Combustion (fire) and ignition

Fire is one of the major accident hazards and effects from flammable releases contribute to the risk from the process plant.

The three essential conditions for fire are fuel, oxygen and heat. The nature and presence of each of these decides the combustion process and the effects from it. Depending on the release phase (solid, liquid or gas), release velocity (flammable mixture available for combustion), and availability (and nature) of source of ignition the chemical reaction could result in different forms. The fire may be classified as jet flames, liquid fires (pool and running/drain), vapour cloud fires (with or without explosion), solid fires (solid materials, dust), warehouse fires and fires associated with oxygen.

Some potential ignition sources on process plants include naked flames, hot surfaces, hot particles, hot work, chemical energy, engines, vehicles, lightning, smoking, hot materials, hot gases, reactive materials, Pyrophoric materials, radio frequency emission, electrical equipment, static electricity, self-heating, arson and sabotage (more information can be found in Table 16.24, FPLees Loss Prevention and Process Safety, 3rd Edition).

Most of the issues related to explosion prevention and protection are addressed by ATEX Directive [http://ec.europa.eu/enterprise/atex/dir92-en.pdf]
In the UK, the requirements are as per DSEAR [http://www.hse.gov.uk/fireandexplosion/atex.htm]


Estimation of risk and probability of ignition:

Individual risk from an event resulting in fire (flammable effects) can be determined by
Individual risk = failure rate x ignition probability x fractional time of attendance x vulnerability
Where,
failure rate is the estimated frequency (per year) of the event that gives rise to the release scenario.
Ignition probability is the The probability that the release will ignite given that the failure has occurred (normally based on the release rate, see table below)
Fractional time of attendance is the maximum proportion of an individual’s time that is spent inside the building under consideration. This is fixed throughout representing one person for 40 hours a week.

A guide on selection of ignition probability for gaseous release is given in the following table.

IGNITION PROBABILITIES FOR GASEOUS RELEASES
Leak (kg/s) Probability of ignition
Gaseous release
Minor (<1) : 0.01
Major (1-50) : 0.07
Massive (>50) : 0.3

Liquid release
Minor (<1) 0.01
Major (1-50) 0.03
Massive (>50) 0.08

Courtesy: Cox, A.W., Lees, F.P. & Ang, M.L.; Classification of Hazardous Locations; IIGCHL, IChemE.; 1990

More information on ignition probability, is available in the IP research report., Ignition probability review, model development and look-up correlations, Published by Energy Institute, London, Jan 2006

Note: This is a small note on a factor that need to be considered while quantifying risk in process industry.