Page 187 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

Chapter 3 Scientific principles

a different pressure. For example, if a cistern measuring 1 m long × 0.5 m
wide × 0.7 high was placed in a roof space, then what pressure would it exert if:

l it was placed on its bottom
l it was placed on its side
l it was placed on its end?

IMPROVE YOUR MATHS
Before we can attempt these calculations, we must first find the mass of the
cistern in kg. The formula for this is:

length × width × height = volume in m 3
From earlier calculations, we know that to find the force of an object we use the
formula:

kg × gravity = N
Since 1 litre of water has a mass of 1 kg, a cistern measuring 1 m × 0.5 m × 0.7 m
has a force of 3433.5 N. ACTIVITY
The formula for finding pressure is:
Pressure
force
= N/m 2 What is the pressure
area exerted by a block of lead
From these calculations we can see that the greater the surface area for a given with a cross-sectional area
mass, the less force will be exerted by that mass. This is of particular importance of 4 m and a mass of
2
when placing large cisterns in roof spaces since the greater the surface area we 4000 kg?
can rest the cistern on, the more we can spread the load of the cistern.

Static pressure of water (head)
The unit of water pressure is the pascal. The pressure exerted by water is
due to its mass and is determined by the height of the column of water.
For instance, if the pressure exerted by a water main is 300 kilopascals
(kPa) it will balance a column of water about 30 m high. This pressure is
equivalent to a head of
water of 30 m. Therefore, 10 m of head = 100 kPa.
Water pressure in plumbing systems is usually measured in bar pressure.
Static head measured from
Static head of water in plumbing systems is measured from the bottom of the bottom of the cistern
exerts a pressure of 10 kpa
the water source, i.e. the cistern, to the outlet, as shown in Figure 3.20. per metre of head.
10 m
ACTIVITY 10 kpa = 0.1 bar = 1 m

Static pressure of water
If the vertical distance between the bottom of a cold water cistern
and the tap is 16 m, what is the pressure at the tap in:
a kilopascals
b bar?

Static head at the tap is
100 kpa = 1 bar = 10 m
p Figure 3.20 The head of pressure on a cold
water system

175

9781510416482.indb 175 29/03/19 8:55 PM