Why is it impossible to pump water from very deep in the ground with a surface pump?

Asked by: Curt Fahey

Answer

The key to understanding this is realizing that suction is not a force, but simply removing an opposing force to the force of air pressure which is already there.

When you stick a pipe down a deep hole into a pool of water at the bottom of a well, air inside the pipe is pushing down on the water in the pipe, and air outside the pipe is pushing down on the water outside the pipe, which in turn pushes up on water inside the pipe. All is in balance.

But now lets say you suck out the air inside the pipe. The water is pushed up the the same as it was before, but there is no counter acting force pushing the water down, so it begins to rise inside the pipe.

So far so good, but why does the water stop rising? Well the water is pulled down by gravity; the more water in the pipe the more it weighs. Since the force of the air outside the pipe is not changing, eventually the weight of the water is equal to the air pressure outside the pipe, and everything is in balance again.
Answered by: Rob Landolfi, Science Teacher, Washington, DC

Water is pumped from a well by creating a partial vacuum above the water by the pump. The amount of vacuum, in inches of mercury, is equal to the weight of the column of water from the water table to the surface.

Atmospheric pressure at sea level is 29.92 inches (approx. 76 cm) of mercury. This is equivalent to a column of water 406.7 inches or 33.9 feet (approx. 10.3 m). Therefore, a total vacuum could only pump water from a depth of just under 34 feet or 10.3 meters.

Actually, a total vacuum cannot be created over water. As the pressure is reduced, the boiling point of the water is lowered, producing a layer of water vapor between the water's surface and the pump. The water vapor reduces the ultimate vacuum and the maximum pumping depth, but only by about 0.7 inches (1.8cm) at 20°C.
Answered by: Scott Wilber, President, ComScire - Quantum World Corporation