Sidebilder
PDF
ePub

directly over the fire. These plugs should be made of brass with hexagon heads and standard pipe threads, in sizes 2, 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 a 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 at piece of 1⁄2 or 3/4 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 the 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

nipple entering the boiler should have a long thread cut on the end which screws into the sheet, and to this end inside the boiler there should be connected another pipe which shall extend horizontally at least twothirds of the length of the boiler, resting on top of the tubes, and then discharge. Or, what is still better, allow the internal pipe to extend from the entering nipple at the front end to within a few inches of the back head, then at right angles across the top of the tubes to the other side, and from there discharge downward. By this method the feed water is heated to nearly, if not quite, the temperature of the water in the boiler before it is discharged. One of the objections to this system is the liability of the pipe inside the boiler to become filled with scale and finally plugged entirely. In such cases the only remedy is to replace it with new pipe. But the great advantage of having the water thoroughly heated before being discharged into the boiler will much more than compensate for the extra expense of piping, and the general idea of introducing the feed water at the top instead of at the bottom of the boiler is therefore recommended as being the best.

The diameter of feed pipes ranges from 1 in. for small sized boilers, up to 1% and 2 in. for boilers of 54 to 72 in. in diameter. It is not good policy to have the feed pipe larger than necessary for the capacity of the boiler; because it then acts as a sort of cooling reservoir for the feed water, and may cause considerable loss of heat.

For batteries of two or more boilers it is necessary to run a main feed header, with branch pipes leading to each boiler. The header should be large enough to supply all the boilers at the same time, should it ever

become necessary to do so. The header can be run along the front of the boilers just above the fire doors with the branch pipes running up on either side, clear of the flue doors and entering the front connection, or smoke arch, and the boiler head at a point two inches above the tubes. There should always be a valve in each branch pipe between the check valve and the header for the purpose of regulating the supply of water to each boiler, and also for shutting off the pump pressure in case of needed repairs to the check. valve. Another valve should be placed between the check valve and the boiler.. By this arrangement it is always possible to get at the check valve when it is out of order.

Blow off Pipes. Blow off pipes should always be connected with the lowest part of the water space of a boiler. If there is a mud-drum, then of course the blow off should be connected with it; but if there is no mud-drum, the blow off should connect with the bottom of the shell, near the back head, extend downwards to the floor of the combustion chamber, and thence horizontally out through the back wall, where the blow off cock can be located.

The best blow off cocks are the asbestos packed iron-body plug cocks, which are durable and safe. A globe valve should never be used in a blow off pipe, because the scale and dirt will lodge in it and prevent its being closed tightly. A straight way or gate valve is not so bad, but an asbestos packed plug cock is undoubtedly the best and safest.

In order to protect the blow off pipe from the intense heat, a shield consisting of a piece of larger pipe can be slipped over the vertical part before it is connected.

Blow off cocks should be opened for a few seconds once or twice a day, to allow the scale and mud to be blown out. If neglected too long they are liable to become filled with scale and burn out. A plan which is said to give good results is to connect a tee in the horizontal part of the pipe, and from this tee run a I in.

[graphic][merged small]

pipe to a point in the back head at the water level. It is claimed that this will cause a circulation of water in the pipe and prevent the formation of scale.

A surface blow off is a great advantage, especially if the water is muddy or liable to foam. By having the surface blow off connected on a level with the

« ForrigeFortsett »