Getting to Grips With... Surge control
Surge vessels play a vital role in water networks, but incorrect sizing and a lack of maintenance can cause problems if not managed carefully
by Tim Harper, Operations Manager, Quantum Engineering Developments
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 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 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?
Often 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 3m3 discovered it needed to fit a much larger vessel of 7.3m3 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 often 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 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 drift, the level of surge protection being provided will also drift. 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.
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