The terms “DN (Diameter Nominal)” and “PN (Pressure Nominal)” are frequently found in the piping engineering literature. These are very important terms related to piping system design. In this article, we will discuss the meaning and significance of DN and PN.
What is DN in Pipes?
“DN” is an abbreviated form of Diameter Nominal (DN). It is a dimensionless number that denotes the pipe size in the metric unit system. The DN pipe size designator is developed by the International Standards Organization or ISO and is widely found in Europe and other metric-based countries.
The DN value does not provide actual physical pipe dimensions but is used as a reference number that indicates the size of a pipe or fitting. The pipe sizes are designated by DN followed by a number to indicate standard pipe size. For example, DN50 indicates a 2″ NPS pipe; Similarly, DN80 indicates a 3″ NPS pipe. The pipe manufacturers print the DN number on the pipe surface which makes the identification easier. DN is measured in millimeters which is an approximate internal diameter of the pipe.
DN vs NPS
NPS or Nominal pipe size is also a dimensionless pipe size designator and is widely used in North America. A specific pipe size is indicated by NPS followed by a number. For example, NPS 2 indicates a pipe having an outside diameter of 2.375 inches. The pipe outside diameter is more than its NPS designator for NPS 12 and smaller pipes. However, NPS 14 and larger pipes have the same pipe OD in inches as the NPS designator. This means an NPS 24 pipe has an OD of 24 inches. NPS is associated with the ASME B36.10 and ASME B36.19 standards.
So, the main difference between DN and NPS is that DN indicates the approximate pipe dimensions in mm whereas the NPS indicates the pipe dimension in inches. For pipes larger than NPS 3.5, the DN number is calculated by multiplying the NPS size designation number by 25. For example, NPS 4 is the same as DN 100.
Table 1 below provides typical co-relations between NPS and DN pipe size designations.
NPS
1/8
1/4
1/2
3/4
1
1-1/4
1.5
2
2.5
3
3.5
4
X (>4)
DN
6
8
15
20
25
32
40
50
65
80
90
100
25X
Table 1: DN vs. NPS Pipe Size Designation
What is the PN Rating Number?
Pipes are usually classified based on the pressure-temperature rating system used for flange classification. The PN number system is a similar pressure rating designator. “PN” stands for the French term Pression Nominal which means pressure nominal. The PN number is indicated by the term PN followed by a designation number. For example, PN 10, PN 50, etc. The value that follows the term PN indicates the approximate pressure rating in bars. Note that the PN rating does not provide a proportional relationship between different PN numbers.
PN basically represents the maximum virtual pressure that a pipe or tube can withstand under standard operating conditions. The piping components are designed based on the reference PN number value. The PN rating number is most widely used for Plastic pipes, HDPE, PVC, and PE pipes. PN is generally used in the European standards BS, DIN, EN, ISO, and so on. Different PN rating indicates different pipe thickness for the same pipe DN.
PN is an alphanumeric designation used for reference purposes, comprising the letters PN followed by a dimensionless number relating to component pressure/temperature capability, taking into account component material mechanical properties and component dimensional characteristics.
What is PN in HDPE Pipe?
The PN abbreviation for pressure nominal in Polyethylene pipes indicates the maximum pressure that the pipe can withstand without failure at a specified temperature range. For example, PN 16 pipe means the pipe can withstand a pressure of 16 bar without failure. Similarly, PN 20 pipe means the pipe is designed to withstand a pressure of 20 bar at the maximum design temperature.
For PE/HDPE pipes the PN rating pressure usually denotes the pressure capability at a temperature of 20°C. PN values are generally mentioned in the Pipe Diameter, Thickness, and Weight chart for HDPE/PE pipes. With an increase in PN number the pipe thickness increases.
Pipe Class Rating vs. PN Numbers
Piping Class Ratings are based on the ASME B16.5 or ASME B16.47 standard whereas The PN rating is based on ISO 7005 standard. Table 2 below provides a cross-reference for the PN rating and ASME Class rating.
Pipe Class
150
300
400
600
900
1500
2500
Pipe Pressure Nominal (PN)
20
50
68
110
150
260
420
Table 2: Piping Class rating vs. PN rating
Differences Between DN, PN, and NPS
Here’s a tabular comparison of DN, PN, and NPS with respect to piping design:
Nominal diameter of pipes, valves, and fittings, typically used in metric systems.
The nominal pressure rating of a pipe system is used to indicate the pressure capability of a component.
Nominal size of a pipe in inches, typically used in imperial systems.
Units
Millimeters (mm)
Pressure in bars (e.g., PN 16 means 16 bars)
Inches (in) for NPS; OD (outside diameter) varies with schedule.
System of Measure
Metric
Metric
Imperial
Usage
Commonly used in European countries and countries using the metric system.
Used globally in conjunction with DN, particularly in Europe.
Commonly used in the United States, Canada, and other countries using the imperial system.
Pipe Diameter Relation
DN gives a rough estimate of the internal diameter of a pipe. It doesn’t directly correlate to the actual OD or ID but is close to ID for larger pipes.
Not directly related to pipe diameter but is an indicator of pressure handling capability.
NPS is roughly equal to the pipe’s OD for NPS 14 and above; for smaller sizes, NPS is not equal to OD.
Pressure Rating
Does not indicate pressure rating.
Directly indicates pressure handling capability, e.g., PN 10 means the pipe can handle 10 bars of pressure.
Does not indicate pressure rating. Pressure capacity is usually defined by the schedule.
Typical Range
DN 6 to DN 2000
PN 2.5 to PN 100
NPS 1/8 to NPS 80 (or larger)
Schedule Relation
DN is independent of the pipe schedule.
PN is independent of the pipe schedule.
NPS must be paired with a schedule to determine pipe wall thickness and pressure rating.
I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.
5 thoughts on “What is DN and PN Number for Pipes? Pipe Class Rating vs PN Numbers”
Consult, I am creating a WEB in Chile of Mechanical Engineering and Piping, I would like to include your Web “WHAT IS PIPING” as reference information, can you tell me if there is any requirement to do so?
Hello sir I am a BTech mechanical engineer completed my graduation in recently ,sir I want to advice from you how to become a piping engineer , after BTech what can I do to become piping engineeing
Can you send me a chart if you have prepared for PVC -O.
For different DN and PN . Along with Wall thickness.
If so kindly share it.
I will be very grateful to you.
Thanks And Regards
Ujjwal Shrivastava.
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ASME B31.3, the "process piping" code, provides guidelines for the pressure leak testing of piping systems. The primary goal of these tests is to ensure the integrity and safety of the piping system...
Consult, I am creating a WEB in Chile of Mechanical Engineering and Piping, I would like to include your Web “WHAT IS PIPING” as reference information, can you tell me if there is any requirement to do so?
You can include this website on your reference page.
Hello sir I am a BTech mechanical engineer completed my graduation in recently ,sir I want to advice from you how to become a piping engineer , after BTech what can I do to become piping engineeing
Hi ,Do you have laatest DN,ISO and EN standard?
Hello Anup Ji
Can you send me a chart if you have prepared for PVC -O.
For different DN and PN . Along with Wall thickness.
If so kindly share it.
I will be very grateful to you.
Thanks And Regards
Ujjwal Shrivastava.