Realistic modelling of pressure-dependent leakage

CCWI 2017 Pressure & Leakage Workshop


Academics and Practitioners use the same theory

for different applications

Two papers at the CCWI 2017 Conference in Sheffield, elaborated at a post-conference Workshop, showed how a recent collaborative research effort to improve the understanding of pressure:leak flow rate relationships anticipated, and successfully bridged, the gap between academic research and fast-track applications by practitioners. The 20 Workshop delegates – a mix of international academics and consultants, UK Utility practitioners, researchers and equipment suppliers – recorded very high satisfaction ratings for overall assessment (92%), expectations met (97%) and applicable knowledge gained (92%). How was this knowledge transfer achieved?

The increasing international application of pressure management of distribution systems, to reduce excess pressure, leak flow rates and bursts, has meant that the simplifying assumption that leak flow rate varies with pressure to a fixed power N1 (between 0.5 and 1.5) needs to be modified in some circumstances, notably in systems with lower pressures.

Professor Kobus van Zyl and colleagues at University of Cape Town have been researching pressure:leak flow rates through laboratory testing of holes and cracks in pipes for several years, with some input by Allan Lambert of Water Loss Research & Analysis (UK) and Dr Richard Collins at University of Sheffield. Their jointly authored open access peer reviewed paper in ASCE Journal of Hydraulic Engineering / Vol 143 Issue 9 – Sept 2017 provides robust academic validation of the authenticity of John May’s 1994 concept of Fixed and Variable Area Discharges (FAVAD) Article 1994M, that areas of cracks and splits in pipes vary linearly with pressure, to greater or lesser degrees.

The challenge was then to convert the full FAVAD concept into fast-track applications, so that practitioners in Utilities could quickly assess if the widely used ‘fixed N1’ assumption remains reasonably valid for their numerous diverse systems and operating pressures, and identify individual distribution zones needing a fast-track approach based on N1 field tests. Pressure management equipment suppliers and data base providers may also wish to understand how their equipment and software may need to be modified.

This aspect of research was carried out by Water Loss Research & Analysis Ltd, liaising with Professor van Zyl and practitioner consultants Dr Marco Fantozzi, Mark Shepherd and Julian Thornton. In their paper to the IWA Water Efficient Conference in July 2017 (Paper 2017L, Presentation 2017K), Lambert, Fantozzi and Shepherd had shown how to quickly derive zonal relationships between N1 and Average Zone Pressure (AZP) from N1 tests using the full FAVAD concept, and had recommended that a quoted N1 value should always be accompanied by a quoted AZP pressure, as is evident from the graph below. Image Blur?

The open access proceedings of the CCWI 2017 Conference at Sheffield University in early September include a paper by van Zyl et al on the hydraulic theory. A paper by Lambert et al shows the fast-track practitioner approach for deriving the equation relating zonal leak flow rate to AZP pressure, from occasional N1 tests.

The leak flow rate vs AZP equation can then be used to split the zonal inflow into leakage and consumption on a continuous basis. A Case Study example from a Mexican Utility with roof tanks and occasional intermittent supply is shown below. Image Blur?

A Workshop to disseminate the results of this collaborative research in more detail was held immediately following CCWI 2017.

In the 1st Workshop Session, Professor van Zyl described, step by step, the evolution of the hydraulic theory. An enhanced version of his presentation with written commentary is currently being prepared for open access publishing on the LEAKSSuite website as Presentation 2017S.

In the 2nd Workshop Session, after outlining a historical perspective on pressure:leak flow rate relationships from 1980 onwards, Allan Lambert showed the steps, using a good quality N1 field test, to quickly define the equation relating N1 and measured AZP pressures, and the equation relating leak flow rate and AZP pressure, for the mix of fixed and variable area leakage paths at the time of the test. Leak flow rate can then be continuously predicted if the AZP pressure is continuously measured, with automated recalibration by N1 tests at an appropriate frequency – comparable to going to the Doctor’s for blood pressure tests.

The ‘fast track approach’ has also been applied to calculation of Night-Day Factors (NDFs) to convert night leakage in m³/hour into leakage in m³/day. As the graph below shows, NDFs can vary very widely depending upon the ratio of average AZP to AZP at the time of minimum night flow. NDFs are also known as Hour-Day Factors (HDFs) in the UK, where they are a fundamental aspect of calculations of annual leakage.

For more information on these approaches, see Presentation 2017T from the Workshop.

In the 3rd Workshop Session, Allan Lambert explained the evolution of understanding of pressure: bursts relationships since 1995. International examples were used to develop the initial simplified ‘Straw that breaks the Camel’s back’ approach in 2006, to identify Zones with high potential for reductions in burst frequency on mains, or services, or both. Large scale implementation of pressure management in Australia during the millennium drought (2000 to 2011) as part of the WSAA (Water Services Association of Australia) PPS-3 Asset Management Program has provided much of the data to further improve the understanding of pressure:bursts relationships which are now used internationally for such predictions.

In addition to principal presenters Professor Kobus van Zyl and Allan Lambert, their co-authors Dr Richard Collins, Dr Marco Fantozzi and Mark Shepherd, and David Pearson also attended the Workshop to provide support and international experience. Julian Thornton, who has also contributed much to these developments and to promoting pressure management internationally , could not be present due to other international commitments.

Attendees came from Imperial College (London) and the Universities of Adelaide, Ferrara, Padova and Sheffield; three UK Utilities, Suez Group (France), and the KWR Research Institute (Netherlands). A UK pressure management equipment specialist also attended.

Evaluation Form comments included:

‘Very informative, some exciting new developments’ ; ‘Good mix of theory and data’; ‘Excellent update’; ‘Clear and interesting’; All very interesting’; ‘A very good and interesting Workshop’ . Helpful suggestions to assist content and session length of future Workshops will be taken into consideration in upcoming Workshops on these topics.

Regarding ‘Most useful/interesting items’, these varied between delegates but covered the whole range of topics, with pressure:bursts relationships received the highest positive response, and historical context the least.

Thanks to all those who attended the Workshop, which is the first of a number of international Workshops covering this particular range of pressure:leakage topics, organised by Water Loss Research & Analysis Ltd (WLR&A Ltd) and presented by international experts with practical experience. If you are interested to participate in future WLR&A Ltd Workshops and Masterclasses, on this and other topics, in the UK and elsewhere, please Contact Us.

24th Sep 2017

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