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Reliability and failure rate analysis of pressure, vacuum and gravity sewer systems based on operating data
EFA-02635; No of Pages 9 Engineering Failure Analysis xxx (2015) xxx–xxx
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Reliability and failure rate analysis of pressure, vacuum and gravity sewer systems based on operating data Katarzyna Miszta-Kruk Warsaw University of Technology, Faculty of Environmental Engineering, Department of Water Supply and Wastewater Management, 20 Nowowiejska Str, 00-653 Warsaw, Poland
a r t i c l e
i n f o
Article history: Received 30 September 2014 Received in revised form 5 July 2015 Accepted 21 July 2015 Available online xxxx Keywords: Pipeline failures Sewer systems Reliability analysis Sewer pipes Failure analysis Pressure sewer system Vacuum sewer system Gravity sewer system Vacuum pipeline
a b s t r a c t This paper presents an assessment of operational reliability of elements of pressure, vacuum and gravity sewer systems, based on research focused on 7 different systems. The article discusses both traditional (gravity) sewer systems and unconventional sewer systems that are implemented on a large scale. The analysis and assessment were based on the data collected during investigations of real sewer systems over a period of 3 to 5 years. On the basis of the analyses, this paper presents the system elements that were found to be most susceptible to failure as well as the most common types of failure events. This analysis allowed the duration of the identified failure events to be determined and yielded a reliability assessment in the form of failure rates (λ) calculated for each highlighted element. An analysis of the results in terms of the functions of the objects most susceptible to failure allows one to assess their operational probability. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Theories of network systems' reliability (including sewage disposal networks), but especially of water supply network systems, have been developed since the beginning of the '70s. Corresponding theories of risk and safety were developed in the mid-1990s. In the world literature on the subject, and especially as relating to the reliability of water supply and wastewater disposal networks, these issues are to a large extent investigated as a result of activities of IWSA (International Water Supply Association) or AWWA (American Water Works Association). Generally, they focus on the assessment of reliability of water supply systems. This domain is better explored through investigations and development of particular theories. However, the intuitively comprehensive approach to this subject should cover an analysis of the reliability of wastewater disposal systems as well. Both systems together provide a comprehensive service of supplying water to and disposing of sewage from users. Almost at the same time, as the start of reliability analysis of water supply systems [16,28,29], research efforts also began to focus on wastewater disposal systems. However, such evaluations were often treated as investigations of secondary importance. Only a few researchers, however, decided to face the challenge related to the assessment of the level of reliability of sewer systems. Among them, one could count the following: [1,3,4,7].
E-mail address: [email protected].
http://dx.doi.org/10.1016/j.engfailanal.2015.07.034 1350-6307/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article as: K. Miszta-Kruk, Reliability and failure rate analysis of pressure, vacuum and gravity sewer systems based on operating data, Engineering Failure Analysis (2015), http://dx.doi.org/10.1016/j.engfailanal.2015.07.034
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Reliability of sewer systems is most often considered in terms of the need to dispose wastewater from users [3,5,6,8]. An intensive development of reliability theory creates a wide range of possible applications aimed at solving problems related to the design and operation of sewer infrastructures. Network systems constitute one of the typical arrangements where application methods are sought for [9]. In the case of sewer systems designed both as traditional solutions and as unconventional ones, it is also important to determine unequivocal criteria, in order to rationalize a reliability assessment. Relevant proposals can be found in the following publications: [2,4,11–15,3,6,10]. The methods proposed in the last three texts are used in practice to assess reliability of different network systems in Poland. 2. Structure of sewer networks under investigation Pressure and vacuum sewer systems belong to so-called unconventional wastewater disposal systems. In pressure sewer systems, as in vacuum ones, the medium flow is forced by the pressure differential generated in small wastewater pumping stations or, in the case of vacuum sewer systems, by means of vacuum pumps. In this, they differ from gravity sewer systems (referred to as traditional), where the flow of sewage proceeds generally in partly filled conduits by the force of gravity. Concepts of pressure and vacuum wastewater disposal appeared already by the end of the 19th century, when technological developments allowed engineers to construct devices to pump or draw sewage. At this point, it is worth mentioning, the first pressure sewer system in Poland was designed and constructed in 1899 in Olsztyn. At that time, the network serviced 93% of the town and disposed of about 3000 m3 of sewage per 24 h [11,13]. The earliest functioning vacuum sewer systems reach back to the 1990s. The first wastewater collection system of this type was patented in 1888 in the United States by Adrian LeMarquand. It was called the system of wastewater collection by barometric depression [EPA/625/1-91/024 1991, 16]. This was the beginning of modern unconventional sewer systems. At present, such systems raise high interest as alternatives for gravity sewer systems. It is advisable to implement unconventional sewer systems primarily in areas characterized by relatively dispersed development (single-family houses, village settlements, etc.) with small changes in altitude or flat areas. Both the pressure and vacuum sewer systems are especially applicable wherever sewage is intermittently generated, namely in camping sites, holiday centers, in the case of exceptionally poor soil and groundwater conditions, in protected areas or on the fringes of water reservoirs. Both pressure and vacuum sewer systems may also be implemented to collect wastewater in underground railway tunnels, underpasses from parking lots, in large industrial halls, construction sites, as well as to handle wastewater transmitted from ships in docks and from passenger airplanes or ships. In the pressure sewer system, wastewater is transported under pressure generated by pumps. The system consists of the following elements: chambers, controlled pumps, and networks of pressurized conduits with accessories. The pipes of a pressure sewer system, as opposed to sewers of a gravity sewer system, are laid at a constant depth below the lower frost penetration limit (about 1.2 m) and it is not necessary to ensure their proper inclination gradients. A pump may force wastewater for the distance of up to several kilometers and up to the height of 45 m · wc. A receiver chamber allows wastewater to enter another sewer system or constitutes the last element before wastewater enters a wastewater treatment plant. The goal of this system is to carry wastewater from internal installations of buildings and facilities in a given area to a point of wastewater disposal (a gravity sewer system or a wastewater treatment plant) (Fig. 1). The vacuum sewer system operates by the force of a negative pressure (0.6–0.7 bar) generated by vacuum pumps in a pumping station. The negative pressure is transmitted to household collection chambers via conduits laid to assume a saw-toothed profile. At the moment of drawing wastewater and its associated mixing with the air in collecting chambers (fitted with discharge valves),
Fig. 1. Construction scheme of a pressure sewer system with identifying system elements.
