15.2: Chemical Injection Pumps

Chemical Injection Pumps

To inject chemical solutions into a pressurized irrigation water stream requires the following equipment components: An injection pump, a chemical supply tank, injection tubing and associated valves, and calibration devices (Figure 15.1). There are three main types of pumps that are commonly used for chemical injection: piston, diaphragm, and venturi injectors (Figures 15.2 and 15.3). Piston and diaphragm pumps are classified as positive displacement pumps where pump discharge is not greatly influenced by the level of pressure in the irrigation pipeline. The lower pressure in the throat of venturi injectors leads to the chemical solution being inducted into the water stream. The required pressure differential across the venturi makes the injection rate sensitive to the irrigation system pressure. In-line venturi injectors can be used for smaller irrigation pipelines while for larger systems, a by-pass line equipped with pressure reducing valves on the irrigation pipeline or an auxiliary booster pump in the by-pass line are necessary to create the required pressure differential for the venturi to function properly (Figure 15.3). Tests conducted by Kranz et al. (1996) found that regardless of the type of injection device, on-site calibration is necessary under the inlet and outlet pressure conditions at the site. Thus, the chemigation application system should be equipped with a calibration device, usually a calibration tube plumbed at the tank outlet.

For smallholder farmers who use gravity water supply systems such as those shown in Figure 14.7 an injection pump is not necessary. In this case the chemical can be mixed with the irrigation water in the supply reservoir and distributed with the irrigation system. Essential characteristics of the injection pump include material compatibility with the chemical being injected, flow adjustment capability within the range of its maximum capacity, and metering accuracy. Additional desirable characteristics may include flow proportional pump controllers and adaptability to precision agriculture. With flow proportional control the chemical injection rate can be made proportional to the flow rate in the irrigation pipeline. This is especially useful for chemical application accuracy when the land area irrigated per unit time varies, e.g, center pivot systems equipped with end guns and swing-boom corner water systems. Eisenhauer and Bockstadter (1990) found that without flow proportional injection rates, chemical application rate errors can exceed 20% with these scenarios. This error can be reduced to 4% or less when using flow proportional injection. Flow proportional injection also allows for variable rate chemical application when used with variable rate irrigation systems for either sector control or zone control (Lo et al., 2018). Injection pumping systems are available for sector control variable rate chemigation and do not require simultaneous variable rate irrigation.

There are many options for powering the chemical injection pump including belt connection to the power shaft of internal combustion engines (Figure 15.2a), electric motors (Figure 15.2b), and oil hydraulic motors. It is desirable that these power sources are connected to the irrigation power sources in such a way that if the irrigation system shuts off, the injection pump will shut off simultaneously, a one-way interlock. This prevents concentrated chemical from continuing to be pumped into the irrigation pipeline. Also, it is desirable, especially for continuously moving irrigation systems, to have the irrigation system shut off in the event that the injection system shuts off inadvertently. This two-way interlock will prevent untreated areas in the field.

Figure 15.1. Chemigation system (drawing on left modified from Eisenhauer and Hay, 1989).

Figure 15.2. Positive displacement injection pumps: (a) piston, (b) diaphragm.

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b)

Figure 15.3. Venturi injectors (a) with valve in line to create pressure differential and (b) with booster pump to create pressure differential. Images courtesy of Mazzei Injector Company.

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b)

Tanks and Chemical Injection Tubing

The chemical supply tank and chemical injection tubing, associated fittings, and all backflow and safety devices should be made of non-corrosive and chemically resistant materials. In addition, it is important that plastic tanks be made of sunlight resistant materials since it is common for them to be exposed to sunlight for long periods of time. Tank failure and tubing failure can result in a spill of concentrated chemicals resulting in expensive chemical losses and significant soil contamination near the injection site. Depending on the chemical and tank volume secondary containment may be required by regulations.

A common feature of chemical supply tanks is an agitator for mixing purposes. To avoid the accumulation of precipitates in the irrigation system the compatibility of the chemical with the irrigation water should always be evaluated before injecting the chemical. A simple “jar test” can be conducted. In a clear glass jar mix the chemical with the irrigation water to a concentration slightly higher than the planned concentration to be applied. After allowing the jar to sit undisturbed for 24 hours examine the contents for cloudiness, scums, and sediments, indicators of potential chemical precipitation problems.