Page 410 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

The City & Guilds Textbook: Plumbing Book 1

Expanded water Expanded water
Air charge Air charge Air charge

A Diaphragm B C
p Figure 6.44 The workings of a diaphragm-type expansion vessel

Internal expansion
With internal expansion, an air pocket is formed as the hot water storage vessel
is filled. A floating baffle plate provides a barrier between the air and the water
so that there is minimum contact between the air and the water in the cylinder.
When the water is heated, the expansion pushes the baffle plate upwards in a
similar manner to an expansion vessel.

Over a period of time, the air within the air bubble will dissipate as it is leeched
into the water. When this happens, expansion cannot take place and the pressure
relief valve will start to discharge water. However, this will only occur as the
water heats up. Once the cylinder is at its full temperature, the pressure relief
valve will close and will only begin to discharge water again when expansion is
taking place. Because of this, manufacturers of bubble top units and packages
recommend that the cylinder is drained down completely and refilled to recharge
the air bubble. This should be done on an annual basis or as and when required.
Hot water dip pipe

Air
Floating baffle plate

Floating baffle plate moves
upwards as the water expands

p Figure 6.45 Integral air bubble

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