84 TECHNICAL
Density The density of a fluid is the mass per unit volume. The density of water varies with temperature:
• One cubic metre of water at 4 º C has a mass of 1 000kg.
• One cubic metre of water at 82 º C has a mass of 967kg.
Water is most dense at 4 º C and becomes less dense when it freezes at 0 º C. It also becomes less dense when heated.
Practical application The theory that hot water rises and cold water subsides in a mass of water can be used in the design of plumbing systems to cause natural‘ thermosiphon’ conditions such as in a solar hot water system or to aid efficient circulation of hot water in a circulating hot water system.
Expansion Water expands when it is heated.
An empirical formula for thermal expansion in water is: V2 =( T2 – T1) ×( V1 × 0.0004)
Where: V2: Volume of expansion( litres) T2: Temperature to which water is raised
( degrees Celsius) T1: Ambient water temperature( degrees Celsius) V1: Volume of water heated( litres) 0.0004: Average volumetric expansion coefficient of water for temperature and pressure ranges in geysers.
Example If a volume of water— 100 litres— is heated from 15 ° C to 60 ° C, the water will expand to a volume of 101.84 litres.
V2 =( 60 – 15) ×( 100 × 0.0004) = 1.84l
Note Water also expands when it freezes( becomes less dense). That is why ice floats on water. An unusual phenomenon but a very fortunate one, since if water in a solid state( ice) was denser, then eventually the seas and some lakes would become solid ice all the way down to the bed in some areas on the Earths’ surface and the marine and aquatic life would die there too!
Practical application A customer complains that the pressure control valve( expansion relief valve part of the pressure control valve) is leaking. This is normal and is the correct function of the valve.
But how much should it expand for each complete heating cycle? We will use the formula above to determine what is normal, for example a 200-litre geyser should have an expansion of approximately 3.6 litres for each complete heating cycle.
The fact that he / she has noticed this and that it is causing a problem, is probably because the geyser installation is non-compliant in terms of SANS 10254, which states that:“ The expansion relief discharge pipe shall be piped to a suitable point of discharge where the drip will be visible but not cause a nuisance or hazardous condition or damage to any structure.”
You can also counter the argument of water wastage that this is not a‘ wastage of water’ in the following way:
To fill the 200-litre geyser, 200 litres of cold water are paid for, which in turn is changed into 200 litres of hot water. There are still 200 litres of water to be used, but in a heated state. The expanded volume is for free and can be used to water plants or for other purposes.
The fact that water expands in its solid state( ice) will remind us to take care when installing pipework that is exposed to freezing conditions in parts of the country where it gets bitter cold and freezing is likely. Proper lagging and protection against freezing will go a long way in averting burst piping and the associated water wastage and unnecessary maintenance that goes with it. In biting cold areas, you might even consider the installation of antifreeze valves.
August 2018 Volume 24 I Number 6 www. plumbingafrica. co. za