132. The safety devices in common use consist of safety, or relief, valves, to relieve the boiler of excessive pressure, and thus prevent explosion; low-water alarms, to notify the engineer, by a whistle, when the water is too low in the boiler, and thus prevent overheating of the boiler plates; and fusible plugs, which are so constructed and attached to the boiler that they will burn out and thus allow steam and water to blow into and extinguish the fire before any damage is done to the boiler by overheating dry plates.

133. Damper Regulators

Damper Regulators. The regulators which are used to control the fire in steam-heating apparatus are essentially different from those used in hot-water and hot-air heating. In the former case, the regulator is actuated by variations in the pressure of the steam; but, in the latter cases, the only thing available for governing purposes is variation in the temperature of the heating agent.

The damper regulators used in steam heating are of two kinds, one being operated by a flexible diaphragm and the other by a piston. The former is used on low-pressure boilers, and the latter for high-pressure, or power, boilers.

134. Boiler Feeders

Boiler Feeders. The boilers attached to steam-heating apparatus are usually replenished by returning all the water of condensation to them, the pipes being arranged so that it will flow back by gravity. When this is impracticable, the water is returned by means of a steam pump, a return trap, or a steam loop. When fresh feedwater is required, it becomes necessary to employ either a steam pump or an injector, if the water pressure is not greater than the boiler pressure.