5.2. Software methodologies
5.2.3. A method to perform RAM analyses with software support
Availability and reliability issues have been comprehensively studied in various fields (e.g., Relex Software Corporation, 2009a). Very comprehensive studies have been executed in aeronautics and astronautics, electronics and especially in IT.
There are well-known applications in the process industries, including petrochemical plants, oil refineries, and power generation. A contribution was done by the Author that exploits the methodologies and software tools developed for these fields and assessed their implementation into MSWM (Sikos and Klemeš, 2008a).
One of the most promising ways to determine availability and reliability issues of systems is to examine failures. Waste management systems have many failure types including component failure, i.e. the unacceptable gap between the expected and actual performance of components (Ramachandran et al., 2005), mechanical failures of main equipment items, equipment blockages, control system failures (or malfunction), failures to detect faults, changeover failures, waste collection failures, lack of manpower, operator errors, instrument failures etc.
An important type is the service failure, which is a kind of failure that may result in fatalities, injuries to personnel, damage to property, shutdown of entire plants, loss of production, ecological problems such as release of hazardous materials.
The aim of availability analysis is to identify the items that might affect the operation of the system, from waste collection to waste treatment.
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The availability of waste management chains depends on the frequency of system component breakdowns and the required time for repairing them. Failure time and repair time are necessary to know, estimate or guess. Further data that are required are the time while the system operates normally before failure occurs, and the total time of system off-line for repair.
There is an advanced approach that is for the benefit of computer aided process engineering, namely modelling availability and reliability issues of waste management using computer software. A wide variety of general availability and reliability software packages are offered on the market. A proper choice should be made to reach the solution of related tasks.
A comprehensive study was conducted in order to provide guidelines and assessment for various software tools (Sikos and Klemeš, 2008b). Specialised programs that are designed for covering one specific problem only are rather rare.
The comprehensive reliability package Relex Reliability Studio (Relex Software Corpora-tion, 2009b) is one of the most promising candidates for reliability prediction and failure mode and effects analyses of MSW systems. It can also be applied for modelling LCC breakdown structures, manage maintenance tasks and generate fault trees, RBDs, event tree diagrams and tables, fault tree diagrams and tables, and Weibull trees (as described earlier in Chapter 3.4.11.1).
Figure 5.3 Scheme of waste management system tree.
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Table 5.2 Main system components (European Commission, 2006; EVECO Brno, 2009).
Subsystem Level 1 Subsystem Level 2 Typical component(s) pre-shipment waste analyser analytical laboratory waste analyser
waste receiver subsystem analytical laboratory, truck waiting area, gate, drum
preparation drum unloading and storage,
tank farm, bulk waste and waste preparation
drums, tanks waste treatment biological/physical/chemical
treatment, stabilisation plant steam generator, waste preparatory, regenerator solid outputs management biological/physical/chemical
treatment, stabilisation plant filters, landfill cells incinerator plant waste entry subsystem weighbridge
storage subsystem containers
pre-treatment subsystem evacuated boxes, grinding, mixing, filtration heat utilisation unit adsorption on carbon or
zeolitic sorbents, chemical absorption
staged flue gas cleaning gas cleaner process measuring and
control system multistage control system, data collection and storage, monitoring equipments complementary units desalting evaporator, turbine
power and backup power generator, laboratory equipment
off-gas and flue cleaning heat utilisation equipment heat exchangers equipment for collecting
particulate matter fabric filters, electrostatic precipitators, cyclone separators
equipment for disposal of
combustible polluting species combustion unit equipment for combined
biomass unit boiler house heat distribution appliances
steam generator steam distribution units power and heat cogeneration
unit heat cogenerator
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Figure 5.4 Availability diagram of the major parts of waste management chains.
The related issues can be analysed through many factors. The first step is to determine the list of system components and construct the system tree (Fig. 5.3). The Author studied the field and developed a methodology for availability and reliability analysis of waste management
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chains. The procedure covers data collection, the calculations of component and overall availability and reliability with availability table and diagram as well as RBD table and diagram.
Complete systems should be treated as collections of subsystems that contain components for system reliability calculations. The relationship between the reliability of waste management systems and that of their components is often oversimplified. A gross simplification is that the the sum of component reliabilities in any given time is often handled as the equalent of overall system reliability for that time which is definitely not true.
There are many availability and reliability related issues in waste management chains because these systems are quite complex. They have numerous subsystems and components (Table 5.2).
Using the list of subsystems and their components it is possible to generate the list of reliability blocks. Furthermore, the reliability block diagram of the complete system can be constructed.
Going one step forward, the reliability-wise arrangement of components is directly related to the mathematical description of the system, i.e. the system reliability function. It is a promising way to represent related data since the failure properties of components are best described by statistical distributions (Chapter 2.1.4).
The availability values of all the major parts have to be taken into account when constructing the availability block diagram of waste management chains. These values are the waste production availability, the collection and transport availability, the recycling/re-use availability, the treatment/processing availability and the availability of disposal facilities (Fig.
5.4).
All of these can be divided into further availability values of subsystems and components.
However, energy and material recycling are quite difficult to represent logically on the availability diagram.
A single block of the RBD might represent multiple identical blocks in series or parallel configuration by using the Multi Blocks feature of BlockSim. The items of Multi Blocks are separate entities with identical reliability characteristics.
This feature is useful because waste management systems generally contain both series and parallel configurations (except some really simple subsystems).
Figure 5.5 RBD of a waste management system.
Let us assume that the reliability block diagram of a waste management system is generated in BlockSim (Fig. 5.5). The main feature to consider is that there are both series and parallel parts within the system structure. In this case the system has 13 failure modes (A to M, between the initial state and the overall system reliability). A is the reliability of waste production; B is the bin reliability which is a requirement of the reliability of waste trucks (C).
Although most of the subsystem reliability values depend on machinery, service quality and materials, human factors should be considered as well (manpower reliability, D and K). The reliabilities of recycling and re-use are wide-ranging (reliability of E – material, F – energy, G – compost). The various treatments have their own reliability factors (H – mechanical treatment reliability, I – biological treatment reliability, J – chemical/thermal treatment reliability). The reliability of disposal facilities depends on the applied subsystem (L – reliability of landfill cells, M – emission treatment reliability). A failure of the entire system will occur if mode A
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occurs, modes B and C occur simultaneously or D occurs, modes E, F or G occur, modes H, I, J or K occur, or L or M occur.
When the structure is ready, the reliability equation can be obtained quite easily by using the identical reliability characteristics of system components. The type of subsystems should be taken into account.
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CHAPTER 6
CASE STUDIES
Several case studies were performed during the PhD research in order to test and prove the effectiveness of the introduced methodologies and achieve the least sophisticated picture possible.
Some of the studies were investigated as software evaluations, assessments or comparisons while others were industrial case studies that analysed real life failure data.
Three different configurations were used as testing environments:
• Configuration 1: AMD Athlon XP 1600+ 1.4 GHz, 512 MB RAM, 2x80 GB HDD 7200 RPM, Windows XP Professional
• Configuration 2: Intel Pentium 4 3 GHz, 1.25 GB RAM, 80 GB HDD 7200 RPM, Windows Vista Business/Fedora 10 Desktop Edition
• Configuration 3: Intel Core 2 Duo T7700 (4MB L2 cache/FSB667) 2.4 GHz, 2 GB RAM (667 MHz), 100 GB SATA HDD 7200 RPM, Windows Vista Ultimate
The reason was the consideration of the different computational capacities of older and newer computers.