Backflow prevention

By Chris Kyle

Backflow prevention is protection against the pollution of potable water in water installation systems, either within individual installations, or from premises to the municipal water supply network.

Pollution can be defined as any relative degradation of the quality of potable water. Polluted water is water that is of a lesser quality than that specified by SANS 241 (potable water quality), such as that which may be obtained from sources like boreholes, rainwater, and recycled grey water.

The quality of water can also be severely affected by many varying factors such as chemical processes in factories or laboratories, the use of toxic substances, and medical and veterinarian procedures. All of which have the potential of contaminating water supply installations, either on site or within the municipal network.

With the current strain on our precious water resources, we should all be consciously concerned with the quality and use of our water resources and do everything possible to protect them.

How backflow occurs

Backflow can be attributed to a variation in a pressure difference that causes a consequent inversion of the normal direction of flow at a certain point in an installation.

Simply put, backflow of water in a plumbing system will occur in a number of instances:

  • During maintenance of a system and a siphon is created in part of the system, causing a reverse flow.
  • When the pressure downstream of the source of supply is increased, causing water to flow in the opposite and unintended direction.
  • When pressure is lost at the source of supply or a pipe bursts, causing the system to drain to the low-pressure area.
  • During firefighting, when there is a large draw off on a communication or service pipe (rare).

Fluid categories

For purposes of risk assessment, fluids contained in installations are classified into five categories according to the degree of risk they pose to human health and safety. These categories range from 1, with no human health hazard, to 5, the most hazardous.

  • Category 1: Water to be used for human consumption coming directly from a potable water distribution system.
  • Category 2: Fluid presenting no human health hazard, as per 1, but the quality of which could have undergone a change in taste, odour, colour, or temperature.
  • Category 3: Fluid representing some human health hazard due to the presence of one or more harmful substances.
  • Category 4: Fluid presenting a human health hazard due to the presence of one or more ‘toxic’ or ‘very toxic’ substances or one or more radioactive, mutagenic, or carcinogenic substances.
  • Category 5: Fluid presenting a human health hazard due to the presence of microbiological or viral elements.

According to these classifications, suitable fit-for-purpose devices, designed to protect the public water supply, must be specified and installed based on their operating principle and minimum and maximum requirements.

It is crucial that all of us take adequate measures to protect our precious water resources and act responsibly.

Protection devices – groups and types

Protection devices are grouped in eight families, identified by the letters A, B, C, D, E, G, H, and L, each of which has one or more variants, called types, also identified by the letters A, B, C or D. Devices are then specified for each application and condition according to the fluid category for which protection is required.

Now, this all sounds very technical and convoluted but in essence, usually the selection of a suitable device is of the B/A type, which caters for fluid categories 1, 2, 3, and 4. Should the device need to cater for fluid categories 2, 3, 4, and 5, five being the worst-case scenario, then devices falling into the A/A type, which incorporate an unrestricted air gap design, are required, but this is very seldom the case.

Misconceptions

There are many misconceptions when the topic of backflow prevention is brought to the table for discussion. Frequently, the argument given is that a simple non-return valve or a dual non-return valve installation can be used to prevent back siphoning or backflow of polluted water. This is not true, since there is no way of monitoring the positive seal or performance of simple non-return valves, be they of the spring loaded or flap type.

Note: A non-return valve falls under type EB, fit only to prevent backflow of category 1 fluids.

National regulations

National regulation SANS 10252 -1 (water supply installations for buildings). As an interesting and informative exercise, refer to 7.4 of SANS 10252-1, ‘Preservation of water and water quality’, where it states that reduced pressure zone backflow preventers must be installed in any installation where there is a risk of contaminated water or harmful substances being back-siphoned or flowing back into a potable water supply line.

The regulations state that backflow preventers shall be installed in certain types of installations and buildings, for example medical treatment of people and animals, pharmaceutical or chemical research, and manufacturing, dairies, nurseries, stadiums, abattoirs, combined fire systems and so on.

Go and research this!

Risk assessment

Given the potential dangers of the phenomenon and the requirements of current regulations, the risk of pollution caused by backflow must be assessed for the type of system and the characteristics of the fluid to be conveyed.

A suitable backflow prevention device must be selected based on the outcome of the assessment performed by the system designer. In the case where it is not possible to clearly verify the risk level, it is necessary to hypothesise the greatest risk, and specify and install accordingly.

Alternate water supply sources

With many areas suffering severe droughts coupled to the lack of water supply infrastructure upgrades, many organisations and homeowners are seeking alternate water supply sources, such as the sinking of boreholes, rainwater harvesting, and recycling or use of grey water.

Municipalities are working with them and setting guidelines for this and have clearly communicated directives and procedures of what is deemed safe and what is not. For example, because of the many varying qualities of water extracted from boreholes, they have limited the use of borehole water to flushing of toilets and irrigation (see Figure 1).

Alternate water systems are subject to City approval, while groundwater and surface water use are subject to Department of Water & Sanitation (DWS) licensing.

