Engineering task and calculation objective
The EN17 module performs the simplified fatigue assessment for pressure fluctuations to EN 13445-3, Clause 17. Pressure vessels that are cyclically loaded and unloaded – such as buffer vessels downstream of reciprocating compressors, adsorbers in swing operation or autoclaves – must be verified against material fatigue in addition to the static strength verification. The simplified method accomplishes this on the basis of the pressure range, without requiring a detailed stress analysis of the component.
From the pressure range between maximum and minimum pressure, the maximum allowable pressure of the component and the stress factor of the design detail under consideration, the module calculates the pseudo-elastic stress range. This is corrected with factors for the influence of wall thickness and temperature (from the assumed mean cycle temperature) and compared against the fatigue curve of the selected class (32 to 90 for welds). The results are the allowable number of load cycles for the corrected stress range Δσ*, the allowable stress range for a specified number of load cycles, and the comparison with the endurance limit.
A distinction is made between ferritic and austenitic materials and between welds in the as-welded condition and flush ground butt welds. If you want to calculate the number of load cycles of a pressure vessel to EN 13445, this delivers a conservative, code-compliant fatigue estimate with only a few inputs – along with the criterion for whether a detailed analysis per Clause 18 is required.
Calculation workflow
- Define material and component data: Design temperature, material with design strength and safety factor, material group (ferritic or austenitic) as well as the nominal wall thickness with allowances for wall thickness deviation and corrosion are recorded; the analysis wall thickness follows from these.
- Define the load cycle: The pressure range is formed as the algebraic difference of the maximum and minimum pressures of the cycle; the minimum and maximum temperatures during a cycle yield the assumed mean cycle temperature. Optionally, a required number of load cycles is specified.
- Classify the detail: The fatigue-critical design detail is classified via the stress factor and the fatigue class (32 to 90); flush ground butt welds receive a more favorable class than welds in the as-welded condition. The constants of the associated fatigue curve are adopted.
- Calculate and correct the stress range: From the pressure range, the maximum allowable pressure and the stress factor, the pseudo-elastic stress range follows. Correction factors for the influence of wall thickness and temperature lead to the corrected stress range Δσ*.
- Determine the allowable number of load cycles: Δσ* is inserted into the fatigue curve of the selected class: if the stress range lies above the endurance limit, a finite allowable number of load cycles results, which is compared with the specified number of cycles. Alternatively, the module reports the allowable stress range for the required number of load cycles.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Calculation temperature | T | °C |
| Material | Nr | - |
| Design strength | K | N/mm² |
| Safety factor | S | - |
| Manufacturing allowance | c1 | mm |
| Corrosion allowance | c2 | mm |
| Nominal wall thickness | en | mm |
| Constant in fatigue curves for | N≤5·106 C1 | MPa |
| Constant in fatigue curves for | N>5·106 C2 | MPa |
| Constant in equations of fatigue design curves | C3 | MPa |
| Temperature influence factor | CT | - |
| Class (32...90) Info: F3 | Kl (32...90) | - |
| Kerbfaktor | Kt | - |
| Allowable number of load cycles for Δσ* | N | - |
| Transition radius at junction of walls | r | mm |
| Minimum operating temperature during a cycle | Tmin | °C |
| Maximum operating temperature during a cycle | Tmax | °C |
| Pressure range calculated from the algebraic difference of max. and min. pressures | ∆P | MPa |
| Allowable stress range | ∆σ | N/mm² |
| Endurance limit for | N=5·106 ∆σD | N/mm² |
| Stress factor of component Info: F3 | η | - |
| Maximum allowable pressure Info: F3 | Pmax | MPa |
| Allowable number of load cycles | N | - |
| Allowable stress range | ∆σ | N/mm² |
Calculated results
| Quantity | Symbol | Unit |
|---|---|---|
| Correction factor to account for influence of wall thickness on fatigue resistance | Ce | - |
| Temperature influence factor | CT | - |
| Effective stress concentration factor | Kf | - |
| Analysis wall thickness | ea | mm |
| Allowable number of load cycles for Δσ* | N | - |
| Assumed mean cycle temperature | T* | °C |
| Pseudo-elastic stress range | ∆σ | N/mm² |
| Corrected stress range | ∆σ* | N/mm² |
| Allowable stress range | ∆σ | N/mm² |
| Allowable number of load cycles | N | - |
| Allowable stress range | ∆σ | N/mm² |
| Fictitious stress range | ∆σ* | N/mm² |
| For given number of load cycles | N = | - |
| Remark | _Bemerkung | - |
Frequently asked questions
When is a fatigue assessment per Clause 17 actually required?
EN 13445-3 provides a threshold criterion: a fatigue assessment only becomes mandatory once the number of equivalent full-pressure cycles exceeds a limit (on the order of 500 cycles). Vessels with infrequent start-up and shutdown operations are usually exempt. For swing operation, pulsating pressures or frequent batch changes, however, the verification is regularly required – first with the simplified method per Clause 17, and if necessary in detail per Clause 18.
What does the fatigue class 32 to 90 mean?
The class denotes the characteristic stress range in N/mm² that the weld detail can endure at a reference number of load cycles – the higher the class, the more fatigue-resistant the detail. Full penetration welds, low-notch weld toes and welds ground flush with the plate lead to high classes; fillet welds, single-sided welds and details with large misalignment to low ones. Classification follows the detail tables of the standard and is one of the most common sources of error: an overly optimistic class overestimates the service life several-fold, since the number of load cycles scales with the third power of the stress range.
Why is the stress range corrected with wall thickness and temperature factors?
Thicker components show a lower fatigue strength at the same nominal stress (statistical size effect and higher residual stresses), which is why a reduction factor is applied above a limiting thickness. Elevated temperature additionally lowers the modulus of elasticity and the fatigue strength; the temperature factor is derived from the assumed mean cycle temperature, separately for ferritic and austenitic steels. Both corrections act on the stress range to be applied and thus directly on the allowable number of cycles.
Is the simplified method conservative – and what if the verification fails?
Yes, the method is deliberately built to be enveloping: it assumes that the full pressure fluctuation acts at the most unfavorable detail and works with lump-sum stress factors. If the verification fails, a closer look is worthwhile first – realistic cycle counting, better weld execution (grinding, higher class), a lower stress factor through design improvements – before switching to the detailed fatigue analysis per Clause 18 with local structural stresses, which requires considerably more effort but can uncover substantial reserves.