15.3: Backflow Prevention and Other Safety Devices

As with all chemical applications in agriculture, there is always a concern about the potential for environmental contamination and for worker safety. With chemigation a primary concern is chemical contamination of the irrigation water source due to backflow of the waterchemical mixture in the irrigation pipeline and/or the flow of concentrated chemical from the supply tank to the water source. Another important matter is soil contamination with concentrated chemical in the injection area. Flow of chemical to the water source is not an issue as long as the irrigation system is operating since the flow direction is away from the water source. When the irrigation water flow stops there is potential for backflow.

Figure 15.4. Soil and water source contamination scenarios with chemigation systems (modified from Eisenhauer and Hay, 1989).

In Figure 15.4 several soil and water source contamination possibilities that occur with chemigation are illustrated. In the first scenario (Figure 15.4a) the injection system could shut off unexpectedly while the irrigation pump continues to operate, causing water backflow through the chemical injection system and an overflow of the supply tank. This can lead to soil contamination near the injection site with subsequent potential for leaching to the groundwater, overland runoff of chemical, or flow of chemical to groundwater via the gravel pack of a well. Another possible occurrence is the flow from both the irrigation water supply and the injection system stopping resulting in backflow of the water-chemical mixture to the water source (Figure 15.4b). The most environmentally hazardous scenarios are when concentrated chemical is allowed to flow directly to the irrigation water source. This can occur by gravitydriven flow from the supply tank to the water source (Figure 15.4c) when both the irrigation water flow stops and the injection pump stops. Probably the worst case scenerio occurs when the irrigation water flow stops but the injection pump continues to operate (Figure 15.4d). Eisenhauer et al. (1988) found that there was over 400 times as much pesticide in a full supply tank than was present in the water-chemical mixture in a 130-acre center pivot irrigation system lateral. This not only illustrated the environmental value but the monetary incentive of retaining the concentrated chemical in the tank.

The risk of contaminating the water source and soil near the injection site can be minimized by using the proper backflow prevention and chemigation safety devices (Eisenhauer and Hay, 1989, Kranz et al., 2015, and Threadgill et al., 1990). These devices will be discussed in the following sections.

Irrigation Pipeline Safety Devices

Backflow prevention in the irrigation pipeline reduces the risk of direct chemical contamination of the water source caused by the scenarios illustrated in Figures 15.4 b, c, and d. The chemigation check valve assembly (CCVA, Figure 15.5) is the most common method of backflow protection on irrigation pipelines that are connected to privately owned wells or single function irrigation water supply districts that are not used as a potable water supply. This is not an acceptable device for irrigation pipelines that are directly connected to public water supply distribution systems. The CCVA is designed for both backpressure and backsiphonage conditions. The check valve in the CCVA is usually an internal spring-loaded valve with a swing gate that is fitted with a resilient-gasket. This valve closes automatically when the irrigation water flow stops. The location of chemical injection must be downstream of the CCVA. Usually incorporated in the CCVA are a vacuum relief valve, a low pressure drain and an inspection port, all located upstream of the check valve. When irrigation water flow stops the check valve closes preventing backflow of the waterchemical mixture in the irrigation pipeline (Figure 15.6a). The vacuum relief valve allows air into the system preventing the creation of a vacuum that could lead to siphoning. In the event that the check valve fails, the low pressure drain can discharge small leakage rates away from the water source (Figure 15.6b). The inspection port (Figure 15.7) can be used by the irrigator or regulatory agency personnel to visually check for leakage from the check valve.

Figure 15.5. Chemigation check valve assembly. (Bottom image courtesy of Kranz et al., 2016.)

Figure 15.6. (a) Backflow prevention operations of CCVA check valve and (b) low pressure drain valve. (Images courtesy of DeLynn Hay, Nebraska Extension.)

a)

b)

An air gap is an acceptable method and alternative to the CCVA. An air gap is created by discharging the irrigation water supply into a tank, reservoir, or farm irrigation ditch in a manner such that there is a free atmospheric vertical separation between the discharge from the water supply pipeline and the water surface in the reservoir (Figure 15.8). The recommended air gap vertical distance is two times the inside diameter of the supply pipeline with a minimum distance of 1 in. (AWWA, 2015). The chemical is then either mixed in the water in the reservoir such as would be possible in the smallholder system illustrated in Figure 14.7 or injected into the water downstream of the reservoir or into the farm irrigation ditch. Depending on the reservoir elevation an irrigation pump may be necessary downstream of the reservoir.

