Let’s talk about backflow prevention

By Richard Bailie

With so much attention being focused on the standards and regulations that surround the generation of hot water in a home (think of the recent changes to SANS 10254 and the installation requirements of heat pumps and solar installations), you would be forgiven for letting something like backflow prevention slip your mind.

It is, after all, one of those trivial things we have all heard of, and we know it resides somewhere within the standards, but nobody really pays it any mind.

The truth of the matter is that these oft overlooked requirements are vital to any sound plumbing system. May I remind you that backflow prevention as a concept plays a big part in ensuring we all have clean water running from our taps. Without it, our potable water network would be rendered all but useless.

So, when we talk backflow prevention, it means just that: the prevention of a substance (in this case water) from flowing in a direction within a system that is opposite to the intended direction of flow. In essence, we are permitting the water to enter a space and once it has passed a certain point, we deny it a path back.

Double check detector assemblyBut why? Well, in all cases where backflow prevention is required, there would be a possibility of contamination. In other words, in these situations, water can come into contact with hazardous substances, for example chemicals or waste, and once mixed with the water, these cannot be allowed to flow backwards to enter the potable supply, and for this purpose there are several technologies we employ.

Double check detector assembly. Credit: Richard Bailie

When do you need to install a backflow preventer, and which type must be installed? This is determined by the standards. For installations that do not require a rational design, we refer to SANS 10252-1 and Annexure D of the same.

SANS 10252-1 (7.4) is a section called “Preservation of Water and Water Quality”, and it has several subsections worth noting. Under “General”, it tells us that we are to take adequate measures to prevent the deterioration of the quality of water in any water installation, including preventing any substance entering that could adversely affect the potability of the water. It goes on to say that when storing water, we are to ensure that the quality of that water is maintained as well.

The onus is on us, the installers. Remember that when we issue CoCs, we are stating that we as registered professionals have given that particular installation the okay as per the requirements of the various mandatory standards. If you determine that a design has fallen short of the requirements, you should be engaging with the designer to educate them and have it changed.

As is sometimes the case, the standards can be cumbersome to decipher and translate into practical guides, and they tend to jump around due to the way in which they are set out. So, if you want to find out about backflow prevention, you cannot simply go to a section of that name and find everything you need, you need to look up what is written about backflow prevention under the various sections of the standard. In the case of SANS 10252-1, there are five sections after the Scope, Normative References, and Abbreviations — eight in total. They are:

  • Section 4: Initial Considerations for Design
  • Section 5: Materials, Pipe, Fittings, Components and Fixtures
  • Section 6: Layout
  • Section 7: Design
  • Section 8: Installation

References to backflow prevention are made in sections 5, 6, 7, and 8.

Subsection 7.4.2 is “Connections” and it speaks of the general rules surrounding which connections are allowed. So, for example, it prohibits the connection between:RPZ discharge 2

  1. a general installation conveying water from the supply main and an installation conveying water from any other source of supply;
  2. any water installation conveying potable water to any drain or sewer;
  3. any water installation pipe and any service pipe upstream from any backflow preventer installed in terms of 7.4.3; and
  4. a general installation and any fire installation when they are supplied through separate communication pipes.

This section also has rules surrounding when pumped connections are included in the design as well as water meters.

An RPZ discharge. Credit: Richard Bailie

Subsection 7.4.3 is called “Prevention of Backflow” and is the best place to start if you want to have a good idea of when you would require backflow prevention: Adequate measures shall be taken to prevent the back-siphonage of water into the following:

  1. a) a communication pipe from any of the following water installations:

1) a fire installation or combined installation;

2) a general installation serving any of the following activities:

  1. i) medical treatment of people or animals;
  2. ii) medical, pharmaceutical or chemical research and manufacturing;

iii) agriculture, including dairies and nurseries;

  1. iv) laundering and dry-cleaning;
  2. v) photographic processing;
  3. vi) metal plating; or

vii) treatment of hides and skins;

3) a general installation serving any of the following locations:

  1. i) mortuaries;
  2. ii) abattoirs;

iii) sewage purification plants;

  1. iv) refuse pulverizing works;
  2. v) harbours;
  3. vi) oil processing and storage facilities;

vii) wineries, distilleries, breweries, yeast and soft drink factories;

viii) sports fields; or

  1. ix) any other premises on which an activity is carried out that can affect the potability of water; and
  1. b) an installation, in all cases where

1) the design of terminal fittings installed (including any hose bibcocks, laboratory taps or movable shower units) is such that a hose or any other flexible pipe can be attached to the fittings,

2) fire hose reels are installed in a combined installation,

3) an underground irrigation system can provide contact between polluted water and water within the installation, or

4) any other fittings can provide contact between polluted water and water within the installation.

NOTE: Backflow can be caused by the following:

  1. the development of negative pressure or the lack of pressure in a water supply system (i.e. when a main is drained for maintenance operations, or when fire-fighting is in progress nearby, or because of a burst water main); or
  2. the downstream pressure exceeding that of the supply. (This is for example possible in an installation such as a heating system, an elevated tank, and a pressure producing system such as a boiler. In the case of a fire installation, non-potable water carried in a fire tender can be introduced into the installation via a pumping connection.) To prevent the possibility of backflow of water in any of the water installations as given in, or in any other case where such a possibility exists, and unless other approved measures have been taken,

  1. a) a storage tank shall be installed such that
  • the entry of water into the storage tank is solely from a pipe that discharges at a height, above the flood level rim of the tank, of the greater of at least 75mm and twice the nominal diameter of such pipe; and
  • water required for any activity that can pollute the water supply is withdrawn only from the outlet of such storage tank, or b) appropriate backflow prevention measures shall be selected from those listed in column 1 of table 12, for the degree of the potential hazard contemplated in column 2 of table 12.

