Drip Irrigation for Greenhouse Crops
Drip Irrigation System Components
Drip irrigation systems can be arranged in a number of ways. The arrangement of components in Figure 14.1 represents a typical layout. Basic components can include the pumping station, pipe network, emitters, filters, a pump and power unit, a backflow prevention device if chemicals are used with water, a filter, a water distribution system, and some devices for controlling the volume of water and pressure in the system. If the water source is from a city/municipal/rural water supply, a direct connection is possible.
The pumping station consists of the power unit (internal combustion engine or electric motor) and a centrifugal, deep-well turbine, or submersible pump and appurtenances. The purpose of a pump is to deliver water evenly at the correct flow rate and pressure.
Once the water has been filtered it is delivered to the greenhouse through the mainline and submain pipelines that feed into the lateral irrigation lines.
Main and Manifold Pipelines
The main line carries water from the pump to the manifold. The manifold is usually connected to the main line at the midpoint of the manifold in order to provide more uniform pressure to all laterals in the system. A globe valve or an automatic pressure regulating valve is installed between the main line and manifold to allow for pressure adjustment so all laterals have approximately equal pressure. Install a pressure gauge immediately downstream from the globe valve to measure at the midpoint of the manifold. Set the appropriate operating pressure by adjusting the globe valve. Maximum pressure loss in the manifold plus loss in the lateral should generally be less than 20 percent the emitter operating pressure.
The drip laterals are connected to the sub-mains or the manifolds. The laterals are made of LDPE (Low Density Polyethylene). There are different types of connectors between the sub-mains/manifolds and the laterals. The connectors have to withstand the working pressure as well as pressure spikes and water hammers. The lateral may be laid on soil surface or underground. Two basic types of drip laterals are used: Thick-walled laterals with line source or point source emitters and thin-walled tapes with turbulent flow inherent water passageway molded into the tape during the extrusion process.
It is important to measure the amounts of water beneficially used and delivered to a greenhouse in order to document improvements in irrigation efficiency due to management changes and/or upgrades in the irrigation system. Careful measurements of crop water use make it possible to determine the volume of water beneficially used.
Line-source emitters are suitable for closely spaced row crops, with the rows separated several feet apart, as with most vegetable crops. Line source emitters are available in two variations:
Thin-Walled Drip Line. A thin walled drip line, also known as drip tape, is a thin-walled polyethylene product, collapses when not pressurized, and has emitters formed into its seam during manufacturing (See Figure 14.2). The thin-walled drip tape inflates upon pressurization. Drip tape systems can be applied to crops in beds where plants are growing at a specified spacing. Often drip tape is used with high value crops, such as strawberries, vegetables, flowers, or nursery stock. Drip tapes are made of LDPE or other soft PE materials available in wide range of diameters, wall thickness, and emitter spacing and flow rates. The choice of tape thickness, measured in millimeters (mils), is based on how long the tape is expected to last and the highest.
Thick-Walled Drip Line. The thick walled drip hose (See Figure 14.5) is a robust variation of the thin walled drip line. The internal emitters are molded or glued to the drip hose. It is more durable because of its considerable thickness.
Point-source emitters (See Figure 14.6) are typically installed on the outside of the lateral line. The installer can select the desired location to suit the planting configuration or place them at equally spaced intervals. Point-source emitters dissipate water pressure through a long narrow path and a vortex chamber or a small orifice before discharging into the air. Emitters are typically manufactured in 3 flow rates; ½, 1, and 2 gallons per hour (1.9, 3.8, and 7.6 L/hr respectively).
Filters are essential to the operation of a drip irrigation system in preventing suspended materials in the water from causing system blockages, which can reduce the efficiency of the system. Suspended materials in irrigation water may be inorganic (sand, silt, and clay), organic (algae, bacteria, plant debris, fish, insects, insect larvae, and slimes), or any other floating or suspended matter. Two sets of filters are often recommended. The primary filter removes suspended material from the water. A smaller-capacity secondary filter installed downstream from the primary filter protects the irrigation system if the primary filter fails. There are a number of filter types available for different needs as discussed in the following sections.
Sand Separator (centrifugal or hydrocyclone) Filters
Centrifugal sand separators (See Figure 13.6), in theory, are not actually filters but are used as pretreatment devices for other types of filters. A centrifugal sand separator removes larger particles of sand, silt, or other abrasive grit particles that can lead to the premature degradation of irrigation system components. These contaminants can reduce the efficiency of the irrigation system equipment by plugging and clogging valves and emitters.
