Explosion and Pressure-Vessel Failure

Q: What is a BLEVE and what causes it?

A boiling liquid expanding vapour explosion (BLEVE) occurs when a vessel containing a liquid above its atmospheric boiling point fails catastrophically. When the vessel ruptures, the sudden pressure drop causes the superheated liquid to flash to vapour almost instantaneously, producing a rapidly expanding pressure wave. If the liquid is flammable, the released vapour also ignites, producing a fireball. The most common cause is external fire heating a pressurised vessel.

Q: How do investigators use fragment trajectories to reconstruct a vessel explosion?

When a pressure vessel fails, fragments are propelled outward at velocities and angles determined by the vessel's internal pressure, the fracture geometry, and the mass of each fragment. Investigators map fragment landing positions relative to the vessel origin using total-station survey. Fragment mass, landing distance, and direction can be back-calculated using ballistic equations to estimate the minimum internal pressure at failure, which is compared against design pressure and operating records.

Q: What is the engineering difference between deflagration and detonation?

In a deflagration, the combustion front moves through the fuel-air mixture at a speed below the speed of sound in that medium, typically metres per second. In a detonation, the front moves at supersonic speed, producing a coupled shock wave and reaction zone. Deflagrations produce sustained overpressure; detonations produce a sharp, impulsive spike followed by a negative phase. Buildings subjected to detonation show inward-wall failure and debris thrown in the direction of blast propagation, while deflagration damage is more diffuse.

Q: What is corrosion under insulation and why is it a pressure-vessel hazard?

Corrosion under insulation (CUI) occurs when water infiltrates the thermal insulation on a piping or vessel exterior, creating a wet environment against the metal surface that is hidden from visual inspection. The combination of moisture, oxygen, and elevated temperature drives corrosion that reduces wall thickness without any external indication. CUI is one of the leading causes of undetected wall-thinning failures in oil and gas facilities.

Q: What happened at the Texas City refinery in 2005?

On 23 March 2005, the isomerisation unit at BP's Texas City refinery experienced a catastrophic overpressure event during a startup operation. A blowdown drum was overfilled, and liquid hydrocarbons overflowed to grade, forming a flammable pool that ignited. The explosion and fire killed 15 workers and injured 180. The incident was attributed to missing level instrumentation on the blowdown drum, an open-to-atmosphere vent stack, and process procedure violations.

BLEVEs, vessel fractures, and overpressure patterns give forensic engineers a physical record of how an explosion unfolded. This topic connects the mechanics of deflagration and detonation to investigation methodology and the regulatory frameworks that govern pressure-equipment design.

Last updated: 10 Jun 2026

A pressure vessel holds energy in a form that is invisible until it releases. A storage sphere holding liquid propane at several bar, a refinery tower running at elevated temperature, a domestic water heater: each is a controlled system whose failure converts stored energy into mechanical work in milliseconds. When that happens violently enough, it is called an explosion, and what is left of the vessel and its surroundings becomes the evidence.

Forensic engineers investigating explosions and pressure-vessel failures work backward from the physical evidence: the pattern of vessel fragments, the overpressure damage to surrounding structures, the fracture surface on the vessel wall, and the process records that describe what was happening inside the vessel at the moment of failure. That combination of physical and documentary evidence allows an investigator to establish failure mode, estimate pressure at failure, and test whether the failure was caused by design deficiency, operating error, corrosion, or external fire.

This topic covers the mechanics of BLEVE events, the use of vessel fragments and overpressure patterns as physical evidence, the engineering distinction between deflagration and detonation, the specific hazard of corrosion under insulation in process plant, and the Texas City refinery disaster of 2005, which remains the single most studied process-safety failure of the past two decades. The regulatory frameworks of ASME Boiler and Pressure Vessel Code and API 579 (Fitness-For-Service) provide the engineering baseline against which failures are assessed.

