Backflow Prevention and Safety Devices for Chemigation

by Kenneth H. Solomon and David F. Zoldoske


CATI Publication #981201 Copyright August 1998, all rights reserved


ABSTRACT
This paper discusses potential hazards to the water source when "chemigating" under various conditions of irrigation system and circumstance. Current U.S. Environmental Protection Agency requirements are presented for backflow prevention and safety devices to protect the water source when chemigating. A discussion of alternate permissible devices to achieve specific purposes is included. The differences in requirements for protection of urban, potable water supplies and typical agricultural water supplies are noted, and specifications for an agricultural "chemigation valve" incorporating necessary safety functions are given. Field experience with failure rates of double check valve assemblies is reviewed. Other aspects of the necessary safety considerations, such as system interlocks and flow checks, are also discussed.

INTRODUCTION
"Chemigation" is a general term used to describe the addition of a variety of agricultural chemicals to an irrigation system, usually for the purpose of distributing the chemicals throughout the irrigated area along with the irrigation water. Other, more specific terms are used to describe the application of given materials through an irrigation system, with the prefix to "-gation" indicating the type of material applied: fertigation (fertilizers); fungigation (fungicides); insectigation (insecticides); and herbigation (herbicides) (Smith, 1990). Chemigation may also include the introduction of chlorine, acids or other chemicals for the purpose of water treatment or cleaning of the irrigation system components. Benefits of chemigation include economical application, the potential for frequent, precise applications to match seasonal crop needs, and a reduction in soil compaction and mechanical damage to the crop (due to reduced tractor and sprayer operations in the field) (Zoldoske and Jorgensen, 1990).

From the standpoint of backflow prevention and safety, the type of material injected into the irrigation system may be significant. Different levels of toxicity and concentration lead to different safety risks. For example, some government agencies (local, state or national) allow fertigation, but prohibit other forms of chemigation on the grounds that the chemicals (other than fertilizers) used are either inherently toxic (biocides: fungicides, herbicides, insecticides, nematicides, etc.) or dangerous (acids). Other agencies allow all forms of chemigation, but allow a lower level of backflow prevention and safety assurance when only fertigation is to be done.

The Center for Irrigation Technology position is that all systems employing any form of chemigation should incorporate backflow prevention and safety devices commensurate with the injection of the most hazardous class of chemicals, i.e., biocides. The rationale is that since the equipment for injection of fertilizers is the same as for the injection of biocides, fertigation systems are potentially chemigation systems. Government agencies cannot assess the intent of the applicator from the equipment configuration alone. Unless an inspector is present during each irrigation event, there is no way to enforce a ban on the injection of biocides. Therefore all injection systems should be treated as biocide injection systems, and protected accordingly.

WATER SOURCE CONTAMINATION HAZARDS WITH CHEMIGATION
In the absence of properly installed and functioning backflow prevention and safety devices, contamination of the water supply can occur in the event of certain failures in the irrigation or injection system. Potentially hazardous circumstances include the following (ASAE, 1994):

(1) Any shutdown of the irrigation pumping plant. If the main irrigation pump shuts down, the direction of main water flow could reverse. This reverse flow could be driven by water from an elevated system exerting a backpressure on the pump or injection point, or by water draining towards a lower level source (water falling back down a well), creating back-siphoning which could draw chemicals into the water source.

(2) Operation of the injection equipment after an irrigation pumping plant shutdown. If the main irrigation line is not pressurized, the injection pump could force concentrated chemicals or a mixture of water and chemicals to flow into the water source.

(3) A failure of the injection equipment while the irrigation pump is still operating. Pressurized water from the irrigation line could flow back through the chemical holding tank, possibly causing its overflow and spillage of chemicals. The chemical spill could pose a hazard in itself, or could drain towards and enter the water source.

AGRICULTURAL VERSUS POTABLE WATER SUPPLY PROTECTION
Backflow prevention equipment for protecting potable water supplies has been available for many decades (Anon., 1993). However, these devices are generally far too expensive for widespread implementation in agricultural installations. Fortunately, some of the requirements for potable supply protection devices are more strict than may be necessary in an agricultural setting. It is this fact that allows agricultural "chemigation" valves to be manufactured and commercially available at reasonable prices. For example (Zoldoske, 1995), an agricultural chemigation valve of 100 mm (4 inch) nominal size may be purchased for approximately one-fourth the cost of a reduced pressure principle device (RP) of the size suitable for potable water supply protection (ag chemigation valve ~$435; RP device for potable water ~$1,645). The difference is even more pronounced at the larger sizes. For example, an agricultural chemi-gation valve of 250 mm (10 inch) nominal size may be purchased for approximately one-seventh the cost of a reduced pressure principle device (RP) of the same size suitable for potable water supply protection (ag chemi-gation valve ~$840; RP device for potable water ~$5,980.

