The water loss is a phenomenon frequently observed within water distribution systems. A considerable part of water losses occurs either because of the incorrect assembly of joints or because of the fatigue and ageing of the material used to ensure a watertight seal. Moreover, such a leakage is very difficult to detect and to assess. In this work, we present a novel formulation for modelling the pressure effect on the background leakage through the joints. The proposed approach is based on the preliminary evaluation of the enlargement Δω, due to pressure, of the existing space between the outer side of the spigot end of a pipe and the inner wall of the hub end of the adjacent pipe (which is characterised by the area ωatm at atmospheric pressure). Furthermore, the whole procedure is based on the evaluation, by field data or calibration, of a parameter ξ representing the rate of enlarged area ω that, for several reasons, may be not covered by the gasket, ω being the value, at pressure p, of the area above defined.
The volumes of water introduced into a water distribution system (WDS) often exceed the volumes actually supplied to consumers and accounted for. The difference between the volumes of water introduced into the network and those actually accounted for consists of many contributions, commonly known as ‘illegitimate uses, under-metered uses, unmetered uses, losses at storage facilities, physical losses from pipes and physical losses from joints’ (van Zyl & Clayton 2005; Puust et al. 2010). Sometimes, physical losses from pipes and joints (leakage) constitute the greatest part of the total water loss, amounting to a significant percentage of the total volume of water introduced into the WDS.
When pipelines are connected by joints, besides the losses from holes and longitudinal or circumferential cracks, a considerable part of water losses occurs either because of the incorrect assembly of joints or because of the fatigue and ageing of the material used to ensure a watertight seal or because of insufficient mass of material forming the seal. The knowledge of the actual operating conditions of the pipe joints after their implementation is crucial in order to determine the total volume of water lost (Cassa et al. 2010).
To clarify the specific contribution that the authors intend to provide in the present work, a preliminary distinction must be made between leakage bursts and background leakage. Leakage bursts are localised water losses, detectable by direct monitoring systems, e.g., leakage detection systems (Geiger 2008; Puust et al. 2010) and/or by indirect analysis, e.g., computational pipeline monitoring (API RP 1130 2007; Puust et al. 2010). Leakage bursts can also be found because of their adverse effects (for example, ground level subsidence caused by earth erosion due to piping phenomena). As a consequence of the typically short time interval associated with burst activity before their detection and, at the same time, of the much more extended presence of the leakages from joints, holes and cracks, most of the physical water losses at yearly scale can be attributed to the background leakages, which are very difficult to individuate and evaluate.
In all cases, the leakage discharges depend on the local pressure and can be modelled by means of appropriate pressure-discharge equations. This observation suggests that the correct management of undetectable leaks (background leakages) could be achieved by minimising the nodal pressures throughout the WDS (Ulanicki et al. 2000; Thornton 2003; Lambert & Thornton 2011; Lenzi et al. 2013), almost to the necessary values to ensure proper service of the water network (Gargano & Pianese 2000; Pirozzi et al. 2002; Cozzolino et al. 2005). To this aim it is necessary to identify the relations between pressures at different points of the network and losses that there can be realised.
While numerous works related to the evaluation of water losses from holes and longitudinal cracks in the pipes can be found in literature (Ogura 1979; Osterwalder & Wirth 1985; Greyvenstein & van Zyl 2007; Cassa et al. 2010; Cassa 2011; Ferrante 2012), a reduced number of works has been devoted to losses from junctions (May 1994; Milano 2006).
The lack of knowledge of the amount of losses which occur at the joints, together with the uncertainty about the formula to be used to relate these losses to the pressure, represents a serious limitation for those studies aimed at the correct identification of the interventions to be undertaken in order to reduce losses from several water distribution networks. Indeed, in the past, most urban WDSs have been assembled using iron pipes or cast/ductile iron pipes with hub and spigot joints (Figure 1).
These pipe categories exhibit durability and the ability to withstand traffic and trench loads; moreover, hub and spigot joints ensure quickness of installation and an easy replacement. Watertight connections are obtained by inserting elastomeric gaskets in the ring space existing between the hub end and the spigot end. Of course, ageing causes undesired polymerisation of the elastomeric gaskets, which can crack as a result of fatigue under the daily cycles of pressure loads or may be no longer suitable to follow the enlargements of the internal walls of the junctions and to cover the annular space between the outer wall of the spigot end and the inner wall of the hub end. Based on these considerations, in this work a new conceptual discharge formula aimed at evaluating the discharges lost through the above described joints is proposed. The analytical structure of the proposed formula is suitable to correctly interpret the background leakage, in relation to which the field observations show a specific link between the local pressure heads and the discharges flowing out from the pipes (Thornton & Lambert 2005). Furthermore, the adoption of such a formula is strongly suitable in the context of analysis of the type ‘pressure driven’.
