Failure analysis and assessment on the exemplary water supply network☆
Introduction
Water supply network constitutes more than sixty percent of the water supply company property value. An important element influencing the lifespan of the water supply system is to monitor failures and detect these elements which cause the failures most often. Assuming that the water pipes' amortization period ranges fifty years, a strategy for water pipes renewal should be taken at the level of about two percent per year. When planning the renewal at first the network failure analysis should be performed, taking into consideration types of damages. Currently, in Poland, 288,000 km of water supply network are being operated, 21.4% of which is steel, 35.8% cast iron, 18.2% PVC, 15.9% PE, 4,9% asbestos cement, 1.7% ductile iron and 2.1% others (reinforced concrete, composites) [1]. Taking into account that the age of the iron and steel network is now 70 to 80 years, the problem of choosing a renewal strategy for water pipes exists and not recognizing it can only lead to expanding the repair brigades and the costs associated with it [2].
Failure of the water supply network means breaking the continuity of its operation or becoming partially or totally out of operation which can result in disruption of water supply to consumers with the proper quality and the appropriate pressure. Many factors can influence the occurrence of the water mains' failure, obviously the great importance is the pipe age and failures associated with its wear, as well as the material from which the network is made and too high pressure [3], [4], [5], [6].
Failure monitoring may contribute to specify projects, methods, procedures and tools of system control, thus preventing failure from occurring. The character of the system control should distinguish the importance of determining the consequences of an undesirable event, the explanation of the cause of the event, which aims to illustrate the future course of the system functioning.
Considering failure occurrence in water supply system it can be concluded that no general classification of failure allows for a clear assessment of causes and sources of failure and possible human errors. However, we can distinguish the following causes of failures [7], [8], [9], [10], [11], [12]:
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design errors, mechanical or material defects,
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internal or external corrosion,
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operational errors,
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bursting with overload,
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transverse or longitudinal cracks,
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fractures,
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other or unknown causes.
Failures can also be caused by:
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earth moves or other natural hazards [13],
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external impact or other random events [14],
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errors in connection [15]. The formation of failure requires analysing several reasons, it is related to errors in the design, construction and operation of water supply network [12]. Based on the survey conducted in water supply companies the causes of failure are the following: environment (5%), materials (30%), the degree of protection (20%) and man (45%) [16].
Some of the significant failure factors that contribute to failure in water supply network are the age [17], diameter and material of pipe [18], [19], ground conditions, operating pressure [20], temperature in the supply network [21], climatic change [22], [23], possible external load [24] or combined internal pressure and external loading [25], random events [26], [27], temporal clustering [28] and the course of failure [29], [30], [31].
In the analysis of reliability and safety assessments of water supply network, four main groups of theoretical models can be specified [12]:
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probabilistic — used for failure analysis, as random events using statistical data,
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causal — shows that accurate identification of factors affecting the failure allows for its prevention,
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systemic — based on analysis of all dependencies and relationships through time and space able to cause a failure,
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behavioural — based on a risk assessment made by the operator or water consumer and its possible acceptance.
Failure analysis is performed as to achieve guaranteed sustainability, which can be disturbed by various undesirable events and factors: bad technical condition due to the use of low quality materials and/or negligent workmanship, leaks in connections and water losses associated with it, the deterioration of water quality as a result of its secondary contamination, and adverse changes in static and endurance parameters. The failures on the exemplary water supply network in the south of Poland, associated with such events, in the years 2000–2012, were analysed.
The aim of this study is to develop a methodology of failure analysis and assessment of system operation.
Section snippets
Material
The considered water distribution system consists of mains, distributional pipes with a total length of 623 km. It is continuously modernising as a result of the increasing demands that relate to connecting new recipients.
The water source is the river having a catchment area of 2546 km2, on its left bank there is one water intake with a total production capacity of 63,000 m3/d and water treatment plants: WTP I — 33500 m3/d and WTP — II 29500 m3/d. WTP was repeatedly modernised as to deliver to the
Results and discussion
Long term use of water pipes can cause “fatigue” of the material, i.e. due to fluctuations in pressure and flow the materials which construct network elements (iron, steel, asbestos cement, plastic) decrease their strength. The material from which the network is made can also affect the failure rate. Fig. 1 shows the number of failures related to the material and the most frequent damages to pipes. The most common cause of failure was corrosion of pipes made of cast iron and steel, a large
Conclusions
Based on the analysis of the results of the studies, the following conclusions and statements were made:
The unit failure rate calculated as the number of failures per unit length of pipe allowed the comparison of failure frequency of individual pipes with the European standards. The unit failure rate for the mains in the considered period was λMavg = 0.98 km− 1 a− 1. The high failure rate of the mains indicates poor technical condition of pipes. The majority of the network is old and needs
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This Article belongs to the special issue of "The Sixth International Conference on Engineering Failure Analysis".