Table 12 Unless other approved measures are taken to prevent the backflow of water from terminal water fittings that discharge water into a sink, wash-hand basin, bath or other similar appliance,

  1. a) such terminal water fittings shall be so installed that the vertical distance between the point of discharge of the fitting and the spill-over level of the receiving appliance is at least equal to the distance given in table 13 for the size of the fitting concerned;
  2. b) a double check valve shall be incorporated as close as is practicable to the point of discharge; or
  3. a combined check valve and vacuum breaker shall be so installed that the vacuum breaker is at least 300 mm above the spill-over level of the receiving appliance. 

RPZ 2Look at section above and think about Nikki spouts in baths. It clearly says here that for fittings that discharge into for example a bath, there must be a gap between the outlet and the spill-over level of the appliance (not the overflow outlet level — the spill-over level means the rim of the bath) with a certain vertical distance separating them.

A Nikki spout does not have this gap, so we need to install either ‘b’ or ‘c’ as above. How many of us do this? As a side note, in Annexure D of SANS 10252-1 (D.1.6.1), it says that if the terminal water fitting already has a backflow prevention device incorporated within the fitting that is suitable for the hazard generated, we need not install additional backflow prevention. Given that a single check valve is not a suitable backflow prevention device, how many of us have actually installed Nikki spouts that comply?

An RPZ valve. Credit: Richard Bailie

In essence, our role here is quite simple: we need to determine the hazard level and from that, we need to decide on the type of backflow preventer we need to install, if any. Note that a break tank with an air gap is also a suitable means of backflow prevention, but not easily employed. Note also that if there is no break tank, the two most common backflow prevention measures are given in table 12 — either an RPZ valve or a double-check valve. Not a single check valve, as this is not considered a backflow preventer.

In many cases, it is not enough to simply install a backflow preventer on the main incoming supply — we need to prevent cross-contamination within the premises as well. That is to say a section or process within an installation that constitutes a hazard must be isolated from the rest of that installation, so the backflow preventer(s) must be installed immediately upstream of the process or area. Have a look at Figure 1.

Figure D1

Figure 1: Typical backflow prevention set-up.

You will notice that all parts of the installation downstream of the backflow preventers are deemed to be non-potable (NP). Therefore, multiple backflow preventers are required in these cases.

RPZ cut away 1Backflow prevention devices must be tested and maintained according to manufacturer’s details, but these intervals shouldn’t be more than 12 months. In Annexure D of SANS 10252-1 (D.3.6.3), it states that if there is no such maintenance programme in place (again, this onus is on us as installers), that backflow preventer shall not be employed, and the standard air gap requirements shall apply, as this set-up is pretty much maintenance-free.

Schematic diagram of RPZ cut away. Credit: Richard Bailie

I wouldn’t mention this fact except to illustrate the importance of backflow prevention and its ongoing operation. You do not see this insistence for a maintenance regime in the geyser installation standards, for example. It is recommended, but not enforced to the point where you are prohibited from installing one for lack of a maintenance regime.

Now, lets revisit the various mentions of backflow prevention in SANS 10252-1. Remember how the different sections in the standard are organised? 6 for layout, 7 for design, and so on? It would then stand to reason that each reference to backflow prevention in these sections are done so within a certain context, so for example:

5.4.15 (materials and the like) says that check valve shall comply with SANS 1808-10, and that swing-type metal-to-metal check valves shall not be used in potable systems in buildings.

6.3 (layout) gives various minimum and maximum distances between components, configurations, and other positional requirements.

7.4.3 (design) appears earlier in this article and speaks to when and where backflow preventers should be used.

8.4.2 (installation) lists the following installation requirements, which I think are worth listing here:

  1. that it can be inspected;
  2. that a condition of backflow can be detected;
  3. that it is readily accessible without having to alter the surrounding structures;
  4. that it cannot be flooded by any liquid; and
  5. that vacuum breakers are installed on anti-syphon loops.

We can see the importance the creators of the standards place on backflow prevention. Local authorities are usually very strict on this as well, considering the legal ramifications for them when it is shown that they were unable to guarantee the potable water quality provided, as is their mandated duty. It boggles my mind therefore that we as plumbers do not pay more attention to this. I am just as guilty of this, something I aim to change going forward.

About Richard Bailie

Richard Bailie completed his apprenticeship in Kimberley and qualified as a plumber in the early 1990s. In 2000, he opened his own plumbing business, Springtide Plumbers, in Cape Town. He serves on the national executive and technical advisory committee for the PIRB. He is the vice-chairman of IOPSA in the Western Cape and has been auditing projects for the PIRB for approximately six years. He conducts training for IOPSA and the PIRB through webinars and classroom-based platforms, delivered to plumbers, architects, BCOs, and Builders Warehouse staff.

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