Pressure Rating Standard of Industrial Flange Fittings
Publication Date:
Jul 30,2026
Author:
Dacheng Jiaye
Pressure class is a core technical index when selecting flange pipe fittings, representing the maximum allowable working pressure under specified reference temperature.
Pressure class is a core technical index when selecting flange pipe fittings, representing the maximum allowable working pressure under specified reference temperature. Global industries adopt two major pressure marking systems, Class series and PN series. Class standards represented by ASME B16.5 include Class150, Class300, Class600, Class900, Class1500 and Class2500, widely applied in North America, petroleum and chemical export projects. PN pressure series defined by European EN standards and Chinese national standards contain PN10, PN16, PN25, PN40, PN63, PN100 and other grades, commonly used in municipal water supply, heating and domestic chemical projects. Many buyers mistakenly believe that pressure grade is a fixed constant pressure value. In fact, the allowable working pressure of flanges declines as operating temperature rises. Material mechanical performance drops under high temperature environments, so pressure derating calculation must be conducted for high-temperature pipelines. A flange marked Class300 can only reach its nominal pressure at reference temperature. When medium temperature exceeds the limit value, the safe working pressure will decrease significantly. Flange pressure rating is determined by raw material, structural design, flange thickness and bolt layout. Flanges of identical dimensions with different materials cannot share the same pressure grade. Carbon steel flanges and stainless steel flanges with identical appearance size have different temperature-pressure bearing curves. During pipeline system design, the whole pipeline should match unified pressure grade standards. Flanges, pipes, valves and gaskets must satisfy or exceed the system maximum working pressure. If low-pressure flanges are assembled in high-pressure pipeline sections, permanent deformation, flange face cracking and serious leakage accidents may happen under pressure impact. Blind flanges need special attention on pressure grade selection. Pipeline pressure testing requires blind flanges bearing test pressure, which is higher than normal operating pressure. Purchasers should select blind flanges with sufficient safety margin. Interchangeability between Class and PN flanges cannot be guaranteed. Even if nominal pressure values seem similar, differences in flange outer diameter, bolt circle diameter, bolt quantity and sealing surface size make direct assembly impossible. Engineering mistakes caused by mixed usage of different standard flanges frequently occur in overseas engineering projects. Professional pipeline designers will confirm drawing standards and pressure grades before procurement. Manufacturers mark pressure class clearly on each flange product. End users are advised to reserve flange technical documents for equipment overhaul, component replacement and system safety assessment in the future.
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