Calculation example for the Wall Thickness conform EN 13445

Determining wall thickness under EN 13445 Part 3 requires more than inserting pressure into a formula.

The wall thickness depends on:

• Allowable material stress at design temperature
• Weld joint coefficient
• Corrosion allowance
• Applicability limits (e / D ≤ 0.16)
• Clear distinction between analysis and design thicknesses

Here is a structured walkthrough aligned with the procedure in Clause 7.4.2 of EN 13445-3. This is based on a worked example from our training course on the EN 13445.

As sketched in the Figure below there is a pressure vessel of 1000mm diameter, to be made of P235GH. Here, the nominal wall thickness (e_n) will be calculated.

The conditions for calculating the wall thickness.

1. Define the Design Basis Clearly

Before applying EN 13445 Section 7.4.2, let's confirm the governing inputs.

In this example:

-Outside diameter (D_e)​ = 1000 mm

-Internal design pressure (P) = 10barg (or 1 MPa)

-Design temperature = 200°C

-Corrosion allowance (c) = 3 mm

-Weld joint coefficient (z) = 0.85

-Plate material is P235GH

2. Determine the Allowable Design Stress (f)

As the required wall thickness (e) is unknown we need to use Formula 7.4-2.

e=PD_e/(2fz-P)

All of the values except for the design stress (f) are known at this point. Under EN 13445, the allowable design stress must be derived from material properties at the design temperature. Material properties are not defined in EN 13445, for plate materials such as P235GH, the EN 10028-2 material standard is consulted.

In EN 10028-2 it is seen that at 200^oC the 0.2% proof strength of P235GH is 182 MPa and that the tensile strength is 360MPa.

Applying the safety factors defined in EN 13445-3 6.2 of 1.5 on proof stress and 2.4 on tensile strength, results in an allowable based on proof stress at 200^oC of 121MPa and based on tensile strength of 150 MPa.

The lowest value governs, so the design stress (f) is 121 MPa

3.Calculate the Required Wall Thickness

Using the Equation 7.4-2 the required thickness (e) can be calculated.

• P=1 MPa
• D_e=1000 mm
• f=121 MPa
• z=0.85

e=1 x 1000/((2 x 121 x 0.85)-1)=4.9mm

4 Required Thickness (e) to Nominal Thickness (e_n)

Reviewing the Figure below, it can be seen that the required thickness (e) from Eq. 7.4.2 does not include the corrosion allowance (c) of 3mm.

The required thickness (e) shall be added to the corrosion allowance (c), which means that the plate thickness needs to be more than 7.9mm (4.9mm+3mm).

Assuming a Normal Tolerance Class A from EN 10029 the maximum permitted tolerance is (δ_e) is 0.5mm for a plate of 8 to 15mm thickness. This means that the nominal thickness (e_n) needs to exceed 7.9mm + 0.5mm = 8.4mm

In the metric system the next lowest standard plate thickness would typically be 10mm. The nominal thickness e_n is therefore 10mm and the extra analysis thickness (e_ex) is 1.6mm.

For further stages in the design process the analysis thickness (e_a) is 6.6mm (e+e_ex).

It is noted that it may be preferable to use a larger thickness for example for handling reasons or if there are a large number of nozzles that would otherwise require reinforcement plates.

5. Verify Applicability of the Thin-Walled Approach

The calculation approach here is valid for thin-walled cylinders where the definition as per EN 13445-3 Clause 7.4.1 is that the ratio of e/D_e is not larger than 0.16.

In the current case this ratio is 4.9/1000=0.005 which is far lower than the limit of 0.16. This check should always be shown explicitly in calculation notes.