One Pipe System in Single Stack System Q6

Q.
The single stack system is the simplification of the one pipe system. Explain the following components of the system:

i) soil pipe
ii) air vent
iii) water traps

(25 marks, 2013 Q6)

A.
(i) Soil Pipe

Ref:
Figure 6 from South African National Standard, The application of the National Building Regulations Part P: Drainage. Published by SABS Standards Division, available at
https://law.resource.org/pub/za/ibr/za.sans.10400.p.2010.html
Soil Stack from Wikipedia, available at
https://en.wikipedia.org/wiki/Drain-waste-vent_system

(ii) Air Vent

[Air vent in the Single Stack System is called Stack Vent - see above diagram in (i). The purpose of the air vent is to allow suction of air to displace the space of sewage draining out to the sewer below.

Imagine a bottle filled with water. When it is inverted, the water drains out from the opening below due to gravity. As the water drains, it creates a vacuum inside the bottle. This vacuum stops the water draining below as outside pressure forces the water back to the bottle. Only when air is allowed into the vacuum (bubbles - air raising to replace the vacuum) that the water draining off is eased. If the bottom of the bottle (now inverted) has openings (holes), the draining of the water becomes smoother as air enters through the top openings. That is how the air vent of a stack (soil pipe) is supposed to work. To release vacuum created when sewage drains off into sewer.

See below another diagram of a double storey building with waste stack, vent stack and secondary stack.

The air vent is coloured in green, and the waste stack (soil stack) is coloured in brown. There is a secondary stack, which is not the design of Single Stack System. This is due to the fact that the branch drainpipe from the kitchen is long and therefore, further vent is required to displace the vacuum when the pipe drains into the main stack. This secondary stack vent would solve this problem.]

Ref:
Understanding Your Drain-Waste-Vent System from DIY Advice, available at
http://www.diyadvice.com/diy/plumbing/prep/drain-system/

(iii) Water Traps

Traps may be defined as fittings placed at the ends of the soil pipes (human waste) or waste (sullage) pipes (kitchen waste) to prevent passage of foul gases from the pipes to the outside. The traps maintain water seal which is a standing water between the pipe and the outside to facilitate this. The water seal does not allow gases to escape to the outside of the pipe. The efficiency of a trap will depend on the depth of water seal. Greater the depth of water seal will be more effective. The depth of water seal generally varies from 20 mm to 75 mm.