Gas Turbine Maintenance: Compressor Washing to Hot Section

Gas Turbine Maintenance: Compressor Washing to Hot Section is the disciplined, section-by-section upkeep of an industrial gas turbine, spanning inlet air filtration, axial compressor washing, the combustion system, and the hot-section blades and vanes that carry thermal barrier coatings and suffer thermal fatigue. The engine is a single aero-thermodynamic chain: fouling at the front raises heat rate everywhere, and a burnt hot-section part can wreck the rows behind it. Effective maintenance treats each section on its own degradation clock while watching the whole machine through vibration and exhaust-temperature data.

Inlet air filtration: the first defence

Every kilogram of air the turbine breathes passes the inlet house. A large industrial unit ingests hundreds of thousands of cubic metres per hour, so even trace dust, salt, hydrocarbons or pollen accumulate fast on compressor aerofoils. Multi-stage filtration (weather hoods, coalescers, pre-filters and high-efficiency EPA or HEPA grade final filters) keeps particulate out and protects blade coatings. Maintenance here is simple but non-negotiable: track filter differential pressure, replace elements before the pressure drop starves the compressor, and inspect for wet-salt bypass in coastal and offshore sites.

Compressor fouling and water washing

Airborne contaminants that pass the filters deposit on compressor blades, changing their aerodynamic profile. The result is lost mass flow, falling pressure ratio, higher heat rate and reduced output, often several percent of power before anyone notices. Water washing restores the surfaces. There are two modes:

• Online washing is done at load with demineralised water, sometimes with an approved detergent. It slows the fouling rate and stretches the interval between shutdowns but only cleans the front stages the spray can reach.
• Offline (crank) washing is done with the unit shut down and cooled, cranked on the starter at low speed. Detergent soak plus rinse cycles clean the full compressor and recover most of the lost performance. It requires a drain path and adherence to OEM water-quality limits to avoid corrosion and hot-section contamination.

A sound programme uses frequent online washes to hold performance and periodic offline washes to reset it, timed by measured compressor efficiency rather than the calendar.

The combustion system

Combustors, fuel nozzles, liners, transition pieces and igniters live in the hottest steady-flame zone. Dry low-emission (DLE, also called dry low NOx) systems are especially sensitive: worn nozzles or cracked liners shift the flame, raise emissions and disturb the temperature pattern entering the turbine. The combustion inspection is the shortest-interval major task, focused on crack detection, coating condition on liners and transition pieces, and nozzle flow checks. Uneven fuel distribution shows up downstream as a widening exhaust temperature spread long before hardware fails.

The hot section: blades, vanes and coatings

Turbine nozzles (vanes) and buckets (blades) endure gas temperatures above the base-metal melting point, survived only by internal cooling and coatings. The failure mechanisms are thermal fatigue from start-stop cycling, creep under sustained load, high-temperature oxidation and hot corrosion, plus loss of the thermal barrier coating. TBCs are typically yttria-stabilised zirconia over an MCrAlY or aluminide bond coat; once the ceramic spalls, the alloy overheats and burns. Hot-section maintenance means borescope inspection, coating restoration or blade replacement, and strict tracking of low-cycle fatigue life. For the coating side of this work, see our guidance on coating inspection.

Borescope inspection and EOH intervals

Because opening a turbine is costly, the borescope is the primary condition tool. Through dedicated ports an inspector examines compressor blades, combustor hardware and the first turbine stages without a full teardown, catching cracks, coating loss, rubs and foreign-object damage. Overhauls are scheduled in equivalent operating hours (EOH), which weight fired hours and add a penalty for each start, since thermal cycling drives fatigue. Actual figures are OEM- and model-specific; the values below are representative heavy-duty industrial intervals, not a substitute for the manufacturer's maintenance manual.

Section, degradation and maintenance action

The table below maps each section to its dominant degradation and the response, the mental model a maintenance planner uses to build the outage scope.

Vibration and exhaust temperature spread monitoring

Between outages the turbine is watched continuously. Shaft vibration on the compressor and turbine bearings flags unbalance, rubs, bearing wear and blade damage; rising trends justify a borescope before the next scheduled window. Exhaust temperature spread, the difference across the ring of exhaust thermocouples, is the hot-section and combustion early warning: a single high or low channel points to a specific burnt or blocked nozzle. Reading these signatures together is where vibration spectrum analysis earns its keep, and the same monitoring discipline carries over to steam turbine maintenance on combined-cycle plants.

Tying filter changes, wash cycles, borescope findings and EOH counters into one plan is exactly what a maintenance platform should do. Book a Fabrico demo to see EOH-based intervals and condition data managed in one place.

Frequently Asked Questions

What is the difference between online and offline compressor washing?

Online washing runs at load and cleans mainly the front compressor stages to slow fouling. Offline (crank) washing is done shut down and cranked slowly, cleaning the full compressor and recovering most lost performance. Most sites use both.

Why are gas turbine intervals counted in equivalent operating hours?

Starts cause far more thermal-fatigue damage than steady running. EOH converts fired hours and starts into one number, adding a penalty per start, so a peaking unit with many starts reaches inspections sooner than a base-load machine.

What does a widening exhaust temperature spread indicate?

An increasing spread across the exhaust thermocouples usually signals uneven combustion, a worn or blocked fuel nozzle, or hot-section damage. It is a leading indicator that warrants a combustion inspection or borescope before failure.

What is a thermal barrier coating and why does it matter?

A TBC is a ceramic layer, typically yttria-stabilised zirconia over a metallic bond coat, that insulates blades and vanes from combustion gas. When it spalls, the alloy overheats and oxidises, so coating condition drives hot-section life.