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TMB - Plumbing Engineer - Features: November 2006: Backflow Prevention and Cross Connection Control

Backflow Prevention and Cross Connection Control:

A Primer

By Susan R. Ecker, Technical Editor

Plumbing Engineer

Proper backflow prevention and cross connection control is vital to the protection of our water supplies, both inside and outside a building. Backflow preventers can be installed in order to protect the city water supply or they can be installed to protect the water supply within a building. There are numerous, well documented cases where cross connections have been responsible for contamination of drinking water and have resulted in the spread of disease. The problem is ongoing, because piping systems are continually being installed, altered or extended.

Because there are so many players involved, there seems to be some confusion as to which authority has jurisdiction when it comes to backflow prevention. There are three such authorities in most jurisdictions: the plumbing inspector and/or the building department, the water utility or the public health department.

Recently, I had a number of conversations with the water utility about the type of backflow prevention device required on both the domestic and fire water services on a particular project. The information that was initially provided did not make sense, so additional telephone conversations were initiated. After speaking with a supervisor at the water utility, it was determined that the backflow devices that I had originally thought were required on the incoming water services were indeed required.

The design engineer cannot just take the information provided at face value. If the information sounds incorrect, it is up to the engineer to pursue the issue. There may be times when the utility has less stringent requirements than are anticipated, but this is doubtful.

Engineers need to have the background required in order to determine whether the information that an authority is providing makes sense for a particular application, and they need to pursue the issue until a sensible resolution is achieved. On one hand, we don't want to spend an owner's or client's money on backflow prevention devices that may not be warranted. On the other hand, we need to be able to protect not only the city water supply but also the water supply within the building. Backflow prevention is just one health issue among many that plumbing engineers deal with, and, in this article the basics of backflow prevention and cross connection control will be discussed.

The following definitions are from the American Water Works Association (AWWA).

Cross connection: any unprotected actual or potential connection or structural arrangement between a public or consumer's potable water system and any other source or system through which it is possible to introduce into any part of the potable system any used water, industrial fluid, gas, or substance other than the intended potable water with which the system is supplied.

It should be noted that, under certain circumstances, atmospheric vents and relief valves on backflow preventers could allow the entry of aerosols, toxic fumes or airborne solid particles into the system. Bypass arrangements, jumper connections, removable sections, swivel or changeover assemblies, or any other temporary or permanent connecting arrangement through which, or because of which, backflow can or may occur, are considered to be cross connections.

Backsiphonage: backflow caused by negative or reduced pressure in the supply piping

Backpressure: A potable system connected to a non-potable supply operating under higher pressure by means of a pump, boiler, elevation difference, air or steam pressure has a high risk that non-potable water may be forced into the potable system whenever these interconnections are not properly protected.

Cross connection is the link or channel connecting a source of pollution with a potable water supply. The polluting substance, in most cases a liquid, tends to enter the potable supply if the net force acting upon the liquid acts in the direction of the potable supply. Two factors are essential for backflow: first, there must be a link between the two systems; second, the resultant force must be toward the potable supply.

Hazard Classification
A health hazard is defined as a cross connection or potential cross connection involving any substance that could, if introduced into the potable water supply, cause death or illness, spread disease or have a high probability of causing such effects.

An example of this is an aspirator. Depending on the application, if fertilizer or laboratory chemicals were able to enter the potable water system via an aspirator, these chemicals could have an adverse health effect on the building occupants and, possibly, on occupants of neighboring buildings.

A non-health hazard is considered a cross connection or potential cross connection involving any substance that generally would not be a health hazard, but would constitute a nuisance or would be aesthetically objectionable if introduced into the domestic water supply.

An example of this is a coffee urn directly connected to the potable water supply. If backflow occurs, the potable water may begin to turn brown, and this may disturb building occupants.

Partial List of Plumbing Hazards
According to the Environmental Protection Agency (EPA) the following is a partial list of plumbing hazards that require a backflow prevention device.

Fixtures with Direct Connections

  • air conditioning, air washer
  • air conditioning, chilled water
  • air conditioning, condenser water
  • air line
  • aspirator, laboratory
  • aspirator, medical
  • aspirator, weedicide and fertilizer sprayer
  • autoclave and sterilizer
  • auxiliary system, industrial
  • auxiliary system, surface water
  • auxiliary system, unapproved well supply
  • boiler system
  • chemical feeder, pot-type
  • chlorinator
  • coffee urn
  • cooling system
  • dishwasher
  • fire standpipe or sprinkler system
  • fountain, ornamental
  • hydraulic equipment
  • laboratory equipment
  • lubrication, pump bearings
  • photostat equipment
  • plumber's friend, pneumatic
  • pump, pneumatic ejector
  • pump, prime line
  • pump, water operated ejector
  • sewer, sanitary
  • sewer, storm
  • swimming pool

