One of the most common mistakes in flange selection is assuming the PN (Pressure Nominal) rating printed on a flange is the same as its maximum safe operating pressure. It isn’t — and understanding the difference matters for both safety and procurement accuracy.

What PN Rating Actually Means

PN rating refers to the drilling standard — the bolt circle diameter, bolt hole pattern, and flange face dimensions defined by EN 1092 / DIN 2501. It tells you how the flange will physically connect to a mating flange or pipe fitting. It does not, on its own, define the maximum pressure the flange can safely handle.

What Determines Actual Operating Pressure

Maximum operating pressure depends on the flange’s material construction, wall thickness, and design — not just its drilling pattern. That’s why two flanges with the same PN rating can have different maximum operating pressures depending on how they’re built.

A Practical Example

A flange sized DA 225 might be drilled to DIN 2501 / EN 1092 PN10 — but still carry a maximum operating pressure rating of 16 bar, well above what the drilling standard alone would suggest. This is common with reinforced encapsulated flange designs like Stu Piping’s PPDA and DF2DI ranges, where the internal ductile iron insert and glass-reinforced polypropylene shell add strength beyond the base PN classification.

Why This Matters for Your Project

Relying on PN rating alone when specifying flanges can lead to either:

• Over-speccing — paying for a higher-rated flange than the application requires, or
• Under-speccing — assuming a lower safety margin than the flange actually provides

Always check the manufacturer’s published maximum operating pressure alongside the PN/drilling standard before finalizing an order.

A Simple Safety Recommendation

Regardless of PN rating, proper installation matters. Stu Piping recommends using washers below the screw head and nut on all flange installations to protect the flange surface and maintain consistent bolt tension.