The failure of a gas appliance safety device while conducting a gas safety check is classed as Immediately Dangerous. The engineer will ask your permission to turn off and disconnect the appliance from the gas supply.
Gas safety devices come in a number of forms the two main types are:-
- Flame Failure Devices. (Flame Supervision Device)
- Oxygen Depletion Devices.
There are other types such as Vitiation Sensing, Atmospheric Sensing and Pressure Sensing devices.
Flame failure devices detect the absence of a flame and use electrical or mechanical means to isolate the gas supply a simple example of this is a thermocouple on a hob.
You hold down the burner knob on initial lighting. The gas burner flame on the hob heats the thermocouple which because it is bi metallic it generates a small current. This current is used to hold open an electro magnet (solenoid) keeping the gas valve open.
If the flame goes out the thermocouple cools and is unable to keep the solenoid open, which then springs shut to shut off the gas to the burner.
This why the flame doesn’t stay in if you release the cooker knob too early as the thermocouple hasn’t had time to warm up enough. Once the burner is lit if it goes out for any reason the thermocouple will cool and the gas valve will shut.
The use of a thermocouple was/is common on many appliances .
A more advanced take on the flame failure device is seen on modern boilers. This is known as Flame Rectification. Flames conduct electricity. The PCB controller on the boiler passes a current through the flames between two probes. If the boiler PCB detects the absence of a current there is no flame and the PCB shuts the gas valve.
Oxygen depletion devices are most commonly seen on gas fires in conjunction with a thermocouple. The device is designed to warm a thermocouple with a pilot flame. If there is a lack of combustion oxygen available the pilot flame will weaken and lift and the thermocouple will cool and close the pilot.
This is far from an exhaustive list of the various safety devices that have been used over the years but gives you an idea of the things an engineer has to consider when performing a gas safety check on appliance safety devices.
Failure for appliance safety devices to meet gas safety check may result in various levels of action depending upon the severity of the problem.
Continue reading about areas of inspection during a gas safety checks such as gas safety check for correct operation and installation which is another aspect of the gas safety check.
MTS Heating Services provides the highest possible standards in gas safety checks for Appliance Safety Devices inspections. Call today on 07971-784046 to arrange your safety check.