Page 287 - 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 5 Cold water systems

The operation of an accumulator can be broken down into three stages, as
follows.

1 When the pump operates, it forces water into the Schrader valve under
protective cap
accumulator bladder, compressing the air surrounding it
to a pressure greater than the vessel’s Compressed
pre-charge pressure. This is the source of the stored air
energy.
2 When the bladder expands due to water being forced in
Synthetic
by the pump, it deforms in shape and the pressure within rubber
bladder
the accumulator increases. Bladder deformation stops
when the water and the now compressed air charge
become balanced. Water
3 When a tap is opened, the pressure within the system
drops and the compressed air forces the water out of the
accumulator. When all of the water inside the accumulator
is used and the pressure falls to a predetermined level, the
pump energises to recharge the accumulator water storage
and pressure, and the cycle begins again.

Probably the most important consideration when applying an Connection to the system
accumulator is calculating the correct pre-charge pressure. p Figure 5.38 A bladder-type accumulator
The following points must be considered:
● the type of accumulator being used
● the work to be done
● the system operating limits.

IMPROVE YOUR MATHS
The pre-charge pressure is usually 80–90 per cent of the minimum system
cut-in pressure (the pressure at which the pump energises), to allow a small
amount of water to remain in the vessel at all times. This prevents the bladder
from collapsing totally. To calculate the pre-charge pressure, follow this simple
procedure:
If the minimum working pressure of a cold water system is 2 bar, then:

2 × 0.9 (90%) = 1.8 bar
Pre-charge pressure = 1.8 bar

The accumulator air charge must be lower than the mains pressure for water to
enter the vessel and, on average, a pressure differential of around 1.5 bar lower
than the supply pressure would be acceptable (but no more than 2 bar and no
less than 0.8 bar). This means that, if the supply pressure is 3.5 bar, then the air
charge within the accumulator must be around 2 bar; a supply pressure of
4.5 bar would require a 3 bar air charge, and so on. Air pressure can be checked
and topped up as necessary at the Schrader valve (a tyre valve where you put
the air in) situated at the top of the accumulator.

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