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Capillary Action

  • Only occurs in S-Trap arrangements and is not a regular occurrence
  • If a thread of material becomes lodged then water can be drawn up this through capillary action

Self-siphonage

Compression

Why traps lose their seal

  • This is most common is wash basins
  • Their shape allows the water to escape quickly
  • As the water escapes a plug of water is formed creating a partial vacuum (negative pressure) in the water pipe between the water plug and the basin
  • This is enough to siphon the water out of the trap
  • ensuring that the waste pipe is within the lengths allowed, to the correct fall, or that it is ventilated should help prevent self-siphonage
  • resealing traps (anti-vac) should also prevent this
  • As the water is discharged into the main stack (usually a W.C at first floor level) it compresses at the base of the stack causing back pressure
  • This can be enough to force the water out of the trap, causing loss of seal
  • The use of large radius bends and minimum of 450mm length between the invert drain and lowest pipe are defined in the regulations in order to prevent this
  • The main reasons for traps losing their seal are bad pipework design and poor installation.

Ways for Trap-Seal Loss

  • Trap seals can be lost as a result of poor practice and by not following the regulations
  • There are three main ways in which trap seal may be lost due to poor practice:
  • 1 self-siphonage
  • 2 induced-siphonage
  • 3 compression

Capillary Action

  • Traps can loose their seal via capillary action
  • Water can travel up-through a materiel such as cotton or hair via capillary action

Induced siphonage

Momentum

  • This is caused by a discharge of water from another appliance that is connected to the same waste pipe as other appliances
  • As the water plug flows past the second appliance connection, negative pressure is created between the pipe and the appliance that siphons the water out of the pipe
  • This arrangement is not acceptable on primary ventilated stack
  • The seal of the trap is removed by the force of water
  • This can happen if you pour a bucket of water down a basin or a toilet

Wavering

  • Wind pressure above the top of the soil and vent pipe, particularly in exposed areas causes the water in the trap to waver and wash over the weir of the trap
  • This does not happen often, and can be ovoided by fitting a 90 degree elbow or cowl at the top of the vent pipe

Trap Seal Losses

Evaporation

  • The most common form of natural seal loss
  • Occurs in very warm, dry weather when an appliance has not been used
  • The water simply evaporates
  • It is unlikely to occur in traps with a 75mm seal
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