Plumbing Connection Autumn 2016

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TION AUTUMN 2016 43
For convenience, many fluid system pressures are
referred to in metres Head (of the fluid in question). This
is really as simple as it sounds. For example, 25m Head of
water is the pressure that would be acting on the base of the
tank in Figure 2 if the water level was 25m, instead of 3.5m
as shown.
Water pressure expressed in kPa can be approximately
converted to metres head of fluid by simply dividing the
kPa value by 10. For example, a standpipe rated at 250kPa
is approximately equal to 25m head of water. However,
please note that this conversion will not work for fluids with
densities significantly different from water. For example, a
pipeline carrying kerosene (ư = 810kg/m3) at a pressure of
270kPa, would be equal to 34m head of kerosene. Luckily,
most of us are generally dealing with water, so this quick
conversion method can be useful to remember. Pumps are
also generally rated by the amount of Head they produce.
For example, a pump rated at 15m Head means it can
generate a maximum Pressure of about 150kPa (of water).
A good understanding of working pressures in water
supply mains is important for plumbers as it significantly
affects flowrates through pipes and fixtures. Mains water
is often supplied by water towers and the water pressure
at a dwelling is often governed by the height of the tower
above the dwelling and the proximity of the dwelling to the
tower, as well as a variety of other variables. For example,
the maximum (theoretical) water Pressure available at the
shower rose shown in Figure 3 is the difference in Head
between the water level in the tower and the shower rose
on the second floor of the house. I say maximum theoretical
pressure because there are a number of other factors that
will cause pressure losses such as pipe friction and form
losses. However, we will discuss these in more detail in
future issues. Where the available Pressure in a system
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FIGURE 2 – WATER PRESSURE EXAMPLE 1
Fresh Water
Base Pressure?
3.5m