GAS TURBINES

Six steps to successful

repair of GT components
By Hans van Esch, TEServices

E
ditor’s note: Hans van Esch, before disassembly (Fig 1). The infor- rotating part rubs a stationary compo-
founder of Turbine End-user mation compiled is helpful in selecting nent, such as at blade-tip and angel-
Services Inc (TEServices), the repair and coating vendor, devel- wing locations. Wear also results from
developed this primer on the oping the repair and bidding specifica- prolonged slow rolling of the turbine
six critical steps for refurbishing indus- tions, and avoiding rework caused by rotor on turning gear. So-called “blade
trial gas-turbine parts, using material fit-up problems. rock” is caused by an increase in clear-
from his respected three-day training Checking dimensions. Before ances between blade roots and the
course, “Metallurgical Aspects of IGT removing any parts, check clearances mating root slots in the wheel (Fig 3).
Component Refurbishment.” First pub- at critical locations (Fig 2). Recall that The problem gets worse with time as
lished as a series in consecutive issues hot-section components are subject to shutdowns accumulate.
of the COMBINED CYCLE Journal creep, which causes them to deform In most cases, manuals provided by
between 2Q/2005 and 1Q/2006, “Six and reduce as-built clearances. This the OEM describe in detail how plant
Steps” was updated in September is especially true for unsupported staff can measure clearances, shroud
2014 because of ongoing demand for components exposed to high tempera- lift, DSD and blade rock. Alterna-
the information. It is presented here tures and stresses—such as shrouded tively, a repair vendor or IGT consul-
in its entirety. blades (second-stage buckets) and tant can assist plant staff. The data
second-stage vane segments (a/k/a gathered, coupled with a thorough
Step 1: How nozzles), so-called shroud lifting and
downstream deflection (DSD).
visual inspection, helps determine if
dimensional correction is necessary.
to assess the Wear can increase clearances Such work can involve blade-tip and

condition of GT
beyond those recommended by the angel-wing restoration, DSD correc-
OEM (original equipment manufac- tion, restoration of blade clearances
parts onsite turer). It occurs most often where a (shroud blocks and blade tip heights),
and/or application of an
Owners and operators that take a anti-rock coating.
proactive role in defining industrial The information com-
gas turbine (IGT) repair and coating piled also is valuable in
requirements ensure receipt of qual- the verification both of
ity refurbished parts at a competitive dimensions taken dur-
price. Plus, top-quality components ing the repair vendor’s
are conducive to maximizing the time incoming inspection and
between overhauls, thereby reducing of its proposed repair
O&M costs. process. Doing the job
Refurbishment of components correctly the first time
should begin with parts inspection avoids unnecessary
and condition assessment at the plant repairs, late deliveries,
and time-consuming fit-
1. Onsite inspection of nozzles and transition piec- up problems during reas-
es provides information helpful for selecting a repair sembly.
and coating vendor
2. Checking clearances at criti-
cal locations prior to disassembly
is extremely important. Impact of
creep is to close clearances; wear
increases clearances (left)
3. Slow rotation of the turbine
rotor causes blades to “rock,”
opening clearances between blade
roots and mating rotor slots. Blade
rock should be measured during or
prior to disassembly (right)
34 COMBINED CYCLE JOURNAL, Third Quarter 2014
GAS TURBINES Different
Fatigue limits life mechanisms
4. This bucket limit life
requires some TLC.
Note wear, “bare” Failure region
spots where coating
has flaked off, and Design
cracks life

nit
it Oxidation,
un

ing u
5. Assessment of creep,
n ge corrosion,
equipment condition -ra

Pe a k
and wear
should determine the id limit life
M
need for refurbish-
ment, not theoretical unit
Starts -load
assumptions based Base
on starts and operat-
Design
ing hours as shown Hours life
When evaluating the proposed
repair process, avoid the temptation
to save money by taking shortcuts. For specified and the selection of a coat- dimensional check, metallurgical
example, restoration of a component ing to accommodate actual operating evaluation of coating and base mate-
and not its matching partner (such as conditions should increase the life of rial, mechanical testing, etc) against
bucket and shroud block) can lead to the part and extend the time between operating conditions (fired tempera-
problems during startup and/or abnor- overhauls. ture and hours, starts, etc) and iden-
mal operating conditions and not per- Importance of operating histo- tify the component that is driving the
forming rejuvenating heat treatments ry. The actual number of starts, type of need to initiate an overhaul. Develop
can shorten the ultimate usable life of shutdown (normal or trip), and hours a plan to extend the operating life of
components. of operation are used by the OEM to this part through better weldment,
Regular borescope inspections determine when it believes an overhaul coating selection, heat treatment, etc,
are valuable for periodic monitoring of is necessary (Fig 5). This calculation thereby extending the time between
parts condition. Findings can be con- can be further defined based on theo- planned outages.
firmed and any deterioration—such retical assumptions such as firing tem- Key to this approach is the devel-
as deposits, erosion, oxidation, corro- perature and fuel. It is not in the best opment of a meaningful database.
sion, melting, wear, impact damage, interest of most owners and operators Carefully track and record the oper-
etc—can be better evaluated with the because results often suggest overhaul ating history of all critical parts,
naked eye or with aid of a magnifying earlier than may be necessary. When making sure to include such infor-
glass when components are disassem- overhauls are governed by an OEM’s mation as parts coatings and base
bled (Fig 4). Keep in mind that visual long-term service agreement, you may materials, hours at temperature,
inspection by itself can be misleading have no other option. use of water or steam for emissions
and should be used primarily as a tool However, absent an LTSA, a better control and power augmentation,
for identifying areas in need of further method might be to base overhauls etc. Access to such an information
assessment with NDE (nondestructive on the actual condition of critical resource facilitates decision-making
examination), dimensional inspection, components. Plot your assessment of on outage scheduling, identification
and metallurgical evaluations. component condition (based on NDE, of components to repair and those
Evaluation of information compiled
during the visual examination and
dimensional checks is particularly Acronyms defined PT—Liquid penetrant test, also
helpful for identifying damage that called LPT
APS—Air plasma spraying
may require further analysis before MT—Magnetic particle test
AR—As received
final decisions are made regarding CNC—Computer numeric control RT—Radiographic test
parts replacement or reconditioning. UT—Ultrasonic test
CVD—Chemical vapor deposition
When component deterioration is so
DSD—Downstream deflection VT—Visual test
severe that it dictates the overhaul
EDM—Electrical discharge Nadcap (see footnote)
cycle, a root-cause analysis (RCA) is
in order. This should be conducted machining OEM—Original equipment manufac-
before any work, such as cleaning, GT—Gas turbine turer
is performed on the components that EB—Electron beam
RCA—Root-cause analysis
might compromise the investigation. HAZ—Heat-affected zone
HIP—Hot isostatic pressure SEM—Scanning electron microscope
Detailed operating data is needed to
fully understand why the damage HVOF—High-velocity oxygen fuel TIG—Tungsten inert gas (same as
occurred. IGT—Industrial gas turbine GMAW)
Your repair vendor may be capable ISO—International Organization for VPS—Vacuum plasma spraying
of such metallurgical analysis. If not, Standardization NADCAP began as an acronym for National
an independent laboratory can conduct LPPS—Low-pressure plasma Aerospace and Defense Contractors Accredita-
the damage assessment. The labora- spraying tion Program. Today that program is internation-
al in scope and known simply as Nadcap. The
tory selected should have a good under- LTSA—Long-term service agreement Performance Review Institute (www.pri-network.
standing of IGTs in general, as well as MCrAlY—Nickel, cobalt, iron, or a org/Nadcap), Warrendale, Pa, administers the
specific knowledge of your engine type combination of these elements, program, which “provides unbiased, inde-
and running conditions. Such analy- plus chromium, aluminum, and pendent manufacturing process and product
sis often provides guidance on how to assessments and certification services for the
yttrium purpose of adding value, reducing total cost,
improve component performance, the NDE—Nondestructive examination and facilitating relationships between primes
required inspection cycle, and ulti- ET—Eddy current test and suppliers.”
mate usable life. Thus the repair work
36 COMBINED CYCLE JOURNAL, Third Quarter 2014
GAS TURBINES
Stage 1 Stage 2 Stage 3 Stage 4

