Safety Report Assessment Guide: Chemical warehouses - Hazards

6 Thermal Radiation from a burning warehouse

A warehouse fire, which is not burning fiercely and is covered by the roof does not radiate large quantities of heat to the surrounding area. The smoke it emits may be hazardous to members of the public at some considerable distance from the site, but the danger of escalation is limited if the local fire fighting services are able to confine the blaze to one building. A raging fire on the other hand has only a minor effect on members of the public, but may radiate sufficient heat to damage adjacent buildings. Of particular concern are nearby: -

• wooden buildings
• diesel storage tanks
• LPG storage tanks
• flammable liquid storage tanks (adjacent site)
• flammable liquid/gas pipelines
• residents houses

Predictions of thermal radiation emitted by a warehouse fire are necessarily imprecise because the burning behaviour of the fire is uncertain and the rate of release of heat varies temporally and spatially. The controlling factors governing the size and intensity of the flames are:-

• The state of the roof (has it collapsed)
• The number of failed roof lights
• Whether or not the doors are open
• The effect of sprinklers
• The strategy adopted by the fire service

Wooden pallets on steel racking, which burn relatively slowly compared to cardboard packaging material and plastic, can provide a "structure" for the fire and enhance the burning rate by maximising oxygen availability. If the fire is not controlled by the sprinkler system, it quickly becomes very hot with flames leaping to a height of several tens of metres. After collapse of the pallets and failure of many plastic bottles containing flammable liquids, it resembles a pool fire with debris dispersed in it. Under these circumstances a rough estimate of the height of the flame pillar can be obtained using the Thomas correlation, assuming that the effective diameter is equal to the shortest side of the warehouse.

The flame length is weakly dependent on the burning rate of the pool, which cannot be accurately determined for mixtures. An approximate burning rate using data for the pure flammable liquids that make up the bulk of flammable pesticides stored in the warehouse can be derived as follows.

The burning rate for a pure substance is given by:-

where

mdot = burning rate (kg/s.m²).
h_c = heat of combustion (MJ/kg).
h_v = heat of vaporization (MJ/kg).
C_p = specific heat (MJ/kg.K).
T_b = boiling point (K).
T_a = ambient temperature (K).

For a mixture of solvents the pool burning rate is: -

where

M_T = total average burning rate (kg/s.m²).
V_s1 = volume of flammable liquid 1 in the warehouse
V_T = total volume of flammable liquid in the warehouse.
m_s1 = mass burning rate of flammable liquid (kg/s.m²).

If the warehouse does not contain appreciable quantities of flammable liquid, a stack mass burning rate for the Thomas correlation can be estimated as follows: -

where

skM_T = effective stack mass burning rate (kg/s).
DH_c = average heat of combustion of stack (MJ/kg).
DH_i = heat of combustion of i'th substance in stack in (MJ/kg).
h_i = height of i'th substance in stack.
Q = heat generation of i'th substance in stack (MW/m.m²).

Where part of a stack contains two or more substances for which a net heat release rate is not available, a simple average value is used.

Configuration factors are notoriously difficult to calculate for most practical situations, but in the case of a tilted flame pillar a standard textbook expression may be appropriate. Alternatively the flame can be modelled as a point radiator that radiates of the order of 30% of the total heat released by combustion.