- Nuclear Eng Design revised from Sandia Natl Lab Rep SAND88-2253C, and Nuclear Regulatory Commission Rep NUREG/CP-0097 5, pp 1–25 Pahl G, Beitz W (1996) Engineering design. - Elsevier, New York Simon HA (1981) The sciences of the artificial. - 294 3 Reliability and Performance in Engineering Design Steele AD, Leitch RR (1996) A strategy for qualitative model-based diagnosis. - Zadeh LA (1973) Outline of a new approach to the analysis of complex systems and decision processes. - Availability and Maintainability in Engineering Design. - Abstract Evaluation of operational engineering availability and maintainability is usually considered in the detail design phase, or after installation of an engineering design. - It deals with the prediction and assessment of the design’s availability, or the probability that a system will be in operational service during a scheduled operating period, as well as the design’s maintainability, or the probability of system restora- tion within a specified downtime. - This chapter considers in detail the concepts of availability and maintainability in engineering design, as well as the various criteria essential to designing for availability and designing for maintainability. - Availability in engineering design has its roots in designing for reliability. - If the design includes a durability feature related to its availability and reliability, then it fulfils, to a large extent, the requirements for engineering design integrity. - Availability in engineering design is thus considered from the perspective of the design’s functional and opera- tional characteristics, and designing for availability, particularly engineering process availability, considers measurements of process throughput, output, input and cap- acity. - Designing for availability is a ‘top-down’ approach from the design’s systems level to its equipment or assemblies level whereby constraints on the design’s func- tional and operational performance are determined. - Maintainability in engineering design is the relative ease and economy of time and resources with which an engi- neered installation can be retained in, or restored to, a specified condition through scheduled and unscheduled maintenance. - In this context, maintainability is a func- tion of engineering design. - Therefore, designing for maintainability requires that the installation is serviceable and can be easily repaired, and also supportable in that it can be cost-effectively and practically kept in or restored to a usable condition.. - Maintainability is fundamentally a design parameter, and designing for maintain- ability defines the time an installation could be inoperable.. - Maintainability and Safety in Engineering Design, c Springer 2009. - 296 4 Availability and Maintainability in Engineering Design. - The foregoing chapter dealt with the analysis of engineering design with respect to the prediction, assessment and evaluation of reliability and systems functional per- formance, without considering repair in the event of failure. - This chapter deals with repairable systems and their equipment in engineering design, which can be restored to operational service after failure. - Evaluation of operational availability and maintain- ability is normally considered in the detail design phase, or after installation of the engineering design, such as during the design’s operational use or during process ramp-up and production in process engineering installations.. - Availability in engineering design has its roots in designing for reliability as well as designing for maintainability, in which a ‘top-down’ approach is adopted, pre- dominantly from the design’s systems level to its equipment level (i.e. - Avail- ability in engineering design was initially developed in defence and aerospace de- sign (Conlon et al. - 1982), whereby availability was viewed as a measure of the degree to which a system was in an operable state at the beginning of a mission, whenever called for at any random point in time.. - The ability to answer this question for a particular system and its equipment rep- resents a powerful concept in engineering design integrity, with resulting additional side-benefits. - One important benefit is the ability to use availability analysis during the engineering design process as a platform to support design for reliability and de- sign for maintainability parameters, as well as trade-offs between these parameters.. - Availability is intrinsically defined as “the probability that a system is operating satisfactorily at any point in time when used under stated conditions, where the time considered includes the operating time and the active repair time” (Nelson et al. - While this definition is conceptually rather narrow, especially concerning the repair time, the thrust of the approach of availability in engineering design is to initially consider inherent availability in contrast to achieved and operational avail- ability of processes and systems. - A more comprehensive approach would need to include a measure for the quantification of uncertainty, which involves considering the concept of availability as a decision analysis problem. - Economic incentive is the primary basis for the growing interest in more deliberate and systematic availability analysis in engineering design.. - Ensuring a proper analysis in the determination of availability in engineering de- sign is one of the few alternatives that design engineers may have for obtaining an increase in process and/or systems capacity, without incurring significant increases in capital costs. - From the definition, it is evident that any form of availability anal- ysis is time-related.. - Figure 4.1 illustrates the breakdown of a total system’s equipment time into time- based elements on which the analysis of availability is based. - It must be noted that the time designated as ‘off time’ does not apply to availability analysis because, during this time, system operation is not required. - It has been included in the il- lustration, however, as this