Yes, there are two (2) types of BLEVEs, and much like the myth that BLEVE’s only happen to Pressure Vessels with liquified flammable gases, COLD BELVEs account for nearly 25% of all BLEVEs. So what is the difference between the two? Super GROK explains it better than I could…
A BLEVE (Boiling Liquid Expanding Vapor Explosion) occurs when a vessel containing pressurized liquid above its atmospheric boiling point suddenly ruptures. This causes rapid depressurization, leading to explosive boiling and vapor expansion as the liquid flashes to gas.
The terms hot BLEVE and cold BLEVE distinguish the scenarios based on the liquid’s temperature relative to its superheat limit, the cause of failure, and the resulting effects (especially for flammable materials).
Hot BLEVE
- Cause: External fire or heat source engulfs or impinges on the vessel. This heats the liquid (raising its temperature and pressure) and often weakens the tank wall (especially the vapor space above the liquid level).
- Conditions: The liquid temperature exceeds the superheat limit (or is significantly superheated) when rupture occurs. Burst pressure exceeds the normal operating/saturated vapor pressure.
- Mechanism: Prolonged heating causes the tank to fail catastrophically. Upon rupture, the highly superheated liquid flashes violently to vapor, producing a strong blast wave. For flammable substances (e.g., propane, LPG), the released vapor ignites immediately (often from the existing fire), creating a large, rising fireball with intense thermal radiation.
- Effects:
- Stronger overpressure/blast.
- Dramatic fireball (can rise high, last seconds to tens of seconds).
- High thermal radiation hazard.
- Flying fragments (rocketing tank pieces).
- Common in: Fire-engulfed LPG/propane tanks, rail cars, or storage vessels. This is the classic “fire-induced BLEVE” firefighters train for.
Cold BLEVE
- Cause: Mechanical failure, impact, material defect, overfilling, or sudden loss of containment without significant external heating. The tank fails at or near its normal operating pressure.
- Conditions: Liquid temperature is lower (below or near the superheat limit) but still above its atmospheric boiling point. Burst pressure ≈ operating pressure.
- Mechanism: Sudden rupture drops pressure, causing the (superheated but less extremely so) liquid to boil and expand rapidly. There is still flash vaporization, but less violent homogeneous nucleation than in hot cases. For flammables, a vapor cloud forms and may ignite, but the fireball is typically weaker, more ground-level, or transitional to a pool fire.
- Effects:
- Weaker blast overpressure compared to hot BLEVE.
- Smaller or ground-hugging fireball (less rise, lower thermal radiation in some models).
- Still produces a significant pressure wave, vapor cloud, and projectiles.
- Can occur with non-flammable liquids (e.g., CO₂, water, ammonia), resulting in a purely physical explosion without fire.
- Common in: Tank impacts, corrosion failures, or sudden depressurization of pressurized liquefied gases at ambient or moderately elevated temperatures.
Key Differences Summary
| Aspect | Hot BLEVE | Cold BLEVE |
|---|---|---|
| Trigger | External fire/heat | Mechanical failure/no major heat |
| Liquid Temp | Highly superheated (> superheat limit) | Moderately superheated |
| Blast Strength | Stronger | Weaker |
| Fireball (flammable) | Large, rising, intense radiation | Smaller, ground-level, weaker |
| Pressure at Rupture | Burst > operating | Burst ≈ operating |
| Example Substances | LPG/propane in fire | CO₂, LPG from impact |
Notes:
- The distinction is not always absolute; some sources use “cold” for lower-temperature events with reduced effects, while others tie it strictly to failure mode.
- Both can produce hazardous projectiles and vapor clouds. For flammables, ignition risk exists in both, but hot BLEVEs are generally more energetic and visible.
- Prevention focuses on vessel integrity, pressure relief, fire protection (e.g., water spray, separation distances), and avoiding impacts.
This classification helps in risk assessment, modeling consequences, and emergency response planning. Real incidents often involve elements of both, depending on exact conditions.