CC BY
6
Engineering technological systems
6. Singh, W. R. The preferential composting of water fern and a reduction of the mobility of potential toxic elements in a rotary drum reactor [Text] / W. R. Singh, A. S. Kalamdhad, J. Singh // Process Safety and Environmental Protection. - 2016. - Vol. 102. -P. 485-494. doi: 10.1016/j.psep.2016.05.002
7. Di Maria, F. Energy production from mechanical biological treatment and Composting plants exploiting solid anaerobic digestion batch: An Italian case study [Text] / F. Di Maria, A. Sordi, C. Micale // Energy Conversion and Management. - 2012. - Vol. 56. -P. 112-120. doi: 10.1016/j.enconman.2011.11.023
8. Kumar Singh, Y. Maturation of primary stabilized compost from rotary drum composter [Text] / Y. Kumar Singh, A. S. Kalamdhad, M. Ali, A. A. Kazmi // Resources, Conservation and Recycling. - 2009. - Vol. 53, Issue 7. - P. 386-392. doi: 10.1016/j.resconrec.2009.02.004
9. Shevchenko, I. Justification of external radius the drum aerator compost mixes [Text] / I. Shevchenko, A. Kovyazin, V. Haritonov // Tekhnichni systemy i tekhnolohiyi tvarynnytstva: Visnyk kharkivs'koho Natsional'noho tekhnichnoho universytetu sil's'koho hos-podarstva imeni Petra Vasylenka. - 2013. - Issue 132. - P. 292-296.
10. Shevchenko, I. Teoretycheskoe obosnovanye parametrov ^ratora kompostnykh smesey [Text] / I. Shevchenko, A. Kovyazin, V. Haritonov, I. Velychko // Zb. nauk. prats' IMT NAAN «Mekhanizatsiya, ekolohizatsiya ta konvertatsiya biosyrovyny u tvaryn-nytstvi». - 2012. - Issue 2 (10). - P. 92-109.
11. Pavlenko, S. Novitni tekhnichni zasoby pererobky orhanichnykh vidkhodiv [Text] / S. I. Pavlenko, O. O. Lyashenko, A. A. Povo-lots'kyi, Yu. A. Filonenko // Visnyk KhNTUSH im. P. Vasylenka «Tekhnichni systemy i tekhnolohiyi tvarynnytstva». - 2013. -Issue 132. - P. 193-200.
12. Pavlenko, S. Monitoring of the market and technical means of production solid organic fertilizer [Text] / S. Pavlenko, V. Dudin, R. Aki-menko // Visnyk KhNTUSH im. P. Vasylenka «Tekhnichni systemy i tekhnolohiyi tvarynnytstva». - 2016. - Issue 170. - P. 34-45.
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Дослиджено причини вiдмов та значення пито-мого параметра потоку вiдмов металевих водо-провидних труб. Аналiз статистичних даних щодо вiдмов дозволив побудовати залежностi питомого параметра потоку вiдмов вид диаметра трубопровода. Виконано порiвняльний аналiз отриманих даних та розроблет вiдповiднi виснов-ки та пропозици. Отримаш даш корисн для роз-рахунтв надiйностi систем водопостачання
Ключевi слова: водопостачання, водопровода мережi, металевi труби, надштсть, безвгдмов-
тсть, причини вiдмов труб
□-□
Исследованы причины отказов и значения удельного параметра потока отказов металлических водопроводных труб. Анализ статистических данных об отказах позволил построить зависимости удельного параметра потока отказов от диаметра трубопровода. Выполнен сравнительный анализ полученных данных и разработаны соответствующие выводы и предложения. Полученные данные полезны для расчетов надежности систем водоснабжения
Ключевые слова: водоснабжения, водопроводные сети, металлические трубы, надежность,
безотказность, причины отказов труб -□ □-
1. Introduction
Water supply networks are the most extended water supply system element, and, therefore, the most vulnerable in terms of reliability. Emergency situations in water supply networks lead to many negative consequences for both utilities companies and consumers of water. On the one
© A. Matyash, I. Usenko, R. MyagkoNib, S. Kostenko, 2017
UDC 628.14
|DOI: 10.15587/1729-4061.2017.101262|
ESTIMATION OF FAILURE-FREE OPERATION OF METAL WATER PIPES
A. M a tyas h
PhD* I. Usenko
PhD, Associate Professor*
E-mail: irina__usenko@ukr.net
R. Myag kohl i b
PhD, Associate Professor Department of Heat and Gas Supply, Ventilation and Heat Power Engineering** E-mail: myagkohlib@gmail.com S. Kosten ko Assistant* *Department of hydraulics, water supply and sewerage** **Poltava National Technical Yuri Kondratyuk University Pershotravnevyi ave., 24, Poltava, Ukraine, 36011
hand, damaged sections in the existing water supply networks lead to the loss of a valuable resource - water, and additional financial expenses to eliminate accidents. On the other hand, it causes discontent of the population due to the shortfall of water in required amount to meet their needs in a timely manner. No less distrurbing this issue for industrial enterprises when a break in water supply leads to losses due
to unsatisfied demand for products. Given this, consumers of water, both legal entities and physical persons, always put forward requirements regarding the reliability of water supply. These requirements can be formulated differently, but, at present, it is impossible to perform a comparison of the specified requirements to the normative parameters of reliability of water supply. Thus, DBN V.2.5-74:2013 «Water supply. Outdoor networks and facilities» [1] formulates requirements only for the duration of interruptions in water supply (Table 1) and does not indicate the frequency of such interruptions.
