Creating a piping class – step by step with an example

A class applies to a range, not to a line. To be defined are the fluid or fluid group, the design pressure, the design temperature – including the lowest – the fluid group under the Pressure Equipment Directive and the range of nominal sizes.

The owner’s requirements belong in the same place: preferred materials, approved component types, excluded gasket materials, an existing designation system. Learning of them after the calculation means calculating twice. If a value is missing, one is proposed and visibly marked as an assumption.

For the example: hot water, design pressure 25 bar, design temperature 200 °C, DN 25 to DN 200, code EN 13480.

The material follows from fluid and temperature, the corrosion allowance from fluid, material and intended service life. Both are design decisions, not code values. The allowance applies uniformly to all components of the class.

The strength values at calculation temperature are taken from the material standard. For non-austenitic steels the design stress to EN 13480-3 is the lower of the elevated-temperature yield strength divided by 1.5 and the tensile strength divided by 2.4.

For the example we assume: seamless pipe in carbon steel for elevated temperature, yield strength at 200 °C of 185 N/mm², tensile strength 360 N/mm². These are assumptions for this example, not standard values – a real design uses the values in the material standard for the chosen grade and thickness. This gives f = min(185 / 1.5; 360 / 2.4) = min(123.3; 150) = 123.3 N/mm². Corrosion allowance c0 = 1.0 mm.

The minimum thickness of straight pipe is e = pc · Do / (2 · f · z + pc). With pc = 2.5 N/mm², f = 123.3 N/mm² and z = 1 for seamless pipe the denominator is 2 · 123.3 + 2.5 = 249.2 N/mm².

For DN 150 with an outside diameter of 168.3 mm: e = 2.5 · 168.3 / 249.2 = 1.69 mm. The corrosion allowance is added. We assume a negative tolerance of 12.5 per cent of the ordered thickness; it is therefore not added but taken into account by division: (1.69 + 1.0) · 100 / (100 − 12.5) = 3.07 mm. There is no fabrication allowance c2 for straight pipe.

The table shows the same calculation for five sizes. In every row the selected thickness is well above the required one – at 25 bar it is not internal pressure that sets the wall but the commonly available dimension. That is exactly what the calculation is meant to confirm: it shows how much margin the class has and up to which pressure the pipe is not the weakest link.

For the flanges, the PN designation is sought whose allowable pressure at design temperature reaches the design pressure. The pressure-temperature table of EN 1092-1 for the material group of the flange is used for this. At 200 °C the allowable pressure is below the numerical value of the PN designation – whether PN 25 is enough or PN 40 is needed is decided by the table, not by the designation.

With the rating, flange type and facing are settled, together with matching gasket and bolting. For gaskets, chemical resistance and temperature limit count; for bolting, the material at temperature. The pressure-temperature curve found in this way is the limit of the whole class.

Bends, tees, reducers and caps are specified by dimensional standard and type. For butt-welding fittings to EN 10253 this is the type with full pressure capability if the fittings are to have the same pressure limit as the pipe.

The branch table states for every combination of run and branch size which design is permitted: tee, reducing tee, welded-in branch with or without reinforcement. Where a welded-in branch is intended, the reinforcement of the opening is verified to EN 13480-3 – once, for the class.

Valves are described by type, pressure rating, body material and end connection, not by make. For testing, the class states the extent of non-destructive testing of the welds – the joint coefficient depends on it – and the pressure test.

For the hydrostatic test the Pressure Equipment Directive requires the greater of 1.43 times the maximum allowable pressure and 1.25 times the loading in service, converted by the ratio of strength values at test and design temperature.

Two documents result. The class sheet is the table for everyday use: header with service range, pressure-temperature limits, components per size, branch table. The calculation record is what lies behind it – every wall thickness, every opening, every assumption.

Both carry the same revision status and name the codes with their editions. Even years later it is then possible to trace why the table says what it says – and what has to be checked if the service range changes.