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77
charge stack. Continuous small rainwater flows can cause excessive pressure
fluctuations in buildings of about 30 storeys. Flooding of the stack during a
blockage would cause severe damage.
11. Pumped or pneumatically ejected sewage lifting: the discharge stack
is gravity-drained into a sump, from where it is pumped into a street sewer at
a higher level. A separate vent is used for the sump chamber and pumped
sewer pipe to avoid causing pressure surges.
12. Capillary: lint or hair remaining in a trap may either block the cap-
illary or empty it. Additional maintenance is carried out in high-risk loca-
tions.
13. Leakage: leakage can occur through mechanical failure of the
joints or the use of a material not suited to the water conditions.
When excessive suction pressure occurs in the waste pipe, some of the
water in the trap is syphoned out. When the central ventilation passage becomes
uncovered, it connects the inlet and outlet static air pressures. This returns the
waste pipe to atmospheric pressure and the syphonage ceases. Sufficient water
remains in the trap to maintain a hygienic seal.
Drainage installations should remove effluent quickly and quietly, be free
from blockage, and be durable and economic. They are normally expected to
last as long as the building and be replaced only because of changed require-
ments or new technology. Blockages occur when the system is overloaded with
solids, becomes frozen, suffers restricted flow at poorly constructed bends or
joints, or has building material left inside pipe runs. Each section of discharge
pipework must be accessible for inspection and internal cleaning.
Transport of solids from WCs is the controlling problem in the design, in-
stallation and maintenance of sloping drains. Swaffield and Wakelin (1976)
showed that, to maintain the flow from a WC and avoid deposition of solids in
the drain, the ratio of the length L of sloping drain (metres) to the gradient G
must be
L/G = 35
2
Pipe bends produce rapid deceleration of solids downstream, followed by
velocity regain as the remaining flush water catches up with and accelerates the
solids with minimal loss of inertia. When minimum gradients are used, solid
deposition could occur at a bend. To avoid this, the equivalent length of a bend
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