Conduct visual Apply internal

Blend
inspection, Disassemble and external
repair/prep
check purchase coating
order

Chemical and Weld, braze, Treatment of

Metallurgical mechanical and coupon external
evaluation stripping/cleaning repair coating

NDT:
Check Inspect external Machine Check coating
dimensions and internal and blend and cooling
surfaces passages
7. Onsite inspection identified the
NDT: Check
Internal HIP or dimensions need to repair this Model 7FA second-
engineering pre-weld Assemble stage nozzle
review heat treatment
Post-weld
heat
treatment Check
Report NDT, including dimensions
findings and flow check
conclusions,
and verify NDT: Check
dimensions
Add moment
Dimensional weight or other
inspection

Touch-up weld
and blend repair Clean, inspect,
Engineering and pack
review for shipment
8. Dimensional
6. What to include Pre-coating checks pinpoint
in repair specs engineering
review deformation and
is outlined in flow Final
Report engineering other problems
chart
findings and review that should be
conclusions, addressed in
and verify Report
findings and your repair spec
conclusions, (above)
and verify Issue final
report; final 9. Visual inspec-
verification tion alone some-
times can identify
parts that cannot
to replace, etc, based on your plant’s are a good starting point for developing be repaired—like
actual needs. Currently there are the specific component repair specs you this Frame 7
software programs and online main- need for the next overhaul. If you have bucket (right)
tenance services available to assist no experience in writing specifications
the IGT operator with this. and have no good examples available facility to report its findings and rec-
for reference, call a knowledgeable ommendations after each stage and
Step 2: Preparing colleague or an IGT consultant spe-
cializing in this work. Avoid “help”
not proceed with the work before you
or your representative approves. In
meaningful from repair facilities that might be on most cases, dividing the repair process