Key terms

BLEVE: Boiling liquid expanding vapour explosion. Catastrophic failure of a vessel containing a superheated liquid, producing an expanding pressure wave and, if the liquid is flammable, a fireball.
Deflagration: Combustion in which the reaction front propagates at subsonic velocity through a fuel-air mixture. Produces a pressure pulse that rises and falls over tens of milliseconds.
Detonation: Combustion in which a coupled shock wave and reaction zone propagate at supersonic velocity. Produces a sharp impulsive overpressure spike followed by a sub-ambient negative phase.
Overpressure: Pressure above ambient atmospheric pressure produced by an explosion. The peak overpressure and its duration (impulse) together determine structural damage and injury severity.
Corrosion under insulation (CUI): External corrosion of a pipe or vessel surface that occurs beneath thermal insulation, hidden from visual inspection, driven by moisture infiltrating the insulation system.
API 579: The American Petroleum Institute's Fitness-For-Service standard, which provides assessment methods for determining whether a damaged or corroded pressure vessel or piping component can continue in safe service, and when repair or replacement is required.

BLEVE mechanics and physical evidence

When a pressure vessel fails from external fire, it does not just leak. It launches.

A BLEVE requires two conditions: a liquid stored above its atmospheric boiling point (meaning it is superheated by the pressure of the vessel), and a sudden loss of containment. The most common trigger is external fire heating the vessel wall above the metal's yield strength at the liquid-vapour interface, where liquid cooling is absent and the wall loses structural integrity. When the wall tears, the pressure drops instantly to atmospheric. The superheated liquid flashes to vapour, expanding rapidly and producing the characteristic pressure wave.

If the liquid is flammable (LPG, liquefied natural gas, fuel oil), the released vapour cloud ignites and produces a fireball whose diameter and duration are roughly proportional to the mass of fuel involved. The fireball is a thermal hazard distinct from the overpressure wave, and the two together produce the combined blast-and-thermal effects associated with large industrial BLEVEs such as the Mexico City PEMEX LPG facility fire of 1984.

Fragment evidence: vessel fragments are propelled as missiles, often landing hundreds of metres from the failure point. Fragment landing positions, mapped by survey, allow back-calculation of minimum internal pressure at failure using ballistic models.
Fracture surface: the tear in the vessel wall begins at a stress-concentration point, often a weld seam or a corrosion pit. Fractographic analysis of the fracture surface can reveal whether failure was ductile (material had remaining plastic capacity), brittle (rapid fracture at stress below yield), or caused by a pre-existing fatigue crack.
Overpressure damage field: the pattern of structural damage to surrounding buildings, particularly inward collapse of blast-facing walls, allows estimation of peak overpressure at various distances using empirical blast-scaled distance correlations.

BLEVE sequence from external fire heating to fireball.

Deflagration versus detonation: damage pattern differences

The type of explosion leaves a distinct signature in every structure it touches.

Explosions in industrial and forensic contexts are almost always either deflagrations or detonations, and the distinction matters both for understanding what happened and for interpreting the structural damage. A fuel-gas leak that accumulates in a building and ignites typically deflagrates. A high explosive placed as a charge detonates. The two produce fundamentally different pressure-time histories and different damage patterns.

Feature	Deflagration	Detonation
Flame speed	Subsonic (< 340 m/s)	Supersonic (1500-8000 m/s depending on explosive)
Pressure rise rate	Gradual, over tens of milliseconds	Near-instantaneous shock front
Peak overpressure	Typically 0.1-0.9 MPa in confined spaces	Can reach tens of MPa adjacent to charge
Negative phase	Usually mild	Significant, causes secondary inward collapse
Structural damage pattern	Windows out, walls pushed outward globally	Close-in cratering, directional fragmentation
Evidence fragments	Debris scattered by pressure wave	High-velocity fragments in line of detonation

In practice, a deflagration can transition to detonation (DDT) in a long pipe or duct if the conditions are right, producing a hybrid damage sequence that can confuse the initial assessment. Investigators use the spatial pattern of window failure, wall direction of displacement, and fragment velocities to reconstruct the pressure-time history and determine whether DDT occurred. Soot deposition patterns and burn marks on surfaces also help distinguish the thermal phase of a deflagration from the blast-only effects of a detonation.

Pressure-vessel fatigue and corrosion-under-insulation

The failure that was growing for years before the explosion.

Not all pressure-vessel explosions are caused by single acute events. Many result from long-running degradation mechanisms that reduce the vessel's effective wall thickness or introduce crack initiation sites, until the vessel can no longer sustain normal operating pressure. The two most common mechanisms in industrial process plant are cyclic fatigue and corrosion under insulation.

