Difference between the standard DN flange and the PN flange?

How are Flanges Classified?

Flanges are crucial components for connecting pipes and valves in industrial applications. However, not all flanges are the same, and they come in various shapes and sizes to suit different requirements. Flange classification is based on several factors: shape, size, pressure grade, and material type, such as carbon steel and alloy steel types, commonly used for industrial applications. Flanges are also classified mostly based on their mechanical properties, such as resistance to corrosion and other environmental factors. Roughly, flanges can be categorized into slip-on, threaded, socket weld, blind and flat flanges (ff), each suitable for specific piping needs. A flange joint is a kind of pipe attachment where the connecting bits have flanges which lock the components with the help of bolts. The plates of flanges are produced by various manufacturing methods, which will affect the quality of the flanges.

Many common stainless steel flange standards are to be found around the globe. These have standardized dimensions to allow easy interchangeability and functionality. ASA / ASME (USA), PN / DIN (EU), BS10 (British / Australian), and JIS / KS (Japanese / Korean) are General world standards. In 1996, the American National Standards Institute stopped publishing B16.5 in the USA, and the equivalent is ASME B16.5.

There are two pipe types of nomenclature: decimal and metric. In the imperial system, they use pounds per square inch(psi) to refer to pressure or nominal pipe size(NPS) to refer to the length and sizes of the pipes. The pressure usually be measured in kilopascal (kPa), while pipe diameter is noted in the metric system's DN, depending on the international unit scale.
What does DN stand for?

DN is the "Diameter Nominal" for flanges, a standard to denote a flange size with a corresponding bolt circle diameter. The DN is not a measurement of the flange's size but an industry standard to ensure uniformity and compatibility between manufacturers. The DN system is commonly utilised for flanges manufactured according to BS EN 1092, a European standard that specifies the requirements for circular steel flanges, which can be used for low and higher temperature and pressure applications. Apart from the DN, it is essential to consider the material, pressure, and application when selecting the appropriate flange for a specific purpose.
What does PN stand for?

What is PN? The term PN means "Pressure Nominale". The PN is a prefix to the pressure rating of the Flanges. For example, the flange PN16 can operate up to 16 levels at 20-degree Celsius. Normal scores: PN6, PN10, PN16, PN25, PN40, PN64, PN100. International flange standards such as ISO 7005-1 or DIN EN 2501, PN10, PN16, PN25, and PN40 series specify pressure classes and metric flange dimensions using the metric measuring system. PN flanges developed to ANSI standards, including AWWA, ASA or old British specifications measured in inches, will not necessarily fit with the specification of flanges.

This is probably a good idea to suggest the actual working pressure when defining a PN flange specification as it's customary to allow a similar PN, say PN16, flange drilling on a fitting or valve to suit neighbouring flanges, but the pressure level is lower than PN16.

DN refers to a pipe's internal diameter. All measures of a piping board, together with the approximate pressure level and type, e.g. The flange measurements, are specified according to the DN.

It should be noted that the true internal diameter, in many instances, deviates by several millimetres. Only when the DN is specified concerning the equivalent DIN norm can it be accurately concluded that pipes from different manufacturers can be mixed.

The DN complies with EN ISO 6708 and must followed by a dimensionless number equivalent to the real internal diameter in millimetres. For reference, a DN 50 pipe indicates an external diameter of 60.3 mm and a wall thickness of 3.65 mm according to EN 10255 (resulting in an indoor diameter of 53 mm).