System Layout and Components

Basic components of a microirrigation system are the pump, filtration equipment, controllers, main pressure regulators, control valves, water-measuring devices, and chemical injection equipment, which usually are centrally located at the pump/filtration station; the delivery system, including the main and submain pipelines that transfer water from

Very sensitive Sensitive Low sensitive

Extremely sensitive filters < 80 /xm Very sensitive filters ^ 80 /xm filters ~ 100 /xm

Low sensitive filters ^ 125 ¡xm

Very low sensitivity filters ^ 150 ¡xm

Table 5.11. Classification of emitter coefficient of manufacturing variation, Ct

Classification

Cv Range

Point source

Excellent Average Marginal Poor

Unacceptable Line source Good Average

Marginal to unacceptable >0.20

the source to the manifolds, which also may have filters, pressure regulators, and control valves; the manifolds, which supply water to the laterals; and the laterals, which carry water to the emitters. Figure 5.34 shows the layout of a typical system.

Pump/Filration Station

Elements that may be required at the central supply point include (see [11, 98, 123, 127-130]):

• a reservoir to store water if the water supply varies in rate or time from the system requirements;

• a pump to pressurize the water (unless the water supply is adequately pressurized);

• a backflow prevention valve to prevent the backward flow of water that may contain chemicals or fertilizers into the pump or main supply system or well;

• a chemical injection system for injecting fertilizer and chemicals into the microirrigation system;

• primary, and possibly secondary, filters to clean particulates from the water;

• control valves to manually or automatically regulate flows or pressures;

• an air-release/vacuum relief valve, located at a high point, used to release any air before it enters the delivery system and relieve vacuums following pump shutdown or valve closure; and

• a flow measurement device.

Filtration. Filters are critical to reduce clogging of drippers by solid particles suspended in the water. Prefiltration also may be required to prevent damage to the pump and valves. Burt and Styles [124] give details about filtration needs and designs.

Prefiltration usually is accomplished by settling in a reservoir and/or some type of prescreening. Water residence time in reservoirs should be 1-2 h to settle out all sand-sized particles and a portion of the silts. Reservoirs may introduce organic contaminants such as algae into the water supply. Prescreens include manual or automatically brushed trash racks and coarse screens, self-cleaning gravity overfall screens such as turbulent fountain screens [61], and manual and self-cleaning cylindrical screens for pump intakes.

Four main types of filters are used with microirrigation systems. The vortex sand separator (hydrocyclone) can be used to remove dispersed solid material, particularly sand. The hydrocyclone is a conical container, wider at the top. Water enters tangentially at the top and flows down at a high rotational velocity. This pushes the solid particles against the wall of the container from where they are carried downward toward the collecting chamber. The clean water at the bottom reverses direction and flows upward axially through the center of the container and out the discharge line. The accumulated sand is discharged periodically from the lower-end chamber. Sand separators should not be used as the final filter because it removes only a portion of the particulates.

Cylindrical screen filters use finely woven screen to prevent passage of particles. Screens work best for mineral particles (sands and silts) and for water without a large particulate load. The screens usually are cleaned by flushing water through the upstream chamber. However, the flow velocity across the screen surface is seldom sufficient to clean the screen. More thorough cleaning requires backflushing clean water through the screen, or by manually removing the screen cylinder and cleaning it. Screen filters are available with an automatic vacuuming mechanism that cleans the screen.

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Figure 5.36. Media filiters: (a) Normal operation; (b) reverse flow for filter cleaning.

Figure 5.36. Media filiters: (a) Normal operation; (b) reverse flow for filter cleaning.

Disc filters comprise a series of grooved discs that are stacked together. When pressed together, the rings form a cylindrical filtering body. They work well for removing mineral and organic matter. They are cleaned by separating the disks and washing them off. Self-cleaning disc filters are available that automatically release the tension on the discs and backflush water through them.

Media filters use sand and gravel in tanks to filter out both mineral and organic particles (Fig. 5.36). The sand is sized to achieve the required filtration. Media filters are recommended when the water is relatively dirty. They are cleaned by diverting the inlet and backflushing clean water through the media. Effective cleaning requires sufficient backflush water velocity to lift the sand media without washing it away.

Filters are designed on the basis of flow rate and allowable pressure drop, the amount and type of suspended particles in the flow, and the size of particulates that are allowed to pass. Some self-cleaning screen and disc filters require as much as 220 kPa pressure to operate effectively, and may require downstream pressure-sustaining valves.

Emitter manufacturers should specify the filtration size required. Typically, the system must filter to at least one-fifth, and preferable to one-tenth, the smallest opening in the emitter. For example, water for a microspray with a 0.4-mm orifice would need to be filtered to at least 80 ¡m. Common filters for microirrigation filter to between 80 and 150 ¡m.

When water carries a large amount of suspended material, the filter should clean automatically. Automatic cleaning can occur in response to pressure loss across the filter and/or at preset time intervals. In media and disk filters, the flow through the filter must be reversed, and so, multiple filters must be used in parallel so that, as the filters are cleaned in turn, the others can operate normally to continue to discharge to the fields and to supply clean backflush water.

Where the water is relatively dirty, use of two filters in series is desirable. This prevents the microirrigation system from being ruined if one of the filters fails. A common combination is to have an automatic, self-cleaning media filter backed up by a manual screen filter. The backup filters also can be placed at the inlets to the manifolds in the field, to protect against particulates that might enter the pipelines during repairs or from scale or algae that might originate in the pipelines.

