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Engineered to Vent: How a Blow-Out Panel Keeps a Blast Out of the Room

RS Rand Simulation · Applications Engineering AI · June 2026 · 12 min read

An arc-fault inside a switchgear cabinet releases an enormous amount of energy in milliseconds. The job of the enclosure isn't to contain that — it's to direct it. A deliberately weak blow-out panel gives the pressure a path to the outdoors, away from the operator standing at the front. We modeled exactly that in LS-DYNA with a multi-material ALE blast: a small charge detonating inside a steel room with an X-shaped relief vent on the back, and tracked both where the overpressure goes and what the panel actually does.

Overpressure (p − p_atm) on a horizontal section through the charge. The blast is held by the sealed steel walls and escapes only through the X-vent on the back (outdoor) face; the vented wave then wraps around the outside of the sealed cabinet and runs along the building wall — the plumes near the top and bottom of the frame are that wrapped outdoor wave on the cabinet's side walls (it first appears at ≈256 µs, when the shock reaches the vent — this is the permanently-open-X run, so nothing “opens”; 256 µs is wave arrival), not pressure crossing the steel. The room side (front, left) stays well below the vented jet.

The same event in 3D: pressure fronts on the two symmetry planes inside the wireframe cabinet, showing the X panel venting the blast outdoors (back, right) while the sealed room front (left) stays well below the vented jet. The slice is clipped to the cabinet interior plus the legitimate vented jet, so the rendered plane stops at the steel walls — there is no false wrap-around bleeding through the intact top and bottom. (The walls genuinely seal; the diagnosis confirmed the apparent top/bottom “leak” in an earlier unclipped render was the vent jet wrapping around the box plus an unclipped render slice, not pressure passing through the steel.)

The same solve as a 3D iso-pressure view — nested constant-overpressure surfaces (0.05 / 0.15 / 0.40 MPa over ambient, Ansys structural-rainbow color) shown inside an idealized translucent cabinet, mirrored from the quarter model to the full box. The pressure builds inside the sealed enclosure, then breaks out through the back X-vent (orange arrow, +z = outdoors) and the vented wave wraps around the outside of the still-sealed box. The sealed walls are never breached — exterior pressure first appears only at ≈256 µs, when the shock reaches the vent plane (the X is a fixed opening here, so nothing “opens”; 256 µs is wave arrival) — so every isosurface beyond the steel is the vented blast wrapping around, not a leak. (The volume is lightly Gaussian-smoothed for surface legibility; the field and its peak magnitudes are unchanged.)

The setup: a blast you can aim

A 25 g TNT-equivalent charge sits at the geometric center of a sealed steel enclosure. The cabinet interior is 500 × 700 × 400 mm (0.14 m³), the walls are 3 mm sheet steel, and the blow-out panel is a 0.8 mm sheet on the back (outdoor) face. The X-shaped relief vent occupies ~0.123 m² — about 35% of the 0.35 m² back face. The charge-to-back-panel standoff is 0.20 m, a scaled distance Z = 0.68 m/kg^1/3. The air is carried as an ideal gas (γ = 1.4, ambient 0.1 MPa absolute) on a multi-material ALE (MMALE) mesh; the high explosive is a programmed-burn JWL. The structure — cabinet walls and the surrounding room — is a Lagrangian shell coupled to the fluid through a fluid-structure-interaction (FSI) constraint. The one feature that makes the whole demo work is the X-shaped relief vent: a deliberately weak path designed to open before the sealed walls are loaded, turning the enclosure from a bomb into a directed vent. We model it two ways — as a permanently open X (to isolate the fluid mechanism) and as a deformable scored panel (to ask whether it actually triggers).

What the physics says the panel should feel

For an energetics audience a solver result is only worth as much as the back-of-envelope check it survives. Three independent hand calculations bracket what the interior and the back panel should see, all reproducible from the inputs above, and all cross-checked against the project's own blast_spec sanity bands (UFC 3-340-02 / Kingery-Bulmash / Kinney-Graham).

