GE Frame 5 Service Manual

							7
Service Factors
While GE does not subscribe to the equivalency of starts to hours, 
there are equivalencies within a wear mechanism that must be 
considered. As shown in Figure 8, influences such as fuel type and 
quality, firing temperature setting, and the amount of steam or 
water injection are considered with regard to the hours-based 
criteria. Startup rate and the number of trips are considered with 
regard to the starts-based criteria. In both cases, these influences 
may reduce the maintenance intervals. 
Typical baseline inspection intervals (6B.03/7E.03):
Hot gas path inspection  24,000 hrs or 1200 starts
Major inspection  48,000 hrs or 2400 starts
Criterion is hours or starts (whichever occurs first)
Factors affecting maintenance:
Hours-Based Factors
•	 Fuel type
•	 Peak load
•	 Diluent (water or steam injection)
Starts-Based Factors
•	 Start type (conventional or peaking-fast)
•	 Start load (max. load achieved during start cycle, e.g.   
part, base, or peak load)
•	 Tr i p s
Figure 8  . Maintenance factors
When these service or maintenance factors are involved in a unit’s 
operating profile, the hot gas path maintenance “rectangle” that 
describes the specific maintenance criteria for this operation is 
reduced from the ideal case, as illustrated in Figure 9 . The following 
discussion will take a closer look at the key operating factors 
and how they can affect maintenance intervals as well as parts 
refurbishment/replacement intervals.
Fuel
Fuels burned in gas turbines range from clean natural gas to 
residual oils and affect maintenance, as illustrated in Figure 10 . 
Although  Figure 10 provides the basic relationship between fuel 
severity factor and hydrogen content of the fuel, there are other 
fuel constituents that should be considered. Selection of fuel 
Incr easing Hydr ogen Content in Fuel
Increasing Fuel Severity Fact orNatural Gas
Distillates
ResidualLight
Heavy
Figure 10 . Estimated effect of fuel type on maintenance
4
8 12
160
202428
1,400
1,200
1,000
800
600
400
200 0
Hours-Based Fact ors
•  Fuel type
•  Peak load
•  Diluent 
Star
ts-Based Fact ors
•  Star t type
•  Star t load
•  Trips
Star ts
Thousands of Fir ed Hours
Maintenance Fact ors Reduce Maint enance Interval
Figure 9 . GE maintenance intervals
GE Power & Water | GER-3620M (00015001200140018
)  
						

							8
severity factor typically requires a comprehensive understanding 
of fuel constituents and how they affect system maintenance.  
The selected fuel severity factor should also be adjusted based   
on inspection results and operating experience. 
Heavier hydrocarbon fuels have a maintenance factor ranging 
from three to four for residual fuels and two to three for crude   
oil fuels. This maintenance factor is adjusted based on the   
water-to-fuel ratio in cases when water injection for NO
x 
abatement is used. These fuels generally release a higher   
amount of radiant thermal energy, which results in a subsequent 
reduction in combustion hardware life, and frequently contain 
corrosive elements such as sodium, potassium, vanadium, and   
lead that can cause accelerated hot corrosion of turbine nozzles 
and buckets. In addition, some elements in these fuels can   
cause deposits either directly or through compounds formed   
with inhibitors that are used to prevent corrosion. These   
deposits affect performance and can require more frequent 
maintenance.
Distillates, as refined, do not generally contain high levels of   
these corrosive elements, but harmful contaminants can be 
present in these fuels when delivered to the site. Two common 
ways of contaminating number two distillate fuel oil are: salt-
water ballast mixing with the cargo during sea transport, and 
contamination of the distillate fuel when transported to site in 
tankers, tank trucks, or pipelines that were previously used to 
transport contaminated fuel, chemicals, or leaded gasoline. GE’s 
experience with distillate fuels indicates that the hot gas path 
maintenance factor can range from as low as one (equivalent 
to natural gas) to as high as three. Unless operating experience 
suggests otherwise, it is recommended that a hot gas path 
maintenance factor of 1.5 be used for operation on distillate oil. 
Note also that contaminants in liquid fuels can affect the life   
of gas turbine auxiliary components such as fuel pumps and   
flow dividers.
Not shown in Figure 10  are alternative fuels such as industrial 
process gas, syngas, and bio-fuel. A wide variety of alternative 
fuels exist, each with their own considerations for combustion in 
a gas turbine. Although some alternative fuels can have a neutral 
effect on gas turbine maintenance, many alternative fuels require 
unit-specific intervals and fuel severity factors to account for their 
fuel constituents or water/steam injection requirements. As shown in Figure 10
, natural gas fuel that meets GE specification 
is considered the baseline, optimum fuel with regard to turbine 
maintenance. Proper adherence to GE fuel specifications in   
GEI-41040 and GEI-41047 is required to allow proper combustion 
system operation and to maintain applicable warranties. Liquid 
hydrocarbon carryover can expose the hot gas path hardware to 
severe overtemperature conditions that can result in significant 
reductions in hot gas path parts lives or repair intervals. 
Liquid hydrocarbon carryover is also responsible for upstream 
displacement of flame in combustion chambers, which can lead   
to severe combustion hardware damage. Owners can control   
this potential issue by using effective gas scrubber systems and   
by superheating the gaseous fuel prior to use to approximately 
50°F (28°C) above the hydrocarbon dew point temperature at 
the turbine gas control valve connection. For exact superheat 
requirement calculations, please review GEI 41040. Integral to the 
system, coalescing filters installed upstream of the performance 
gas heaters is a best practice and ensures the most efficient 
removal of liquids and vapor phase constituents.
Undetected and untreated, a single shipment of contaminated   
fuel can cause substantial damage to the gas turbine hot gas   
path components. Potentially high maintenance costs and loss 
of availability can be minimized or eliminated by:
•	 Placing a proper fuel specification on the fuel supplier.
For liquid fuels, each shipment should include a report that
identifies specific gravity, flash point, viscosity, sulfur content,
pour point and ash content of the fuel.
•	 Providing a regular fuel quality sampling and analysis program.
As part of this program, continuous monitoring of water
content in fuel oil is recommended, as is fuel analysis that,
at a minimum, monitors vanadium, lead, sodium, potassium,
calcium, and magnesium.
•	 Providing proper maintenance of the fuel treatment system
when burning heavier fuel oils.
•	 Providing cleanup equipment for distillate fuels when there
is a potential for contamination.
In addition to their presence in the fuel, contaminants can 
also enter the turbine via inlet air, steam/water injection, and 
carryover from evaporative coolers. In some cases, these 
sources of contaminants have been found to cause hot gas path  
						

