Page 564 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

The City & Guilds Textbook: Plumbing Book 1

ACTIVITY
The gradient curve
Try the gradient curve for
yourself. Determine the
answers to the following
questions:
1 A waste pipe has a Invert of
length of 1.5 m. What the drain
is its gradient? 450 mm for low-rise buildings
2 A waste pipe has a 750mm minimum for multi-
length of 750 mm. storey buildings
What is its gradient? p Figure 9.3 Branch connections at the invert of the drain
3 A waste pipe has a
gradient of 120 mm/m. Bends and off-sets
What is its length?
Bends at the base of discharge stacks should be large radius, the minimum
radius being 200 mm. Two 45° bends can be used as an alternative. This ensures
KEY TERM the smooth flow of water and solid waste into the drainage system. Tight
bends can cause a problem called compression, where the water hitting the
Compression: the process
of water hitting a bend at bend forces a shock wave of air upwards, which can blow the water out of waste
forces that cause a shock pipe traps, causing them to lose their seal and let obnoxious smells into the
wave of air upwards. dwelling.

200mm minimum

Large-radius bend Double 45° alternative

p Figure 9.4 Large-radius bends at the base of the stack

Off-sets in the wet part of the stack should be avoided if possible. Where
there is no option, again large-radius bends should be used, with no branch
connections within 750 mm of the off-set. If an off-set is to be placed in the wet
part of a soil stack, in a building of up to five storeys, then the stack must be
ventilated both above and below the off-set.

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