Pipes and Fittings
Domain 4 is about pipe sizing, thread types, the fittings used for supply and for drainage, and the supports and expansion allowances that keep a run serviceable.
Sizes and threads
- Nominal bore for GI pipe: 15, 20, 25, 32, 40, 50, 65, 80 and 100 mm.
- Plastic pipes are marked by outside diameter, for example 110 mm soil pipe, plus a pressure class.
- BSP thread with a 55° angle is standard in India; BSPT tapers and BSPP is parallel. NPT is 60° and will not seal against BSP.
Fittings
- Supply: socket, elbow, tee, reducer, union, nipple, cap, plug, bushing, bib cock, gate valve.
- Drainage: 45° junction, bend, door bend, P-trap, floor trap, gully trap, inspection chamber, vent cowl, rainwater head, shoe.
- Any change of direction in a drain uses two 45° bends or a rest bend, not a single 90° bend.
Supports and expansion
Support horizontal pipe at regular intervals, provide expansion loops on hot lines, keep supports clear of joints, and never hang a run from another service pipe.
Worked example
A 110 mm soil stack must turn to run horizontally under a floor. Reasoning: a single 90° bend makes a sharp turn where solids catch and nothing can be cleaned, so use a rest bend or two 45° bends, add a rodding eye at the base of the stack, and support the horizontal run so the gradient is not lost.
Traps
Mixing BSP and NPT; using a tee instead of a 45° junction for a soil branch; forgetting the rodding eye; buying 110 mm plastic pipe for a 4-inch GI line without checking the connection.