Chapter 02Maintenance MaterialsArticle 2.1
Availability, MTBF and MTTR
Materials readiness influences maintenance performance because an unavailable spare can turn a repair task into extended downtime.
- ProblemA short repair becomes long downtime while the spare is found.
- SignalsFailure consequence, spare availability, lead time, repairables.
- DecisionWhich spares to hold, where, and in what condition.
- ControlSpare readiness treated as part of restoring service.
01Business problem
When equipment fails, the repair itself may take hours. Getting the right spare to the job can take days or months. From the plant's point of view the equipment is down for the whole time, whatever the reason.
That is why materials management sits inside maintenance performance: not as a support function at the end of the chain, but as one of the things that decide how long a failure actually costs.
02Core concept
Three standard terms describe the picture:
- MTBF (mean time between failures): the average operating time between failures of a repairable item.
- MTTR (mean time to repair or restore): the average time needed to restore the item once it has failed.
- Availability: the proportion of time the item is able to perform when required.
From the materials side, the useful view is the sequence after a failure: the equipment fails, a spare is needed, the spare is or is not available, there is waiting time, the repair is done, the equipment returns to service. Waiting for a spare is not part of the hands-on repair, but it can become part of the time it takes to restore service.
03Decision rule
Hold a spare when the consequence of waiting for it is worse than the cost of holding it. Decide that from the failure consequence, the lead time and the installed population, not from consumption alone.
The same logic decides where and in what condition. A spare that exists but is unidentifiable, unpreserved or at the wrong location adds waiting time just like a spare that does not exist.
04Signals and inputs
- Failure consequence
- Safety, production or environmental effect of the equipment being down.
- Lead time
- How long a spare takes if it is not held.
- Installed population
- Number of identical units the spare can serve.
- Redundancy
- Whether standby equipment covers the failure.
- Repairable items
- Spares that can be repaired and returned to stock.
- Stockout risk
- Likelihood that the spare is not there when needed.
05Model
- 01Equipment failureThe item stops performing its function.
- 02Need for a spareDiagnosis identifies the part required.
- 03Spare availabilityIn stock, at another location, on order, or not held.
- 04Waiting timeTime spent obtaining the spare if it is not ready.
- 05RepairHands-on restoration work.
- 06Return to serviceThe equipment is available again.
This is a simplified, steady-state relationship. It describes the inherent availability of a repairable item from its failure and repair characteristics alone. Real operational availability also depends on logistic and administrative delays, on waiting for spares, people and permits, and on planned maintenance downtime. Those delays are often longer than the repair itself.
Waiting for a spare does not change the intrinsic MTTR of the equipment. It does lengthen the actual time to restore service, which is what operations experiences.
06Worked example
Illustrative example
The same repair, different readiness
A pump fails on average every 2,000 operating hours. The repair takes 8 hours when the seal kit is on the shelf. If the time to restore service is used in place of the repair time:
- Spare ready: about 8 hours down per failure, 2,000 / (2,000 + 8) ≈ 99.6%.
- Spare not held, 10 days to obtain it: about 248 hours down per failure, 2,000 / (2,000 + 248) ≈ 89.0%.
The repair task is identical. The difference is spare readiness. Whether holding the kit is justified depends on the consequence: with a standby pump the 10 days may be acceptable; without one they may not be.
07Professional judgement
Spare availability is part of reliability performance, but it is not the whole of it.
It would be wrong to claim that every material shortage changes the reliability of the equipment. It does change how long a failure keeps the equipment out of service. The judgement is in deciding where that delay is acceptable and where it is not.
Insurance spares are the hardest case: rarely used, expensive to hold, and decisive when needed. Consumption history cannot justify them. Failure consequence, lead time and redundancy can, and the reasoning should be written down, because the next inventory review will ask why the item never moves.
08Failure modes
Critical spares judged by consumption
Insurance spares are removed because they never move.
ControlReview criticality and lead time separately.
Lead time unknown
A failure waits for a spare nobody expected to take months.
ControlKeep lead times current for critical spares.
Repairables not tracked
Repaired units are lost or bought again as new.
ControlTrack repairable items through repair and return.
Installed population ignored
One spare is expected to cover too many units.
ControlCheck how many identical units each spare serves.
Spare not ready for use
Stock exists but is unidentified, unpreserved or at the wrong location.
ControlTreat identification and preservation as part of readiness.
09Field note
10Practical checklist
- Failure consequence and redundancy are known for the equipment.
- Critical and insurance spares are identified separately from consumption items.
- Lead time is current for spares not held locally.
- Repairable spares are tracked through repair and return.
- Spares can be found through the equipment BOM.
- Preservation and storage location support a fast issue.
11Connected intelligence
Experience
See this principle in practice
- Preservation Governance and Storage ReadinessOperational experience
- Material Master, BOM and SPIL Data QualityOperational experience
12Evidence and basis
MTBF, MTTR and availability follow standard dependability terminology; the availability formula is the usual simplified steady-state model. The worked example uses illustrative values. Basis: maintenance materials, critical spares and preservation work in the CV roles. The interpretation of materials readiness is the author's view.
Sources & References
- IEC 60050-192:2015 International Electrotechnical Vocabulary, Part 192: DependabilityInternational Electrotechnical Commission (IEC) · Official Standard · 2015
- ISO 14224:2016 Petroleum, petrochemical and natural gas industries: Collection and exchange of reliability and maintenance data for equipmentInternational Organization for Standardization (ISO) · Official Standard · 2016
The definitions of MTBF, MTTR and availability follow the standards listed. The link between spare readiness and the time to restore service is the author's professional interpretation, not a statement of those standards.