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Fluid Journal : Early Spring 2013
9 The Fluid Journal Early Spring 2013 of microirrigation system components that can cause a pressure differential are a partially closed valve or a pressure- reducing (pressure regulating) valve. The rate the material is injected is influenced by the flow rate through the tank and the concentration of injection material in the tank at any given time. The batch tank is filled with the chemical (most frequently, fertilizer) to be applied. During irrigation, water is allowed to flow into the batch tank where it displaces some of the tank's contents, forcing them into the irrigation system. Initially, the liquid leaving the batch tank is of high chemical concentration, but with time, the concentration in the batch tank becomes more diluted as it mixes with water. The chemical injection starts out at a higher concentration and declines in concentration during the injection period. Batch tanks are appropriate if the objective is to inject a total amount of chemical (e.g., fertilizer) during a long injection period. Batch tanks are not appropriate if a constant injection concentration is required. Venturi injection. Injection devices using the venturi principle (Figure 2) have been used for many years in a wide variety of industrial and agricultural applications. A venturi is a specially shaped constriction in a device's water flow path. As the flow passageway at the venturi section becomes smaller, the velocity of the flowing fluid increases such that a vacuum is formed at the venturi's throat section. An opening located in the venturi's throat allows air or a fluid to be "sucked" in and mixed with the water stream. To create this effect, the inlet pressure to the venturi must be at least 15 to 20 percent greater than the outlet pressure. This is achieved by plumbing the venturi device across a system pressure differential (e.g., a partially closed valve, Figure 3) or using a small pump that draws water from the microirrigation system and forces it through the venturi injector (Figure. 4). The injection rate of a venturi-type injector depends on the size of the venturi section (1/2-inch to 2-inch sizes are available) and on the pressure difference between the inlet and the outlet of the venturi. The venturi injector delivers a more constant chemical injection rate than does a batch tank. However, the injection rate of a venturi injector can change (or even stop) if the pressure changes upstream or downstream in Figure 1. Batch tank. Figure 2. Venturi Injector
Late Spring 2013