Calculating piping classes to ASME B31.3

B31.3 “Process Piping” is one section of the ASME Code for Pressure Piping. It deals with piping in refineries, chemical and pharmaceutical plants, in the paper and semiconductor industries, in cryogenic plants and related processing facilities. Power piping has its own code, B31.1.

In Europe B31.3 is met mainly where an international owner or licensor specifies it – or where a plant is designed for export. In this world the piping class is the central document: the piping material specification fixes all components for each class, and isometrics and bills of material refer to it.

Before any calculation, the fluid service has to be determined – and under B31.3 that is the owner’s responsibility. Material selection, permitted components and extent of examination depend on this classification.

B31.3 does not give the allowable stress S as a formula that the user fills with material properties but as a table: for every listed material the values are given against temperature in Appendix A. Below the creep range they essentially go back to one third of the tensile strength and two thirds of the yield strength.

For straight pipe under internal pressure, paragraph 304.1.2 gives:

t = P · D / (2 · (S · E · W + P · Y))

P is the design pressure, D the outside diameter, E the quality factor of the pipe seam – 1.0 for seamless and 0.85 for electric resistance welded pipe – 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 diameter.

The required minimum thickness is tm = t + c. The term c contains the corrosion and erosion allowance and mechanical allowances such as thread depth. The mill tolerance of the pipe then has to be considered: many seamless pipes to ASTM specifications may be up to 12.5 per cent below nominal wall thickness, so the nominal thickness must be at least tm / 0.875.

Pipe dimensions are given in ASME B36.10M for steel pipe and B36.19M for stainless steel pipe. The size is called NPS and stated in inches, the wall thickness by a schedule number or a weight class – STD, XS, XXS. Schedules with the suffix S, such as 10S or 40S, belong to the stainless series.

The schedule number is a designation, not a dimension. NPS 2 in Schedule 40 has a wall of 3.91 mm, NPS 6 in Schedule 40 has 7.11 mm. A piping class therefore states the schedule for every size – and it may change within a class, because the calculation gives a different result for small sizes than for large ones. A heavier schedule at small sizes is typical, where threads, socket welds and mechanical robustness matter.

ASME B16.5 covers flanges up to NPS 24 in pressure classes from Class 150 to Class 2500; above that B16.47 applies. As with PN, the class is a designation: a Class 150 flange in carbon steel of material group 1.1 carries 19.6 bar at ambient temperature, one in Class 300 carries 51.1 bar – and less in each case as temperature rises.

The pressure-temperature ratings in B16.5 are arranged by material group. For a piping class the curve of the chosen group is compared with the service range; as a rule it sets the limit of the class, not the pipe wall.

The system includes further dimensional standards referenced by B31.3: B16.9 for butt-welding fittings, B16.11 for forged socket-welding and threaded fittings, B16.20 and B16.21 for gaskets, B16.34 for valves. Components to these listed standards are considered qualified within their pressure-temperature ratings.

A plant put into service in the European Union is subject to the Pressure Equipment Directive 2014/68/EU – regardless of the code used for the calculation. ASME B31.3 is not a harmonised standard. Anyone designing to it has to show separately that the essential safety requirements of the directive are met.

In practice this concerns three points above all: materials, which without a harmonised standard need a particular material appraisal; inspection documents; and the pressure test, for which the directive sets the test pressure independently. A piping class for such a project therefore states both references – and is checked against both.

The most common source of error is mixing the systems: an EN material with ASME stresses, a PN flange on schedule pipe, a joint coefficient from one world in the equation of the other. Both codes are self-contained. Allowable stress, factors, allowances and extent of examination are matched to one another and only valid together.

Where a plant touches both worlds – an ASME unit within a European site – the boundary is defined: at which flange, with which transition piece, and which class applies on which side.