Page 409 - 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 6 Hot water systems

The bladder (bag) type expansion vessel
Also known as the bag-type expansion vessel, this is usually made from steel
and contains a neoprene rubber bladder to accept the expanded water. At no
time does the water come into contact with the steel vessel as it is contained at
all times within the bladder.
The inside of the steel vessel is filled with either air or nitrogen to a predetermined
pressure. The initial pressure charge from the manufacturer is usually made with
nitrogen to negate the corrosive effects on the steel vessel’s interior. A Schrader valve is
fitted to allow the pressures to be checked and to allow an air ‘top-up’ if this becomes
necessary. Figure 6.42 shows the workings of a bladder-type expansion vessel.

Expanded water Expanded water
Air cushion Air cushion Air cushion

Bladder-type membrane
A B C
p Figure 6.42 Workings of a bladder (bag) type expansion vessel

● Diagram A shows the bladder in its collapsed state; this is because the only
pressure is the air/nitrogen charge compressing the empty bladder. There is
no water in the bladder.
● Diagram B shows that water under pressure has entered the bladder during
the initial cold fill of the storage cylinder, causing the bladder to expand
and pressurising the air in relation to the water pressure. The bladder has
expanded because the water pressure is greater than the pressure of the air.
● Diagram C shows the bladder fully expanded due to the hot water expansion
when the system is heated.
With some bladder expansion vessels, the bladder is replaceable in the event of
bladder failure. A flange at the base of the vessel holds the bladder in place. By
releasing the air and removing the bolts, the bladder can be withdrawn and replaced.

The diaphragm-type expansion vessel
Diaphragm expansion vessels are used where the water has been deoxygenated
by the use of inhibitors or because the water has been repeatedly heated, such
as in a sealed central heating system. They must not be used with UHWSS
because the water is always oxygenated and comes into direct contact with the
steel of the vessel.
They are made in two parts with a neoprene rubber diaphragm separating
the water from the air charge. Again, like the bladder-type expansion vessel, p Figure 6.43 Bladder (bag) type
a Schrader valve is fitted to allow top-up and testing of the air pressure. expansion vessel
Figure 6.44 shows the workings of a diaphragm-type expansion vessel.

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