Table 1
Categories of centralized water supply systems by reliability or by a degree of water supply provision
Category of the centralized water supply systems Conditions of system operation by permissible limits
Water supply decrease Break in water supply
% time
1 <30 <3 days Allowed during period of shutdown of damaged and switching on the reserve system elements <10 min
2 <30 <10 days Allowed during period of shutdown of damaged and switching on the reserve system elements and the repair <6 hours
3 <30 <15 days Allowed during period of shutdown of damaged and switching on the reserve system elements and the repair <24 hours
At the same time, in accordance with the Decree of Cabinet of Ministers of Ukraine [2], it is stipulated that: the absence of water for consumers should not last lpnger than 6 hours per day and not oftener than 2 times per month. The given Decree is generic in nature and it does not indicate for which consumers (an individual person or a populated area) these requirements are valid. The above aspects limit working out measures to improve the reliability of water supply system as whole.
2. Literature review and problem statement
At present, the issue of reliability of separate structures of water complex is rather important for both scientists and specialists in the field of water supply This factor is important because it is directly linked to the efficiency and quality service of water supply to consumers.
Development of methods and techniques for calculating the reliability was addressed by a significant amount of scientific papers. It is proposed to determine the level of reliability for water supply systems by the coefficients of readiness KR or failure-free operation KFO. [3]. However, in order to determine a coefficient of readiness, it is necessary to know the average duration of the elimination of breakdown (mean duration of repair). Article [4] proposed an improved classification of consumers and water-supply systems by reliability (Table 2) than that of DBN V.2.5-74:2013 and it establishes regulatory requirements concerning the reliability of water supply as a technological process. The techniques, devised in [3, 4], to calculate reliability of the water supply facilities, make it possible to calculate the quantitative indicators of reliability. This, in turn, allows comparing them to the normative requirements. On the other hand, it is necessary to continue to develop methods and procedures for calculating the reliability of water supply facilities with regard to the specified requirements. Calculations of reliability of separate elements of the water supply systems in [5, 6] are performed based on theoretical principles only. A technique for the calculation of reliability in [7] can only be used for water supply systems with distributed networks. In order to see an actual picture of reliability of metal water pipes, it is necessary to carry out research additionally for each city [8].
Considerable attention of scientists is paid to collecting and analyzing statistical data on failures in water-supply networks. Papers [9, 10] established the causes of failures in water supply pipes and the main factors influencing the reliability of separate structures in the water system of Moscow (Russia) and Babylon (Iraq). Based on the collected statistical data, an analysis of pipeline failures depending on the diameter and material of pipes is conducted (Fig. 1).
