The Water Distribution Subsystem Failures Affects on Krakow s Water Supply System

Transcription

1 Paper N 0 : IV. The Water Distribution Subsystem Failures Affects on Krakow s Water Supply System Izabela Zimoch Abstract: The increase of regulation requirement regarding water quality delivered to consumers challenges administrators of water supply systems. Operations intend to improve quality of water and reliability of its supply cannot only limit to subsystem of water production. Nowadays the level of water treatment technology allows reaching good quality of water, much better than formal requirements. Nevertheless the majority of subsystems of water distribution do not fulfill criteria that allow maintaining such quality during delivering it to a consumer. This paper presents the results of preliminary reliability analysis of Krakow water distribution system operating. The analysis was made on the ground of exploitation information, which was concerned the real failures of water distribution system in the year The reliability estimation contained calculations of basic parameters such as: average operation time without failure, failure intensity value and average repair time. This analysis of water distribution system of Krakow city includes investigation results of dependency of water-pipe network failures upon their diameters, materials and importance consumer s water supply. The failure reasons and effects, which appeared in water distribution system, are showed in this article too. Keywords: water distribution subsystem, reliability analysis, availability factor. Introduction Application of the reliability theory in engineer system evaluation in Poland and in the world is very common and contains different fields of economy as: military, transport or building. Using that theory for analysis of water supplying systems or some subsystems or elements is already acknowledged as standard. Nevertheless it is still a field of science that is not fully recognized.

2 352 Zimoch The inspection and reliability estimation of a distribution system is a complicated and difficult question. It requires considering a wide range of random events. First of all it results from multifunctionality of such system: supplying water to buyers in needed amount, quality, under proper pressure and at the any moment. Moreover structure of the system, multitude of different elements consisting of, is another aspect that influences a difficulty of estimating reliability. Nowadays, water demand in the system has considerably decreased, the reliability inspection are performed in particular to estimate weak points of the water supply system, where frequency of accidents and type of effect is the greatest. The above inspections gives in addition possibility of marking out such point of the system where exists a potential risk of secondary water contamination, caused by change of network running hydraulic parameters resulting from lack of water demand stability in the system. Small reliability and occurring damages may lead not only to disable operation of water supply system (WSS), but also can risk consumers health and lead to other damages of water-pipe enterprises. Qualitative and number estimation of negative events lets draw more precise conclusions and use an engineer performance having in view eliminating them to improve exploitation conditions of system functionality. Hence, reliability estimations of WSS, corresponding not only to engineer structure but also allowing for qualitative aspect of the product, determine essential information for effective management and exploitation of WSS. This paper is a try of presenting reliability estimation of a water supply system. Analysis was performed for distribution systems of Krakow city. Basic of investigation was a wide base of information related with damages of the water distribution system in exploitation period Reliability estimation has included determining basic parameters such as: parameter of failure flux, mean time of running without damage or mean time of repair. It was also analyzed different dependencies between possibility of pipe damage and their diameter, material and if their aim was supply water to a buyer. Elaboration consist also of results of causes and effects analysis in the pipe network. The paper was written within the framework of realization of research project KBN 5T07E under the title Determination of reliability operation model of water supply system (WSS) in the aspect of secondary water contamination in a water pipe network. 2. Water distribution subsystem of Krakow city The beginning of the water-pipe network of Krakow is dated to 5 February 90. The network was then running in the system with the initial reservoir, from which water was gravitationally delivered to city network by cast iron pipeline 750 of length 2,870 meters. At the beginning of the last century a substance of water-pipe network in Krakow were pipelines of diameter mm. At the starting of operation of the water-pipe in the city there were only 206 home service pipes and 43 street wells, giving possibility to inhabitants of Krakow of general access to water. At the end of the first year of exploitation, total length of the water-pipe network was 80,967 m (Rafalski L. and others, 993).

