Pressure transients – seeing is believing
Hidden transients cause leaks, raise repair costs,
reduce asset life
Would you like to see what a pressure transient looks like – not just as a graph, but as a visible reality? Read on to access a startling media clip of transients generated in an above ground mains test rig, and then consider what damage may be done to underground distribution systems by pressure transients. International leakage management specialists who contributed to this blog are very well aware of the adverse effects of such events. However, many Utilities do not routinely take short-period pressure measurements to check for the presence of transients; or they may consider that if transients exist, they probably do not significantly influence leakage, burst frequency and infrastructure life.
You can view or download the media clip at https://1drv.ms/u/s!Al21dfsV9658dsHvi7X4o-j1_1M. It runs for 4 minutes and shows the effect of a sudden valve operation generating pressure transients in above-ground water mains. The spoken words (in German) at the start of each section of the clip state that the steady flow velocity is 4 metres per second, with no ‘brake’ (transient control, pressure relief) or non-return valve. The media clip shows that sensors have been installed to measure transients, but the source and that data are not known to the authors of this blog. If any reader is aware of the source of the media clip, please Contact Us.
The 1st section of the clip shows the valve and the effect of transients at the valve when it is closed. The 2nd section shows the effect the transients on a vertical 90º bend. The 3rd section shows the layout of the longest parallel pipe lengths and the effect of the transient on the horizontal 180º bend at the end. There appears to be a pipe clamp repair on the short straight section of the larger diameter pipe, between the two 90º bends.
After seeing the media clip, some Utilities may wish to review their opinion on transients, and take a more active approach to identifying and controlling them. For further recommended reading on this topic, see Rezaei et al (2013) ‘Pipe failure analysis and impact of dynamic hydraulic conditions in water supply networks.’
Checking for transients: active pressure control, to reduce leak flow rates and burst frequencies from excess static pressure, continues to be increasingly widely practised internationally, in particular since the relationships between maximum pressure and burst frequency on mains (and on services) has become better understood, see Article 2013M and other material in Influences of Pressure. However, continuous pressure measurements within distribution systems mostly tend to be limited to Zonal inlet points.
Where additional zonal pressures are measured (for example, at Critical Points and Average Zone Points), pressures are often averaged over 15 minute periods for calculation purposes, which masks the presence of positive and negative transients lasting only a few seconds. So pressure transients may be occurring frequently in a distribution system, without the Utility being aware of them. In some cases, the pressure management which has been installed (with the intention of reducing maximum static pressures for several hours at night) can itself cause transients to occur because the pressure changes (reduction, and then subsequent increase}, are being carried out too rapidly.
So it is necessary to check periodically for transients in every part of the distribution system, using special loggers which record over very short time periods. This applies even in Zones where management of maximum static pressures is taking place, and those in which there is no pumping, as valve operation and activities by customers (particularly large industrial customers) are known to sometimes cause substantial transients.
The following graph shows an example of booster pumping of mains in a Queensland Utility; the recording interval not known. The test identified, in a 10 hour period, 3 positive transients of 24 metres, and 3 negative transients of up to 30 metres, superimposed on an average pressure of around 70 metres. The transient problem was dealt with.
Benefits of controlling transients: the authors are personally aware of numerous international examples of previously unsuspected transients, including examples where reduction of these transients has resulted in documented reduction of burst frequencies. However, few of these examples are available in a published or summarised format which could be used to encourage others to identify and manage burst frequencies and extend (or achieve) expected asset life.
As the purpose of this blog is to draw general attention to this problem, rather than to list specific success stories, a sub-section can be created within the ‘Influences of Pressure’ section of the LEAKSSuite website, where such examples can be made available, free to all. If you are willing to provide such examples, please Contact Us.
Thanks to Blog contributors: Allan Lambert, Marco Fantozzi, Joerg Koelbl, Pank Mistry, Mark Shepherd, Julian Thornton, Kobus van Zyl
17th May 2017