Gas turbines: borescope inspection and acceptance criteria

In one line

In a gas turbine, "is this crack serious?" is not a judgment call — the manual gives the exact number (longer than 1 inch and 0.13 mm in diameter) that separates an acceptable defect from one that is not.

Diagnosis by borescope inspection

Borescope inspection lets you observe the internal combustion parts, the turbine's hot sections and the compressor blades without having to open the casings. The points checked by borescope are: the combustion chamber, the fuel injectors, the compressor blades and the expander blades.

Typical mechanical defects and their acceptance limits

In the combustion elements, the typical defects are coating detachment, flame-impingement damage, plastic deformation from temperature, and wear of the anchoring elements. The guidelines for deciding whether a defect is acceptable are concrete:

• Acceptable: when the defects do not carry a risk of material detachment
• Not acceptable: when they produce cracks in critical areas longer than 1 inch with a diameter greater than 0.13 mm, or when they could cause a partial failure, or when there is a risk of material detachment if the crack progresses
• Foreign-object damage: the impact notch must not exceed 50% of the wall thickness
• Corrosion or erosion: acceptable as long as the material removed from the blades is less than 50% of the wall thickness

On the compressor blades, the most common findings are soot deposits and impact marks from foreign objects.

Documented major failures

The manual lists real cases of major gas turbine failure:

• Melting of a combustion chamber from a blowtorch effect caused by a fuel leak
• Deformation of the first-stage compressor blades from rubbing against the casing, due to compressor surge
• Deformation of the 1st- and 2nd-stage compressor blades from contact with the casing, as a result of differential expansion between the casing and the rotor
• Failure of compressor stages from a foreign object entering the intake
• Failure of turbine stages from a 1st-stage blade breaking
• Failure of the train's radial bearings
• Premature, widespread failure of the hot section parts (blades and vanes) from hot corrosion

Electrical and control failures

These are generally due to field-instrument failures or electrical failures in motors or systems that cause the machine to stop, lose power, or malfunction. The typical ones are: failure of the gas valve position detectors, failure of the solenoid valve feeding oil to the torque converter, DC system failure, failure of the anti-surge valve actuator, fire-suppression system tripping due to a sensor failure, position failures in the steam injection system's limit switches, and incorrect servo-valve operation from water contamination of the oil.

Maintenance: daily vs. with the turbine stopped

The manual sums up something worth stating first: the technological complexity of gas turbines means that, in practice, their maintenance has to be contracted out to the manufacturer. Within that framework, there are two types of maintenance:

Daily, with the machine running (done by the equipment's own owner):

• Power generated
• Exhaust gas temperature
• Startup and shutdown time
• Frequency of stops
• Vibration map
• Turbine efficiency

Reviews with the turbine stopped:

• Cleaning the air filters
• Inspecting the combustion elements
• Inspecting the hot section parts
• A general inspection that requires opening the turbine, roughly every 50,000 hours

Routine maintenance tasks (monitoring parameters, oil analysis, replacing intake filters, calibrating instrumentation, cleaning the compressor, checking the fire-suppression system) are clearly different from the major overhauls (changing blades, changing the combustion chamber, manually cleaning the compressor blades, a full review of instrumentation and lubrication, rotor balancing, turbine alignment).

What having numerical limits, not just judgment, is for

A 0.9-inch crack with a 0.10 mm diameter and a 1.2-inch crack with a 0.15 mm diameter are not "roughly similar": by the manufacturer's own criteria, the first is acceptable and the second is not. Having the exact number keeps the decision to stop a turbine or not from depending on the subjective experience of whoever is looking through the borescope that day.

Benefits

• Borescope inspection detects internal damage without disassembling the turbine, avoiding unnecessary shutdowns just to look
• Numerical acceptance limits turn a judgment call into an objective, repeatable decision across different inspectors
• Separating daily tasks from reviews with the machine stopped makes clear what the plant's own staff can monitor and what requires the manufacturer

Limitations to keep in mind

• Relying on the manufacturer for major maintenance is, at the same time, a quality guarantee and a limitation on cost and spare-parts availability
• The acceptance limits (1", 0.13 mm, 50% of wall thickness) belong to this manual — each manufacturer may set its own depending on the model and material
• The general inspection every 50,000 hours is approximate: the real interval depends on each installation's duty cycle and environmental conditions

In summary

A gas turbine is inspected by borescope without opening it, and the manual gives concrete numerical limits — not just "at the technician's judgment" — to decide whether a defect is acceptable. Daily maintenance is done by the equipment's own owner by watching operating parameters; major overhauls, with the turbine stopped, are almost always done by the manufacturer because of the equipment's technological complexity.