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