situation is often found in complex integrated systems, where the reliability concept of ‘redundancy’ is related to the availability concept of. - Up Time + Down Time (4.1) Analysis of availability is accomplished by substituting the time-based elements defined above into various forms of the basic relationship, where different combi- nations formulate various definitions of availability.. - Designing for availability predominantly considers whether a design has been configured at systems level to meet certain availability requirements based on spe- cific process or systems operating criteria. - Designing for availability is mainly con- sidered at the design’s systems and higher equipment level (i.e. - assembly level, and not component level), whereby availability requirements based on expected sys- tems performance are determined, which eventually affects all of the items in the systems hierarchy. - Similar to designing for reliability, this approach does not de- pend on having to initially identify all the design’s components, and is suitable for the conceptual or preliminary design stage (Huzdovich 1981).. - 298 4 Availability and Maintainability in Engineering Design However, it is observed practice in most large continuous process industries that have complex integrations of systems, particularly the power-generating industry and the chemical process industries, that the concept of availability is closely related to reliability, whereby many ‘availability’ measures are calculated as a ‘bottom-up’. - In such cases, availability in engineering design is approached from the design’s lower levels (i.e. - assembly and/or component levels) up the systems hi- erarchy to the design’s higher levels (i.e. - Clearly, this ap- proach is feasible only once all the design’s equipment have been identified, which is well into the detail design stage.. - In order to establish the most applicable methodology for determining the in- tegrity of engineering design at different stages of the design process, particularly with regard to the development of designing for availability, or to the assessment of availability in engineering design (i.e. - ‘top-down’ or ‘bottom-up’ approaches in the systems hierarchy respectively), some of the basic availability analysis techniques applicable to either of these approaches need to be identified by definition and con- sidered for suitability in achieving the goal of this research.. - Furthermore, it must also be noted that these techniques do not represent the total spectrum of availability analysis, and selection has been based on their application in conjunction with the selected reliability techniques, (reliability prediction, assess- ment and evaluation), in order to determine the integrity of engineering design at the relative design phases.. - The definitions of availability are qualitative in distinction, and indicate signifi- cant differences in approaches to the determination of designing for availability at different levels of the systems hierarchy, such as:. - evaluation of operational availability based on measures of time that are subject to delays, particularly with respect to anticipated values of administrative and logistics downtime.. - Maintainability in engineering design is described in the USA military handbook. - ‘Designing and developing maintainable products and systems’ (MIL-HDBK-470A 1997) as “the relative ease and economy of time and resources with which an item can be retained in, or restored to, a specified condition when maintenance is per- formed by personnel having specified skill levels, using prescribed procedures and resources, at each prescribed level of maintenance and repair. - Maintainability refers to the measures taken during the design, development and manufacture of an engineered installation that reduce the required maintenance, re- pair skill levels, logistic costs and support facilities, to ensure that the installation meets the requirements for its intended use. - A key consideration in the maintain-. - Therefore, the inherent maintainability char- acteristics of the system and its equipment must be assured. - Designing for maintainability.. - Maintainability analysis includes the prediction as well as the assessment and eval- uation of maintainability criteria throughout the engineering design process, and would normally be implemented by a well-defined program, and captured in a main- tainability program plan (MPP).. - Maintainability analysis differs significantly from one design phase to the next, particularly with respect to a systems-level approach during the early conceptual and schematic design phases, in contrast to an equipment-level approach during the later schematic and detail design phases. - These differences in approach have a significant impact on maintainability in engineering design as well as on contrac- tor/manufacturer responsibilities. - Maintainability, from a maintenance perspective, can be defined as “the proba- bility that a failed item will be restored to an operational effective condition within a given period of time”.. - The item’s operational effective condition in this context is also considered to be the item’s repairable condition. - Corrective maintenance action is the action to rectify or set right defects in the equipment’s operational and physical conditions, on which its functions depend, in accordance with a standard. - 300 4 Availability and Maintainability in Engineering Design through restorative corrective maintenance action through some or other repair ac- tion. - Maintainability is thus a measure of the repairable condition of an item that is determined by MTTR, and is established through corrective maintenance action.. - Maintainability modelling for a repairable system is, to a certain extent, a form of applied probability analysis, very similar to the probability assessment of uncer- tainty in reliability. - It can be compared to the problem of determining the occupancy, arrival and service rates in a queue, where the service performed is repair, the server