Periodic inspection of boilers and turbines in thermal power plants

A thermal power plant is a system built around a boiler, a steam turbine and a generator, running for thousands of hours without interruption under high pressure and temperature. The integrity of this equipment governs both personnel safety and production continuity. Periodic inspection is therefore not merely a legal obligation. It is an engineering discipline that catches damage mechanisms early, predicts remaining life, and puts maintenance decisions on a data-driven footing.

The sections below cover the boiler, turbine and generator side at the level of component, damage mechanism and applicable standard, with field practice in mind.

1. Regulatory and standard framework

A periodic examination program starts with selecting the right set of standards. In practice the following references complement one another:

Design and construction codes: ASME BPVC Section I for power boilers, Section VIII (Div. 1/2) for pressure vessels, EN 12952 for water-tube boilers, and ASME B31.1 for power piping.
Welding and NDE: welding procedure qualification and welder performance qualification follow ASME BPVC Section IX; nondestructive examination methods and acceptance criteria follow Section V.
In-service inspection: API 510 for pressure vessels, API 570 for piping, and NBIC (NB-23) for general in-service inspection, repair and alteration. (Note: in-service examination of fossil plants does not fall under ASME Section XI. Section XI covers in-service inspection (ISI) of nuclear facilities only — a frequent source of confusion.)
Damage, fitness and risk: damage mechanisms in API 571, Fitness-for-Service in API 579-1 / ASME FFS-1, and risk-based inspection in API 580/581.

A sound examination program asks "which damage mechanism do I expect on this equipment?" before it asks "which method do I apply?" The method follows the mechanism.

2. Expected damage mechanisms

The mechanisms that dominate in a thermal plant, by component:

Creep: permanent deformation and micro-void formation at high temperature — that is, above roughly 40% of the absolute melting temperature (homologous temperature). Creep is the primary life limiter for superheaters and reheaters, steam headers and main steam lines.
Thermal and mechanical fatigue: driven by start-stop cycles and load swings, especially in thick-wall headers, drum connections and turbine rotors.
Corrosion and erosion: flue-gas side erosion and fly-ash wastage on the economizer and waterwall tubes; on the water side, oxygen corrosion and pitting corrosion.
Hydrogen damage and caustic embrittlement: water-chemistry driven, on the inside surface of waterwall tubes.
Stress corrosion cracking (SCC): on turbine discs and last-stage blades, in chloride or caustic environments.
Dissimilar metal weld (DMW) damage: carbon migration and interface cracking at ferritic-to-austenitic transition welds.
Grade 91/92 Type IV cracking: creep cracking in the heat-affected zone (HAZ) of welds in modern 9Cr martensitic steels, concentrated in the fine-grained and intercritical region (FGHAZ/ICHAZ) — a critical monitoring item on current plants.

3. Examination on the boiler side

The boiler sees the highest pressure and temperature in the system. Plan the examination component by component, driven by the expected damage mechanism.

Critical components

Waterwall tubes: flue-gas side erosion and wastage, internal deposits and hydrogen damage on the water side; map the tube wall thickness.
Superheater / reheater (SH/RH): these run at the highest metal temperature in the boiler, so creep and steam-side oxidation dominate. Measure the steam-side oxide scale thickness by ultrasonics to estimate long-term metal temperature and remaining creep life.
Economizer: low temperature, dew-point corrosion and erosion.
Drum and headers: thick walls bring thermal fatigue; look for cracking at nozzles and ligaments, and examine inside surfaces and weld seams.
Main steam / feedwater lines and DMWs: creep, fatigue and the integrity of dissimilar metal welds.

NDE methods applied

Method	Abbreviation	Application
Visual examination	VT	First-pass survey, deposits, deformation, corrosion
Ultrasonic thickness / phased array	UT / PAUT	Tube wall and shell thickness, internal discontinuities, volumetric weld examination
Time-of-flight diffraction	TOFD	Crack sizing in weld seams
Magnetic particle	MT	Surface and near-surface cracking in ferromagnetic material
Liquid penetrant	PT	Surface-breaking cracks (all materials)
Radiography	RT	Internal weld discontinuities, thin walls
Internal rotary inspection system	IRIS	Internal examination of boiler and exchanger tubes
Oxide scale UT	—	Metal temperature and creep life on SH/RH tubes
Replication + metallography	—	Creep cavitation, spheroidization
In-situ hardness testing	—	Material degradation, heat treatment verification
Positive material identification	PMI	Alloy mix-ups, verification of the correct grade

For examinations after welding and repair, take acceptance criteria from ASME Section V / VIII / B31.1, and verify welder performance qualification and welding procedure qualification through Section IX.

