Engineering task and calculation objective
The Pmax module determines the maximum allowable pressures for the complete vessel of a project. While the individual strength-calculation modules – for the cylindrical shell, dished end, flange connection or opening reinforcement, for example – each deliver the allowable pressure for their own component, Pmax consolidates these individual results and reports the smallest allowable pressure, which governs the entire vessel.
In practice, this evaluation is needed whenever the maximum allowable pressure of a pressure vessel has to be determined: when setting the set pressure of safety valves, when checking whether an existing vessel can continue to be used under changed operating conditions, or when demonstrating margins above the design pressure. The weakest component determines the allowable pressure of the complete vessel – and it is exactly this component that the module makes visible.
The evaluation draws on the component calculations already performed in the project according to the applicable code (e.g. the German AD 2000 code or EN 13445); Pmax itself does not perform an independent strength calculation but consolidates the results at the vessel level.
Calculation workflow
- Perform the component calculations in the project: First, all pressure-bearing components of the vessel are calculated with the corresponding strength modules; in addition to the verification for the design pressure, each module also delivers the maximum allowable pressure for its component.
- Collect the allowable pressures of the components: Pmax reads in the maximum allowable pressures of all items contained in the project – shells, heads, openings, flanges and further components – each referred to its associated design temperature.
- Take the minimum and identify the governing component: From the individual values, the minimum is determined as the maximum allowable pressure of the complete vessel; the associated component is the pressure-limiting item.
- Document the result: The overview of all allowable pressures with the governing minimum is output for documentation and can serve as the basis for safety-valve setting, re-rating checks or margin assessments.
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
How does the maximum allowable pressure differ from the design pressure?
The design pressure is the specification for which the vessel is designed. The maximum allowable pressure is the pressure the components as actually built – with their selected wall thicknesses and materials – may sustain at design temperature; because of wall thickness allowances, standard plate thicknesses and rounding in design, it usually lies above the design pressure. The concept corresponds to the MAWP (maximum allowable working pressure) of the ASME world.
Why does a single component determine the allowable pressure of the entire vessel?
The pressure acts in all pressure-bearing components simultaneously. Allowable is therefore only the pressure at which even the weakest component still satisfies its verification – typically an opening with marginal reinforcement, a flange connection or a thin-walled dished end. The overview shows which item is limiting and where a reinforcement would have the greatest effect.
Does the determined value apply at all temperatures?
No. The allowable pressures of the components depend on the design temperature through the material strength values. The reported value applies to the load-case temperatures stored in the project; where there are several load cases (e.g. operation and test), each case must be considered separately.