Matt Smillie - Quest Integrity NZL Ltd - Extending inspection intervals of large industrial gas turbines

Dr Matthew Smillie

Extending Major Inspection Intervals of Large Industrial Gas Turbines

16th Annual Australian Gas Turbines Conference

www.QuestIntegrity.com2

Fundamentals of extending intervals

• Why are outages needed?

• What controls service life of GT parts?

• What are OEM inspection interval strategies?

• How can intervals be extended?

• The risks and rewards of extending inspection intervals


Definitions

• Outage

– Scheduled (or unscheduled) break from machine availability in order to carry out maintenance or repair

• Inspection Interval

– The period of time between scheduled inspection outages

• Minor Inspection

– Shorter outage, generally focusing on inspection/replacement of combustor hardware

• Major Inspection

– Longer outage, generally focusing on inspection/replacement of turbine parts. Can involve rotor removal


Why have an outage?

• GT use damages components when operating normally:

– High speed rotation, fluid flow can lead to high cycle fatigue, will lead to wear

– Operational cycles will lead to low cycle fatigue, thermal fatigue, thermo-mechanical fatigue

– High temperatures and stresses will lead to creep

– High temperatures and environmental factors will lead to oxidation and may lead to corrosion

– High temperatures will change the microstructure

• GT components can be damaged when not operating!

– Standby corrosion

– Barring wear


Fatigue - types and locations

• High cycle fatigue

– Low stress range, high frequency (hours – minutes –based).

– Vibration, harmonics, passing response.

– Blades, vanes

– Initiates off other damage – secondary damage!

• Low cycle fatigue

– High stress range, low frequency (starts based)

– Mechanical (rotational) and thermal stresses

– Rotors, discs, casings

• Thermo-mechanical fatigue

– Thermal stresses with a hold time (starts and hours based)

– Affects rigid parts that get hot: turbine blades and vanes, disc rims, turbine casings

– Creep-fatigue interaction


Creep

• Function of temperature, stress and time

– Mobility of atoms/defects in crystal structure

• Inevitable in alloys running at high temperature under load

• Where is it hot enough in a GT?

– Latter stages of compressor

– Combustor hardware

– Turbine

– Exhaust casing/ducting

– Rims of turbine discs


Corrosion and oxidation

• Aqueous Corrosion

– Wet end of compressor

– Chlorine, sulfur

– Rust, pitting

– Standby corrosion

• High Temperature – Hot Corrosion

– Sulfur plus alkali metals (Na, K, V)

– Combustion chambers, buckets, vanes

– In temperature bands

• Type 1: 850 °C to 1000 °C

• Type 2: 600 °C to 800 °C, maximum attack at 650 °C

• Oxidation

– Dominates at 1000 °C or more


• Erosion from particles

– Dust

– Water droplet erosion

• Wear between surfaces

– Seals

– Shafts

– Blade tips/shrouds

– Combustion hardware

– Disc/Blade interface

• Tends to be linear with time service, except when caused by upsets

Erosion and wear


• Precipitate formation and

growth

• Precipitate dissolution

• Softening

• Embrittlement

• Diffusion of coatings

Metallurgical degradation

CoatingBucket


Type of operation = type of damage

Factored

Hours

Factored

Starts

Starts-based

maintenance

interval –

LCF, TMF,

Standby

Peaking duty –

starts based

Hours-based maintenance

interval – creep, wear,

corrosion, oxidation

Base load

duty – hours

based

Intermediate duty –

hours starts


Standard outage intervals

• OEM’s recommend standard outage intervals in order to:

– Inspect for damage at known critical locations

– Replace parts deemed to be at end of service life - not fit for (further) service

• Most OEM’s have minor inspections every year (8000 hours) and major inspections every 3 years (24k hours) as a baseline

• OEM’s offer extensions to intervals, depending on parts installed and operation of machine.

– GE “Extendor” parts

– Alstom (GE?) “XL” extended life operation

– Siemens “41MAC” and rotor life extension

• Margin of +10% often supported by OEM for reliable units


OEM heavy duty interval philosophies

GE (factored hours or

factored starts) Siemens

Westinghouse (60Hz) –

equivalent base hours or

equivalent starts

Adjusted Hours

Ad

jus

ted

Sta

rts

Siemens and

GE Alstom ABB

– Linear EOH

MHI – EOH,

adjusted starts

Siemens

Alstom –

elliptical

EOH


Affected components

• Hot Gas Path

– Combustor, turbine and exhaust

– 8000/12000/24000/36000/48000 hours service life

– High capital cost (lots of expensive alloys)

• Rotor/Casings

– Shaft/discs, compressor blading, casings

– 100000+ hours

– 5000 starts typical

– Large capital cost and long lead times (18+ months) for large forgings/castings

• Balance of Plant

– Fuel skids, lube system, controls, inlet, protection systems etc.

– Ad-hoc basis, inspected/maintained during and outside outages.

