Backflow – a reversal of the normal flow of water in a pipe system – can have comical results. In the late 1960s wine flowed through the taps of the surprised and delighted residents of Cincinnati, USA, when a local winery left a water supply valve open after flushing its fermentation tanks. But backflow can also be dangerous, causing drinking water contamination… or worse. For example, a maintenance man connected a large liquid petroleum gas tank to a water main to wash it out. Unbeknown to him, there was still gas under pressure in the tank. By opening the water supply valve, he back pressurised the water supply main with gas, causing three nearby houses to explode. There are horror stories in the hvac market too. In one case, water backflowed into the air conditioning system of a Philadelphia hotel, resulting in conditions that fostered the growth of the bacteria that cause Legionnaires’ Disease.
These examples are extreme. But backflow is a real threat – the US Environmental Protection Agency reckons that at least 100,000 cases of cross contamination occur in the USA every day. That makes it a pressing health & safety issue, and its prevention a priority.
Backflow is caused by one of two factors
* Back siphonage, which occurs when the pressure on the supply end of a water pipe drops dramatically. This causes a vacuum which can suck fluids back through connections upstream thus contaminating the water system.
* Back pressure, the result of reversed water pressure on an outlet from a water main connection exceeding the mains supply pressure.
One solution to these problems is the reduced pressure zone (RPZ) valve. It employs spring check valves create an intermediate zone of pressure that is lower than the supply pressure. If back pressure builds up and the outlet check valve fails to hold, the intermediate zone will pressurise and cause a relief valve to open, draining the entire assembly, thus preventing possible contamination back into the supply.
Installers in the USA have used RPZ valves since the early 1970s when Watts Industries launched its 909 RPZ. Now the British Government has decided to allow their use in this country. The new Water Regulations which replace the Model Water Byelaws free installers to fit RPZ valves here for the first time.
Until these regulations, UK installers have had to install a break tank with a ball valve. However, pressure is lost from the mains with this arrangement so a pump also has to be fitted complete with a level switch to control the pump. This is complex, bulky and expensive, compared with the RPZ valve.
The RPZ valve can only be operated once its type of protective use has been approved by the local Water Regulations Inspector and its correct fitting and operation has been independently checked by one of the 150+ RPZ-licensed inspectors operating in the UK. However, its simplicity, low cost compared to break tanks and ease of installation make it an attractive option for installers