Water & Wastewater Treatment

30 | FEBRUARY 2015 | WWT | www.wwtonline.co.uk In the know Getting to grips with network surge ejected back out of the air/vacuum valve(s) and are eventually stopped when these two slam shut. Both events – the re–joining of water columns and the abrupt flow stop at the check–valve(s) generate an additional change of velocity and con- sequently, a high pressure wave ('up- surge'). The high pressure is reflected as waves, initiating a water flow in the original, normal direction, which con- sequently causes a new low pressure wave ('down surge') being generated again. This sequence may continue to oscillate several times until eventually, the friction factors of the network dis- sipate the energy and the system rests at static pressure conditions. "High pressure surges may not only rupture the pipework, but potentially cause damage to accessories such as check-valves, water meters and air valves" What does the surge valve prevent? A surge–anticipating valve (located on a branch of the main pipe, downstream of the check valve(s) in the pump sta- tion) opens instantly when the initial 'down surge' materializes, and stays in a fully open position until the returning flow reaches its location. Instead of slamming to a halt at the closed check–valve(s), the return–flow gets discharged out of the system, thus preventing the sudden velocity change that generates the high pressure wave. As the pressure rebuilds, the valve slowly closes, gradually reducing the returning flow in the system. Although the valve does not prevent the low pres- sure waves and the negative values, it can, when sized and positioned cor- rectly, be combined with air/vacuum valve(s), to prevent these conditions. How is the water in a surge vessel discharged? The surge vessel is a tank that is connected to the main pipe downstream of the check valve(s). The tank is partially filled with water and the remaining volume is filled with air that is compressed to the pump's normal operation pressure value. When the 'down surge' material- izes, the air expands and the water in the tank is discharged (pushed) into the mains, instead of the flow from the pump (which has now stopped). If sized large enough, the surge vessel maintains the pressure in the pipeline to the value that prevents the creation of unwanted negative values anywhere in network. The returning flow re–compresses the air, which dissipates the energy of the returning water mass, much like a spring. What about testing and water loss? Air/surge vessel(s) require a minimum of yearly testing and require the pumps to be stopped as per an emergency situation. However, there is a definite risk, because if the surge vessel(s) are faulty then the entire network will then be put under a full surge event. With surge-anticipating valves, periodic testing is possible and can be under- taken while the system is running in its normal operating state, so there is no threat in causing a surge event. In ad- dition, air/surge vessel(s) do not release water from the network, so they cannot relieve any unwanted high pressures from the network. In contrast, a surge- anticipating valve can allow water to be released from the network and returned back to the storage tank or a drainage point. What are the system cost comparisons? For smaller networks, an air/surge vessel is a more cost-effective solution. However, the required tank sizes in- crease with the capacity of the pipeline, so in longer pipelines they generally re- quire a larger vessel(s). Higher volume systems usually mean higher costs. For surge-anticipating valves there is a lower cost for larger systems because the valve size is reduced as the pipeline gets longer. Long mains have higher frictional losses that minimise the returning flow. What type of accessories are required? An air/surge vessel requires a compres- sor, plus a back-up compressor. To maintain the appropriate air vs water volume in the vessel, a sophisticated control system is also needed. Apart from a drain pipe or piping to return the flow back to the storage tank or soak-away, a surge-anticipating valve does not require external accessories. IVL Flow Control is in the process of developing an alarm system for any high risk air valve locations. This new alarm will alert water utilities to any air valve that is submerged and in danger of causing a network contamination event. More information is available at wwtonline.co.uk Surge-anticipating valves have several advantages over air/surge vessels

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