The traditional safety measure derived from the time of oil fired boiler houses is the use of a fusible link placed above the each boiler. The fusible link is intended to melt if it becomes overheated through a fire starting (normally through ignition of oil leaking outside the boiler). This causes an alarm to be raised and the fuel supply to be shut off. In a boiler house where gas is the major fuel used then it is no longer appropriate to use fusible links because the hazard is very different.
If a gas leak develops there is no heat to melt the fusible link. The gas will not ignite until the lower explosive limit (LEL) of the gas is reached. At this point when the gas is ignited by a suitable source an explosion will occur causing more serious consequences. The ignition source does not necessarily have to be a defined flame such as the boiler flame or the pilot light but sufficient energy can reside in hot surfaces or be produced through normal operation of unrelated electrical apparatus (light switch/fitting, switch panel, additional plant etc).
On dual fuel systems using oil or gas it is seen that both fusible link and gas detection are necessary safety precautions.
WHY GAS DETECTION?
The main purpose for a gas detection system is to raise an alarm when a leak develops. The gas detection system is reading gas levels in its immediate area and by monitoring from zero gas to the LEL allows alarm points to be activated before an explosive level of gas has built up.
As most boiler houses and plant rooms are unmanned or visited at irregular intervals this allows the environment to be monitored and for any potential leaks to be detected that could otherwise go undetected for a long time. A benefit of the detection system is its ability to provide an opportunity to deal with the leak before the level of gas has reached a dangerous level. Alarm levels allow relays to be activated and slam shut valves can operate isolating the leak from the gas source. This removes the continuation of the risk and as a further benefit will reduce fuel gas wastage and cost.