Why is there a difference between the two pressure terms, PN and MPa?

AUTO Instruments Co., Ltd. has recently summarized and analyzed pressure transmitter orders to better understand customer needs and the details in order execution. Through this process, we identified some small details that could be easily overlooked or misunderstood. While these details might seem minor, they could affect customer experience and order accuracy. To ensure that all relevant personnel can handle orders more precisely and efficiently, we decided to share and explain these issues in detail.

The instruments involved include: Differential pressure transmitters, Absolute pressure transmitters, Flange level transmitters, Radar level gauges, Ultrasonic flow meters, etc.

1. Different Definitions:

· PN: PN stands for "Nominal Pressure," which represents the nominal pressure value. It is a standardized reference value, not the actual measured pressure, and is used for classification and identification purposes.

Note: PN is defined based on the standard temperature (usually 20°C) and standard materials (such as steel). In design and use, PN reflects not only the pressure resistance but also considers material strength, temperature effects, and other factors.

· MPa: MPa stands for "Megapascal," which is a unit of pressure (1 MPa = 10⁶ Pa). It describes the actual pressure applied to a unit area and represents the real force exerted.

2. Different Uses:

· PN: PN is mainly used for pressure classification and matching of pipes and fittings (such as flanges, valves), commonly used in Europe and Asia (in the US, Class is used instead).

The unit for PN is bar, where 1 bar = 0.1 MPa. For example, PN10 represents a nominal pressure of 10 bar, which is equivalent to 1.0 MPa.

· MPa: MPa is commonly used to measure the actual working pressure, such as the fluid pressure inside pipes or the output pressure of pumps and pressure equipment.

MPa is an internationally recognized engineering unit that directly represents a physical quantity and is related to the actual operating conditions of the equipment.

3. The Relationship Between PN and MPa

In practical applications, PN and MPa are interconnected:

· Selection of Pipes and Fittings: When the actual fluid pressure (MPa) is known, an appropriate PN level is selected to ensure safety. For example, if the working pressure of a pipe is 0.8 MPa, PN10 (1 MPa) pipe fittings should be selected.

· Temperature Effects on PN: PN is usually the nominal value at 20°C. As the temperature increases, material strength decreases, and the actual pressure resistance may be lower than the PN value. For example, a PN40 flange at 200°C may have an actual pressure resistance of around 2.5 MPa (refer to standard tables).

· Safety Factor: When selecting pipes and equipment, a safety margin is typically added to the PN value. For example, if the actual working pressure is 1.2 MPa, PN16 fittings may be chosen.

Summary

In simple terms, PN is a man-made concept used for categorizing and classifying pipes and fittings, with the unit in bar. It is mainly used in Asian and European standard systems. On the other hand, MPa is a unit that describes the actual pressure magnitude and is directly related to the physical operating conditions of the equipment.