3.4.3 Application Modelling Outcome The acid plant failure data do not suitably fit the Weibull distribution, with 89% model accuracy.. Engineered Installation Downtime Table 3.27 Total
Trang 1Fig 3.83 Design specification FMECA—final absorption tower
Total failures + suspensions = 72
Mean time to failure (MTTF) = 2.35 (days)
The Kolmogorov–Smirnov goodness-of-fit test The Kolmogorov–Smirnov (K–S)
test is used to decide if a sample comes from a population with a specific distribu-tion The K–S test is based on the empirical distribution function (e.c.d.f.) whereby,
given N ordered data points Y1,Y2, YN, the e.c.d.f is defined as:
where n (i) is the number of points less than Yi, and the Yiare ordered from smallest
to largest value This is a step function that increases by 1 /N at the value of each
ordered data point An attractive feature of this test is that the distribution of the K–S test statistic itself does not depend on the cumulative distribution function being tested Another advantage is that it is an exact test; however, the goodness-of-fit test depends on an adequate sample size for the approximations to be valid The K–S test has several important limitations, specifically:
• It applies only to continuous distributions.
• It tends to be more sensitive near the centre of the distribution than at the tails.
Trang 2284 3 Reliability and Performance in Engineering Design
Table 3.26 Acid plant failure data (repair time RT and time before failure TBF)
Failure time RT
(min)
TBF (day)
Failure time RT
(min)
TBF (day)
Failure time RT
(min)
TBF (day)
7/31/01 0:00 148 1 9/29/01 0:00 346 2 11/20/01 0:00 336 10
8/13/01 0:00 580 1 10/10/01 0:00 52 2 12/6/01 0:00 124 1 8/14/01 0:00 897 1 10/10/01 0:00 39 0 12/11/01 0:00 25 5 8/15/01 0:00 895 1 10/11/01 0:00 55 1 12/12/01 0:00 120 1 8/16/01 0:00 498 1 10/12/01 0:00 36 1 12/17/01 0:00 35 5 8/17/01 0:00 308 1 10/14/01 0:00 10 2 12/26/01 0:00 10 9 8/19/01 0:00 21 2 10/18/01 0:00 1,440 4 1/2/02 0:00 42 7 8/21/01 0:00 207 2 10/19/01 0:00 590 1 1/18/02 0:00 196 16
8/23/01 0:00 110 1 10/24/01 0:00 107 2 2/9/02 0:00 455 11 8/25/01 0:00 26 2 10/29/01 0:00 495 5 2/10/02 0:00 435 1
• The distribution must be fully specified—that is, if location, scale, and shape
parameters are estimated from the data, the critical region of the K–S test is no longer valid, and must be determined by Monte Carlo (MC) simulation.
Goodness-of-fit results The K–S test result of the acid plant data given in
Table 3.26 is the following:
Kolmogorov-Smirnov (D) statistic = 347
Critical value of modified D = 1.094
Tabled values of K–S statistic = 0.113 0.122 0.132 0.141
Mean absolute prob error = 0.1058
The hypothesis that the data fit the two-Weibull distribution is rejected with 99% confidence.
Trang 3Fig 3.84 Weibull distribution chart for failure data
Three-parameter Weibull fit—ungrouped data (Fig 3.84):
Shape parameter BETA = 1.63
Scale parameter ETA = 1.74 (days)
Characteristic life = 2.21 (days)
Standard deviation = 0.98 (days)
Test for random failures The hypothesis that failures are random is rejected at 5%
level.
3.4.3 Application Modelling Outcome
The acid plant failure data do not suitably fit the Weibull distribution, with 89% model accuracy However, the failures are not random (i.e the failure rate is not constant), and it is essential to determine whether failures are in the early phase or
in the wear-out phase of the plant’s life cycle—especially so soon after its
Trang 4installa-286 3 Reliability and Performance in Engineering Design tion (less than 24 months) The distribution must be fully specified—that is, the K–S test is no longer valid, and must be determined by Monte Carlo (MC) simulation However, prior to simulation, a closer definition of the source of most of the failures
of the critical systems (determined through the case study FMECA) is necessary Table 3.27 shows the total downtime of the acid plant’s critical systems The down-time failure data grouping indicates that the highest downdown-time is due to the hot gas feed induced draft fan, then the reverse jet scrubber, the drying tower blowers, and final absorption.
