By Patrick Gordon of Calafrica (technical@calafrica.co.za)
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.
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.
Images supplied by Patrick Gordon
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.
Operating principle:
The valve opens the outlet on reaching the settings for:
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.