Please cite this article as: K. Miszta-Kruk, Reliability and failure rate analysis of pressure, vacuum and gravity sewer systems based on operating data, Engineering Failure Analysis (2015), http://dx.doi.org/10.1016/j.engfailanal.2015.07.034
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Fig. 2. Construction scheme of a vacuum sewer system with identifying system elements.
wastewater is transmitted through pipes in the direction of a vacuum pumping station. There, wastewater is collected in tanks. Then it is transferred to a wastewater treatment plant or another sewer system. A scheme of a vacuum sewer system is presented in Fig. 2. The gravity sewer system presented in Fig. 3 can be defined as a system of wastewater disposal by means of natural land gradients of at least 3–5% which will allow gravity to drive the wastewater flow. However, there are locations, with extreme elevation profiles, where a classical gravity sewer system design is erroneously attempted. This has a significant impact on the reliability of system operation. Traditional sewer systems consist of the following elements: chambers that receive wastewater from buildings by force of gravity; and networks of conduits — laid with a gradient that ensures gravity-driven wastewater flow — that carry wastewater to a point of wastewater disposal or another sewer system or to a wastewater treatment plant. The above characterization of sewer systems is sufficient for the purposes of research and assessment of their reliability. As follows from the above definitions, both in unconventional and in traditional systems, one can distinguish the elements (or direct objects, depending on the objective of the reliability research) as presented below in Table 1. In terms of flow variability in pressure sewer systems, one most commonly differentiates between high-pressure sewer systems based on positive-displacement pumps and low-pressure sewer systems based on centrifugal pumps. In the case of vacuum sewer systems, one can differentiate two additional categories: vacuum systems and vacuum and siphon systems. In traditional gravity sewer systems, the following system categories are differentiated: separate sewer systems with distinct separation of sanitary and storm water sub-systems, combined sewer systems and semi-separate sewer systems. Previously, analyses of sewer systems have usually been of the overview study type, aimed at describing systems and devices, principles of their operation as well as problems in and experiences resulting from their operation [11,17–27]. Some of these studies focus
Fig. 3. Construction scheme of a gravity sewage system.
Please cite this article as: K. Miszta-Kruk, Reliability and failure rate analysis of pressure, vacuum and gravity sewer systems based on operating data, Engineering Failure Analysis (2015), http://dx.doi.org/10.1016/j.engfailanal.2015.07.034
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Table 1 Elements/objects of the unconventional sewer systems and of the traditional gravity sewer system. System element
Unconventional sewer systems Vacuum sewer system
Pressure sewer system
Gravity sewer system
1 2
Gravity sewer lateral Collecting chamber with a vacuum interface valve (automatic control unit) Network of negative pressure conduits arranged to form a saw-toothed profile Vacuum-pumping station with its elements System of interceptor conduits
Gravity sewer lateral Small pumping stations (with pump and controls) Network of pressurized conduits
Gravity sewer lateral Collecting chamber
Expansion chamber Point of disposal or chamber that opens into another system
Point of disposal
3 4 5
Traditional sewer system
Network of gravity conduits
primarily on proprietary solutions [17,19], while others focus on design problems [22,24,26]. There are also interesting documents focused on the question of selecting a sewer system, while taking into account technical and economic analysis, as well as treatises that delineate the range of system applicability [11,26,27]. The few published reliability studies of unconventional sewer systems focus primarily on identifying the causes of failures, especially in small pumping stations [4,5,7], and on assessing the functional reliability of a selected system, based on the method of a comprehensive survey and coefficients determined in operational tests [1,12]. 3. Operation research of sewer system reliability 3.1. The nature, objective and scope of research This section presents the results of a study focused on random variability of operational states that describe susceptibility to failure of sewer systems, both traditional (conventional) and unconventional. The study stressed primarily the analysis of elements identified in Table 1, functioning in three selected systems. The performed research belongs to the category of operation reliability studies, i.e. research conducted in natural conditions of system operation and functioning. The research belongs to the category of reliability studies, i.e. research conducted under natural operational conditions. Research of this type considerably extends the scope of available data (hitherto rather modest) on sewer system functioning problems. However, it is extremely important for determining optimum methods of system design and operation. The main objective of the research was to assess, quantitatively, operation states that describe susceptibility of sewer system elements to failure and to identify the causes and effects of element defects. The reliability research was carried out according to a pre-established program. The research drew on operational data, such as working modes and states of repair, overhaul and heavy repairs carried out by waterworks and wastewater disposal company service personnel in the towns under examination. Data records were usually stored in GIS type databases, while part of the data was found in hardcopy documentation. All the data acquired from these sources were additionally verified by the technical personnel of a given company. Data, indispensable for the assessment of susceptibility to failure, were collected by means of failure charts prepared especially for this purpose. Each chart covered one failure event. 3.2. Research objects Data availability and conditions and duration of system service constituted the basic conditions for selecting systems to be examined. The selection was based on an analysis of numerous sewer systems that operating in Poland. These activities showed that, in
Table 2 Technical data of sewer systems under examination. Characteristics Chamber/pumping station
Pump type/valve type Amount Construction Diameter/dimension
pcs – mm
Material
–
Diameter range Length
mm m
Pump control/discharge control Alarm signaling Pressure/vacuum/gravity conduits
System commissioning year
Pressure sewer system
Vacuum sewer system
Gravity sewer system
Screw pump 58–120 Concrete 800–1500 Hydrostatic floats/float connecting bars Light signal PE
pneumatic valve 130–278 Concrete 1000 × 1000 –
– 1350 Concrete 800–1200 –
Light signal PE
40–90 8800–10,700 2004
90–280 2800–12,500 2002
– PVC, cast iron, stoneware, PE 63–1200 82,000 1964–2010
Please cite this article as: K. Miszta-Kruk, Reliability and failure rate analysis of pressure, vacuum and gravity sewer systems based on operating data, Engineering Failure Analysis (2015), http://dx.doi.org/10.1016/j.engfailanal.2015.07.034
