Flame supervision device
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For gas appliances, a flame supervision device (FSD) – alternative name: flame failure device (FFD) – is a general term for any device designed to stop flammable gas going to the burner of a gas appliance if the flame is extinguished. This is to prevent a dangerous buildup of gas within the appliance, its chimney or the room. Causes of flame failure include chimney downdraught, temporary interruption of the gas supply, gas under-pressure, liquid overspill on cooker hotplates or the draught from an oven door being opened and closed.
There are many types of FSDs utilising different technologies such as thermoelectric valves, flame conductance, flame rectification, ultraviolet sensing devices and liquid expansion valves. FSDs are found on all modern gas appliances.
Cooker hotplates may not have a FSD on each burner. If a hotplate is to be used in a multi-occupancy building every burner must have its own FSD.
When the FSD activates it should stop (or reduce to safe levels) gas flow to the burner until it is reset manually.
Older devices, such as bimetallic strips, were used in conjunction with pilot lights. The pilot light is no longer used in new devices, but may still be encountered on old appliances still in service. Pilot lights were withdrawn for two reasons: their continual small flame represented a waste of fuel and also the FSD protected only the main burner, not the pilot. If the pilot was extinguished, there was still the risk of a small gas leakage.
Regular testing of FSD is a part of routine maintenance for gas appliances.
Other safety devices may be fitted in addition to an FSD. One type of these are the Vititation Sensing Devices, that detect an adequate supply of oxygen for efficient combustion, thus avoiding the production of poisonous carbon monoxide. As well as detecting a blocked supply of oxygen, these must also detect a blocked exhaust or reversed flow in the exhaust flue owing to wind conditions. Two methods are used to provide these, an Oxygen Depletion System (ODS) that measures the availability of oxygen for combustion or an Atmospheric Sensing Device (ASD) measures excess heat rise in the exhaust.
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