Table 2
Classification of consumers and water-supply systems by reliability
No. of entry Groups and categories of water supply systems Values of essential and basic reliability indicators
1 First group Subgroup A - water supply systems of dangerous industries Maximum probability of failure-free operation during service life max P(Tl)
Subgroup B - fire protection water supply systems Maximum coefficient of operational readiness over 3 hours of fire site localization KOR.=KR • P(t)
Second group Centralized water supply systems of populated areas at number of residents Time to failure T, hours Average time of operation recovery Tr, hours Coefficinet of readiness Kr
2 town category 1, N > 50 thousand people T > 2160 hours (3 months) TR < 3 hours 0.99861
town category 2, 10 thousand people < N < 50 thousand people T > 360 hours (0.5 month) TR < 6 hours 0.98361
settlement category 3, N < 10 thousand people T > 360 hours (0.5 month) TR < 24 hours 0.93750
Fig. 1. Dependence of specific parameter of failure rate on the pipe diameter
However, using these results of reliability of water supply pipes for other cities is difficult. When obtaining reliable results for reliability in other localities, there are a number of local factors: water corrosion, operation quality of water supply networks, geological conditions and others.
It is a relevant task to model failures of water supply system structures depending on the many influences: period of operation, a change in pressure in a water supply network, maintenance of systems, human factor, and others. Articles of scientists [11, 12] presented analyses and methods that make it possible to establish causal relationships of failures in the form of a «failure matrix tree». Assessing the reliability of water supply system as a renewable technical system made it possible to build statistical models of failure intensity considering time and to display graphically in the form of a reliability curve [13]. Any theoretical research [11-13] should be verified by practical experiment. For this purpose, it is necessary to analyze the types of failures in separate facilities of water supply system. Paper [14] conducted a detailed analysis of failures of metal pipes under the influence of corrosion. However, the practice of exploiting the pipelines suggests that the causes of failure in pipelines may be due to a number of other reasons.
Theoretical methods of calculating the reliability are based on the existing mathematical models of the process of functioning of renewable elements. However, in order to verify theoretical hypotheses about the failures and renewal of separate elements, it is necessary to collect statistical data based on the operational practice. Practical methods are based on statistical data on reliability of the elements obtained during operation. Therefore, the more statistical data on operation the better results of calculating the reliability of water supply facilities.
3. The aim and tasks of research
The aim of present work is to evaluate reliability of water supply pipes on the example of water supply system in the city of Kremenchuk, Poltava oblast (Ukraine).
To accomplish the set aim, the following tasks had to be solved:
- to define the main factors that lead to the failures of metal water pipes (qualitative analysis);
- to calculate basic indicators of failure-free operation of metal water pipes (quantitative analysis);
- to determine the dependences of parameter of failure rate on the diameter of pipes based on statistical data on the damages of metal pipes.
4. Materials and methods for examining the failure-free operation of metal water pipes
4. 1. Materials and methods for examining the failures of metal water pipes
In order to perform research into evaluation of reliability of metal water pipes, we used information on the damages of water supply networks of the examined object. A local «Vodokanal» registers information about the failures of water system elements at the end of each day by the foremen of respective stations and submits these data to a unified database. To evaluate a failure-free operation of pipes in the sections of water networks, we have collected and processed statistical data from the city of Kremenchuk, Poltava oblast (Ukraine). We employed data on the damages to metal pipes with a diameter of 50...300 mm over 7 years. Prior to combining statistical data into the general sample, we verified separate samples on deviation from the accepted law of distribution of the observation results.
In order to process and analyze obtained statistical data on failures of metal water pipes, we applied the following methods:
- mathematical statistics in the analysis of statistical data on the damages to metal pipes;
- theory of reliability when performing qualitative and quantitative analysis of failure-free operation of water supply networks.
4. 2. Analysis of statistical data on the failures of metal water pipes
4. 2. 1. Plan of check tests
In the theory of reliability [15], when choosing possible plans of check tests, the number of elements (N) is taken into account that undergo the test, duration of the test (t) and character of the test:
- test with a renewal (V) is the test in which every failed element is replaced with the new one;
- test without a renewal (B) is the test in which the element that failed during examination is not replaced.
Time of completing the tests can be assigned in various ways:
T - plans, under which tests are carried out within the assigned time T;
r - plans, under which tests are conducted prior to the occurrence of the r-th failure;
(r, T) - plans, under which tests are carried out either to the time tr of the occurrence of the r-th failure, if tr < T, or to the time T, if tr > T:
1. [N, B, T] - the plan, under which N elements are put to test; elements that failed are renewed (B); the test is conducted to the previously specified time T.
