Engineering guide
The S30 module performs the general safety verification for pressure vessels to AD 2000-Merkblatt S3/0 of the German AD 2000 pressure vessel code. This Merkblatt sets out the principles by which additional loadings – weight loads, nozzle loads, moments from attachments, wind and connection forces – are assessed together with the internal pressure when the component Merkblätter of the B series alone are not sufficient. The verification is based on given equivalent stresses, which may come from a hand calculation or from a detailed analysis.
The core of the method is the comparison of the equivalent stresses with the allowable values, which may be set at different levels depending on the stress category and load case: membrane stresses are limited more strictly than local bending or peak stresses. The module contains a material database for determining the nominal design strength and the safety factor at design temperature, and provides selection criteria for the application of the limit load method or the shakedown method, by which locally limited exceedances of the elastic limits can be assessed as acceptable.
In practice, S3/0 acts as the umbrella over the specific stability Merkblätter S3/1 to S3/4: whoever has to verify additional loads on vessels that do not match any of the standard situations (skirt support, saddles, legs, brackets) carries out the strength verification according to the principles of this Merkblatt.
Scope
- General safety verification for pressure vessels regulations
Typical workflow
Compile load cases and actions
For operation, testing and erection, all simultaneously acting loads are recorded: internal or external pressure, dead weight and filling, forces and moments from nozzles, supports, anchorages and attachments, including torsion.
Determine the equivalent stresses
For the governing cross-sections, the stresses from the superposed loads are calculated and combined into equivalent stresses – according to the distortion energy (von Mises) or maximum shear stress (Tresca) hypothesis, separated into membrane, bending and total components.
Establish the allowable stresses
The nominal design strength K and the safety factor S at design temperature are determined from the material database. Depending on the stress category and load case (operation, test, erection), different allowable multiples of K/S apply.
Check the strength condition
The existing equivalent stresses are compared with the allowable values. Membrane stresses must not exceed the basic strength; elevated limits apply to locally confined bending and peak stresses.
Apply the limit load or shakedown method
If the elastic verification is not sufficient, the selection criteria of the Merkblatt indicate whether the limit load method (verification against plastic collapse with a global safety factor) or the shakedown method (verification that the component settles into purely elastic behaviour after a few cycles) may be applied.
Frequently asked questions
When is S3/0 needed in addition to the B-series Merkblätter?
The B-series Merkblätter size components for pressure as the principal load. As soon as significant additional loads act – large nozzle forces from piping, moments from agitators, wind and seismic loads, support reactions outside the standard cases S3/1 to S3/4 – their superposition with the pressure must be verified separately. S3/0 provides the principles, stress limits and safety factors for this.
What distinguishes the limit load method from the shakedown method?
The limit load method demonstrates that the load lies with a sufficient safety margin below the plastic limit capacity of the component – suitable for predominantly static loading. The shakedown method demonstrates that, under repeated loading, the component behaves purely elastically after initial local yielding, i.e. no progressive plastic deformation (ratcheting) occurs – decisive for cyclic loading. The Merkblatt states selection criteria for which method is admissible.
Why are membrane stresses limited more strictly than bending stresses?
A membrane stress acting through the entire wall thickness causes the whole cross-section to plastify immediately when the yield limit is reached – there is no redistribution reserve. Bending stresses initially plastify only the outer fibres; the cross-section continues to carry load until a plastic hinge forms. Local peak stresses are relevant only for fatigue. The different allowable multiples of K/S reflect this graded severity.
Technical content source: LV-Solved-Problems-Content 628d313247e7