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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 5 6 = 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
square root of 12.73 and the result is 3.6 in. - diameter 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 Aue boiler the proper location for the fusible plug is in the back head
about 1/2 or 2 in. above the top row of tubes. Ir firebox boilers the plug can be put into the crown sheet
airectly over the fire. These plugs should be made of brass with hexagon heads and standard pipe threads, in sizes 72, 34, i in., or even larger if desired. A hole drilled axially through the center and counter sunk in the end that enters the boiler is filled with an alloy of such composition that it will melt and run out at the temperature of the dry steam at the pressure carried in the boiler. Thus, if the water should get below the plug the dry steam, coming in contact with the fusible alloy, melts it and, escaping through the hole in the plug, gives the alarm, and in case of fire-box or internally fired boilers the steam will generally extinguish the fire also. The hole is counter sunk on the inner end of the plug so as to retain the fusible metal against the boiler pressure.
These plugs should be looked after each time the boilers are washed out, and all dirt and scale should be cleaned off in order that the fusible metal may be exposed to the heat.
Another type of fusible plug consists of a small brass cylinder into one end of which is screwed a plug filled with a metal which will fuse at the temperature of dry steam at the pressure which is to be carried in the boiler. The other end of the cylinder is reduced and fitted with small stop valve and threaded to screw into a brass bushing inserted into the top of the boiler shell. This bushing also receives at its lower end a piece of 72 or 34 in. pipe which extends downwards to within 2 in. of the top row of tubes, or the crown sheet if the boiler is internally fired. The principle of the device is that in case the water falls below the lower end of the pipe, steam will enter, fuse the metal in the plug, and be free to blow and give warning of danger. Some of these appliances are
fitted with whistles which are sounded in case the steam gets access to
them. But even with such devices no engineer can afford to relax his own vigilance and depend entirely upon the safety appliances to prevent accidents from low water.
Domes and Mud-Drums. As a general proposition, both mud drums and domes are useless appendages to steam boilers. There are, no doubt, instances where they may serve a purpose, but as a rule their use is of no advantage to a boiler. Neither are the so-called circulating systems, sometimes attached to return tubular boilers, of any real value. These consist of one or more 4 to 6 in. pipes extending under the boiler from front to back through the furnace and the combustion chamber and connected to each end of the boiler.
Feed Pipes. Authorities differ in regard to the proper location of the inlet for the feed pipe, but upon one point all are agreed, namely, that the feed water, which is always at a lower temperature than the water in the boiler, should not be allowed to come directly in contact with the hot boiler sheets until its temperature has been raised to within a few degrees of the temperature of the water in the boiler. Certainly one of the most fruitful sources of leaks in the seams and around the rivets is the practice of introducing the feed water into the bottom either at the back or front ends of boilers, as is too often the case. The cool water coming directly in contact with the hot sheets causes alternate contraction and expansion, and results in leaks, and very often in small cracks in the sheet, the cracks extending radially from ihe rivet holes. It would appear that the proper method is to connect the feed pipe either into the front head just above the tubes, or into the top of the shell. The