A CONCEPTUAL DISCHARGE FORMULA FOR THE EVALUATION OF LEAKAGES FROM HUB AND SPIGOT JOINTS
Consider a hub and spigot joint, where Di and De are the diameters of the outer wall of the spigot end and of the inner wall of the hub end, respectively, at atmospheric pressure patm (see Figure 1). The size of the empty space existing between the outer wall of the spigot end and the inner wall of the hub end is . Let ωatm be the area of the annular section whose thickness is λatm. When the absolute pressure pabs increases with respect to the atmospheric value patm, both Di and De increase, becoming and , respectively.
Indeed, considering types of pipes commonly available on the European market, the construction of which is regulated by specific European Community rules, if the pressure head ranges in the interval [20, 100] m (typical values within a WDS), then:
for ductile iron pipes (E ≅1.2 × 105 N/mm2) with nominal pipe size (NPS) in the range [60, 1,600] mm, regulated by EN 545, Δλ is varying in the range [0.00036, 0.05577] mm;
for PVC pipes (E ≅ 3.5 × 103 N/mm2) with NPS in the range [40, 1,000] mm and pressure nominal (PN) in the range [0.6, 2.5] N/mm2, regulated by EN 1452, Δλ is varying in the range [0.00257, 0.06241] mm (having already considered that, for PVC pipes characterised by a nominal pressure equal to 0.6/0.8 N/mm2, the pressure head cannot overcome 60/80 m, respectively).
As a consequence of the widening due to the pressure increase from patm to pabs, a variation of the initial annular cross section ωatm, which becomes ω at pressure pabs, takes place.
In particular, when the values of the pressure head h = p/γ (with γ specific weight of water) fall within the range [20, 100] m, the Δω values vary as follows:
For ductile iron pipes having NPS in the range specified above, Δω ranges within [0.11, 454.06] mm2;
For PVC pipes having NPS and PN values specified above, Δω ranges within [0.50, 1,185.67] mm2.
As a consequence, the enlargements Δω caused by the pressure heads usually existing in WDSs are not negligible.
The values of Δω, evaluated for several commercial pipes regulated by European legislations (EN 1452, for PVC pipes; EN 545, for ductile iron pipes, respectively), are reported in Figure 2(a) and 2(b), where Δω is drawn for different materials as a function of Di (Figure 2(a)) and, with reference to PVC pipes PN 10, for different values of Di and h (Figure 2(b)), respectively.
For these pipes, very useful relationships between leakage and pressure or leakage and pressure head could be obtained to evaluate the leakage from hub and spigot joints.
It is possible to observe that Δω and Δωappr. (Equation (5)) are in good agreement, regardless of the diameter of the pipe and its constituent material.
Thus, in this work Equation (5) has been adopted to evaluate Δω in all subsequent relationships between leakage from a hub and spigot joint and pressure or between leakage from a hub and spigot joint and pressure head.
Let us consider that at atmospheric pressure the annulus of area ω is completely occupied by a water stop ring, made of a material that can be considered originally elastic. In consequence of this behaviour, the annulus is initially occupied by the sealing material (Figure 4(a)–4(c)), even though the pressure increases from patm to pabs. Therefore, at this time there is no loss from the junction.
After a few years of operation, the material used for the gasket may alternatively: (i) continue to be perfectly elastic; (ii) behave as completely inelastic (such as when the seal becomes fully cured); or (iii) preserve only part of its elastic behaviour (as often occurs in reality). It can be assumed that, at atmospheric pressure, the polymerised gasket, though aged, is able to completely close the annular space of area ωatm existing between the jointed pipe ends (see Figure 4(a)–4(c)). Similarly, it can be assumed that, in the case of a non-zero relative pressure, a completely (or partially) polymerised gasket cannot (perfectly) fit the enlarged ring-space Δω existing between the jointed pipe ends, allowing leakage to occur.
Generally speaking, also the coefficient ξ should be a function of the internal pressure. In fact: (i) for constructional reasons, because of the difficulty of assembly of the pipes that would result from the use of very thick gaskets, usually the material constituting the elastomeric gasket is not sufficient to cover the area Δω; (ii) in consequence of the aging of the material that constitutes the gasket, because of internal pressure this last tends to widen less than Δλ. In both cases, the percentage of the annular space Δω not covered by the elastomeric gasket should increase with the internal pressure.
CALIBRATION OF THE PARAMETER ξ
The coefficient ξ introduced in Equations (6)–(8) accounts for the actual degree of joint opening due to the internal pressure. This coefficient is 0 for elastic non-polymerised gaskets with perfect joint implementation, and is 1 for a completely inelastic seal. Although the approach proposed in this paper does not seem to take into account the effects of viscoelasticity that can take place in elastomeric gaskets and pipes, as shown in Ferrante (2012), the approach may easily extended to incorporate these effects. To consider these effects and to verify the suitability of Equations (7) and (8), field data given by Milano (2006) for one ductile iron pipe and one PVC pipe were used, both just laid. The ductile iron pipe had the following characteristics: NPS = 300 mm; Dint. = 311.6 mm; Di = 326.0 mm; s = 7.2 mm; De = 366 mm; E = 1.07 × 105 N/mm2; while the PVC pipe had the following characteristics: NPS = 355 mm; Dint. = 341.0 mm; Di = 355.0 mm; s = 7.0 mm; De = 370.2 mm; E = 3.0 × 103 N/mm2.