component repair
your bidders’ list. The flow chart (Fig into the four stages illustrated in the
6) lists some of the items you’ll want flow chart will produce positive results.
specs to include.
Key to your specification develop-
The steps are:
Stage 1 Receive and conduct the ini-
Preparation of component repair ment is information compiled during tial inspection.
specifications is the first step in ven- visual inspection (Fig 7) and check- Stage 2 Disassemble components,
dor selection and these specs should ing of clearances (Fig 8) discussed clean/strip, heat treat, and
be part of your bid package. The in Step 1. Also, factor in previous inspect.
specs also provide a framework for repair experience with the same Stage 3 Repair, heat treat, and inspect.
evaluation of work in progress and components and the experience of Stage 4 Coat, reassemble, and make
for conduct and verification of criti- others on similar parts—things you a final inspection.
cal inspections from receipt of parts might have learned at a user-group Stage 1: Receive, inspect. Start
by the contractor through project meeting, for example. your repair specification with receiv-
completion. To ensure quality work, divide the ing, where the parts are visually
Existing specifications from previ- repair process—and your specs—into inspected to ensure that no handling
ous repairs or end-user organizations logical stages. You want the repair and transport damage has occurred.
38 COMBINED CYCLE JOURNAL, Third Quarter 2014
 GAS TURBINES
will be successful in regaining desired and diffusion heat treatments, the
properties post repair. Sometimes metallurgical condition and mechani-
special heat treatment—such as hot cal properties can differ significantly
isostatic pressure (HIP)—is required among repair vendors.
to make parts serviceable. When specifying the type of NDE
External and internal (cooling for your components, keep in mind
cavities) surfaces also require evalu- that ultrasonic (UT) and eddy-cur-
ation. Consider these possibilities: rent testing (ET) can detect sub-
The internal coating can be in such surface indications and, therefore,
10. Metallurgical evaluation helps good condition that stripping and may have advantages over visual
pinpoint problems such as this inter- recoating may not be necessary. By inspection and liquid-penetrant test-
nal oxidation attack to IN 738 material contrast, uncoated internal surfaces ing (PT). Also, these technologies can
can be so heavily attacked by oxida- determine wall thicknesses at critical
Set this dimension to nominal tion that oxidation products penetrate locations. Once again, the inspection
the grain boundaries (Fig 10) and team should verify that cooling pas-
ultimately form cracks that reach the sages are open.
external surface. More detailed dimensional checks
Evaluation of the external sur- are required at this stage—such as
face can reveal if the coating system determining the downstream deflec-
provided the required protection for tion of nozzle or vane segments (Fig
H I LA D the component or if another coating 11). This step completes the so-called
is required. In the case of uncoated “incoming” inspection and another engi-
external surfaces, the amount of deg- neering review with the owner, and/or
radation can be determined; it must its IGT consultant, is recommended to
B be removed before repairs are made ensure that the scope of work still will
C
K D or the coating is applied. provide the desired outcome.
Following incoming inspection and Stage 3: Repair, heat treat,
Known dimension Y metallurgical evaluation, an engineer- inspect. The repair process can start
11. Dimensional checks conducted ing review should be conducted with after a component is cleaned/stripped,
by the repair shop are necessary to the owner or its representative pres- solution heat-treated, and inspected.
determine such things as downstream ent. Both parties must come away In most cases, gas tungsten arc weld-
deflection of nozzle or vane segments from that meeting confident that the ing (GTAW)—also called TIG (tung-
original scope of work still will result sten inert gas) welding— is used for
in the desired outcome. Adjustments repair, although more and more facili-
in cleaning, stripping, repair, and the ties also use brazing for component
coating process can be made, if neces- restoration, mostly driven by need to
sary; worst case is that components reduce cost.
must be scrapped. Keep in mind that while weld
Stage 2: Disassemble, clean and methods and filler materials are
strip, heat treat, inspect. A more comparable throughout the indus-
thorough inspection of parts received try, braze repairs are proprietary
for repairs requires disassembly and processes. This makes it especially
removal of hardware—such as core important for you to define in your
plugs, impingement sleeves, etc. This spec if brazing is allowed and where
work makes the surface accessible for and under what circumstances it
12. Replacement of sections of cleaning, stripping, and heat treat- can be used. Also, inspection after
components sometimes is necessary ment without risking damage to other prepping the components for welding
to make parts serviceable areas of the component. and brazing, and after application
Most of the coating generally is and heat treatments, is important to
Individual parts should be identified removed by chemical stripping. Then ensure the desired quality is achieved.
and marked, and match the shipping the external surface can be inspected Note: Lately, cost-aware (or driven)
and purchase documentation. Dimen- by heat tinting or macro-etching, so repair facilities, including OEMs, are
sional and visual inspections are next, any remaining coating and oxidation/ replacing PT and visual inspections
followed by disassembly. corrosion products can be identified with visual inspections only.
Flow testing of internal cooling and eliminated by blending. Replacement of sections of your
circuits with rod, water, or air should Important: Specify tight control components—so-called “coupon
be performed at the beginning of the stripping/cleaning processes to avoid repair”—sometimes is required to
repair process to obtain data before unnecessary thinning of the compo- make the parts serviceable (Fig 12).
possible plugging of these holes dur- nent. The replacement material should have
ing cleaning and other operations. NDE. Preparation of components metallurgical condition and mechani-
Visual inspection during early work should include standard pre-weld cal properties the same or better than
can confirm the repairability of a solution heat treatment for nickel the original. The vendor’s quality sys-
component (Fig 9) before more cost is precipitation and cobalt-based super- tem must be capable of ensuring that
incurred. This also assists in selecting alloys. Occasionally, metallurgical correct chemistry and other properties
sacrificial parts—those from which evaluation also suggests the need are within specifications.
representative metallurgical samples for specialized heat treatment. Note: Post-weld heat treatments that
will be removed. For many alloys such as OEM-pat- follow the repairs should be combined
Metallurgical evaluation is vital ented materials, no published heat with the heat treatments required
for characterizing the base material to treatments are available and that for coating application and diffusion.
ensure that standard heat treatment combined with non-standard coating Important variables in the heat-
40 COMBINED CYCLE JOURNAL, Third Quarter 2014
treatment process include time on
temperature, furnace atmosphere,
and heating/cooling rates. Note: The
heat treatments should not negatively
influence the metallurgical condition
and mechanical properties of the base
material and should be discussed and
agreed upon prior to repair being
released by the customer and/or its
IGT consultant.
Be aware that vendors sometimes
subcontract heat treatment and
other critical process steps requir-
ing detailed instruction on their
execution. When preparing repair
specs, ensure that your company has
access to its components at both the
contracted repair facility and all of
its subcontractors until the recondi-
tioned parts are returned after project
completion.
Before coating and reassembly are
permitted, yet another inspection is
necessary. This is especially critical
for items that cannot be inspected or
corrected later in the repair process
because of heat-treatment, coating, or
assembly issues. As for the preceding
steps, another engineering review is
suggested here to decide if coating and
assembly should proceed.
Stage 4: Coat, assemble, inspect.
The application process is as critical to
coating performance as the selection of
the proper coating system. Your spec
should address so-called “first-article”
qualification, process repeatability,
and final inspection. Coating quality
should meet specification requirements
and be as good as that demonstrated
on the same or comparable component
during the qualification trial. Metal-
lurgical evaluation of the qualification
sample should be part of the verifica-
tion process (Fig 13).
The application procedure must
lend confidence that this result can
be repeated as many times as neces-
sary without question. Quality checks
should be conducted to verify process
repeatability.
After coating and inspection are
complete, component reassembly can
proceed. During reassembly, dimen-
sional checks are necessary to ensure
strips, etc. Final inspection should
include dimensional verification, like
Step 3: Selecting
proper installation of core plugs, wear area and harmonics checks of nozzles, the appropriate
moment weight and sequencing for
blades, unrestricted internal cooling vendor to refurbish
passages, and visual confirmation of
a job well done.
turbine parts
The repair process can be con- When selecting a repair facility to
sidered complete after the repair refurbish parts for your IGTs, keep
vendor’s final report is received and in mind that cost—the variable upon
accepted. It should include all certifi- which many decisions are based—is
cations, inspections, and engineering only one of several important consid-
recommendations. This information erations. Shop capabilities, experience
is important should problems arise. of employees, vendor performance on
13. Metallurgical evaluation of coat- Also, it provides valuable input to similar jobs, quality control, sharing
ing quality is a necessary part of the repair specification development for of technical details and documenta-
repair process the next overhaul. tion, communication, and other fac-
COMBINED CYCLE JOURNAL, Third Quarter 2014 41
GAS TURBINES

