Recent Developments in Pressure Management

Pressure management of potable water distribution systems is now undergoing an international renaissance, as Utilities begin to realise the many benefits that it can bring. Thirty years ago, research in Japan and the United Kingdom identified that the average relationship between pressure and flow rates of leaks in distribution systems was approximately linear, rather than a square root relationship. During the last five years, the effects of pressure management on burst frequencies of mains and service connections have become more widely recognised; initially through the published work of the Pressure Management Team of the IWA Water Loss Task Force, and more recently from Utilities reporting their own success stories. Other benefits include deferment of pipe renewals and increase of infrastructure life, reduced costs of active leakage control, reductions of some components of consumption, and improved service to customers from fewer interruptions to supply. Pressure management is now being used not only for leakage control, but also for demand management, water conservation and asset management. Utilities wishing to implement pressure management need to make predictions of these benefits, which vary from one situation to another. Reliable concepts and practical methods are needed to make a sound financial case for such investment, and for prioritising individual pressure management schemes. This paper attempts to summarise the ‘state of the art’ of these concepts and methods, and to promote further international co-operation for improving them where necessary. Introduction Thirty years ago, Japan and the United Kingdom identified that reduction of excess pressure could significantly reduce flow rates of existing leaks and bursts, and they began to practice and promote active pressure management. Some countries and Utilities followed this lead, but even ten years ago many others had not; perhaps because of concerns of possible loss of income from metered customers, or uncertainly about predicting benefits that might not justify the investment costs,. However, during the last five years, the effect of pressure management on burst frequencies of mains and service connections has also become more widely known. Moving from intermittent supply to continuous supply at a lower pressure – the 24/7 policy approach in India – is one example. In systems with continuous supply, rapid reductions in bursts and repair costs are now changing the economics of pressure management and the perception that leaks and bursts can only be managed by repairs or pipe replacement. Utilities that have recently implemented pressure management schemes are now realising that reduced leak flow rates and burst repair costs are not the only benefits. Pressure management is not only a tool for leakage control, but also for demand management, water conservation and asset management. Other benefits including:  deferment of pipe renewals and increase of infrastructure life  reduced costs of active leakage control  reductions of some components of consumption  improved service to customers through fewer interruptions are summarised in Table 1. REDUCED CONSUMPTION REDUCED REPAIR COSTS, MAINS & SERVICES DEFERRED RENEWALS AND EXTENDED ASSET LIFE REDUCED COST OF ACTIVE LEAKAGE CONTROL FEWER CUSTOMER COMPLAINTS FEWER PROBLEMS ON CUSTOMER PLUMBING & APPLIANCES REDUCED FLOW RATES OF LEAKS AND BURSTS REDUCED FREQUENCY OF BURSTS AND LEAKS REDUCED FLOW RATES CONSERVATION BENEFITS WATER UTILITY BENEFITS CUSTOMER BENEFITS PRESSURE MANAGEMENT: REDUCTION OF EXCESS AVERAGE AND MAXIMUM PRESSURES Table 1: overview of range of benefits of pressure management Utilities need to be able make reasonably reliable predictions of all of these benefits – which vary from case to case so as to make a sound financial case for investment in pressure management, and to be able to prioritise individual pressure management schemes. This paper attempts to summarise the present ‘state of the art’ of concepts and methods used for:  predictions of benefits from proposed pressure management schemes  data analysis from completed schemes to assess actual benefits and improve existing prediction methods where necessary How does pressure reduction influence leakage and Real Losses volume? The Background and Bursts Estimates (BABE) concept of Component Analysis of Real Losses splits leaks into 3 categories for purposes of analysis:  ‘Reported’ leaks and bursts (typically high flow rates, but short run times)  ‘Unreported’ leaks (moderate flow rates, run times depend on Utility policies)  ‘Background’ leakage (small non-visible, inaudible leaks, running continuously) Figure 1 illustrates these three components within a Zone of a distribution system as a simplified time series, before and after the introduction of pressure management to reduce excess average pressures and pressure transients. Background leakage runs continuously. Unreported leaks gradually accumulate, at an average rate of rise RR, and economic intervention occurs when the accumulated value of the ‘triangle’ of unreported leakage equals the cost of the intervention; the process then repeats itself. Reported leaks and bursts are superimposed on the other two components. The annual average of all 3 components, representing the annual real losses volume, is shown as a dashed line. 