Q. What care should be taken of the fusable plug? A. Keep it scraped clean, and not allow it to become corroded on top.
Q. What is a fusible plug? A. It is a hollow cast plug screwed into the crown sheet or top of fire box, and having the hollow or center filled with lead or babbit.
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.