Page 596 - 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

Reasons for loss of tap/trap seal
There are various ways in which tap/trap seal loss occurs:

● self-siphonage
● induced siphonage
● compression
● wavering out
● evaporation
● capillary attraction
● momentum
● foaming.

Self-siphonage
INDUSTRY TIP
Self-siphonage occurs when water is discharged from an appliance. The water
Self-siphonage is most forms a plug, which, as it disappears down the appliance waste, creates a partial
common on washbasins due vacuum in the waste pipe between the plug of water and the water in the trap.
to the rapid evacuation of the This then pulls the water from the trap.
water from the bowl and the
small size of the waste pipe. Atmospheric pressure Negative pressure zone 'Plug' of flowing water

Water seal sucked out of trap

p Figure 9.79 Self-siphonage

In most cases, self-siphonage will not occur if the waste pipe length is kept
within the recommended lengths of BS EN 12056. If it does occur, the
installation of a vent on the waste pipe branch may be necessary or an anti-
siphon trap could be fitted.

Induced siphonage

Induced siphonage can occur by one appliance causing the loss of trap seal of
another appliance connected to the same waste pipe. When water is discharged
down an appliance, the water in the trap of the next appliance is drawn out by a
negative pressure as the plug of water passes the branch connection.

584

9781510416482.indb 584 29/03/19 9:07 PM