Table 1: Experience and reputation assessment

Rating points
1 3 5
Field service experience Similar unit < 3 years > 3 years
Conducted overhaul Similar unit < 3 years > 3 years
Performed incoming inspection of part < 2 years < 5 years > 5 years
Performed repair on part 1 year < 5 years > 5 years
Fixtures available for your specific
components (inspection, repair, and coating) In development Available Qualified
Coated part 1 year <3 years Field experience
Technician/supervisor with company
(the person who will direct your repairs) > 2 years > 4 years > 8 years

Quality as seen by customers Not meeting Meets specs, not Meets specs and
specifications expectations expectations
Resolving quality issues Unwilling,
must be forced Willing, but slow Proactive, highest priority

Delivery time Within two weeks Always on time On time, monitored and
of date promised available for review
Summarized and Summarized and complete,
Documentation of repair and coating Critical inspection complete work scope including digital pictures,
steps and final condition HT, and critical dimensions

Table 2: Technology and subcontractor experience assessment

Rating points
1 3 5
Nondestructive examination Technologies, < 3 years Technologies, < 5 years Technologies, > 5 years
Metallurgical laboratory (experience Qualified vendor Internal, > 3 years Internal, > 10 years
of metallurgical engineer) > 10 years
Stripping (similar coating, Qualified vendor Internal, > 5 years Internal, > 10 years
base material, design) > 10 years
Heat treatment: Vacuum Qualified vendor (Nedcap, ISO) Internal Internal (Nedcap, ISO)
Heat treatment: Internal Internal (Nedcap, ISO)
Controlled atmosphere, other
Weld repair: GTAW, plasma > 1 year > 3 years > 5 years
Weld repair: Laser or EB Qualified vendor, > 5 years Internal, > 3 years Internal, > 5 years
Weld method/application Performed Performed to ASME Up-to-date per ASME
qualification Section 9 or AWS 2.1 Section 9 or AWS 2.1
Braze repair > 2 years > 4 years > 8 years
Braze method/application Published, including
qualification Performed External qualification mechanical properties
Machining: Conventional, Conventional And CNC And EDM or laser
CNC, EDM, laser
Coating: Thermal spray Qualified vendor Internal APS, HVOF Internal robotic application
> 5 years > 3 years > 5 years
Coating: Diffusion Qualified vendor Pack segmentation True CVD
> 5 years > 3 years > 3 years

tors are equally important—perhaps of repairing compressor, combustor, differ depending on the IGT com-
even more so. and hot-section parts for your last ponent, work to be performed, and
A thorough evaluation of alterna- outage are necessarily the best part- schedule. For someone with limited
tive service companies is critical to ners for the next overhaul. Likewise, vendor auditing experience, the guid-
selecting the best repair facility for a company rejected previously may ance offered here for evaluation of (1)
the work required in support of your have acquired new capabilities, hired experience and reputation, (2) in-house
next outage. The sector of the IGT new people, etc, and deserves recon- and subcontractor technical capa-
services industry concerned with sideration. The competitive nature bilities, and (3) human resources and
parts refurbishment is dynamic and of the IGT-based generation busi- management systems, will help you
it is common for shops to be acquired, ness suggests the need for continual get started. The scorecards provided
relocated, expanded through acquisi- monitoring or recent audit of service are particularly valuable and should
tion, or closed with little or no public companies. be customized to suit your specific
disclosure. Every experienced plant and main- requirements.
This can have major impact on tenance manager has his or her meth- Experience and reputation.
repair quality, so don’t assume the odology for evaluating alternative Generally, it is in your best interest
facilities that did an acceptable job repair shops—procedures that may to select a repair facility with expe-
42 COMBINED CYCLE JOURNAL, Third Quarter 2014
 Gas Turbine
rience both on your type of IGT and
on the specific components requiring
refurbishment. Such experience helps
the end user because the service pro-
vider has a better understanding of

