What is a Boiling Liquid Expanding Vapor Explosion (BLEVE) on an LNG carrier?
Like the airlocks on LNG carriers (safety systems located between the deck (hazardous area) and the cargo compressor engines (gas-free area)), cargo holds on LNG carriers are equipped with various safety measures to prevent cargo from coming into contact with the atmosphere and overpressurization.
LNG carrier cargo holds are specially constructed using high-strength, low-thermal-coefficient materials and are equipped with insulation and safety systems. A key safety feature in these holds is the pressure relief valve, which helps prevent overpressurization and rupture.
On LNG carriers, the most dangerous component is often vapor, not liquid, which evaporates at extremely low temperatures. This can lead to a BLEVE.
What is a Boiling Liquid Expanding Vapor Explosion (BLEVE)?
A boiling-expanding steam explosion (BLEVE) is a type of steam explosion that can occur when a tank structure fails catastrophically, causing the liquid cargo inside to exceed its boiling point at nominal atmospheric pressure.
Cargo in gas carriers is typically part liquid and part vapor. However, when the tank structure collapses, vapor attempts to escape or leaks through openings, causing a drop in pressure inside the tank. This sudden drop in pressure inside the cargo hold causes the liquid to boil rapidly and increases vapor formation.
Boiling-Expanding Steam Explosion
The escaping vapor’s pressure rises rapidly, generating a shock wave or, in the presence of an ignition source, an explosion that can destroy the entire tank structure and the surrounding area.
The extent of a BLEVE depends entirely on the size, volume, and weight of the container or cargo hold. The amount of liquid cargo remaining in the tank at the time of a BLEVE also affects the scale of the explosion.
Note that a BLEVE can occur even in the absence of flammable materials. For example, liquid cargoes stored at extremely low temperatures, such as liquid helium or other refrigerants or coolants, can experience similar conditions during a boiling liquid vapor explosion (BLEVE); however, these are not generally considered chemical explosions. If the substance or cargo used in a BLEVE is toxic, it can contaminate a large area.
A fireball can form during a BLEVE, similar to a fuel-air explosion, also known as a vapor cloud explosion (VCE), only if the substance used is flammable.
Common Scenarios of a BLEVE
A BLEVE does not occur immediately. It can occur under certain conditions, such as when warning signals are ignored. The following are necessary conditions for a boiling liquid vapor explosion to occur:
Liquid cargo: Vapor alone cannot cause a vapor explosion. The liquid cargo must be present in the tank for a vapor explosion to occur. Even water can cause a vapor explosion, but since water is non-flammable, it does not cause a fire.
Pressure vessel: Liquid cargo must be contained in sealed containers or cargo holds. Even if the container is well ventilated, a vapor explosion can only occur if the ventilation system fails, causing the tank pressure to build up.
Temperatures above the boiling point: A sealed liquid cargo must be heated above its boiling point at atmospheric pressure to cause a vapor explosion. When a pressurized cargo hold or tank is heated, vapor pressure increases. This increase in boiling point is accompanied by an increase in vapor pressure. Structural failure: The liquid needs a path to escape from a pressurized cargo hold and convert to vapor, and this can only occur if there is a structural failure in the hold or tank.
Common Causes of BLEVEs
The most common cause of a BLEVE is a fire near a cargo hold containing a gas cargo such as propane. Due to the high ambient temperature, the cargo hold begins to heat up, creating excessive pressure inside. High pressure within the tank is typically relieved via a pressure relief valve.
However, if the pressure increases rapidly due to the high ambient temperature and high rate of heating, the tank’s weakest point can collapse, exposing the pressurized flammable vapor to an open flame and causing the expanding liquid vapor to explode.
Causes of Tank Structural Failure
- Improper Tank Maintenance
- Corrosion of the Tank Structural Structure
- Faulty or Blocked Tank Safety Valve
- Mechanical Damage to the Tank
- Physical Failure
- Severe Tank Structural Exposure, Flames, or Fire
Different Stages of a BLEVE
As previously mentioned, a BLEVE does not occur suddenly or in the absence of any of the aforementioned elements. The following are the steps that may lead to a BLEVE:
Tank Failure/Sealing: Tank failure can occur due to a variety of reasons, including increased internal pressure and failure of the weakest component.
Phase Change: When a tank fails, the pressure of the liquefied gas suddenly drops. The liquid-gas mixture, thermodynamically saturated above its boiling point, superheats, and the initial pressure within the tank/storage vessel drops to atmospheric pressure within milliseconds. Liquid-Vapor Splash: When the superheat limit (SLT) temperature is exceeded, rapid gas bubble formation begins within the tank, causing the liquid-gas mixture to violently erupt into the atmosphere.
Explosion: The pressure drop, accompanied by a violent phase change in the superheated state, causes the liquid to boil, followed by gas bubble formation, ultimately leading to an explosion. BLEVE warning signs:
- Buzzing sound from the metal frame
- Discoloration of the tank
- Small metal parts falling off
- Blisters or bulges on the tank surface
- Sudden increase in tank pressure
- Preventative measures to avoid BLEVE
- Maintain cargo holds regularly
- Always ensure the pressure relief valve is in proper working order
- The pressure relief valve to be installed must be sized in accordance with the International Gas Code
- Emergency preparedness for all ship personnel
- During gas cargo transfers, avoid handling normal cargo, stores, and compressed gases near the gas loading process
- No refueling or lubrication operations are allowed during or prior to loading
- Continuously check mooring lines to ensure the vessel is securely moored to the berth/pier
- A liquefied petroleum gas (LPG) detector must be available for berth use
- Any cargo transfer operations must be conducted at night or during dark hours with adequate lighting both in port and on board
- Connection areas, stop valves, etc. must be clearly visible. Firefighting equipment must be available and easily accessible at all times during transfers. Adequate warning signs must be posted to ensure that all personnel, both shore and on board, are aware of cargo loading and unloading procedures and necessary precautions.
- Safety is always our top priority.
