Availability
Availability refers to the readiness of a system or service to be used when needed. In the context of technology and engineering, availability often pertains to the reliability and uptime of systems, networks, and applications. It is a critical aspect of ensuring that technology resources are accessible and operational for users, minimizing downtime and disruptions.
6 Key excerpts on "Availability"
eBook - ePub- Louis J. Gullo, Jack Dixon, Louis J. Gullo, Jack Dixon(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
...15 Design for Availability James Kovacevic 15.1 Introduction Would you like to know if your assets are in an operational state when they are needed and can be used at any given point in time? If so, then you are interested in Availability. Availability is a measure of the degree to which an item is in an operable state and can be committed at the start of a mission when the mission is called for at an unknown (random) point in time [ 1 ]. Availability considers the time difference between the times when an asset is expected to be operating compared with the time when an asset is not able to be operated. In addition, Availability may also be defined as the percentage of time that the asset is in an operable state. Typically, Availability is measured at the asset level, which may be the system level, the platform level, the equipment level, or the product level. The definition and configuration of the asset must be clearly understood before considering Availability as a requirement. Availability is a covariance function of how reliable the asset is and how easy the asset is to maintain. Reliability is based on how often failures of the asset occur. Maintainability is not only how quickly the asset can be restored to a functional state, but also considers how often preventive maintenance (PM) is performed, and how long the asset is down for the planned and unplanned maintenance. One form of Availability considers any logistical delays in restoring the asset to a functional state or while performing preventive maintenance. Occasionally, individuals may confuse Availability with reliability. To ensure the difference is clear, here is a simple way to remember the differences: Availability focuses on time utilization Maintenance focuses on process restoration Reliability focuses on failure elimination 15.2 What is Availability? Availability comes in many different variations, each with its own use...
eBook - ePubDefence Logistics
Enabling and Sustaining Successful Military Operations
- Jeremy Smith, Jeremy Smith(Authors)
- 2018(Publication Date)
- Kogan Page(Publisher)
...Examples will be used to demonstrate the importance of reliability, maintainability and particularly logistics, in achieving Availability and capability. Availability Availability in general There are different definitions of Availability and equations for how it can be calculated. As a start point, in essence users of equipment want that equipment to work when required. Availability is effectively all the time during which the equipment has not failed, so, for illustration: the washing machine is plumbed in and ready to work when needed; the main battle tank is in the armoured squadron’s assembly area, serviceable, fuelled, and ready for use when the commander needs it; the aircraft is on the flight line ready to be tasked for operations. Some authors write about the system functioning (Kumar, 2000) or about its functionability (Knezevic, 1997), but however Availability is expressed, the system is where it’s required, ready to perform the functions that the users demanded of it when they articulated their requirements (see discussion of user requirements and system requirements in Chapter 6). There are three main types of Availability within procurement and support, and each has a different meaning and is calculated in a different way. Hence, their use is different. The three types are: intrinsic Availability (A i), which is also known as inherent Availability; achieved Availability (A a) ; and operational Availability (A o). For the user of the equipment operational Availability is the most important, but is also the most difficult to calculate. In very general terms, Availability is calculated as shown in Equation 8.1. From this point forward, this general expression of Availability will be referred to as classic Availability. In simple terms, the uptime is when the equipment is performing its designed function...
eBook - ePubPractical Power Plant Engineering
A Guide for Early Career Engineers
- Zark Bedalov(Author)
- 2020(Publication Date)
- Wiley-IEEE Press(Publisher)
...Many times, this is the type of Availability that companies use to report the Availability of their products (e.g. computer servers) because they see downtime other than actual repair time as out of their control and too unpredictable. The CM reflects the efficiency and speed of the maintenance personnel, as well as their expertise and training level. It also reflects the characteristics that should be of importance to the engineers who design the system, such as the complexity of necessary repairs, ergonomics factors, and whether ease of repair (maintainability) was adequately considered in the design. For a single component, the inherent Availability can be computed by (21.13) (21.14) MTBF = uptime/number of system failures MTTR = CM downtime/number of system failures The higher the MTBF value is, the higher the reliability and Availability of the system. MTTR affects Availability. This means that if it takes a long time to recover a system from a failure, the system is going to have a low Availability. High Availability can be achieved if MTBF is very large compared to MTTR. One should keep in mind that until steady state is reached, the MTBF calculation may be a function of time (e.g. a degrading system). In such cases, before reaching steady state, the calculated MTBF changes as the system ages and more data are collected. Thus, the above formulation should be used cautiously. Furthermore, it is important to note that the MTBF defined here is different from the MTTF (or more precisely for a repairable system, MTTFF : mean time to first failure). 21.6.2 Operational Availability, A o Operational Availability is a measure of the “real” average Availability over a period of time and includes all experienced sources of downtime, such as administrative downtime, logistic downtime. The operational Availability is the Availability that the customer actually experiences. It is essentially the a posteriori Availability based on actual events that happened to the system...