Filters do not, as a rule, completely solve the problem of clogging. Microirrigation laterals should be flushed periodically to remove accumulated fine particles. Calcium carbonate deposits in the system can be dissolved with a solution of nitric or hydrochloric acid (2 to 5 L acid per m3 water). Deposits of organic material can be removed by flushing the system with water for about 15 min and then with compressed air under high pressure. When surface waters are used, microorganisms and small algae may pass through the filter and later accumulate and grow within the system. Treatment with chlorine (1 to 5 mg L-1) continuously or periodically will prevent algae growth.

Some substances in the water, such as iron and manganese, pose special clogging problems [124]. An analysis of irrigation water for inorganic constituents is recommended before design of the filtration/chemigation system.

Chemical Injection. Fertilizer and other soluble chemicals such as acids and chlorine can be added to the pressurized water by several methods. The simplest method is to bypass a small portion of the flow around a fitting that creates a small head loss or pressure differential (<5 kPa), through a chemical storage tank. As the water flows into the tank, the solution is forced out the opposite end and into the system. A valve regulates the flow rate. The tank must be able to hold the maximum expected system pressure. Depending on the density and mixing of the solution, the chemical concentration of the tank outflow will decrease with time. This simple method is used only if the concentration of the added chemical can vary with time. A variation on this pressure-tank concept is to pressurize the tank with a compressed gas (usually N2) to force the solution into the irrigation system.

Electrically driven injection pumps are an accurate method for adding chemicals at a constant rate. The flow rate of these diaphragm or piston pumps is regulated with a motor speed controller, or by mechanical adjustment of the piston displacement. The pump materials must be compatible with the chemicals used. Good-quality injection pumps are relatively expensive and require good maintenance, but they deliver an adjustable, constant flow and require no pressure loss in the water delivery system. Hydraulically driven injection pumps are also available. Some hydraulic pumps can automatically adjust flow rate proportional to the system flow rate to maintain constant chemical concentrations.

The third method to inject chemicals is through a venturi. All or a portion of the flow is diverted through a venturi. High-velocity flow through the venturi throat, such that the flow velocity head exceeds the water pressure, creates suction that draws solution from the chemical tank. Chemical injection venturis are available in a range of sizes and configurations. A large venturi in the delivery line creates a substantial (15%-30%) pressure loss. A valve in the delivery line to create the bypass flow through a small venturi is simple and inexpensive, but requires more than 70 kPa pressure drop in the water delivery system, and thus can be very expensive in terms of energy cost. A less-energy-consuming alternative is to use a small booster pump to bypass a small portion of the water through the small venturi.

Fertilizers and chemicals that will not cause filter damage should be injected upstream of the final filter in case particulates are introduced with the solution. Chemical application should begin after the lateral lines are fully pressurized and cease with sufficient irrigation time remaining to flush the entire system with clean irrigation water.

Control/Monitoring Devices. Pressure regulators are required if the inlet pressure to the system may vary beyond an acceptable range. Reasons for varying pressure include fluctuating water supply pressures, such as fluctuating well-water depths, and varying system flow rates. Constant system pressure will ensure constant irrigation application rates to all manifolds. Pressure regulation is critical if the pressure can exceed the burst pressure of system components. A pressure relief valve can protect against system overpressure.

If system hydraulics or topography result in nonuniform pressure at the various manifolds, regulators may be required at the manifold inlets. Pressure-compensating emitters may eliminate the need for system pressure regulation. Pressure regulators normally require 40 to 80 kPa pressure loss to operate properly and thus may increase the system pressure requirements [116].

Microirrigation allows precise water application. Automatic control of microirrigation provides precise water application without high labor. Automatic control of irrigation times can be accomplished in a number of ways. Commonly, a master-control unit is installed at the central pump/filtration station and is connected via buried wires to field stations located along the main pipeline. Each field station in turn controls a number of automatic valves (fitted with solenoids) at the inlets to submains or manifolds. The system automatically activates successive groups of manifolds to apply any desired amount of water and fertilizer. The control unit records any problems, such as pump failures, that may occur.

Flow measurement is important to ensure that the system is operating as designed. Decreased flow rate is often the first indication that emitters are plugged. Increased flow may indicate system failures such as lateral breaks. Flow totalizers document power outages. Flow should be measured at the pump/filtration station, and may be measured at submains. Because microirrigation water is clean, several types of mechanical and electronic devices can be used. Pressure measurement also is recommended to help verify proper system operation.

Distribution-Pipe Network

A manifold with its attached laterals is the basic system subunit and the last control point in the system. Manual valves, automatic valves, pressure regulators, filters, and flow measurement can be provided at manifold inlets. These controls also can be provided upstream at bifurcations between the main and submain units. Upstream from pressure control points, the allowed pressure variation for the subunit does not affect pipe-size selection. Therefore, the selection of pipe sizes for the main water supply lines should be based primarily on the economic trade-off between energy costs and costs for pipe and installation.

Normal operating pressures being low, the pipe pressure class rating will be determined more by structural strength than by system requirements. Air-vacuum release and pressure-relief valves should be considered and incorporated into the main system at the appropriate locations. Means to flush and drain the pipelines also should be provided.

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