This month’s article takes us back to the basics. In this and the articles to follow, we will be talking about pumped water systems. Because this is a ground water publication, when we use the term “pumped water system,” we are referring to systems that move ground water from a well to its intended point of use – the types of systems likely to be encountered by well drilling contractors and other ground water professionals. We’ll start with a look at the heart of any pumped water system – the pump.
Water can be moved in many ways. If water is above where you want it, gravity
can get it to its destination. If you need to get it higher than where it is,
you’ll need some sort of pump. There are ram pumps, piston pumps, gear pumps,
diaphragm pumps, lift pumps, centrifugal pumps, etc., each having its own place
in the sun (or under the ground, as the case might be). Some are positive
displacement pumps capable of moving relatively small amounts of fluids at very
high pressures, way more than is needed in most water system applications.
Others, like the centrifugal pump, don’t produce much pressure or lift, but can
move very large amounts of water. However, if configured properly, as will be
pointed out later, centrifugal pumps can produce a respectable amount of
There are six types of pumps commonly used in ground water applications. They
are the straight centrifugal, the shallow-well jet pump, the deep-well jet
pump, the submersible pump, the submersible turbine and the vertical line-shaft
turbine. In this series of articles, we will focus on the first four types of
pumps, which are the ones used in residential systems. Turbine pumps are the
types used in agricultural and municipal systems.
All four residential types of pumps will be described in detail in the coming
months. So that you can have a mental image of each, here are brief
descriptions and pictures of each one. As you will see, all four types are
variations of the basic centrifugal pump.
The straight centrifugal consists of a motor to provide the energy, an impeller
to spin and accelerate the water, a volute to concentrate the water and build
pressure, and a housing or case to contain the pressure and provide a place to
attach the pipes to the pump. A common variation used in ground water
applications is called the end-suction centrifugal because the suction pipe is
connected to the end of the pump opposite the motor. An end-suction centrifugal
pump can draw water from as deep as 20 feet.
A shallow-well jet pump is an end-suction centrifugal pump with an attachment
on the front of the pump housing, which boosts the pressure. The attachment,
called a jet assembly, uses a venturi to convert some of the energy from the
centrifugal portion of the pump into additional pressure, both positive pushing
pressure and negative suction pressure. A shallow-well jet pump can draw water
from as deep as 25 feet.
A deep-well jet pump is similar to a shallow-well jet, but the jet assembly is
located down in the well where it pushes the water to the surface instead of
pulling it. As you will learn later, there are natural laws governing the depth
from which we can pull water to the surface, so having a pump that pushes the
water allows us to pump from much deeper wells. A deep-well jet pump can pump
from as deep as 150 feet.
A submersible pump has the same components as a straight centrifugal pump, but
the parts are all small enough in diameter, even the motor, to fit into a well.
Although there are single-impeller submersible pumps used for special
applications, most submersible pumps have more than one impeller/volute
assembly. Each impeller/volute assembly is called a stage, and each stage adds
to the pressure produced by the one before it. For very deep wells, it is not
uncommon to have more than 50 stages. A multi-stage submersible pump can pump
from well over 1,000 feet deep.
As we said, most residential water systems pumps, whether they are multi-stage
submersible pumps, shallow- or deep-well jet pumps, or end-suction centrifugal
pumps, are based on the principle of the centrifugal pump. In a centrifugal
pump, an impeller, rotating at high speed, causes water to accelerate as it
moves from the inlet, or eye at the center of the impeller, to the outlet at
the outside edge of the impeller, creating pressure at the outlet and a partial
vacuum at the inlet. This acceleration is caused by centrifugal force, thus the
name centrifugal pump (see Figure 2 on p. 17). The amount of pressure that can
be produced by a centrifugal pump increases as the diameter of the impeller
increases, and as the speed at which the impeller is spinning
The other major component at the wet-end of a centrifugal pump (as
distinguished from the motor end) is the pump housing, called a volute in an
end-suction centrifugal and a diffuser in a submersible or jet pump. Its
purpose is two-fold – to convert the velocity of the water leaving the impeller
into pressure, and to direct the water to the pump discharge or to the inlet of
the next impeller in a multi-stage pump.
The closeness of the fit between the inlet of the impeller and the pump housing
affects the pump’s performance. If there is too much clearance, some of the
water that has been pumped through the impeller simply will leak around the
impeller and back to the inlet, reducing the discharge pressure and the amount
of water delivered. Not only is this fit an important element in the design and
manufacture of a pump, it can deteriorate during normal usage, due to corrosion
or erosion of the pump impeller and housing, decreasing a pump’s performance.
Next month, we will continue this discussion on pump basics with a look at how
centrifugal pumps lift water and make pressure. ’Til then ….
Tech Topics: How Pumps Work - Part 1
May 1, 2010