2. Contents
Definition
Relationship with Maintainability and Reliability
Classification
Availability and Number of Nine
Component Availability Flow Chart
Calculationa of Total Availability
Improving Techniques
References
3. Consider a Situation-
1. Today, you are in an industry
2. The shift is of 8 hours(10:00 am to 6:00 pm)
3. It has so many machines
4. Consider one machine –A Lathe Machine
5. The machine should work for 8 continuous hours
6. Suppose a component fails at 12:00 noon and it stops working
7. It took 30 minutes in maintenance of that machine
8. Again it stopped working for 30 minutes at 3:30pm.
How long the Lathe Machine did actually work in a given shift?
4. Is that 7 hours?
Exactly 7 hours is the actual time when machine worked without
any kind of failure.
The machine worked for 7 hours with zero failure
Failure and maintenance time 1 hour
Total Time = 7+1=8 hours
What will you call the ratio 7/8?
It is Availability.
5. Definition of Availability
From Situation given, during 8 hours of shift, 1 hour is considered for any
kind of failure and its maintenance, the Availability of that machine will be 7/8
or 0.875 or 87.5% for that particular shift on specified date.
Availability is the probability that the system/component is
operational at a given time, without any kind of failure.
Availability could also be understood with ‘Readiness’.
Availability does not show how many times the maintenance
was performed.
8. Inherent Availability
The steady state availability when considering only the corrective
downtime of the system.
For a single component, this can be computed by:
- For a system, the Mean Time Between Failures, or MTBF, is used to
compute inherent availability:
9. 9
Achieved Availability
Achieved Availability is similar to Inherent Availability except Preventive
Maintenance (PM) is also included.
The steady state availability when considering the corrective and preventive
downtime of the system computed by looking at the Mean Time Between
Maintenance actions, MTBM and the Mean Maintenance Downtime:
10. 10
Operational Availability
Operational Availability is the percentage of calendar time to which one can
expect a system to work properly when it is required.
Expression of User Need rather than just Design Need.
Operational Availability is the ratio of the system Uptime and Total time.
Mathematically, it is:
DowntimeUptime
Uptime
Ao
• Includes all experienced sources of downtime, such as administrative downtime and
logistic downtime to restore the system.
• It is also called Operational Readiness.
11. 11
Basic System of Availability
Inherent Availability and Achieved Availability are controlled by the system
designer/manufacturer.
Operational Availability is not solely controlled by the manufacturer due to
variations in location, resources and logistics factors under the province of
the end user of the product.
12. Availability and Number of 9
Availability is typically described in nines notation.
For Example- Availability of a system is considered over one year.
13. Factors Affecting Total Availability
Hardware
Hardware failures like File System Full error, Kernel In-Memory Table Full error, disk
full, power spike, power failure, and LAN infrastructure problem.
Software
Complexity of software, Size of software, Experience of team developer, Percentage
of the code which is used before in a stable project, How much test/debug is done
before releasing the product.
Environment
These fault can be occur by power outages, fires, earthquakes, tornadoes and other
events.
The point is that these events cannot be predicted and when encountering with
them, the whole system becomes down for some hours or even months depending
on the damages they impose.
Human Errors
15. Calculation of Availability
Total availability calculations
are same as calculation of
Reliability.
There are three systems-
Parallel
Series
Mixed
Parallel System
Total Availability = 1-[(1-A1)(1-A2)]
=1 − [(1− 0.005)(1-0.005)]
= 0.999975
Series System
Total Availability = A1xA2
=0.995x0.995
=0.990025
16. Improving Techniques
Proper training of plant employees
Consistent cleaning of machines and equipments
Proper maintenance scheduling
Good quality lubricants
The 5S
Safety measures
Automation
Monitoring availability
Uninterrupted Power Supplies (UPS)
Standby Systems
Logistics
17. References
Elsayed, E., Reliability Engineering, Addison Wesley, Reading, MA, 1996.
Hoda Rohani, Azad Kamali Roosta, Information Services Organization
KLM-Air France, Amsterdam
Wendai Wang et al.,2002, IEEE, USA