Low-wa′ter a-larm′.
The devices for this purpose may he classed under four heads.
- 1.
Those in which a float in the boiler subsides with the water, and is the means of opening a feedvalve, turning a steam-cock to the whistle, and indicating on a dial the condition of the boiler.
- 2. Those in which the subsidence of the water uncovers the end of a pipe, and admits steam to a rod or tube whose increment of heat and consequent expansion operate mechanically upon a valve, and allow steam to pass to a whistle.
- 3. Those in which steam admitted to a pipe, as just stated, melts a fusible plug, and so allows steam to pass to a whistle.
- 4. Those in which the subsidence of the water uncovers the end of a pipe and allows water to flow from a reservoir on the end of a lever, the steam from the boiler occupying the place thus vacated.
The change of weight incident to the substitution causes the motion of a lever, which is made to open a valve or turn a stop-cock and emit steam to the alarm-whistle.
1. The
float movement.
A. The float is attached to an arm, and is immersed in water in the chamber which communicates by pipes with the boiler.
Should the end of the lower pipe be uncovered by the subsidence of the water to that level, the water leaves the chamber and the float falls, depressing the valve and admitting steam to the whistle.
When the water rises again, the upper valve is moved to allow steam to depart, and the normal condition is re-established.
B. The hollow steam-tight case has a central hub and a sector space, occupied by the arm of a float, which rises and falls with the changes of level of the water in the boiler.
An indicator on the same axis moves with the float.
C. A float is placed on the crank-arm of the valvestem, and rises and falls with the changes of the water-level, bringing a stud on the stem against an inclined socket, and raising the valve from its seat.
This allows steam to pass to the whistle.
2. The
thermostat movement.
D. When the water subsides below the end of the vertical pipe, which extends downward into the boiler, the water contained therein is discharged and steam substituted.
The increased heat, due to the presence of steam, elongates the thermostatic rod in the tube and acts upon the lever to lift the valve from its seat; the steam rushes out and sounds the whistle.
E. This acts similarly to the foregoing, except that the effect is due to the expansion of the steampipes when the water is emptied and steam admitted.
The lever has its fulcrum on a post between the pipes; the expansion of one of the latter depresses the valve-stem and withdraws the valve from its seat; the expansion of the other pipe withdraws the seat from the valve.
The action of the two is cumulative.
The passage of steam sounds the whistle, as in other devices for the same purpose.
3. The
fusible-play action.
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Low-water Alarms. |
F. After the boiler has been filled to the waterline and put in operation, the pressure of the steam forces the water into the pipe and air-chamber
c. As there can be no circulation of water in the pipe as long as the lower end of it is under the water-line, the
disk d will continue solid; but when the water in the boiler evaporates below the end of the pipe to
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the alarm water-line, then the water in it falls of its own weight into the boiler, and steam at once takes its place, melts the plug, and notice of low water is given by the sounding of the whistle.
There are many variations of the fusible-plug device, some depending upon the direct heat of the steam, and others, as above, upon the admission of steam to a pipe which is normally charged with water.
4. The
gravity movement.
G. A vertical pipe passes into the boiler, and its open end is at the level at which it is desired notice shall be given.
This pipe forms a communication between the boiler and a reservoir on the end of a hollow arm and axis.
In the normal condition this reservoir is filled with water; but when the end of the pipe is uncovered by the subsidence of the waterlevel in the boiler, the water runs out of the reservoir and steam takes its place.
The change of weight in the reservoir, due to the substitution of steam for water, causes the arm to be lifted by the weighted lever and raise the valve which admits steam to the whistle.