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Time (years) --------> Le ve l o f l ea ka ge -------> <-------------AFTER PRESSURE MANAGEMENT -----------------------> Background leakage Background leakage Unreported leakage Unreported leakage Unreported leakage Unreported leakage Frequency and flow rates of reported leaks reduce Rate of rise of unreported leakage reduces Frequency and cost of economic intervention reduces Background leakage reduces Reported leaks and bursts Figure 1: Influence of pressure management on BABE components of Real Losses Source: Fantozzi & Lambert (2007) Predicting Reductions in Leak Flow Rates The Pressure Management Team of the IWA Water Loss Task Force (WLTF) recommends use of the FAVAD (Fixed and Variable Area Discharges) Concept, proposed by May (1994) for these types of predictions. Japanese research (Ogura, 1979) showed that leak flow rate L in individual sectors of a distribution system varies with pressure P, where the exponent N1 averaged 1.15 but could vary from 0.5 to more than 2.0. The FAVAD concept attributes this variability to some types of leaks having fixed areas (N1 = 0.5) and others having areas that vary with pressure, resulting in N1 values of 1.5 or more. The basic FAVAD equation for analysing and predicting changes in leak flow rate (L0 to L1) as average pressure changes from P0 to P1 is L1/L0 = (P1/P0) ..............(1) It is the ratio of average pressures and assumed N1 exponent that influence the reliability of the predictions. Tests in different countries have shown that:  N1 is usually close to 1.5 for background leaks, and splits in flexible pipes that increase in area as pressure increases  N1 is close to 0.5 for detectable leaks from cracks and holes in rigid pipes  N1 is often close to 1.0 for large systems with mixed pipe materials, i.e. a 10% change in average pressure produces a 10% change in leak flow rates N1 values can be assessed from tests at night when average pressure is reduced and changes in night leakage are measured; or using an empirical prediction equation (Thornton & Lambert, 2005) based on Infrastructure Leakage Index (ILI) and % of rigid pipes (p%) : N1 = 1.5 – (1 – 0.65 / ILI) x p/100 .............(2) Further explanation of night tests and the use of equation (2) will be provided in the WLTF Pressure Management Team Guidelines scheduled for publication in 2011. The simplest possible basis for roughly estimating N1 is as follows:  if you know nothing about the pipe materials or type of leaks in your system or zone, assume N1 = 1.0 (linear) with confidence limits of +/0.5  for systems with rigid pipes, N1 falls from 1 to 0.5 as leakage increases; but if background leakage is very high N1 values could still be close to 1.0  for systems with flexible pipes with many splits, assume N1 is close to 1.5 Predicting Reductions in Frequency of New Bursts During the 1990’s, a few Utilities and individuals in a few countries started to collect data on the number of leaks and bursts before and after pressure management in individual Zones. Many of the results were impressive – in Torino, a 6 metre (9%) reduction in maximum piezometric pressure, in a system with pumping at night, resulted in a 46% reduction in leaks, that has been maintained for at least 6 years Attempts (mainly in the UK) to derive correlations between average pressure and mains burst frequency for large sets of grouped data were generally inconclusive. However, in 2004, following another impressive example in a Zone in Gold Coast, Australia (a 75% reduction in bursts on both mains and services), the IWA WLTF members provided ‘before’ and ‘after’ data from 50 individual pressure management schemes in Australia, Brazil, Italy and the UK; many of these data sets showed substantial reductions in frequency of new leaks. Pearson et al (2005) found that a basic FAVAD equation (burst numbers vary with P) was not appropriate to analyse this data, but the concept of failure envelopes and duty points in this paper was fundamental in developing a conceptual approach to pressure:bursts relationships. A data set of 112 example from 10 countries was then collected by the WLTF Pressure Management Team (Thornton & Lambert, 2006), for mains and/or service connections. The summarised data were simply presented as graphs of % reduction in pressure against % reduction in new burst frequency. Although the separate graphs for mains and service connections were similar (Figure 2), this does not mean that in an individual Zone, both respond by the same % to pressure management. Figure 2: Influence of pressure management on break frequency of m