Q. Is such a plug a protection to a boiler?
A. It is if kept in proper condition.
Q. Can you explain the principle of the fusible or soft plug
as it is sometimes called?
A. It is placed directly over the fire, and should the water
fall below the crown sheet the lead fuses or melts and allows
the steam to flow down on top of the fire, destroys the heat and
prevents the burning of crown sheet.
Q. Why don't the lead fuse with water over it?
A. Because the water absorbs the heat and prevents it
reaching the fusing point.
Q. What is the fusing point of lead?
A. 618 degrees.
Q. Is there any objection to the soft plug?
A. There is, in the hands of some engineers.
Q. Why?
A. It relieves him of the fear of a dry crown sheet, and gives
him an apparent excuse for low water.
Q. Is this a real or legitimate objection?
A. It is not.
Q. What are the two distinct classes of boilers?
A. The externally and internally fired boilers.
Q. Which is the most economical?
A. The internally fired boiler.
Q. Why?
A. Because the fuel is all consumed in close contact with the
sides of furnace and the loss from radiation is less than in the
externally fired.
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