Issue: 1/05

Editor’s Note: “Back to Basics” is a column that will run periodically in PME reviewing the basic principles of plumbing engineering.

The basic principles of cross-connection control are very simple, yet there are thousands of variables that enter the formula in designing and engineering backflow prevention into any potable water system. It is a complete understanding of the elementary aspects of cross-connection control that will allow us to engineer and design systems that achieve the goal of preventing backflow in an effective and cost-efficient manner.

The goal is to assure a backflow event does not cause an unintentional reversal of flow of any substance into the potable water system that can affect water quality by means of a cross-connection. Backflow can happen because of two hydraulic conditions, backpressure and/or backsiphonage. Backpressure is a condition where a greater pressure is created on the outlet side of a piping system than from the inlet side. Pumps, thermal expansion or elevated storage tanks are different examples that can cause this condition to occur. Backsiphonage is a reversal of flow caused by a negative or sub-atmospheric pressure. Broken water mains often cause negative pressures in a potable water system by increasing demand to a point where a negative pressure can be created.

The hydraulic conditions of backpressure and backsiphonage can only cause a problem if there is a passageway from the unwanted material and the drinking water. This passageway is called a cross-connection. There are two types of cross-connections that can be created: either an actual (direct) or potential (indirect) connection. An example of an actual connection would be the feed line from the potable water supply connected to the boiler feed. An example of a potential connection would be a janitorial sink faucet with a hose thread outlet. This has the potential of connecting an open-end hose into the sink of soapy water or dangerous chemicals.

Once we know the hydraulic condition we are trying to prevent and the type of connection involved, we must evaluate the degree of hazard of the unwanted material we are connected to. There are two degrees of hazard to evaluate: low hazard (pollutant) or high hazard (contaminant). A low hazard would be a substance that would be an objectionable substance that could affect the aesthetic conditions of the potable water. The black stagnant water in a wet-charged (nonchemical) fire system would be an example of a low hazard substance.

A high hazard (contamination) substance is one that can present a health hazard. This high hazard substance can take many forms, such as sewage, toxic, biological or chemical. It breaks down to this: will the substance cause a health problem if a backflow condition occurs and the substance is transferred to the potable water through a cross-connection? If yes, it is a high hazard.

Now that we have identified the problem, we must look at how we can prevent this unwanted condition from affecting our potable water. This can be achieved by the proper application of various types of backflow preventers. I contend there are five basic means of backflow protection.

I know, I know, I can hear you screaming. You’re right, there are many methods, devices and assemblies to use for the purpose of backflow prevention, but if you break them down, they will all fall under one of the five basic means of prevention. These five basic means of protection are: air gap, dual check valve, vacuum breaker, double check valve assembly and reduced pressure principle backflow prevention assembly. For those that will insist, there is the barometric loop. The barometric loop is a continuous section of the supply line that rises a minimum of 35' at sea level, above the highest point of the water used downstream, and returns back to the original level. The barometric loop works on the principle that a vacuum can only draw a head of water so far up a water column, and if the column is tall enough, a backsiphonage cannot draw the unwanted material back into the potable water.

Air gap is the highest level of protection, as it creates a physical separation from the potable water and the unwanted substance. An air gap can protect against both high and low hazards, as well as backsiphonage and backpressure. The air gap is an unobstructed vertical separation between the discharge end of a potable outlet and the flood level rim of a non-pressurized receiving receptacle. An air gap separation must be a minimum of two times the diameter of the outlet but never less than 1". If the air gap is installed next to an adjacent wall, the minimum separation is three times the diameter, and if adjacent to two adjoining walls, the minimum separation is four times the diameter.

A dual check valve is a device with two independent operating spring-loaded checks enclosed in the same body. A vented dual check is a variety of a dual check that has an atmospheric vent separating the checks. They are used for low hazard only, and protect against both backsiphonage and backpressure.

Vacuum breakers come in all shape and sizes. There are pipe atmospheric, pressure, hose bibb, laboratory, spill resistance and more. You will need to study the exact application requirements and installation criteria for each type. All have these common traits: they protect against low and high hazards, and are used to protect against backsiphonage only. Some vacuum breakers are assemblies and are testable, such as the pressure vacuum breaker.

The double check valve assembly is an assembly consisting of two independently operating internally loaded check valves with tightly closing shut-off valves on each side inlet and outlet, as well as tests cocks properly located for testing each of the check valves. This assembly protects against low hazards only, and both backpressure and backsiphonage.

A reduced pressure principle backflow prevention assembly offers the highest level of mechanical backflow prevention. The assembly consists of two internally loaded independent check valves, with a hydraulically operating independent pressure differential relief valve located between the two check valves. The assembly has properly located tests cocks to facilitate testing of the check and relief valve. Resilient seated shut-off valves are on the inlet and outlet of the assembly. The reduced pressure assembly protects against all levels of hazards, and against both backpressure and backsiphonage.

It is at this point that effective backflow prevention design and engineering becomes a bit more of a challenge. To implement the items we just discussed, we need to be aware of some local restrictions on use and application of backflow prevention equipment. These local restrictions can come from federal, state and local rules and regulations covering plumbing, fire and health codes. Product standards, approvals and listings may restrict your choices. Other jobsite environmental conditions and hydraulic concerns may also need to be analyzed, such as the effects of chemical and metrological reactions and biological environments, stagnant water and thermal expansion, just to mention a few.

To help in the selection and designing of backflow protection, ask yourself these 12 questions:

1. Is the application a low or high hazard?

2. Is the application subject to backpressure and/or backsiphonage?

3. Is there a danger of water damage?

4. Is the application subject to freezing?

5. Does the application require continuous service?

6. Will there be control valves downstream of the device?

7. Is there electrical equipment located near the installation?

8. Is there sufficient drainage for the application?

9. What will the pressure loss be through the device?

10. Is the device to be used at the point of use?

11. Is there acceptable room for maintenance and testing?

12. Can a remote location be utilized?

These are the fundamentals of good design and engineering criteria and are based on my experience in the design, engineering and providing of backflow protection in potable water systems. There is no “Backflow 101” in any of the engineering classes. At best, backflow is a footnote in a textbook, and must be learned and shared as we gain experience.