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 chamber 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.

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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.

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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 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

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Pump Sizing

1. What is the size of the inside diameter of the well?
2. What is the pumping level? I the pump is installed away from the well on higher ground, this elevation must be included.
3. 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.
4. 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.

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Foot Valve

1. It receives the water into the system and feeds the pump.
2. It filters sediment and debris out of the well or cistern water and keeps it away from to the pump.
3. It maintains the pump prime and prevents the pump from running dry by turning it off if there is no water in the valve.
4. 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.

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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.

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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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