Page 314 - 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

The idea behind the DC pipe interrupter is to create an air inlet should a
back-siphonage situation occur. When water begins to backflow upwards due
to back siphonage, the DC pipe interrupter allows air into the system to break
the siphonic action, thus preventing contamination.
The DC pipe interrupter is a non-mechanical fitting. It does not contain any
moving parts. They are manufactured from corrosion-resistant brass. Typical
uses include WCs and urinal installations. The following points should be noted.
● The valve should be fitted in the vertical position, discharging downwards.

● It must be installed at least 300 mm above the overflowing level, or 150 mm
p Figure 5.75 Typical DC pipe
interrupter if fitted above a urinal.
● No tap or valve should be installed downstream of the interrupter.
● Pipe size reductions downstream of the interrupter are not allowed.
● The length of the pipe downstream after the interrupter should be as short
as possible.
● The interrupter should be accessible for replacement and repair.
● DC pipe interrupters must comply with BS 6281 Part 3.

Mechanical backflow prevention devices

An air gap is the most effective method of preventing contamination of the
water supply; most installers will try to achieve this within their installations and
designs but there are many cases where air gaps are not practical as a method of
protection. In these instances, installers may opt to install a mechanical backflow
prevention device. These provide a physical barrier to backflow. However, it must
be remembered that mechanical backflow prevention devices have limitations
and can be subject to failure.

In this section, we will look at some of the more common mechanical backflow
prevention devices and where we can install them (see Table 5.9).

 Table 5.9 Schedule of mechanical backflow prevention arrangements and fittings and their
respective fluid category protection
Description of backflow prevention Suitable for protection
Type arrangements and devices against fluid category
Back Back
pressure siphonage
a BA Verifiable backflow preventer with reduced pressure 4 4
zone
b CA Non-verifiable disconnector with difference between 3 3
pressure zones not greater than 10%
c DB Pipe interrupter with atmospheric vent and moving X 3
element
d EA/EB Verifiable and non-verifiable single check valves 2 2

e EC/ED Verifiable and non-verifiable double check valves 3 3
f HA Hose union backflow preventer; permitted only 2 3
for use on existing hose union bib tap in house
installations
➜

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