3 The Water Distribution Subsystem Failures Affects on Krakow s Water Supply System 353 Now the subsystem consists of the complex water-pipe network with numerous reservoir of pure water. Krakow is divided into separate water-pipe zones fed from independent sources, which are 4 top water intakes supplied from rivers: Raba, Rudawa, Dłubnia and Sanka and one underground intake placed in Mistrzejowice. Localization and engineer solutions of supply systems with water purification plants ensure (on normal conditions of exploitation) reliable operation of the water supply subsystem to the city. The present Krakow WSS is classified as so called systems with excess that means the system has a reserve of not used production capacity in the level of 9% with regard to actual water requirement in the city. In the case of a stoppage of water supplies from one intake (e.g. as a result of contamination) there is possibility of a damage supply to some part of city using remaining sources. Division into supply zones results from configuration of city terrain, determining the position and the range of the zones. In Krakow there are 3 different pressure zones incorporating areas from 225 m above sea level to 366 m above sea level in northern region of the city. The reliability of water supply to the inhabitants of high zones is ensured by hydrophones and intermediate sewages (Archival materials, ). Actually (at the end of 2004) total length of the water-pipe network in the area served by Krakow s Water Company is,849 km. The extensive water distribution system of Krakow consists of transit network of pipes 400, it amount totally to 8 km length of the system. The main network of the system is pipes of diameter and total length 265 km. The greatest part of the system (in regard to length) is distributing pipes of diameters from range , which total length is 00 km of water-pipe network. The last linear element of the discussed system is service pipes, being a system of 466 km pipes of diameter With regard to over 00-year exploitation of water-pipe network, it is characterized by essential age differentiation. In the city there still exist pipelines from the first decades of the distribution system operating period, of the age over 50 years The many-year exploitation and development of the water-pipe network contributed to its material differentiation. The most significant part in material structure take steel pipes, being 32% of total length of the network (600 km) and next pipes made of cast-iron (26%, 479 km), PCV (23%, 426 km) and others materials (9%, 344 km). The integral part of the water-pipe network is all kinds of utilities. Plumbing fixtures allow using them according to its destination and makes easier servicing, controlling and exploitation of the system. Efficiency of the system distribution functionality depends also on exploitation of the network stilling-reserve reservoirs. The reservoirs are designed for storing excess of water and next to supplement water supply in the periods of increased requirement. The additional task some of the reservoirs is stabilization of pressure in the supply areas, which changes regarding to different part of a day, depending on water usage in the city. In the first years of last century it has started running the first network reservoir Kościuszko of capacity 5 thousand m 3. The reservoir in 987 was developed and now it has total store ability at the level m 3. With the water-pipe network developing it was increasing the amount and capacity of the reservoirs. Now in Krakow there run reservoirs complexes of total capacity 276.2

4 354 Zimoch thousands m 3. They are mainly round terrain reservoirs with single- or multi-chamber system of reinforced concrete construction. Particular attention should be paid to the greatest reservoirs complex Siercza storing 58.5 thousands m 3 of water from the Raba supply system, what is almost a daily requirement in the city. So much treated water stored in the city raises reliability of the WSS of Krakow and decrease a water-supplying enterprise s risk of not fulfilling its primary function - supplying water to buyers (Archival materials, ). 3. Factors of the reliability analysis Most of objects and devices consisting of the water supply system are classified as so called renewable elements that mean such that can be taken in processes of running and renovation. A determining exploitation condition, definite as reliability states, is a ground of selection and estimation of suitable reliability factors of these objects. Two reliability states are distinguished like: state of running, it means full ability and state of partial or complete inability. Taking into consideration above exploitation states and specification of objects and water-pipe devices in the reliability analysis of their operating there are chosen from under named defined factors (Kwietniewski M. and others,993). Mean operational time between failures is defined as expected value of random variable T p determining operational time between following failures: T k = t k + z i= p pi + z t 3. where: k number of operational periods of failing objects, t pi value of i th operational period [d], t observation period [d], z number of operational periods of unfailing objects, and: z = N-M, and N is a number of tested objects, and M is a number of failing elements. Mean time of being failed is an expected value of random variable T o determining time of renewal, considering expectation time and time of removing occurred damage: T o = no n o i= t oi 3.2 where: n o number of failures during tested period, t oi time of lasting i th renewal [h]. Parameter of failure flux characterizes reliability of renewal and two-stage objects, it is an unconditional probability of occurring an object failure in time period t, independently whether at the beginning of period it operates correctly or is failed, in case of linear objects:

5 The Water Distribution Subsystem Failures Affects on Krakow s Water Supply System 355 () t ( t, t + t) n = L t ω 3.3 where: n(t, t + t) failure number in period t, t length of time period that observation period was divided [year], N number of tested objects L length of tested pipes [km], (Kwietniewski M. and others,993). Availability factor is probability that object, system or subsystem will be ready to operate at the moment t. In practice this factor means probabilistic estimation of object, system or subsystem availability in the range of execution of a given task: K g T p T p = T o where: T p mean time of operation between failures [d], T o mean renewal time[d], (Kwietniewski M. and others,993). Operating probability of an element R(t) is defined as probability that in the interval of time (0, t) between following failures, beginning from starting operation after failure, object will not fail. Flux of water-pipe objects failure is flux without results, single and stationary, and renewal process is Poisson process, for which operating time has exponential distribution. With such assumptions object operating probability has form: ( t) = ( ω t) R exp 3.5 Mean repair intensity is a parameter defines number of inability removed a time unit: () t = r µ 3.6 t n where: r failure number of renewed elements, t n summary time of repair of tested objects in observation period [h]. Practical use of abovementioned formulas in reliability estimation of operation of WSS base on indispensable information obtained from water-pipe exploitation. Source date are failure protocols, failure cards, exploit books, operating books of a machine, log of standby service, registers of failures etc. Above documentation should contains data such as: date and hour of failure, repair, overhaul etc., lasting time, description of the event containing type of failure and its effects for the subsystem or whole WSS. Unfortunately, information obtained form exploitation are not always enough, what considerably makes difficulty to perform full analysis. They are not systematic and sometimes allow only stating a fact of failure without any numerous data useful in finding appropriate factors.