Sand Media Filters
Sand media filters (See Figure 14.7) have been used extensively for drip irrigation systems. They consist of fine gravel and sand of selected sizes placed in cylindrical-pressurized tanks. The main body of the tank contains sand, which is the active filtering ingredient. The sand is placed on top of a thin layer of gravel, which separates it from an outlet screen. Contaminants are filtered from the water as it flows through the sand and gravel media. Media filters are often used to remove organic materials (bacterial slimes and algae), fine silt, or other fine organic or inorganic materials from ponds and surface water. Due to the three-dimensional nature, media filters have the ability to entrap large amounts of contaminants.
Screen filters (See Figure 14.8) are most frequently used for removing physical contaminants. They are efficient in removing very fine sand from the irrigation water, but tend to be clogged rapidly by heavy loads of algae and other organic material and are not efficient as sand media filters. Screen filters are sometimes used as secondary filters, located downstream of sand media filters.
Disc filters (See Figure 14.9) are relatively new devices that possess traits of both sand media and screen filters. Disc filters are better than screen filters for retaining algae. The screening element of a disc filter consists of stacks of thin, doughnut-shaped, grooved discs, forming a three-dimension filter cartridge. The stack is enclosed in corrosion and pressure resistant housing. Each individual disc contains grooves, molded into its surface. These molded grooves provide for the mesh (or micron) rating of the filter.
Valves for Drip Irrigation Systems
As with any drip irrigation system, proper selection and placement of valves is critical in a irrigation system. Water flow rate and pressure throughout the drip irrigation system should be precisely controlled to ensure efficient and timely water application. Valves play key roles in controlling pressure, flow and distribution under different conditions to optimize performance, facilitate management, and reduce maintenance requirements in drip irrigation systems. Valves used in a complete a drip irrigation system include check/backflow valves, shut off valves, electronic remote control valves, pressure regulators, and air/vacuum relief valves.
A backflow prevention valve prevents water, chemicals, and other contaminants from flowing backward from the irrigation system into the water supply. There are several types of backflow prevention devices using various mechanical designs to operate.
Shut-off Valves or Stop Valves
They are most widely used valves, manually operated. Usually installed between the ends of two pipes they serve to start or stop the flow of fluid in the pipeline. Stop valves are primarily designed for just two extreme situations: either to be completely open, to freely pass the full flow of fluid, or to be completely closed, to prevent any flow.
Electronic Remote Control Valves
Electronic remote control valves are normally used in drip irrigation systems for automatic control of water flow. Diaphragm type electronic valves are widely used in irrigation systems because of their simplicity. Parameters for selecting valves are pressure range, flow rate range, inlet and outlet sizes, maximum temperature, and electrical specifications. Electronic remote control valves typically use 24 volts to operate, and most of them have a flow adjustment device on the bonnet.
System pressure varies due to pressure loss through pipes, valves and fittings, and elevation changes. To obtain uniform water distribution throughout the subsurface drip irrigation system, pressure regulators should be installed on each zone to maintain a constant pressure and flow rate. Another reason to use pressure regulation is to reduce system stress and prevent drip lines and fittings from blowing apart.
Air/Vacuum Relief (AVR) Valves
Air/vacuum relief (AVR) valves help prevent negative suction pressure, which can cause serious clogging problems—especially if laterals are buried or in constant contact with settled soil. The presence of free air in water installations causes many difficulties in the piping system at start-up, during operation, and when draining the system.
Water Flow Meters
An important device for measuring water movement between the water source and the greenhouse is the water flow meter. Water flow meters can measure the flow rate of water or the total water that has passed by the measuring point. Flow rate is the volume of water per unit of time moving past the measuring point. Nearly all flow meters record the total flow passing through them in gallons (gal), acre-inches (ac-in), acre-feet (ac-ft), cubic feet (ft3), or some similar volume measurement. Some flow meters also register the instantaneous flow rate in gallons per minute (gal/min), cubic feet per second (cfs), or other similar units of measure.
Propeller Flow Meter
The most commonly used flow meter is the propeller meter (See Figure 13.10). The speed of the propeller is a function of the flow rate. The register accumulates the number of turns of the propeller and converts this to the flow rate or volume of water passing the propeller. It requires installation in a straight section of pipe and for the pipe to flow at full capacity in order to register accurately. They should be installed in a section of pipeline that is straight and unobstructed for 8 to 10 times the pipe diameter upstream of the meter and 4 to 6 times the pipe diameter downstream.
Magnetic Flow Meter
Magnetic flow meters do not have an obstruction in the pipe so there is no opportunity for debris to get clogged in the meter (See Figure 13.11). Unlike propeller meters, readings are not affected by a loss in pressure.
Most greenhouse operations are going to want to have their drip irrigation system set up for fertigation or chemigation (applying fertilizers, pesticides, or herbicides through the irrigation system). Chemigation requires a large tank to hold the chemicals that are to be injected (usually in liquid form), a pump and an injection port that mixes the chemicals with the water.
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