TECHNICAL
33
What is the difference between a PCV and a PRV ?
By Patrick Gordon of Calafrica ( technical @ calafrica . co . za )
A PCV is a “ Pressure Control Valve ” and a PRV is a “ Pressure Reducing Valve ”. So , what does that mean ?
A PRV only has a single function and that is to reduce the incoming water pressure . If for some reason the pressure in the system increases downstream the valve cannot react to this . In this instance an ERV ( Expansion Relief Valve ) needs to be fitted to the system to react to this situation .
A PRV only has two connection points . A PCV has a dual function : 1 ) it reduces the main ’ s pressure to approximately 80 % of the rated pressure of the valve and 2 ) relieves any excess pressure that builds up downstream of |
PRV |
ERV |
PCV |
the valve . There is a pressure gap of 20 % between set and relief pressure . This enables the system to |
absorb some of the expansion pressure generated by the heating of the Hot Water Cylinder . The PCV |
normally has three connection points . |
Nominal rated system pressure
Pressure control closing pressure ( below 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
Expansion relief discharge pressure ( system pressure )
The PCV is the common one that is used in most domestic installations . You have to remember though that the position of the PCV is very important as you are not allowed to have an isolating valve or check valve between the Hot Water Cylinder ( HWC ) and the PCV as this will cut off the expansion function of the system . So , if you require the valve to be situated outside and have an isolation valve at the HWC an Expansion Relief Valve needs to be installed between the HWC and the isolating valve . In this way of installation it would be more cost effective to have a PRV outside the building and the ERV at the HWC . By doing the installation this way it makes any maintenance very convenient as you only isolate the HWC while the rest of the installation still remains commissioned .
Remember that SANS 10254 requires all HWC installations to have 2X Vacuum Breakers , a PRV , an ERV and a Temperature and Pressure Safety Valve .
I hope that this has cleared up any confusion that may have existed . PA
July 2022 Volume 28 I Number 5 www . plumbingafrica . co . za