Cyclic fatigue in pressure vessels operates on the same principles as in any other structure. Vessels that undergo repeated pressurisation and depressurisation cycles (start-up and shutdown cycles, process pressure fluctuations, hydraulic surge) accumulate fatigue damage at stress-concentration points such as nozzle attachments, weld toes, and thickness changes. ASME Boiler and Pressure Vessel Code Section VIII Div. 2 requires fatigue analysis for vessels above specific cycle counts, but many older vessels were designed to earlier codes without explicit fatigue provisions.

Corrosion under insulation (CUI) is a concealed external corrosion mechanism. Water from rain, process leaks, or condensation penetrates the thermal insulation, becomes trapped against the metal surface, and drives corrosion at rates that depend on metal temperature, water chemistry, and oxygen availability. The most aggressive CUI zone for carbon steel is roughly 60-120 degrees Celsius, where the surface stays wet but is warm enough to accelerate the electrochemical reaction. Stainless steel suffers stress-corrosion cracking under CUI rather than simple pitting.

ASME BPVC and API 579: the regulatory baseline

Every pressure-vessel failure is measured against the code it was designed to meet.

The ASME Boiler and Pressure Vessel Code (BPVC) sets design rules for the construction of pressure vessels in the United States and many international jurisdictions. It is organised into sections by vessel type: Section I for power boilers, Section VIII for unfired pressure vessels (Divisions 1, 2, and 3), and Section X for fibre-reinforced plastic vessels, among others. The code specifies design margins, material allowable stresses, weld joint efficiencies, and required inspection and testing at fabrication.

API 579 (Fitness-For-Service) operates at the other end of the vessel life cycle. It provides assessment methods for a vessel that has developed a defect, corrosion, or damage during service, allowing engineers to determine quantitatively whether continued operation is safe and under what conditions. Relevant sections include Level 1, 2, and 3 assessments for general metal loss (corrosion thinning), pitting, and crack-like flaws. The standard is widely used both as an engineering tool and as a reference for forensic assessment of why a vessel that had been inspected and cleared subsequently failed.

The Texas City refinery disaster: case study

A startup procedure gone wrong, a blowdown drum with no level indicator, and fifteen deaths.

On 23 March 2005, workers at BP's Texas City refinery in Texas were restarting the isomerisation unit after a maintenance shutdown. The raffinate splitter tower was being filled with hydrocarbon feedstock and brought to operating temperature. During the startup, liquid levels in the tower were not correctly monitored. The tower was overfilled, and hot liquid hydrocarbon was routed through the overhead system into a blowdown drum that was already full.

The blowdown drum, a cylindrical vessel designed to receive vapour and small amounts of liquid from process upsets and vent them to atmosphere through a stack, had no liquid-level instrumentation. Liquid overflowed the stack and fell to the ground as a flammable pool. Vapour from the liquid formed a cloud that found an ignition source, likely a running vehicle engine nearby. The deflagration ignited the liquid pool and the resulting explosion and fire killed 15 people in the trailer complex adjacent to the unit and injured 180 others.

The US Chemical Safety Board (CSB) investigation, published in March 2007, identified multiple layers of failure: the blowdown drum design with no liquid-level indication, the open-to-atmosphere stack that discharged flammable vapour at grade level rather than to a flare system, a startup procedure that was not followed, a level-indicator on the tower that was defective, and organisational factors including cost-cutting and inadequate safety culture. The incident drove changes to ASME process-safety design guidance and API standards for blowdown systems, including requirements for closed systems (flare or scrubber connection) rather than open atmospheric stacks in hydrocarbon service.

Reconstructing an explosion: methodology

Going from wreckage to a pressure and failure sequence.

An explosion investigation proceeds through a defined evidence-collection and analysis sequence that mirrors the broader failure-investigation process described in ASTM E860.

