FEATURES
The NBR also has a list of other related standards that need to
be studied, such as piping and fixtures. It also lists the standards
for underground and above-ground piping. The above-ground
types of plastic piping must be ultraviolet (UV) resistant, which is
essential and often gets neglected in installations. Not using the
correct material is a sure way to get the installation failed. You
cannot use below-ground plastic pipes above ground because
below-ground pipes do not get manufactured with UV-resistant
stabilisers. This essentially means that when exposed to sunlight,
these below-ground pipes will not last the required length of time,
and they will become brittle, crack, or burst.
The installation of above-ground piping is specified in detail in
SANS10400-P and the most important part is the fixing of the
piping in a stable position with saddles, pipe clamps, brackets,
supports, hangers, and the like, but it must always allow for
expansion and thermal movement. The best option is to use pre-
manufactured systems that have been designed to consider this.
SANS1200 is a civil engineering standard for construction, but
valuable for the installation of below-ground piping and the
related elements such as the construction of manholes and
inspection or cleaning chambers. It also describes the excavation,
the pipe bedding, the laying of the piping, and the material
required around and above for backfilling. Both SANS10400-P
and SANS1200 also specify the testing requirements.
WHAT IS THE CORRECT METHOD OF CALCULATING THE FLOW
OF WASTE?
A plumber is allowed to carry out a deemed-to-satisfy-rule design
(DTSR), and there are a number of tables in SANS10400-P that
indicate how to size the various types of pipe, such as a drain
(which shall not have a gradient less then 1 in 60 in the case of the
DTSR design), and to size the stack pipe and branch pipes.
One of the tables indicates the number of discharge units (DU) per
fixture, such as one DU for a washbasin and eight DUs for a water
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closet (WC). The design of a drainage pipe relates to DUs and not
litres per second as you will find in water system designs. This
system is considered ‘old’ and originated many years ago but worked
well in the case of a DTSR design for simple building systems.
The existing tables are outdated and render an over-design;
however, it is still within safe design parameters. The original tables
were based on a toilet flushing volume of 12–13 litres per flush;
this volume has been significantly reduced to between three and
six litres. Today, there are systems that flush with two litres.
The critical element of a drainage system is not only the flushing
of the WC and the volume of water to flush the bowl clean, but
the volume of water required to flush the effluent down the piping
and carry it away to the municipal sewers, and further. The DUs
relate to the volume of water to flush the effluent down the drain
and then through the municipal sewer — this has not yet been
addressed and therefore not revised to suit current requirements.
Ultimately, the answer to this question is that you as the plumber
are obliged to comply with SANS10400-P as per the latest deem-
to-satisfy rules, to size the various pipes or have a rational design
done by the engineer, which is not necessary for a house.
In the case of a complex building, it would be better to do a
rational design by a registered engineer who can then calculate
the pipe sizes in terms of hydraulic design principles, and the
same for the municipal sewers.
If you analyse the pipe diameters of most of the non-complex
buildings, then you will find that most of the pipe systems consist
of common sizes between 50mm and 100mm. The 50mm is
mostly for the wastewater and the 100mm is the minimum for
soil piping. It is recommended to work according to the present
tables until a formal revision is carried out. PA
Part 2 continued in the next issue.
April 2019 Volume 25 I Number 2