Pipe wall thickness calculation

A pipe wall thickness calculation answers one question: how thick must the wall be for the pipe to carry the internal pressure at design temperature? The result is a minimum thickness without any allowance.

It says nothing about loads from weight, restrained thermal expansion or nozzle loads – that is the job of a pipe stress analysis. Nor does it cover external pressure or vacuum, where the pipe may buckle, or bends, branches and reducers, for which the codes provide separate rules.

For straight pipe under internal pressure with an outside-to-inside diameter ratio up to 1.7, EN 13480-3 gives:

e = pc · Do / (2 · f · z + pc)

Here e is the minimum wall thickness, pc the calculation pressure, Do the outside diameter, f the design stress of the material at calculation temperature and z the joint coefficient of the weld. Pressure and stress use the same unit, normally N/mm² (MPa); 1 bar equals 0.1 MPa.

The design stress f follows from the strength values in the material standard. For non-austenitic steels in the time-independent range it is the lower of the elevated-temperature yield strength divided by 1.5 and the tensile strength divided by 2.4. In the creep range, time-dependent values apply. The joint coefficient z is 1 for seamless pipe and for welds whose quality is fully verified by testing, 0.85 with random non-destructive testing and 0.7 with visual inspection only.

Nobody orders the calculated minimum. Three allowances are added: c0 for corrosion and erosion over the service life, c1 for the permitted negative tolerance of the pipe and c2 for thinning during fabrication, for example by bending or threading.

If the negative tolerance is given as a percentage x of the ordered thickness, it is not added but divided: the ordered thickness must be at least (e + c0 + c2) · 100 / (100 − x). The next larger standard wall thickness is then selected.

An example: pipe with 168.3 mm outside diameter, calculation pressure 40 bar, assumed design stress 130 N/mm², seamless. The minimum thickness is 4 · 168.3 / (2 · 130 + 4) = 2.55 mm. With 1 mm corrosion allowance and a 12.5 per cent negative tolerance, (2.55 + 1.0) / 0.875 = 4.06 mm must be ordered – the standard thickness chosen is 4.5 mm.

For straight pipe under internal pressure ASME B31.3 uses t = P · D / (2 · (S · E · W + P · Y)). S is the allowable stress from the code tables, E the quality factor, W the weld joint strength reduction factor at elevated temperature and Y a coefficient taken as 0.4 for ductile steels up to 482 °C as long as the thickness is less than one sixth of the outside diameter.

The equations look related; the results need not be. Each code derives its allowable stress differently. A design is therefore carried out entirely to one code – with its material values, its factors and its allowances.

The calculation is short; the errors sit in the input values.