2. [N, B, r] - the plan, under which N elements are put to test; elements that failed are renewed (B); the test is held until the occurence of the r-th failure.
3. [N, B, (T, r)] - the plan, under which N elements are put to test; elements that failed are renewed (B); tests are carried out either to the time tr of the occurrence of the r-th failure, if tr < T, or to the time T, if tr > T.
4. [N, B, T] - the plan, under which N elements are put to test; elements that failed are not renewed (B); tests are carried out to the specified time T.
5. [N, B, r] - the plan, under which N elements are put to test; elements that failed are not renewed (B); tests are carried out to the time of the occurrence of the r-th failure.
6. [N, B, (T, r)] - the plan, under which N elements are put to test; elements that failed are not renewed (B); tests are carried out either to the time tr of the occurrence of the r-th failure, if tr < T, or to the time T, if tr > T.
Water pipes relate to the renewable elements; observation period to obtain statistical data made up 7 years. Given the above, we selected the first check test plan [N, B, T].
4. 2. 2. General characteristic of statistical data
Generalized statistical data on the damages of metal water pipes are given in Table 3.
Table 3
General statistical data on the damages of metal water pipes in the city of Kremenchuk, Poltava oblast (Ukraine)
Pipe sections of the network Diameter D, mm Total length of analyzed pipes, L, km Number of registered damages in the sections of network, n
50 0.400 15
100 14.490 398
150 12.110 234
Cast iron (grey iron) 200 9.920 174
250 11.095 96
300 12.760 98
Total 60.775 1015
50 1.0500 40
100 9.979 177
150 4.3900 64
Steel 200 1.1410 14
250 5.5000 57
300 6.6800 63
Total 28.74 415
Corrosion 9.0 %
Damaged by excavation equipments0 %
Cement outlet from bell joints 68.0 %
Corrosion 7.0 %
Damaged by excavation equipment 4.0 %
Iraiisvers e
fistulas
71.0%
b
Fig. 2. Types of damage to metal water pipes in the city of Kremenchuk (Ukraine): a — cast iron pipe (grey cast iron); b — steel pipes
5. 2. Quantitative analysis of the damage to pipes
As the main indicator of failure-free operation of metal water pipes we accepted the mean time to failure T and the magnitude inverse to it - the average value of failure rate parameter ra. In order to calculate reliability of failure-free operation in the sections of water network, it is necessary to apply a specific parameter of failure rate ra0 - this is a failure rate parameter relative to 1 km of a pipeline. To calculate the mean value of specific failure rate ra0, we applied formula:
(1)
t X L
Study into the failures in water-supply networks of the examined object revealed that over the total length of cast-iron water pipes of 60.775 km, the number of failures reached 1015, and in the steel water pipes of length 28.74 km - 415.
5. Results of examining the failure-free operation of metal water supply pipes
5. 1. Qualitative analysis of the damage to pipes
An analysis of statistical data allowed us to highlight the main types of damage to the metal water pipes (Fig. 2). For the cast-iron pipes:
- cement outlet from bell joints - 68 %;
- transverse fractures - 19 %;
- corrosion - 9 %;
- damaged by excavation equipment - 4 %.
For the steel pipes:
- transverse fistulas - 71 %;
- breach of welds - 18 %;
- corrosion - 7 %;
- damaged by excavation equipment - 4 %.
For the cast-iron pipes, the largest damage is done by the cement outlet from bell joints (68 %). For the steel pipelines, the biggest factor in failures is the transverse fistulas (71 %).
where n is the number of failures of section of a water network; t is the duration of observation; ZL is the total length of sections of the water network of corresponding diameter, km.
Specialists in water suply [4, 8] adopted a statistical hypothesis of exponential distribution of time to failure T per 1 km of a pipeline. An analysis of statistical data on water supply of the examined object was conducted during a fixed period and the received values of specific failure rate parameter ra0 were random. Interval estimations for the specific failure rate parameter were calculated by the following formulas:
- the lower interval estimation of failure rate parameter:
(2)
the upper interval estimation of failure rate parameter:
(3)
where ri, r2 are the coefficients to determine the interval estimates in the case of exponential distribution, which are accepted in accordance with [16] for confidence probability y = 0.95.
