN 20 | 2.0 | 20 | 20.4 |
300 | PN 50 | 5.0 | 50 | 51.0 |
400 | PN 68 | 6.8 | 68 | 69.4 |
600 | PN 100 | 10.0 | 100 | 102 |
900 | PN 150 | 15.0 | 150 | 153 |
1500 | PN 250 | 25.0 | 250 | 255 |
2500 | PN 420 | 42.0 | 420 | 428.4 |
3500 | PN 590 | 59.0 | 590 | 601.8 |
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
- 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.
- 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.
- 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.
- 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.
- 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.