Steam boiler is dangerous machinery, and for that reason, law is required to control every aspect of the boiler. Factories & Machinery (Steam Boilers and Unfired Pressure Vessel) Regulations, 1970, provided stringent requirement on safety aspects and controls on boilers. This Code stipulates that any boiler should consolidate thirteen essential fittings so that boilers can be operated safely.
The thirteen essential fittings can be grouped into three categories. The first category is safety fitting. Safety fittings include safety valves, gauge glasses, pressure gauge, low water alarm, low-water fuel cutout, and fusible plug. The second category is control fittings, which incorporate blowdown valves, main stop valve, feed check valve, and feed pumps. The final category is legal fitting, which incorporate inspector’s test attachment, registration plate, and manufacturer’s name plate. In this article, I would discuss some safety fittings.
Safety valve is the most important safety fitting on a boiler. It is fitted to prevent excessive pressure buildup in the boiler which could lead to boiler explosion. FMA Regulations 1970 stipulates that every boiler having a heating surface surpassing 100 sq.ft must be equipped with two safety valves, one of which must be direct spring-loaded type, mounted vertically as close as practical to the steam boiler shell without shut valve placed in between. The relieving capacity of safety valves is determined by the capacity of the boiler at peak load. The safety valves must be able to effectively discharge all steam with a rise of pressure of not greater than ten percent of the authorized safe working pressure (ASWP). The safety valves must be set to blow at different pressures. The first valve should open when the pressure exceeds three percent above the boiler working pressure while the other valve should open at a pressure slightly higher but must not be higher than the ASWP. The working pressure of a boiler is usually set at seven percent below ASWP. The spindles, disk, and other moving parts must not be made of materials that corrode easily.
Pressure gauge is the fitting that measures steam pressure inside the steam boiler. The Bourdon tube is the most common pressure gauge used on a boiler. The Code specifies that a pressure gauge should be installed on the steam space and provided with a shutoff cock and a siphon which develops and maintains a water seal, protecting direct contact of live steam with the Bourdon tube. A pressure gauge is usually mounted in front of the boiler in such a position that the boilermen can conveniently read it from the firing floor. FMA (Steam Boilers and Unfired Pressure Vessels) Regulations, 1970 stipulates that the diameter of the dial must not be less than 6 inches and must show pressure within a tolerance of two percent the authorized safe working pressure. The scale on the dial of a code pressure gauge must be graduated to not less than one and one-third and not more than twice ASWP.
A fusible plug is used in addition to other forms of low water protection. This device is not required by ASME code but is still included in FMA Regulations 1970. The fusible plug in watertube boiler is placed 3 inches below lowest water level while in firetube boiler, fusible plug is placed 3 inches above top tubes. In the event of low water, fusible plug will melt, the pressure inside the boiler is released, and the sound of escaping steam will produce high whistling sound, alerting the boilermen. However, the capability of fusible plug to quench the fire in the furnace is debatable because the steam rushed out is too small to have quenching impact. The hole of fusible plug is only 1/2″. Fusible plug is made of brass or bronze and contains a tapered hole. The hole is filled with tin which has a melting point of 230 deg.C. According to code, the fusible alloy should melt at a temperature of not more than 70 deg.C higher than the saturated steam temperature corresponding to the ASWP. For example, for a steam boiler of 10 bar, the saturated steam temperature is 180 deg.C; therefore the fusible alloy should melt at 250 deg.C. The use of fusible plug is limited to solid fuel and steam boiler having a pressure of 17 bar, which corresponds to tin’s melting temperature. Higher pressure would require low water alarm. That is why in a modern boiler, fusible plug sometimes cannot be found as it is replaced with low water alarm which serves exactly the same function.