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Wall Thickness Definitions and Corrosion Allowance in EN 13445
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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 (en), analysis thickness (ea) 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.
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The wall thickness definitions in EN 13445
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1. Nominal Thickness (en) vs Analysis Thickness (ea)
EN 13445 distinguishes between different thickness definitions.
Nominal thickness (en)
The ordered plate thickness or the thickness after forming.
Analysis thickness (ea)
The thickness used in calculations conform EN 13445 to determine the MAWP (Maximum Allowable Working Pressure). This is found as:
Analysis Thickness (ea) = Nominal thickness (en) - corrosion allowance (c) - negative manufacturing tolerance (δe)
Here the corrosion allowance and manufacturing tolerance are given in absolute values in mm.
This analysis thickness (ea) 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 ea clear, the following should be explicitly documented in the reporting:
- Corrosion allowance used
- Plate tolerance used
- Resulting analysis thickness
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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. emin is the Minimum Fabrication Thickness
The minimum fabrication thickness is the nominal thickness (en) minus the negative tolerance (δe). In other words it is the same as ea, but then before corrosion allowance (c) has been deducted.
4. Required Thickness (e) versus Analysis Thickness (ea)
Which wall thickness is relevant depends on the starting point of the analysis. Is this the design pressure or the nominal wall thickness.
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The different uses of required thickness (e) and analysis thickness (ea)
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Nominal Wall Thickness as starting point
Here the analysis thickness (ea) 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.
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Extra Thickness (eex)
The possible plate thickness will always be a discrete value (en). This leads to the value eex which is the extra thickness to make up the difference between the nominal plate thickness and that required from the calculations as per the Figure above.
For a correctly designed vessel ea will always be larger than e, and the MAWP will be larger than the required design pressure.
5. Making Thickness Calculations Review-Ready
To reduce ambiguity in any review of your EN 13445 design calculations:
- Clearly separate nominal, analysis, and design thickness
- Show corrosion allowance explicitly in equations
- State tolerance assumptions
EN 13445 is structured and consistent in these thickness definitions, but it assumes the engineer defines certain inputs transparently.
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