Results of calculations are given in Table 4.
a
n
0
M
Mn ' =
M
0
upper _
0=
M
Table 4
Calculation of interval estimates for specific failure rate parameter ra0
Network sections pipes Diameter D, mm Sample size n Confidence probability y Coefficients for determining the interval estimates Specific failure rate parameter mo, 1/year - km
lower r1 upper r2 mean value interval estimate
lower upper
Cast iron (grey iron) 50 15 0.95 1.4 0.74 5.36 3.83 7.24
100 398 1.09 0.918 3.92 3.60 4.27
150 234 1.15 0.87 2.76 2.40 3.17
200 174 1.19 0.85 2.51 2.11 2.95
250 96 1.23 0.822 1.24 1.00 1.50
300 98 1.225 0.824 1.10 0.90 1.33
Steel 50 40 0.95 1.33 0.755 5.44 4.09 7.21
100 177 1.16 0.868 2.53 2.18 2.92
150 64 1.28 0.79 2.08 1.63 2.64
200 14 1.4 0.74 1.75 1.25 2.37
250 57 1.32 0.77 1.48 1.12 1.92
300 63 1.4 0.738 1.35 0.96 1.83
The weighted average value of specific failure rate parameter ra0 regardless of diameter is calculated by formula:
ra0iLi + ®02L2 + ... + ^0nLn Lj + L2 +... + Ln
(4) Z
The weighted average value of specific failure rate parameter con regardless of diameter for a water supply system of the examined object is
co™d = 2.98 1 /year ■ km for the cast iron pipes;
C0olid = 2.06 1 /year ■ km for the steel pipes.
5. 3. Analytical dependences for the failure-free operation of metal pipes
As shown by an analysis of statistical data (Table 4), specific failure rate parameter of metal pipes decreases with increasing diameter. This allowed us to evaluate and predict the level of reliability of metal water pipes depending on a diameter.
As a result of mathematical processing of statistical data, we received dependences of specific failure rate parameter ra0 on the pipe diameter. Construction of functions based on statistical data is conducted using the «Microsoft Excel SR-1» software.
Dependence graphs of specific failure rate parameter ra0 on the pipe diameter are shown in Fig. 3, 4.
Cast iron pipes (grey cast iron)
g 8,0
ï g
<3 16,0
% Sa 5,o
. 4»
I -3,0
g 2.0 C5
fc 1,0
0,0
50
100 150 200 Pipe diameter, mm
250
300
8,0 7,0 6,0 5,0 4,0 3,0 2.0 1,0 0.0
SteeLpipes
\ upper estimate ♦ \ -
M„= 95,16D 8 75i
50
250
Fig. 4,
100 150 200 Pipe diameter, mm
Analytical dependence a0 = f(D) for the steel pipes
300
Fig. 3. Analytical dependence a0=f(D) for the cast iron pipes
The obtained analytical functions for the mean values of specific failure rate parameter ra™d for a water supply network of the examined object take the form:
- for the cast iron pipes rayid = 206.5D-0888;
- for the steel pipes ra{Tid = 95.16D-0 756, where D is the pipe diameter in mm.
6. Discussion of results of examining the failure-free operation of metal water pipes
6. 1. A comparative analysis of causes of damage to water pipes
Determining the main factors of failures of water supply metal pipes is the actual direction of research into reliability on the territory of Ukraine. Condition of water-supply pipelines is characterized by a high percentage of deterioration (over 90 %) and low rates of renewal of water supply networks.
The main causes of failures in metal pipes of water supply networks (Fig. 2) allowed us to establish that the dominant factors are the following:
- for the cast iron (grey iron) pipes - transverse fractures and cement outlet from bell joints;
- for the steel pipes - transverse fistulas, and damage of welded joints.
The results obtained on the causes of failures of the cast-iron and steel water pipes do not disagree with data from the articles of known foreign authors [17, 18]. According to [17, 18], based on statistical data of accidents from five water supply companies, the physical and environmental factors that lead to failures in water-supply networks were established. An analysis of basic damage made it possible to categorize and isolate the main types of damage that are similar to those explored in Ukraine:
- for the cast-iron pipes (grey cast iron): longitudinal crack and pitting corrosion;
- for the steel pipes: transverse fistulas.
