The Locomotive Of Today

42 The Boiler: Safety Valves. The casing for the spring is made in two lengths, one sliding within the other, and the springs are attached to plates at the ends of each cylinder. The smaller cylinder is g'raduated to show the steam pressure in Ibs. per square inch, so that as tension is applied to the springs by a screw at the top it pulis the larger cylinder higher up the inner one and exposes the various figures. Two springs are generally arranged in each case, coiled right and left handed one within the other, the lower end of the spring case is attached by suitable pins to a bracket fixecl to the boiler, as shown at D, which represents this form of safety valve in section. To set the valves to blow off at any required pressure steam is raisecl as before, and the springs screwed up to the required tension, when the distance between the nut and the shoulder at the base of the screw is accurately measured, then the pressure relieved, and a ferrule made to fit over the screw which cannot be removed while steam is up, it is then easy to release the valves and allow steam to blow off before the maximum pressure is reaclied, but more tension cannot be applied to the spring to enable a higher pressure being carried. This form of valve is simple and not liable to get out of order, but is not so good as the Ramsbottom, the valves not being able to get an opening large enough to release steam rapidly, as the distance between it and the end of the lever is necessarily great. Another dis- advantage is the possibility of it being' held down to secure increased pressure, and boilers when fitted with this valve should also have a smaller one locked up as an auxiliary and a safeguard. A directly loaded valve for use in conjunction with the above is shown at C. In this case the valve is also of the wing type, and is held down by a coiled spring bearing on a plate fitted to a spindle, which has at its base a point engaging the valve, the upper end being carried through a hole in the holding down plate as a guide. The proper tension on the spring is obtained as describecl for the other cases, by trial, ferrules being fitted over the holding' down bolts between the top and bottom plates. The spring loaded valves above describecl possess a serious disadvantage, inasmuch as when the valves lift the load on them increases, due to the increasing tension on the spring, just the reverse of what theory would tell us to be the desired action of a perfect relief valve. With the Ramsbottom valve it is necessary to set it to blow off just under the limited figure, and even then it is quite possible to exceed it by io-lbs. per sq. in. when biowing off hard. Many attempts have been made to overcome this disadvantage both in the spring-