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Fire Effects

Background

Direct effects of fire on personnel are generally regarded as fatal. The predominant, non-direct effect of thermal radiation can vary greatly depending on factors such as the heat output of the source, the distance of the personnel from the source, the duration of exposure and the atmospheric conditions. The effect of fire on personnel is an important factor in the design and operation of process plant, as it impacts greatly on the outcome of safety case assessments.

If a pressurised vessel is attacked by fire, its temperature rises and this reduces the strength of the vessel. This, combined with the pressure within the vessel, may lead to vessel failure within a short time (minutes) with catastrophic consequences.

Strategy objectives

Current knowledge of fire effects

Effect on people

In general, the direct contact of personnel with fire is relatively simple to determine. It is much more difficult to assess the non-direct effect of fires on personnel

Smoke and toxic gases

Carbon dioxide is toxic above 5% concentration and causes hyperventilation above 2% where the subject may inhale large quantities of other toxic components contained in smoke. Hydrogen cyanide will incapacitate a subject within minutes and sulphur dioxide and nitrous oxide have similar effects. Virtually all hydrocarbons will generate smoke. Dense smoke production will obscure escape routes. Smoke inhalation may cause death some hours after exposure.

Hot gases and hot objects

Objects with temperatures above 45°C may cause pain if in contact with skin for more than 10s and those with temperatures above 100°C will cause burns within seconds. Convected hot air or hot gases above 120°C will result in skin pain after 10 minutes. Below this temperature cooling by sweating is possible, giving longer endurance times.

Thermal radiation

Physiological effects of thermal radiation may involve voluntary exposure over relatively long times (e.g. many minutes). The effects typically include high pulse rates, increased and laboured respiration, increased sweating and increased body temperature. The effects may be increased with increasing temperature, up to the point where pain/injury occurs.

At skin temperatures above 44°C, pain is felt and injury continues whilst the temperature remains above this point. The rate of injury increases by a factor of 3 for every degree above 44°C, such that at 50°C, the injury rate is ~100 times that at 44°C. In addition, for heat fluxes greater than 12.5 kW m-2, 33% of the final burn occurs during cooling.

The extent of injury is often related to the thermal dose, which may involve a high heat flux over a short duration. Thermal radiation is a particular hazard in the event of fire balls or where escape routes are blocked by fire.

Process plant

Heat loads from fire attack, implicit in the current guidance (i.e. API 521), are much lower than can be expected in severe fires (up to 350 kW m-2), that may occur on offshore installations and thus the process blowdown system may not guarantee vessel protection.

Areas of uncertainty

In general, there is insufficient information available on the failure modes, times and conditions for offshore vessels. The main area of concern relates to the effects of fires on pressurised systems and improved guidance, which identifies the key parameters that should be used in design, is needed.

The effectiveness of safety measures such as water deluge, passive fire protection and blowdown need to be carefully evaluated particularly if they feature in an ALARP demonstration.

Industry practice in assessment of fire effects

Heat effects on personnel are evaluated using simple and pessimistic Rule Sets based on human response to 5, 12.5 and 37.5 kW m-2. Escape is assumed at 5 kW m-2 but fatalities within minutes assumed at 12.5 kW m-2 and instantaneous death at 37.5 kW m-2.

Fire attack on process plant is currently calculated based on the API 521 guidance that underestimates the heat load from pool and jet fires.

Time to failure determines the time available for evacuation and for shutdown or inventory dumping before escalation occurs.

Strategy development issues

Updated 2012-12-13