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Pump Basics
How A Centrifugal Pump Works
A centrifugal pump is of very simple design.  The only moving part is an impeller attached to a shaft that is driven by the motor.

The two main parts of the pump are the impeller and diffuser.  The impeller can be made of bronze, stainless steel, cast-iron, polycarbonate, and a variety of other materials.  A diffuser or volute houses the impeller and captures the water of the impeller.

Water enters the eye of the impeller and is thrown out by centrifugal force.  As water leaves the eye of the impeller a low pressure area is created causing more liquid to flow toward the inlet because of atmospheric pressure and centrifugal force.  Velocity is developed as the liquid flows through the impeller while it is running at high speeds on the shaft.  The liquid velocity is collected by the diffuser or volute and converted to pressure by special designed passageways that direct the flow to discharge into the piping system; or, on to another impeller stage for further increasing to pressure.

The head or pressure that a pump will develop is in direct relation to the impeller diameter, the number of impellers, the eye or inlet opening size, and how much velocity is developed from the speed of the shaft rotation.  Capacity is determined by the exit width of the impeller.  All of these factors affect the horsepower size of the motor to be used; the more water to be pumped or pressure to be developed, the more energy is needed.

A centrifugal pump is not positive acting.  As the depth to water increases, it pumps less and less water.  Also, when it pumps against increasing pressure it pumps less water.  For these reasons it is important to select a centrifugal pump that is designed to do a particular pumping job.  For higher pressures or greater lifts, two or more impellers are commonly used; or, a jet injector is added to assist the impeller in raising the pressure.

What Pump Do I Need?
the two most popular types used for private well systems or low flow irrigationapplications are jet pumps and submersible pumps.

Jet Pumps are mounted above ground for the end with the water out of the ground through a suction pipe.  Jets are popular in areas with high water tables and warmer climates.  There are two categories of jet pumps and pump selection varies depending on water level.  Shallow well installations go down to the well depth of about 25 feet.  Deep wells are down 150 feet to water, where surface pumps are involved.

The jet pump is a centrifugal pump with one or more impeller and diffuser with the addition of a jet injector.  A JET INJECTOR consists of a matching nozzle and venturi.  The nozzle receives water at high-pressure.  As the water passes through the jet, water speed (velocity) is greatly increased, but the pressure drops.  This action is the same as the squirting action you get with a garden hose as when you start to close the nozzle.  The greatly increased waters speed plus the lower pressure around the nozzle tip, is what causes suction to develop around the jet nozzle.  Water around a jet nozzle is drawn into the water stream and carried along with it.

For a jet nozzle to be effective it must be combined with a venturi.  The venturi changes the high-speed jet strained back into a high-pressure for delivery to the centrifugal pump.  The jet and venturi are simple in appearance but they have to be well engineered and carefully matched to be efficient for various pumping conditions.  The jet nozzle and venturi are also known as ejectors/ejector kits.

On a shallow-well jet pump the injector kit (jet nozzle and venturi) is located in the pump housing in front of impeller.

How a jet provides pumping action
Water is supplied to the Jet ejector under pressure.  Water surrounding the jet stream is lifted and carried up the pipe as a result of the jet action.
When a jet is used with a centrifugal pump a portion of the water delivered by the pump is returned to the jet ejector to operate it.  The jet lifts water from the well to a level where the centrifugal pump can finish lifting it by suction.
A portion of the suction water is recirculated through the injector with the rest going to the pressure tank.  With the injector located on the suction pipe of the pump, the suction is increased considerably.  This enables a centrical pump to increase its effective suction lift from about 20 feet to as much as 28 feet.  But, the amount of water delivered to the storage tank becomes less as the distance from the pump to the water increases...   more water has to be recirculated to operate the injector.

The difference between a deep-well jet pump and a shallow-well jet pump is the location of the injector.  The deep-well injector is located in the well below the water level.  The deep-well injector works in the same way as the shallow-well ejector.  Water is supplied to it under pressure from the pump.  The ejector then returns the water plus an additional supply from the well, to a level where the centrifugal pump can lift it the rest of the way by suction.

A convertible jet pump allows for shallow-well operation with the ejector mounted on the end of the pump body.  This time of pump can be converted to a deep-well jet pump by installing the injector below the water level.  This is of particular value when you have a water level that is gradually lowering.  This will probably require a change of venturi to work efficiently.
Because jet pumps are centrifugal pumps, the air handling characteristics are such that the pump should be started with the pump and piping connections to the water supply completely filled with water.

With a shallow-well jet pump, the ejector is mounted close to the pump impeller.  With a deep well jet pump, the ejector is usually mounted just above the water level in the well, or else submerged below water level.

Centrifugal pumps, both the shallow-well and deep-well types have little or no ability to pump air.  When starting, the pump and suction lines need to have all of the air removed.  An air leak in the suction line will cause the pump to quit pumping ... or sometimes referred to as "losing its prime".

