506 APPLIED MECHANICS 
of the air chamber for a double-acting pump may, however, be less than — 
for a single-acting one. 
On the suction side of a pump the water in the suction pipe requir 
time to acquire, under the action of the pressure of the atmospher 
sufficient velocity to make it flow into the pump | 
barrel and completely fill the space left by the 
bucket or plunger ; and when the suction pipe is 
long, or when the speed of the pump is high, the 
-amount of water entering the barrel during the 
suction stroke may not be sufficient to fill it. 
Again, at the end of the suction stroke the suction 
valve is suddenly closed, and the water in the 
suction pipe is suddenly brought to rest and a 
shock: is produced. To remedy these defects a 
vacuum chamber V (Fig. 809) is placed on the 
suction pipe near to the suction valve 8. This 
vacuum chamber is not entirely devoid of air. When the water is ab 
rest the pressure of the air in the vacuum chamber, together with the 
pressure due to the head of water in the suction pipe, is equal to the 
pressure of the atmosphere. 
The theory of the action of the vacuum chamber is similar to that oft 5 
the air chamber, already discussed, but while the air chamber converts 
the intermittent discharge of the pump into a continuous flow in the — 
delivery pipe, the vacuum chamber converts a continuous flow in the 
suction-pipe into an intermittent flow in the pump. oa 
The volume of the vacuum chamber may be about half that of the ait 4 
chamber. he 
439. Pump Valves.—In nearly all pumps in which valves are Bs 
essential, the valves are operated automatically by the pressure of the 
water passing through them. The valves permit the water to pass freely — 
in one direction, but when the actuating force is removed, the valves close _ 
and prevent the return of the water. Kinematically, these valves are the — 
same as the pawl which permits a ratchet wheel or rack to move in one © 
direction only. dl 
There are many designs of valves in use in pumps, but it will only be 
necessary to refer to a few of them here. A simple conical direct lift valve 
Fig. 809. 
IONS Fl SS) 
ay 
Fia. 810. Fig. 811. Fig. 812. 
is shown in Fig. 810, The body of this valve is a slightly arched dise — 
with a conical edge, which forms the face of the valve. The valve face 
beats on a corresponding conical seat, formed on a bush or on a part of | 
Fie. 813. 
