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.

Gas dispersion

Gas dispersion (or vapor dispersion) is used to determine the consequences of a release of a toxic or flammable material. Typically, the calculations provide an estimate of the area affected and the average vapor concentrations expected. In order to make this determination, one must know the release rate of the gas (or the total quantity released) and the atmospheric conditions (wind speed, time of day, cloud cover).

The steps required to utilize a gas dispersion model are:
1. Identify the scenario. What can go wrong to result in the loss of containment of the material?
2. Develop an appropriate source model to calculate the release rate or total quantity released based on the specified
3. Use an appropriate gas dispersion model to estimate the consequences.
4. Determine if the resulting consequence is acceptable. If not, then something must be changed to reduce the consequence

Calculations and experiments have demonstrated that even the release of a small quantity of toxic or flammable material can have a significant consequence. Thus, it is clear that the best procedure is to prevent the release in the first place. However, release mitigation must be a part of any process safety program. Release mitigation involves: (1) detecting the release as early as possible, (2) stopping the release as quickly as possible, and (3) invoking a mitigation/emergency response procedure to reduce the consequences of the release.

Parameters Affecting Gas Dispersion

A wide variety of parameters affect the dispersion of gases. These include: (1) wind speed, (2) atmospheric stability, (3) local terrain characteristics, (4) height of the release above the ground, (5) release geometry, i.e. from a point, line, or area source, (6) momentum of the material released, and (7) buoyancy of the material released.

As the wind speed is increased, the material is carried downwind faster, but the material is also diluted faster by a larger quantity of air. Atmospheric stability depends on the wind speed, the time of day, and the solar energy input. During the day, the air temperature is at a maximum at the ground surface as a result of radiative heating of the ground from the sun. At night, radiative cooling of the ground occurs, resulting in an air temperature which is low at ground level, increases with height until a maximum is reached, and then decreases with further height. Terrain characteristics affect the mechanical mixing of the air as it flows over the ground. Thus, the dispersion over a lake is different from the dispersion over a forest or a city of tall buildings.

Gaussian Dispersion

Gaussian dispersion is the most common method for estimating dispersion due to a release of vapor. The method applies only for neutrally buoyant clouds and provides an estimate of average downwind vapor concentrations. Since the concentrations predicted are time averages, it must be considered that local concentrations might be greater than this average; this result is important when estimating dispersion of highly toxic materials where local concentration fluctuations light have a significant impact on the consequences.

HAZARDS OF INERT GASES

The use of inert atmospheres should be considered to prevent fires and deflagrations when using flammable materials. However, inert atmospheres can be dangerous to personnel. One of the most important concerns in the use of an inert atmosphere is that it can kill if a person breathes it. The air we normally inhale contains about 21 percent O2, 79 percent N2, and small amounts of other components. Inhaling air containing less than about 16 percent oxygen causes dizziness, rapid heartbeat, and headache. One or two breaths of pure nitrogen and some other gases containing no oxygen can be lethal. Other gases of this type include methane, ethane, acetylene, carbon dioxide, nitrous oxide, hydrogen, argon, neon, helium, and some others. Oxygen in the lungs is washed out and replaced by gas containing no oxygen. Blood from the lungs receives insufficient oxygen and flows to the brain, where tissues rapidly become deficient. Within five seconds of inhaling only a few breaths of oxygen-free gas, there can be mental failure and coma. Symptoms or warnings are generally absent. Death follows in two to four minutes. However, a coma due to lack of oxygen is not always fatal. Cardiopulmonary resuscitation techniques should be used on persons who are not breathing due to lack of oxygen (Ventilation for Acceptable Indoor Air Quality, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, Ga.; Zabetakis, Flammability Characteristics of Combustible Gases and Vapors, Bulletin 627, Bureau of Mines, 1965).

Gases which act as simple asphyxiants, such as nitrogen and helium, merely displace oxygen in the atmosphere so that the concentration falls below that needed to maintain consciousness. There are also chemical asphyxiants, such as carbon monoxide, hydrogen sulfide, and hydrogen cyanide, which have a specific blocking action and prevent a sufficient supply of oxygen from reaching the body. Most deaths due to short-term gassing are caused by carbon monoxide (Lees, Loss Prevention in the Process Industries, Butterworths, London, 1980, p. 646).

Effects of Low Oxygen Levels
There are many factors which can affect the ability of human beings to adjust to lower oxygen levels. For example, two men were accidentally exposed to a low oxygen level in a vessel. One of them died, and one survived without permanent injury. The one who died had been in poorer general health and it is believed that this factor may have made the low oxygen level fatal for him, while the other person, who was in good health, survived. It is well known that people accustomed to living near sea level can take several days to adjust fully to the lower amount of oxygen available in mountainous regions such as Denver, Colorado. Anyone who has traveled to the top of Pike’s Peak knows how the altitude can make one tired, lethargic, and even sick. People react differently, however, and one cannot generalize as to exactly how a person will react to lower oxygen levels and higher altitudes.

