In addition to the safety valve with spring balance, however, a pop safety valve is employed on the part of the dome that is shown broken away, the construction of this pop valve being shown in the outside view, [Fig. 3326a], and a sectional view, [Fig. 3326b], the casing being removed from the latter. In the valve seat b is a recess a, and upon the circumference of the valve is a threaded ring c′. When the valve lifts, the steam is somewhat confined in the annular recess of the valve, and the extra valve area thus receiving pressure causes the valve to lift promptly and the steam to escape freely. The degree of this action is governed as follows:
The sleeve c′ is threaded upon the upper part of the valve, so that by screwing it up or down upon the valve the amount of opening between the annular recess a a, and the lower edge of the sleeve c′ c′, is increased or diminished at will; the less this opening, the more promptly the valve will rise after lifting from its seat.
To secure the sleeve or ring in its adjusted position, the ends of the screws l, l seat in notches cut in the upper edge of the sleeve. In many engines pop valves alone are used, and in some cases levers are provided by means of which the pop valve can be raised from its seat to test if it is in working order.
Referring again to [Fig. 3326], h is the handle for operating the injector, and w a rod for opening the injector overflow.
We now come to the automatic air brake; steam for the steam cylinder of which, is received from the steam drum through the pipe j, passing through the pump governor, or regulator g. The exhaust pipe for the steam cylinder of the air pump passes into the smoke box. The air cylinder receives its supply of air through the small holes at k, k, and delivers it through the pipe c into the air reservoir or tank, from which it passes through the tank pipe up to the threeway cock or engineer’s brake valve, whose handle is shown at m. The brakes are kept free from the wheels and out of action so long as there is air pressure in the air reservoir and in the train pipe, hence the normal position of the handle m is such as to let the air pass from the air reservoir up the pipe x and into the train pipe. When the brakes are to be applied, handle m is moved so that there is an open connection made between the train pipe and the pipe to open air, which releases the air pressure and then puts on the brakes not only on each car, but also on the engine, because the engine brake cylinders receive their air pressure from the pipe shown leading to the train pipe. From the tank pipe x a pipe h leads to the top of the pump governor g, whose action is to shut off the steam from the steam cylinder of the air pump whenever the pressure in the air reservoir or tank exceeds 70 lbs. per square inch. A small pipe leads up from pipe h to the air pressure gauge.
For regulating the draught of the fire there is a damper door at each end of the ash pan, and to increase the draught, a pipe leads from the steam drum into the smoke box, where it passes up alongside of the exhaust pipe, its end being shown at z. This is called the blower, and its pipe is on the other side of the engine. The plate shown at p, p in the smoke box checks the draught in the upper tubes, and therefore distributes it more through the lower ones.
Fig. 3327.
There are two sand valves, both of which are operated by one rod, the construction being shown in [Fig. 3327], which is a plan showing the bottom of the sand box broken away to expose the gear for moving the valves. The two valves v, v for the sand pipes are on raised seats e, e, and are fast on the same shafts as the segments s, s, but the valves are obviously above, while the segments are beneath the bottom of the sand box. The gear wheel w is pivoted to the under side of the bottom of the sand box, and the arm l is fixed to the wheel. At t are pieces of wire, which, being fast in the spindle, revolve with it and stir up the sand when the valves are moved. As shown in the figure, the two sand pipes a, a are open, but suppose the rod is moved endways and l will revolve w, which will move s, s and the valves v, v, causing the latter to move over and cover the pipes a, a, and shut off the sand from the pipes.