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Well Pumps and Pressure Tanks
All private water systems, including wells, must conform to the same
standards -- including delivery, as municipal water systems, only on a
smaller scale.
Shallow Well Pumps
Shallow well vacuum pumps are designed to extract water
from cisterns and from wells no deeper than 25 feet.
They can be either centrifugal pumps or jet pumps but
jet pumps are more common.
Centrifugal pumps are designed for low suction heads
(vertical lifts) and high capacities. As a rule they develop low pressures,
usually 45 to 55 pounds per square inch maximum.
Pumping action is created by a means of a high-speed
impeller that literally throws the water or mixture out of the pump by
means of centrifugal force.
They are most frequently used as water pumps when high
volumes and low pressures are required.
All centrifugal pumps must be primed by filling them
with water before they can operate and they are limited in the pressure
they can deliver.
A Jet pump is really a combination of two pumps: the
injector jet and a centrifugal pump.
Jet pumps are often incorrectly called centrifugal
pumps.
If the jet is located in the pump itself, it is known
as a shallow well pump and will lift water up to about 25 feet.
If the jet is located down in the well below the water
level then it is a deep well pump.
They utilize an impeller mounted on the drive shaft
that spins and thrusts the water from the inlet to the outlet pipe similar
to a centrifugal pump but at increased pressure.
At the inlet nozzle, before the water reaches the
impeller, it passes through a venturi, which is a tapered constriction in
the pipe. As the water passes through this constriction, it builds
pressure and the velocity of the water increases. As the water is released
into the widening section of the venturi, pressure drops, creating a
suction effect at the constriction, and increasing the flow of water into
the pipe.
Venturi pictured at right:
the flow of the water is from left to right and a suction effect is
created at point B as the water expands to the right of that point, thus
drawing more water in from the left.

A diffuser following the mixing chamber slows down the water and converts
velocity head into pressure head
Jet pumps are self-priming, have no moving parts and do
not require lubrication.
Their efficiency is typically low (on average about
40%) and they provide low flows at high pressure.
They also have a tendency to clog from scale and to
collect mineral deposits in the pipes, which can break loose into the
water supply. The use of copper tubing or plastic pipe in the well tends
to reduce the clogging problem. Since the motors are above the water, they
are easily accessible for service.
Deep Well
Pumps
Deep well pumps are simple in operation.
The foot valve below the jet holds water in the system.
Above the surface is a standard impeller-diffuser type pump. The output of
the pump is split, and half to three-fourths of the water is sent back
down into the well through a pressure pipe.
When the motor starts, pressurized water is pumped down
to the venturi through this pipe (at point A in fig. 1 above).
This pressure causes an even higher velocity stream to
pass through the nozzle and enter the venturi throat section, thus
creating a more powerful vacuum than normal at this point. This
sufficiently increases pressure head beyond the impeller to lift the water
out of much deeper wells.
The ideal working lift for a single stage deep well jet
is 30 to 75 feet, although deeper levels are possible, while a shallow
well jet is limited to about 25 feet. Fig 2 below illustrates how a
typical well is installed.
Submersible
Pumps
Submersible pumps are vertical turbine type pumps.
They place all the major pump components at the bottom
of the well. This allows much greater efficiency in the operation of the
motor, reduces friction, allows the water to cool the motor, and enables
pumping from much greater depths. (The Sta-Rite Signature 2000 series
shown above is rated for heads up to 1150 feet.)
The name, deep-well turbine pump, is applied only to
pumps operating on the centrifugal principle and having diffuser vanes
within the bowl or case.
They can be single-stage or multistage for higher-pressure
applications. Pump bowls, which contain impellers and diffusers, are
located below the water surface, and they should be submerged under
pumping conditions.
Submersible pumps eliminate the drive shaft and bearing
systems of centrifugal pumps, thus reducing the mechanical complexity and
required maintenance.
Submersibles in deep settings are much more cost
effective that other pumping means. Submersibles also do not require
structures to enclose them and do not produce surface noise.
Standard submersible motors are water-filled and rely
on water as the internal lubrication for the motor. These motors are
extremely reliable when applied within their design limits of temperature,
hydraulic loading, and power requirements
Pump
Sizing
The answer to four basic questions will help select the
proper pump:
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What is the size of the inside diameter of the well?
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What is the pumping level? I the pump is installed
away from the well on higher ground, this elevation must be included.
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What should the average discharge pressure be? Normal
pressure is 40 lb, but if the pump is installed away from the well on
higher ground or when the house is above the pump, more pressure will be
required.
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What flow capacity is required?
Refer to Sta-Rite's
website for help in calculating this need.
