Irrigating Greenhouse Crops
Water Delivery Systems
Water delivery systems begin with a pump or a public water meter. For the public water supply source, the water supplier will specify the type of water meter according to water needs and usage of the greenhouse(s). If the water supply is on-site, a ground water pump must be selected to supply the irrigation system at an optimum pressure for the system operation. The water supply must be able to supply water at a given flow rate and volume per day.
Groundwater Wells
A well is an opening stretching from the ground surface to the underground aquifer, where the groundwater is located. The depth of the well may vary from a few feet to several hundred feet. Wells are typically drilled with special drilling equipment able to penetrate the various layers of the ground, such as sand, clay, bedrock, and the like. Inside the drilled hole a casing (pipe) is typically installed, which prevents the well from collapsing around the pump. Below the casing and in line with the aquifer is another “casing” with fine slots. This is the well screen, where the slots allow the water to enter the well. It holds back sand and larger particles trying to enter the well. To improve the filtering function, the borehole typically features a diameter that is 2 to 3 inches larger than the casing. A fine sand gravel pack filter is placed between the casing and the aquifer. Some casings come with a pre-made gravel pack filter. Made correctly, this filtering method prevents sand and silt from entering the well.
Backflow Prevention
Many operators use irrigation systems to apply fertilizer and chemicals, a practice called chemigation. Certain precautions must be taken to prevent contamination of the irrigation well should an unscheduled shutdown of the irrigation pump occur during chemigation. If the pump stops while chemical products are within the irrigation pipeline, backflow of contaminated water into the well can contaminate the ground water supply. Backflow prevention devices are required by federal law when toxic chemicals are applied through irrigation water.
Pressure Tanks
Smaller watering systems not on a municipal water supply may need a pressure tank to supply the needed pressure to the irrigation system. A pressure tank is placed between a pump and the point of use of a water system to allow the water to become pressurized in the tank. The pump forces water into the tank, compressing the air in the tank. As the air compresses, the air and water pressure in the tank increases.
Pumps
Pumps used for irrigation include centrifugal, deep-well turbine, and submersible pumps. Actually, turbine and submersible are special forms of a centrifugal pump. Table 16.1 gives some general indication of the advantages and disadvantages of different kinds of pumps.
Centrifugal Pumps
Centrifugal pumps are used to pump from reservoirs, lakes, streams and shallow wells. They also are used as booster pumps in irrigation pipelines. All centrifugal pumps must be filled completely with water or “primed” before they can operate. The suction line, as well as the pump, has to be filled with water and free of air. Airtight joints and connections are extremely important on the suction pipe. Priming a pump can be done by hand-operated vacuum pumps, an internal combustion engine vacuum, motor-powered vacuum pumps or small water pumps that fill the pump and suction pipe with water. Centrifugal pumps are designed for horizontal or vertical operation.
Deep-Well Vertical Turbine Pumps
Deep-well vertical turbine pumps are adapted for use in cased wells or where the water surface is below the practical limits of a centrifugal pump. Turbine pumps also are used with surface water systems. Because the intake for the turbine pump is continuously under water, priming is not a concern. Turbine pump efficiencies are comparable to or greater than most centrifugal pumps. They usually are more expensive than centrifugal pumps and more difficult to inspect and repair.
Submersible Pumps
The submersible pump is simply a turbine pump close-coupled to a submersible electric motor attached to the lower side of the turbine. Both pump and motor are suspended in the water, thereby eliminating the long-line shaft and bearing retainers that are normally required for a conventional deep-well turbine pump. In this way, the pump is always filled with water (primed) and ready to pump.
Power Units for Pumping Water
Power units used for irrigation pumping include electric motors and internal combustion engines. Power units should be selected to match the power requirements of the pumping application. Overloading a power unit may shorten its useful life significantly, while power units oversized for the job operate at reduced efficiency. The efficiency of electric motors ranges from 85 to 92 percent. Large electric motors (above 15 to 20 horsepower) are more efficient than small electric motors.
Electric Motors
Many irrigation-pumping plants use electric motors. The electric motor provides flip-the-switch convenience along with minimal service and attention requirements. They have a long-expected life, require minimal maintenance, and are very reliable. An electric motor, properly selected and protected, can be expected to supply many years of trouble-free power if properly designed and operated, including correct mounting, rodent protection, good ventilation, adequate shelter from the elements, and safety devices against overloading, under-voltage, and excessive heating.
Internal Combustion Engines
There are several factors in selecting an internal combustion engine for a pumping application, including portable or stationary, air cooled or water cooled, fuel type, speed, size, efficiency, emissions, maintenance costs, expected life, and initial cost. Liquid-cooled engines can be cooled by either a radiator or a heat exchanger. The heat exchanger can recover up to 8 percent more horsepower than the radiator because it does not require the engine to run a fan to pull air through the radiator. The heat exchanger needs to be sized to the engine for adequate cooling even on the hottest days, but not so large that the engine over cools and forms excessive internal sludge. The manufacturer’s recommendation on engine temperature should be followed closely to prevent problems from overheating or under-heating, and a properly rated coolant thermostat be used to regulate coolant temperature.
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