In plant maintenance, two common strategies are used: TBM (Time-Based Maintenance) and CBM (Condition-Based Maintenance).
Even if these terms sound familiar, many sites struggle with how to apply them properly. In particular, when TBM and CBM are mixed without a clear definition, maintenance effectiveness can decline and troubleshooting can become more complicated.
This article explains the practical differences between TBM and CBM and provides a structured way to avoid confusion in real-world plant operations.
Definitions may vary slightly depending on interpretation, but what truly matters is internal consistency. If your maintenance system is clearly organized and shared within the organization, it works. The difficulty lies in that organization.
Which to Choose, TBM or CBM? | For Chemical Plants
TBM, CBM, BM: Understanding Maintenance Terminology in Plants
TBM vs. CBM — Basic Concepts
First, let’s review the common understanding:
- TBM: Replace components at fixed time intervals
- CBM: Collect condition data regularly and decide replacement timing based on equipment condition
TBM is often associated with “time,” and CBM with “condition.”
This sounds simple. In practice, however, the distinction is not always clear.
A more detailed interpretation would be:
TBM
Applies to equipment that must be overhauled or repaired at predetermined intervals. Operational diagnostics may be performed, but they are treated as reference information.
CBM
Replacement timing has general guidelines but is flexible. Equipment condition is monitored during operation, and priorities are adjusted based on degradation trends.
Even this explanation does not eliminate ambiguity. Let’s look at practical examples.
Is TBM Always “Mandatory Replacement”?
TBM does not necessarily mean forced replacement.
For example, during a scheduled shutdown, equipment may be opened for inspection. If internal components are clean and in good condition, they may simply be cleaned and reassembled without replacement.
Is this TBM or CBM?
I consider it TBM — because the inspection itself was scheduled periodically.
Some argue it is CBM since no mandatory replacement occurred. But the shutdown and inspection were time-based decisions, which aligns more closely with TBM logic.
In chemical plants, true TBM (strict time-based replacement without condition review) is rare. Budget constraints and operational realities often push originally TBM-intended equipment into CBM-style management.
In my view, distinguishing whether the plant must be stopped or not provides a clearer practical framework:
| Inspection Type | Inspection Timing | Replacement Decision |
|---|---|---|
| TBM | Scheduled shutdown | Predetermined |
| CBM | During operation (or flexible timing) | Based on condition |
Equipment vs. Components
Confusion also arises when discussing entire equipment systems versus individual components.
An entire pump system may be managed under CBM, while its bearings or seals are effectively managed as TBM — or vice versa.
Maintenance discussions at the plant level often classify equipment as TBM or CBM.
However, in daily practice, maintenance engineers focus on components, which blurs the classification.
Failure to clearly separate equipment-level strategy from component-level strategy is one reason TBM/CBM discussions become confusing.
Practical Examples
Shutdown Inspection (SDM) → TBM
Opening equipment during a scheduled shutdown is typically TBM.
The timing is driven by production schedules rather than condition monitoring.
Vibration Monitoring → CBM
Monitoring pump vibration during operation is a clear example of CBM.
Replacement timing depends on measured deterioration trends.
Even if the actual replacement happens during a scheduled shutdown, the decision logic remains condition-based — therefore CBM.
Seal and Bearing Replacement → CBM
Mechanical seals and bearings are generally replaced after trend monitoring indicates degradation.
Opening them usually implies replacement, which differentiates them from inspection-only TBM cases.
Thickness Measurement → CBM
Wall thickness measurement during operation is CBM because it monitors condition without necessarily stopping the plant.
If inspection requires shutdown and is scheduled periodically regardless of condition, it may resemble TBM.
In practice, however, thickness inspection tends to be condition-driven and therefore CBM.
Belt Replacement → TBM
Belts are often inexpensive and potentially hazardous to inspect during operation.
Regular scheduled replacement (TBM) is typically safer and more practical.
Summary
Trying to rigidly apply “TBM = time” and “CBM = condition” often creates confusion.
In reality:
- Pure TBM is difficult to implement strictly.
- Many systems drift toward CBM-style management.
- Equipment-level and component-level strategies differ.
- Definitions vary among individuals and organizations.
The key is not strict textbook classification, but internal clarity.
If your organization shares a consistent understanding and operates accordingly, the maintenance system will function effectively.
Discussing TBM and CBM openly within your team may reveal new insights about your current maintenance strategy.
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