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SmartScreenLevel 2 NVQ Diploma in Plumbing and Domestic Heating Unit 010 Handout 4

2012 City and Guilds of London Institute. All rights reserved.

www.SmartScreen.co.uk

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Unit 010: Sanitation systemsHandout 4: Traps

Requirements for a trap

The purpose of a trap is to retain an amount of water contained in a trap fitting which iscalled a plug of water. The aim is to prevent foul air from the sanitation and drainagepipework systems from entering the room and causing unhygienic and unsanitaryconditions.

The depth of this volume of water plug is called the trap seal depth.

There are requirements laid down under the Building Regulations which relate to theminimum depth of trap seals. The purpose of this minimum depth is to ensure that thewater seal is not removed when the sanitations system is in operation. If the trap seal islost then foul smells can enter the building.

Figure 1 P trap and S trap

Figure 2 Depth of trap seal

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The table below shows the minimum trap seal depth and trap diameter for differentsanitary appliances as identified under the building regulations

Minimum trap sizes and seal depths

Appliance Trap diameter

(mm)

Depth of seal

(mm)Washbasin1

Bidet32 75

Bath2

Shower240 50

Food wastedisposal unitSinkWashing machine2

Dishwashingmachine2

40 75

WC pan less than

80mm

WC pan over 80mm

75

100

50

50

1. The depth of seal may be reduced to 50mm with flush gratedwastes without plugs when spray tap basins are used

2. Whenever an appliance discharges direct to a gully the sealdepth may be reduced to 38mm

Trap seal loss

The purpose of well-designed above ground sanitary pipework systems is to ensure thatan effective water seal is retained in the appliance trap when subjected to operatingconditions. If a trap fails to retain an effective trap seal then foul smells can enter thebuilding which create a nuisance and a potential health risk to the occupants.

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The primary causes of trap seal loss

1. Self siphonage

This type of trap seal loss commonly occurs in basins when a partial vacuum is createdin the discharge (waste) pipe between the water plug and the basin. This negative(suction) pressure can siphon the water out of the trap, particularly if the water emptiesquickly. Self-siphonage can be avoided by ensuring that branch pipes are not too long

and are laid to the correct fall (not too steep). If self siphonage occurs it can be also beeliminated by fitting ventilating pipework to the appliance or more commonly by fitting aresealing trap.

2. Induced siphonage

Figure 3 Self siphonage taking place

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As the water flows past the second appliance connection, it completely fills the pipe andnegative pressure or suction effect is created between the pipe and appliance which cansiphon the contents of the second trap. This is the reason why care needs to be takenwhen connecting two appliances into a common discharge (waste) pipe with the primaryventilated stack (single stack) system. The problem can be cured by installing ventilationpipework to the two appliances or fitting resealing traps.

3. Compression

As water is discharged into a stack from upper floor levels it can create a compressioneffect (back pressure) at the base of the stack which can be sufficient to remove a trapseal. There are therefore specific restrictions on the type of bend used at the foot of thestack and the location of any low level connections.

Figure 4 Induced siphonage taking place

Figure 5 Compression taking place

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4. Other forms of trap seal loss

Other forms of trap seal loss in above ground systems that are not largely affected bysystem design include the following:

evaporation usually occurs in warm weather when the trap is not being used forperiod of time and the water in the seal simply evaporates

wavering out is caused by excessive wind pressures occurring across the top ofthe stack vent which in turn creates a wave motion in the trap (usually WCs) andthen causes the water to wash over the weir of the trap

momentum using a large volume of water such as a bucket of water to flush thecontents of a WC pan can completely remove the trap

capillary attraction can occur in S traps and is caused by a thread of material

overhanging into the trap seal and continuing into the discharge pipe, water is thenlost from the seal by capillary attraction.

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