What is a check valve? A Check Valve is a one-way valve, allowing flow in one direction only.
What's the difference between check valves, flush valves, backflow preventers and relief valves? Each of these items have a specific purpose and because they have many different names we tend to get confused on what the purpose is of each one.
The word "valve" should make us think of stopping and/or starting the flow of something (liquid or gas). Check valves are used in many applications but we will focus on what we use in irrigation. No human intervention is required for the check valve to do its job, it is operated by flow and differential pressure. Some areas of use include: protecting equipment such as pumps and filters from backflow damage; stopping low head drainage in a sprinkler system with elevation changes; preventing contamination of water source from back siphoning in the irrigation system; even providing pressure relief in liquid or air (gas) applications. As with any valve or pressure sensitive component you will want to match the item specifications to fit your system design, especially flow rates and opening and closing pressures.
Inline check valves are used anywhere you want to prevent water draining from the system after shutting down. This might be in overhead watering in a greenhouse or a low sprinkler head in your yard. It is most common in an irrigation application that the check valve is open in positive pressure systems and closes when system pressure drops, to prevent reverse flow.
A Flush Valve is a device used at the ends of irrigation lines or in low spots in your drip lines. Its purpose is to perform a “quick flush” at the start-up of your irrigation cycle. When your system starts, before it fully pressurizes, the valve is open and a few good spurts of water are ejected out the valve, flushing away any debris that may have been siphoned in through the emitters at the last cycle shutdown. An added benefit is the valve opens again as your system depressurizes (shuts down) and does help water drain out if installed at low points. See our selection of Flush Valves. A similar item for sprinkler systems is the Auto Drain Valve.
Here is a video showing how the Perma-Loc Flush Valve works
Many Backflow Preventers are a combination of a normally closed air vent and a check valve, usually used near the source of an irrigation system to stop the reverse flow (back siphonage) of irrigation water back into the water source system. This prevents contamination of the potable water source. These are considered isolation protection devices and are installed where the protection is needed. Backflow preventers must not be exposed to more than 12 hours of continuous service as this increases the risk of failure. Many municipalities have specific codes for what type of backflow device is required when installing an irrigation system. Always check your local codes before selecting a backflow protection device.
A commonly used backflow preventer in residential applications is a hose vacuum breaker (HVB). This would be installed on each outdoor hose spigot to protect your potable water source. A properly installed HVB vents water to the atmosphere when a back flow condition occurs. Here's how these work. When the water is turned on, the pressure opens a spring-loaded check valve allowing water to pass through into the hose. When this pressure is interrupted the spring-loaded check valve closes at the same time the air gap vents open to the atmosphere. Any back flow of water is released out the vents to the ground.
Pressure Relief Valves (PRV) are safety valves designed to control or limit pressure within your system. These spring loaded valves are set at specific levels, so when excessive pressure develops in a piping system the spring valve opens allowing water (or air) to escape, thus decreasing pressure.
Air/Vacuum Release (Relief) Valves are pretty much just what the name sounds like, a device to release air into and out of your irrigation lines. Properly installed vents prevent air lock or vacuum situations. There are several types available and each performs specific operations when appropriately placed within your irrigation system. These are recommended in large systems, any subsurface irrigation system, and when there is uneven terrain. These are not a check valve, they do not stop the flow of water in any direction.
Click this link for more information about Air Vents and Vacuum Relief.