Engineering task and calculation objective
What pressure can the apparatus really withstand? This module answers that question for the entire project: it determines the maximum allowable pressure (Pmax) for every single component of a vessel or piping system and identifies the weakest link. The basis are the design rules of DIN EN 13445-3 for pressure vessel components, DIN EN 13480-3 for piping components, and DIN EN 12516-2 for valve bodies.
Unlike the design calculation, which delivers the required wall thickness for a given pressure, the calculation here is inverse: from the actually present wall thicknesses, materials and temperatures of each component follows the pressure each can sustain. The minimum over all components — shell courses, heads, nozzles, flanges, pipe sections, valves — determines the maximum allowable pressure of the overall apparatus.
This evaluation is needed when establishing the maximum allowable working pressure PS, when coordinating the set pressure of safety valves, for change-of-service assessments and re-rating of existing apparatus, and for the question of how much reserve a project has over its design pressure.
Standard and calculation basis: DIN EN 13445-3: 2021-12 & DIN EN 13480-3: 2017-12 & DIN EN 12516-2: 2015-01
Calculation workflow
- Capture the components of the project: The module takes over all calculated components of the project — cylindrical shells, cones, dished and flat ends, openings and nozzles, flange connections, piping parts and valves — with their actual wall thicknesses, materials and design temperatures.
- Back-calculate the maximum allowable pressure per component: For each component, the design equations of the applicable code (EN 13445-3, EN 13480-3, EN 12516-2) are solved for the pressure: from the available wall thickness minus allowances, the allowable stress at design temperature and the joint efficiency, the sustainable pressure Pmax of each part is obtained.
- Distinguish load cases: The evaluation is carried out per load case: for the operating condition, the nominal design stresses at design temperature apply; for the test condition, the increased allowable stresses at test temperature. This yields consistent maximum pressures for operation and pressure testing.
- Identify the weakest component: The minimum of the component values determines the maximum allowable pressure of the overall project. The module shows in a table which component limits — frequently a flange connection, an opening or the thinnest shell course — and how large the margin of the remaining components is.
- Use the result: The determined value serves as the basis for establishing PS, for setting the safety devices and for assessing reserves. If the allowable pressure is to be increased, the evaluation immediately shows which component must be reinforced or re-assessed first.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Load case | Lastfall | – |
| Maximum permissible pressure | Pmax | MPa(p) |
| Applicable EN-Modules in this project | NEN | – |
| Are modules selected? | ausgewählt? | – |
| Is Pmax for at least one selected module available? | verfügbar? | – |
| Can additional modules get included? | werden? | – |
| Maximum number of calculable modules | Nmax | – |
| Refresh is required. | erforderlich | – |
| Refresh already executed. | durchgeführt | – |
| Perform iteration | aus | – |
| Beschreibung | Beschreibung | – |
| Beschreibung | Beschreibung | – |
| Beschreibung | Beschreibung | – |
| Beschreibung | Beschreibung | – |
| Beschreibung | Beschreibung | – |
| Beschreibung | Beschreibung | – |
| Beschreibung | Beschreibung | – |
| Beschreibung | Beschreibung | – |
| Beschreibung | Beschreibung | – |
| Beschreibung | Beschreibung | – |
| Beschreibung | Beschreibung | – |
| Beschreibung | Beschreibung | – |
| Beschreibung | Beschreibung | – |
| Beschreibung | Beschreibung | – |
Calculation options
Load case
Operation · Testing
Frequently asked questions
How does Pmax differ from the design pressure of the project?
The design pressure is a specification of the design process; the components are dimensioned so that they can at least sustain it. Since wall thicknesses are rounded up to available products, the actually sustainable pressure Pmax is practically always above the design pressure. The difference is the hidden reserve of the apparatus — knowing it is valuable for process changes, re-ratings and the assessment of upset scenarios.
What role does Pmax play in overpressure protection with safety valves?
The set pressure of the safety device must not exceed the maximum allowable pressure PS of the vessel, and PS in turn must not lie above the calculated Pmax of the weakest component. The project-wide Pmax evaluation thus delivers the reliable upper limit for establishing the set pressure and the allowable pressure rise during discharge.
Why is the maximum allowable pressure determined component by component and not with a single blanket value?
Every component has its own limit load capacity, depending on geometry, material and temperature: a torispherical head limits differently than a cylindrical shell, a reinforced opening differently than a flange connection with gasket restrictions. Only the component-wise back-calculation to the applicable code reveals which element is truly governing — a blanket value would either give away reserves or miss the weak point.
Are valves in the project also considered?
Yes. For valve bodies, the module applies DIN EN 12516-2, which governs the strength calculation of steel valve shells. Thus valves and gate valves with their allowable pressure-temperature ratings also enter the project-wide minimum — important because valves are frequently the limiting component in piping systems.