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availability
please note:
- the content below is remote from Wikipedia
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{{Short description|Term in reliability engineering}}{{distinguish|Availability (thermodynamics)|Availability heuristic}}{{Redirect|Available}}In reliability engineering, the term availability has the following meanings: - the content below is remote from Wikipedia
- it has been imported raw for GetWiki
- The degree to which a system, subsystem or equipment is in a specified operable and committable state at the start of a mission, when the mission is called for at an unknown, i.e. a random, time.
- The probability that an item will operate satisfactorily at a given point in time when used under stated conditions in an ideal support environment.
Representation
The simplest representation of availability (A) is a ratio of the expected value of the uptime of a system to the aggregate of the expected values of up and down time (that results in the “total amount of time” C of the observation window)
A = frac{E[mathrm{uptime}]}{E[mathrm{uptime}]+E[mathrm{downtime}]} = frac{E[mathrm{uptime}]}{C}
Another equation for availability (A) is a ratio of the Mean Time To Failure (MTTF) and Mean Time Between Failure (MTBF), or
A = frac{MTTF}{MTTF + MTTR} = frac{MTTF}{MTBF}
If we define the status function X(t) as
X(t)=
begin{cases}
1, & text{sys functions at time } t
0, & text{maintenance}
end{cases}
therefore, the availability A(t) at time t > 0 is represented by
1, & text{sys functions at time } t
0, & text{maintenance}
end{cases}
A(t)=Pr[X(t)=1]=E[X(t)]. ,
Average availability must be defined on an interval of the real line. If we consider an arbitrary constant c>0, then average availability is represented as
A_c = frac{1}{c} int_0^c A(t),dt.
Limiting (or steady-state) availability is represented byElsayed, E., Reliability Engineering, Addison Wesley, Reading, MA,1996
A = lim_{c rightarrow infty} A_c.
Limiting average availability is also defined on an interval [0,c] as,
A_infty =lim_{c rightarrow infty} A_c = lim_{c rightarrow infty}frac{1}{c} int_0^c A(t),dt,quad c > 0.
Availability is the probability that an item will be in an operable and committable state at the start of a mission when the mission is called for at a random time, and is generally defined as uptime divided by total time (uptime plus downtime).Methods and techniques to model availability
Reliability Block Diagrams or Fault Tree Analysis are developed to calculate availability of a system or a functional failure condition within a system including many factors like:- Reliability models
- Maintainability models
- Maintenance concepts
- Redundancy
- Common cause failure
- Diagnostics
- Level of repair
- Repair status
- Dormant failures
- Test coverage
- Active operational times / missions / sub system states
- Logistical aspects like; spare part (stocking) levels at different depots, transport times, repair times at different repair lines, manpower availability and more.
- Uncertainty in parameters
Definitions within systems engineering
Availability, inherent (Ai) WEB, Inherent Availability (AI),dap.dau.mil/glossary/Pages/2045.aspx, Glossary of Defense Acquisition Acronyms and Terms, Department of Defense, 10 April 2014, dead,web.archive.org/web/20140413164657/https://dap.dau.mil/glossary/Pages/2045.aspx, 13 April 2014, The probability that an item will operate satisfactorily at a given point in time when used under stated conditions in an ideal support environment. It excludes logistics time, waiting or administrative downtime, and preventive maintenance downtime. It includes corrective maintenance downtime. Inherent availability is generally derived from analysis of an engineering design:- The impact of a repairable-element (refurbishing/remanufacture isn’t repair, but rather replacement) on the availability of the system, in which it operates, equals mean time between failures MTBF/(MTBF+ mean time to repair MTTR).
- The impact of a one-off/non-repairable element (could be refurbished/remanufactured) on the availability of the system, in which it operates, equals the mean time to failure (MTTF)/(MTTF + the mean time to repair MTTR).