Some of the key points of difference are serviceability, pressure rating, operating time and potential risk. It is required (Anon., 1993) that potable water supply devices can be repaired without removal from the line. This isn't necessary within an agricultural setting. Devices for potable water supply are usually rated at 12 Bars     (175 PSI). Agricultural chemigation valves may use a lower rating. Most potable water devices operate, or at least are under pressure, nearly continuously and endure far more hours of operation per year and per life cycle than agricultural chemigation.

Finally, the agricultural condition is inherently far less risky than the potable water supply condition. If chemicals are introduced into a potable water supply, potentially they may be ingested directly by humans. Contamination of an agricultural water source, such as a well, is less problematic. The contamination will be further diluted in the ground water, or as it is re-pumped from the well, and it should not be directly consumed by humans.

BACKFLOW PREVENTION AND SAFETY REQUIREMENTS
An air gap separation is generally regarded as the highest form of protection against backflow contamination (Anon., 1993; Zoldoske and Jorgensen, 1990). An air gap is a physical separation between the discharge end  of water supply pipe and an open receiving vessel, for example a reservoir or standpipe. The separation must be at least twice the diameter of the supply pipe, but in no case less than 25 mm (1 inch). An air gap separation is regarded as sufficient protection for even potable water supplies against chemigation backflow hazards. Air gap protection is often well suited for surface irrigation systems. A disadvantage of air gap protection is that the receiving vessel in not pressurized, and so the water may need to be re-pumped to overcome elevation differentials or to achieve the operating pressure required by a pressurized irrigation system.

In the absence of a suitable air gap separation, water source protection from chemigation systems may be achieved with specific combinations of equipment and system interlocks. U.S. Environmental Protection Agency (EPA) regulations specify backflow prevention and related equipment to be used when chemigating. The following descriptions of equipment and assemblies are based on current EPA regulations as cited by Smith (1990) and on the relevant standard, EP 409.1, from the American Society of Agricultural Engineers (ASAE, 1994).

1. Suggested Irrigation Pipeline Equipment

1a. A check valve must be installed between the water source and injection point, to prevent water in the irrigation pipeline from moving back towards the water source.

1b. A low pressure drain must be installed upstream  of the check valve, usually directly below the vacuum relief valve (1c), to dispose of small volumes of fluid which may leak past the check valve.

1ab'. An alternate which substitutes for check valve (1a) and low pressure drain (1b) is a gooseneck pipe loop in the main water line between the irrigation water pump and injection point. The gooseneck pipe loop must contain a vacuum relief (or combination air and vacuum relief) valve at the apex of the loop. The bottom of the pipe at the loop apex must be at least 0.6 m (24 in) above the highest water emitting device in the system. The chemical injection port must be downstream of the apex of the pipe loop and at least 0.15 m (6 in) below the bottom of the pipe at the loop apex.

1c. A vacuum relief valve must be installed upstream of the check valve, usually directly above the low pressure drain (1b), or at the apex of the gooseneck pipe loop, to prevent formation of a vacuum which could cause backsiphonage.

1c'. An alternate which substitutes for the vacuum relief valve (1c) is a combination air and vacuum relief valve.

1d. A low pressure sensor (or flow sensor) must be installed downstream of the irrigation pump to identify pressures too low for proper chemical application or low pressure conditions signifying irrigation pump stoppage or failure.

2. Suggested Injection Line Equipment (when using a metering type injection pump, such as a positive displacement or diaphragm pump, between chemical holding tank and injection point)

2a. An anti-backflow injection valve (a spring loaded check valve with an opening pressure of at least 70 kPa [10 PSI]) must be installed at the point where the injection line joins the main irrigation line to prevent water from flowing back into the chemical holding tank should the injection pump fail, and to prevent gravity drainage from the chemical holding tank when the irrigation line is not pressurized.