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many cases, the data concerning susceptibility of a sewer system object to failure were not controlled or recorded. As a result, three pressure sewer systems functioning since 2004, three vacuum sewer systems characterized by very similar service conditions and one gravity sewer system were selected to be examined (Table 2). 3.3. Research results and failure event analysis It should be underscored that the research covered all systems, without assessing their comparative reliability, while analyzing only the susceptibility of particular elements to failure. Thus, one can identify elements most susceptible to failure in particular systems: – in pressure sewer systems: small wastewater pumping stations (with a tank, pump and control unit); – in vacuum sewer systems: collecting chambers with vacuum interface valves; – in gravity sewer systems: conduits. This is shown in Fig. 4. Damaged pumps in small pumping stations were responsible for over 90% of all failure events in pressure sewer systems. Of these failures, the control unit failed most often (67% of failures). This category of failures included defects of automation subassemblies in the control box as well as damages to the boxes themselves. Failures of this type constituted from 4 to 33% of the failure events. In a pressure sewer system, failures caused by power failure constituted an important failure event category. In a number of the investigated cases, this resulted in discontinued wastewater service (from 5 to 13% of all failure events) (Fig. 5). The highest number of failures in vacuum sewer systems was recorded for collecting chamber failures. In this case, the number of chamber failures was about 6 times the number of failures of pumping stations and almost 50 times the number of failures that affected vacuum conduits. Failure to open a valve, failure to close a valve tightly, defective valve closing mechanism and flooding the valve control mechanism (controller) were the most frequent failures affecting the collecting chamber. They were responsible for 92% of all observed failures that affected collecting chambers. In the case of a vacuum station, the dominant failure event was prolonged operation of vacuum pumps, caused by leakages in the system. Power failure resulted in pump failures or discontinued operation of the entire equipment in a vacuum station, which led to shutting down the entire wastewater collection system (Fig. 6). In the case of the gravity sewer system, practically all failure events were related to wastewater conduits. In the system examined, one observed primarily duct blocking (about 99% of failure events in the network), irrespective of duct material and its diameter. 3.3.1. Interrelations between types and effects of failures and their causes Decidedly, damages to unconventional sewer systems resulted most frequently in overfilling small pumping stations with wastewater, due to discontinued pump operation (pressure sewer systems), or in failure to close the valve tightly, while vacuum pumps continued to operate (vacuum sewer systems). Conduit blocking in gravity sewer systems resulted most frequently in wastewater releases or in the inability to receive wastewater from clients. Failures of the elements most susceptible to failure in each system were primarily caused by improper use of the wastewater disposal system by its users. This was responsible for about 70% of the failures in the pressure sewer system and 25% in the vacuum sewer
Fig. 4. Comparison of elements most susceptible to failure in each of the tested sewer systems.
Please cite this article as: K. Miszta-Kruk, Reliability and failure rate analysis of pressure, vacuum and gravity sewer systems based on operating data, Engineering Failure Analysis (2015), http://dx.doi.org/10.1016/j.engfailanal.2015.07.034
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K. Miszta-Kruk / Engineering Failure Analysis xxx (2015) xxx–xxx
Fig. 5. Comparison of the most frequent failures (failure types) occurring in the element of a pressure sewer system that is most susceptible to failure.
system. Most of the failure events (47%) in vacuum sewer systems were due to a defect of a valve element (the mechanism that opens or closes the access way to negative pressure conduits). Irrespective of the identified types, effects and causes of damages, it is interesting to examine the interrelations between these events. These interrelations are illustrated with the data collected for a pressure sewer system and vacuum sewer system (tables 3 and 4). It follows from Table 3 that the main failure type — i.e. pump damage — resulted to the same extent (37.2% of failure cases) in tank overfilling with wastewater in a small wastewater pumping station and in continuous operation of alarm signaling controls (while it was impossible to switch them off automatically). In the case of vacuum sewer systems, in turn, failure to close the valve tightly or even valve damage (amounting to 34% of failure cases) resulted in continuous valve operation or discontinued valve operation in the total of 36% of failure events. As shown in Table 4, pump failures in small pumping stations were caused primarily by the human factor, understood as improper use of the sewer system by residents. It was the cause of not less than 67.5% of all cases of pump damage. Only 10% of the cases of pump damage were caused by wear of structural pump elements. A material defect and the human factor were the most frequent causes of failure events affecting collecting chambers with vacuum interface valves in vacuum sewer systems. These factors were responsible for 45% and 22% of failure events, respectively. As to the gravity sewer system, it should be underscored that the recorded failure events were primarily of the conduit blockage type. They covered not less than 99% of all observed events affecting the sewer network and sewer laterals. Two types of events were distinguished in this set, namely total conduit blockage (40%) and partial conduit blockage (59%). The remaining few cases (about 1%) of other damage types cover transversal cracking of conduits (0.011 failure/km year) and collapsing of conduit bottom (0.10 failure/km year). The human factor was here the main cause of failure. 4. Reliability analysis Reconditioning times — assumed to be the failure duration time between a failure report and failure removal — were also investigated for the systems in question. It was necessary to make such simplifying assumption, due to lack of alarm state monitoring. A cumulative presentation of failure duration time distribution is provided in Fig. 7. It follows from the analysis of collected data that the reconditioning times in each system in most cases do not exceed 2 h. Failure removal took up to 2 h in 77% of recorded failure events affecting pressure sewer systems, 86% of events affecting vacuum sewer
Fig. 6. Comparison of the most frequent failures (failure types) occurring in the element of a vacuum sewer system that is most susceptible to failure.
Please cite this article as: K. Miszta-Kruk, Reliability and failure rate analysis of pressure, vacuum and gravity sewer systems based on operating data, Engineering Failure Analysis (2015), http://dx.doi.org/10.1016/j.engfailanal.2015.07.034
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Table 3 Types and effects of the most frequent damages affecting a sufficiently small wastewater pumping station and a chamber with a vacuum interface valve in a pressure sewer system and a vacuum sewer system (values in the table are in %) [7,12]. Damage
Pressure sewer system
Effect
Level sensor failure
Pump failure
Control system failure
No power supply
Power supply failure
2
37.2
11.1
3
–
– –
37.2 –
– –
3 –
– 1.5
Pressure sewer system
Vacuum sewer system
Tank overfilling with wastewater/ pumps do not operate Continuous operation of alarm signaling controls Impermanent, short-lasting pump operation Continuous valve operation Discontinued valve operation Valve flooded with wastewater
Vacuum sewer system Valve not closed tightly
Valve not opened
Failure of the valve closing mechanism
23.7 18 –
– 18.3 2.6
10.5 3.8 –
Table 4 Types and causes of the most frequent failures of a small wastewater pumping station in a pressure sewer system and a collecting chamber with a vacuum interface valve in a vacuum sewer system (values in the table are in %) [7,12]. Damage
Pressure sewer system
Cause
Level sensor failure
Pump failure
Control system failure
No power supply
– –
67.5 –
– 9
1 3
– 1.5
10 –
– – 2
– –
Pressure sewer system
Vacuum sewer system
Human factor Electric installation and automatic control unit — damaged or penetrated with water Wear of pump elements Neglected maintenance Devastation No power supply Material defect Element defect Neglected maintenance Human factor Product defect
Vacuum sewer system Valve not closed tightly
Valve not opened
Failure of the valve closing mechanism
Valve controlling mechanism flooded with wastewater
4.1 11 0.3 22.1 0.7
1.5 15.2 1.3 – 1.3
1 12.1 – 0.2 0.7
0.3 6.2 0.7 1.3 1.8
2
systems and 58% of events affecting the gravity sewer systems. Average values of the reconditioning times in the systems under considerations are very similar. In the majority of cases, they fall within the range between 40 min and 2 h. The longest failure event occurred in the gravity sewer system and lasted 7.5 h. The shortest failure events occurred in a vacuum and pressure sewer system and lasted 20 and 40 min, respectively. Reconditioning times were determined for each of the systems and it was assumed that the process of functioning and operation of sewer system elements can be treated as a bistable process. This means that there are two states during use and operation of an
Fig. 7. Reconditioning time distribution for particular pressure, vacuum and gravity sewer systems.
