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Irrigation Pumping Systems

Irrigation Pumping Systems. Objectives. Identify pumps commonly used in irrigation pumping systems List the steps involved in selecting the proper size pump Calculate pump pressures required for an irrigation system Select a pump from a sample pump curve Describe common pump controls.

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Irrigation Pumping Systems

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  1. Irrigation Pumping Systems

  2. Objectives • Identify pumps commonly used in irrigation pumping systems • List the steps involved in selecting the proper size pump • Calculate pump pressures required for an irrigation system • Select a pump from a sample pump curve • Describe common pump controls

  3. Warning: • A number of catalogs, websites, and stores sell pumps that they call "irrigation" pumps. • These are typically a small centrifugal pump. • Most of these pumps will run a single small sprinkler head, but not a sprinkler system. • Typically pumps that are suitable for sprinkler systems are marketed under the label "high pressure pump".

  4. Many pumps are sold with misleading performance labels. • Often a pump package will say on the box or the sign, "XX GPM, XX PSI". • Or it may say "XX GPM, XX feet of lift". • In most cases this means one "or" the other, not one "and" the other. • Do not buy a pump until you have designed your irrigation system!

  5. Types of Pumps • Displacement Pumps • Force the water to move by displacement • Examples include: piston pumps, diaphragm pumps, roller-tubes, and rotary pumps. • These pumps are used for moving very thick liquids, or creating very high pressures. • They are used in fertilizer injectors, spray pumps, air compressors, and hydraulic systems for machinery.

  6. Types of Pumps • Centrifugal Pumps • Almost all irrigation pumps fall into this category. • A centrifugal pump uses an "impeller" to spin the water rapidly in a "casing", "chamber", or "housing" • This spinning action moves the water through the pump by means of centrifugal force. • Centrifugal pumps may be "multi-stage", which means they have more than one impeller and casing, and the water is passed from one impeller to another with an increase in pressure occurring each time.

  7. Each impeller/casing combination is referred to as a "stage". • All centrifugal pumps must have a "wet inlet", that is, there must be water in both the intake (inlet) pipe and the casing when the pump is started. • They must be "primed" before the first use.

  8. Types of Centrifugal Pumps • End-Suction Centrifugal Pump • The most common type of pump. • Typically "close-coupled" or mounted to an electric motor's drive shaft and the pump case is bolted straight into the motor so that it looks like a single unit. • The water typically enters the pump through a "suction inlet" centered on one side of the pump, and exits at the top.

  9. Almost all portable pumps are end-suction centrifugals. • This type of pump needs to be installed on a pad above the high water level if pumping from a lake or river.

  10. Submersible Pumps • Installed completely underwater, including the motor. • The pump consists of an electric motor and pump combined in a single unit. Typically the pump will be shaped like a long cylinder so that it can fit down inside of a well casing. • Submersibles don't need to be primed. • They also tend to be more efficient because they only push the water, they don't need to suck water into them.

  11. Most submersible pumps must be installed in a special sleeve if they are not installed in a well, and sometimes they need a sleeve even when installed in a well. • The sleeve forces water coming into the pump to flow over the surface of the pump motor to keep the motor cool. • Without the sleeve the pump will burn up. • Because the power cord runs down to the pump through the water it is very important that it be protected from accidental damage.

  12. Turbines and Jet Pumps. • A turbine pump is basically a centrifugal pump mounted underwater and attached by a shaft to a motor mounted above the water. • The shaft usually extends down the center of a large pipe. • The water is pumped up this pipe and exits directly under the motor. • Turbine pumps are very efficient and are used primarily for larger pump applications. • They are typically the type of pumps used on municipal water system wells.

  13. The turbine pump is mounted in a large concrete vault with a pipe connecting it to the lake. • The water flows by gravity into the vault where it enters the pump. • The pump motors are suspended over the vault on a frame. • A jet pump is similar to a turbine pump but it works by redirecting water back down to the intake to help lift the water.

  14. Booster pump • Most pumps are used to take water from a standing (or non-pressurized) source and move it to another location. • A booster pump is used to increase the water pressure of water that is already on its way somewhere. • Example: If you have a sprinkler system that needs 80 PSI of pressure to operate, but the water line coming onto your property only has 50 PSI of pressure. • In this case you would install a booster pump to raise the pressure from 50 PSI up to 80 PSI for your sprinkler system.

