DESIGN: DEAR MR PLUMBER
19
Engineering and assumptions
Engineering design is applied science based on assumptions and
mathematical calculations, and assumptions are mostly not exact.
By Vollie Brink, Pr Eng
Most engineering projects are based on assumptions that eventually
become decisions that form the basis of the design, and on which
calculations are based, that determine the outcome of the project. meal time and also how much water
the kitchen will need, and the laundry
(if there is one).
The assumptions are the most important element of the design as they
dictate the success of the project. If the design was based on incorrect
assumptions, then the project will be a failure. The storage of cold water must also be
assumed and calculated for the total
hotel and gardens.
The engineer can make their own assumptions and base these
calculations on it to render a product that complies with the specified
performance or objective of the proposed project, as required by the client.
The engineer can follow any formal institutional standard or regulation or
rules to deliver the same product. Assumption also dictates the design
of pipe sizes and the flow rate in the
piping, and relates to how many fixtures
are connected to a supply pipe or
system, but more important, how many
fixtures are used simultaneously. We
have developed an equation for how to
calculate the probable-simultaneous-
demand, but the design engineer must
still check it by means of assumptions.
The important issue is that in engineering design, the assumptions are not
always exact, whether assumed or whether formally regulated.
When we developed SANS10252-1, we had to deal with this phenomenon
specifically in the tables for quantities of water usage and flow rates, and
therefore you will see that we don’t give exact figures,but rather a range.
The idea was to allow the design engineer to make assumptions to suit
the specific project.
The flow rates of water in piping depend on the usage and the persons,
the climate, the socio-economic situation, the water availability, the type of
building, and even the age of the users, as well as many other elements.
Even the hot water volume cannot just be a number that you expect to be
exact. This also applies to the temperature and the velocity of the water in
the piping, the pressure, and so on.
An example of how the assumptions dictate the design is the hot and cold
water design of a hotel. A hotel consists of a number of bedrooms and
most bedrooms are usually double, but these bedrooms are not always
fully occupied with two persons per room.
Even the choice of materials and
equipment is based on assumptions
and past experiences, as well as the
service and inputs from so-called
specialist sub-contractors.
Vollie Brink
Vollie Brink is one of the
industry’s longest-serving
wet services engineers.
He continues to serve on
SABS committees and
has been involved in the
Green Building Council’s
Green Star rating
system. Brink continues
to consult for various
organisations while
enjoying a well-earned
retirement.
The choice of pipe material has
become a serious assumption to be
made in terms of quality, longevity,
guarantees and after-installation service. Because of all these variants we
decided not to give exact figures in the tables. This allows the engineer to
intelligently make assumptions on which to base their design, according
to the specific project and environment, how it is going to be used, and for
what application. This is the basis of a rational design by an engineer.
All of this is why SANS10252-1 cannot just be declared to be the
domestic water part of the National Building Regulations (NBR) as is.
The design engineer must make a number of assumptions to be able to
design the quantity of hot water to be generated and stored. The volume
of hot or cold water is always related to the number of people using
the water. This document was designed to be a design manual and was originally
published as a Code Of Practice. As a design manual it does not include
the regulations or even deem-to-satisfy-rules.
The following assumptions must be made:
What is the expected occupation rate — how many people are normally
going to occupy the rooms? Some hotels work on 1.5 people per room
and others may work on full occupation. It is important to understand that the regulations in the NBR are only those
listed in the Annexure A of every part. The rest are only deem-to-satisfy-
rules (DTSR) which are not compulsory unless the owner decides to follow
them,; then the designer shall fully apply these.
From this you can calculate the volume of hot water to be generated, but
this is not the full story as you must also make an assumption of how
many people will shower or bath in the morning and how many in the
evening. You have to calculate how many meals will be served for each
www.plumbingafrica.co.za
Please see the various application forms.
Remember:
Volume: is related to number of people.
Flow: is related to number of fixtures.. PA
July 2019 Volume 25 I Number 5