40 TECHNICAL
Valve pressure
By
Patrick Gordon , Calafrica
Q : I have a 400kPa valve but the pressure reading on my gauge is only 300kPa . Is the valve faulty ?
A : In a system that is rated 400kPa , that is the maximum allowed pressure and thus the valve will ensure that the pressure never goes above that rating . During the operation of the system , there will be various amounts of expansion due to the increase in temperature while the hot water cylinder is heating . If the valve reduces the mains pressure to the maximum allowed , the relief valve will always be purging excess pressure .
be anywhere between 280kPa and 400kPa , depending where in the heating cycle it is , and be completely within specifications .
Feel free to contact your valve manufacturer for more information on valve fault finding or look out for a future article on this topic . PA
By having the valve set pressure a little lower than the system pressure it allows for some expansion to take place without any water being purged . The set pressure vs relief pressure is calculated to a fixed ratio . ( see the chart below )
After studying the chart , you will notice that the system pressure on a 400kPa installation , the gauge reading can
Nominal rated system pressure
Pressure control closing pressure ( below system pressure )
Expansion relief discharge pressure ( system pressure )
100kPa ( Blue ) 70 – 85kPa 95 – 100kPa
200kPa ( Black ) 140 – 170kPa 190 – 200kPa
300kPa ( Brown ) 210 – 255kPa 285 – 300kPa
400kPa ( Red ) 280 – 340kPa 380 – 400kPa 600kPa ( Green ) 420 – 510kPa 570 – 600kPa
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