Any Accident Types of Gas Risk in Vessel

The assessment of the hazards of a major fire event, requires a relationship between the thermal load (a function of the radiation intensity and exposure time) and the effects on people. The issue can be addressed from two perspectives.

1. How long can a worker continue to operate in an emergency situation whilst exposed to a given level of radiation?
2. What fraction of the population will die or sustain serious injury given exposure to a certain dose of radiation.

Gases are commonly stored in large pressurised vessels as liquefied gasses. If these vessels are subjected to engulfing pool or impinging jet fires significant amounts of heat may be transferred to the vessel. If the fire exposure lasts for sufficient time, the vessel may fail catastrophically, resulting in a Boiling Liquid Expanding Vapor Explosion (BLEVE). In these events, it is the temperature rise and subsequent loss of strength of the steel wall which determine the time to failure. Although vessels are usually protected with pressure relief valves, failure can occur in just a few minutes. The use of water deluge systems or passive fire protection (PFP) materials decrease heat flow to the vessel contents and can reduce or eliminate the risk of a BLEVE occurring36. In order to be able to assess this behavior and the hazards posed from fire-engulfment of liquefied gas storage vessels it is important to understand the mechanism of failure and to be able to predict the response of vessels under such conditions.

The HEATUP scenario has been developed in order to model the behavior of vessels containing liquefied gases exposed to fire and produce suitable data as input for hazard consequence analysis tools. HEATUP quantifies the thermodynamic properties in the vapor and liquid phase of the contents of vessels exposed to a range of fire scenarios. The code allows for fluid loss though a PRV whenever the set pressure of the valve is exceeded and it can also be set up to model vessels with PFP coatings. By calculating the thermodynamic properties of the fluid remaining inside the tank, at the point of catastrophic tank failure, HEATUP effectively determines the source terms essential to evaluating the hazards associated with the resulting BLEVE. The tank pressure, liquid fill level, fluid and wall temperatures and fluid enthalpy in the liquid and vapour zones are all predicted up to the point of vessel failure.

There are many different physical processes occurring when a flame interacts with a vessel containing liquefied gases due to the complex behavior of the flame, the vessel and the vessel contents. The important processes occurring during jet-fire impingement on vessels containing liquefied gas include:

1. Heat transfer between the fire and outer surface of the vessel, in the vapor and liquid 'zones', by radiation and convection.
2. Heat transfer through the vessel walls by conduction. The wall may comprise of an outer passive fire protection (PFP) coating plus the underlying steel wall.
3. Heat transfer into the vessel fluids by predominantly radiation in the vapor space, and by natural convection or nucleate boiling in the liquid phase. Mass transfer from the bulk liquid or vapour to the outside environment through any holes in the vessel.
4. Mass transfer out of the vessel through any open or partially open pressure relief valves (PRVs).
5. Mass transfer within the liquid phase by flow of heated fluid into a stratified 'hot' layer lying above the bulk liquid. The hot layer may or may not be stable.
6. Mass transfer between the liquid and vapor phases by evaporation.
7. Pressure, enthalpy and liquefied gas composition changes in the fluid during each of the above processes.
8. Catastrophic vessel failure resulting in a possible BLEVE.