TECHNICAL
51
Valves and
plumbing systems
Valves play an important, and in some cases critical,
function in a plumbing system to manage risk in terms of
safety of the system as well as direct and indirect users.
By Richard Wiederhold, managing director of Advanced Valves
The three main South African National Standards
(SANS) related to valves in this article are contained
in SANS198: Functional-control valves and safety
valves for domestic hot and cold water supply
systems, 10252-1: Water supply and drainage
for buildings, and SANS10254: The installation,
maintenance, replacement and repair of fixed electric
storage water heating systems.
SAFETY VALVES IN GENERAL
The function of the temperature probe on a safety valve
is to cause the valve to open when the temperature of the
water exceeds the preset temperature (which is between
93-98°C). So, when the water in the cylinder reaches that
temperature, the micro-crystalline wax (which is what the
probe is made out of) changes from a solid into a liquid
which causes expansion of 15%. This, in turn, drives a
pin upward to open the safty valve. Super-heated water
then escapes through the outlet, thus allowing cold water
to enter through the inlet, cooling the water down and
creating a safe condition which can be maintained. If a
safety valve is faulty or damaged, it should be replaced
because it may become a very dangerous component
which is meant to act as the safety mechanism of a hot
water cyclinder.
VACCUUM BREAKERS
Something that some plumbers don’t know about vacuum
breakers is that when you drain a geyser or hot water
system, you get what is called thermosiphonic action.
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SANS 198 contains a clause in paragraph 6.2 that states
that functional control valves may be maintained and
repaired, but safety valves shall not be repaired and and
it is vitally important that plumbers don’t tamper with
these valves as it can cause the water to become “super
heated” which means the water reaches boiling point.
The geyser can also burst and obviously this creates a
dangerous hazard for both the occupant, and plumber
who will need to perform his work.
A design example of a cutaway showing the inner workings of a pressure reduction valve.
What this means in reality is that if you take a 10-storey
building, and work on the water mains is being carried
out and a mains valve is opened, the water in the pipes
of building will weigh in excess of two tons and would
suck the water out of the system. The weight of the
water may also start to crumple the geysers. The vacuum
July 2019 Volume 25 I Number 5