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Pool Diameter Dragged Diameter

θ

Flame Tilt

Parameters Defining Pool Fire Shape 13.5.1 Flame Dimensions

The physical dimensions of pool fires including flame tilt, dragged pool diameter and flame length are dependant of the properties of the material (mass burning rate, vapour density), and on environmental factors (wind speed, air temperature, humidity). Pool diameter is often based on physical constraints such as bund dimensions. Flame height is only constrained in particular scenarios such as tunnel fires. Numerous models are available based on experimental observations for a large range of materials and pool sizes. R2A use the following correlations available from the SFPE Handbook of Fire Protection Engineering (1995):

Flame Tilt: American Gas Association Dragged Diameter: Welker & Sliepcevich Flame Length: Thomas Equation

13.5.2 Surface Emissive Power

The surface emissive pool diameter and physical properties of the burning product. Experimental data indicates that larger pool fires have a lower surface emissive power, due in part to a loss in combustion efficiency in larger fires. Smoke and soot particles also reduce the surface emissive power of pool fires, with soot having a SEP of around 20kW/m2, and clean flame around 140kW/m2. Typical averaged SEPs are in the order of 25-90kW/m2.

13.6 Jet Flames

Jet fires can liberate large amounts of energy. According to Chamberlain a release rate of 100kg/s over a few seconds would produce a flame about 65m long in moderate winds and release some 5000MW of combustive power which is more than two and a half times the output of Loy Yang power station. The model developed by G.A. Chamberlain (1987), of Shell assumes that the surface of the flame can be treated as a frustum for the purpose of calculating the Surface Emissivity Power (SEP). The dimensions of the flame can be defined in terms of the flame lift-off, tilt, length, frustum length, base width & tip width:

Jet Flame Frustum 13.6.1 Release Rates

Gaseous release rates are calculated using an analytical solution assuming adiabatic flow of gas leaving an orifice. Different relationships are used if the flow is "choked" (critical) or "un-choked" (sub- critical). Under choked flow, the gas exits the pipeline at greater than atmospheric pressure, and continues to expand downstream of the release. For full bore ruptures, choked flow occurs when sonic velocity is achieved, which is the maximum possible velocity in the pipe.

The calculation used gives a good estimation for the release rate of a gas leaving an orifice, but as hole sizes approach the pipeline diameter the calculation begins to over predict the release rates. This makes the analysis somewhat conservative. The following graph shows how the release rate drops as a function of pipeline length for a 100mm diameter pipeline rupture at transmission pressure:

10 15 20 25 30

13.6.2 Surface Emissive Power

The net heat release rate of a flame, Q (kW) is simply the product of the heat of combustion (∆Hc) of the gas (kJ/kg), and the rate of gas release (kg/s). Jet flames have a much higher surface emissive power than pool fires, owing to the more efficient combustion as a result of turbulent gas flow.

The fraction of the total heat that is radiated is a function of the gas jet velocity (u), and can be determined from the following expression:

Fr=0.21 exp(-0.00323u) + 0.11 Typically the emissivities of jet flames are in the order of 100-400kW/m2.

13.7 Explosions

The energy released in an explosion is normally due to stored chemical energy, fluid expansion energy or vessel strain energy. For all explosion types, the energy released is equal to the work done by the expansion of gas from its initial to its final state:

W = − PdV

1 2

∫

The effects of an explosion are determined using a scaling law, and an equivalent number of tonnes of TNT (W). For a particular criterion, the scaled distance (z) is determined, which can then be used to find the actual distance (r) to the overpressure using the following formula:

r = zW

1 3

13.7.1 Scaled Distance

The scaling is a function of the overpressure, and is usually determined from a graph based on empirical studies. The following chart for vapour cloud explosions is based on the equation in "Major Industrial Hazards technical papers" from the Warren Centre, University of Sydney, sourced from the 2nd report UK Advisory Committee on Major Hazards:

0 100 200 300 400 500 600 700 800 900 0 10 20 30 40 50 60 70 OverpRessure (kPa) Scaled Distance

13.7.2 TNT Equivalence

The equivalent quantity of TNT is calculated based on a heat of combustion of 4600kJ/kg. For vapour cloud explosions, energy release is based on complete combustion of the explosive material.

In determining the equivalent mass of TNT, a yield factor is applied. Energy in the blast wave of an explosion is generally a small fraction of that theoretically available, with kinetic energy of shrapnel, potential energy in products, and residual energy in air also occurring. Typically, 1-10% of the available energy of an explosion is in the blast wave. The yield of the Flixborough explosion in which 30-40 metric tonnes of cyclohexane were released was estimated to be 4-5%.

13.7.3 Effects of Explosive Overpressure

The following table outlines the typical observable effects of explosive overpressures.

Explosion Overpressure Effect

3.5 kPa (0.5 psi) * 90% glass breakage.

* No fatality and very low probability of injury.

7 kPa (1 psi) * Damage to internal partitions and joinery can be repaired. * Probability of injury is 10%. No fatality.

14 kPa (2 psi) * House uninhabitable and badly cracked. 21 kPa (3 psi) * Reinforced structures distort.

* 20% chance of fatality to a person in a building.

35 kPa (5 psi) * 50% chance of fatality for a person in a building and 15% chance of fatality for a person in the open.

70 kPa (10 psi) * Threshold for lung damage.

* 100% chance of fatality for a person in a building or in the open. * Complete demolition of house.

Some Effects of Explosion Overpressure (after HIPAP No 4:1992) 13.8 Toxic Gas Clouds

Many calculation intensive computer programs exist to determine the toxic "footprint" as a function of time in the event of a release of a heavier than air toxic gas. Major factors affecting the impact of such releases are discussed below.

13.8.1 Release Type

The manner in which a material is released will have a large bearing on the toxic cloud footprint. Sudden releases of liquefied gas tend to result in result in a large initial cloud due to aerosol particles and flashing liquid, which will rapidly drop back to a steady state size. Continuous releases will take longer to achieve a maximum cloud size, which is often the same size as the steady state cloud formed by a sudden release. The steady state cloud size is limited by the rate of mass transport from the liquid pool. This is influenced by factors such as heat transfer from the ground, solar radiation levels, and the surface area of the pool (which can be limited by bunding).

For gaseous releases, high pressure causes forced mixing of air and gas, resulting in a long narrow plume. Lower pressure releases tend to be wider as natural dispersion is more influential. For an equivalent release rate, low pressure scenarios are likely to have more far reaching impacts. 13.8.2 Meteorological Data

D a y N i g h t

Wind Speed (m/s) Solar Radiation Cloud Cover Fraction S t r o n g M o d e r a t e S l i g h t <0.5 0 . 5 - 0 . 8 >0.8 <2 A A - B B F E D-E 2 - 3 A - B B-C C F E D-E 3 - 5 B B-C C E D D 5 - 6 C C-D D D D D >6 C D D D D D Atmospheric Stability 13.8.3 Surface Roughness

Effective surface roughness (in metres) characterises the ground conditions over which a plume will travel. Surface roughness generally varies between 0.005 and 1.5m, with the lower end representing a surface such as a spill over water, and the upper end forested or built up urban areas. Increased surface roughness reduces the impact area of toxic clouds.

13.8.4 Probit Relationships

Probit equations for toxic exposure take that same form as that for heat radiation exposure used by Eisenberg, Lynch and Breeding:

Y = A+ Bln(toxic load)

Toxic load or dose are interchangeable terms for the integration over time (t) of the concentration of a toxic substance (C), raised to a power termed the dose exponent (n).