Effect of integrating polymeric pipes on the pressure evolution and failure assessment in cast iron branched networks

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Highlights

  • A transient solver is developed to study water hammer in branched hydraulic networks.

  • The integrity of HDPE pipes is assessed using a semi-empirical model.

  • Failure hazard of cast iron pipes is investigated using the SINTAP code.

  • The integrity of metallic and HDPE pipes is studied for different crack geometries.

Abstract

Pressure response and structural integrity assessment of cast iron and HDPE pipes were investigated in this study. The failure analysis of pipes in a branched network was considered taking into consideration the maximum stress generated by hydraulic transient events and the presence of an initial external semi-elliptical defect. The numerical study was conducted based on a developed non-conventional transient solver that incorporates the viscoelasticity of polymeric pipes. Considering the replacement of pipes located in sensitive zones with high-density polyethylene (HDPE) pipes, the pressure evolution of the network was evaluated for different network states at different locations. The integration of polymeric pipes in the originally cast-iron network provided remarkable pressure wave damping and dispersing in some locations. Since the failure of pipes with a corrosion crater is likely to occur when the admissible stress is reached, a structural integrity analysis was conducted. Failure Assessment Diagram (FAD) using the SINTAP code was considered to calculate the safety factors for cast iron pipes with different metallic-plastic combinations, whereas a semi-empirical model that calculates the J-integral is used for HDPE pipes. The results of this study conclude that high attention should be paid when a longitudinal crack is located in the pipelines. It may be possible for integrated polymeric pipes to alleviate the risk of failure in the network. However, under large crack geometries, additional measures should be taken.

Introduction

Hydraulic transient phenomenon is often encountered in fluid transmission lines. It is defined as the result of pressure-flow disturbances caused, for example, following fast closure or opening of valves, sudden stopping and starting of pumps, etc … Various studies of hydraulic transient in water distribution systems (WDSs) have been conducted [1], [2]. For instance, [3] have tested the reliability of transients due to pump trip as a powerful tool for the pre-localization of anomalies in real pipe systems. They have developed a Lagrangian model to simulate pressure wave propagation associated with a genetic algorithm –and to locate possible anomalies – associated with wavelet analysis. The results of hydraulic transient analysis have shown that the intensity of these events is very high in these networks. In other terms, these events may lead to great damages to pipelines and hydraulic equipment [4], [5], [6], [7]. With the increasing demand for WDSs, it has become vital to protect these facilities against such a threat. In this context, hydraulic engineers and researchers often use different transient control strategies [8]. A classical solution would be the usage of water hammer control devices (Pressure relief valves (PRV), surge tanks, air vessels…). Several studies have been conducted to investigate the performance of such devices. Presented results have focused on studying the advantages and limitations of the simple and combined usage of various anti-ram devices [9]. Nonetheless, it has been also stated that the usage of the aforementioned transient control devices, due to different reasons, is not always possible. To address such limitations, researchers have proposed new water hammer control techniques based on the usage of polymeric pipes [10], [11].

Since their introduction in the world market, polymeric pipes, specifically High-Density Polyethylene pipes (HDPE) and Polyvinyl Chloride pipes (PVC) have been widely utilized. Precisely, the high resistant properties of HDPE and PVC materials were essential factors contributing to its increasing popularity [12]. These polymeric pipes exhibit a viscoelastic response under transient pressure situations. On this matter, researchers have conducted several experimental studies to further explore the material’s behavior [13], [14]. Obtained results have demonstrated that the rheological behavior of the pipe wall highly affects the transient response in a pipe system. Henceforth, to account for this material property, new numerical models have been introduced in the literature [15], [16], [17], [18], [19]. Accounting on its pressure damping, researchers proposed the usage of polymeric sections in originally metallic networks. Ghilardi and Paoletti [11] first investigated the response of a reservoir-pipeline-valve system that includes a polymeric section, under water hammer events. It was evinced that the proposed technique contributed to the damping of pressure waves in the original network. Pezzinga et al. [20] have pointed out the differences between transients in viscoelastic and elastic pipes by considering a 2D model, in which an elastic behavior is assumed for the pipe material. They have shown that the elastic model is unable to capture the main characteristics of the transients. It has been concluded that the faster decay of pressure oscillations and velocity profiles are attributed to the viscoelasticity because of the time-shift between pressure oscillation and retarded circumferential strain. Gargouri et al. [21] have numerically studied the impact of the pipe wall viscoelasticity on pressure surge in a quasi-rigid branched network. Results have shown the aptitude of the technique in attenuating the pressure surge in the network. Hadj Taieb et al. [22] have proposed the integration of HDPE pipes in an existing branched polymeric network. The numerical results have shown that the implemented polymeric pipe, in the zones of high flow disturbances, have high potentials in reducing the negative and positive pressure waves. It was also evinced that cavitation hazard has been eliminated. Recently, a numerical model has been progressively refined by Meniconi et al. [23] to study the role of a HDPE branch in a metallic pipe. They have discussed the effect on transient pressure signals of the unsteady friction and difference in elastic and polymeric pipe. It has been concluded that less important but still significant improvement is obtained when the unsteady friction is taken into account. On the contrary, the role of the viscoelasticity becomes relevant only when the length of the polymeric branch is appreciable. Gong et al. [8] undertook the study of pressure wave damping utilizing Metallic-Plastic-Metallic pipe configurations. The proposed study considered a pipeline network with combined pipe materials; metallic, and plastics. The aim was to answer if a plastic pipe that replaces a metallic section can cause pressure wave attenuation. The findings have indicated that larger and longer polymeric pipes diameters and lengths contribute to better water hammer damping.