Recommendations for backflow prevention when irrigation systems are connected to a public water supply system are presented by AWWA (2015). In general irrigation connections are considered a high hazard by AWWA (2015) which recommends four acceptable methods for backflow prevention: an air gap (discussed above), reduced-pressure zone backflow prevention assembly, a pressure vacuum breaker assembly, or an atmospheric vacuum breaker assembly.

Figure 15.7. Inspection for check valve leakage in CCVA.

Figure 15.8. Air gap separation backflow prevention.

The reduced-pressure zone (RPZ) backflow prevention assembly (Figure 15.9a) consists of two independently-acting internally-loaded check valves in series with one another and a differential pressure relief valve located in the chamber between the check valves and lower in elevation than the upstream check valve. The RPZ device is capable of preventing backflow in the event of either a backpressure or backsiphonage conditions. The loading or opening pressure of the upstream check valve (minimum 3 psi) creates a differential pressure across the valve. The relief valve opening pressure (minimum 2 psi) is less than the differential pressure created across the first valve. In a backsiphonage condition the lower pressure upstream of the first check valve will cause the relief valve to open to the atmosphere and prevent backflow to the water source even in the event of failure of the second (downstream) check valve. The relief valve will also open in a backpressure condition if the second check valve fails and allows leakage of pressure into the middle chamber.

Figure 15.9. Backflow prevention assemblies, (a) reduced-pressure zone and (b) pressure vacuum breaker.

a) b)

The pressure vacuum breaker (PVB) assembly is the third alternative for preventing backflow in a high hazard environment (Figure 15.9b). It is only applicable in a backsiphonage condition. The assembly includes an internally loaded check valve and an internally loaded air-inlet vacuum relief valve that opens to the atmosphere. The PVB must be positioned so that the elevation of the discharge pipe is a minimum of 12 inches higher than the elevation of the highest irrigation outlet. Like the PVB the atmospheric vacuum breaker (AVB) can also be used under high hazard backsiphonage conditions, but it has more limited application in irrigation systems because shutoff valves downstream of the AVB are not allowed. Under normal irrigation flow conditions, the poppet in the AVB seals on the air-inlet seat. Under a back-siphonage condition the poppet opens and drops to seal on the check valve seat. The poppet is not spring loaded so if a closed downstream valve causes the poppet to be seated at the air inlet for long periods of time there is a risk for it to stick to the seat and not open when backsiphonage occurs.

Public suppliers of potable water and local plumbing codes almost invariably have their own requirements and specifications for backflow prevention which are usually based on recommendations by the American Water Works Association and the Foundation for Cross-Connection Control and Hydraulic Research at the University of Southern California.

Chemical Injection Pipeline Safety Devices

As discussed above it is imperative to stop the flow of concentrated chemical from the supply tank when the irrigation water flow stops. The one-way interlock between the irrigation water supply and the injection device, as discussed in Section 15.2.1, stops the injection pump when the water flow stops preventing the situation illustrated in Figure 15.4d. But the interlock will not stop the flow caused by gravity illustrated in Figure 15.4c. A chemical injection line check valve (Figure 15.10) will help prevent this flow. The check valve is internally loaded, usually by spring, so that it has an opening pressure of 10 psi or greater. At 10 psi the valve would block flow until 23.1 feet of water head is exceeded. The chemical injection line check valve will also stop the backflow through the injection system in the event that the injection system stops but the irrigation water continues to flow (Figure 15.4a). The chemical injection line check value is usually an integral part of the injection port with the discharge end near the center of the irrigation pipeline as would be the case for the valve shown in Figure 15.10a.

Figure 15.10. Chemical injection line check valve. (Image b courtesy of DeLynn Hay, Nebraska Extension.)

a) b)

Whenever possible it is helpful to place the point of chemical injection at an elevation higher than the maximum liquid level in the supply tank Figure (15.11b). This will provide more protection against flow caused by gravity. If this is not possible another technique for additional protection is to create a vertical pipe loop with a vacuum relief valve at the apex. The apex must be at an elevation higher (minimum 12 in) than the maximum elevation of the liquid level in the supply tank (Figure 15.11a). The vacuum relief valve will break the siphon and stop the flow from the tank.

