Page 310 - 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

Type AG air gap with minimum size circular overflow
determined by measure or vacuum test

This is a non-mechanical backflow prevention arrangement of water fittings
with an air gap, together with an overflow, the size of which is determined
by measure or a vacuum test. This arrangement gives protection against
fluid category 3.
The type AG air gap fulfils the requirements of BS 6281: Part 2 1982 Devices
without moving parts for the prevention of contamination of water by
backflow. Specification for type B air gaps. In a cistern that is open to the
atmosphere, the vertical distance between the lowest point of discharge and the
critical water level should comply with one of the following requirements:
● It should be sufficient to prevent back siphonage.
● It should not be less than the distances specified in Table 5.7, depending
on cistern type.
Air gap Critical water level

Warning/overflow pipe

Outlet
Suitable for protection against fluid category:
3 Back 3 Back
pressure
siphonage
p Figure 5.68 Type AG air gap

The following points about type AG air gaps should be noted.
● The air gap is related to the size of the inlet supply and is the minimum
vertical distance between the critical water level and the lowest part of
the discharge outlet of the float-operated valve, as specified in Table 5.7.
● The critical water level is the level that is reached when the float-operated
valve has failed completely and the water is running freely at maximum
full-bore flow rate and pressure.
● AG air gaps must comply with the requirements of BS 6281.
Where storage cisterns are installed, it is likely that the critical water level
would differ from installation to installation because of the varying flow rates
and pressures of the incoming supply, and the differing lengths and gradients
of the overflow pipe. With this type of installation, the type AG air gap is not
practical because the critical water level cannot be calculated accurately. It is

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