Sanitary Fittings and Drainage
What this domain tests
This section tests the drainage vocabulary: soil, waste and vent pipes, traps and water seals, one-pipe and two-pipe systems, inspection chambers, manholes, septic tanks and gradients. Numerical questions on drain fall and pipe capacities are common.
Core rules and concepts
- Soil pipe carries faecal matter from water closets; waste pipe carries sullage from sinks, basins and baths; a vent pipe protects water seals.
- One-pipe system uses a single soil-cum-waste stack with a separate vent; two-pipe system keeps soil and waste stacks fully separate.
- Every fixture has a trap with a water seal, commonly 50 mm for soil fittings, to stop sewer gas and insects entering the building.
- House drain runs below the building to the inspection chamber, then the house sewer; an intercepting trap is placed at the boundary before the public sewer.
- Drain gradient for a 100 mm pipe is usually between 1:40 and 1:60, giving a self-cleansing velocity of about 0.6-0.75 m/s without scouring.
- Septic tank followed by a soak pit is used where no public sewer exists; never discharge septic tank effluent directly into an open drain.
Worked example
Fall in a 12 m long 100 mm drain laid at 1:60. Fall = length ÷ gradient denominator = 12 ÷ 60 = 0.20 m, i.e. 200 mm over the run. The invert at the downstream inspection chamber must be 200 mm lower than at the upstream end, and the chamber must have benching to channel the flow.
Traps that cost marks
Confusing soil and waste pipes; quoting a slope steeper than 1:40 for small drains, which separates solids from water; omitting the intercepting trap at the boundary; forgetting the vent pipe on tall stacks.