When a fire protection system (non-potable water system) is connected to the public water supply, the systems are said to be cross connected. In some localities, cross connections may be prohibited or closely regulated by health authorities.  Improperly protected water systems have the potential to lead to illness and even in some cases death. Federal regulations require states to provide quality water when it is intended for public consumption. Because of this, states and municipal governments have taken various steps to protect the potable water supply, such as requiring backflow prevention when the fire protection system will be supplied by a potable water source.

Backflow preventers are installed to prevent contaminants from traveling from the non-potable source to the potable public drinking supply via back siphonage and back pressure.

Back siphonage is backflow caused by a negative pressure in the supply piping. This negative pressure in the supply piping is similar to drinking water through a straw. The water from the non-potable system is pulled into the supply piping.

Backpressure is backflow caused by a pressure in the non-potable water system being greater than the pressure in the potable water supply piping. This higher pressure causes water in the non-potable system to be pushed back into the supply piping.

Its important to note here that the requirement for backflow prevention in a fire protection system comes from the local water authority and not from any NFPA standard. For example, NFPA 13 does not require a backflow preventer for an automatic sprinkler system, however, if one is required, it provides additional requirements to ensure it is installed in a manner that limits its impact on system operation and provides for a means to test the system.

There are a few different types of backflow preventers available, and the type of backflow preventer required by the water authority is going to be based on the degree of hazard posed by the cross connection. The degree of hazard may be classified differently, but the two main degrees include high hazard and low hazard. A high hazard is a system that could introduce waterborne disease organisms, or harmful chemical, physical, or radioactive substances into a public water system, and which presents an unreasonable risk to health. An example of this may be a system that contains an additive, such as a fire protection system with antifreeze, or a foam system. A low hazard is a system that could cause aesthetic problems or have a detrimental secondary effect on the quality of the public potable water supply, an example of this could be a fire sprinkler system that contains stagnant water or contains microbiologically influenced corrosion (MIC).

The Double Check Valve Assembly (DCVA) and the Reduced Pressure Zone Assembly (RPZA) are the most used backflow preventers for fire protection systems, but I will discuss all the most common backflow preventers used in plumbing systems.

An air gap is the most effective type of backflow prevention. This method utilizes a physical air space between the potable and non-potable systems. The most common example of this would be a faucet and a sink. This may be a backflow prevention method used to fill a water supply tank.

Air gaps can be used to protect low and high hazards under both back siphonage and backpressure.

An Atmospheric Vacuum Breaker Assembly contains an air inlet valve and a check seat. When water flows through, the air inlet valve closes, but when the water flow stops, the air inlet valve falls against the check seat and stops back siphonage, while at the same time letting air into the system.

AVBs can only protect against a low or high hazard under back siphonage.

The Pressure Vacuum Breaker Assembly is like an atmospheric vacuum breaker, but it contains a spring-loaded air inlet valve and check valve, two shutoffs, and two test cocks. When water is flowing, the check valve is open and air inlet valve is shut, when water stops flowing, the check valve shuts, and air inlet valve opens. The addition of the shutoff valves and test ports allows for this assembly to be field tested.

The PVB only protects against low or high hazards under back siphonage.

A Double Check Valve Assembly (DCVA) contains two spring-loaded check valves with two shut off valves and four test cocks. In the event of a backflow the first check valve will close, if that check valve fails then the other check valve will close. The addition of the shutoff valves and test ports allow this assembly to be tested.

A DCVA can be used to protect against low hazards under both back siphonage and back pressure.

A double check valve detector assembly is the same as a DCVA, but it also includes a bypass for the installation of a water meter to monitor for incidental water use that is also protected with a smaller DCVA.

A Reduced Pressure Zone Assembly (RPZA) provides the maximum protection and along with the DCVA is the most common type of backflow prevention used in fire protection systems. This assembly contains two spring-loaded check valves with a differential relief valve between them and two shut off valves and four test cocks. The RPZA operates like a DCVA with the addition of a relief valve, if there is a backflow the check valves will close, and the relief valve will open, resulting in a reduced pressure zone and air gap between the check valves. The two shut off valves and four test cocks allow this assembly to be field tested as well.

The RPZA can be used to protect high and low hazards under both back siphonage and back pressure.

A reduced pressure zone detector assembly is the same as a RPZA, but it includes a bypass for the installation of a water meter to monitor for incidental water use that is also protected with a smaller RPZA.

As you can see, there are a few different types of backflow preventers, and the selection of the right preventer is going to depend on the requirements from the local water authority as well as the hazard. When the design of a fire protection system includes a backflow preventor, the designer must make sure that they account for the backflows impact on the available water supply pressure. If a backflow preventor is installed on a fire protection system, it is also important that proper inspection testing and maintenance (ITM) be performed (such as a forward flow test) to ensure that the backflow remains operational and does not seize up, which could impair the fire protection system.