Minimum Oxygen Limits
Oxygen limits are set at 19.5 percent minimum as recommended by OSHA and the American Standards Institute. Michigan has adopted these guidelines as well and has defined grade D air for O2 to be 19.5 percent to 23.5 percent as an obligation to the employee by their employer. The Ontario Ministry of Labour designates enclosures containing less than 18 percent O2 as hazardous.

Inerting Monomer Storage Tanks with Nitrogen
It is good practice to keep the vapor space of flammable liquids out of the flammable range. Monomers that can potentially polymerize require special consideration. The vapor space above some monomers, such as styrene and methyl acrylate, should be kept below about 10 percent oxygen in warm weather to be below the flammable range. For many of these monomers, a small amount of oxygen is required to maintain the activity of the inhibitor and to avoid polymerization in storage tanks, which could lead to overheating and explosions and fire. An oxygen concentration of 5 percent in the vapor space is recommended as a safety factor to stay out of the flammable range and maintain inhibitor activity. Maintaining an inert atmosphere for these applications can be difficult, since usually nitrogen is available as a high-purity gas, and it is necessary to add a small amount of oxygen (usually air) to the nitrogen to achieve the desired oxygen concentration. Mixing air and nitrogen has not proven to be a reliable method of maintaining the proper inert pad in the past. This is because instrument failure has caused high nitrogen concentration, which in turn has caused storage vessels to polymerize. One alternative to consider is the use of membrane systems, such as those sold by Generon Systems and other suppliers. This system can produce 95/5 percent nitrogen/oxygen for inerting, using plant compressed air available at 65 psig (449 kPa gauge). This system has an inherently stable output when operating at a specific pressure drop because the pressure drop across the membrane module sets the nitrogen purity.

Nitrogen is often the preferred gas for providing an inert atmosphere. In general, most organic combustible compounds will not propagate flame if oxygen in the mixtures of the
organic vapor, inert gas, and air is below about 10 percent and 13 percent, with nitrogen and carbon dioxide, respectively, as the inert gases. With carbon dioxide, the minimum oxygen concentration is higher than with nitrogen because carbon dioxide has a higher specific heat. Carbon dioxide is fairly soluble in many liquids and will react with alkaline
materials, so its use as an inerting material is limited. Heavy gases such as carbon dioxide provide superior inerting of vent stacks to prevent air entry. Water vapor is a good inerting gas if the temperature is high enough (above about 80 to 85°C [176 to 185°F]).

The use of an inert atmosphere can virtually eliminate the possibility of explosions and fire with flammable materials. However, inerting systems can be quite expensive and difficult to operate successfully and can be hazardous to personnel. Before using inert systems, alternatives should be explored, such as using nonflammable materials or operating below the flammable range.

Source: Perry 1999

Radiation Heat Flux from Pool Fires

The radiation from the hot gases and incandescent soot deep within the flame passes through the visible "surface" to external objects. The amount of heat emitted varies with the distance over which emission occurs and the concentration and type of emitting species within that path. As the thermal radiation passes through the atmosphere outside the flame it is attenuated by absorption of energy in the infrared wavelengths corresponding to the absorption bands of the atmospheric gases (principally carbon dioxide and water vapour). The attenuating effect on radiation is significant even over path lengths of a few tens of metres. Water sprays, mists and smoke can also strongly attenuate radiation.

The radiative heat emission process is modelled by assuming that the radiation comes from the visible surface. The surface emissive power, SEP, of a flame is the heat radiated outwards per unit surface area of the flame. Thus the use of SEPs is a two-dimensional simplification of a very complex three-dimensional heat radiation problem. These models can be used for reliable prediction of external radiative heat fluxes to within about a flame length of the fire. They cannot be used for near impingement conditions however.

The model average SEP depends on the fuel type and on the pool diameter. A uniform SEP is used over the whole of the sides of the model flame shape. This results in underprediction of the radiative heat flux near the base of large pool fires because the model fails to take account of the small bright, highly emissive region at the base of the flame. The following formulae are used in Shell FRED. For land-based LNG pool fires (Shell FRED 2004):

The physical effects modelled by this equation are the increase in SEP with increasing emitting path length, given by the first exponential term:

And the appearance and increase in the amount of dark obscuring smoke on the outside of the flame, given by the second exponential term:

For LPG and propane the SEP is given by:

The second term is only included if the pool diameter exceeds 18m. For butane, gasoline and kerosene, the SEP is given by:

Above figure shows the variation of SEP with pool diameter. There is a peak SEP for all three classes of fuels, representing a fire when the emission from bright areas of flame is at a maximum, but before significant amounts of smoke have appeared outside the flame. The peak occurs earlier for heavier hydrocarbons because the flames contain higher soot concentrations.

Above figure shows variation of model average SEP with pool diameter for different fuels!