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TECHNICAL
We continue with our valve series
By Patrick Gordon of Calafrica ( technical @ calafrica . co . za )
Q – How does an ERV work ?
The purpose of an “ Expansion Relief Valve ” ( ERV ) is to protect the system from becoming over-pressurised ( higher than rated system pressure ).
The pressure in the system may increase due to several reasons :
1 . The temperature of the water increasing and thus causing an increase in the size of the water molecules . When the water molecules are heated the electrons have more energy to overcome the attraction of the nucleus on them and thus can move further away . This effectively increases the diameter of the molecule . Because the size of the molecules has increased in the system , it results in pressure building up in the system . 2 . Due to incorrect plumbing installations which cause interaction between dissimilar pressure rated systems ( unbalanced pressure installations on mixers ).
Images by Calafrica unless otherwise stated
The valve defaults to the normally closed position by means of a spring that holds the sealing surface down .
As the pressure builds up under the sealing surface , it overcomes the pressure of the spring and allows the sealing surface to lift off the seat . This then allows some of the system pressure to escape . As soon as the force of the spring is stronger than the system pressure , the relief valve will again close to maintain the desired system pressure .
The discharge of excess pressure should roughly be the same as the rate of the pressure increase . Normal expansion should be evident in the form of a drip and not a strong flow .
Some ERVs have an easing gear on the valve to facilitate the flushing of foreign materials from the main seat .
Illustration taken from : Weird Science : Macroscopic Changes in Liquid Water Volume .
For any further information regarding this topic please feel free to contact the writer . PA
www . plumbingafrica . co . za @ plumbingonline @ plumbingonline @ PlumbingAfricaOnline March 2023 Volume 29 I Number 1