Basic example
If we are using equipment which has a mean time to failure (MTTF) of 81.5 years and mean time to repair (MTTR) of 1 hour:
MTTF in hours = {{math|1=81.5 Ã 365 Ã 24 = 713940}} (This is a reliability parameter and often has a high level of uncertainty!)
Inherent availability (Ai) {{math|1= = 713940 / (713940+1) = 713940 / 713941 = 99.999860% }}
Inherent unavailability {{math|1= = 1 / 713940 = 0.000140%}}
Outage due to equipment in hours per year = 1/rate = 1/MTTF = 0.01235 hours per year.Literature
Availability is well established in the literature of stochastic modeling and optimal maintenance. Barlow and Proschan [1975] define availability of a repairable system as “the probability that the system is operating at a specified time t.” Blanchard [1998] gives a qualitative definition of availability as “a measure of the degree of a system which is in the operable and committable state at the start of mission when the mission is called for at an unknown random point in time.” This definition comes from the MIL-STD-721. Lie, Hwang, and Tillman [1977] developed a complete survey along with a systematic classification of availability.Availability measures are classified by either the time interval of interest or the mechanisms for the system downtime. If the time interval of interest is the primary concern, we consider instantaneous, limiting, average, and limiting average availability. The aforementioned definitions are developed in Barlow and Proschan [1975], Lie, Hwang, and Tillman [1977], and Nachlas [1998]. The second primary classification for availability is contingent on the various mechanisms for downtime such as the inherent availability, achieved availability, and operational availability. (Blanchard [1998], Lie, Hwang, and Tillman [1977]). Mi [1998] gives some comparison results of availability considering inherent availability.Availability considered in maintenance modeling can be found in Barlow and Proschan [1975] for replacement models, Fawzi and Hawkes [1991] for an R-out-of-N system with spares and repairs, Fawzi and Hawkes [1990] for a series system with replacement and repair, Iyer [1992] for imperfect repair models, Murdock [1995] for age replacement preventive maintenance models, Nachlas [1998, 1989] for preventive maintenance models, and Wang and Pham [1996] for imperfect maintenance models. A very comprehensive recent book is by Trivedi and Bobbio [2017].Applications
Availability factor is used extensively in power plant engineering. For example, the North American Electric Reliability Corporation implemented the Generating Availability Data System in 1982.WEB,www.nerc.com/pa/RAPA/gads/Publications/GADS---Mandatory%20Reporting%20of%20Conventional%20Generation%20Performance%20Data%20Final.pdf,www.nerc.com/pa/RAPA/gads/Publications/GADS---Mandatory%20Reporting%20of%20Conventional%20Generation%20Performance%20Data%20Final.pdf," title="ghostarchive.org/archive/20221009www.nerc.com/pa/RAPA/gads/Publications/GADS---Mandatory%20Reporting%20of%20Conventional%20Generation%20Performance%20Data%20Final.pdf,">ghostarchive.org/archive/20221009www.nerc.com/pa/RAPA/gads/Publications/GADS---Mandatory%20Reporting%20of%20Conventional%20Generation%20Performance%20Data%20Final.pdf, 2022-10-09, live, Mandatory Reporting of Conventional Generation Performance Data, North American Electric Reliability Corporation, Generating Availability Data System, July 2011, 13 March 2014, 7, 17,See also
- Reliability engineering
- Safety engineering
- List of system quality attributes
- Spurious trip level
- Condition-based maintenance
- Fault reporting
- High availability
- RAMS
References
{{Reflist}}Sources
- {{FS1037C MS188}}
- K. Trivedi and A. Bobbio, Reliability and Availability Engineering: Modeling, Analysis and Applications, Cambridge University Press, 2017.
External links
- Reliability and Availability Basics
- System Reliability and Availability
- Availability and the Different Ways to Calculate It
- How to track and improve Technical Availability?
- content above as imported from Wikipedia
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- "availability" does not exist on GetWiki (yet)
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