Heat detector: Fusible Loop System

Heat detector: Fusible Loop System

In addition to the flame, fire also produces smoke and heat. So a heat detector can be used to detect a fire. Compared to a flame detector and a smoke detector, a heat detector is slow in detecting a fire because it only activates when the fire produces enough heat for the heat detector to respond to. However, a heat detector can provide a useful backup or alternative to flame and smoke detectors. One way to sense heat is to use a fusible loop system.

The fusible loop system consists of the following parts:

- A fusible plug that melts at a set temperature

- Pipe network

- Charging panel with fusible loop

During startup, the fusible loop fills to allow air from the instrument air system to pressurize the loop. During normal operation, the pressure remains constant. If a fire occurs and the ambient temperature rises, the fusible plug will melt. The air in the loop is released and the pressure inside the tubing is reduced. A loss of loop pressure will be detected by a pressure transmitter in the fusing loop charging panel and signaled to the control room. Another action in the control system can be fire alarm activation, ESD activation, flood valve opening, etc.

The fusible plug is a non-resettable type. Restoring the system to normal after activation will require replacement of parts, i.e. fusible plugs.

Another disadvantage of the fusible loop is that the plug can be melted by a heat source other than fire, such as hot surfaces, motors, etc.