Confined quasi-static gas pressure. Once the shock has reverberated and the products fill the box, the enclosure holds a quasi-static overpressure Δp = (γ−1)E/V. With E = 25 g × 4.52 MJ/kg = 113 kJ and V = 0.14 m³, E/V = 807 kPa; for γ between 1.2 and 1.4 that is Δp = 0.16 to 0.32 MPa gauge. This is a sustained load, not a transient spike, and it sits in the same order as the blast_spec quasi-static anchor (~0.10 MPa).

Reflected shock at the back panel (Kingery-Bulmash). The panel sits 0.20 m from a 25 g charge: W^1/3 = 0.292 kg^1/3, so Z = 0.20 / 0.292 = 0.68 m/kg^1/3. At that scaled distance the Kingery-Bulmash reflected curves give a reflected pressure of order 8–12 MPa and a reflected impulse of order 100–130 Pa·s. This standoff is inside the 0.3–0.5 m (Z = 1.03–1.71) window the blast_spec bands are quoted for, so those bands (side-on 0.3–1.0 MPa, reflected 1–8 MPa) under-predict the panel load; 8–12 MPa reflected sits just above the band's 8 MPa ceiling, exactly as a shorter standoff should.

What that does to a 0.8 mm panel. The panel's areal mass is 7850 kg/m³ × 0.8 mm = 6.3 kg/m². A reflected impulse of 100–130 Pa·s delivers a velocity kick of order i/m = 16 to 21 m/s — centimetre-scale motion within the captured window. Statically it is worse: a thin plate carries bending stress σ ≈ 0.3 q(L/t)², so a 0.8 mm / 250 MPa panel reaches yield at only q ≈ 53 kPa on a 0.1 m span and ~6 kPa on a 0.3 m span. The confined quasi-static pressure alone (160–320 kPa) is 3× to ~50× over that. Both regimes say the same thing: this panel yields. A longer run cannot rescue a “did not open” verdict — the panel is past its static capacity before any dynamic amplification.

Three probes: where the blast actually goes

The headline numbers are easier to trust as a time history than as three static figures. We pulled the overpressure trace from three cells in the ALE field of the open-X run: one in the room directly in front of the sealed cabinet face (where the operator stands), one in the jet just behind the X vent, and one deep in the room. The story is in the timing as much as the magnitude.

Overpressure-vs-time at the three probes (open-X run, real ALE field). The vented jet behind the X spikes to **+1.53 MPa gauge** at ~600 µs as the internal pressure finds its exit; the operator front face sees **+0.183 MPa gauge** (183 kPa ≈ 26.5 psi) at ~470 µs — about **8× below** the vented jet, but a serious exposure in its own right: 183 kPa gauge is above the ~35 kPa eardrum-rupture threshold and inside the ~100–200 kPa 50%-rupture band, approaching short-duration lung-injury levels. The deep room never leaves ambient — a flat **0.10 MPa absolute** line. **The insight:** directing the vent puts the largest pressure event on the *other side of the wall* and cuts the operator-face peak ~8× — a real safety gain — but reduced is not safe: the front-of-cabinet exposure still warrants hearing protection and standoff. (The +1.53 MPa single jet-cell peak is the same physics as the +1.38 MPa bulk-region max quoted below — one cell versus a regional max.)

Getting the seal right was the hard part

Two different coupling lessons shaped this study, and it is worth separating them. In the open-X run the walls are rigid and the only question is whether the ALE field leaks through them: in an MMALE model the Lagrangian structure holds pressure only where the ALE faces are aligned with it, and a misaligned seam leaks the blast straight through a wall that should be solid. Aligning the ALE faces to the cabinet seam is what separates the room side from the vented jet; without it, any “containment” is an artifact of a leaky mesh. The second lesson — transferring load onto the deformable panel in the rupture-X run — is the one this revision is built around, and it is below.

Peak-overpressure instant (t ≈ 8 µs): the 25 g charge has just detonated and the entire field is still *inside* the sealed cabinet — the walls hold the peak; venting through the X-panel comes later. The bright spot at the cabinet/wall junction is a localized FSI penalty spike, discussed below.

