Engineering task and calculation objective
The Nzul module determines the maximum allowable number of load cycles for the complete vessel to AD 2000-Merkblatt S1 of the German AD 2000 pressure vessel code. While the fatigue verification to S1 is initially carried out component by component, what the operator and the inspection authority ultimately need is a single statement: how many pressure and temperature cycles may the complete pressure vessel experience over its lifetime? This project-wide evaluation is exactly what the module delivers.
For this purpose, the pressure cycle range and the cycle temperatures of the individual load cycles are recorded for the entire project. From the results of the component-level S1 calculations, an allowable number of load cycles is obtained for each component; the vessel is governed by its weakest link, i.e. the smallest allowable number of load cycles of all components. The module reports this number of maximum allowable load cycles to AD 2000 S1 as the overall result.
In practice, this evaluation is needed for equipment operated under fluctuating loads – such as air receivers, autoclaves, sterilizers or batch-operated reactors – whenever the allowable number of cycles of the vessel has to be documented as part of the design or of a periodic inspection.
Standard and calculation basis: AD 2000 S1
Calculation workflow
- Define the load cycles of the project: The operational cycles of the vessel are described: the pressure cycle range (difference between the upper and lower cycle pressure) and the associated cycle temperatures, from which the governing temperature for the material properties follows.
- Draw on the component-level S1 verifications: For every fatigue-relevant component of the project, an allowable number of load cycles is available from the S1 calculation, taking into account pressure, temperature and design factors (welded/non-welded, wall thickness).
- Take the minimum over all components: The module evaluates the individual results and determines the smallest allowable number of load cycles – this counts as the allowable number of cycles for the complete vessel to AD 2000 S1.
- Compare with the number of operating cycles: The expected number of pressure and temperature cycles over the planned service life is compared with the allowable number of load cycles. If it is exceeded, a detailed verification to AD 2000 S2 or a design modification is required.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| 1 | 1 | – |
| 2 | 2 | – |
| 3 | 3 | – |
| 4 | 4 | – |
| 5 | 5 | – |
| 6 | 6 | – |
| 7 | 7 | – |
| 8 | 8 | – |
| 9 | 9 | – |
| 10 | 10 | – |
| 11 | 11 | – |
| 12 | 12 | – |
| 13 | 13 | – |
| 14 | 14 | – |
| 15 | 15 | – |
| 16 | 16 | – |
| 17 | 17 | – |
| 18 | 18 | – |
| 19 | 19 | – |
| 20 | 20 | – |
| 21 | 21 | – |
| 22 | 22 | – |
| 23 | 23 | – |
| 24 | 24 | – |
Frequently asked questions
When does a fatigue verification to AD 2000 S1 have to be carried out at all?
AD 2000 S1 provides the simplified verification when the vessel is predominantly subject to fluctuating loading and the number of pressure cycles may exceed the limits stated in the Merkblätter. For purely static loading with few cycles (e.g. rare start-up and shutdown operations), no separate verification is needed. Fatigue is particularly critical for high-yield-strength steels, because the static design permits higher stresses there.
Why does the weakest component determine the number of cycles for the complete vessel?
Every cycle acts on all pressure-bearing components simultaneously. Crack initiation starts at the detail with the smallest allowable number of load cycles – typically at low-class welds, nozzle transitions or thick-walled regions with temperature gradients. The minimum of all component-level values is therefore the only meaningful characteristic for the vessel.
What can be done if the allowable number of load cycles is not sufficient for the planned operation?
First, the detailed verification to AD 2000 S2 can be carried out, which works with real stress ranges and mean stress influence and usually yields higher allowable cycle counts than the simplified method. Alternatively, design measures help: weld dressing (a higher weld class), thicker walls to reduce the stress range, or limiting the pressure cycle range in operation.