The Locomotive Of Today

28 The Boiler : Scale, Biast Pipe. strated that scale of an in. thick demands an increase of 15 per cent, of fuel, and scale J-in. thick 60 per cent., and scale ^-in. thick 150 per cent, more fuel, and so on; but the circumstances to be considered are not well understood, and are too varied to admit of exact calculation, as the matters that go to form the incrustation are so diverse in their character. A large number of Chemical substances have been intro- duced with a view to secure the solubility of the salts contained in the scale by decomposing them and their use has been attended by varying- degrees of success. The most common of these is the ordinary soda of commerce ; white ash or soda ash is often used as it is cheaper, but it is less effective. It is usually clissolved and introduced into the feed water, the quantity being found by experiment and varying with the water usecl. From i-lb. to 2^-lb. per day is a usual quantity, but it depends upon the amount of work done, etc. More soda ash and less caustic soda would be required, but though good for detaching scale, most of these substances have a tendency to increase priming. Frequent washing out with water under pressure is the best means of preventing the formation of scale n locomotive boilers. The exhaust or biast pipe should stand central with the chimney so that the escaping steam shall be dischargeel clirectly through it. The most common position for the top or nozzle of the biast pipe is just above the level of the top row of boiler tubes, but this is by no means universal as instances may be found of positions varying from the bottom row ot tubes up to the chimney base. It has been observed that the higher the nozzle the more the fire will be burnt at the front of the firebox, and by lowering it the fire will be burnt more at the back. In this country the orifice is always circular in shape, and its diameter depends upon the work the engine has to do, the size of the cylinders, the grate area, and the quality of fuel used. For all ordinary engines having cylinders of 16-in. diameter and upwards, it will be found to be between 4i-in. and 5 i-in. in diameter. The metal employecl is generally cast iron, but some will be noticecl made of copper, with the base and nozzle either of cast iron or brass. It is common practice to make separate nozzles with a flange, and form the top of the pipe with a similar flange, to which they can be attacheel; the outlet is thus easy to remove for cleaning, and can also be readily changed should it be found necessary to alter the size. These separate nozzles are often cast in brass, with a hollow chamber round the top, to which steam can be admitted and clischarged