Why are fusible links not the answer for gas fired boiler room hazard monitoring?

The most common fuels in use for space and water heating are Oil, Natural Gas and Liquid Petroleum Gas (LPG). By far the most popular, clean and cost effective solutions are utilising Natural Gas or LPG.

The traditional safety measure derived from the time of oil fired boiler houses is the use of a fusible link placed above 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.

Methods of Gas Detection?

The low-cost sensing device for gas detection of flammable gas leakage is based on the semi conductor sensor.

The semi conductor detector can proficiently detect flammable gas however the major concern is its unfortunate trait that it will also detect many other gases and vapours within an atmosphere and will even provide false gas readings through variations in temperature and humidity.

The traditional low-cost sensing system is using catalytic gas detectors. These devices actually use a burning principle to detect gas and are thus immune from the sensitivity issues of semi conductors mentioned previously. The catalytic burning effect only responds to flammable gases and is a controlled reaction of what the detection system is there to prevent and consequently takes all atmospheric effects into account without further sophisticated electronics. The technique is quick and has been utilised for over 90 years. The robust nature of the sensor housing and its uncomplicated connection makes them very simple devices to install, commission and service.

The most expensive solution is to utilise Infra-red absorption as the method of detection which responds quickly and can be more sensitive than catalytic detectors. Internal within the detector is a beam of Infra-red radiation that passes from a source to a detector. The detector will show if there is any loss of energy between source and detector. The size of this loss is a measure of the amount of gas along the path of the beam.

This detection system can derive speed and accuracy benefits but unfortunately carries a high price level and can derive a more extensive and expensive maintenance regime.

Location of Sensors and Control Equipment

The first consideration is placing the control panel. Normally this is located outside of the area of detection allowing people to view the alarm situation without endangering themselves. This may be just outside the Boiler Room or in a normally manned area like a Reception Area or Security Lodge.

It is then dependant on the nature of the gas being consumed that determines the placement of gas detectors.

Natural gas (which is largely methane) is lighter than air and therefore upon release will predominately rise towards the highest point in the boiler room.

In a lot of cases boiler houses and plant room ceilings are a network of pipe work and ducts and it is common to place the detectors at a lower level over the leak source utilising a collecting cone to enhance gas gathering to the sensor.

LPG is heavier than air and therefore upon release will tend to fall to the floor and follow any sloped and drains in the surface. It is recommended that these detectors are placed 6-18″ (15 to 45cm) from floor level and a splashguard is utilised to prevent accidental water ingress.

The most likely points in the gas installation for leaks to occur are:

  • The gas meter
  • The mechanical shut off valve (if fitted)
  • The solenoid shut-off valve
  • The pressure booster if fitted
  • The gas train
  • The gas burner

As a general rule it is common to utilise a 4 meter radius of detection for gas detectors. In a small boiler room all the above points can be close enough that only one sensor will be needed. On installations of one or more large boilers as in hospitals, factories or large blocks of flats it may be necessary to fit one sensor at each of the above points.

System Outputs

Sensors are usually wired individually to a control panel, which will give a visual indication that preset alarm levels have been reached.

On installations with a number of sensors it may be cost effective to install an addressable  system which enables sensors to be wired via a single highway cable providing savings in material and labour.

For both system types when a present alarm level is exceeded, alarm relays change state to begin any required action, by means of volt free contacts.

In practice a lower alarm relay normally activates audio/visual alarms:

  1. Outside the entrance to the boiler house
  2. Site Reception Area
  3. A remote site such as a security gatehouse/caretaker’s office

The higher level contact may cause the gas supply to be shut off and raise an alarm to evacuate the area.

Alternatively these alarms may be fed through to a Building Management System (BMS) for integration to the sites overall management tool.


Further Considerations

Oil Fired Boiler Systems are not immune from gas detection requirements as many of the oil fired systems rely on LPG to provide the initial firing of the burners. Once the Oil is alight then the LPG is withdrawn.

Be aware that as stated earlier LPG is heavier than air and will fall upon release requiring sensor locations closer to the floor.

On dual fuel systems using oil or gas it is seen that both fusible link and gas detection are necessary safety precautions.



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