2b. A normally closed solenoid or hydraulically operated valve must be installed between the chemical holding tank and the injection pump to prevent unwanted flow into or out of the chemical holding tank.

2b'. An alternate which substitutes for the normally closed valve (2b) is a check valve to prevent water from flowing back towards the chemical holding tank. The check valve shall be spring loaded with an opening pressure of at least 70 kPa (10 PSI) to prevent gravity drainage from the chemical holding tank.

2b''. Another alternate which substitutes for the normally closed valve (2b) is a vacuum relief valve between the injection pump and the anti-backflow injection valve (2a). The vacuum relief valve must be positioned at least 0.3 m (12 in) above the highest fluid level possible inside the chemical holding tank, and must be the highest point in   the injection line. The valve must open at 0.15 m  (6 in) water vacuum or less. The vacuum relief valve may be a combination air and vacuum relief valve.

3. Suggested Injection Line Equipment When Using A Venturi Injector (Note: The venturi injector may be inserted directly into the main irrigation line, into a bypass line, or into a bypass line boosted with an auxiliary water pump.)

3a. A check valve must be installed in the line from the chemical holding tank to the venturi to pre-vent flow back towards the chemical holding tank.

3b. A normally closed solenoid or hydraulically operated valve must be installed between the chemical holding tank and the injection point to prevent unwanted flow into or out of the chemical holding tank.

3ab'. In bypass systems, as an alternate to placing both valves (3a and 3b) in the injection line, the check valve (3a) may be installed in the bypass line upstream of the venturi, and the normally closed valve (3b) may be installed in the bypass line downstream of the venturi.

4. Suggested Power and System Interlocks

4a. Interlock the injection pump or venturi bypass booster pump with main irrigation pump so that the injection or booster pump automatically shuts off when the main irrigation pump stops.

4b. Interlock the normally closed solenoid or hydraulically operated valve (2b or 3b) with the injection pump or venturi bypass booster pump so that the valve closes upon shut down of the injection  pump or venturi bypass booster pump or system.

4c. Interlock the normally closed solenoid or hydrau-lically operated valve (2b or 3b) with the low pressure or flow sensor (1d) so that the valve opens only when the main irrigation line is adequately pressurized.

SPECIFICATIONS FOR AGRICULTURAL CHEMIGATION VALVES
Agricultural "chemigation" valves are commercially available that combine the recommended main line check valve, low pressure drain and air/vacuum relief valve in one product. Specifications for these valves are as follows.

CIT suggests that the chemigation valve's check  shall be spring loaded so that a pressure greater than  2 kPa (6 inches of head, 1/4 PSI) is required to open the valve. The check must be able to withstand for one minute an internal hydrostatic pressure of twice the rated pressure of the valve. It shall withstand for 16 hours, without leakage at the valve seat, an internal hydrostatic pressure equivalent to the head of a 1.5 m (5 ft) column of water retained within the downstream portion of the valve body. No leakage past the clappers is acceptable in either case.

The combination air relief and vacuum relief valve shall have a nominal size at least 25 percent the nominal size of the chemigation valve. The vacuum relief valve must be open at pressures below 10 kPa (1.5 PSI).

The automatic low pressure drain valve shall be a minimum 20 mm (3/4 inch) nominal size attaching to the chemigation valve at the lowest point of the upstream side of the check valve. It shall be pressure activated with an opening point of 10 3 kPa (1.5 0.5 PSI). The drain valve must close above 7 kPa (2 PSI), and must be able to be closed externally to facilitate testing.

The chemigation valve will have an inspection port of at least 100 mm (4 inch) diameter on the top side of  the valve, directly above the low pressure drain. This inspection port may be combined with the mounting of the combination air/vacuum relief valve.

The chemigation valve shall have test ports on each side of the check valve, at the elevation of the centerline of the chemigation valve. The chemigation valve shall be rated at 8.5 Bars (125 PSI) or higher.

The chemigation valve shall be marked with the following: name or identifying symbol; nominal size; model number or catalog designation; rated pressure; mark indicating date of manufacture; arrow showing direction of flow; intended orientation (horizontal or vertical) if this affects the operation of the valve; and a unique serial number for identification purposes.