Services
the condition of the components, what
the critical dimensions are, what the
typical damage and failure modes are,
which coating systems will provide the
level of protection desired, etc.
If a candidate facility does not have
direct experience on your machine,
perhaps it has done comparable work
on models similar in design, base mate-
rial, and coating and cooling systems.
The facility’s reference list of projects
completed over the last three to five
years pertinent to the work you require
should contain adequate detail for
follow-up (check the references with
your colleagues), due diligence, and
decision-making.
Valuable insights on the perfor-
mance of repair facilities can be gained
by interviewing the candidates’ cus-
tomers—past and present. One of the
most important things to learn is how
the prospective facility resolves qual-
ity issues. All repair facilities will run
into problems at some point, but how
quickly the company communicates
and responds to problems, and the
methods it uses to address them, is of
vital importance to you.
Your interviews will reveal that
most repair facilities are willing to
develop repair methods and coating
applications to suit specific needs.
This is positive; however, if your com-
ponents are the first articles repaired
by a new method, more time and
resources must be put against this
effort than required by a proven pro-
cedure. And don’t forget to consider
the end-user’s risks associated with Gas Turbine Services
any new procedure. 19 British American Blvd. | Latham, NY 12110
As part of your assessment, be sure
to inquire about the fixtures available ph. 518-399-3616 | www.MDAturbines.com
to facilitate the inspection and repair of
components (Table 1). Fixtures can pay
dividends in the inspection of complex PARTS | SERVICES | REPAIRS
components, such as transition pieces,
(Fig 14), while reducing shop time and
accuracy of the required check to be During the request-for-quotation OEM on new parts. For example, in
performed. Also, the availability of process the evaluation of repair facili- the past, MCrAlY often was applied
fixtures offers an indication of how ties can be confirmed. Facilities with on advanced blades and buckets with
much work a particular shop does, or good experience will come up with LPPS or VPS; today, however, HVOF-
has done, on specific components for some intelligent questions and offer applied MCrAlY typically is used for
a given IGT model. some options to reduce the costs or repairs, and even new parts.
Record-keeping is another area to prolong the lives of your components. Information gathered from the
investigate. After repairs are com- Technology and subcontract- onsite visual inspection, and from the
plete, you want documentation on ing. The assessment of IGT compo- AR (as-received) condition NDE and
the condition of your components (ID nents conducted as part of Step 1, metallurgical inspection at the repair
by serial number) “as received” and and the repair scope developed in facility, sometimes suggests a custom-
“final,” in addition to certifications Step 2, provide information of value ized repair and coating solution that
for important process steps—such as for determining the optimum tech- can reduce the expected cost of refur-
heat treatments, stripping, coating, nologies for parts refurbishment. bishment and/or extend the service life
shot-peening, and most important At this stage, keep in mind that it of components.
of all, critical dimensions before and is not always necessary to apply the Make sure you don’t miss an oppor-
after repair. same coating system provided by the tunity for competitive advantage by
COMBINED CYCLE JOURNAL, Third Quarter 2014 43
GAS TURBINES

Table 3: HR and management system assessment

Rating points
1 3 5
Years since last organizational change
(location, ownership, and/or management) >2 >4 >8
Available and Available and up to date and
Organization chart Available up to date portraying a quality organization
Available and Available and up to date and
Job descriptions Available up to date portraying a quality organization
Available and Available and up to date and
Quality manual Available up to date portraying a quality organization
Quality system audit to ISO, Nedcap, etc Internal audit External audit External audit, > 5 years
Available and Available and up to date
Procedures (welding, coating, inspection) Available up to date and practiced
Available, signed, Available and up to date
Workscopes Available and up to date and practiced
Materials/parts tracking (replacement Order requires
parts, filler and coating materials) certification Verification Traceability
Subcontractor work First-article
(heat treatments, coating, shot peening) Vendor audited verification Verification of work performed
Engineering department One person Three people Five people
(mechanical, metallurgical, process) > 10 years > 10 years (avg) > 10 years (avg)
Inspection department One Level II Three Level II More than three Level II and
(VT, PT, UT, MT, RT, ET) > 3 years per > 5 years per one III with > 5 years per
discipline discipline discipline
TIG welding (experience/qualifications More than two More than five welders
welders, > 3 years > 5 years Five welders
for each material) > 5 years qualified
experience (avg) experience (avg)
Two blenders More than two
Blenders and experience > 3 years blenders, > 5 years
Two operators More than two Two robotics operators
Coating operators and experience
> 3 years operators, > 5 years > 5 years

selecting a service provider that many capable engineers and

will partner with you to advise repair technicians. Some of
unselfishly on repairs and coating these people joined forces and
systems (Table 2). An indepen- started small specialty shops,
dent IGT consultant could also be contributing to the relatively
helpful in this. large number of repair facili-
Refurbishment of IGT compo- ties—well over a dozen in the
nents for late-model machines Houston area alone.
often requires sophisticated Small shops do not always
stripping, cleaning, repair, coat- have their organizational
ing, and inspection technologies charts, job descriptions, qual-
that are expensive to install ity manuals and systems, pro-
and support. Thus many shops cedures, and qualifications in
are not equipped to perform all place. Important to remember
these processes and steps in- with respect to small organiza-
house for all IGT models and tions is that an experienced core
they rely on subcontractors for staff is capable of performing
certain tasks. quality repairs and coatings at
To illustrate: Chemical strip- a competitive price on specific
ping of IGT components poses GT components. However, if
environmental and health con- the repair process increases in
cerns and is often subcontracted to 14. Special fixtures facilitate repair complexity—as it does for advanced
a specialty shop. Recently, however, and inspection of transition pieces components—or when the organiza-
the most experienced subcontractor tion grows too quickly, quality issues
was taken over and later closed down tions. This requires expertise on, and can arise.
for these same reasons, leaving the experience with, a process not present When turn-around time is impor-
repair facilities scrambling for an at your vendor. And, since the subcon- tant, a small company may not have
alternative. tractor is an extension of the primary the resources to repair and coat com-
To have certain repair or coat- company, it should be audited as part ponents to meet your schedule. On
ing steps performed by a specialized of your capabilities assessment. the other hand, larger companies may
subcontractor can be advantageous. Human resources and manage- not always give the attention that you
However, your repair facility is still ment systems. In the early 2000s, and your components deserve. Some-
responsible for final product quality competition in the IGT services sector times the best result can be achieved
and must monitor the subcontractor forced the closure or sale of marginal by dividing the component repair and
to ensure conformance to specifica- companies and drove out of work coating work to multiple small, special-
44 COMBINED CYCLE JOURNAL, Third Quarter 2014
GAS TURBINES