Please cite this article as: K. Miszta-Kruk, Reliability and failure rate analysis of pressure, vacuum and gravity sewer systems based on operating data, Engineering Failure Analysis (2015), http://dx.doi.org/10.1016/j.engfailanal.2015.07.034
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K. Miszta-Kruk / Engineering Failure Analysis xxx (2015) xxx–xxx
Table 5 Failure rates λ (failure/km year or failure/unity year) of identified elements of pressure, vacuum and gravity sewer systems. System
Element
Min. λ
Average λ
Max. λ
Pressure sewer system
Small pumping stations (with a pump and control equipment) Network of pressurized conduits Collecting chamber with a vacuum interface valve Network of negative pressure conduits arranged to form a saw-toothed profile Vacuum-forcing pumping station (vacuum pump)
– – – – –
– – – – –
System of interceptor conduits Gravity sewer lateral
– 7.4 In 2006 2.49 In 2006 1.13 In 2008 and 2009
0.48 0.015 0.617 0.43 0.24 (1.8) 0.11 10.28
Vacuum sewer system
Gravity sewer system [30]
Sewer network Collecting chamber
3.05 1.24
– 15.08 In 2008 3.58 In 2009 2.83 In 2010
element, namely: the state of failure-free operation and the state conventionally referred to as “failure”. A failure state in the context of this research is understood simply as an event that results in a failure of an element to fulfill its functions according to its intended purpose. The duration of such an event is counted from the moment when it is recorded down to the moment the relevant repair is completed and the element is put back into service. For such a reliability model, the scope of the reliability analysis covered all elements identified in the classification presented in Table 1, for which failure data could be collected. Linear elements (sewer network, conduits, and pipes) and the so-called point elements (chambers, small pumping stations and chambers with vacuum interface valves) were analyzed separately. Some elements were disregarded in further investigations due to lack of the relevant failure data. The values of failure rates calculated for the examined elements are presented in Table 5. The failure rates presented confirm the analysis of susceptibility to failure and allow operational probabilities to be determined for particular elements. Fig. 7 presents operational probability functions for two elements, namely a small wastewater pumping station in a pressure sewer system and a chamber with a discharge valve in a vacuum sewer system as the two system elements that are most susceptible to failure (Fig. 8). In the case of both sewer system elements A and B, different operational probabilities P(t) were obtained. In the case of a small wastewater pumping station in a pressure sewer system, one can see that this element will operate at a probability level of 50% after about 2 years and 9 months of service. In vacuum sewer system, a collecting chamber with a discharge valve will operate at a probability level of 50% already after less than 2 months. The wide confidence interval in diagram A) (Fig. 7) results from a small sample size, which involves a high probability of assessment error. For all sewer systems under examination, the operational probability values for these elements decrease in time.
5. Conclusions and proposed operation process rationalizations As a result of the analysis performed and assessment of the failure susceptibility of sewer system networks, one can arrange the objects of investigation in terms of failure risk. Failures constitute conditions that endanger proper functioning of sewer networks and realization of their goals. In the case of a sewer network, this risk involves failure to receive anticipated amounts of wastewater from buildings.
Fig. 8. Probabilities of failure-free operation and confidence intervals (for β = 0.95) for a small wastewater pumping station in a pressure sewer system (A) and a chamber with a vacuum interface valve in a vacuum sewer system (B).
Please cite this article as: K. Miszta-Kruk, Reliability and failure rate analysis of pressure, vacuum and gravity sewer systems based on operating data, Engineering Failure Analysis (2015), http://dx.doi.org/10.1016/j.engfailanal.2015.07.034
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The research performed extends to the area of service-related experience which covers the operation of not only traditional systems, but also unconventional sewer systems, especially in the domain of assessing failure states of such systems. The analysis of acquired results allows the following final conclusions to be formulated: • The main elements that fail in the investigated sewer systems: pumps and control units in pressure sewer systems pumping stations; collecting chambers with discharge valves in vacuum sewer systems; conduit assemblies in the gravity sewer system. • Typical results of failures in unconventional sewer systems: overfilled wastewater tanks resulting in continuous alarming. • Basic causes of failures: improper use of sewer installations by residents (almost 2/3 of all causes in system types) in pressure and vacuum sewer systems investigated. • The average reconditioning time was within the range of about 40 min to 2 h in each of the systems investigated. Essentially, up to 80% of all failure events were remedied within this range. • The values of P(t) obtained encourage one to determine and compare operational probabilities for particular system elements, and thus to determine system reliability, while comparing reliability of the same elements in different systems. The analyses performed also lead to a formulation of actions proposed to improve efficiency of the operation of sewer systems under examination. Among the most important measures, one could list the following: 1. Constantly improve residents' technical awareness of how to properly use wastewater disposal installations. 2. Implement a system of constant monitoring of working parameters of sewer system elements (if lacking), especially the monitoring of pumps. Such a system should make it possible to automatically control operational states, collect data for improving the efficiency of sewer system elements.
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Miszta-Kruk, Failure of water supply networks in selected Polish towns based on the field reliability tests, Eng. Fail. Anal. 35 (2013) 736–742. [29] M.J. Fadaee, R. Tabatabaei, Estimation of failure probability in water pipe network using statistical model, Eng. Fail. Anal. 18 (2011) 1184–1192. [30] J. Chudzicki, Z. Czechowicz, M. Kwietniewski, K. Miszta-Kruk, Operational reliability rating sewerage network elements, X Scientific-Technical Conference New Technologies. Networks And Installations Water Supply And Sewage, Ustron, 2012.
