The common pump (lift pump)
Pumps were used successfully to raise water from wells long before their action was properly understood, and are still to be seen in country villages, carefully preserved as relics of the past. They consist of a cylindrical metal barrel with a side tube near the top to act as a spout (Fig. 11.2).At the bottom of the barrel, where it joins a pipe leading to the well, there is a clack valve B. The latter is a hinged leather flap weighted by a brass disc so that it normally falls shut. A plunger carrying a leather cup and fitted with a second clack valve A is moved up and down inside the barrel by a handle H.
To start the pump working it is first primed by pouring some water on to the to of the plunger. This makes a good air seal and prevents leakage of air past the plunger during the first few strokes which are needed to fill the pump with water. Once the pump is filled the action is as follows:
The downstroke (Fig. 11.2 (a)) When the plunger moves downwards the valve B closes owing to the force of gravity on it and the weight of water above it. At the same time water inside the pum passes upwards through the valve A into the space above the plunger. The upstroke (Fig. 11.2 (b)).
On the upstroke the valve A closes owing to the force of gravity on it and the weigh. of water above it. Also as the plunger rises, water is pushed up the pipe through the valve B by atmospheric pressure acting on the surface of the water in the well. At the same time, the water above the plunger is raised and flows out of the spout. Limitations of the common pump Owing to the fact that the atmospheric pressure cannot support a column of wa more than about 10 m long, it follows that 10 m is the theoretical maximum the to which water can be raised by a common pump. An imperfect vacuum, however. usually obtained owing to bubbles from dissolved air forming near the top of water column. For this reason the practical working height of a pump is rather less than 10 m.
Occasionally one finds a pump which can lift water to a height greater than the theoretical maximum. This will occur if air can leak into the pipe near the bottom. Air bubbles then rise in the pipe and break up the water column into a series of
shorter columns. Thus, although the total length of water in the pipe is not more than about 10 m, the total length of water plus air is greater than 10 m. Consequently, water can enter the pump. A similar thing can happen when one is using a pipette. If the pipette is inadvertently lifted out of the liquid while it is being filled air will enter and the bubbles will carry the liquid up into the mouth almost immediately.