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Warnings that negative pressure in pipelines can lead to contamination of the water supply come as no surprise to engineers who specialise in surge vessels. Many people, even those working in the water industry, do not realise that negative pressure can be just as damaging to the network as high pressure.
Researchers from the University of Sheffield have now demonstrated that a fall in pressure can suck in groundwater bearing bacteria and viruses, leading to contamination of the water supply and creating a health risk. This shows how important it is for water engineers to be fully aware of the need for well-designed and well-maintained surge vessels that can protect the water network from negative pressure and contamination of supply.
Negative pressure can occur when flow rates change due to valve operation, pump trip or – in the worst-case scenarios – power failure. Each change in flow rate leads to a series of alternating high and low pressure spikes, or transients, risking the creation of negative pressure along the main.
As the scientists from Sheffield showed, if negative pressure develops near to a leaking pipe joint there is a real risk of pulling contaminated water into the system. An added risk is that in the event of a complete vacuum, extreme high pressure is created as the vacuum is collapsed by the returning water column. Such pressure spikes can cause stress on pipe joints, substantially increasing the risk of bursts.
Quantum Engineering Developments (QED) has been manufacturing, refurbishing and maintaining surge vessels for more than 30 years. We frequently find neglected surge vessels on water utility sites which are full of water or have been isolated from the system.
In many cases existing surge vessels are 20 or 30 years old but have been engineered to suit a system where the hydraulic profile of the main has changed beyond recognition. Surge vessels play a key role in maintaining the correct level of pressure within the system. This protects not just the main, but the entire network from leaks.
Water companies have a legal obligation to comply with Drinking Water Inspectorate guidelines that demand that all potable water systems operate at a positive pressure. Most water companies have introduced a minimum of 0.2 bar, while some have set the level at 0.5 bar.
Our experience, which comes from visiting many different sites for many different water companies, is that we regularly see surge vessels which are outdated, incorrectly controlled or isolated from the network. In many cases the main can be suffering bursts a few miles down the road, with no one making a connection with the way the surge vessel is functioning.
By ensuring that surge vessels are appropriate to the main they are protecting and that they are correctly controlled, water companies will often see a huge reduction in leakage – both in the affected main and elsewhere in the network.
As leakage engineers know, conserving water that has been pumped and treated represents considerable carbon and cost saving to utilities. Making the network more resilient can also have a positive impact on customer service incentive mechanism (SIM) scores.
Well-maintained and well-designed surge vessels should be a key part of water companies’ drive to reduce water wastage and protect our increasingly precious supplies.
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