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Nominal pressure: Nominal pressure is what is commonly referred to as PN pressure. It is a numerical value that represents pressure; it is a convenient round number used for reference, and it is usually indicated on pipes. All pipe fittings with the same nominal diameter and the same nominal pressure PN value have the same connection dimensions appropriate to their end connection type. The nominal pressure of a pipe indicates its pressure resistance at the standard temperature; therefore, it is conditional on specific operating conditions. For example, PN20 means that the pipe can withstand a working pressure of 20 bar at the material’s standard temperature (20 bar = 2 Mpa). If the operating temperature changes, the operating pressure should be reduced according to different temperature drop coefficients. Many people think that 2 Mpa is the maximum pressure that a pipeline can withstand, but this is incorrect. This requires an explanation of the difference between work pressure and design pressure. Working pressure refers to the maximum pressure specified based on the highest operating temperatures of the media transported through the pipeline system, in order to ensure the safe operation of such systems over a certain period of time. Design pressure shall be not less than the pressure under the most severe combination of pressure and temperature conditions that may occur during operation; such extreme pressure may only appear momentarily. Relationship among the three: Nominal pressure > Design pressure > Operating pressure