Pool fires on the installation

The following information is provided for pool fires on an installation:

• The expected flame extent, so that items or personnel within that range can be identified and the consequences of flame engulfment considered.

• The mass burning, so that the duration of a fire following a spillage might be assessed and to provide input to calculations of the incident radiation field.

• The Fraction of Heat Radiated, F, so that estimates of the far field incident radiation hazard can be made using Equation 5-3 in Section 5.3.1, where the rate of fuel combustion, m& , is taken as the mass burning rate times the area of the pool.

• The CO level and soot concentration in the smoke produced.

• The total heat flux to an engulfed object together with the radiative and convective components, so that calculations of the object heat-up can be performed (see Section 5.5). Note that these fluxes represent the initial values when the engulfed object is cold. Values of typical flame temperature, emissivity and convective heat transfer coefficients are also provided.

• The effect of deluge in terms of the reduction in the heat flux to engulfed objects and the enhanced attenuation of incident radiation to the surroundings using the Effective Fraction of Heat Radiated, F’ (see Equation 5-4).

• The effect of confinement on fire characteristics and the combined effect of confinement and deluge.

This information is presented in Table 5.3.

Table 5.3 Pool fires on the installation

Twice pool diameter Up to twice pool diameter

Pool fire

Considerable fire control and potential extinguishment can be achieved. Expect a reduction in flame coverage (and hence flame size) of up to 90 % within 10minutes.

Rapid extinguishment with AFFF.

Up to 50 % reduction in radiative heat flux to engulfed objects.

In far field take F’ = 0.8F for 1 row of water sprays, F’=0.7F for 2 rows and F’=0.4F for

>2 rows at 12 l min^-1m^-2

The following information is provided in Table 5.4 for hydrocarbon pool fires on the sea:

• The expected flame extent, so that items within that range can be identified and the consequences of flame engulfment considered.

• The mass burning, so that the duration of a fire following a spillage might be assessed and to provide input to calculations of the incident radiation field.

• The Fraction of Heat Radiated, F, so that calculations of the far field incident radiation hazard can be made using Equation 5-3, where the rate of fuel combustion, m& , is taken as the mass burning rate times the area of the pool.

• The CO level and soot concentration in the smoke produced.

• The total heat flux to an engulfed object together with the radiative and convective components, so that calculations of the object heat-up can be performed (see Section 5.5 “Heat transfer”). Values of typical flame temperature, emissivity and convective heat transfer coefficients are also provided.

The gas outflow from a sub-sea pipeline will depend on the pressure and the pipeline size.

The release will also vary with time; this variation depending upon the length of pipeline which is depressurising. Similarly, the area at the sea surface over which the gas emerges will depend on the depth and the gas release rate. Furthermore, depending on the gas outflow and the depth, the gas plume at the sea surface may not be within flammable limits. For these reasons, simplified guidance cannot be readily provided and the use of a model is recommended. This topic is an area of some uncertainty and model predictions vary considerably. For illustrative purposes, predictions of the fire hazard following the rupture of a long 24” diameter natural gas pipeline operating at 100 barg at a depth of 50 m suggest that the fire diameter might be of the order of 100 m with a flame length of 150-200 m. On the basis that the fire is a low velocity laminar flame, it can be regarded as a large pool fire and the

Table 5.4 - Hydrocarbon pool fire on the sea

Pool fire on sea parameters Value

Typical Pool Diameter (m) >10

Flame Length (m) Up to twice diameter Mass Burning Rate (kg m 2 s 1) Crude – 0.045-0.060

Diesel – 0.055 Kerosene – 0.060 Condensate – 0.100

C3/C4s – 0.200 Fraction of Heat Radiated, F 0.12 CO level (% v/v) and Smoke

Concentration (g m^-3)

CO < 0.5 Soot 0.5 – 2.5

Total Heat Flux (kWm^-2) 250

Radiative Flux (kWm^-2) 230

Convective Flux (kWm^-2) 20

Flame Temperature (K) 1460

Flame Emissivity, 0.90

Convective Heat Transfer Coefficient, h (kWm^-2K^-1)

0.02

5.4.2.5 BLEVEs

BLEVEs are highly transient events in which a fixed inventory is instantaneously released. The subsequent combustion gives rise to a fireball which grows in size to a maximum before burning out as all the fuel is consumed. Consequently, the key parameters of interest in terms of a consequence assessment are the extent of the flame and the incident radiation hazard to personnel outside the flame. These parameters are also highly transient. In relation to incident radiation levels outside the fireball, both the maximum level experienced and the ‘dosage’ over the duration of the event are of interest in order to determine the effect on people.

Consequently, Table 5.5 presents the following data in relation to BLEVEs:

• Typical maximum fireball diameter (assuming unconfined) based on the mass of fuel involved in the BLEVE, and the maximum flame volume calculated assuming a spherical geometry. Hence the area of a module which would be expected to be engulfed in flame can be assessed by dividing the volume of the fireball by the height of the module.

• The expected duration as a function of the mass of fuel involved in the BLEVE.

• The Modified Fraction of Heat Radiated F*, which can be used to calculate the maximum incident radiation received at a location d, remote from the fireball (more than one fireball diameter distant from edge of fireball) using the equation:

-2

2 kWm

,max 4

*

d

F M H

q d t

τ

= π ... Equation 5-11 where: t is the duration of the BLEVE event (s)

τ is the atmospheric transmissivity

M is the mass of fuel involved in the BLEVE (kg) H is the calorific value (kJ kg^-1)

d is the distance from the centre of the fireball where the dosage is experienced (m)

Various correlations have been developed relating the maximum diameter (D), maximum height (h) and duration (t) of the fireball following an unconfined BLEVE to the mass of fuel released, for example CCPS Guidelines for Chemical Process Quantitative Risk Analysis [5.1]

suggests that:

D = 6.48 x M^0.325 h = 0.75 x D t = 0.825 x M^0.26

These equations have been used to derive the values presented in Table 5.5 for maximum diameter and duration. Comparisons with large scale data showed reasonable agreement.

Table 5.5 - BLEVEs

Parameter Characteristic expressed as function of BLEVE fuel mass (kg of fuel)

Maximum Diameter (m) D = 6.48 x M^0.325

Maximum Flame Volume (m³) V = 142.47 x M^0.975

Duration (s) t = 0.825 x M^0.26

Modified Fraction of Heat Radiated, F*

0.35

(ONLY for use in Equation 5-11 to derive maximum incident radiation at a locations remote

from the fireball)