  15. Pressure vs. Flow • The performance of a pump varies depending on how much water the pump is moving and the pressure it is creating. • An important relationship not only because it determines whether the pump is suitable for your irrigation system, but also because it is these pump characteristics which allow you to control the operation of your pump. • The primary relationship to understand is that as the flow INCREASES, the pressure DECREASES

  16. Standard formulas used to estimate flow, pressure, and horsepower • FT.HD. = HP x 2178 / GPM • GPM = HP x 2178 / FT.HD. • HP = GPM x FT.HD. / 2178 • HP is brake horsepower • GPM is gallons per minute of flow • FT.HD. is pressure in feet of head (PSI x 2.31 = FT.HD.) • Note: these formulas have been simplified to assume a pump efficiency of 55% which is a good average figure to work with if you don't know the exact efficiency of your pump. Pressure for pumps is always measured in feet of head, one foot of head is equal to 0.433 pounds per square inch.

  17. Selecting a Pump • Here's the basic procedure to follow if you're selecting a pump for a new irrigation system. This is presented here as an overview to help you see where we are going with all of this: • Estimate your flow (GPM) and pressure (feet of head) requirements and select a preliminary pump model to use. • Using your preliminary pump information, create a first draft irrigation design.

  18. Once you have a first draft of your irrigation you may be able to fine tune your pump selection based on that design. Return to the pump selection process and re-evaluate your pump selection. Make your final pump selection. • Return once again to your irrigation design. Can it be fine tuned to better match your final pump selection? Make any necessary adjustments.

  19. Pump Pressure • First you will need to find out the "Dynamic Water Depth" of the water in your well. • Dynamic Water Depth is the depth of the water below the top of the well, in feet, when the pump is running. • Note that the term "draw-down" is often erroneously used in place of Dynamic Water Depth. • When the pump is running, the water level in the well drops below the water table. It may drop a few inches or more than 100 feet depending on the type of soil (or rock) the well is drilled into.

  20. Elevation Difference • Figure out the elevation difference between the top of your well and the highest point in the area to be irrigated • This may be a negative number if the well is higher than the irrigated area.

  21. Irrigation System Operating Pressure • This pressure will be calculated as part of the irrigation design process and if you have a design already it should be noted on the irrigation design. • If you have an existing irrigation system that you want to add a new pump to, then you can try measuring the water pressure with a gauge at the point where you plan to tap the new pump into the system. This is one of those unusual cases where you want to measure the dynamic pressure, not the static pressure. • So when you measure the pressure make sure that one of the irrigation system valves turned on and the sprinklers are running

  22. Chances are you don't have an irrigation system yet, or even a design. In this case you will need to make an "educated guess". Minimum Pressures for Irrigation Systems Drip Irrigation =70 feet head (30 PSI) Spray Type Sprinkler Heads =93 feet head (40 PSI) Rotor Type Sprinkler Heads =104 feet head (45 PSI)

  23. To finish up your pressure requirement calculations you add the values of the Dynamic Water Depth, elevation head, and operating pressure head together to get the total head required. • Example: • You measure a Dynamic Water Depth of 25 feet in your well. • The irrigation system is 10 feet higher than the top of the well. Your going to use rotor type sprinkler heads so you select an operating pressure of 104 feet head. • Your total head required would be 25 + 10 + 104 = 139 feet of head!

  24. Selecting a Pump • Pump Curves • Graph which shows the performance characteristics of a particular pump. • Pump curves are created by the pump manufacturer and the manufacturer should be able to provide you with performance curves for the pumps you are considering. • Remember, there is always an inverse relationship between pressure and flow. Higher pressures mean lower flows. Lower pressures result in higher flows.

  25. Pump Controls • Most pump control circuits are designed using a relay circuit that isolates the user from the pump voltage. • Most relays use 12 or 24 volts, a few use 120 volts.

  26. Automating the Control Circuit Common types include: • Timer • Pressure Switch • Most well pumps are controlled this way • A typical installation will have a pressure tank which stores pressurized water so that the pump doesn't cycle on and off rapidly due to pressure changes. • The tank acts as a reservoir to stabilize the pressure in the system.

  27. Irrigation Controller • Most irrigation controllers have a pump start circuit built into them. • You simply connect the wires that go to the switch to the controller pump start terminals, and the irrigation controller will turn the pump on whenever it turns on a valve. • Do not hook up the controller to an existing relay circuit.

  28. Flow Switch • Sometimes used on booster pumps. • When the switch detects flow in the pipes it turns on the pump. • Combination of one or more of the above switches. • A very common application is one relay controlled by both a pressure switch and a manual override switch, and a second relay operated by the irrigation controller.

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