Fusible Plug is a fitting used in Steam Boilers to enhance the safety during operation. It’s working depends upon the water level in the Boiler shell. A fusible Plug is a threaded metallic cylindrical object, with a tapered hole in it which goes through its entire length. It is usually of brass, bronze or even gunmetal. In a smoke tube steam boiler, it is fitted on the front tube sheet, just above the tubes (used for flue gas flow) arrangement but below the perforated feed line inside the shell.
But the question is how exactly this little thing helps in preventing a huge disaster. It’s working is dependent only on the temperatures not on pressures. What happens is that during steam boiler operation, the feed pumps to supply water for steam conversion in the Boiler shell on the basis of the signal sent to it by the electrical panel. And electrical panel receives this signal from the mobrey (automatic water level controller) based on the fluctuating water levels due to continuous steam generation.
So, if anyhow, this loop of supplying water to the Steam boiler does not work (because of any fault) during Boiler operation then after some time the water level in the Boiler shell will first go down the low water level, then the perforated line. Now because of the reduced quantity of water in the Steam Boiler shell, the steam volume in the shell rises and because of continuous combustion in the furnace the temp of this steam goes up and it gets superheated. When the water level goes below the fusible plug, the lead present inside the Fusible plug gets melted because of superheated steam and now the water from the boiler shell comes out and wets the flue gases and even disrupts the combustion thereby protecting the Steam Boiler from any mishap that may have happened otherwise.
We, at Thermodyne, supply Smoke Tube Steam Boilers with all such desired protections to avoid a tension-free and reliable operation of the Steam Boiler.