The submergible pump is a centrifugal pump.  Because all stages of the pump end (wet end) and the motor are joined and submerged in the water, it has a greater advantage of the over other centrifugal pumps.  There is no need to recirculate or generate drive water as with jet pumps, therefore, most of its energy goes toward "pushing" the water rather than fighting gravity and atmospheric pressure to draw water.

Virtually all submergibles are "multi-stage" pumps.  All of the other impellers of the multi-stage submergible pump are mounted on a single shaft, and all rotate at the same speed.  Each impeller passes the water to the eye of the next impeller through a diffuser.  The diffuser is shaped to slow down the flow of water and convert velocity to pressure.  Each impeller and matching diffuser is called a stage.  As many stages are used as necessary to push the water out of the well at the required system pressure and capacity.  Each time water is pumped from one impeller to the next, its pressure he is increased.

The pump and motor assembly are lowered into the well by connecting pipe to a position below the water level.  In this way the pump is always filled with water (primed) and ready to pump.  Because the motor and the pump are underwater they operate more quietly than above ground installations; and, pump freezing is not concern.

We can stack as many impellers as we need; however, we are limited to the horsepower of the motor.  We can have numerous pumps that have 1/2 HP ratings - pumps that are capable of pumping different flows at different pumping levels; they will, however, always be limited to 1/2 HP.  Another way to look at it is that a pump will always operate somewhere along its design curve.

To get more flow, the exit width of the impeller is increased and there will then be less pressure (or head) that the pump will develop because there will be less impellers on a given HP size pump.  Remember, the pump will always trade-off one for the other depending on the demand of the system.  If the system demands more than a particular pump can produce, it will be necessary to go up in horsepower; thereby, allowing us to stack more impellers or go to different design pump with wider impellers.

Introduction To Pump Curves
A curve simply stated is normal a curved line drawn over a grid of vertical and horizontal lines.  The curved line represents the performance of a given pump.  The vertical and horizontal grid lines represent units of measure to measure that performance.

Let's think of a well all of water.  We want to use the water in our home.  The home is at a higher level than the water in the well.  Since gravity won't allow water to flow uphill, we used a pump.  A pump is a machine used to move a volume of water a given distance.  This volume is measured over a period of time expressed in gallons per minute (GPM) or gallons per hour (GPH).

The pump develops energy called discharge pressure or total dynamic head.  This discharge pressure is expressed in units of measure called pounds per square inch (psi) or feet of head (ft).

NOTE: 1 P.S. I will push a column of water up a pipe a distance of 2.31'.  When measuring a pumps performances, we can use a curved to determine which pump is best to meet our requirements.

Above is shown a grid with the unit of measure in feet on the left-hand side.  We start with 0 at the bottom.  The numbers printed as you go up the vertical axis relate to the ability of the pump to produce pressure expressed in feet.  Always determine the value of each grid line.  Sometimes the measure will say feet head, which is what most engineers call it.
With the pump running a reading was taken from the gauge in PSI and converted to feet ( 1 psi - 2.31 feet ).

We show another unit of measure in gallons per minute across the bottom.  You start with 0 on the left.  The numbers printed as you go to the right relate to the ability of the pump to produce flow of water expressed as capacity -- in gallons per minute [GPM].  Again, always determined the value of each grid line.

To establish a pump curve we run the pump using a gauge, valve and flowmeter on the discharge pipe.  We first run the pump with the valve closed and read the gauge.  This gives us the pump's capacity at 0 capacity and maximum heading in feet.

We marked the grid point 1.  Next we opened the valve to 8 GPM flow, for example, and read the gauge.

We again marked this point on the grid 2.  We continue this process until we have marked all the points on the grid.

We now connect all the points.  This curved line is called a head/capacity curve.  Head (H) is expressed in feet and capacity (C) is expressed in gallons per minute (GPM).  The pump will always run somewhere on the curve.
When the TDH is known, read vertically up the left-hand side of the curve to that requirements, for example, 300 feet.  Then read horizontally to a point on a curve that connects to the capacity needed, for example 26 GPM.  It is then determined that a 3 HP 19 stage pump is needed.

there are many different type curves shown in our catalog.  Here is a composite performance curve (more than one pump) for the submergible.  There is a separate curve for each horsepower size.  Let's compare 2 sizes:

1. First look at the 1 HP, 8 stages (impellers and diffusers).  At 20 GPM capacity this model will make 160 feet.
2. Now look at the 5 HP, 28 stages.  At 20 GPM capacity this model will make 500 feet.

When you add impellers, the pump makes more pressure (expressed in feet).  This allows the pump to go deeper in the well, but also takes a more horsepower.

Provided by A.Y. McDonald Mfg. Co.
Last updated 06/24/2002
Not responsible for typographical errors.