Availability, reliability, maintainability, and capability

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  • Publicado : 7 de marzo de 2012
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Availability, reliability, maintainability, and capability are components of the effectiveness equation. The effectiveness equation is a figure of merit which is helpful for deciding which component(s) detract from performance measures. In many continuous process plants the reliability component is the largest detractor from better performance. Calculation of the components are illustrated by useof a small data set.


is defined by an equation as a figure-of-merit judging the opportunity for producing the intended results. The effectiveness equation is described in different formats (Blanchard 1995, Kececioglu 1995, Landers 1996, Pecht 1995, Raheja 1991). Each effectiveness element varies as a probability. Since components of the effectiveness equation have differentforms, it varies from one writer to the next. Definitions of the effectiveness equation, and it’s components, generate many technical arguments. The major (and unarguable economic issue) is finding a system effectiveness value which gives lowest long term cost of ownership using life cycle costs, (LCC) (Barringer 1996a and 1997) for the value received: System effectiveness = Effectiveness/LCC Costis a measure of resource usage. Lower cost is generally better than higher costs. Cost estimates never include all possible elements but hopefully includes the most important elements. Effectiveness is a measure of value received. Clements (1991) describes effectiveness as telling how well the product/process satisfies end user demands. Higher effectiveness is generally better than lowereffectiveness. Effectiveness varies from 0 to 1 and rarely includes all value elements as many are too difficult to quantify. One form is described by Berger (1993): Effectiveness = availability * reliability * maintainability * capability In plain English, the effectiveness equation is the product of: --the chance the equipment or system will be available to perform its duty, --it will operate for a giventime without failure, --it is repaired without excessive lost maintenance time and --it can perform its intended production activity according to the standard. Each element of the effectiveness equation requires a firm datum which changes with name plate ratings for a true value that lies between 0 and 1. Berger’s effectiveness equation (availability * reliability * maintainability * capability)is argued by some as flawed because it contains availability and components of availability (reliability and maintainability). Blanchard’s effectiveness equation (availability*dependability*performance) has


similar flaws. For any index to be successful, it must be understandable and creditable by the people who will use it. Most people understand availability and can quantify it. Few canquantify reliability or maintainability in terms everyone can understand. The effectiveness equation is simply a relative index for measuring “how we are doing”. Consider these elements of the effectiveness equation for refineries and chemical plants. In many continuous process industries, availability is high (~85 to 98%), reliability is low (~0.001 to 10%) when measured against turnaroundintervals, and maintainability is high (~50 to 90%) when measured against the allowed time for repairs, and productivity is high (~60 to 90%). So what does the effectiveness equation tell about these conditions? The one element destroying effectiveness is the reliability component (Barringer 1996b)—so it tells where to look for making improvements. Can the effectiveness equation be used to benchmarkone business to another? In theory yes, but in practice no. The practical problem lies in normalizing effectiveness data across companies and across business lines. For example, one plant may have an acceptable mission time for their equipment of one year, whereas a second plant may require a five year mission time because of their turnarounds. Similarly, one plant may set a repair time for a...
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