64
TECHNICAL
The difference
between
PCV and PRV
Patrick Gordon
PCV is a pressure control valve, while PRV is a pressure
reducing valve. So, what does that mean?
By Patrick Gordon
Patrick started in the
plumbing retail business
in 1985 with the Boumat
Group. In 2000, he
moved from retail to
manufacturing and joined
Cobra Watertech as a call
centre administrator. He
rapidly moved up in the
company and served as
product support manager
/ product development
manager and in 2005, he
was asked to take over the
product support department
nationally. He then qualified
as a facilitator and assessor.
When LIXIL Africa was
formed and they had to train
all the companies — Grohe,
Cobra, Isca, Vaal, Plexicor
and Libra — the training
department was expanded.
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. A PRV only has two
connection points.
ERV
PRV
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
Conversely, a PCV has a dual function:
1. It reduces the mains pressure to approximately
80% of the rated pressure of the valve; and
2. It relieves any excess pressure that builds up
downstream of the valve.
There is a 20% pressure gap 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.
A pressure control
valve, or PCV.
June 2018 Volume 24 I Number 4
The PCV normally has
three connection points.
The PCV is commonly
used in most domestic
installations. You have to
remember, though, that
the position of the PCV
is crucial, since you are
not allowed to have an isolating valve or check valve
between the hot water cylinder 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 hot water cylinder, an ERV
needs to be installed at the hot water cylinder before
the isolating valve. In this manner of installation, it
would be more cost-effective to have a PRV outside the
building and the ERV at the hot water cylinder. By doing
the installation this way, it makes any maintenance
convenient, as you only isolate the hot water cylinder
while the rest of the installation remains commissioned.
Remember that SANS 10254 requires all hot water
cylinder installations to have two vacuum breakers, a PRV,
an ERV, and a temperature and pressure safety valve.
I hope this has made it all clear. PA
www.plumbingafrica.co.za