Scene documentation and preservation
Total-station and photogrammetric survey of fragment positions relative to the vessel origin. GPS coordinates and photographs for each fragment. Documentation of structural damage to surrounding buildings, including wall displacement directions, window failure patterns, and damage gradients with distance.
Fragment recovery and cataloguing
Each fragment is recovered, weighed, and its original position on the vessel tentatively identified from wall-thickness marks, weld seams, and nozzle locations. Reconstruction of the vessel from fragments, even partial, establishes the geometry of the failure initiation zone.
Fracture surface examination
The fracture surface at the initiation zone is examined macroscopically for beach marks, fatigue striations, corrosion pits, or hydrogen embrittlement features. Scanning electron microscopy provides micrometre-scale detail. Material composition is verified against the original mill certificate.
Pressure reconstruction
Fragment ballistic analysis and overpressure damage assessment at known distances are used to estimate minimum internal pressure at failure. The estimated pressure is compared against the vessel design pressure and the operating pressure at the time of failure from process records.
Failure mode determination
The physical evidence, the pressure reconstruction, and the operational history are synthesised to establish whether the cause was material deficiency, design inadequacy, operating exceedance, external fire (BLEVE), or latent corrosion or fatigue. The determination is tested against the ASME BPVC requirements applicable to that vessel.

Explosion reconstruction methodology from scene to failure mode determination.

Worked example

LPG storage sphere failure: BLEVE reconstruction

Fifteen fragments, three craters, and a pressure estimate that contradicted the operating record.

An LPG storage sphere at a petrochemical terminal failed catastrophically during a process upset. The sphere held approximately 270 tonnes of liquid propane at 7 bar. Adjacent piping had developed a small leak, which ignited and produced a flame impingement on the sphere shell. Emergency responders activated water deluge cooling, but the deluge system coverage of the lower hemisphere was insufficient. The sphere failed approximately eighteen minutes after the initial leak ignition.

Fragment mapping. Fifteen major shell fragments were located within a 350-metre radius. Total-station survey established each fragment's landing coordinate relative to the sphere's foundation ring. Fragment masses were determined on site.
Ballistic analysis. Using the fragment mass and landing distance with a standard ballistic projection model (accounting for aerodynamic drag), the minimum initial velocity at launch was estimated for each fragment. Back-calculation of the kinetic energy at launch, combined with assumptions about the angle of ejection derived from the fracture geometry, gave a range of internal pressure estimates: 11-15 bar.
Design pressure comparison. The sphere's design pressure was 8.5 bar. Operating records showed the vessel had been running at 7 bar. The reconstructed failure pressure of 11-15 bar is consistent with the shell having yielded under combined thermal weakening from fire impingement and internal pressure, at a pressure well below the vessel's nominal burst pressure under ambient conditions but achievable once the shell temperature at the impingement zone exceeded the metal's elevated-temperature yield strength.
Fracture surface. Examination of the fracture edge at the initiation zone showed a ductile fracture with significant necking, consistent with high-temperature yielding rather than brittle fracture or pre-existing defect. No fatigue beach marks or corrosion pits were present at the initiation. This is the expected failure mode for a fire-weakened BLEVE.
Conclusion. The investigation concluded that the BLEVE was caused by fire impingement on a region of the sphere not adequately covered by the deluge system. The failure was not caused by a pre-existing material defect, operating overpressure, or design code violation. The contributing factor was the inadequate deluge coverage specification, which became the basis for the subsequent liability and regulatory review.

Check your understanding

Question 1 of 4· 0 answered

A BLEVE requires which combination of conditions?

Key Takeaways

A BLEVE requires a superheated liquid vessel and sudden loss of containment; the resulting pressure wave and, for flammable liquids, the fireball produce fragment missiles and overpressure damage that are the primary physical evidence.
Deflagrations and detonations produce fundamentally different damage patterns: deflagrations push walls outward diffusely; detonations produce near-field cratering, directional fragmentation, and a significant negative-pressure phase.
Corrosion under insulation is a concealed mechanism that reduces vessel wall thickness without external indication; it is the leading cause of undetected wall-thinning failure in ageing process plant and is assessed using API 579 fitness-for-service methods.
The Texas City 2005 disaster illustrates how multiple simultaneous barrier failures (missing instrumentation, open vent to grade, deviation from procedure) combine to produce a catastrophic outcome that no single root cause can explain.
Explosion reconstruction moves from fragment mapping and ballistic back-calculation through fracture surface analysis to failure-pressure estimation, which is then compared against design and operating records to establish failure mode.

What is a BLEVE and what causes it?

