Question 4 of 7: Fire Detection and Suppression

Question 4: Fire Detection and Suppression (20 marks: 6/7/7)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(i) Fire Detectors

• (a) Heat detectors — respond to temperature. Fixed-temperature types trigger at a set temperature; rate-of-rise types trigger on a rapid temperature increase. Reliable with few false alarms, but slower (respond only once heat builds), so suited to dirty/dusty areas where smoke detectors would false-alarm.
• (b) Smoke detectors — respond to combustion products/smoke: ionization type (sensitive to fast, flaming fires) and photoelectric type (sensitive to smouldering, smoky fires). Faster warning than heat detectors (detect fire in its early smouldering stage), but more prone to false alarms in dusty/steamy environments.

(ii) Sprinkler Systems

• (a) Wet-pipe — pipes always contain water; when a sprinkler head opens (heat-activated), water discharges immediately. Simplest, most reliable, fastest; used where freezing is not a risk.
• (b) Dry-pipe — pipes hold pressurized air; when a head opens, air escapes and a valve admits water. Used in unheated/freezing areas; slight delay before water arrives.
• (c) Deluge — all heads are open; a separate detection system opens the deluge valve and water flows from every head at once. Used for high-hazard areas needing rapid, large-area water application (e.g., flammable-liquid areas).

(iii) Non-Water (Special-Agent) Suppression Systems

• Used where water would be ineffective or damaging (electrical/electronic equipment, flammable liquids, computer rooms). They extinguish by removing one leg of the fire tetrahedron (oxygen, heat, fuel, chain reaction).
• Clean agents / gaseous (e.g., inert gas or halocarbon agents) — displace oxygen or interrupt the chemical reaction; leave no residue, safe for electronics.
• Carbon dioxide (CO₂) — smothers by displacing oxygen (hazard: asphyxiation—person safety precautions needed).
• Dry chemical — powders that interrupt the combustion reaction (Class B/C).
• Foam — blankets flammable-liquid surfaces to exclude oxygen and suppress vapours.
• Selection depends on the fire class, the protected contents, and occupant safety.