Besides producing flame, fire also produces smoke and heat. Hence to detect the existence of fire, heat detector can be used. Compared to flame detector and smoke detector, heat detector is slow in detecting fire because it only activates once a fire has produced sufficient heat of which heat detector could respond. However, heat detector could provide useful back up or as alternative to flame and smoke detectors. One mean to sense heat is by utilizing Fusible Loop System.
The Fusible Loop System consists of the following:
- Fusible plug which melt at set temperature
- Tubing network
- Fusible loop charge panel
During start-up, fusible loop is charged to allow air from instrument air system pressurizing the loop. The pressure remains constant during normal operation. If there is fire and the surrounding temperature rises, the fusible plug will melt. The air in the loop relieves and subsequently the pressure inside the tubing reduces. Loss of pressure in the loop will be detected by pressure transmitter in the fusible loop charge panel and it will signal the control room. Next action in the control system could be activate fire alarm, trigger ESD, open deluge valve, etc.
Fusible Plug is non-resettable type. To restore the system into normal surveillance after activation will require replacement of parts i.e. the fusible plug.
The other disadvantage of fusible loop is that the plug could melt by other source of heat other than fire such as hot surfaces, motors, etc.