A sprinkler backflow preventer is a safety device installed on an irrigation system to safeguard the public drinking water supply. This mechanism is designed to be a one-way gate, ensuring that water flows only from the potable source into the sprinkler system. Its primary purpose is to stop irrigation water from reversing its flow and contaminating the clean water supply. The device is mandated at the point where the non-potable irrigation system connects to the main potable water line.
Understanding the Risk of Backflow
The necessity for this device stems from the concept of a cross-connection, which is any link between a drinking water system and a source of non-potable water. In an irrigation system, the non-potable water contains fertilizers, pesticides, herbicides, and other chemicals, along with stagnant water and bacteria. If this water reverses direction, it can introduce these contaminants into the municipal water supply, creating a serious public health hazard.
Backflow occurs through two mechanisms: back-siphonage and back-pressure. Back-siphonage happens when there is a sudden drop in the supply-side water pressure, such as from a water main break or heavy fire-fighting demand, creating a vacuum effect that sucks water backward into the main line. Back-pressure occurs when the pressure in the irrigation system exceeds the pressure in the public water supply, effectively forcing the contaminated water upstream. For public safety, backflow prevention is a requirement enforced by local plumbing codes and health departments to isolate these two water sources.
Principles of Operation
Backflow preventers operate by creating a pressure differential that blocks reverse flow. At the heart of the device are spring-loaded check valves that allow water to flow freely in the correct direction but immediately close if the water attempts to reverse course. These check valves maintain a higher pressure on the supply side, ensuring the one-way flow of clean water.
In more complex assemblies, a relief valve is positioned between two check valves, creating a pressure-monitored chamber known as the reduced pressure zone. Under normal conditions, the pressure within this zone is maintained at a level lower than the supply pressure. If the first check valve begins to leak or if back-pressure from the irrigation side increases, the relief valve automatically opens to the atmosphere. This action vents the potentially contaminated water out of the system, preventing it from crossing the barrier into the potable supply.
The closure of the check valves and the opening of the relief valve are mechanical responses to changes in pressure dynamics. If the supply pressure drops, the check valves snap shut to prevent back-siphonage. If the downstream pressure is higher than the upstream pressure, the relief valve activates to discharge the water. This maintains the pressure differential necessary to protect the clean water source.
Comparing Common Backflow Preventer Types
Two of the most common assemblies used in irrigation systems are the Double Check Valve Assembly (DCVA) and the Reduced Pressure Zone Assembly (RPZA). The DCVA is composed of two independent, spring-loaded check valves in a series. This assembly is generally used for low-to-moderate hazard situations, such as typical residential irrigation systems where the primary risk is non-toxic stagnant water.
The RPZA adds a differential relief valve positioned between the two check valves. This relief valve provides a fail-safe mechanism by discharging water in the event of a pressure failure. RPZAs offer the highest level of protection against both back-siphonage and back-pressure. They are required for high-hazard installations, such as systems that inject fertilizers or pesticides directly into the water line.
While DCVAs can often be installed below ground, RPZAs must be installed above ground. RPZAs must be placed at least 12 inches above the highest point in the system. This placement allows for the proper function of the relief valve and prevents flooding.
Required Maintenance and Testing
The reliability of backflow preventers is maintained through mandatory procedures. Most jurisdictions require annual testing of the assembly by a certified backflow tester. This testing verifies that the check valves are sealing correctly and that the relief valve, if present, is opening at the precise pressure differential.
The certified tester uses a calibrated gauge test kit to measure the pressure across the device components. The results must be documented and submitted to the local water authority. Failure to complete this annual test can result in fines or the termination of water service. Additionally, these devices are susceptible to freezing, so proper winterization, which involves draining the assembly of all water, is necessary to prevent internal damage.