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