6 356 Zimoch 4. Reliability analysis of WDS s operating for Krakow city The water distribution subsystem of Krakow is a large and extensive one, with complicated topology and different types of utilities. Relatively frequent failures are caused by many-year exploitation of the pipeline, by negative influence of environment and often by low quality of materials, of which the pipeline was made after World War II. In the course of the year Water Company in Krakow registers about 2000 failures of the water-pipe network. Performed analysis of pipe network failure has shown that most common reason of pipeline failures is corrosion damage. Moreover it was stated dependency between intensity of failures and season of the year. Relatively the largest number of failures falls to autumn-winter period Three independent repair brigades, servicing water distribution regions: Centrum, Podgórze and Nowa Huta, remove damages and their effects in the city area. Every region has its own Water-pipe Service, which is due to remove damage after announcement. Efficiently working the Water-pipe Services increase the reliability of WSS operating by minimizing time of elements being failure. Efficient intervention reduces also effects of failure and risk of the enterprise, which operates extensive and complicated WSS. Performed reliability analysis paid attention to estimation and classification of operating conditions of water supply system of Krakow. Above studies were performed using archival system exploitation data in years ,849 km of water-pipeline were analyzed. Studies were bidirectional. First direction of study contained reliability analysis of waterpipelines in regard to performed function in water distribution system in the city (dividing them into distribution, main and transit networks and terminal elements). Second aspect of study was preliminary reliability estimation of separated linear elements of the distribution system. Failure intesity [failure/axkm],4,2 0,8 0,6 0,4 0, Exploitation year whole network steel cast-iron recomended level Figure The damage intensity value of main networks in the years

7 The Water Distribution Subsystem Failures Affects on Krakow s Water Supply System 357 3,5 3 Failure intensity [failure/axkm] 2,5 2,5 0, Exploitation year whole network steel cast-iron PCV PE AC recomended level Figure 2 The damage intensity value of distributive networks in the years ,5 2 Failure intensity [failure/axkm],5 0, Exploitation year whole network steel iron cast PCV PE recommended level Figure 3 The damage intensity value of home terminal in the years Considering specification of studied water-pipeline objects, two states of their operation and Poisson character of renewal process, in reliability analysis of that elements were found following factors: mean operating time between failures T p, parameter of failure flux ω (t), renewal intensity µ (t), mean being failure time T o and availability factor K g and operation probability of object R(t). The results of performed analysis are contained in diagrams figures - 3 and in table no.

8 358 Zimoch Table 3 Comparison of value of reliability parameters of water distribution system of Krakow city Reliability factor Unit Transit and main pipes Network type Distributing pipes Water-pipe terminals Mean failure intensity failure/axkm 0,362 0,844,273 Mean operating time d 52,38 22,85 48,06 Mean being failure time h 9,78 8,36 8,9 Mean repair intensity h - 0,0 0,2 0,2 Availability factor - 0, , ,9869 Operating probability - exp(0,09xt ) exp(0,0438xt ) exp(0,0208xt) 5. Summary According to the recommended level of failure intensity factors the smallest failure intensity characterizes main and transit network. Failure rate of distribution network is over twice greater (0.844 failure/axkm) and home terminal even three times (.273 failure/axkm) with regard to failure intensity of main and transit networks (0.386 failure/axkm). The main part of failures of studied objects is cast-iron pipe and then steel ones, asbestos cement (AC), network made of PE and PCV. Parameter of failure intensity of castiron is λ*(t)>0.890 failure/axkm. Cast-iron pipelines make about 26% of length of water-pipe network, and so often accidents bring about that make the greatest exploitation problems. The most exposed to corrosion are steel pipes, for which failure intensity was λ*(t)=0.376 failure/axkm. On reliability of cast-iron pipes decides mainly joints failures. During the analysis period such failures has been 4,027. In the Krakow distribution system, cast-iron pipes are old, approaching the expiry term of working life (36% of pipes in the network are yearold). Majority were connected with pipe bells traditionally tighten with cord and aluminium foil, which failure intensity is greater then connections using new technologies. From a reliability point of view, it is possible to state that despite the high failure intensity parameter, the water-pipe network generally was characterized by high availability, because stationary factor of availability for pipes of the main and transit network was and for terminals In the case of the distributing network value of that parameter reach a level , what results from high failure-rate of pipes with diameter 00 and 50 mm. References Kwietniewski M. and others.(993). Niezawodność wodociągów i kanalizacji. Arkady, Warszawa 993, pp Rafalski L. and others (993). Woda dla Krakowa, MPWiK S.A. Krakow 993. Archival materials of MPWiK S.A. Krakow, Author Izabela Zimoch D. Eng: Silesian University of Technology, Institute of Water and Sewage Engineering St. Konarskiego8, Gliwice,Polan, izima@poczta.onet.pl or i.zimoch@gpw.katowice.pl