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33
2012-11
Softened water, safety valve
SH Operation
Softened water, safety valve
Initial fill-up with softened water and
enough shut-off valves
All relevant standards demand softened water for
heating systems with larger water volumes.
For the boiler a value of 20,000 lt°dH for system
volume in litres multiplied by the hardness
(in degrees of German hardness) may not be
exceeded.
A system volume of 300 litres (boiler and heating
elements) and very hard water with 30°dH yields
a value of 9,000 lt°dH. In this case, the system can
still be filled with unsoftened water.
For a heating system with a 1000-litre buffer, for
example, the total volume is 1300 litres. Even with
water that is not very hard, such as 20°dH, a value
of 26,000 lt°dH results; that is too high. For a
volume of 1300 litres, the water must be softened
to less than 15°dH (20,000 divided by 1,300).
Around 0.25 kg of limescale will precipitate from
a cubic metre of water with 15°dH, resulting in
a limescale layer 0.2 mm thick on a quarter of
a square metre of the boiler's heat exchanger
surface; this small area in the boiler is where the
lime build-up is concentrated. This may not seem
serious, but with 2 m³ of buffer storage and a
system volume of 0.5 m³, a thickness of 0.5 mm
is reached. Thicker layers hinder the heat flow
through the boiler wall to the extent that it is no
longer cooled sufficiently and heat stress cracks can
form.
In practice, that means the boiler can tolerate a
buffer filled with unsoftened water as long as
no repairs and no leaks in the system (defective
air valves or safety valves that fail to close) force
water to be refilled during the further course of the
system's service life.
To have a sufficient safety reserve for additions,
the new system must be filled with softened
water, meaning that the empty system must really
be filled exclusively with softened water before
starting the boiler for the first time. Changing
the water after the boiler has already been in
operation is too late since the limescale from the
unsoftened water has already precipitated in the
boiler.
To reduce the amount of water replacement for
later repairs, it must be possible to shut off all large
volumes such as buffers, boiler and heating circuits
to keep the lime content to a minimum during
water additions.
Protection against corrosion
To limit corrosion with softened water, the
pH value should be adjusted to between 8 and 9
using suitable inhibitors (trisodium phosphate).
Safety valve against overpressure
Install a safety valve with 3 bar opening pressure
on the boiler (is already installed for PU and
PC boilers). No shut-off valve may be installed
between the boiler and the safety valve. If solar
or other heat sources provide energy to the buffer
storage tank via a heat exchanger, a safety valve
(3 bar maximum) is also required on the buffer
storage tank. Normally an expansion tank that is
too small or defective, or blocked heating lines, are
the cause for activation of the safety valve.
The safety valve must be on top of the boiler in
the flow in order to also discharge heat in an
emergency. Only this way can it discharge heat
when it blows out hot water and also steam.
The discharge must be directed to the sewer via
an easily visible, open flow path (siphon funnel)
so malfunctions and, above all, a non-closing
valve can be recognised. If no sewer is available,
the discharge must be directed into the ground in
a pipe so nobody is endangered by hot water or
steam.