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Water Pump 101

Which Pump Will Work Best For Me?

Jet_Pump_Installation

Graphic courtesy of A.Y. McDonald. One of our leading suppliers! Click to see enlarged version.

The two most popular types of pumps used for private well systems or low flow irrigation applications are jet pumps and submersible pumps. JET PUMPS – Jet Pumps are mounted above ground and lift 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; pump selection varies depending on water level. Shallow well installations go down to a water 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 ejector. A JET EJECTOR consists of a matched 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 water speed plus the low 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.

Centrifugal Pumps

Pump Basics 6-13 (1)

A Centrifugal pump. The impeller is on the inside, and the diffuser is on the outside.

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 ejector is added to assist the impellers in raising the pressure.

A centrifugal pump is a 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 off 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 turning at high speeds on the shaft. The liquid velocity is collected by the diffuser or volute and converted to pressure by specially designed passageways that direct the flow to discharge into the piping system, or on to another impeller stage for further increasing of 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; as the more water to be pumped or pressure to be developed, the more energy is
needed.

Submersible Pumps

submersible

Submersible pump graphic from A.Y. McDonald. Click to see enlarged.

The submersible 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 great advantage over other centrifugal pumps. There is no need to recirculate or generate drive water as there is 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 submersibles are “multi-stage” pumps. All of the impellers of the multi-stage submersible 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

Submersible Pump Impeller

Submersible Pump Impeller

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 is increased. The pump and motor assembly are lowered into the well by connecting piping 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 pump are under water they operate more quietly than above ground installations and pump freezing is not a concern. A.Y. McDonald can stack as many impellers as needed; however, the horsepower of the motor is limited. For instance, numerous pumps 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 more impellers to be stacked or to go to a different design pump with wider impellers.

Pressure Tanks

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Why do I need a tank?   There are four main reasons to include a tank in your system:

  1. To protect and extend the life of the pump by reducing the number of cycles.
  2. To provide storage of water under pressure for delivery between cycles.
  3. To have reserve capacity for periods of peak demand.
  4. To reduce system maintenance.

How do I choose a tank for my system?

Choosing the proper tank for your pumping system will greatly reduce the risk of premature pump failure. Most manufacturers recommend a minimum run time of one minute in order to protect the pump and the pump motor. The larger the tank the longer the running time and fewer pump cycles will result in longer pump life. One HP and larger pumps require longer run times.

To determine the proper size of tank, there are three factors to consider:

  1. Pump flow rate in gallons per minute
  2. Desired run time of the pump
  3. Cut-in and cut-out psi of the pressure switch

From these factors you can determine the tank drawdown with the following equation:

Pump flow rate X run time = tank drawdown capacity required.

Tank drawdown capacity is the minimum amount of water stored and/or delivered by the pressure tank between pump shut-off and pump re-start. This should not be confused with “tank volume.” For example, a pre-charged tank with a tank volume of 20 gallons has only five to seven gallons drawdown capacity depending on the cut-in / cut-out (on/off) setting of the pressure switch.

Pumps with flow rates (capacities) up to 10 GPM should have a tank with a minimum of one gallon drawdown capacity for each GPM delivered by the pump.

Example: 10 GPM pump = 10 gallon “drawdown”.

Pump flow rates from 11 to 20 GPM should have tank drawdowns approximately 1.5 times the GPM rating.

For example, 20 GPM X 1.5 = 30 gallon “drawdown”. Pump flow rates above 20 GPM should have tank drawdowns approximately two times the GPM rating and multiple tanks should be considered.