15. Dye-penetrant testing reveals

cracks in coating
16. Inspection fixtures enable accurate
measure of critical dimensions (right)
ist facilities. tion at the repair facility, as well as Initial visual inspection. Purpose of
When conducting vendor assess- conducting a visual inspection and visual inspection at this stage essen-
ments, be mindful that training and a metallurgical evaluation. An engi- tially is to confirm the degradation
qualification of inspectors, welders, neering report reviewing all findings modes identified during the onsite
blenders, and other technicians is the should confirm that the scope of work inspection. Occasionally, however,
foundation for quality work. The score- will produce the desired end product. the better lighting and higher-mag-
card presented in Table 3 offers a valu- If discrepancies occur, the end user nification inspection tools—and more
able reminder of items to investigate. or its representative should review experienced personnel—available in
Important to remember: OEMs the components, inspection reports, a shop environment reveal damage
recently acquired some of the indepen- and recommendations, and then meet not observed at the plant. Be sure to
dent repair facilities—losing in transi- with the repair vendor. Work should document these items with digital pho-
tion experienced and knowledgeable be stopped until after that meeting. tography and investigate further such
human resources, and transforming A revised scope of work may be nec- important findings as coating cracks
the organization to OEM “standards.” essary. and rimming, oxidation, corrosion and
Another market dynamic: Some Disassembly and inspection. erosion, and wear.
end users have found the most com- After the scope of work is confirmed, Mapping of every defect identified
petitive and capable independent or modified to reflect the findings of in each component and summariz-
repair shops filled during outage the initial inspection, the component ing the information in one drawing
season, because of the increased is disassembled from its retaining ring, could be advantageous for defining
number of engines in service and the blocks, etc. It is important that the the typical wear and damage pat-
higher GT capacity factors resulting work order clearly define which com- tern for that component exposed to
from retirements of coal-fired plants. ponents and parts will be refurbished that operating condition. Although
Several repair facilities responded by and reused and which will be replaced. mapping of defects was common 10 or
increasing shop space and staff. The Next step is to remove hardware such more years ago, additional inspection
negative impact for a few: Overcapac- as seals, pins, core plugs, covers, zip- charges from the repair facility can be
ity, a buyers’ market, and smaller pers, and bolts. expected for this labor-intensive work.
profit margins because expenses Stripping/cleaning of parts slated If the end user’s onsite findings differ
remained roughly the same. Cost- for refurbishment are important steps: significantly from degradation iden-
cutting to boost profit margins can Clean surfaces are required to ensure tified in the shop, the plant should
have a negative impact on quality. successful inspection, repair, and consider redefining its inspection
recoating. Confirmation of surface procedures to bring them in-line with
Step 4: Key stages cleanliness is highly recommended
prior to pre-weld solution heat treat-
current industry practice.
Metallurgical evaluation. In most
in the repair ment. Heat tinting and macro etching cases, extensive metallurgical evalu-

process
are two techniques used to assure that ation—even destructive testing—is
the degree of cleanliness required has necessary to obtain the information
Initial inspection. After assessing been achieved. needed to ensure quality, long-lived
the condition of GT components at the After pre-weld solution heat treat- repairs. Representative metallurgi-
plant site, creating repair and coating ment, NDE should be performed to cal samples, as well as samples from
specifications, and selecting the repair identify any cracking, wall thinning, and locations of special concern identified
vendor, your parts can be released for cooling-hole blockage (Fig 15). Dimen- in the preceding visual inspection, are
refurbishment and recoating. The sional checks define deflection of walls, removed for closer examination. Using
first of four stages in the repair pro- airfoils, shrouds, z-notches, etc. What a microscope, a qualified metallurgist
cess is confirmation of, or assessment follows is a series of thumbnail sketches can accurately assess the condition of
of, component condition (refer to flow describing the key actions required the base material and internal and
chart included in Step 2). during Stage 2 of the repair process to external surfaces.
This involves taking dimensions in assure accurate verification by the repair Further definition of the base mate-
the as-received and assembled condi- vendor of component condition: rial, coatings, environmental attack, and
46 COMBINED CYCLE JOURNAL, Third Quarter 2014
 needs that should be addressed during
refurbishment. Or, in the extreme, it
may suggest that component replace-
ment should be considered.
Beware that some repair facilities
forego this inspection to reduce their
costs. This means critical dimension-
al information that might be required
for intermediate and final machining,
and inspection of the parts, will be
lost. Recently developed coordinate
measurement equipment and white-
or blue-light scanning are replacing
conventional inspection methods;
although these methods can increase
the accuracy and efficiency of the
inspection, caution is required to
ensure the correct and proper dimen-
sions are measured.
Stripping/cleaning and surface
inspection. The stripping plan devel-
oped for a given component should
consider its base material and coating
as well as its condition as determined
by metallurgical evaluation. Most of
the coating is removed by chemical
stripping or grit blasting and lately
more and more water blasting is used
for combustion parts as well as blades
and vanes. Coating material and/or
corrosion/oxidation products remain-
ing after this step are removed by hand
blending following a heat tint of macro-

Small has never been bigger.

etch, a procedure that is repeated until
the surface is completely clean (Fig 17).
Removal of coatings from compo-
Not every job calls for a Goliath sized solution. When it comes to nents with very thin walls demands
small turbine lube oil and compressor reservoirs with contamination special care to ensure that the integrity
problems, the FSTO is sized just right. Combining ICB and VTM of the base material is not compro-
technologies, varnish doesn’t stand a chance. mised. Process usually is time-inten-
sive. Be wary of repair facilities that
hyprofiltration.com/ suggest otherwise. Note that it is not
always necessary to remove all of the
bondcoat on these components. How-
ever, success weighs heavily on having
a proactive owner/operator available to
verify that the coating is removed to
the extent required without damaging
the base material.
Heat treatments typically are per-
other deterioration can be obtained using the identification of manufacturing formed in a vacuum furnace; argon
a scanning electron microscope (SEM) flaws, and the expected lifetimes of and hydrogen furnace atmospheres
equipped with energy dispersive x-ray components based on their current sometimes are viable alternatives to a
microanalysis capability. It is good prac- condition and anticipated future oper- vacuum. Heat-up and cool-down rates,
tice to examine samples, both etched and ating regimes. as well as time at temperature, are criti-
unetched, at low (50X), medium (500X), Dimensional inspection. The initial, cal parameters for achieving the desired
and, in the case of gamma prime, at high as-received inspection helps identify metallurgical and mechanical proper-
(10,000X) magnification. issues associated with the fit-up of ties. To ensure quality heat treatment,
Metallurgical findings should be various components—such as those verify furnace atmosphere, temperature,
used to confirm or fine-tune the refur- associated with clearances, throat pressure, etc, with proper monitoring/
bishment process—for example, to opening, seal grooves, bucket rock, and recording instrumentation. Also, remem-
suggest the proper stripping method, z-notch contact area. Some of this work ber to request feedback from follow-on
the requirement to strip and recoat relies on the use of fixtures (Fig 16) to heat-treatment cycles to ensure that the
internal passages, to help identify replicate the location and position of desired metallurgical and mechanical
the optimum coating system for your a given component within the IGT— properties were indeed achieved.
application, etc. thereby enabling accurate inspection NDE alternatives for assessing com-
The metallurgist’s report also and measurement of critical dimen- ponent condition after pre-weld solution
provides valuable feedback to the sions (for example, wall thickness, heat treatment include visual, liquid
end user regarding the level of suc- deflection, tip height). The information penetrant, ultrasonic, eddy current, and
cess of previous repairs and coatings, gathered helps to identify any special x-ray testing. Depending on component
48 COMBINED CYCLE JOURNAL, Third Quarter 2014
 Final Filters &
(H)EPA Air Filters

