The category of products we'll be discussing here is sometimes called cross connection control devices. Cross connection refers to any linking between a potable water supply (one safe for drinking) and any source of non-potable water--or any other fluid--into a common system. Examples of cross connection piping arrangements include bypass hookups, jumper connections, removable sections, and swivel or changeover devices. The need for cross connection control goes beyond the simple prohibiting of improper hookups themselves; it commonly involves specifying and installing of protective devices to prevent the backflow of contaminated or polluted water supplies in the event that cross connections are (sometimes have to be) made. These products are required by code for plumbing installations today, including municipal water systems, food processing plants, medical facilities and many industrial applications.
(Note: As a point of technical clarification, the term "contamination" refers to an impairment of the water quality that would cause a hazard to the public health through poisoning or disease [sometimes referred to as "high hazard"]. "Pollution," on the other hand, is an impairment of the water quality, which, though undesirable, would not create a health hazard [sometimes referred to as "low hazard"].)
There are two other terms related to cross connections that are sometimes confused: back-siphoning and back-pressure. These may sound like the same thing, but they're not. Most significantly, it is important to understand that a product that can prevent back-siphoning is not necessarily capable of preventing a reversal of flow caused by back-pressure. Back-siphoning is caused by a negative (less than atmospheric) pressure in a supply line that would tend to draw fluid back through an outlet into the piping system. An example often given is the case of a garden hose left submerged in a pail of dirty water at a time a street hydrant is being flushed. The huge volume of water shooting out the hydrant would create a vacuum situation in the supply system of the house, causing that dirty water to be pulled back out of the pail into the same pipes that feed the faucets inside. Back-pressure, on the other hand, is caused by a positive, downstream pressure in a piping system that is higher than the supply pressure itself. An example of this would be a heating system in which makeup supply water is piped directly into the boiler. Since the pressure in the boiler can build to a level higher than that of the makeup supply source, chemically treated boiler water would be forced back into the potable water system.
Products designed to counteract both conditions, back-siphoning and back-pressure, are called backflow prevention devices. Let's look at the basic designs available in this area.
Air GapThis can refer to either a specific product component, or in many cases, a system or installation specification. It entails a physical separation (a gap) through which water travels on its way toward delivery, downstream from a shut-off valve. A common example of an installation air gap is the height placement of a faucet spout above a fixture. To adequately prevent water in the vessel from being sucked back into the outlet during a negative pressure occurrence, the end device (such as an aerator or tub spout opening) can be located no less than one to two inches above the flood level rim, depending on the type (there are other variables that call for even greater gaps). A newer example of an air gap is the type used with reverse osmosis systems. Since the waste water produced is directed to a drain, the result is a direct connection between a potable water supply and this source of contamination. In this case, the air gap is a component, typically housed within the base of a drinking faucet, through which the R.O. waste water is routed before arriving at the drain connection.
Atmospheric Vacuum BreakerVacuum breakers incorporate two basic valving components, both technically check valves. In a regular positive pressure condition, a water check valve is open, allowing normal flow--and an air check valve (usually referred to as an "air vent") is held closed to prevent leakage out of the valve. When a negative pressure condition occurs, the modes of the two valves reverse--the water check now closes, and the air vent opens. The reason this system provides more assurance than a simple check valve is that it accounts for the possibility that the water check could fail to close entirely (often from some debris on the seat, called "fouling"). With this type of design, even such a failure would not cause backflow, since only air--not downstream water--would be drawn into the valve (and the induced air would break the vacuum). Atmospheric vacuum breakers are not designed to handle "locked pressure" for more than a few hours at at time, however, and should not be installed with a shut-off valve downstream.
Pressure Vacuum BreakerThink of this as a heavy-duty version of the atmospheric type (both are technically atmospheric, by the way). Pressure vacuum breakers typically contain a spring-loaded water check valve (sometimes two) and a spring-loaded air vent valve. This type is designed for pressurized use, and in fact, is often provided with integral shut-off valves, located upstream and downstream from the vacuum breaker components. They are typically equipped with connections for periodic testing of the valve's sealing capability (small valves with threaded outlets called "test cocks").
Double Check ValveThis self-explanatory design consists of two independently operating check valves assembled in tandem fashion. The assembly also includes two shut-off valves located at each end and four test cocks. Double check valves are effective in preventing both back-siphoning and back-pressure. While not the ultimate in protection, these products are often specified for applications in which the possible backflow is nontoxic in nature (pollutants, not contaminants).
Reduced Pressure Principle DesignThis type offers maximum protection against all forms of backflow and is especially specified when possible backflow could be toxic in nature. Like the above, it also includes two independently operating check valves, but there is an important additional feature in this case. Located between the two check valves is an automatically operating pressure differential relief valve. The first check valve reduces the supply pressure to a predetermined level within the intermediate chamber so that during normal or no-flow conditions, the pressure there is lower than the supply pressure, thus keeping the spring-loaded differential relief valve closed. In the event of back-siphoning or back-pressure, both check valves will close, and as the pressure between check valves nears the supply pressure level (eliminating the differential factor), the relief valve opens to atmosphere. If the condition is one of back-siphonage, the opening of the relief valve allows the induction of air to break the vacuum. If it is a back-pressure situation, the opened relief valve routes the contaminated water out of the system (installations should provide drainage for such spillage). The body of this type of valve is equipped with two shut-off valves and a series of test cocks.
With backflow prevention devices, it is very important to know whether the mechanism is functioning properly. Since there is no visible indication outside the device to tell if the product is capable of working as intended, there are test procedures specified--in some states required at regular time intervals. Various kits are available for testing the products in this category. As an example, a test kit for a reduced pressure backflow preventer typically consists of a differential pressure gauge and three connecting hoses. With these hoses connected to the test cocks, various procedures are conducted to check the performance of the differential relief valve, the sealing of the #2 check valve, and the static pressure drop across the #1 check valve. Shut-off valves are alternately turned off as part of this test procedure. To check the performance of double check valve assemblies, a test kit usually includes two pressure gauges and three connecting hoses. This apparatus is used to check for the sealing of each check valve in the assembly. Test kits are also available for evaluating the performance of pressure vacuum breakers.
Editor's Note: The "College of Product Knowledge" ran in Supply House Times for three years and resulted in a reprint manual that sold for many years to follow, totaling thousands of copies. It became something of an industry classic, though unfortunately is no longer available. Much of the original training material is still applicable to the products sold today. The purpose of this updated series is to look at what has come along since the first edition.