Fixtures with Submerged Inlets

  • baptismal fountain
  • bathtub
  • bedpan washer, flushing rim
  • bidet
  • brine tank
  • cooling tower
  • cuspidor
  • drinking fountain
  • floor drain, flushing rim
  • garbage can washer
  • icemaker
  • laboratory sink, serrated nozzle
  • laundry machine
  • lavatory
  • lawn sprinkler system
  • photo laboratory sink
  • sewer flushing manhole
  • slop sink, flushing rim
  • slop sink, threaded supply
  • steam table
  • urinal, siphon jet blowout
  • vegetable peeler
  • water closet, flush tank, ball cock
  • water closet, flush valve, siphon jet

Methods of Backflow Prevention
When installing backflow prevention devices, the installation contractor should follow the manufacturer's recommendation. There may be additional requirements from the authority having jurisdiction; these must be followed as well. Please keep in mind that there may be more than one authority that has jurisdiction over the installation. As noted at the beginning of this article, these include the plumbing and/or building inspector, the public health agency and the water utility.

Following is a list of backflow prevention devices. In some cases, installation tips are provided.

Air Gap: the unobstructed vertical distance through the free atmosphere between the lowest opening from any pipe or faucet supplying water to a tank, plumbing fixture, or other device and the flood level rim of the receptacle into which an outlet discharges.

An air gap is the best means available for protection against backflow. The only problem with this method is that it can easily be circumvented by a bypass arrangement.

Reduced Pressure Principle Backflow Prevention Assembly (RPBA): this consists of two independently acting, approved check valves, together with a hydraulically operating, mechanically independent pressure differential relief valve located between the check valves and below the first check valve.

These units are located between two tightly closing resilient-seated shutoff valves as an assembly and are equipped with properly located resilient-seated test cocks. An RPBA is effective against backflow caused by backpressure and backsiphonage and is normally used in locations where an approved air gap is impractical. The main advantage is that, if failure of the assembly or system occurs, there is visible flow of water. Adequate clearance must be provided for removal of the relief valve and/or the check valves. An RPBA shall not be installed in a pit below ground level. If the relief valve port is submerged in groundwater, cross connection is created and may be more serious than the hazard that the assembly isolates.

Double Check Valve Assembly (DCVA): Two internally loaded check valves, either spring-loaded or internally weighted, are installed as a unit between two tightly closing resilient-seated shutoff valves as an assembly.

A DCVA is effective against backflow caused by backpressure and backsiphonage, and it is used to protect the water system from pollutants that would not constitute an actual health hazard but might be objectionable to the water supply system. A DCVA should not be installed below ground level unless provided with adequate drainage to maintain a dry location. Where an assembly must be installed in a location that is susceptible to flooding, such as a basement, the test cocks shall be plugged.

Pressure Vacuum Breaker (PVB): consists of an independently operating internally loaded check valve and an independently operating loaded air inlet valve located on the discharge side of the check valve, with properly located resilient-seated test cocks and tightly closing resilient-seated shutoff valves attached at each end of the assembly.

The PVB is effective against backflow caused by backsiphonage only and should not be used if backpressure could develop in the downstream piping. If the PVB is used to isolate a health hazard, the potential for circumvention of the assembly and/or the possibility of backpressure exists; premise isolation with a RPBA may then be necessary.

Atmospheric Vacuum Breaker (AVB): an assembly that performs similarly to the pressure vacuum breaker. The AVB consists of a float check, a check seat and an air inlet port. A shutoff valve immediately upstream may be an integral part of the assembly.

An AVB is effective against backflow caused by backsiphonage only and should not be used if backpressure could develop in the downstream piping. The AVB should be used for protection against non-health hazards. As with the PVB, if used to isolate a health hazard, additional area or premises isolation may be required. This assembly is not for use at a meter.

In order to provide the proper backflow prevention device, a determination must be made as to the type of hazard that is to be protected against. Once the type of hazard has been established, a device can be chosen. There are a number of documents that can aid the engineer in determining the backflow prevention required for a particular application, including the plumbing code and the documents listed in the reference section of this article. If there is a specific question about your application, discuss your design with the plumbing inspector prior to installation. Backflow prevention and cross connection control are vital in order to maintain a safe drinking water supply.

Susan R. Ecker is a senior plumbing engineer with Rumsey Engineers Inc. in Oakland, Calif. and technical editor of Plumbing Engineer. She has extensive experience in designing plumbing systems for industrial and commercial market sectors. Susan can be reached at

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