Wall Thickness Definitions and Corrosion Allowance in EN 13445

When working under EN 13445 Part 3, wall thickness is not a single value. Nominal, analysis and required wall thicknesses each have a specific definition and serve a different purpose in design and verification.

During internal reviews, comments can arise if nominal thickness (e_n), analysis thickness (e_a) thickness, and required thickness (e) are treated interchangeably.

Understanding these thickness definitions, and how corrosion allowance and tolerances interact with them, reduces ambiguity and ensures a code compliant design. 

All of the thicknesses are defined here in the figure below.

The wall thickness definitions in EN 13445

1. Nominal Thickness (e_n) vs Analysis Thickness (e_a)

EN 13445 distinguishes between different thickness definitions.

Nominal thickness (e_n)

The ordered plate thickness or the thickness after forming.

Analysis thickness (e_a)

The thickness used in calculations conform EN 13445 to determine the MAWP (Maximum Allowable Working Pressure). This is found as:

Analysis Thickness (e_a) = Nominal thickness (e_n) - corrosion allowance (c) - negative manufacturing tolerance (δ_e)

Here the corrosion allowance and manufacturing tolerance are given in absolute values in mm. 

This analysis thickness (e_a) is used to calculate the maximum allowable pressure in equations such as in EN 13445-3 Equation 7.4-3 for cylindrical shells

To make the calculation path for e_a clear, the following should be explicitly documented in the reporting:

• Corrosion allowance used
• Plate tolerance used
• Resulting analysis thickness

2. Corrosion allowance (c)

EN 13445 treats all material wastage during operation as corrosion. This includes:

• Oxidation and scaling
• Erosion
• General surface wastage

However, the code does not prescribe corrosion allowance values. The value must be defined by the user based on:

• Service environment
• Fluid characteristics
• Inspection philosophy
• Project life

Typically the corrosion allowance for the designer will be defined in the specification for the project.

3. e_min is the Minimum Fabrication Thickness

The minimum fabrication thickness is the nominal thickness (e_n) minus the negative tolerance (δ_e). In other words it is the same as e_a, but then before corrosion allowance (c) has been deducted.

4. Required Thickness (e) versus Analysis Thickness (e_a)

Which wall thickness is relevant depends on the starting point of the analysis. Is this the design pressure or the nominal wall thickness.

The different uses of required thickness (e) and analysis thickness (e_a)

Nominal Wall Thickness as starting point

Here the analysis thickness (e_a) is a known quantity determined by the chosen plate with corrosion allowance and under tolerance. Combining this with the material properties and the diameter of the vessel allows the MAWP to be determined.

Design Pressure is the starting point

In contrast the required wall thickness (e) is found using a sizing calculation which determines for a given material, pressure and diameter what the required wall thickness (e) is.

In initial sizing of a pressure vessel component the required wall thickness (e) can then be combined with the corrosion allowance and under tolerance to determine the required plate size.