condition, one or more methods is examination. That step complete,

suitable for identifying cracks on or repairs now can be made where
below the material surface, pinpoint- required; also, the components can
ing cooling-system problems, and be recoated. For a snapshot of key
measuring wall thickness. Special steps, refer to the flow chart in
methods, such as flow and frequency Step 2.
tests, can be helpful in identifying Preparation for welding,
problems such as overheating and brazing, coating. Before any of
high-cycle fatigue. these processes can be performed,
The owner/operator should the surface must be prepared to
compare the repair vendor’s NDE ensure that it is free of cracks,
results to its expectations based on coating, oxidation and corrosion
visual and dimensional (clearances) products, and other contaminants.
inspection at the plant and compo- For weld repair, cracks generally
nent operating history. If discrepancies 17. Heat tinting of liners should con- are removed manually with air tools,
occur, the end user or its representa- firm that all coating has been stripped followed by PT and visual inspection
tive should review the components, to ensure that the end of the crack has
inspection reports, and recommenda- work, specifications, and schedule, be been reached. Next, the penetrant,
tions, and then meet with the repair sure to update the verification points developer, oil, and other contaminants
vendor. Work should be stopped until in the repair process as well as final are removed to enable proper welding.
after that meeting. acceptance terms—if applicable. You’ll Note that that the removal of
Engineering review of Stage 2 sleep better knowing you have done cracks is a critical task in Step 5
inspection results is necessary to everything possible to ensure reli- and the temptation for taking “time-
ensure that the existing scope of work able operation of your IGT through saving shortcuts” should be avoided.
for the refurbishment effort is condu- the next cycle. Be aware, too, that some shops have
cive to achieving the desired results. substituted visual inspection only for
As described in Stage 1, repair work
should not proceed until the end user, Step 5: Verification stationary components, eliminating PT
to reduce cost. This may have quality
or its representative, reviews the during the implications. Premature cracks can

