Page 316 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

The City & Guilds Textbook: Plumbing Book 1

Most RPZ valves consist of three separate elements:

1 two check valves
2 a differential relief valve
3 three test points.

The first check valve is spring loaded to generate a specific pressure drop across
this part of the valve. This creates a reduced pressure zone downstream in the
middle chamber of the valve and on the downstream side of the differential relief
valve. The incoming mains supply maintains supply pressure on the upstream
side of the differential valve and, as long as the mains pressure is higher, the
differential relief valve will remain closed.
If, under static conditions, the mains pressure reduces to where it is just 0.14
bar above the pressure in the reduced pressure zone, the differential relief valve
will open and release the contents of the middle chamber to drain. Should
backflow occur past the first check valve element, the pressure on both sides
of the differential valve will equalise and the differential relief valve will open
to discharge the water.
If complete mains failure occurs, the contents of the middle chamber are
discharged to drain, provided that both check valve elements are functioning
correctly. However, should the upstream check valve become faulty, the
pressure in the middle chamber will equalise to that of mains pressure and the
differential relief valve will open and continuously discharge water at a steady
rate. If the downstream check valve fails under zero mains pressure conditions,
the differential relief valve will open and water will discharge from the
downstream side of the system until the pressure here also becomes zero.
Testing, commissioning, maintenance and annual inspection can be carried out
only by a trained and approved installer. Anyone who tests RPZ valves must be
certificated. Specialist training is available from various test centres across the
UK. Further recommended reading is the Water Regulations Advisory Scheme
Information and Guidance Note No. 9-03-02.

Type CA non-verifiable disconnector with difference
between pressure zones not greater than 10 per cent

These are very similar to BA devices (RPZ valves) in that they provide a positive
disconnection chamber between the downstream water and the upstream
water. The disconnection area between the two main check valves is open to
the atmosphere under fault conditions, thereby maintaining an air gap should
a loss of upstream pressure occur. Like the RPZ valve, any water discharged
would run to drain via a tundish. They are suitable for fluid category 3.

A typical use of a type CA disconnector is as a permanent connection
p Figure 5.77 A type CA backflow between a sealed central heating system and the water undertaker’s cold
preventer water supply.

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