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Fluid Journal : Early Spring 2013
10 The Fluid Journal Early Spring 2013 the microirrigation system. This can occur if irrigation sets (applications) with different flow rates are operated from the same water supply pump. A venturi injector installation in which the venturi is plumbed across a pressure differential (see Figure 3) is particularly sensitive to such changes, and it is often inconvenient to readjust the injector. The injector is usually installed parallel to the microirrigation system pipeline, with valves that can be closed to isolate the injector from the irrigation system when injection is not occurring (see Figures 3 and 4). A venturi injector installation using a small pump in conjunction with the venturi eliminates the need to install the venturi across a pressure drop, and it also minimizes the venturi's sensitivity to irrigation system pressure fluctuations. However, this type of installation requires an electrical or gasoline power source. Positive displacement pumps have a relatively small capacity and deliver a very constant rate of injection. The pumps can use a cylinder-piston configuration or a flexible diaphragm to inject a liquid at a pressure higher than that of the irrigation system. Electrically- driven, gasoline-engine-driven, and water-driven pump injectors are available. Positive displacement pumps provide the most accurate and constant injection rate, but they are also the most expensive injection devices. They do not need to be installed across a pressure drop since they are externally powered. Positive displacement pump injectors are available as constant-rate pumps and as proportional pumps. Constant- rate pumps inject at a set rate (often adjustable) no matter what the flow rate is in the irrigation system. Proportional pumps (frequently water-driven) inject at a rate dependent on the flow rate passing through the injector or through the irrigation system. For the electrical or gasoline-driven pumps, the injector is linked to the irrigation system via a flow meter. For example, a proportional positive displacement injector set at 1:250 would inject 1 gallon of material for every 250 gallons of water passing through it. The proportional rate setting can be adjusted, as can the stock tank mixture concentration, to control the injected material's concentration in the irrigation system. Solutionizer machines. With the relatively recent advent of solutionizer machines, a wide variety of less- soluble solid materials is now being Figure 3. Venturi "across a pressure drop" layout Figure 4. Venturi with a small pump
Late Spring 2013