Page 595 - 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 9 Sanitation systems

The self-sealing trap has certain advantages over conventional traps:
● The valve removes the problems associated with negative pressure within
a system by opening to allow air in, in much the same way as an air
admittance valve. This creates a state of equilibrium within the system, and
means that air admittance valves and extra vent pipes are not required.
● Because there is no water in the valve, the problems of self-siphonage and
induced siphonage are eliminated. p Figure 9.77 The self-sealing trap
● The valve operates silently. This eliminates the noises generally associated
with water-filled traps.
● The valve allows a greater number of appliances to be installed on the same
discharge system without the risk of compromising system efficiency.
● The valve can withstand back pressures equivalent to ten times greater than
those experienced in a typical sanitary pipework system.

KEY POINT
The self-sealing trap is so
50 mm 50 mm 50 mm effective that it can be
A range of washbasins installed on a ventilated discharge branch system used safely on primary
ventilated stack systems
and ventilated discharge
branch systems.

40 mm 40 mm 32 mm
The same installation using self-sealing valves
There are no ventilation pipes and the main waste pipe is of smaller diameter

p Figure 9.78 Multiple installations of the self-sealing trap

Loss of trap seal
Provided the recommendations in BS EN 12056:2002 are followed, problems
of trap seal loss should be avoided. Most trap seal problems occur even before
water has been let down the trap, simply because they can be attributed to
design and installation issues with the sanitary pipework system. When loss of
trap seal occurs, obnoxious smells will permeate the dwelling. Most trap seal
problems can be traced back to the following faults:
● waste pipes that are too long
● waste pipes that are too small for the appliance
● waste pipes that are laid to an incorrect fall
● incorrect bends at the foot of the soil stack
● too many appliances on the same waste branch
● too many changes of direction.

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