4. Examination on the turbine side

A steam turbine runs at high speed with tight tolerances. Its examination covers alignment, clearance and dynamic behavior as much as mechanical integrity.

Rotor and rotor bore: ultrasonic examination (PAUT) of the center bore and the body, scanning for fatigue and creep cracking. The rotor is the most critical, longest-lead-time part of the machine.
Blades and blade roots: phased array UT and eddy current (ET) on fir-tree and dovetail root geometries for fatigue and SCC cracking; moisture erosion on last-stage LP blades.
Discs: SCC and fatigue cracking in the bore and rim regions.
Casing and seals: thermal distortion, sealing performance, clearance measurement.
Internal examination: borescope examination for access without disassembly.
Bearings and alignment: bearing clearance, journal condition, coupling alignment.
Balancing and vibration: balance quality to ISO 21940 (formerly ISO 1940); vibration assessment to ISO 20816 (formerly ISO 10816). Unbalance and shaft bow are early warnings of failure.

5. Generator and electrical examination

Turbine examination cannot be separated from the generator it drives. Run the electrical integrity checks in the same outage window as the mechanical work:

Stator core: EL-CID (Electromagnetic Core Imperfection Detector) or a full flux ring test to detect breakdown of the lamination insulation.
Stator winding wedges: tap testing for looseness, and partial discharge (PD) measurement for insulation degradation.
Rotor: RSO (Recurrent Surge Oscillography) for shorted turns; checks of the slip ring and brush system.
Insulation tests: insulation resistance and polarization index (PI), DC high-potential (hi-pot), and dissipation factor (tan δ) measurements.

Thermal plant examination is not limited to pressure equipment. A fault at the generator end stops generation just as completely.

6. Remaining life and risk-based inspection

The modern approach prioritizes by risk instead of imposing the same calendar on every piece of equipment.

Risk-based inspection (API 580/581): Risk = Probability of Failure (PoF) × Consequence of Failure (CoF). Direct resources to the highest-risk equipment and circuits, and set examination scope and interval from that matrix.

Fitness-for-Service (API 579-1 / ASME FFS-1): engineering calculation decides whether equipment carrying a detected flaw — local thinning, cracking or creep damage — can stay in service. It turns the "run, repair or replace" call into an objective decision.

Remaining life logic:

A simple estimate for corrosion and thinning:

Remaining life = (current thickness − minimum allowable thickness) / corrosion rate

Calculate the corrosion rate both long-term (from the original construction thickness) and short-term (between the last two measurements), then use the more conservative of the two. For high-temperature components, assess life with the Larson–Miller parameter, which combines temperature and stress, together with replication findings.

Set the next examination interval at whichever is smaller: half the remaining life, or the maximum period defined in the standard. The reliability of this calculation depends on keeping the measurement points — condition monitoring locations (CMLs) — fixed and repeatable.

7. Inspection planning and documentation

A sound program does more than take readings; it builds a traceable history:

Year-by-year trending of thickness and findings for every CML,
Traceability of WPS/PQR and NDE reports for welds and repairs (ASME IX / V),
PMI records confirming material identity,
Periodic updating of the RBI matrix.

From the field

The problem you hit most often is not technical — it is data continuity. Readings taken in different years, from different points, with different probes simply cannot be compared, and that alone can sink a remaining life analysis. The most valuable output of an outage is not the report you hand over at the end. It is the consistent data set that builds up over the years. That is where the real engineering decision comes from.

Türkiye-specific note

In Türkiye (Turkey), periodic inspection of pressure equipment and installations is mandated by the Regulation on Health and Safety Conditions in the Use of Work Equipment, which transposes EU Directive 2009/104/EC. The regulation sets both the requirement for periodic inspection and the competence required of the person performing it. It sits alongside, not instead of, the ASME and API references above.

Take it to the field: To keep these NDT methods, standard references and field steps in your pocket — completely offline and free — take a look at the Doawise NDT Guide app.

At DoaWise we carry out periodic examination of boilers, turbines, generators and piping systems in thermal power plants to international ASME, API and EN standards, producing recordable, auditable results. With API 579 Fitness-for-Service and remaining life studies, and API 580/581 risk-based inspection work, we support operators in their planned maintenance and investment decisions.