Please cite this article as: K. Miszta-Kruk, Reliability and failure rate analysis of pressure, vacuum and gravity sewer systems based on operating data, Engineering Failure Analysis (2015), http://dx.doi.org/10.1016/j.engfailanal.2015.07.034
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Reliability and failure rate analysis of pressure, vacuum and gravity sewer systems based on operating data Katarzyna Miszta-Kruk Warsaw University of Technology, Faculty of Environmental Engineering, Department of Water Supply and Wastewater Management, 20 Nowowiejska Str, 00-653 Warsaw, Poland
a r t i c l e
i n f o
Article history: Received 30 September 2014 Received in revised form 5 July 2015 Accepted 21 July 2015 Available online xxxx Keywords: Pipeline failures Sewer systems Reliability analysis Sewer pipes Failure analysis Pressure sewer system Vacuum sewer system Gravity sewer system Vacuum pipeline
a b s t r a c t This paper presents an assessment of operational reliability of elements of pressure, vacuum and gravity sewer systems, based on research focused on 7 different systems. The article discusses both traditional (gravity) sewer systems and unconventional sewer systems that are implemented on a large scale. The analysis and assessment were based on the data collected during investigations of real sewer systems over a period of 3 to 5 years. On the basis of the analyses, this paper presents the system elements that were found to be most susceptible to failure as well as the most common types of failure events. This analysis allowed the duration of the identified failure events to be determined and yielded a reliability assessment in the form of failure rates (λ) calculated for each highlighted element. An analysis of the results in terms of the functions of the objects most susceptible to failure allows one to assess their operational probability. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Theories of network systems' reliability (including sewage disposal networks), but especially of water supply network systems, have been developed since the beginning of the '70s. Corresponding theories of risk and safety were developed in the mid-1990s. In the world literature on the subject, and especially as relating to the reliability of water supply and wastewater disposal networks, these issues are to a large extent investigated as a result of activities of IWSA (International Water Supply Association) or AWWA (American Water Works Association). Generally, they focus on the assessment of reliability of water supply systems. This domain is better explored through investigations and development of particular theories. However, the intuitively comprehensive approach to this subject should cover an analysis of the reliability of wastewater disposal systems as well. Both systems together provide a comprehensive service of supplying water to and disposing of sewage from users. Almost at the same time, as the start of reliability analysis of water supply systems [16,28,29], research efforts also began to focus on wastewater disposal systems. However, such evaluations were often treated as investigations of secondary importance. Only a few researchers, however, decided to face the challenge related to the assessment of the level of reliability of sewer systems. Among them, one could count the following: [1,3,4,7].
E-mail address: [email protected].
http://dx.doi.org/10.1016/j.engfailanal.2015.07.034 1350-6307/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article as: K. Miszta-Kruk, Reliability and failure rate analysis of pressure, vacuum and gravity sewer systems based on operating data, Engineering Failure Analysis (2015), http://dx.doi.org/10.1016/j.engfailanal.2015.07.034
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Reliability of sewer systems is most often considered in terms of the need to dispose wastewater from users [3,5,6,8]. An intensive development of reliability theory creates a wide range of possible applications aimed at solving problems related to the design and operation of sewer infrastructures. Network systems constitute one of the typical arrangements where application methods are sought for [9]. In the case of sewer systems designed both as traditional solutions and as unconventional ones, it is also important to determine unequivocal criteria, in order to rationalize a reliability assessment. Relevant proposals can be found in the following publications: [2,4,11–15,3,6,10]. The methods proposed in the last three texts are used in practice to assess reliability of different network systems in Poland. 2. Structure of sewer networks under investigation Pressure and vacuum sewer systems belong to so-called unconventional wastewater disposal systems. In pressure sewer systems, as in vacuum ones, the medium flow is forced by the pressure differential generated in small wastewater pumping stations or, in the case of vacuum sewer systems, by means of vacuum pumps. In this, they differ from gravity sewer systems (referred to as traditional), where the flow of sewage proceeds generally in partly filled conduits by the force of gravity. Concepts of pressure and vacuum wastewater disposal appeared already by the end of the 19th century, when technological developments allowed engineers to construct devices to pump or draw sewage. At this point, it is worth mentioning, the first pressure sewer system in Poland was designed and constructed in 1899 in Olsztyn. At that time, the network serviced 93% of the town and disposed of about 3000 m3 of sewage per 24 h [11,13]. The earliest functioning vacuum sewer systems reach back to the 1990s. The first wastewater collection system of this type was patented in 1888 in the United States by Adrian LeMarquand. It was called the system of wastewater collection by barometric depression [EPA/625/1-91/024 1991, 16]. This was the beginning of modern unconventional sewer systems. At present, such systems raise high interest as alternatives for gravity sewer systems. It is advisable to implement unconventional sewer systems primarily in areas characterized by relatively dispersed development (single-family houses, village settlements, etc.) with small changes in altitude or flat areas. Both the pressure and vacuum sewer systems are especially applicable wherever sewage is intermittently generated, namely in camping sites, holiday centers, in the case of exceptionally poor soil and groundwater conditions, in protected areas or on the fringes of water reservoirs. Both pressure and vacuum sewer systems may also be implemented to collect wastewater in underground railway tunnels, underpasses from parking lots, in large industrial halls, construction sites, as well as to handle wastewater transmitted from ships in docks and from passenger airplanes or ships. In the pressure sewer system, wastewater is transported under pressure generated by pumps. The system consists of the following elements: chambers, controlled pumps, and networks of pressurized conduits with accessories. The pipes of a pressure sewer system, as opposed to sewers of a gravity sewer system, are laid at a constant depth below the lower frost penetration limit (about 1.2 m) and it is not necessary to ensure their proper inclination gradients. A pump may force wastewater for the distance of up to several kilometers and up to the height of 45 m · wc. A receiver chamber allows wastewater to enter another sewer system or constitutes the last element before wastewater enters a wastewater treatment plant. The goal of this system is to carry wastewater from internal installations of buildings and facilities in a given area to a point of wastewater disposal (a gravity sewer system or a wastewater treatment plant) (Fig. 1). The vacuum sewer system operates by the force of a negative pressure (0.6–0.7 bar) generated by vacuum pumps in a pumping station. The negative pressure is transmitted to household collection chambers via conduits laid to assume a saw-toothed profile. At the moment of drawing wastewater and its associated mixing with the air in collecting chambers (fitted with discharge valves),
Fig. 1. Construction scheme of a pressure sewer system with identifying system elements.