refurbishment
inspection reports and recommenda- develop during pre-weld activities—
tions and discusses them with the such as heat treatment—or later dur-
repair vendor.
Don’t be reluctant to make neces-
process ing IGT operation when preparation
activities are rushed.
sary changes to the scope of work at Step 4 offered guidance for proac- A few repair facilities use the same
this point in the refurbishment pro- tive users during the initial inspec- prep processes for brazing as they do
cess. However, be sure to factor any tion process—including metallurgical for welding. However, most rely on the
commercial and delivery-time conse- evaluation, disassembly, cleaning and hydrogen, vacuum, or vacuum/hydrogen
quences into your analysis. Finally, if tripping, post-weld heat treatment, partial pressure for cobalt-based materi-
any changes are made to the scope of and dimensional and nondestructive als and fluoride ion cleaning processes
COMBINED CYCLE JOURNAL, Third Quarter 2014 49
GAS TURBINES
for precipitation-hardened nickel-base Surface preparation using grit of and not disclosed to the customer. But
materials. Their effectiveness on inter- controlled quality and size applied even when disclosed, only limited met-
nal surfaces exposed by cracks only can at the proper pressure is important. allurgical and mechanical data—such
be verified by conducting a metallurgical Use of new grit is recommended as stress rupture, ductility, and impact
inspection of a representative sample before coating application; metallic and crack resistance—are provided.
processed with each batch. grit before brazing. The time interval Further, if weld repair is required after
between prep and the welding, braz- brazing it can be challenging.
ing, or coating stages should be short However, the recent development of
and the environment controlled to brazes containing boron and hafnium,
prevent surface contamination. Keep which act as melting-point depressants,
in mind that outsourcing component have been able to overcome most of the
cleaning to a specialty shop can be in ductility and weld-repair concerns. If
vain unless extreme care is taken to your contractor uses braze technology
ensure contaminant-free handling, during the repair process (refer to Step
packaging, and transport. 3), demand complete first-article test
It is particularly important for results. For later lots, verify use of the
the end user or its representative to same procedure that was applied to the
ensure that the contractor adheres to first article. Also, that proper inspection
its procedures and scope of work as the steps are in place—including metallur-
first batch of components is repaired, gical evaluation.
even if that means several visits or a Coating. Today, most coatings
prolonged stay. Items to include on used in repair processes are applied
your audit checklist: quality of person- by robotic thermal spraying. Diffusion
nel training, visual inspection of the coatings find greatest application on
end results, and the care displayed internal surfaces. At recent industry
18. Temperature control of the in handling components to prevent meetings addressing thermal spray
repair process is critical for achiev- contamination. coatings, the importance of repeat-
ing desired results Welding. One of your tasks during ability was emphasized by several
the repair-facility selection phase was presenters. According to the experts,
to verify independent qualification of more than 300 parameters can influ-
both the welding process and the weld- ence the quality of a thermal coating.
ers. Components repaired by trained Thus process repeatability should be
and certified personnel following the a primary focus of your vendor audit.
parameters outlined in the qualifica- This includes making sure technicians
tion documentation generally assures performing the work follow instruc-
that job quality and performance will tions to the letter.
meet expectations. Monitoring of process quality and
Welding using a low-heat-impact repeatability includes the following:
method is recommended to enable n Verify coating thickness using eddy-
a high-quality repair. It minimizes current, weight-gain, or physical
component deformation and the size measurement methods.
of the heat-affected zone (HAZ). Time n Take bond-strength and metal-
constraints, such as those imposed by lurgical samples to confirm use of
19. Quality is a concern with braze lead-time or commercial issues, some- specified procedures.
repairs because shops generally are times suggests the use of a heat input Perform a destructive metallurgical
reluctant to disclose process details that is higher than desirable. Caution evaluation of an actual part (Fig 20)
is advised here: It can contribute to to evaluate coating quality. When this
deformation or premature cracking at is not possible, verify that specifica-
the HAZ (Fig 18). tions for grit cleaning, time between
Even components that can pass the cleaning and coating application, etc,
final inspection may be of substan- are followed to ensure a quality equal
dard quality if the welding process is to the quality demonstrated on the
rushed. An unannounced visit during first article.
weld repair is suggested to verify the Pre-coat inspection. Finally, it is
repair facility’s compliance with its important to verify findings of inspec-
procedures, quality specifications, tions that cannot be conducted and/or
and scope of work. A benefit of having corrected after coating and reassem-
complete confidence in the job being bly—such as area and check, harmon-
done comes at the end of the project ics, and wall thickness. The repair
when you might be able to save time by facility should provide these results
just reviewing the results of the final to the end user or its representative
nondestructive testing rather than for review.
by witnessing the PT inspection after
post-weld heat treatment.
Brazing raises quality concerns, Step 6: Verifying
particularly among end users having
no experience with the process or the
final inspection
20. Destructive metallurgical evalu- repair facility recommending it (Fig The last step of any final-inspection
ation is suggested for evaluating 19). Reason: Braze compositions and verification should be a review of all
coating quality procedures typically are confidential quality records for conformance to
50 COMBINED CYCLE JOURNAL, Third Quarter 2014
specifications. Things to remember:
n Ensure that all records are signed
by the appropriate technician,
inspector, or engineer, and dated.
n Be sure documentation includes
certificates for materials and
specifics of all subcontracted pro-
cesses.
n Verify traceability of materials—
including weld filler and braze—to
their certs.
n Confirm proper heat treatment
based on recorded temperatures,
time on temperature, heating and
cooling rates, and quality of atmo-
sphere.
n Check coating certifications, includ-
ing metallurgical evaluations, and
inspection documents—NDE,
dimensional, flow, frequency, area,
and harmonics, as well as moment-
weight and sequence charts.
Some of the quality records may
reveal items out of spec or iden-
tify other concerns that should be
addressed during final verification.
Keep copies of critical records for
your files.
NDE verification. Confirm the
condition of your components as soon
as practicable after final heat treat-
ment. Inspection at this stage gives a
good indication of workmanship and your final report and factored into The final visual. Before your
overall condition of your components. future decisions regarding vendor components are loaded for shipment,
However, keep in mind that the new selection. conduct a final visual inspection to
coating can limit the effectiveness of If specifications agreed upon with ensure that the parts are clean and
liquid-penetrant testing and mask the repair facility are not achieved, not susceptible to handling damage.
cracking that could occur during heat for future projects you may want to This is especially important for the
treatment. PT and/or other NDE consider making performance require- fragile coating edges. Also, be aware
methods should be conducted after ments less demanding and/or changing that the coating and the last grit
each heat treatment, but especially the scope of work. Alternatively, you cleaning operation can close up cooling
after the last one. Verify the quality of might consider a new vendor. holes. They should be inspected using
any touch-up work with PT and retain Other. Depending on the compo- wire, light, water, and/or air methods.
documentation. nent, other measurements may be Finally, be sure that shipping contain-
Consider being present in person needed to confirm the quality of work ers are sturdy enough to withstand
or through a representative dur- done—including cooling-air flow, fre- the rigors of transport and that your
ing all final NDEs—or at least until quency, moment weight, area, and components are well supported in
good insight is obtained regarding harmonic checks. Well-defined speci- these containers. ccj
practices in use at the repair facility fications should be developed for these
during days, nights, and weekends. operations because industry standards
Hans van Esch
Keep an open mind regarding blend typically are not available. Suggestion: (hvanesch@teser-
repairs versus weld repair if inspection Let the repair facility repeat some of vices.us, 281-867-
results suggest rework: It’s the quality the measurements required to be sure 1500) has more
of the repair that’s important, not its it can achieve an acceptable level of than three decades
appearance. repeatability and that all measure- of experience in
Dimensional verification of ments are within spec. the development of
critical measurements—including This information may be of greater repairs and coat-
roundness, clearances, height, thick- value in the future when experience ings for industry
ness, throat openings, etc—provided indicates what components are most leaders—including
Sulzer Elbar, Hick-
by the repair facility is strongly susceptible to failure and when work- ham, and Chromalloy. He founded TE-
recommended. The validity of these ing with advanced components. Later Services to support plant owners and
measurements depends in large part review of quality records—such as operators with training, component and
on the dimensional accuracy of fix- heat-treatment charts—also may metallurgical evaluations, repair and
tures used as part of the repair pro- warrant a closer look at some of the bid specifications, third-party inspec-
cess. Recall that fixture qualification repair and inspection processes used tion and audits, etc. Over the years,
was conducted as part of Step 4. True and possible changes to those pro- other experts in component repairs and
verification of dimensional accuracy cesses. The value of lessons learned rotor inspections—such as Shaheen
comes when components are reas- is that they can be incorporated Hayatghaibi and Mike Beauchamp—
have joined the company, increasing its
sembled into the GT. Feedback from into future repair work to improve experience base and service offerings.
this work should be incorporated into quality.
COMBINED CYCLE JOURNAL, Third Quarter 2014 51