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connections between the water glass and the boiler is choked or plugged with scale, is when there is no perceptible movement of the water in the glass. When the connections are free and the boiler is being fired,

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there is always a slight movement of the water up and down in the glass, and when there is no perceptible movement it is time to look for the cause at once. Many instances of burned tubes have occurred, and

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even explosions caused by low water in boilers while the gauge glass showed the water to be at a safe height. But owing to the connections having become plugged with scale, the water in the glass had no connection whatever with that in the boiler, and the water column was therefore worse than useless.

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Steam Gauges. As water columns are made at present the steam gauge is usually connected at the top of the column. This makes a handsome and convenient connection, although theoretically the proper method would be to connect the steam gauge directly with the dome or the steam space of the shell. There should always be a trap or siphon in the gauge pipe in order

to retain the water of condensation, so as to prevent the hot steam from coming in contact with the spring.

If at any time the water is drained from the siphon, care should be exercised in turning on the steam again by allowing it to flow in very slowly at first until the siphon is again filled with water.

The steam gauge and the safety valve should be compared frequently by raising the steam pressure high enough to cause the valve to open at the point for which it is set to blow.

Safety Valves. The modern pop valve is generally reliable, but, like everything else, if it is allowed to stand idle too long it is likely to become rusty and stick. Therefore it should be allowed to blow off at least once or twice a week in order to keep it in good condition.

Most pop valves for stationary boilers are provided with a short lever, and if at any time the valve does not pop when the steam gauge shows the pressure to be high enough, it can generally be started by a light blow on the lever with a hammer.

The ratio of safety valve area to that of grate surface is, for the old style lever and weight valve, I sq. in. of valve area for each 2 sq. ft. of grate surface, and for pop valves I sq. in. of valve area for each 3 sq. ft. of grate surface.

Each boiler in a battery should have its own safety valve, and, in fact, be entirely independent of its mates as regards safety appliances.

One example of safety valve computation will be given. Suppose the grate surface of a boiler is 56 30 sq. ft, what should be the diameter of the lever, safety valve? The required area of the valve is 30+ 2 = 15 sq. in. Then 15.7854-19, which is the

square of the diameter of the valve. Extracting the square root of 19 gives 4.35 in. diameter of valve. In actual practice one 5 in. or two 3 in. lever safety valves would be required. If a pop valve is to be used the required area is 30+ 3 = 10 sq. in. Then 10+.7854 = 12.73 = square of diameter of valve. Extract the

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diam

square root of 12.73 and the result is 3.6 in. eter of valve. In practice a 4 in. valve would be

required.

Fusible Plugs. A fusible plug should be inserted in that part of the heating surface of a boiler which is first liable to be overheated from lack of water.

In a horizontal tubular or return flue boiler the proper location for the fusible plug is in the back head

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about 11⁄2 or 2 in. above the top row of tubes. Ir firebox boilers the plug can be put into the crown sheet

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