Please cite this article as: K. Miszta-Kruk, Reliability and failure rate analysis of pressure, vacuum and gravity sewer systems based on operating data, Engineering Failure Analysis (2015), http://dx.doi.org/10.1016/j.engfailanal.2015.07.034
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Fig. 2. Construction scheme of a vacuum sewer system with identifying system elements.
wastewater is transmitted through pipes in the direction of a vacuum pumping station. There, wastewater is collected in tanks. Then it is transferred to a wastewater treatment plant or another sewer system. A scheme of a vacuum sewer system is presented in Fig. 2. The gravity sewer system presented in Fig. 3 can be defined as a system of wastewater disposal by means of natural land gradients of at least 3–5% which will allow gravity to drive the wastewater flow. However, there are locations, with extreme elevation profiles, where a classical gravity sewer system design is erroneously attempted. This has a significant impact on the reliability of system operation. Traditional sewer systems consist of the following elements: chambers that receive wastewater from buildings by force of gravity; and networks of conduits — laid with a gradient that ensures gravity-driven wastewater flow — that carry wastewater to a point of wastewater disposal or another sewer system or to a wastewater treatment plant. The above characterization of sewer systems is sufficient for the purposes of research and assessment of their reliability. As follows from the above definitions, both in unconventional and in traditional systems, one can distinguish the elements (or direct objects, depending on the objective of the reliability research) as presented below in Table 1. In terms of flow variability in pressure sewer systems, one most commonly differentiates between high-pressure sewer systems based on positive-displacement pumps and low-pressure sewer systems based on centrifugal pumps. In the case of vacuum sewer systems, one can differentiate two additional categories: vacuum systems and vacuum and siphon systems. In traditional gravity sewer systems, the following system categories are differentiated: separate sewer systems with distinct separation of sanitary and storm water sub-systems, combined sewer systems and semi-separate sewer systems. Previously, analyses of sewer systems have usually been of the overview study type, aimed at describing systems and devices, principles of their operation as well as problems in and experiences resulting from their operation [11,17–27]. Some of these studies focus
Fig. 3. Construction scheme of a gravity sewage system.
Please cite this article as: K. Miszta-Kruk, Reliability and failure rate analysis of pressure, vacuum and gravity sewer systems based on operating data, Engineering Failure Analysis (2015), http://dx.doi.org/10.1016/j.engfailanal.2015.07.034
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Table 1 Elements/objects of the unconventional sewer systems and of the traditional gravity sewer system. System element
Unconventional sewer systems Vacuum sewer system
Pressure sewer system
Gravity sewer system
1 2
Gravity sewer lateral Collecting chamber with a vacuum interface valve (automatic control unit) Network of negative pressure conduits arranged to form a saw-toothed profile Vacuum-pumping station with its elements System of interceptor conduits
Gravity sewer lateral Small pumping stations (with pump and controls) Network of pressurized conduits
Gravity sewer lateral Collecting chamber
Expansion chamber Point of disposal or chamber that opens into another system
Point of disposal
3 4 5
Traditional sewer system
Network of gravity conduits
primarily on proprietary solutions [17,19], while others focus on design problems [22,24,26]. There are also interesting documents focused on the question of selecting a sewer system, while taking into account technical and economic analysis, as well as treatises that delineate the range of system applicability [11,26,27]. The few published reliability studies of unconventional sewer systems focus primarily on identifying the causes of failures, especially in small pumping stations [4,5,7], and on assessing the functional reliability of a selected system, based on the method of a comprehensive survey and coefficients determined in operational tests [1,12]. 3. Operation research of sewer system reliability 3.1. The nature, objective and scope of research This section presents the results of a study focused on random variability of operational states that describe susceptibility to failure of sewer systems, both traditional (conventional) and unconventional. The study stressed primarily the analysis of elements identified in Table 1, functioning in three selected systems. The performed research belongs to the category of operation reliability studies, i.e. research conducted in natural conditions of system operation and functioning. The research belongs to the category of reliability studies, i.e. research conducted under natural operational conditions. Research of this type considerably extends the scope of available data (hitherto rather modest) on sewer system functioning problems. However, it is extremely important for determining optimum methods of system design and operation. The main objective of the research was to assess, quantitatively, operation states that describe susceptibility of sewer system elements to failure and to identify the causes and effects of element defects. The reliability research was carried out according to a pre-established program. The research drew on operational data, such as working modes and states of repair, overhaul and heavy repairs carried out by waterworks and wastewater disposal company service personnel in the towns under examination. Data records were usually stored in GIS type databases, while part of the data was found in hardcopy documentation. All the data acquired from these sources were additionally verified by the technical personnel of a given company. Data, indispensable for the assessment of susceptibility to failure, were collected by means of failure charts prepared especially for this purpose. Each chart covered one failure event. 3.2. Research objects Data availability and conditions and duration of system service constituted the basic conditions for selecting systems to be examined. The selection was based on an analysis of numerous sewer systems that operating in Poland. These activities showed that, in
Table 2 Technical data of sewer systems under examination. Characteristics Chamber/pumping station
Pump type/valve type Amount Construction Diameter/dimension
pcs – mm
Material
–
Diameter range Length
mm m
Pump control/discharge control Alarm signaling Pressure/vacuum/gravity conduits
System commissioning year
Pressure sewer system
Vacuum sewer system
Gravity sewer system
Screw pump 58–120 Concrete 800–1500 Hydrostatic floats/float connecting bars Light signal PE
pneumatic valve 130–278 Concrete 1000 × 1000 –
– 1350 Concrete 800–1200 –
Light signal PE
40–90 8800–10,700 2004
90–280 2800–12,500 2002
– PVC, cast iron, stoneware, PE 63–1200 82,000 1964–2010
