TECHNICAL
Friction loss increases exponentially with an increase
in the speed of the water, roughly by the square of the
speed; that is, if the water speed doubles, friction loss
will be roughly four times as much.
Different piping materials have different frictional loss
characteristics due to the wall thicknesses of the various
materials as well as the smoothness of the internal
surface, known as the ‘roughness factor’.
PRACTICAL EXERCISE
Determine the friction loss on a given pipe run and
make a pipe size selection
The task/brief
You are asked to install a galvanised mild steel (SANS
62-1 medium class) pipeline feeding 10 high-pressure
toilet flushing valves (Cobra Junior Flushmasters)
located in a new ablution block on a warehouse site.
The routing of the piping is in the ground from an
existing on-site 100mm water supply.
The length of the new run is 100m from the source of
supply to which you will connect. The lowest expected
on-site water pressure at the point where you will
connect in this case is given as 400kPa — but you
must always check this before commencing work,
using a pressure gauge. The lowest measured pressure
over a period should be used.
The newly installed Flushmasters are located on the first
floor of an existing building and are 3m (elevation) above
the source of supply (remembering that 1m of ‘head’ is
equivalent to 10kPa either gained or lost. In this case, it
is a loss of 30kPa).
Since the pipe run will be installed in the ground, noise
is not a factor, so in terms of SANS 10252:1, a water
velocity of 3m/sec. is acceptable for buried pipelines.
The architect has specified Junior Flushmasters and
top-entry W/C pans. These flush valves require 65ℓ/min.
at 200kPa flow pressure to work properly (see Figure 1
— a manufacturer’s specifications).
The expected flow demand on the new line is 130ℓ/min.,
which you might estimate as follows:
Each of these valves requires a flow rate of
65ℓ/min. There are 10 valves, but it is highly
improbable that all 10 could ever be flushed at
once. Most probable would be two being used
simultaneously, resulting in an assumed and
reasonable probable flow demand of 130ℓ/min.
This concurs roughly with SANS 10252:1 as well as
simultaneous usage flow demand tables given by
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the manufacturer for this product and the project
type; that is, a warehouse facility.
Note: The calculation of flow demand (Q) is always
based on reasonable assumptions and is not an exact
science. There are many ways of doing this. SANS
10252:1 (4.2.2 Probable (or design) Flow Demand) gives
reference to one method among many others.
To summarise the above information:
• Pressure available: 400kPa
• Required flow pressure at the terminal fittings:
200kPa
• Required flow demand: 130ℓ/min.
• Length of pipe: 100m
• Pipe material: medium-class galvanised mild steel
(SANS 62-1)
• Elevation above source: 3m (height of new fittings
above water supply source).
For this example, a few simple sums are necessary:
Available pressure 400kPa
Less elevation of 3m
− 30kPa
Subtotal 370kPa
Less required pressure
− 200kPa
Affordable pressure loss in pipe
170kPa
If we divide the affordable pressure loss by the pipe length
plus 25% (25% to allow for future encrustation, bends,
tees, and other in-line fittings such as line strainers), then:
Affordable pressure loss (170kPa) ÷ pipe length
(100m + 25% = 125m)
= Permissible friction loss of 1.36kPa per metre of
pipe run
Now refer to the friction loss table, Figure 2, to determine
a suitable pipe size.
Result
By entering Figure 2 from right to left at the 130ℓ/
min. mark, it can be seen that a 32mm galvanised
pipe (red line) will yield a friction loss of 2m/m,
whereas from our sums above, we can only afford to
lose 1.36kPa/m.
The next pipe size up is a 40mm pipe (green line), which
will serve well since the friction loss will only be 1kPa/m.
In both instances, the water velocity is in order; that is,
approximately 2.2m/sec. on the 32mm pipe and 1.7m/
sec. on the 40mm pipe, but the friction loss is too high
on the 32mm pipe.
May 2019 Volume 25 I Number 3