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75
pears towards the end of discharge. Separation between the water attempting to
remain in the P-trap and the plug falling into the soil stack causes an air pocket
to form. The static pressure of this air will be subatmospheric. Air from the
room and the ventilated soil stack bubbles through the water to equalize the
pressures and a noisy appliance operation results. The inertia of the discharge
may be sufficient to syphon most of the water away from the trap, leaving an
inadequate or non-existent seal.
The problem is avoided by using 32mm basin waste pipes when the
length is restricted to 1.7 m at a slope of 20 mm/m run.
The sloping waste pipe can be up to 3 m long if its diameter is raised to
40 mm after the first 50 mm of run. This allows aeration from the stack along
the top of the sloping section. Longer waste pipes with bends and steeper or
even vertical parts have a 25 mm open vent pipe. Vertical soil and vent stacks
are open to the atmosphere 900 mm above the top of any window or roof-light
within 3 m. Underground foul sewers are thus atmospherically ventilated. Wa-
ter discharged into the stack from an appliance entrains air downwards and es-
tablishes air flow rates of up to a hundred times the water volume flow rate. Air
flow rates of 10-1501/s have been measured. The action of water sucking air
into the pipe lowers the air static pressure, which is further reduced by friction
losses.
Water enters the stack as a full-bore jet, shooting across to the opposite
wall, falling and establishing a downward helical layer attached to the pipe sur-
face. Restricted air passageways at such junctions further lower the air static
pressure by their resistance to flow. Atmospheric pressure will be re-established
at the base of the stack because of the flow of air into the low-pressure region.
The falling fluid tends to fill the pipe near the base and positive air static pres-
sures can be generated. Appliances connected to such a region may have their
water seals intermittently forced out.
The maximum permitted pressure is 375 Pa as this is equivalent to the
recommended trap depth of 75 mm water gauge for single-stack drain installa-
tions. When suction of this magnitude is applied to a 75 mm water seal, some
of the water is sucked from the trap, leaving about 56 mm. This is sufficient to
stop fumes entering the building.
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