TECHNICAL 35
Hot water cylinder ( HWC ) installations explained using SANS 10254 ( Part 5 )
By
Patrick Gordon
In the last article we discussed where and why a Temperature and Pressure Safety Valve ( TP & S ) should be installed . In this article we discuss how it works .
Operating principle : The valve opens the outlet on reaching the settings for :
The TP & S valve controls and limits the temperature and pressure of the hot water contained in a domestic storage heater and prevents it from being able to reach temperatures of over 100 ° C , with the resultant formation of steam . It also limits the system pressure from over pressurizing . When the system pressure reaches one and a half times ( 1.5X ) the rated system pressure , the valve will open to discharge some pressure to bring it back to an acceptable level .
On reaching the settings , the valve discharges enough water into the atmosphere so that the temperature and pressure returns to within the system ’ s operating limits .
Each HWC is tested for SANS compliance with a suitable pressure rated TP & S valve , therefore , the pressure rating of the TP & S valve should never be changed . The Pressure Control Valve ( discussed in the April issue of Plumbing Africa ) may be rated less that the HWC but never more .
Images supplied by Patrick Gordon
Temperature : The thermostat compound inside the temperature sensor 1 ) submerged in the hot water storage heater , expands as the temperature increases . This expansion causes a thrust pin to move and act on the main seat 2 ) opening the valve . The valve is set to open at temperatures of 93 ° C .
Pressure : The main seat , opposed by a set spring , raises on reaching the pressure setting and opens the outlet completely . The pressure setting is specified by SANS 198 as One and a half times the rating of the valve and is chosen according to the maximum permissible pressure in the system .
As the temperature and pressure decrease , the opposite action occurs with the valve subsequently reclosing within the set tolerances .
The pressure rating of the system is controlled initially by the pressure reducing portion of the Pressure Control Valve . This setting must be between 70 – 85 % of the valve rating . The second safety control is the expansion control function of the Pressure Control Valve . This setting must be between 95 – 100 % of the valve rating . The last safety setting is the TP & S valve that is 150 % of the rating of the valve .
You may well ask if this arrangement is not unsafe . This is not the case as the pressure limit of the HWC is 200 % of the stated rating . This stepped rating is carefully designed to completely protect the HWC and highlights the importance of having these valves present on all installations .
If you have enjoyed this article , look out for next month ’ s article . PA
August 2024 Volume 30 I Number 6 www . plumbingafrica . co . za