TECHNICAL
53
the municipal supply pressure in
South Africa can be anywhere from
about 500-1000kPa.
A design example of a vacuum
breaker valve.
breaker (which is essentially a one-way
valve) lets air into the hot water cylinder
or system and then you don’t get this
crumpling effect (thermosiphic action).
The other function that a vacuum breaker
peforms is that when it is installed (300mm
above the geyser) on a new installation
(there needs to be one on the hot water
going out and the cold water going in and
300mm above the top of the geyser), what
happens with products like focus mixers
is that you shut the water off very quickly,
so, you will often get a shudder and shock
throughout the whole piping system. This
happens because the compressed water in
the pipe has nowhere to go, and in simple
terms it shocks the system. A vacuum
breaker acts as a shock absorber by
allowing the air to compress in the upstand
pipe just below the vacuum breaker, and
this is like a shock absorber. This protects
water systems and especially geysers from
this shock action. Always install two vacuum
breakers – one on the hot water in, and one
on the cold water out.
PRESSURE REDUCING VALVES
Gravity fed systems are usually fed by a
holding tank on the roof of a building. If you
were 20 storeys up the pressure between
the top floors and the bottom floors would
be significant and a pressure reduction valve
(PRV) would be necessary.
Most valves on the market have a dual
function. The control part of a PRV is to
reduce the municipal supply pressure.
This protects the geyser from over
pressurisation. If a pressure reduction
valve is not installed, it will damage the
geyser and shorten the lifespan. A 400kPa
geyser needs to be fitted with a 400kPa
rated valve. It would reduce the pressure
down to between 280kPa and 340kPa. The
relief side of the valve will release excess
pressure of the geyser to between 380kPa
and 400kPa. This happens because cold
water is ‘smaller’ than hot water in density
measures. When the element warms the
water up, the extra ‘size’ of the water needs
to escape otherwise the geyser will burst
and therefore the need for a relief valve.
A 600kPa valves operates between
420kPa and 510 kPa with release between
570kPa and 600kPa. For easy reference,
the standards (SANS10254) stipulate that
600kPa valves must be green, and 400kPa
valves are red. You do also get 200kPa and
100kPa valves.
NORMALITIES OF A PRESSURE VALVE
If you consider a 150ℓ geyser, you would
experience expansion that will produce
between 1.5ℓ and 2ℓ per day. This should
be intermittent because every time the
thermostat picks up that the water is cool,
it turns the element on and heats the water
like a kettle would. This creates expansion
and therefore it is completely normal that
an expansion valve should ‘leak’ on a
heating cycle but then stop leaking when
the geyser is hot enough.
The function of a PRV is a two-part answer.
Depending on where the site is located
(either up on a hill or down in a valley) Many homeowners don’t know this and
when they have a new valve installed they
see the ‘release drops’ as something is
wrong with the valve, but in fact if it does
not leak the geyser will burst. The overflow
pipe should also always be taken to a
suitable outlet where it can be visible – you
need to see the drip and the homeowner
needs to see the drip to identify if there
is a problem as continual dripping would
indicate a problem.
www.plumbingafrica.co.za July 2019 Volume 25 I Number 5