Boiler Fittings, Mountings and Accessories
Fittings versus accessories
A candidate must distinguish mountings, which are essential for safe operation and are fitted directly to the boiler, from accessories, which improve performance and are usually external. Mountings include safety valves, main steam stop valve, feed check valve, water level gauges, pressure gauge, blowdown valves, fusible plugs, low-water alarms and cut-outs, and the nameplate. Accessories include economisers, air heaters, superheaters, feed pumps, injectors and draught plant.
Safety valves and water level
Understand why at least two safety valves are normally required, how they are set and sealed, and why the lowest set pressure governs. Know the purpose of the easing gear and why the valve must not be gagged. For water level, know the required number and type of gauge glasses, the function of the protector or shut-off balls, and how to prove a gauge glass clear. Understand the trial cock and the correct blowdown sequence.
Traps
Questions often test which device is a mounting and which is an accessory, or which fitting must be locked or sealed. Another trap is the setting of safety valves: they are set in descending order from the lowest, and the boiler pressure must not exceed the lowest setting. Do not confuse the feed check valve, which prevents backflow, with the feed stop valve. Remember that a fusible plug melts at a set temperature to warn of low water, not to relieve pressure.
Worked example
If three safety valves are fitted with settings of 10, 10.5 and 11 bar, the maximum permissible working pressure is the lowest setting, 10 bar. Reasoning from the lowest upward prevents overpressure of the boiler shell.
Rule of thumb
If it protects the boiler from unsafe pressure, level or temperature, it is a mounting; if it improves efficiency, it is an accessory.