As a result of quantitative analysis of statistical data on failures of water metal pipes we obtained numerical values and interval estimates for specific failure rate parameter in a range of diameters D=50...300. We also received weighted average values of failure rate parameter ra0 regardless of diameter for the water supply system in the city of Kremen-chuk by formula (4). An analysis of published studies by other authors [9, 19] allowed us to perform a comparison of failure-free operation of water supply pipes by specific failure rate parameter ra0. Comparison is represented by diagrams (Fig. 5), which show a significant difference in the reliability of pipes in different countries. Comparison to [9, 19] reveals that the failure-free operation of Ukrainian metal water pipes is much lower than that in Sweden and Germany and about 2 times lower than that in Poland.
2,5
2 -
1,5 -
=
a
0,5
Fig.
□ cast iron pipes (grey cast iron)
□ steel pipes
Sweden
Germany
Poland (the city Ukraine (the city of Jaslo) of Kremenchuk)
5. Comparison of the failure free operation of pipes by the parameter of failure rate
An analysis of statistical data on failures allowed us to build dependences ra0 = f(D) and subsequently predict the failure free operation of water supply networks for other assortments of metal pipes. Fig. 6, 7 show analytical dependence graphs for the city of Kremenchuk (Ukraine) and other cities from foreign countries [9, 10]. The obtained dependences confirmed a known fact that with an increase in the diameter of metal pipes, specific failure rate parameter decreases. However, analytical dependences take different forms for the specified cities. This can be caused by either insufficient accuracy of registering the damage or there are local operational factors, which are different in the indicated cities (pressures in network pipes, soils, hydrogeological conditions, etc.).
Obtained dependences of the specific failure rate parameter on the diameter of pipes (Fig. 6, 7) allow us to easily perform calculations of reliability of pipe lines for other assortments of diameters of pipes.
7 6 -
В * -3
к 4
w
2 -1 -
0
Cast iron pipes (grey cast iron)
Fig.
б i
i E i
1 J3 *
t b
£ я
ai ч _
J_ -С1
E -
Я
* 1 H
100 150 200 Diameter, mm
6. Analytical dependences ra0=f(D) for the cast iron pipes
300
Steel pipes
+ Kremenchuk (Ukraine)
j G$id= 95,16 D*™
Moscow (Russia) ^s.
Babylon (Iraq)
fif = 192,3D-1'1
150 200
Diameter, mm
Fig. 7. Analytical dependences ra0 = f(D) for the steel pipes
7. Conclusions
1. An analysis by the methods of mathematical statistics of data on the failures of metal water pipes in the city of Kremenchuk, Poltava oblast (Ukraine) confirmed the fact that with an increase in the diameter of metal pipes, specific failure rate parameter decreases.
2. Conducted statistical analysis allowed us to isolate the main types of damage to the metal water pipes. For the cast-iron pipes:
- cement outlet from joints - 68 %;
- transverse fractures - 19 %;
- corrosion - 9 %;
- damaged by excavation equipment - 4 %.
For the steel pipes:
- transverse fistulas - 71 %;
- breach of welded joints - 18 %;
- corrosion - 7 %;
- damaged by excavation equipment - 4 %.
3. Analytical dependences obtained for the mean value of specific failure rate parameter of metal pipes take the following form:
- the cast-iron pipes (grey cast iron)
rnmid = 206.5D-0888 1/yearkm;
- the steel pipes
rnmid = 95.16D-0756 1/yearkm,
where D is the pipe diameter in mm.
The obtained dependences allow us to calculate the frequency of failures in the sections of a network D = 50...300.
4. Generalized mean values of specific failure rate parameter ramld regardless of diameter for the city of Kremen-chuk, Poltava oblast (Ukraine) make up:
- for the cast-iron pipes (grey cast iron)
ramid = 2.98 1/year km;
- for the steel pipes ramid = 2.06 1/yearkm.
5. A comparison we performed to foreign data reveals that the failure free operation of Ukrainian metal water pipes (cast-iron pipes - 2.98 1/year ■ km, steel pipes - 2.06 1/year ■ km) is much lower than that in the Western European countries:
- Poland (cast iron pipes - 1.75 1/year ■ km, steel pipes -1.43 1/year km);
- Germany (cast iron pipes - 0.25 1/year ■ km, steel pipes - 0.18 1/year ■ km);
- Sweden (cast iron pipes - 0.1 1/year km, steel pipes -0.113 1/year km).
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