Water & Wastewater Treatment

In the know Getting to grips with... surge control appropriate. A surge vessel may be decades old and there is no way that the hydraulics within the system are the same as they were when it was first installed. The pumping station may now have to serve many more households and new treatment works may have been added to the original network. Regulatory changes may have led to treatment processes being added to a network which have altered the demands made on the system. How are surge systems correctly maintained? Once surge vessels are installed, correct control of the IAV is critical to ensure correct surge protection is maintained to protect the pumping main. If the IAV is allowed to dri•, the level of surge protection being provided will also dri•. Maintenance of any air valves along the pumping main is also critical to the effective performance of the surge system. It is vital for the hydraulic study and associated pumping main data to be kept up-to-date to take into account any changes, modifications or upgrades within the system. There is a common misconception that surge control vessels fitted with bladders can be installed and then ignored. However, over time rubber bladders, which are permeable, will allow the air to bleed away, and become filled with water. Why is surge control important? Understanding surge control can help utilities cut maintenance costs, reduce the risk of contamination and reduce water waste. It is important to take surge control seriously. By looking at the surge control assets they have and assessing whether they are working as they should, utilities could save themselves a great many problems in the future. Utilities have a duty of care to their customers and should be doing everything they can to prevent leaks, cut the risk of contamination, and reduce the risks of interruptions to water service. A thorough understanding of efficient methods of surge control is essential. About the author: Tim Harper is one of the leading experts in the water industry on the use of surge vessels and the uses of compressed air. He has worked for Worcestershire based Quantum Engineering Developments (QED) for 19 years. drinking water systems. Appropriate, properly controlled surge control systems can ensure drinking water remains in the pipe while bacteria and contaminants are kept out. How is surge risk evaluated? One of the most important tools when it comes to risk management of the pipeline is the initial hydraulic survey. Quantum Engineering Developments works closely with Hydraulic Analysis Ltd of Leeds who will assess each system and recommend the most appropriate type of hydraulic study. The extent of the hydraulic investigation is usually governed by the availability of built data for the assets. Where little data is available, onsite pressure and flow measurement may be required. A desktop modelling exercise will be undertaken to simulate worst-case operating scenarios and optimise any necessary surge suppression system. A hydraulic study usually takes about three weeks to carry out. During that time a full hydraulic investigation takes place, with experienced hydraulic engineers looking at every aspect of how a pumping station and a pipeline work together. The hydraulic modellers study the effects of normal and irregular pump start/stops, determine the magnitude of maximum/minimum pressures and loads, along with their respective locations, before optimising the design to ensure the pressures in the system remain within acceptable parameters. What can be done to improve surge control? O•en the installation of a surge vessel, or the replacement or refurbishment of an existing vessel is recommended. The hydraulic study will specify both the total surge vessel volume and critical initial air volume (IAV) necessary to protect the pumping main. It will also identify locations along the pumping main whether or where air valves should be installed and their operating characteristics. The hydraulic survey can establish exactly what volume of surge vessel and IAV is needed. Fitting the correct volume of surge vessel can dramatically improve the way the pipeline is managed over its working life. In one recent project a UK utility which was using a surge vessel of 3m 3 discovered it needed to fit a much larger vessel of 7.3m 3 in order to protect the main. But a correctly-sized surge vessel is not necessarily larger than the legacy infrastructure. In another recent case where the utility needed to replace a surge vessel, it discovered the existing vessel was twice the size it needed to be. Thanks to the findings of the hydraulic survey the water utility was able to fit a replacement vessel smaller than expected, and therefore reduce the carbon footprint and save money. Why do surge vessels fail? It is not unusual for operators to believe a surge system is working faultlessly despite the vessel being full of water, empty, or even isolated, providing zero protection to the pumping main it is connected to. Further investigation o•en reveals regular main bursts taking place, sometimes many miles from the pump station, with no link being made to the inappropriately controlled surge vessel as a possible cause. Surge vessels are designed according to the individual needs of each particular pipe network. If there have been big population changes and many more connections have been added to the main, then the original hydraulic analysis may no longer be A correctly working surge vessel will expel water into the pumping main to maintain a positive pressure following a pump trip before accepting the returning surge wave. 34 | FEBRUARY 2016 | WWT | www.wwtonline.co.uk

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