Page 317 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

Chapter 5 Cold water systems

System boiler

Type CA
disconnector

Water undertaker' s
Flow Return cold water supply
p Figure 5.78 Use of a type CA backflow preventer

Type DB pipe interrupter with atmospheric vent
and moving element
The type DB pipe interrupter is a backflow prevention device specifically
designed for fluid category 4 applications. The concept of the DB interrupter is
very simple. Water enters a tube that has one end blanked off. Around the tube
are a series of small holes over which a flexible rubber membrane is stretched.
As the water flows into the tube, it is forced through the holes and this flexes
the rubber membrane to allow water to flow. If the supply pressure suddenly
stops, then the membrane contracts against the holes to effectively prevent
backflow. Any back-flowing water is then released to atmosphere through
another series of holes in the outer casing of the device. They are approved
for use as protection against back siphonage but not back pressure.

Direction of flow

The outer
The vents open
membrane to
closes off atmosphere
the outer when a
vent holes sudden loss
during of pressure
normal occurs. This
water flow prevents
back
siphonage

p Figure 5.80 Type DB pipe
p Figure 5.79 Type DB pipe interrupter schematic interrupter

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