Due to the health risks, no alternative water may be used for drinking, cooking, or body washing, and even swimming pools must be filled with water that meets the SANS 241 specification. Complete off-grid uses of alternate water will only be permitted for large consumers such as big residential estates and corporates, where very expensive and costly in upkeep filtration systems are installed and maintained at prescribed and regular intervals to ensure that the quality of water provided meets the standards of SANS 241. In all of the above instances, reduced pressure zone backflow preventers must be installed.

Chris Kyle Figure1Figure 1: Suggested alternate water supply application.
Credit: Chris Kyle

What is a reduced pressure zone backflow preventer? B/A type

Commonly referred to as RPZs, they usually consist of a DZR brass or bronze body and have an upstream and downstream high-quality spring-loaded check valve. They comprise an operating chamber, a reduced pressure zone (hence, the valve’s name RPZ), a diaphragm system that divides these two zones, a spindle and discharge port and seat, as well as a spring. It also has three test ports on top of the valve used to monitor and test for proper operation (not shown in Figure 2).

The function of an RPZ is to prevent the backflow of polluted water by way of the two check valves, but most importantly, to dump any backflow that may find its way past the downstream check valve via the discharge port to a safe point of discharge, thus preventing contamination of the upstream water supply. No rocket science.

Chris Kyle Figure2Figure 2: An illustration of an RPZ valve cross section.
Credit: Chris Kyle

How do they work?

By applying the formula for pressure — that is, Pressure = force/area — and Newton’s Laws, and following these simple sketches, you will be able to determine how they work.

Chris Kyle Figure3Figure 3: Flow conditions.
Credit: Chris Kyle

Chris Kyle Figure4Figure 4: No flow conditions.
Credit: Chris Kyle

Chris Kyle Figure5Figure 5: Backflow conditions.
Credit: Chris Kyle

How they are installed

All RPZs must be installed with a positive shut-off isolating valve (that is, an approved shut-off valve that will ensure no leakage when closed) upstream and downstream of the RPZ. This is required not only for servicing purposes but is needed for testing purposes, too. A line strainer must be installed between the upstream isolating valve and the RPZ.

Should the municipal water pressure exceed 600kPa, then it would be advantageous to install an adjustable pressure-reducing valve with a pressure gauge between the line strainer and the RPZ. Do some research on SANS 10252:1 to find out what the regulation says about supply pressure considerations 7.1.1.2. Remember that one of the primary methods of reducing water usage and system maintenance is to reduce supply pressure. We should all be concerned with the efficient use of our water resources.

This group of fittings is termed ‘the protection unit’. The point in the system at which the protection unit is installed is called the protection point. The protection point is usually located on the service pipe serving a building before any tee-off from the service pipe, downstream of the municipal water meter but on the erf premises. In larger, more complex installations, depending on the nature of activity and processes taking place, additional protection points may be required to protect the overall on-site water reticulation system as well.

The protection unit must be installed with the discharge port of the RPZ at 30cm above a suitable drain point.

Chris Kyle Diagram6Diagram 6: An example of the installation of an RPZ valve.
Credit: Chris Kyle

An interesting point

It’s amazing sometimes what answers you get when the question is asked what the purpose of a vacuum breaker is, as installed on a SANS 10254 geyser installation. The fact is that it is a backflow prevention device that is installed on an anti-siphon loop on the cold inlet and hot outlet of a properly executed geyser installation.

All a vacuum breaker does is to allow air into the system when a negative pressure (vacuum — sufficient enough as to create a siphonic action) is caused on any part of the installation, thus breaking the siphon. Have you ever tried siphoning fuel from a petrol tank using a pipe with holes in it? It doesn’t work. This not only precludes the geyser being drained during shut down of the municipal water supply or when other maintenance is done on any part of the system, but also protects the quality of the water in the system from backflow of polluted water into the potable lines.

Figure 7 shows how without vacuum breakers installed, the bucket containing insecticide could be back siphoned into the potable water lines via the hosepipe and the connection to the municipal main supply or any other part of the system.

Many municipalities insist on vacuum breakers or B/A backflow preventers to be installed at all garden taps for this reason. A vacuum breaker thus in terms of backflow prevention device classification is an anti-vacuum valve D/A type.

Chris Kyle Figure7Figure 7: Vacuum breakers prevent back siphoning of water.
Credit: Chris Kyle

Conclusion

It is crucial that all of us in the industry take adequate measures to protect our precious water resources and to act responsibly on the understanding that we must be accountable for our actions.

We must also take cognisance of the South African National Regulations and understand the reasons behind them having been written. Rules and laws are written to protect the public and are sometimes written in anticipation of something nasty happening, or after a disaster has occurred. They are based on knowledge and common sense. Without knowledge, common sense cannot prevail.

Already, in our expanding population, there have been a great number of cases where people have been exposed to waterborne diseases and poisoning caused by irresponsible practice. The Romans understood this well and knew that to establish a strong and healthy nation, clean water supplies and proper sanitation were key. Some of these ancient systems are still in use today. They understood the theory of backflow prevention and many of their water supply systems incorporated methods and simple devices to prevent the backflow or ingress of polluted water into their clean water storage systems.

About the author

Chris Kyle is the general manager at Calafrica Hydronic Solutions.


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