Please cite this article as: K. Miszta-Kruk, Reliability and failure rate analysis of pressure, vacuum and gravity sewer systems based on operating data, Engineering Failure Analysis (2015), http://dx.doi.org/10.1016/j.engfailanal.2015.07.034
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many cases, the data concerning susceptibility of a sewer system object to failure were not controlled or recorded. As a result, three pressure sewer systems functioning since 2004, three vacuum sewer systems characterized by very similar service conditions and one gravity sewer system were selected to be examined (Table 2). 3.3. Research results and failure event analysis It should be underscored that the research covered all systems, without assessing their comparative reliability, while analyzing only the susceptibility of particular elements to failure. Thus, one can identify elements most susceptible to failure in particular systems: – in pressure sewer systems: small wastewater pumping stations (with a tank, pump and control unit); – in vacuum sewer systems: collecting chambers with vacuum interface valves; – in gravity sewer systems: conduits. This is shown in Fig. 4. Damaged pumps in small pumping stations were responsible for over 90% of all failure events in pressure sewer systems. Of these failures, the control unit failed most often (67% of failures). This category of failures included defects of automation subassemblies in the control box as well as damages to the boxes themselves. Failures of this type constituted from 4 to 33% of the failure events. In a pressure sewer system, failures caused by power failure constituted an important failure event category. In a number of the investigated cases, this resulted in discontinued wastewater service (from 5 to 13% of all failure events) (Fig. 5). The highest number of failures in vacuum sewer systems was recorded for collecting chamber failures. In this case, the number of chamber failures was about 6 times the number of failures of pumping stations and almost 50 times the number of failures that affected vacuum conduits. Failure to open a valve, failure to close a valve tightly, defective valve closing mechanism and flooding the valve control mechanism (controller) were the most frequent failures affecting the collecting chamber. They were responsible for 92% of all observed failures that affected collecting chambers. In the case of a vacuum station, the dominant failure event was prolonged operation of vacuum pumps, caused by leakages in the system. Power failure resulted in pump failures or discontinued operation of the entire equipment in a vacuum station, which led to shutting down the entire wastewater collection system (Fig. 6). In the case of the gravity sewer system, practically all failure events were related to wastewater conduits. In the system examined, one observed primarily duct blocking (about 99% of failure events in the network), irrespective of duct material and its diameter. 3.3.1. Interrelations between types and effects of failures and their causes Decidedly, damages to unconventional sewer systems resulted most frequently in overfilling small pumping stations with wastewater, due to discontinued pump operation (pressure sewer systems), or in failure to close the valve tightly, while vacuum pumps continued to operate (vacuum sewer systems). Conduit blocking in gravity sewer systems resulted most frequently in wastewater releases or in the inability to receive wastewater from clients. Failures of the elements most susceptible to failure in each system were primarily caused by improper use of the wastewater disposal system by its users. This was responsible for about 70% of the failures in the pressure sewer system and 25% in the vacuum sewer
Fig. 4. Comparison of elements most susceptible to failure in each of the tested sewer systems.
Please cite this article as: K. Miszta-Kruk, Reliability and failure rate analysis of pressure, vacuum and gravity sewer systems based on operating data, Engineering Failure Analysis (2015), http://dx.doi.org/10.1016/j.engfailanal.2015.07.034
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Fig. 5. Comparison of the most frequent failures (failure types) occurring in the element of a pressure sewer system that is most susceptible to failure.
system. Most of the failure events (47%) in vacuum sewer systems were due to a defect of a valve element (the mechanism that opens or closes the access way to negative pressure conduits). Irrespective of the identified types, effects and causes of damages, it is interesting to examine the interrelations between these events. These interrelations are illustrated with the data collected for a pressure sewer system and vacuum sewer system (tables 3 and 4). It follows from Table 3 that the main failure type — i.e. pump damage — resulted to the same extent (37.2% of failure cases) in tank overfilling with wastewater in a small wastewater pumping station and in continuous operation of alarm signaling controls (while it was impossible to switch them off automatically). In the case of vacuum sewer systems, in turn, failure to close the valve tightly or even valve damage (amounting to 34% of failure cases) resulted in continuous valve operation or discontinued valve operation in the total of 36% of failure events. As shown in Table 4, pump failures in small pumping stations were caused primarily by the human factor, understood as improper use of the sewer system by residents. It was the cause of not less than 67.5% of all cases of pump damage. Only 10% of the cases of pump damage were caused by wear of structural pump elements. A material defect and the human factor were the most frequent causes of failure events affecting collecting chambers with vacuum interface valves in vacuum sewer systems. These factors were responsible for 45% and 22% of failure events, respectively. As to the gravity sewer system, it should be underscored that the recorded failure events were primarily of the conduit blockage type. They covered not less than 99% of all observed events affecting the sewer network and sewer laterals. Two types of events were distinguished in this set, namely total conduit blockage (40%) and partial conduit blockage (59%). The remaining few cases (about 1%) of other damage types cover transversal cracking of conduits (0.011 failure/km year) and collapsing of conduit bottom (0.10 failure/km year). The human factor was here the main cause of failure. 4. Reliability analysis Reconditioning times — assumed to be the failure duration time between a failure report and failure removal — were also investigated for the systems in question. It was necessary to make such simplifying assumption, due to lack of alarm state monitoring. A cumulative presentation of failure duration time distribution is provided in Fig. 7. It follows from the analysis of collected data that the reconditioning times in each system in most cases do not exceed 2 h. Failure removal took up to 2 h in 77% of recorded failure events affecting pressure sewer systems, 86% of events affecting vacuum sewer
Fig. 6. Comparison of the most frequent failures (failure types) occurring in the element of a vacuum sewer system that is most susceptible to failure.
