Traps are an important component of a plumbing system. They prevent ingress of foul air, insects and vermin from the sewers into the building and resist the spread of disease. Traps are constructed, so that they retain a body of water which acts as a water seal.
Traps should be of the self-cleansing type. They should generate enough velocity from the available flow to have a self-cleansing effect, a smooth finish and a full uniform bore.
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Foul gasses produced in the sewers, drains, waste-pipes may cause a nuisance by entering in houses through house-connecting pipes if their passage is not checked by some suitable devices. The appurtenances which are used to stop entering of foul gasses inside the houses are known as traps.
A gully trap is provided outside the building before connecting to external sewerage line. It also collects waste water from the kitchen sink, wash basins, baths and wash area.
Gully Trap is provided to prevent the foul gases entering in to the building by providing water seal. It has deep water seal of minimum 50 mm depth and it also prevents entry of bugs and insects from sewer line to waste water pipes.
In combined system i.e. what rainwater and building sewerage flow in single drainage, rainwater pipes may be connected to the drainage line through gully traps only.
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The total depth of gully trap shall not be less than 30 cm. And depth of water shall not be less than 15 cm and with a minimum water seal of 50 mm. Each gully trap shall have one cast iron grating 15 cm x 15 cm and it must be embedded in cement concrete foundation properly and housed in a brick masonry chamber. Chamber must be well plastered and one water and air tight C.I cover fixed over frame 30 cm X 30 cm inside dimensions.
Gully traps receive sullage from baths, wash basins, kitchen sinks etc and pass it on into the sewer carrying discharge from W.C. Gully trap, therefore, disconnects the sullage drain from sewer line of the house. When gully trap is provided it may not be necessary to provide individual traps for baths, basins and kitchen sinks.
Cleaning of gully trap at frequent intervals is necessary to prevent choking.