[Fig. 190] represents the same pump running at a comparatively low speed. It will be noticed that the valves have not been raised as high as in [Fig. 191], because a longer time being allowed for the discharge of the water, a smaller orifice is sufficient. It will be seen also that the piston, moving at a lower velocity, cannot travel as far in [Fig. 190] before the valves seat, and consequently a vacuum can be created in the cylinder earlier in the stroke, and a larger volume of water can therefore be drawn in during the return stroke. In the latter case it is evident that the volume of water drawn into the cylinder will be nearly equal to a cylinderful and consequently the loss by slippage must be correspondingly less.
In order to reduce the loss by slippage several valves are used instead of a single valve of equal area. A flat disc valve will rise a distance equal to one-fourth the diameter of the port or of the opening in the seat to discharge the same volume of water that can flow through the port in the same time. In practice the rise exceeds this proportion of one-fourth a trifle, owing to the friction of the water, and this is especially true at high speeds.
Fig. 193.
Reinforced pump valves. Where pure gum has been used for pump valves it has always proved too soft and when it has been compounded with other substances it has been found too hard to withstand the severe duty to which it is subjected as a material for pump valves.
In the accompanying Fig. 192 is shown the Braden pump valve, which is made of composition of rubber having wire rings embedded in the center of the disc. The composition has been removed from a section to show these rings. A ferrule of composition metal forms a hub around the center through which the bolt or stud passes to guide the valve and to prevent excessive wear of the hole.
Its wire coil frame work clothed with rubber maintains a due amount of stiffness, with a degree of flexibility which prevents its bulging into the holes in the seats, or sticking therein, and thus impairing the suction and discharge. Both sides, the upper as well as the lower, are made available for service. These qualities of stiffness and flexibility combined, permit this valve to adjust itself to form a water-tight seat.
Figs. 194, 195, 198.
