Pollution problem prevented at rail station
Honeywell BA298 RPZ valves (Reduced Pressure Zone back flow preventers) are being used to prevent a pollution problem at Leeds City Station as part of the Leeds 2000 Development Project. The valves stop non-potable water from flowing back into the mains when water is supplied to the carriages of the high-speed trains that run between Leeds and London writes Andrew Stokes-Roberts of Honeywell Control Systems.
Honeywell RPZ valves have been used in numerous circumstances where back flow prevention is required and it is advantageous to fit a valve instead of a traditional break tank with booster pump.
In this case, for both cost and space reasons, the Honeywell RPZ valve was the ideal solution for back flow prevention control, which utilises mains water pressure and therefore there are no running costs to consider.
Moreover, the RPZ valve negates the need for expensive testing procedures for the existence of legionella, which can be the case with an unprotected break tank system incorporating a type ŒA air gap.
Back flow can occur for two main reasons over-pressure and back siphonage. If the pressure in a non-potable water system exceeds the water supply pressure and there is a direct mains connection, the result will be pollution of the water main, for example, a pressurised fire sprinkler system, where a pressure vessel is fitted to give a boost to the fire fighting supply as soon as a fire is detected.
During periods of high usage of the water main, the high flow rate conditions cause the pressure in the main to fall below that of the pressure vessel. In this situation, if measures are not taken, contaminated water from the sprinkler system could flow back into the mains supply. In a back siphonage situation, a physical rupture of the main or pump failure at a booster station may cause the condition. In either case, if the connection point is on higher ground than the mains fault then water will travel back down the main in the opposite direction back siphonage. Depending on what fluid is connected to the main at the higher point, the result of this could be disastrous for the public water supply.
To prevent any such contamination of the mains supply it is therefore imperative to fit an effective protection device to prevent back flow. Furthermore, in the case of potable water supplies it is essential to employ the correct protection device for the fluid category risk identified. Under the new Water Regulations, due to be effective from July 1999, the minimum back flow requirement is to be determined by an assessment of the fluid category downstream of the back flow protection device. There are five listed categories; category four covers the applications suitable for the RPZ valve. The main fluid categories and examples of relevant applicable back flow protection devices are as follows:
During the design of the new platform W, Railtrack engineers identified a problem in providing a water supply to the carriages of the high speed trains which would be regularly using the platform to deliver and collect passengers to and from London. The level of risk was assessed and Railtrack consultants CEDG specified a Honeywell BA 298-100A four inch RPZ valve as the best and most cost-effective method of preventing back flow into the mains.
The BA 298 RPZ feeds a number of hose points, used to fill water-holding tanks for each carriage. Each hose point also has a special Honeywell low pressure D06FN regulating valve, to maintain a maximum outlet pressure of 7 psi. This additional special requirement was to protect against the risk of water jets reaching the 25,000 volt cable lines above the track.
The BA 298-100A RPZ valve and the D06FN low pressure regulating valves were supplied by INPLAS Plastic Pipe Systems. Additional requirements for the installation of the RPZ valve are:
- A strainer to be fitted before the RPZ valve assembly:
- Isolation valves must be installed either side of the RPZ valve
- A drain connection is to be fitted to the discharge valve assembly on the underside of the valve and
- An accredited RPZ valve tester must test the RPZ valve assembly.