DOUBLE CHECK VALVE FAILURE RATES
Data reported to the Pacific Northwest Section of the American Water Works Association (Bratton, 1992) over the three-year period 1979-1980-1981 is summarized in Table 1. The data were collected within the jurisdictions of the Portland Water Bureau of Oregon; the Seattle Water Department, Tacoma Water Department, and Modern Electric & Water Company of Washington; and the Richmond Water Department and Vancouver Water Department of British Columbia. Each device is required to be tested at least annually (Anon., 1993), so the failure rates listed may be considered annual rates.

Table 1 implies that while there is some risk reduction due to the redundancy offered by the second check valve, it is not the one or two order of magnitude reduction that might be expected if failure of the two check valves were in fact independent. Conditions that tend to foul the first check valve will also tend to foul the second: one-third of the occasions when the first check failed, the second one did also.

The devices referred to in Table 1 are designed for backflow protection in potable water supply systems. Typically, their operating conditions differ from agricultural installations in that (1) they are dealing with relatively clean water, and (2) they are operating, or at least are under pressure, nearly continuously. It is not known how failure rates of devices under agricultural conditions would compare with the rates in Table 1; however, we believe they would be somewhat higher due to the higher incidence of sediments or contaminants in agricultural waters.

This suggests that failure rates of single check valves in agricultural backflow prevention devices might be on the order of three percent to five percent per service-year. This further suggests that in order to protect agricultural water supplies, backflow prevention devices should be checked at the beginning of each irrigation (chemigation) season, and perhaps once or twice at other times during the season.

ADDITIONAL SAFETY CONSIDERATIONS
An independent water source should be available near the chemical injection site for washing off any chemicals that contact the skin. Note that a hose and/or outlet from the main water supply or irrigation pipeline is not appropriate for this purpose, unless it is also protected from backflow with a suitable device. A hose connected to an outlet downstream of the chemical injection point may discharge chemicals or a mixture of water and chemicals rather than the fresh water needed for washing. A hose connected between the backflow prevention device and the water source will dispense fresh water, but itself constitutes a contamination hazard if not protected from backflow. Consider, for example, what would happen if the hose, used to wash out or dilute chemicals in the chemical holding tank, were left in the tank during a backsiphonage situation. Chemicals to be injected would be drawn back through the hose directly into the water source. One acceptable alternative is to have a separate tank and spigot for fresh water only, not connected to the chemigation system in any way. Another would be to install a small backflow prevention device upstream of the hose connection point.

Workers should use proper goggles and protective clothing when mixing chemicals. Operating instructions and safety precautions for the injection system should be posted where they can easily be seen by workers. Pressurized chemical holding tanks should carry warnings against inadvertent opening when pressurized.

The site should be graded so that any chemical spill runs away from the water source. Chemical storage tanks should be provided with secondary containment, such as an earthen basin, lined with impermeable material, with a capacity at least 125 percent of the chemical storage tanks.

Filters for microirrigation should not be backflushed during chemical injection. Special arrangements may be necessary for filtration systems with automatic backflush cycles.

SUMMARY
With proper selection, installation and maintenance of chemigation equipment, including backflow prevention and safety devices, applying chemicals through an irrigation system can be an effective, economical and safe practice. Commercially available "chemigation" valves, combined with appropriate system interlocks, provide sufficient backflow protection for agricultural water supplies.

Many states and/or counties lack backflow prevention standards on agricultural water supplies. Others maintain strict requirements related to potable water supplies. The purpose of this paper has been to explore safe backflow prevention alternatives between these two potentially extreme positions.

REFERENCES
Anonymous. 1993. Manual of Cross-Connection Control, 9th Edition. Foundation for Cross-Connection Control and Hydraulic Research, University of Southern California, Los Angeles, California, 307 pages.

ASAE. 1994. ASAE Engineering Practice EP 409.1 (Dec. 93): Safety Devices for Chemigation. American Soc. of Agricultural Engineers, St. Joseph, MI, 3 pages.

Bratton G.N. 1992. Personal Communication. Schaefer & Bratton Engineers, Coupeville, Washington, USA.

Smith E.M. (editor). 1990. Chemigation Workbook. Texas Agricultural Extension Service Bulletin B-1652, College Station, Texas, 61 pages.

Zoldoske D.F. 1995. Conversations With Industry Sources. Center for Irrigation Technology, California State University, Fresno, California.

Zoldoske D.F., and Jorgensen G.S. 1990. Careful Chemigation Could Help Growers. Western Fruit Grower, April 1990, 110(4):26,28.

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