A boiling liquid expanding vapour explosion (BLEVE) occurs when a vessel containing a liquid above its atmospheric boiling point fails catastrophically. When the vessel ruptures, the sudden pressure drop causes the superheated liquid to flash to vapour almost instantaneously, producing a rapidly expanding pressure wave. If the liquid is flammable, the released vapour also ignites, producing a fireball. The most common cause is external fire heating a pressurised vessel.

How do investigators use fragment trajectories to reconstruct a vessel explosion?

When a pressure vessel fails, fragments are propelled outward at velocities and angles determined by the vessel's internal pressure, the fracture geometry, and the mass of each fragment. Investigators map fragment landing positions relative to the vessel origin using total-station survey. Fragment mass, landing distance, and direction can be back-calculated using ballistic equations to estimate the minimum internal pressure at failure, which is compared against design pressure and operating records.

What is the engineering difference between deflagration and detonation?

In a deflagration, the combustion front moves through the fuel-air mixture at a speed below the speed of sound in that medium, typically metres per second. In a detonation, the front moves at supersonic speed, producing a coupled shock wave and reaction zone. Deflagrations produce sustained overpressure; detonations produce a sharp, impulsive spike followed by a negative phase. Buildings subjected to detonation show inward-wall failure and debris thrown in the direction of blast propagation, while deflagration damage is more diffuse.

What is corrosion under insulation and why is it a pressure-vessel hazard?

Corrosion under insulation (CUI) occurs when water infiltrates the thermal insulation on a piping or vessel exterior, creating a wet environment against the metal surface that is hidden from visual inspection. The combination of moisture, oxygen, and elevated temperature drives corrosion that reduces wall thickness without any external indication. CUI is one of the leading causes of undetected wall-thinning failures in oil and gas facilities.

What happened at the Texas City refinery in 2005?

On 23 March 2005, the isomerisation unit at BP's Texas City refinery experienced a catastrophic overpressure event during a startup operation. A blowdown drum was overfilled, and liquid hydrocarbons overflowed to grade, forming a flammable pool that ignited. The explosion and fire killed 15 workers and injured 180. The incident was attributed to missing level instrumentation on the blowdown drum, an open-to-atmosphere vent stack, and process procedure violations.

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Explosion and Pressure-Vessel Failure

Last updated: 10 Jun 2026

Feature

Deflagration

Detonation

Flame speed

Subsonic (< 340 m/s)

Supersonic (1500-8000 m/s depending on explosive)

Pressure rise rate

Gradual, over tens of milliseconds

Near-instantaneous shock front

Peak overpressure

Typically 0.1-0.9 MPa in confined spaces

Can reach tens of MPa adjacent to charge

Negative phase

Usually mild

Significant, causes secondary inward collapse

Structural damage pattern

Windows out, walls pushed outward globally

Close-in cratering, directional fragmentation

Evidence fragments

Debris scattered by pressure wave

High-velocity fragments in line of detonation

Key Takeaways

A BLEVE requires a superheated liquid vessel and sudden loss of containment; the resulting pressure wave and, for flammable liquids, the fireball produce fragment missiles and overpressure damage that are the primary physical evidence.

Deflagrations and detonations produce fundamentally different damage patterns: deflagrations push walls outward diffusely; detonations produce near-field cratering, directional fragmentation, and a significant negative-pressure phase.

Corrosion under insulation is a concealed mechanism that reduces vessel wall thickness without external indication; it is the leading cause of undetected wall-thinning failure in ageing process plant and is assessed using API 579 fitness-for-service methods.

The Texas City 2005 disaster illustrates how multiple simultaneous barrier failures (missing instrumentation, open vent to grade, deviation from procedure) combine to produce a catastrophic outcome that no single root cause can explain.

Explosion reconstruction moves from fragment mapping and ballistic back-calculation through fracture surface analysis to failure-pressure estimation, which is then compared against design and operating records to establish failure mode.

What is a BLEVE and what causes it?

How do investigators use fragment trajectories to reconstruct a vessel explosion?

What is the engineering difference between deflagration and detonation?

What is corrosion under insulation and why is it a pressure-vessel hazard?

What happened at the Texas City refinery in 2005?

Your journey to becoming a forensic professional starts here.

Key Takeaways

Key Takeaways