A practical example at South West Water’s Network Training Centre
South West Water looks after the water and wastewater services for 1.8 million customers across Cornwall, Devon, the Isles of Scilly and parts of Dorset and Somerset. As part of its commitment to drive improvements in its performance, the company has collaborated with AUMA Actuators Limited on valve speed actuation and calm networks at its specialist Network Training Centre.
South West Water’s Network Training Centre comprises a purpose-built laboratory where network components are installed and tested to record the effect of different operating conditions, and educate network engineers on the importance of calm networks. The work carried out between AUMA Actuators and South West Water focused on the effects on pipeline pressure transients by applying a simple variable speed actuation profile to the closing of a quarter turn valve..
Clive Dumbleton is a member of the Resource & Production team at South West Water. He explains: “South West Water is invested in ‘Calm Networks’ training, in tandem with our focus on pressure management and the basics of pressure control. The research that we conducted with the team at AUMA aimed to determine the effects of variable speed actuation on pipeline pressure transients, and mitigate the risk of water hammer.”
Tests were carried out by firstly closing the valve at a constant speed and measuring the pressure transients upstream of the valve. This is what is referred to as the benchmark. A simple speed profile was then set in the variable speed actuator. First, drive full speed to 20% open, then at the slowest variable speed to closed. Next, drive full speed to 20% open, then at half speed to closed. Finally, drive full speed was set to 20% open, then at quarter speed to closed. The results of the four tests were recorded and the upstream pressure transient and the system flow rate for each of the profiles were compared for a conclusion to be reached.
Colin Borrows is Operations Manager at AUMA Actuators Limited. He worked alongside Clive Dumbleton during the tests: “It was evident that with a constant speed actuator, an unwanted pressure spike was created that was over twice the system pressure. It was also clear that with a slow variable speed profile the pressure transient was removed entirely, but the time taken to close the valve was more than double the fixed speed. The quarter speed variable setting provided the best compromise of speed of response against minimising transient pressure surges, reducing the ‘spike’ from over 100% of the system pressure to a much smaller 17% transient.”
This is a relatively simple test to demonstrate the effectiveness of a variable speed actuator in the mitigation of risk of ‘water hammer’. The actuator can be configured to a more sophisticated speed profile with multiple speed changes through the stroke if needed.
Colin Borrows concludes: “What is clear in this test is that having the flexibility of variable speed in the actuator, the system designer can introduce tolerance in the system both before and after installation making it perfect for new and retrofit applications.
“Variable speed actuators help control aged assets in the field, minimising leaks, damaged valves, and expensive call-outs. This experiment clearly demonstrates the effectiveness of variable speed actuators in calm networks.”
What is water hammer?
Water hammer happens when there is a surge in pressure and flow rate of fluid in a pipeline, causing rapid changes in pressure or force. Very high pressures can result in pipeline failure, such as leaking joints or burst pipes. Support components can also be subjected to strong forces from surges or sudden flow reversal.
Water hammer can be a significant issue in many applications. Valve seats may suffer if they are driven hard to their end positions whenever the valve is closed. A variable speed actuator, gives the control engineer the opportunity to specify the speed of actuation at different points in the cycle. Variable speed actuators can be set so that the valve moves quickly through the majority of its cycle, slowing down as it nears the close position, minimising water hammer problems and extending the life of the valve seat. For emergency situations a ‘fast close’ speed can still be specified.
For further information about AUMA’s comprehensive range of actuators, controls, drives and gearboxes visit www.auma.co.uk
