Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | FEBRUARY 2016 | 33 A well-managed surge control system helps prevent leaks and bursts, reduces water wastage, protects drinking water from contamination, cuts costs and ensures compliance – but there are many misconceptions within the industry Tim Harper OpERATiOns MAnAgER QUAnTUM EnginEERing DEvElOpMEnTs Stopping the surge surge vessels play a vital role in water networks, but incorrect sizing and a lack of maintenance can cause problems if not managed carefully In the know Getting to grips with... surge control This 8.5m 3 surge vessel was discovered to be twice the size it needed to be about the benefits. The aim of surge control in a mains sewerage or drinking water system is to create a shock absorber for pressure spikes created within the system by changes in flow and/or velocity. Pressure spikes can occur in pumping mains during stop/start, valve closures and unexpected power failure events causing pump outages. Each change in flow rate leads to a series of alternating high and low pressure waves or transient surges. Pressure spikes can cause the pressure within the pipework to rise above the maximum recommended levels and also create unacceptable negative pressures. Pressures below atmospheric levels are not permitted in a potable water system because of the potential risk of contamination due to polluted groundwater being drawn into the main through open valves or leaking joints. In addition, severe negative pressures can result in water vapour forming in the fluid which can cause significant vapour cavities. The subsequent collapse of a vapour pocket will cause very high and rapidly applied localised pressure transients that can be sufficient to rupture the pipe and/or the lining and cause rapid corrosion. Power failure events require particular attention from operators as there are systems fitted with electrical type surge suppression devices that could be rendered ineffective and put the system at risk. For example, systems where pumps are fitted with variable speed drives to provide an extended pump ramp-down time may not be protected. For utilities, failure in surge control can lead to leaking pipes, wastage of water, disruption to service and financial losses. Yet there are many misconceptions and misunderstandings within the industry about the benefits of properly calculated and well-maintained surge control. What are the regulatory requirements? Because of the potential health risks of negative pressures, the Drinking Water Inspectorate has stipulated that the pressure within mains used for drinking water must remain positive at all times. Most utilities set a minimum water pressure of 0.2bar, although some have a minimum of 0.5bar as an additional safety margin. Studies carried out recently at the University of Sheffield confirmed the vital importance of maintaining a positive pressure by demonstrating how negative pressure can lead to bacteria and viruses present in groundwater being sucked into

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