– Most common cause of unavailability, but often easiest and cheapest to fix.


• Looking for loss of integrity

– Cracks

– Deformation

– Thinning

– Impact

• Looking for progressive damage mechanisms

– Fatigue

– Corrosion

– Wear

– Creep

• Cannot predict unexpected damage mechanisms and causes

– FOD/DOD

– Fuel upsets

Component Assessment


How to extend inspection intervals?

• AS ISO 21789:2014 Gas turbine applications – Safety

“All modifications and updates to…safety relevant components shall be implemented to achieve the required tolerable level of risk. Replacements of components beyond the requirements of normal maintenance as well as modifications and upgrades of equipment to newer technology requires that a risk assessment be performed to ensure that the resulting level of risk remains tolerable”

“Life predictions for safety related systems/components…shall be performed to establish that the gas turbine can be operated with a tolerable level of risk [throughout its lifetime]”

“Periodic maintenance of…safety relevant components shall be scheduled to ensure the safety of the plant.”


Other safety critical component approaches

• AS/NZS 3788-2006 Pressure equipment—In-service inspection:

“Operating with extended inspection intervals requires a formal process to be in place which advises changes to process conditions which impact on pressure equipment integrity, such as operating temperature, process fluid composition or process fluid velocity. The information justifying an extended inspection interval shall be documented by a competent person.”

“An inspection program based on inspection of sample equipment selected from those at the site may be used if soundly based on competently applied risk based inspection techniques, and on adequate historical data regarding…active causes of deterioration.”

“The inspection interval for pressure equipment may be extended…after establishing the following:

a) The wastage rate, or rate of deterioration of mechanical properties, is predictable.

b) The equipment is to remain in the same service, or the service and operating conditions are to remain unchanged.

c) The equipment is installed so that it is not subject to inadvertent corrosion by contamination, or loss of protective system.

d) No defects are present which would give rise to premature failure.

Extensions shall be justified, supported by a risk assessment, and shall be documented in an auditable manner.”


Summary of approach

• Identify current and/or acceptable risk levels

• Assess critical items affected by major interval extension

– Almost always higher temperature HGP components

– Potentially rotor/casings/compressor later in life

– Identify degradation mechanism(s) in play

– Assess current condition and forecast future condition based on the known progressive deterioration rates

– Use additional or expanded minor inspections to assess condition

• Analysis of effect of extension on risk

– “Likelihood” term dominates risk assessment on GT components

– “Consequence” almost always the same!


Quantitative

• Engineering analysis

– Stress/strain/materials

• Fitness for service (FFS)

– Time/cycles to critical crack size

• Creep life calculations

– Account for actual operation

– Benefit in even small reductions in

actual firing temperature

Qualitative

• Condition assessment

– Actual condition of parts

– Corrosion – pit depth

– Coatings – location and amount of

spallation

• Operator and fleet experience

– Empirical assessment of design,

materials and manufacture

– What have others got away with?

Risk analysis


Benefit of firing temperature reduction

Significant

material creep

life improvement

below nominal

firing temp.

Nominal firing

temperature


• Rotor Lifing– Low cycle fatigue– Creep– Fatigue crack growth

• Thermal model– Field & CFD data

• Stress model– Temperature– CF & pressure loads

• Fracture mechanics– fatigue / creep

• NDT

• Engineering analysis of actual

rotor life beyond standard?

Stress analysis and FFS

Critical

crack size

Crack

growth

Detectable crack size (NDT)

Gather DataCFD/Thermal Analysis

Stress Analysis

Fracture Mechanics


Candidates for interval extension

• Reliable and consistent operation

– BoP, condition monitoring, proactive maintenance

• Clean air and fuel

– Minimise corrosion risk

• Reduced firing temperature

– OEM modification – de-rate to increase component life

– Flexible operational requirements?

• Ability to perform minor off-line inspection

– Videoscope components of interest to monitor degradation

• Mature technology

– Extensive fleet history and knowledge/management of issues


Why, and why not

Pro’s

• Align with process/commercial timings

• Defer maintenance to where/when it is needed

• Encourages considered management of asset

Con’s

• Requires comprehensive analysis of risk

• Requires history of design/part/operation

• Requires consistency of operation

• Not suitable for all units

• Requires informed and agreeable insurer

• May result in scrapped components rather than refurbishable components –cost /benefit analysis required

“Periodic maintenance of…safety relevant components shall be

scheduled to ensure the safety of the plant.”


In summary

• Interval extension has been implemented for heavy industrial

GT’s worldwide, by OEM’s and independently of OEM’s

• A well documented risk analysis should be performed to justify

the ability to extend intervals, without significantly increasing

risk to personal and plant

• Understanding the damage mechanisms and actual condition

of the plant is vital in getting an accurate and safe analysis.

Matt Smillie - Quest Integrity NZL Ltd - Extending inspection intervals of large industrial gas turbines

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