PUMPS 507 
“the valve casing. The valve is guided as it rises or falls by means of the 
central stem, forming part of the valve body, which slides in a hole in a 
bridge stretching across the opening below the seat. The amount of lift 
of the valve is determined by a stop on the casing above the valve. A 
‘modification of this type of valve is shown in Fig. 811. Here the body 
_ is made conical so as to direct the flow of water more gradually towards 
the opening, and thus reduce shock. This valve is guided by three 
east on it, which slide in the opening below the seat. In valves 
_ with conical faces and seats, the slant side of the cone is usually inclined 
at 45° to its axis. 
The valve shown in Fig. 812 differs from the one shown in Fig. 810, 
‘in having its face and seat flat, and in having the central stem above 
instead of below the valve. This stem slides in a guide forming part 
of the valve casing. The interior of this guide is in free communication 
_ with the interior of the casing through the small holes shown at the top, 
otherwise the stem would not rise and fall freely in the guide. The final 
grinding of the valve on its seat should be done when the guide is in 
position. The lift of the valve is limited by the collar on the stem 
_ striking the lower end of the guide. 
, Fig. 813 shows a ball valve. The ball is guided and its lift deter- 
_ mined by the cage surrounding it. 
In the valves just described the width of the seat may be as small as 
zz inch, and it is sometimes as much as } inch. The narrower the seat, 
_ the easier is it to make the valve tight, but the area of the seat must be 
sufficient to prevent the crushing of the material of the valve or seat. 
_ These valves are generally made of brass or gun-metal. 
Referring to Fig. 812, where the seat is flat, d is the diameter of the 
valve, and h its lift. The lateral opening through the valve is rdh, and the 
area through the seat is i” hence when these two are equal, = 
The valve is therefore full open when the lift is one quarter of the 
LYE ti 
Fig. 814. Fig. 815. Fia. 816. 
diameter. In practice the lift of single-beat metal valves working on 
metal seats, and actuated by the fluid, is generally considerably less than 
one quarter of the diameter, especially when the speed of the pump and 
