Page 306 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

The City & Guilds Textbook: Plumbing Book 1

Suitable for protection against
Type Description of backflow prevention arrangements and devices fluid category
Back pressure Back siphonage
g AUK3 Air gaps for taps or combination fittings (tap gaps) discharging over any X 5
higher-risk domestic sanitary appliances where a fluid category 4 or 5 is
present, such as:
● any domestic or non-domestic sink or other appliance, or
● any appliances in premises where a higher level of protection is
required, such as some appliances in hospitals or other healthcare
premises
shall be not less than 20 mm or twice the diameter of the inlet pipe to the
fitting, whichever is the greater.
h DC Pipe interrupter with permanent atmospheric vent X 5
Notes:
1. ‘X’ indicates that the backflow prevention arrangement or device is not applicable or not acceptable for
protection against back pressure for any fluid category within water installations in the UK.
2. Arrangements incorporating type DC devices shall have no control valves on the outlet of the device;
they shall be fitted not less than 300 mm above the spill-over level of a WC pan, or 150 mm above the
sparge pipe outlet of a urinal, and discharge vertically downwards.
3. Overflows and warning pipes shall discharge through, or terminate with, an air gap, the dimension of
which should satisfy a type AA air gap.
Each of the air gaps described in Table 5.7 will have two fluid categories
attached to it: one for back pressure and one for back siphonage. The difference
Because the pressure in the main is zero, between the two is simple to explain.
gravity forces water in the system back
towards the water main ● Back pressure: this is caused when a downstream
pressure is greater than the upstream or supply
pressure in the water undertaker’s main or the
consumer’s potable water supply. Back pressure
can be caused by:
● a sudden loss of upstream pressure, i.e. a burst
pipe on a water undertaker’s mains supply
● an increase in downstream pressure caused by
pumps or expansion of hot water
● a combination of both of the above.
● Back siphonage: this is backflow caused by a
negative pressure creating a vacuum or partial
vacuum in the water undertaker’s mains cold
water supply. It is similar to drinking through a
Up stream Down stream
straw. If a sudden loss of pressure on the mains
supply were to occur while a submerged outlet
was flowing, then water would flow back upwards
Sudden loss of pressure due to a burst on the undertaker's main
through the submerged outlet and down into the
p Figure 5.60 Back pressure water undertaker’s main.

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