All About Engines

Raising Steam 93 genuity, and we shall consider one or two examples of each in turn. The simplest form of stop valve for opening or closing communication between the engine and boiler is the screw-down valve shown in Fig. 48. The valve is in the form of a circular disc, a, with three webs, b, beneath ; these serve as guides while permitting the passage of steam when the valve is raised from its seating.1 Opening and closing is effected by the wheel, c, on the top of the spindle, d, which has a thread working into a fixed bar, while the spindle passes into the valve cham- ber, E, through a stuffing box, F. A valve of this kind is only suitable for small pipes and low pressures, because the full pressure of the steam acts on the upper side and renders it difficult to valve of only 2 inches diameter under a pressure of 100 lb. of steam would be held down by a force of 314 lb., while if it was 3 inches in diameter with a pressure of 160 lb. on the square inch the force would be 1,120 lb. To open a valve of 6 inches diameter against a pressure of 200 lb. on the square inch would involve a force of 5,655 lb., or more than 2J tons! This is equivalent to a 2j-ton screw jack, fully loaded. Fig. 48.—Simple stop valve open. Thus a 1 The valve illustrated is simply a saucer-shaped disc without webs.