Pipe Fitting
This domain tests fittings, thread practice and the allowances that decide whether an assembly closes to the right dimension.
Core rules and concepts
- Common fittings: 90° and 45° elbow, equal and reducing tee, cross, reducer, bushing, nipple, union, coupling, cap, plug, long-radius bend, P-trap, bottle trap, floor (nahani) trap, gully trap.
- Pipes use BSP taper threads; fixture shanks generally use BSP parallel threads. A taper die on a parallel thread, or the reverse, gives a leaking joint.
- Seal threads with two or three turns of PTFE tape wound in the direction of the thread, or with approved jointing compound.
- Use two spanners on a union - one to hold, one to turn - so the pipe is not twisted.
- Support spacing is closer for plastic than for GI, and closer near valves and changes of direction.
Worked example
The face-to-face distance between two fittings is 2.000 m and each pipe end engages 12 mm into its socket. What is the cut length? Cut length = 2.000 + 2 × 0.012 = 2.024 m. Reasoning: the pipe must bridge the clear gap and also enter both sockets, so engagement is added at each end. If the 2.000 m had been measured socket-bottom to socket-bottom, the engagement would be subtracted instead. Write which dimension you are given before calculating.
Traps that cost marks
- Forgetting thread engagement, so the run comes out short.
- Using a parallel die where a taper thread is needed.
- Too much PTFE tape, which splits the socket.
- Replacing a long-radius bend with two 45° elbows and assuming equal flow.
- Fitting a bushing the wrong way round.