Calculating and building piping classes to EN 13480

Designing a single line means calculating for exactly one size, one pressure and one temperature. In a plant with several hundred lines that calculation repeats constantly – with the same materials and the same components. A piping class brings the work forward: for a defined service range every nominal size is verified once, and the result is available as a table.

Piping class calculation is therefore not a method of its own but the application of EN 13480-3 to every component of a class: straight pipe, bend, tee, reducer, branch. Added to that is what the standard does not calculate but presupposes – standard flanges, gaskets and bolting with their own pressure-temperature limits.

EN 13480 “Metallic industrial piping” consists of several parts that build on one another. For a piping class Part 3 is the calculation part, but none of the others can be left out: the design stress in Part 3 presupposes a material to Part 2, and the joint coefficient of a weld depends on the extent of testing in Part 5.

What is binding is not the standard but the Pressure Equipment Directive 2014/68/EU. It applies to piping with a maximum allowable pressure above 0.5 bar and divides fluids into two groups: Group 1 comprises the dangerous fluids – such as flammable, explosive, toxic and oxidising ones – and Group 2 all others.

Fluid group, state, nominal size and pressure lead via the charts in Annex II to the category of the piping: I, II or III. Anything below the thresholds falls under Article 4(3) – sound engineering practice, without CE marking. The requirements for the conformity assessment procedure and for material certification rise with the category.

For a piping class this means: the header states fluid group and state, and for every nominal size it is clear which category a line of this class can reach at most. EN 13480 is harmonised under this directive. Applying it in full gives a presumption of conformity with the essential safety requirements.

The directive allows three routes for pressure-bearing parts: materials to harmonised standards, materials with a European approval, or a particular material appraisal. The first route is the simplest, and a piping class should take it wherever possible.

For pipe these are the EN 10216 (seamless) and EN 10217 (welded) series, for butt-welding fittings EN 10253, for flanges EN 1092-1. Each class also states the type of inspection document to EN 10204 – for the main pressure-bearing parts of the higher categories a certificate of specific product control is required, normally an inspection certificate 3.1.

With fittings to EN 10253 the type deserves a close look: type B carries the full pressure of straight pipe of the same wall thickness, type A only a reduced one. A class that does not specify this has not been verified at that point.

For straight pipe under internal pressure with an outside-to-inside diameter ratio up to 1.7:

e = pc · Do / (2 · f · z + pc)

with the calculation pressure pc, the outside diameter Do, the design stress f at calculation temperature and the joint coefficient z. For non-austenitic steels in the time-independent range f is the lower of the elevated-temperature yield strength divided by 1.5 and the tensile strength divided by 2.4. The joint coefficient is 1 for seamless pipe and fully tested welds, 0.85 with random non-destructive testing and 0.7 with visual inspection only.

The corrosion allowance, the negative tolerance of the pipe and, where relevant, a fabrication allowance are added to the minimum thickness. Only then is the next standard wall thickness selected. The article with the worked example shows what this looks like for a whole range of sizes.

In most piping classes the pipe wall is considerably thicker than internal pressure requires – standard thicknesses, corrosion allowance and robustness in handling see to that. The weakest link is then the flange.

EN 1092-1 assigns allowable pressures as a function of temperature to every PN designation and every material group. PN 16 therefore does not mean that the flange carries 16 bar at any temperature: as temperature rises the allowable pressure falls, by a different amount for each material. The pressure-temperature limit of a piping class is thus normally the curve of the flange – and it belongs on the class sheet as a table or chart.

The flange comes with gaskets to EN 1514 and bolting, the selection of which for pressure equipment is covered by EN 1515-4. Where tightness matters particularly, the flanged joint is additionally verified to EN 1591-1.

The class sheet is the document that design, purchasing and construction work with. It has to be readable without the calculation behind it – and the calculation has to be traceable for every figure.