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Estimating Failure Parameters
Mick Drew – ARMS Reliability Engineers
How important is it to use Failure Parameters in Reliability Analysis and RAMS for
major projects?
Answer: There are different types of Failure Parameters that can be used. There are failure parameters that
are used to assess the reliability of a component or system, and there are failure parameters used to predict
reliability of components or systems.
Assessment: One of the most common parameters used to assess reliability is MTBF. Comparing the MTBF is an
indicator of the time between failures and can also be used to calculate system Availability.
The Mean Time Between Failures is calculated by:- Total Operating hours/No of Failures
The MTBF can be used at equipment level or at System level. The inverse of the MTBF is the Failure rate. For
new projects failure rates can be determined from existing plant, similar plant or there may be published data
availability from various industry bodies. OREDA is often referred to for offshore oil industry, and the GADS
database for more than 6,500 electric generating units. Other published data is available from Reliability Analysis
Centre such as the NPRD handbook for non electronic parts.
Of course the best data is from your own plant or similar plant. In the following I describe the analysis of failure
data for four cases.
© 2009 ARMS Reliability Engineers
2. Empowering Maintenance & Reliability Decision Makers
Case 1
Up
Down
15 30 45 60
In case a system has four failures over the first 70 days of operation. The timeline is shown above. From the
event log the following data is assembled and the MTBF is calculated from 44 operating days divided by 4
failures gives an mtbf of 11 days. Similarly the MTTR the plant is 6.5 days.
Perform
In service TTF Maintenance Duration
0 7 7 4
11 10 21 5
26 12 38 7
45 15 60 10
Total 44 26
Mean 11 6.5
A maintenance analysis finds the first failure was due to severe aging mechanism, whereby it was decided to
perform a PM task every 15 days.
The resulting timeline is shown in (Case 2) and the timeline shows the first failure eliminated.
© 2009 ARMS Reliability Engineers
3. Empowering Maintenance & Reliability Decision Makers
Case 2
Up
Down
15 30 45 60
In this case the MTBF has increased from 11 days to 18 days. A dramatic improvement. Or is it?
When we include the planned outages in the calculation as shown in Case 3, the Mean Time Between Outages
has reduced to only 6.7 days. So whilst the failures have reduced the downtime has actually increased.
Case 3
Up
Mtbo=47/7=6.7days
Down
15 30 45 60 75
In Perform
service TTF Maintenance Duration
0 15 15 2 PM
17 4 21 5 Repair
26 4 30 2 PM
32 6 38 7 Repair
45 0 45 2 PM
47 13 60 10 Repair+PM
70 5 75 2 PM
Total 47 30
Mean 6.7 4.3
© 2009 ARMS Reliability Engineers
4. Empowering Maintenance & Reliability Decision Makers
Investigation work by the Engineering group finds a way to eliminate the Regular PM’S and design out two of the
failures, but the repair time is now greatly increased. But the MTBF increases significantly to 18 days.
Case 4.
Up
Mtbf=36/2=18days
Down
15 30 45 60 75
In Perform
service TTF Maintenance Downtime
0 21 21 24
45 15 60 26
Total 36 50
Mean 18 25.0
© 2009 ARMS Reliability Engineers
5. Empowering Maintenance & Reliability Decision Makers
Discussion of Results
Comparison on the MTBF could indicate that the MTBF of case 4 means that Case 4 is the best case.
Case 1 Case 2 Case 3 Case 4
MTBF (Days) 11 16 6.7 18
This is clearly not the case if one was at all concerned about the amount of downtime.
In order to consider the downtime it is necessary to factor in the MTTR and calculate Availability for each case
which is shown below.
Availability = MTBF/(MTBF+MTTR)
Case 1 Case 2 Case 3 Case 4
MTBF 11 16 6.7 18
Failure Rate 0.09 0.06 0.15 0.06
MTTR 6.5 7.3 4.3 25
Availability 62.9% 68.6% 61.0% 41.9%
From this of course we see that Case 1 is actually better than Case 3 and Case 4 and Case 2 is invalid because the
planned downtime was not taken into account.
Conclusions
• If you are concerned about number of unplanned outages- MTBF can be used as a guide.
• If you are concerned about number of outages- MTBO can be used as a guide.
• If you are concerned about minimising Downtime then Availability can be used as a guide.
• It is very important to consider both planned and unplanned outages when assessing and comparing
systems.
• The use of a fixed time maintenance regime without consideration of operating time is far too
conservative.
• Major projects may start off with maintenance outage data from vendors but the actual outage times
must be determined through maintainability studies that take into account logistics delays, sparing
levels, diagnostic times.
• What about Predicting performance? For the answer to that question we need to go to Reliability
Parameters.
© 2009 ARMS Reliability Engineers