eBook - ePub- Andrew P. Sage, William B. Rouse(Authors)
- 2011(Publication Date)
- Wiley-Interscience(Publisher)
...For systems that are continually used and are providing useful output, Availability is often estimated by calculating the fraction of total “need time” that the system is operational or capable of providing useful output. Operational Availability addresses the system’s readiness to perform its intended function at a particular point in time. The difference between a system’s being up or down, however, is often a function of the customer’s definition of failure, which depends on the use of the system. Thus, if the performance related to a critical attribute is not satisfactory, the customer may consider the system to be “down,” and readiness or Availability from that point until the need ends, or until the deficiency is corrected, is zero. For example, if a radar has a specified range of 50 miles, is the radar considered down if it is effective only to 45 miles? If the 50-mile range is the absolute minimum needed to avoid midair collisions, the aircraft on which the radar is installed would be considered unflyable, and the radar would be considered unavailable for the mission. If the 50-mile range is a goal value and 20 miles is the absolute minimum, a 45-mile range might be acceptable. An Availability calculation could be based on a definition that includes as uptime all periods for which the range is at least 20 miles. In summary, reliability translates into a demand for support resources, while maintainability translates into the range of resources (i.e., people, space) and the time required to support the system operation. The interaction of reliability and maintainability results in the need for support assets to maintain a level of operational readiness or Availability over the time desired by the system user. REFERENCES Bowles, J. B. (2003). Fundamentals of failure modes and effects analysis. In: Tutorial Notes of the Annual Reliability and Maintainability Symposium. Coudert, O., and Madre, J. C. (1993). Fault tree analysis: 1020 prime implicants and beyond...
eBook - ePubElectronic Measurement Systems
Theory and Practice
- A.F.P Van Putten(Author)
- 2019(Publication Date)
- Routledge(Publisher)
...In short, the probability of every possible transfer state has to be defined and calculated. For instance, given that the system is working normally in a certain state, the question ‘what is the probability that the system will achieve another state?’ has to be answered. Therefore forward and backward differential equations in the time domain have to be developed, whereas in non-maintained systems only forward transitions have to be considered. This approach results in an additional figure of merit to describe the reliability of maintainable systems, the so-called Availability, often denoted by A (t). In general, Availability is also a function of time. The Availability of the considered maintainable system is defined as the probability that the system is in an operational state at any time t given that the system was fully operational at the time t = 0. An alternative definition, more easily understood, is based on the ratio of the time that the system is operational, the so-called ‘uptime’, to the total amount of time that the system is needed. If the time for maintenance and repair is called the ‘downtime’, we can write Availability = uptime uptime + downtime. (5.14) The downtime is defined as the product of the number of maintenance and repair actions which require a shutdown of the system and the average time for each such action. The average time for each action is called the mean time to repair (MTTR), and this concept results in the following expression for the Availability: Availability = MTBF MTBF + MTTR. (5.15) If the MTBF and thus the reliability increase, the MTTR can also increase; if the MTBF decreases, the average time for a repair action must also decrease in order to maintain the same Availability. In a great majority of cases, involving many different types of component, equipment and system, here again an exponential distribution of the repair time can be justified...
eBook - ePub- Patrick O'Connor, Andre Kleyner(Authors)
- 2011(Publication Date)
- Wiley(Publisher)
...This can be by servicing, such as cleaning and lubrication, or by inspection to find and rectify incipient failures, for example by crack detection or calibration. Preventive maintenance affects reliability directly. It is planned and should be performed when we want it to be. Preventive maintenance is measured by the time taken to perform the specified maintenance tasks and their specified frequency. Maintainability affects Availability directly. The time taken to repair failures and to carry out routine preventive maintenance removes the system from the available state. There is thus a close relationship between reliability and maintainability, one affecting the other and both affecting Availability and costs. The maintainability of a system is clearly governed by the design. The design determines features such as accessibility, ease of test, diagnosis and repair and requirements for calibration, lubrication and other preventive maintenance actions. This chapter describes how maintainability can be optimized by design, and how it can be predicted and measured. It also shows how plans for preventive maintenance can be optimized in relation to reliability, to minimize downtime and costs. 16.2 Availability Measures In order to analyse a system's Availability it needs to be measured. Depending on the available data and the objectives of the analysis Availability can be expressed in several different ways. 16.2.1 Inherent Availability Inherent Availability is the steady state Availability which considers only the corrective maintenance (CM) (covered in Section 6.7). Assuming that CM actions occur at a constant rate, it can be estimated as: (16.1) MTBF is the mean time between failure and MTTR is the mean time to repair (Chapter 6) which is the same as mean corrective maintenance time. 16.2.2 Achieved Availability Achieved Availability is very similar to inherent Availability with the exception that PM downtimes are also included...
