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!

Pool Fire

Gasoline was distributed by a transmission pipeline to the next transmission station and then carried away to the Public Gas Station by a tanker truck. Since the distribution process by a pipeline is very vulnerable, so a leak may happen and then the gasoline will spilled out, and the consequence is it will form a pool that have a potential to become a Pool fire.


Other Fire Modelling Packages

Several specialist fire packages have been recently developed:
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1. CHIC - compartment fires
CHIC (Combustion Hazards In Compartments18) is a time dependent model which predicts the physical effects arising from the confinement of a jet or pool fire in a compartment, e.g. an offshore module. Internal heat fluxes, smoke layer thickness and temperature, CO and soot concentration and the extent, if any, of external flaming are predicted. The model has been validated by experimental work using propane, diesel and gas condensate as fuel.

2. SEAFIRE - fires on the sea
SEAFIRE predicts the behaviour of a subsea release of product at sea, from e.g. a broken pipeline. The release may occur above/on or below the sea surface and for the latter a new model predicts the behaviour of the rising bubble plume and subsequent surface spreading due to wind, waves and gravitational effects. Release products may be mixtures of gas and oil. The fire characteristics of the burning pool are based on experiment. Run time graphics show the development of fire with time.

3. CLOUDF - cloud or flash fires
Cloud or flash fires are transient in nature and are the product of the delayed ignition of a dispersing cloud in an unconfined environment. Two types of cloud fire are modelled19 with radiation dose predictions being supplied at user specified locations. The first is the delayed ignition of the cloud formed from a vertical release. This results in a fireball dying back to a steady state jet flame from the source. The second is the delayed ignition of the cloud formed from a horizontal negatively-buoyant release. Such a flame, when not accelerated by obstacles, will travel horizontally through the cloud at relatively low speeds (less than 20 m/s). Because of the short duration of the fire, radiation effects are generally not significant outside the burning cloud.

Unconfined Vapor Cloud Explosions

Article below is continuing of previous article:

Unconfined vapor cloud explosions (also known as vapor cloud explosions) in open air often result when accidental releases of vapors or gases to the atmosphere are ignited. In addition, some tests have been so-called meteorological area sources, while the dispersion equations are generally meteorological point sources. (Only concentrations relatively close to the location of discharge of the vapors will be affected by this difference in sources.) Also, the momentary concentration of a combustible gas or flammable vapor is the important duration of a concentration for UVCEs; not all dispersion models specify their averaging time of concentrations. Thus, predictions of concentrations must be treated as estimates.


Pressure Development
Overpressure in a UVCE results from turbulence that promotes a sudden release of energy. Tests in the open without obstacles or confining structures do not produce damaging overpressure. Nevertheless, combustion in a vapor cloud within a partially confined space or around turbulence-producing obstacles may generate damaging overpressure. Also, turbulence in a jet release, such as may occur with compressed natural gas discharged from a ruptured pipeline, may result in blast pressure.

Example
The combustion process in large vapor clouds is not known completely and studies are in progress to improve understanding of this important subject. Special study is usually needed to assess the hazard of a large vapor release or to investigate a UVCE. The TNT
equivalent method is used in this example; other methods have been proposed. Whatever the method used for dispersion and pressure development, a check should be made to determine if any governmental unit requires a specific type of analysis

Prevention and Protection
It is difficult to cope with a potential UVCE once an accidental release has occurred. Consequently, the best procedure to guard against a UVCE is to prevent the release in the first place. Safe piping is essential to protect against UVCEs. Forty percent of all major plant losses are due to piping failures, and corrosion is one of the largest single causes of plant and equipment breakdown. Moreover, mistakenly open valves that caused mammoth emissions of hydrocarbons have resulted in two major UVCEs with a total of 29 deaths in those two instances. Thus, close scrutiny regarding piping and valves is mandatory to help prevent UVCEs. Some other protection methods are summarized as follows:

1. Remotely Operated Shutoff Valves
These should be considered for supply lines and other vulnerable pipelines. Excess flow valves that close when flow exceeds a set amount are possible substitutes, but they are not acceptable to some operators.

2. Flammable Vapor Detectors
These should be installed to warn of leaks, although such devices do not effectively control UVCEs with sudden, massive releases.

3. Elevated or Remote Air Intakes
Elevated or remote air intakes for control rooms will help in reducing ingress of dense, flammable vapors into those rooms. Ordinarily, elevating the tip of the air intake duct 9 m (30 ft) above the ground is sufficient. Installing flammable vapor detectors in the air intake ducts provides additional protection. Controls that automatically stop air to control rooms if vapor concentrations reach 25 percent of their LFL should also be considered.

4. Intentional Ignition
Intentional ignition to ignite a vapor cloud early before it spreads out to a large volume has been used or considered only rarely. Such ignition should not be employed for control of UVCEs solely without thorough study of the ramifications of its use. (In some infrequent cases when the gas is both flammable and particularly toxic, intentional ignition may be warranted.)

5. Large Fans
These could be used to dilute a vapor cloud below its LFL with ambient air (see, for example, Whiting and Shaffer, “Feasibility Study of Hazardous Vapor Amelioration Techniques,” Proc. 1978 Nat. Conf. on Control of Hazardous Material Spills, USEPA, Miami Beach, April 1978). But caution must be exercised because the turbulence produced by fans will likely promote rapid combustion and a resulting UVCE unless vapors are diluted below the LFL. Nevertheless, in new plants, strategic placement of air coolers may provide enough air flow to reduce the risk of a UVCE.

6. Water Sprays and Steam Curtains
These have been used and/or advocated to help protect against a UVCE. Such devices entrain air to dilute a vapor cloud. Also, some claim that water curtains form a physical barrier to stop the flow of the vapor cloud. As with large fans, vapors need to be reduced below LFLs to decrease the possibility of UVCEs from enhanced turbulence by the sprays or curtains. Moodie studied water spray barriers using carbon dioxide to approximate a heavy, flammable vapor cloud Release of a pressurized, liquefied gas to the atmosphere will cause the gas to cool and condense water vapor in ambient air, forming a visible vapor cloud. Firefighters and operators who attempt to move such a cloud away from furnaces and the like with fire hoses and water jet guns are at risk, because of the possibility of a UVCE near them. Plants and governmental agencies who recommend such practices need to reexamine their policies.

7. Structures
Structures that include partially confined spaces and turbulence-producing obstacles such as pipe bridges plus closely packed equipment, promote UVCE. This undesirable architecture-relative to UVCE, is often a product of congestion on a plant. Congestion is an enemy of safety. Thus, the probability of a UVCE, plus property losses and casualties, will likely be greater at a congested plant than at an uncontested site. Vapor cloud explosions also occur indoors when large amounts of flammable vapors are discharged accidentally into buildings.

8. Strong Buildings
Strong buildings may be prudent where people congregate, such as control rooms. For new plants, serious consideration must be given to stronger design of buildings that are vulnerable to a UVCE, compared to past designs.