For the ductile iron pipe, 13 pairs of values (h, Q) are available, while 14 pairs of values (h, Q) are available for the PVC pipe.
The ξ values have been deduced by means of Equations (8) and (10), using the pairs of values (h, Q), and the geometrical and mechanical characteristics of the pipes, respectively, presented in the work of Milano (2006) have been shown to be actually variable with the internal pressure. The changes of the ξ values with the pressure head h are shown in Figure 5(a) (for the ductile iron pipe) and Figure 5(b) (for the PVC pipe), respectively.
Then, by using Equations (8), (10) and (11), it has been possible to estimate the values of the leakages from joints and to compare these values with those actually observed by Milano (2006) during field experiments.
A comparison between the observed and estimated leakages is shown in Figure 6.
It is possible to observe that, regardless of the diameter of the pipe and its constituent material, the agreement between the leakages observed and estimated is sufficiently good, allowing the possibility to use the proposed approach in the technical field.
Generally speaking, ξ must be considered a model parameter, whose value must be calibrated on a case-by-case basis, as explained in the following.
To understand the importance of the conceptual model described above, it should be noted that: (i) a number of analyses, carried out over 100 field tests on sections of Japanese WDSs and Australian, Brazilian, Canadian, Malaysian, New Zealand, UK and US district metered areas, have shown that, to simulate the presence of holes and cracks in the pipe walls, formulas in which the leakage discharge is proportional to pb (or hb) must be used, where b has to be chosen between 0.5 and 2.5 (Thornton & Lambert 2005); (ii) other laboratory experiments have led to values of b ranging between 0.42 and 2.4 (van Zyl & Clayton 2005); (iii) according to Bargiela (1984), based on the experiments carried out by Godwin (1980) and the National Water Council (1980), the value b = 1.18 could be considered (Germanopoulos & Jowitt 1989); (iv) a few tests carried out on WDSs in which all detectable (burst) leaks have been repaired or temporarily shut off, leaving only background (undetectable) leakage, tend to produce (Thornton & Lambert 2005) values of the exponent b that are precisely equal to the value 1.5 in Equations (7) and (8).
Consequently, the simple model proposed in this paragraph, although unable to account for the soil head losses due to the movement of water through the soil after leaving holes or deteriorated joints, is capable of taking into account the background leakage by means of Equations (7) and (8). Therefore, it can be easily used within pressure-driven numerical models.
To reduce losses within WDSs, several approaches may be adopted, such as reducing the pressures existing in the network to those strictly necessary to ensure the delivery of the discharges required by users. To identify actions to be implemented for this purpose, it is necessary to adequately simulate the operation of the network and, in particular, to use appropriate relationships between the pressure (or pressure heads) and leakages, able to allow the evaluation of losses in the network, including the background losses. Concerning this topic, several relationships have been proposed in the last 40 years. Most of them aim at the evaluation of water losses that occur from the holes and the longitudinal lesions present in the pipes. Very few studies are devoted instead to the evaluation of losses at joints. Starting from these considerations, in this work a new conceptual discharge formula has been proposed, aimed at evaluating the discharges and water volumes lost through the ‘hub and spigot’ joints, which are frequently used within the WDSs.
The proposed relationships between the leakage and pressure (or the leakage and pressure head, respectively) are physically based, and contain a single parameter calibration, which, in turn, is of clear physical meaning. For its calibration, different procedures have been illustrated, suitable to be used in realistic application. Among them, a calibration procedure based on experimental or field observations and the use of numerical analysis by a pressure driven approach has been presented.
However, the proposed relationships are far from the overall solution of the joints losses assessment. As a matter of fact, more theoretical and experimental work needs to be done to achieve a clear interpretation of the phenomena and a more precise evaluation of joints leakages.
Indeed, the field data used in this work are taken from just-laid pipes with very small leakages. Therefore, more data are needed to perform further analysis. In this perspective, the relationships proposed in this work seem suitable to provide sufficiently reliable estimates, at least as an order of magnitude, of the leakages at joints, only for just-laid pipes.
The authors would like to thank the Editor, the Associate Editor and the three anonymous reviewers for their valuable observations and suggestions that greatly contributed to improving the paper. The present work was developed with financial contributions from the Campania Region, L.R. n.5/2002 – year 2008 – within the Project ‘Methods for the evaluation of security of pressurised water supply and distribution systems towards water contamination, also intentional, to guarantee to the users, and the optimisation design of water systems', prot. 2014.0293987 dated 29.04.2014 – CUP: E66D08000060002.