A later section cut, at **t = 744 µs** — well after the 608 µs near-vent peak. The cabinet interior has already vented back toward ambient; what remains is the *outdoor* blast **wrapping around the sealed box** and channeling along the building wall. The plumes near the top and bottom of the frame are this wrapped wave on the cabinet's *side* walls — exterior pressure that first appeared at ≈256 µs (shock arrival at the vent, not an opening — the X is a fixed opening in this run), never through the sealed steel. Plotted on a late-time 0–0.15 MPa scale so the decayed plume is visible. **The insight:** the vent keeps directing the flow outdoors as the event plays out, so the interior keeps venting rather than reloading.

The result: the X-vent directs the blast outdoors and the blow-out panel triggers (opens outward) under the 25 g charge. The operator-side (front-of-cabinet) overpressure peaks at +0.183 MPa gauge (183 kPa ≈ 26.5 psi) — about 8× below the +1.38 MPa gauge regional max in the vented jet behind the cabinet (a single jet cell peaks at +1.53 MPa gauge) — while the deep room never leaves ambient (0.10 MPa absolute). The ~8× operator-face reduction is a genuine safety gain; the +0.183 MPa exposure is still above the eardrum-rupture threshold, so directed, not safe, is the honest verdict.

What the panel actually does — and how v1 got it wrong

The open-X run above answers “where does the pressure go” with a permanently open vent — the cabinet and wall are rigid, so the X is simply a fixed opening the blast is free to use. That isolates the fluid mechanism, but it leaves the structural question: if the X were a real scored, rupturing panel, would it open, and what stress would the rest of the cabinet see? So we ran a second model (the rupture-X variant) where the back panel is a deformable 0.8 mm sheet with a kinematic-hardening steel law (250 MPa yield) set to erode at 5% plastic strain along the score line.

The first version of this study reported that the panel barely moved — peak von Mises ~1 MPa, effective plastic strain exactly zero, and 0.008 mm of out-of-plane deflection — and concluded it did not trigger under the 25 g pulse. That conclusion is retracted. It fails every anchor above. Back-solving the reported motion: 0.008 mm over the ~0.35 ms after vent arrival is a mean panel velocity of ~0.02 m/s, a delivered impulse of ~0.14 Pa·s, a time-average panel-face pressure of ~400 Pa — four orders of magnitude below the 0.16–0.32 MPa the box must hold and the 8–12 MPa reflected pulse the panel actually sees. The stated excuse (“the impulse is too brief to drive the thin panel past failure in the time we captured”) also has the dynamics backwards — brief loading does not mean a small response, it means the response arrives after the pulse — and it fails statically too: the panel is over its yield capacity at any plausible span.

The cause was a dead FSI coupling, not panel physics. In the original run the panel absorbed 3.6×10⁻³ mJ while the air carried 1.2×10⁸ mJ; it eroded nothing, and — the tell — it drifted into the room (−z), the wrong side of a blast that should push it outdoors. The penalty coupling never transferred load to the one deformable part. Three deck settings did it: the back-panel shell normals pointed away from the fluid, the coupling was compression-only (DIREC = 2), and it was non-eroding (CTYPE = 4), so failing elements stayed in the mesh and shielded the panel from the flow.

Two checks confirm the diagnosis and the fix. First, driving the panel as a standalone structure under the panel-face pressure history extracted from the fluid gives 2.4 mm of deflection — about 300× the 0.008 mm, and three times the panel's own thickness — with a clean energy balance (ratio 1.0007). Second, the coupled model was rerun with the coupling repaired: CTYPE 4→5 (eroding), DIREC 2→1 (compression + tension), shell normals made consistent toward the fluid (NORM = 1), and DATABASE_FSI output so the interface force is directly observable. It terminated normally at 10 ms. Now the panel (part 3) absorbs ~40 J of internal and ~30 J of kinetic energy — about 10⁷× the original — it erodes along the score line (86 J of eroded internal energy, i.e. elements reaching the 5% failure strain), and its momentum is outward (+z, toward the atmosphere): the correct blow-out direction. Corrected, the blow-out panel triggers and opens under the 25 g charge.

