Failure Modes Explained
Understanding how equipment fails is the first step to preventing failures. Learn the common failure modes, the bathtub curve, and how to design for reliability.
The Bathtub Curve
The failure rate of most equipment follows a pattern known as the bathtub curve. It has three phases:
- Infant mortality (early failures): High failure rate at the start of life. Caused by manufacturing defects, installation errors, or design problems. These failures are identified during commissioning.
- Useful life (random failures): Low, constant failure rate during normal operation. Failures are random and unpredictable. This is the period the equipment was designed for.
- Wear-out (end of life): Increasing failure rate as components age and wear. Failures become more frequent. This is when preventive maintenance and component replacement are most valuable.
Preventive maintenance is most effective in the wear-out phase
During useful life, failures are random and preventive maintenance cannot prevent them. In the wear-out phase, components are degrading predictably, and preventive replacement before failure is effective. The challenge is knowing when equipment enters the wear-out phase. Condition monitoring and trending help identify this transition.
Common Failure Modes
- Wear: Progressive loss of material due to friction, erosion, or abrasion. Common in bearings, seals, impellers, and valve seats.
- Fatigue: Failure caused by cyclic loading. Cracks initiate and propagate until the part fractures. Common in shafts, springs, and structural members.
- Corrosion: Chemical or electrochemical degradation. Types: uniform (general thinning), pitting (localised holes), galvanic (dissimilar metals), crevice (in gaps), stress corrosion cracking (combined stress and corrosion).
- Overload: Failure from a single load exceeding the material's strength. Can be mechanical (impact, excessive pressure) or thermal (thermal shock).
- Thermal degradation: High temperature causes materials to degrade: polymers embrittle, grease breaks down, insulation fails.
- Erosion: Material loss from high-velocity fluid or particle impingement. Common in pipe bends, valve internals, and pump impellers.
- Cavitation damage: Vapour bubble collapse near surfaces causes pitting. Common in pump impellers and valve seats.
- Electrical failure: Insulation breakdown, winding shorts, bearing current damage (EDM), contactor wear.
Failure Mechanisms vs Failure Modes
A failure mode is what happened (e.g., 'shaft broke', 'seal leaked'). A failure mechanism is why it happened (e.g., 'fatigue crack from cyclic bending load'). Understanding the mechanism is essential for preventing recurrence.
If you replace a broken shaft without understanding the mechanism (fatigue from misalignment), the new shaft will fail the same way. Root cause analysis identifies the mechanism, which leads to corrective action that addresses the actual cause, not just the symptom.
Designing for Reliability
- Select appropriate materials for the service conditions (temperature, pressure, corrosivity, erosion).
- Include adequate safety margins in design calculations. Don't design to the limit.
- Consider maintainability: can the equipment be inspected, cleaned, and repaired easily?
- Specify redundant systems for critical functions (two pumps, two sensors, two feed lines).
- Protect against common failure modes: install strainers (erosion), use compatible materials (corrosion), design for adequate NPSH (cavitation), ensure proper alignment (fatigue).
- Plan condition monitoring: include tappings for vibration, temperature, and pressure measurement.
- Document the design basis and expected failure modes. This guides the maintenance strategy.
Tip: When a failure occurs, ask: is this a random failure (during useful life) or a wear-out failure (expected at this age)? Random failures require investigation into the specific cause. Wear-out failures require adjusting the maintenance schedule or upgrading to more durable components. Both require documentation to build institutional knowledge.