Engineered Installation Downtime
Table 3.27 Total downtime of the environmental plant critical systems
Downtime reason description Total hours Direct hours Indirect hours
Monte Carlo simulation With the K–S test, the distribution of the failure data must
be fully specified—that is, if location, scale and shape parameters are estimated from the data, the critical region of the K–S test is no longer valid, and must be determined
by Monte Carlo (MC) simulation.
MC simulation emulates the chance variations in the critical systems’ time be-fore failure (TBF) by generating random numbers that form a uniform distribution that is used to select values from the sample TBF data, and for which various TBF values are established to develop a large population of representative sample data The model then determines if the representative sample data come from a popula-tion with a specific distribupopula-tion (i.e exponential, Weibull or gamma distribupopula-tions) The outcome of the M C simulation gives the following distribution parameters (Tables 3.28 and 3.29):
Time Between Failure Distribution
Table 3.28 Values of distribution models for time between failure
Distribution model Parameter Parameter value
1 Exponential model Gamma 4.409E-03
2 Weibull model Gamma 1.548E+00
Trang 5Repair Time Distribution
Table 3.29 Values of distribution models for repair time
Distribution model Parameter Parameter value
1 Exponential model Gamma 2.583E-01
2 Weibull model Gamma 8.324E-01
The results of the MC simulation are depicted in Fig 3.85 The representative
sam-ple data come from a population with a gamma distribution, as illustrated The
me-dian (MTTF) of the representative data is given as approximately 2.3, which does not differ greatly from the MTTF for the three-parameter Weibull distribution for ungrouped data, which equals 2.35 (days) This Weibull distribution has a shape pa-rameter, BETA, of 1.63, which is greater than 1, indicating a wear-out condition in the plant’s life cycle.
Fig 3.85 Monte Carlo simulation spreadsheet results for a gamma distribution best fit of TBF data
Trang 6288 3 Reliability and Performance in Engineering Design
Conclusion From the case study data, the assumption can be made that the critical
systems’ specific high-ranking critical components are inadequately designed from
a design integrity point of view, as they indicate wear-out too early in the plant’s life cycle This is with reference to the items listed in Table 3.25, particularly the drying tower blowers’ shafts, bearings (PLF) and scroll housings (TLF), the hot gas feed induced draft fan (PFC), the reverse jet scrubber’s acid spray nozzles (TLF), the final absorption tower vessel and cooling fan guide vanes (TLF), and the IPAT SO3 cooler’s cooling fan control vanes (TLF).
Figure 3.85 shows a typical Monte Carlo simulation spreadsheet of the critical systems’ time before failure and MC results for a gamma distribution best fit of TBF data.
3.5 Review Exercises and References
Review Exercises
1 Discuss total cost models for design reliability with regard to risk cost estimation and project cost estimation.
2 Give a brief account of interference theory and reliability modelling.
3 Discuss system reliability modelling based on system performance.
4 Compare functional failure and functional performance.
5 Consider the significance of functional failure and reliability.
6 Describe the benefits of a system breakdown structure (SBS).
7 Give reasons for the application and benefit of Markov modelling (continuous-time and discrete states) in designing for reliability.
8 Discuss the binomial method with regard to series networks and parallel net-works.
9 Give a brief account of the principal steps in failure modes and effects analysis (FMEA).
10 Discuss the different types of FMEA and their associated benefits.
11 Discuss the advantages and disadvantages of FMEA.
12 Compare the significant differences between failure modes and effects analysis (FMEA) and failure modes and effects criticality analysis (FMECA).