Different pumps have different flow capacities, and the
overall rate of flow of water in the system is directly related to the
flow capacity of the pump serving it. A household with a 10 gpm demand
will not be satisfied by a pump that will only deliver 5 gpm. But a 10 gpm
pump will not fill the need if the well cannot produce enough water to
maintain that level.
The Foot valve is mounted below the pump at the
bottom of the well. It is the first mechanical component to contact the
water in the well. It is a crucial element in the system and performs a
number of key functions.
-
It receives the water into the system and feeds the
pump.
-
It filters sediment and debris out of the well or
cistern water and keeps it away from to the pump.
-
It maintains the pump prime and prevents the pump
from running dry by turning it off if there is no water in the valve.
-
It prevents the backflow of water from the system
back into the well.
The Foot Valve is designed with a slightly larger flow
area than the pipe size to insure minimal head loss. They are designed to
be self-cleaning. Fig. 3 illustrates a typical foot valve cross-section.
Pressure
Tanks
Once the well has been drilled, the water in the
well is available for use, but it must be extracted from the well and
delivered under pressure to the building.
This is accomplished by means of a well pump and a
pressurized tank. The pump pressurizes the system as it extracts the water
from the well and conveys it to the tank. The tank acts as a pressure
regulator to the system by maintaining a constant outlet pressure.
One type of pressurized tank is called a bladder tank;
it houses an inflated pre-pressurized polybutyl diaphragm (bladder).
The pump feeds the inlet, and as it pumps, water
stretches the diaphragm and displaces the air space it contained. As the
tank fills with water, air pressure behind the diaphragm then builds
inside the tank. Air pressure controls the water pressure. When the air
pressure in the tank reaches the upward limit, a governing cut out switch
turns the pump off. The system is protected with a pressure relief valve
should the cut out switch fail.
As water is drawn off by a faucet or fixture, the
bladder retracts and the air pressure begins to drop inside the tank. When
it reaches the lower limit of the range, the cut in switch turns the pump
on again to recharge the system.
A typical pressure range is 30 to 50 psi. The pump cut
in switch should be set at 2 psi higher than the pre-set tank pressure.
That establishes the minimum allowable pressure and ensures that the pump
will be turned on before outlet pressure to the house drops below the
desired level.
A pressure regulator controls the outlet pressure to
the house so that it remains constant even though the internal pressure of
the tank varies. The tank serves as a storage device as well as a pressure
regulator.
By storing water under a range of pressures, it permits
small drawdowns such as in the flushing of a toilet or the washing of
hands without turning the pump on. This saves wear and tear on the pump,
which is important because repeated on/off cycling puts unnecessary stress
on the motor.
The volume of water in gallons that can be drawn from
the system before pressure drops to the cut in level is called the draw
down, and the percentage of maximum draw down is called the maximum
acceptance factor.
Tanks are available in a wide variety of sizes and
configurations. Tanks are designed for the location in which they will be
installed, and for the type of pump that will feed them. They are rated as
to size and output. An important concern is the length of time the pump
must run to re-pressurize the system; the shorter the time, the better.
The object is to keep pump run time to a minimum.
Troubleshooting
Cavitation
Pump cavitation is caused by an air pocket in the line between the
impeller and the water being pumped.
These air pockets are zones of partial vacuum which
fill with water vapor as the surrounding water boils due to the reduced
pressure in the line. The air pockets are displaced by water flowing to
outer circumference of the impellers.
As they move toward the circumference, the pressure in
the surrounding water increases, and the pockets collapse against the
impellers with considerable force.
The force created by this collapse often causes erosion
and rapid wear of the pump impellers as well as a characteristic noise
during pump operation.
Cavitation can be caused by any combination of factors,
including inadequate submergence or excessive suction lift, high impeller
speeds, restricted pump intake lines, or high water temperature.
It can occur in all types of pumps and it can create a
serious problem. In some cases of mild cavitation, the only problem may be
a slight drop in efficiency. On the other hand, severe cavitation may be
quite destructive to the pump and result in pitting of impeller vanes.
Since any pump can be made to cavitate, care should be taken in selecting
the pump for a given system and planning its installation.
Inadequate Water Volume (obstructions)
If a pump is sized properly but is still not
delivering an adequate water supply, the cause may be an obstruction in
the system; the foot valve may be clogged, or there may be scale buildup
in the venturi nozzle.
see also: Shallow Well Pumps
Pump Operation
The system must be airtight. Air leaks or air locks will not allow pump to
prime.
The pump must be full of water before starting. Failure
to ensure this will cause damage to the pump seal and result in damage,
leakage, or flooding.
The pump must not be allowed to run against a closed
discharge valve. This will cause the motor to overheat, resulting in
possible pump damage, personal injury, or property damage.
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