Regulation opens market

Danfoss' Ian Ward explains why RPZ valve use may increase.

EU membership means the regulations governing the equipment available to UK water companies is increasingly standardised with other European countries. An example of such changes are the water supply (water fittings) regulations 1999, which took effect on 1 May 2000. Designed to prevent the contamination, waste, misuse and undue consumption of public water, the regulations state under schedule one that any and every fluid back-flow risk can be categorised. The categories are simply ranked one to five, category one represents wholesome drinking water, category five is a fluid which represents a serious health risk.

Until now, categories four and five, or their equivalent, have always been isolated from mains and drinking water networks by using a type-A air gap, more commonly known as a break tank. This system has been used effectively for years and no doubt will continue to be utilised. The water supply (water fittings) regulations 1999, however, make it possible to do away with the break tank. For industrial and commercial applications this means the space taken by the tank can be reclaimed, the need for a power-consuming pump is removed and it is possible to work directly from the mains water supply for category-four fluids.

One effect of this is an expansion in the range of applications for which the reduced pressure zone (RPZ) valve, or BA device, is suitable. Class-five risks, it should be noted, still require the type-A air gap. Compared to a lot of water industry equipment the RPZ valve is relatively unsophisticated, but it can be a major cost-saver if used correctly.

Pressure drop
The three main springs in the valve have different pressures. Assuming flow direction from left to right, the first check valve starts to open at approximately 0.5bar and the second check valve at around 0.25bar. The dump-valve spring pushing the valve to open is about 360mbar. So there is a pressure loss of approximately 0.75bar across the valve.

If, for example, mains pressure is 5bar, the first check valve will reduce the pressure in the centre chamber to 4.5bar. The 5bar mains pressure is also channelled to fill above the main diaphragm. With 5bar pushing the dump valve closed and 4.86bar - 4.5bar pressure in the centre chamber plus the 0.36 bar spring - pushing in the opposite direction, a pressure of only 140mbar is left, keeping the dump valve closed. Regardless of whether the valve encounters differing mains pressures, back flow, or if the valve becomes faulty, this impacts on the pressures required to maintain the dump valve in a closed position. The dumping of the centre chamber produces an air-gap between the first and second check valves. There are good reasons to use this valve instead of a tank and pump, the most obvious being cost. The Danfoss Socla BA2760 valve at 2.54mm is extremely cost-effective compared to a tank and pump system. Running a pump also incurs operational costs which do not affect the RPZ valve. Companies in the food processing industry which use tanks and pumps will be only too aware of the cost and effort involved in meeting tank hygiene standards.

International acceptance
The RPZ valve has been used successfully in the US for over 25 years and in many parts of Europe for more than ten years, which represents a thorough test of the device's abilities. This should help alleviate the resistance to change which deters some from using RPZ valves. However, this is not just a fit-and-forget valve. RPZ valves require and annual inspection by a qualified technician. It is also essential the user's local water authority has categorised and assessed the risk level before a decision to install an RPZ valve can be made.



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