**The retracted v1 result.** Left: von-Mises on the deformable X blow-out panel from the *original* coupled run, on an Ansys-style scale — the X-score geometry is visible and the most-loaded cells (where the arms cross) reach only ~1.1 MPa. Right: the panel's peak von-Mises and peak deflection over time, nowhere near the 250 MPa yield line. It reads like a panel that shrugged off the blast, but the near-zero response was the symptom of a dead coupling, not a design margin: the panel absorbed 3.6×10⁻³ mJ against the air's 1.2×10⁸ mJ and drifted the wrong way. The corrected coupled solve overturns it — the panel yields, erodes, and opens outward. A real switchgear panel is pre-creased and far weaker than a 5%-strain steel sheet, so sizing the scoring to open at the design fault energy is exactly what this kind of run is for — once the coupling is verified to carry load.

Why this one matters

Pressure-relief and blow-out design is everywhere — switchgear, battery enclosures, transformer tanks, vent stacks — and the question is always the same: when it lets go, where does it go, and who is standing there? A coupled blast model answers both — but only if the fluid-structure coupling actually moves load onto the parts you care about. The discipline that makes the answer trustworthy is the unglamorous one: verify the interface force against an integrated probe pressure before you believe any hold-or-trigger claim, and anchor the field against a hand calc before you publish a number. This study is a case in point — the first pass got a headline conclusion inverted precisely because that check was skipped.

Revisions

v2 · Internal reviewThe panel-held conclusion was retracted: a repaired fluid-structure coupling shows the blow-out panel triggers and opens outward, and operator-face exposure was recharacterized as serious.

AI disclosure: RandSim Labs is an experimental AI-driven engineering simulation platform. Content on this site, including simulations, analyses, figures, and written materials, may be generated or assisted by AI using licensed Ansys tools. AI-generated content may contain errors and is provided for educational, informational, and demonstration purposes only. Users should independently verify all results before relying on them for engineering, design, manufacturing, safety, or other production decisions.

Honest scope. This is an engineering demonstration of the venting mechanism, not a certified arc-flash or pressure-vessel rating. The charge is a generic 25 g TNT-equivalent, the air is an ideal gas, and the structure is a shell idealization. One localized worst-cell at the cabinet/wall junction reads high (~2.7 MPa) — a thin-shell penalty-FSI singularity at this demo mesh resolution; the bulk room field is far lower, and resolving the seam at sub-mm scale (or modeling it as a welded/gasketed joint) would shrink it. The containment claim is about the bulk field, with that caveat called out, not buried. On the structural result: The corrected ALE-FSI run conserves energy only approximately — total energy climbs to ~1.5× over the 10 ms window, from the detonation energetics and penalty-coupling work — so the trigger / no-trigger verdict is robust (it is a many-orders-of-magnitude result, corroborated by three independent hand anchors and a standalone structural run), but the absolute peak magnitudes — exact peak pressure, exact deflection before erosion — are indicative, not precise. The cabinet and building wall are still modeled rigid, so the deformable panel is the only part carrying a real stress/strain field. As before, the one non-physical cell is the FSI penalty spike at the cabinet/wall junction noted above — a coupling artifact, not a structural stress.

When your enclosure lets go, do you know which side of the wall the pressure lands on — and how hard? An LS-DYNA multi-material ALE blast — a programmed-burn JWL charge coupled to the Lagrangian shells through a face-aligned, load-transfer-verified FSI seal, probed at the operator face (+0.183 MPa gauge), the vented jet (+1.38 MPa gauge regional max, +1.53 MPa single cell), and the deep room (0.10 MPa absolute), then rerun with a deformable scored panel that triggers — is how simulation shows where the energy goes, and what the operator would feel, before a real arc-fault runs the experiment for you. That's innovation through insight.