According to the above-cited studies, polymeric pipes can act as a surge damper in WDNs. The usage of polymeric pipes as surge suppressors has been proven to be effective. Nonetheless, most presented cases are only limited to small scale hydraulic systems. Additionally, a reservoir-single pipeline-valve system cannot give an overall idea on how inserting a polymeric pipe in a real large-scale system will affect the transient response. As stated by Gong et al. [8], our understanding of the interaction between metallic and plastic pipes is still quite limited. Accordingly, it would be very interesting to simulate transient flow in pipes with different impedances. Moreover, studies on the interaction between metallic and plastic materials in the presence of an external pipe defect are still quite limited. It is evident that the phenomenon of breaking pipes is frequently encountered in urban areas. Under stresses caused by water hammer, a defect is likely to occur in water convoying pipes [24]. Under certain stress conditions, these defects can grow and gradually shorten the residual life of the pipe or initiate a catastrophic failure [25], [26]. To assess the danger of these defects, engineers and researchers often refer to fracture mechanics. For instance, Bouaziz et al. [27] have used the SINTAP procedure to evaluate the structural integrity of cast iron branched pipe network. The transient response of the network was investigated with the presence of semi-elliptical defects. Authors have concluded that, in the case of integrated pumps, the entire branched network does not operate in the security zone, which highlights the need for transient protection devices. In their study, Hassani et al. [28] have presented a risk assessment investigation of cracked pipe subjected to transient pressure. They calculated the safety factor by using the Monte Carlo method and the failure assessment diagram (FAD) method.

In the present study, transient events in a branched WDN, caused by an instantaneous valve closure, are numerically investigated. The numerical analysis is conducted based on a viscoelastic water hammer model coded in MATLAB. This research has two main goals. First, a numerical study is performed to explore the response of the original cast iron network with integrated polymeric pipes. Pipes located in the most sensitive zones, where flow disturbances are likely to occur, are replaced with HDPE pipes. The pressure responses of different network states with different pipe configurations are compared and discussed. The second aim is to assess the structural integrity of cast iron pipes with semi-elliptical external defect. The failure assessment of both cast iron and HDPE pipes is conducted. Results with various conditions are presented and discussed.

Section snippets

Governing equations

Transient flows in closed conduits may be modeled based on the one-dimensional classic water hammer model developed in [29]. Nevertheless, this model is considered inaccurate to model unsteady flows in polymeric pipes [30]. It has been evinced, through various studies, that an additional term should be added to the continuity equation to account for the viscoelasticity of the pipe wall material. Henceforth, the set of two partial differential equations can be expressed as follows [15]:Ht+C2gA

Results and discussions

In the following section, the interactions of the metallic and plastic pipes are investigated relating to water hammer up-surge generated following valves closures. As described previously, when pipelines are damaged in WDNs, polymeric pipes present a good choice for replacement. Therefore, to study the metallic-plastic interaction, an existing WDN was considered for the numerical analysis. The considered WDN is a branched network with, originally, sixteen cast-iron pipes and three reservoirs

Conclusions

The presented study addressed the impact of integrating high-density polymeric pipes on the pressure response and pipeline structural integrity in a branched water distribution system. To this end, a viscoelastic transient solver was developed to simulate the pressure wave propagation in the southern Tunisian water distribution system. To investigate the transient response of the network with metallic-polymeric combinations, different network states, with different combinations of Cast iron and

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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