TECHNICAL 67
Backflow prevention
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.
By
Chris Kyle
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: 1. During maintenance of a system and a siphon is created in part of the system, causing a reverse flow.
2. When the pressure downstream of the source of supply is increased, causing water to flow in the opposite and unintended direction.
3. When pressure is lost at the source of supply or a pipe bursts, causing the system to drain to the low-pressure area.
4. 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-forpurpose devices, designed to protect the public water supply, must be specified and installed based on their operating principle and minimum and maximum requirements.
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
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www. plumbingafrica. co. za April 2018 Volume 24 I Number 2