13 Compare the advantages and disadvantages of the RPN technique with those of the military standard technique.
14 Discuss the relevance of FMECA data sources and users.
15 Consider the significance of fault-tree analysis (FTA) in reliability, safety and risk assessment.
16 Describe the fundamental fault-tree analysis steps.
17 Explain the basic properties of the hazard rate function and give a brief descrip-tion of the main elements of the hazard rate curve.
18 Discuss component reliability and failure distributions.
Trang 719 Define the application of the exponential failure distribution in reliability anal-ysis and discuss the distribution’s statistical properties.
20 Define the application of the Weibull failure distribution in reliability analysis and discuss the distribution’s statistical properties.
21 Explain the Weibull shape parameter and its use.
22 Discuss the significance of the Weibull distribution function in hazards analysis.
23 Describe the principal properties and use of the Weibull graph chart.
24 Consider the application of reliability evaluation of two-state device networks.
25 Describe the fundamental differences between two-state device series networks,
parallel networks, and k-out-of-m unit networks.
26 Consider the application of reliability evaluation of three-state device networks.
27 Briefly describe three-state device parallel networks and three-state device series networks.
28 Discuss system performance measures in designing for reliability.
29 Consider pertinent approaches to determination of the most reliable design in conceptual design.
30 Discuss conceptual design optimisation.
31 Describe the basic comparisons of conceptual designs.
32 Define labelled interval calculus (LIC) with regard to constraint labels, set la-bels, and labelled interval inferences.
33 Consider the application of labelled interval calculus in designing for reliability.
34 Give a brief description with supporting examples of the methods for:
a Determination of a data point: two sets of limit intervals.
b Determination of a data point: one upper limit interval.
c Determination of a data point: one lower limit interval.
d Analysis of the interval matrix.
35 Give reasons for the application of FMEA and FMECA in engineering design analysis.
36 Define reliability-critical items.
37 Describe algorithmic modelling in failure modes and effects analysis with re-gard to numerical analysis, order of magnitude, qualitative simulation, and fuzzy techniques.
38 Discuss qualitative reasoning in failure modes and effects analysis.
39 Give a brief account of the concept of uncertainty in engineering design analysis.
40 Discuss uncertainty and incompleteness in knowledge.
41 Give a brief overview of fuzziness in engineering design analysis.
42 Describe fuzzy logic and fuzzy reasoning in engineering design.
43 Define the theory of approximate reasoning.
44 Consider uncertainty and incompleteness in design analysis.
45 Give a brief account of modelling uncertainty in FMEA and FMECA.
46 In the development of the qualitative FMECA, describe the concepts of logical expression and expression of uncertainty in FMECA.
47 Give an example of uncertainty in the extended FMECA.
48 Describe the typical results expected of a qualitative FMECA.
Trang 8290 3 Reliability and Performance in Engineering Design
49 Define the proportional hazards model with regard to non-parametric model for-mulation and parametric model forfor-mulation.
50 Define the maximum likelihood estimation parameter.
51 Briefly describe the characteristics of the one-parameter exponential distribu-tion.
52 Explain the process of estimating the parameter of the exponential distribution.
53 Consider the approach to determining the maximum likelihood estimation (MLE) parameter.
54 Compare the characteristics of the two-parameter Weibull distribution with those of the three-parameter Weibull model.
55 Give a brief account of the procedures to calculate the Weibull parameters β , μ and γ
56 Describe the procedure to derive the mean time between failures (MTBF) μ from the Weibull distribution model.
57 Describe the procedure to obtain the standard deviation σ from the Weibull distribution model.
58 Give a brief account of the method of qualitative analysis of the Weibull distri-bution model.
59 Consider expert judgment as data.
60 Discuss uncertainty, probability theory and fuzzy logic in designing for reliabil-ity.
61 Describe the application of fuzzy logic in reliability evaluation.
62 Describe the application of fuzzy judgment in reliability evaluation.
63 Give a brief account of elicitation and analysis of expert judgment in designing for reliability.
64 Explain initial reliability calculation using Monte Carlo simulation.
65 Give an example of fuzzy judgment in reliability evaluation.
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