Another valve that is useful on the injection tubing is a bleed valve (Figure 15.10a). Upon system shutdown, pressure is usually locked into the tubing between the injection pump and the chemigation line check valve. The bleed valve can relieve this pressure before the tubing is disconnected preventing the operator from being sprayed with concentrated chemical. The bleed valve can also be helpful for removing air from the injection line when priming the system.

Figure 15.11. Chemical injection line options for providing additional protection from chemical flow due to gravity from the chemical supply tank into the irrigation pipeline. (Modified from Eisenhauer and Hay, 1989.)

a) b)

For further safety, a normally-closed solenoid valve on the inlet side of the injection pump can be electronically interlocked with the injection pump power supply to provide a positive shut off on the chemical injection line if the injection pump stops. This valve is sometimes included in regulatory requirements. Another device that is sometimes required by regulation is a flow sensor positioned in the injection line just upstream of the injection port. The flow sensor safeguards against continued operation if there is a rupture of the injection line, injection pump failure, loss of prime, or the injection port is plugged.

Irrigation Pipline Low Pressure Switch

A low pressure switch on the irrigation pipeline will shut the irrigation system and injection system off if the system pressure drops below a critical point. One potential advantage of chemigation is the uniformity of chemical application which is dependent on the irrigation application uniformity. If the pressure is too low the irrigation uniformity is compromised making it important to stop the application.

Other Safety Items and Considerations

A strainer on the inlet side of the injection device is essential to prevent foreign materials from clogging or fouling the injection pump, chemical injection line check valve, or other injection system safety equipment. For public and applicator safety posting of fields (Figure 15.12) can be helpful and may be required by regulations. To avoid complacency it is usually recommended/regulated that the signs not be permanent but have specific times of posting prior to chemigation and following the event, for example a maximum of 48 hours before application and 48 hours after the pesticide re-entry period.

Chemigation applicators should follow safety procedures that are common to all chemical application systems such as wearing the appropriate protective clothing (gloves, goggle, rain gear, etc) and adhering to re-entry periods that may be specified on a pesticide label. A fresh water faucet located upstream of the chemical injection port and preferably upstream of the CCVA is advised for washing as needed.

Figure 15.12. Field posted for chemigation.

Federal, State, and Local Regulations

In the United States the practice of chemigation is regulated by federal, state, and local government agencies. The intent of the regulations is to reduce the risk of environmental contamination and to protect worker and public safety. Essentially all of the regulations contain some if not all of the backflow prevention and safety equipment discussed above and it is common that the regulations will reference the Engineering Practice, ASAE EP 409.1 Safety Devices for Chemigation (ASABE, 2018).

At the federal level the application of pesticides is regulated by the U.S. Environmental Protection Agency (EPA) through the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). FIFRA requires that pesticides be applied according to the product label. Fertigation is not regulated by FIFRA. The Label Improvement Program, U.S. EPA Pesticide Registration (PR) Notice 87-1 and an updated list of alternatives provides pesticide manufacturers with generic statements that they can incorporate on the label of their product. For example the label will state whether or not the product can be applied using chemigation and if so what specific safety requirements must be followed. Are there field posting requirements? Is chemigation limited to specific types of irrigation systems, such as sprinklers? What are the specific backflow and safety equipment requirements? For products labeled for chemigation it is common to see the requirement of a CCVA, a chemical injection line check valve, a normally-closed solenoid valve on the inlet side of the injection pump, a one-way system interlock, irrigation system low pressure shutoff switch, and field posting. An example of an acceptable alternative device is to substitute the normally-closed solenoid valve with a chemical injection line check that has an opening pressure of 10 psi or higher. If the label allows for application of a pesticide using water from a public water supply it is likely to state that either an approved air gap or RPZ are required and that specific buffer distances from public places must be followed.

Many states in the U.S. have chemigation regulations and usually these apply to both fertilizers and pesticides. The state requirements can be more extensive than federal requirements but not less. An example is that some states require two CCVA assemblies placed in series if pesticides are to be applied. States usually provide lists of approved CCVA’s that are commercially available. Permitting of chemigation sites is often required and the equipment at permitted sites may be regularly inspected for working performance. Chemigation applicator training and competency testing is sometimes required by states. Other items in some state regulations include accident reporting, secondary containment of the supply tank, and specific pre and post time requirements of posting fields.

Local government subdivisions may also have their own more chemigation regulations. For public water supply systems such as municipalities, the local plumbing code usually has specific regulations for connected irrigation systems.