Machinery Casing Impact & Containment
The structural response of machinery casings to internally released component impact — casing deformation, rupture, mount and interface loads, and the structural containment assessment.
The machinery containment problem
Rotating machinery — turbines, compressors, pumps — may release internal components (blades, discs, bolts) during a failure event. The released component impacts the casing from inside at high velocity. The casing must contain the component — prevent it from escaping into the surrounding space — while maintaining structural integrity. The containment assessment determines whether the casing can stop the released component for the specified release conditions (component mass, shape, velocity and trajectory).
Released component as externally specified load
The released component — its mass, shape, velocity and trajectory — is an externally specified input to the containment analysis. The analysis does not predict the release event; it predicts the casing response to the release. The release conditions may be specified by a regulatory requirement, a customer specification or a failure analysis. The specification should include the component mass, the release velocity (typically the rotational speed at the release radius), the component geometry and the impact location on the casing.
Casing impact and local deformation
The casing responds to the component impact through local deformation — the casing wall dents, bends or tears at the impact location. The deformation depends on the casing material, wall thickness, curvature and the component geometry. A cylindrical casing may deform asymmetrically — the impact may produce a local bulge on the opposite side. The casing deformation absorbs the component kinetic energy through plastic work. If the deformation is sufficient to absorb all the energy, the component is contained. If not, the casing ruptures and the component escapes.
Rupture and containment failure
Casing rupture occurs when the local strain at the impact point exceeds the material's failure strain. The rupture may be a tear (ductile), a crack (brittle) or a slot (plugging). The rupture opens a path for the component to escape — containment is lost. The rupture threshold depends on the casing material, thickness, temperature and the component geometry. The analysis must predict whether rupture occurs for the specified impact conditions. If rupture is predicted, the containment is inadequate and the casing design must be improved (thicker wall, stronger material, multi-layer construction).
Mount and interface loads
The impact produces reaction loads at the casing mounts — the bolts, flanges or supports that connect the casing to the machinery frame. The mount loads may be large — the impact momentum is transferred through the casing to the mounts. The mount loads may cause bolt failure, flange damage or frame deformation. The mount loads must be assessed in the analysis — the mounts must be strong enough to carry the dynamic reaction without failing. If the mounts fail, the casing may detach from the frame, creating a secondary hazard.
Structural containment assessment
The structural containment assessment combines the casing response (deformation, rupture), the mount response (loads, failure) and the secondary response (frame, foundation). The assessment determines whether the machinery is contained — the component does not escape, the mounts do not fail and the frame does not collapse — for the specified release conditions. The assessment may be performed by explicit FEA (modelling the component, casing, mounts and frame) or by a combination of analysis and test. The containment assessment is a critical safety analysis for rotating machinery.
Model extent and boundary representation
A machinery casing is coupled to bearing housings, frames, flanges, ducts and mounts. Truncating the model too close to the impact region can make the casing artificially stiff or flexible and can distort both local deformation and mount loads. Use a model extent that captures the dominant support paths and verify the boundary stiffness against the surrounding assembly. Where a detailed full-machine model is impractical, condensed stiffness or validated interface representations can preserve the relevant structural restraint.
Local-to-global coupling
The first part of the event is dominated by local casing deformation and contact, but the momentum transferred to the casing excites global ovalisation, bending and mount reactions. These modes can govern clearances, rotor-stator interaction or secondary structural damage even when the casing contains the released component. Review both the short-duration local response and the later global vibration. A containment assessment that stops at the instant of arrest may miss the structural consequence of the momentum already transferred into the machine frame.
Joints, flanges and mounts
Flanges and mounts should be assessed as structural components rather than treated only as boundary conditions. Bolt groups may experience transient tension, shear and prying; flange contact can separate and re-close; welded or cast transitions can see local plasticity. When attachment integrity forms part of the containment requirement, extract time histories of joint forces and deformations and compare them with appropriately dynamic or conservatively justified capacities. Check whether load redistributes after local yielding instead of assuming that the initial elastic load split remains valid.
Correlation, inspection and residual integrity
Representative containment testing, where available, should be used to correlate local casing deformation, damage morphology, component arrest and mount response. After an event, residual integrity may also depend on cracks, distortion and interface damage that are not captured by a simple no-escape criterion. Link analysis to post-event inspection and, where necessary, a residual static, pressure or alignment assessment. This is particularly important for machinery that may need to remain safe while running down or while adjacent systems are brought to a controlled state.