Two BASIC PRINCIPLES

Two BASIC PRINCIPLES

Two basic principles underlie the workings of virtually all pumps now being installed for private water systems.

The first, often referred to as suction pressure or suction force, actually is the result of atmospheric pressure.

The second, known as centrifugal force, is a factor in the operation of every type of rotary pump.

Atmospheric Pressure

At sea level, the atmosphere has a weight (or pressure) of about 14.7 pounds per square inch (psi).

The weight of this air is responsible for operation of all types of suction pumps.

Visualize a drinking straw in a glass of soda or a pipe inserted in a body of water.

If a partial vacuum is created inside the straw or pipe, liquid will flow into it.

While this action is commonly called "suction," it is actually the result of atmospheric pressure pushing the liquid into the space where a vacuum exists.

Maximum Lift Capacity

How high can such a vacuum lift a column of water?

If the suction were perfect, i.e., a complete vacuum, there would be no force whatever to counteract the 14.7 psi of atmospheric pressure.

One psi can lift a column of water 2.3 ft.

Therefore in a perfect vacuum, suction force could theoretically lift a column 2.3 x 14.7 or 33.9 ft.

However, for a variety of reasons, including the impossibility of a perfect vacuum, 25 ft is the accepted maximum lift of suction-type pumps.

In shallow wells, pumps can operate on the basis of suction pressure alone.

But in wells deeper than 25 ft, this force must be coupled with some other action to bring water to the surface.

Centrifugal Force

The second mechanical principle involved in pumping is centrifugal force.

This is nothing more than the tendency of any rotating object to move outward from the axis or center of rotation.

Again, a common experience is illustrative.

Consider a pail of water being swung in a circle.

Although the pail may be tipped almost horizontal to the earth's surface, the water doesn't spill out because centrifugal force keeps it in place.

Now imagine that a hole is punched in the bottom of the pail.

The water flows out in a steady stream.

If there were a means of replenishing the water by suction, you would have what amounts to a centrifugal pump.

How Centrifugal Pumps Work

In practice, the pump creates centrifugal force with a bladed wheel known as an impeller.

Water enters the pump housing at the hub of the wheel.

The rotating impeller creates a centrifugal effect which causes the water to flow outward from the axis.

This flow in turn creates a partial vacuum or suction, which brings additional water in through the intake.

In one form or another, centrifugal force underlies the operation of all common water systems pumps—jets, submersibles, turbines.

In simplest form, the intake for a plain centrifugal pump is simply connected to the drop pipe into the well.

When the pump is turned on, centrifugal force and suction cause water to flow up the drop pipe.

Such an arrangement, however, is suitable only for very shallow wells—15 ft or less.

For greater depths, a special type of centrifugal unit known as the jet pump is commonly used.