- (a) Describe, with the aid of a sketch, how a pressurised system sprinkler head is constructed.
(5)
(b) Explain the operation of the sprinkler head and bulb assembly described in part (a), stating how the alarm is initiated.
(5)
(a) Construction of a Pressurised System Sprinkler Head (5 marks)
Main parts:
- Body/Frame – metal casting connected to the pressurised water supply pipe.
- Glass bulb (or fusible element) – a small heat-sensitive bulb filled with coloured liquid (colour indicates rated temperature, e.g. red = 68 °C).
- Sealing cap/valve seat – pressed against the water inlet to prevent flow; held in place by the glass bulb.
- Deflector plate – mounted at the outlet; designed to break up the water stream into fine spray droplets for maximum fire coverage.
- Bulb support arms – frame arms holding the bulb in compression, maintaining the seal.
Sketch (exam-style):
- Show pipe connection → sprinkler body → glass bulb across frame → sealing cap at inlet → deflector plate at outlet.
- Label each component.
(b) Operation of Sprinkler Head and Bulb Assembly & Alarm Initiation (5 marks)
Operation:
- The sprinkler system is kept under pressure.
- When fire raises the surrounding air temperature to the rated level:
- The glass bulb liquid expands and the bulb bursts (or fusible link melts).
- The sealing cap is released, allowing pressurised water to flow out.
- Water strikes the deflector plate, creating a fine spray that suppresses the fire.
Alarm initiation:
- The sudden drop in system pressure (caused by water discharge from the opened head) triggers a flow/pressure switch in the sprinkler system.
- This switch activates the fire alarm on the control panel, alerting crew to the sprinkler discharge and location.