Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | FEBRUARY 2015 | 29 In the know Getting to grips with network surge Transient Phenomena Pressure surges within pumping systems can cause damage to the network, but surge- anticipating valves can help provide the answer Any rapid change in water flow velocity creates a transient phenomenon which manifests itself as pressure oscillation in the network, more commonly referred to as 'surges' or 'water hammer'. Mere mention of 'water hammer' can be enough these days to send even the most experienced engineer into a spiral. It's not good. These surges are an array of alternating pressure waves, which can Craig STannerS DiREcToR, iVL FLoW conTRoL be substantially higher or lower than normal network 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. In contrast, low pres- sure conditions may reach negative values that can cause the pipework to collapse due to the external load of atmospheric pressure and soil weight. Also, damaging joints can potentially result in contamination of the water supply, as well as additional cavitation damage to pipework. What causes pressure surges? One of the most common causes of pressure surges is a sudden halt in pumping. Following what could be described as an emergency stop of the pump(s), the pressure at the discharge of the station drops below the static head at the pump location, known as 'down surge'. This is really the only surge event that is unavoidable. This low pressure propagates as a wave at a sonic speed into the down- stream network, reducing the head of the entire system. At local high–points along the pipeline, the hydraulic gradient may be even lower than the pipe elevation, resulting in air valves opening. This then poses a high risk of contamination to the water supply, because whatever is in that chamber will be drawn into the pipeline. How does a change in flow velocity affect the network? Within a system, water–column sepa- ration may occur as the water mass is separated by gas 'pockets', which are either filled by vapour or by air (introduced by an air/vacuum valve). At this stage, because the hydraulic gradient is reversed, a returning flow is generated by the head of the receiv- ing reservoir or the highest point in the network, or from a returning positive surge wave normally caused by the check valve(s) from the pumping sta- tion slamming shut. These returning flows refill the gas pockets which are a surge-anticipating valve (model 31-6-re/eL)

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