is the maintenance func- tion, and the patrons of the queue are the systems and equipment that are repaired at random intervals, coincidental to the random occurrences of failures.. - Applying maintainability models enhances the capability of designing for main- tainability through the appropriate consideration of design criteria such as visibil- ity, accessibility, testability and interchangeability. - Using maintainability prediction techniques, as well as specific quantitative maintainability analysis models relating to the operational requirements of a design can greatly enhance not only the in- tegrity of engineering design but also the confidence in the operational capabilities of a design. - Maintainability predictions of the operational requirements of a design during its conceptual design phase can aid in design decisions where several de- sign options need to be considered. - Quantitative maintainability analysis during the schematic and detail design phases consider the assessment and evaluation of main- tainability from the point of view of maintenance and logistics support concepts.. - Designing for maintainability requires a product that is serviceable (must be easily repaired) and supportable (must be cost-effectively kept in, or restored to, a usable condition). - If the design includes a durability feature related to avail- ability (degree of operability) and reliability (absence of failures), then it fulfils, to a large extent, the requirements for engineering design integrity. - Maintainability is primarily a design parameter, and designing for maintainability defines how long the equipment is expected to be down. - Serviceability implies the speed and ease of maintenance, whereby the amount of time expected to be spent by an appropriately trained maintenance function working within a responsive supply system is such that it will achieve minimum downtime in restoring failed equipment. - In designing for maintainability, the type of maintenance must be considered, and must have an influential role in considering serviceability.. - For example, the stipulation that a system should be capable of being isolated to the component level of each circuit card in its control sub-system may not be justified if a faulty circuit card is to be replaced, rather than repaired. - 4.1 Introduction 301 Supportability has a design subset involving testability, a design characteristic that allows verification of the operational status to be determined and faults within the system’s equipment to be isolated in a timely and effective manner. - Designing for maintainability also needs to take cognisance of the item’s opera- tional durability whereby the period (downtime) in which equipment will be down due to unavailability and/or unreliability needs to be considered. - Relevant criteria in designing for maintainability need to be verified through maintainability design reviews. - These design reviews are conducted dur- ing the various design phases of the engineering design process, and are critical components of modern design practice. - The primary objective of maintainability design reviews is to determine the relevant progress of the design effort, with par- ticular regard to designing for maintainability, at the completion of each specific design phase. - design reviews concerned with designing for reliability, availability, maintainability and safety), maintainability de- sign reviews fall into three distinct categories: initial or conceptual design reviews, intermediate or schematic design reviews, and final or detail design reviews (Hill 1970).. - Initial or conceptual design reviews need to be conducted immediately after for- mulation of the conceptual design, from initial process flow diagrams (PFDs). - The purpose is to carefully examine the functionality of the intended design, feasibility of the criteria that must be met, initial formulation of design specifications at process and systems level, identification of process design constraints, existing knowledge of similar systems and/or engineered installations, and cost-effective objectives.. - Intermediate or schematic design reviews need to be conducted immediately af- ter the schematic engineering drawings are developed from firmed-up PFDs and initial pipe and instrument diagrams (P&IDs), and when primary specifications are fixed. - Final or detail design reviews, referred to as the critical design review (Carte 1978), are conducted immediately after detailed engineering drawings are devel- oped for review (firmed PFDs and firmed P&IDs) and most of the specifications have been fixed. - This review considers evaluation of design integrity and due diligence, hazards analyses (HazAns), value engineering, manufacturing meth- ods, design producibility/constructability, quality control and detail costing.. - The essential criteria that need to be considered with maintainability design re- views at the completion of the various engineering design phases include the follow- ing (Patton 1980):. - 302 4 Availability and Maintainability in Engineering Design. - Evaluation of maintainability trade-off studies. - Evaluation of FMEA results. - Design for maintainability specifications. - Evaluation of physical design factors. - Evaluation of facilities design dictates. - Evaluation of maintenance design dictates. - Evaluation of maintenance strategies. - Evaluation of logistic support facilities.. - 4.2 Theoretical Overview of Availability and Maintainability in Engineering Design. - For repairable systems, availability is generally considered to be the ratio of the actual operating time, to the scheduled operating time, exclusive of preventive or planned maintenance. - Because of this, closed formulae for determin- ing confidence in the case of a twofold uncertainty are not easily established, even in the simplest case when both failure and repair events are exponential. - It is for this reason that the application of Monte Carlo simulation is resorted to in the analysis
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