Pressure Rating and Bolt Circle in PN Series Flanges

A PN rating is more than a mark stamped on a flange; it is the design input that sets the part's geometry. At the same nominal size, as the PN step rises the flange gets thicker, the bolt count goes up, the bolt diameter grows and the bolt circle moves outward, taking the outside diameter with it. That difference is not arbitrary — it is the price of the load the gasket face has to hold.

Why geometry changes with pressure

Bolts in a flanged joint do two jobs at once: they resist the end force pushing the flanges apart, and they keep the gasket squeezed hard enough not to leak. When pressure rises the first force grows directly, while the residual load left on the gasket must not fall. The only way to satisfy both is to increase total bolt area — more bolts, thicker bolts, usually both.

More bolts need more circumference, so the bolt circle grows, and a larger bolt circle demands more material outside the holes. Thicker bolts are tightened harder, and that torque tries to rotate the flange hub, so thickness increases to resist it. The chain runs: pressure → bolting → bolt circle → thickness → outside diameter.

Same size, different rating: they will not bolt up

The practical consequence is simple. If two flanges of the same nominal size carry different PN steps, their bolt circles usually differ and the holes do not line up. On site this surfaces on installation day: both flanges belong to the same pipe and share the same size, yet only a few holes align. The only correct response is to change the flange. Reaming the holes, forcing a fit or running with bolts missing all take the joint outside its design.

The trickier case: pattern matches, rating does not

For some size and rating combinations, neighbouring PN steps share the same drilling. Hole count, hole diameter and bolt circle match exactly, the flanges bolt up easily, the gasket seats, and the fitter has no reason to suspect anything. That is precisely the problem: fitting is not the same as being suitable. What the joint can hold is set by its weakest link, and the lower-rated flange's wall and bolt area were never calculated for the higher design pressure. Two flanges mating is therefore not a verification method; verification comes from the marking on the flange and from the delivery documents.

Face type picks the gasket

PN series flanges come with different sealing faces, and that choice drives the gasket decision. On a flat face flange the gasket is a full-face type extending past the bolt holes; on a raised face flange, sealing happens in a narrow band on the circular projection at the centre of the face.

The difference is not cosmetic. On a raised face the same bolt load acts over a smaller area, so unit stress on the gasket is higher and sealing is easier — in return, that narrow band asks more of the gasket material. A flat face suits brittle bodies, a cast iron pump casing for instance, because it spreads load widely. Mating the two forms is a mistake: the raised face loads the flat flange at its inner edge.

Three lines on the order

Describing a flange completely takes three things together: which standard, which PN step, which face type. Size covers none of them, and putting them on the order is cheaper than measuring from a scaffold on installation day.