Please cite this article as: K. Miszta-Kruk, Reliability and failure rate analysis of pressure, vacuum and gravity sewer systems based on operating data, Engineering Failure Analysis (2015), http://dx.doi.org/10.1016/j.engfailanal.2015.07.034
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Table 3 Types and effects of the most frequent damages affecting a sufficiently small wastewater pumping station and a chamber with a vacuum interface valve in a pressure sewer system and a vacuum sewer system (values in the table are in %) [7,12]. Damage
Pressure sewer system
Effect
Level sensor failure
Pump failure
Control system failure
No power supply
Power supply failure
2
37.2
11.1
3
–
– –
37.2 –
– –
3 –
– 1.5
Pressure sewer system
Vacuum sewer system
Tank overfilling with wastewater/ pumps do not operate Continuous operation of alarm signaling controls Impermanent, short-lasting pump operation Continuous valve operation Discontinued valve operation Valve flooded with wastewater
Vacuum sewer system Valve not closed tightly
Valve not opened
Failure of the valve closing mechanism
23.7 18 –
– 18.3 2.6
10.5 3.8 –
Table 4 Types and causes of the most frequent failures of a small wastewater pumping station in a pressure sewer system and a collecting chamber with a vacuum interface valve in a vacuum sewer system (values in the table are in %) [7,12]. Damage
Pressure sewer system
Cause
Level sensor failure
Pump failure
Control system failure
No power supply
– –
67.5 –
– 9
1 3
– 1.5
10 –
– – 2
– –
Pressure sewer system
Vacuum sewer system
Human factor Electric installation and automatic control unit — damaged or penetrated with water Wear of pump elements Neglected maintenance Devastation No power supply Material defect Element defect Neglected maintenance Human factor Product defect
Vacuum sewer system Valve not closed tightly
Valve not opened
Failure of the valve closing mechanism
Valve controlling mechanism flooded with wastewater
4.1 11 0.3 22.1 0.7
1.5 15.2 1.3 – 1.3
1 12.1 – 0.2 0.7
0.3 6.2 0.7 1.3 1.8
2
systems and 58% of events affecting the gravity sewer systems. Average values of the reconditioning times in the systems under considerations are very similar. In the majority of cases, they fall within the range between 40 min and 2 h. The longest failure event occurred in the gravity sewer system and lasted 7.5 h. The shortest failure events occurred in a vacuum and pressure sewer system and lasted 20 and 40 min, respectively. Reconditioning times were determined for each of the systems and it was assumed that the process of functioning and operation of sewer system elements can be treated as a bistable process. This means that there are two states during use and operation of an
Fig. 7. Reconditioning time distribution for particular pressure, vacuum and gravity sewer systems.
Please cite this article as: K. Miszta-Kruk, Reliability and failure rate analysis of pressure, vacuum and gravity sewer systems based on operating data, Engineering Failure Analysis (2015), http://dx.doi.org/10.1016/j.engfailanal.2015.07.034
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Table 5 Failure rates λ (failure/km year or failure/unity year) of identified elements of pressure, vacuum and gravity sewer systems. System
Element
Min. λ
Average λ
Max. λ
Pressure sewer system
Small pumping stations (with a pump and control equipment) Network of pressurized conduits Collecting chamber with a vacuum interface valve Network of negative pressure conduits arranged to form a saw-toothed profile Vacuum-forcing pumping station (vacuum pump)
– – – – –
– – – – –
System of interceptor conduits Gravity sewer lateral
– 7.4 In 2006 2.49 In 2006 1.13 In 2008 and 2009
0.48 0.015 0.617 0.43 0.24 (1.8) 0.11 10.28
Vacuum sewer system
Gravity sewer system [30]
Sewer network Collecting chamber
3.05 1.24
– 15.08 In 2008 3.58 In 2009 2.83 In 2010
element, namely: the state of failure-free operation and the state conventionally referred to as “failure”. A failure state in the context of this research is understood simply as an event that results in a failure of an element to fulfill its functions according to its intended purpose. The duration of such an event is counted from the moment when it is recorded down to the moment the relevant repair is completed and the element is put back into service. For such a reliability model, the scope of the reliability analysis covered all elements identified in the classification presented in Table 1, for which failure data could be collected. Linear elements (sewer network, conduits, and pipes) and the so-called point elements (chambers, small pumping stations and chambers with vacuum interface valves) were analyzed separately. Some elements were disregarded in further investigations due to lack of the relevant failure data. The values of failure rates calculated for the examined elements are presented in Table 5. The failure rates presented confirm the analysis of susceptibility to failure and allow operational probabilities to be determined for particular elements. Fig. 7 presents operational probability functions for two elements, namely a small wastewater pumping station in a pressure sewer system and a chamber with a discharge valve in a vacuum sewer system as the two system elements that are most susceptible to failure (Fig. 8). In the case of both sewer system elements A and B, different operational probabilities P(t) were obtained. In the case of a small wastewater pumping station in a pressure sewer system, one can see that this element will operate at a probability level of 50% after about 2 years and 9 months of service. In vacuum sewer system, a collecting chamber with a discharge valve will operate at a probability level of 50% already after less than 2 months. The wide confidence interval in diagram A) (Fig. 7) results from a small sample size, which involves a high probability of assessment error. For all sewer systems under examination, the operational probability values for these elements decrease in time.
5. Conclusions and proposed operation process rationalizations As a result of the analysis performed and assessment of the failure susceptibility of sewer system networks, one can arrange the objects of investigation in terms of failure risk. Failures constitute conditions that endanger proper functioning of sewer networks and realization of their goals. In the case of a sewer network, this risk involves failure to receive anticipated amounts of wastewater from buildings.
Fig. 8. Probabilities of failure-free operation and confidence intervals (for β = 0.95) for a small wastewater pumping station in a pressure sewer system (A) and a chamber with a vacuum interface valve in a vacuum sewer system (B).
Please cite this article as: K. Miszta-Kruk, Reliability and failure rate analysis of pressure, vacuum and gravity sewer systems based on operating data, Engineering Failure Analysis (2015), http://dx.doi.org/10.1016/j.engfailanal.2015.07.034
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The research performed extends to the area of service-related experience which covers the operation of not only traditional systems, but also unconventional sewer systems, especially in the domain of assessing failure states of such systems. The analysis of acquired results allows the following final conclusions to be formulated: • The main elements that fail in the investigated sewer systems: pumps and control units in pressure sewer systems pumping stations; collecting chambers with discharge valves in vacuum sewer systems; conduit assemblies in the gravity sewer system. • Typical results of failures in unconventional sewer systems: overfilled wastewater tanks resulting in continuous alarming. • Basic causes of failures: improper use of sewer installations by residents (almost 2/3 of all causes in system types) in pressure and vacuum sewer systems investigated. • The average reconditioning time was within the range of about 40 min to 2 h in each of the systems investigated. Essentially, up to 80% of all failure events were remedied within this range. • The values of P(t) obtained encourage one to determine and compare operational probabilities for particular system elements, and thus to determine system reliability, while comparing reliability of the same elements in different systems. The analyses performed also lead to a formulation of actions proposed to improve efficiency of the operation of sewer systems under examination. Among the most important measures, one could list the following: 1. Constantly improve residents' technical awareness of how to properly use wastewater disposal installations. 2. Implement a system of constant monitoring of working parameters of sewer system elements (if lacking), especially the monitoring of pumps. Such a system should make it possible to automatically control operational states, collect data for improving the efficiency of sewer system elements.
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Please cite this article as: K. Miszta-Kruk, Reliability and failure rate analysis of pressure, vacuum and gravity sewer systems based on operating data, Engineering Failure Analysis (2015), http://dx.doi.org/10.1016/j.engfailanal.2015.07.034