							9
degradation equal to that seen with fuel-related contaminants. 
GE specifications define limits for maximum concentrations of 
contaminants for fuel, air, and steam/water.
In addition to fuel quality, fuel system operation is also a factor in 
equipment maintenance. Liquid fuel should not remain unpurged  
or in contact with hot combustion components after shutdown   
and should not be allowed to stagnate in the fuel system when 
strictly gas fuel is run for an extended time. To minimize varnish 
and coke accumulation, dual fuel units (gas and liquid capable) 
should be shutdown running gas fuel whenever possible. Likewise, 
during extended operation on gas, regular transfers from gas to 
liquid are recommended to exercise the system components and 
minimize coking.
Contamination and build-up may prevent the system from 
removing fuel oil and other liquids from the combustion, 
compressor discharge, turbine, and exhaust sections when   
the unit is shut down or during startup. Liquid fuel oil trapped   
in the system piping also creates a safety risk. Correct functioning 
of the false start drain system (FSDS) should be ensured through 
proper maintenance and inspection per GE procedures.
Firing Temperatures
Peak load is defined as operation above base load and is   
achieved by increasing turbine operating temperatures.   
Significant operation at peak load will require more frequent 
maintenance and replacement of hot gas path and combustion 
components.  Figure 11 defines the parts life effect corresponding 
to increases in firing temperature. It should be noted that this is 
not a linear relationship, and this equation should not be used for 
decreases in firing temperature.
It is important to recognize that a reduction in load does not 
always mean a reduction in firing temperature. For example, in  heat recovery applications, where steam generation drives overall 
plant efficiency, load is first reduced by closing variable inlet 
guide vanes to reduce inlet airflow while maintaining maximum 
exhaust temperature. For these combined cycle applications, 
firing temperature does not decrease until load is reduced below 
approximately 80% of rated output. Conversely, a non-DLN turbine 
running in simple cycle mode maintains fully open inlet guide 
vanes during a load reduction to 80% and will experience over a 
200°F/111°C reduction in firing temperature at this output level. 
 
The hot gas path parts life changes for different modes of 
operation. This turbine control effect is illustrated in Figure 12 . 
Turbines with DLN combustion systems use inlet guide vane 
turndown as well as inlet bleed heat to extend operation of   
low NO
x premix operation to part load conditions.
Firing temperature effects on hot gas path maintenance, as 
described above, relate to clean burning fuels, such as natural 
gas and light distillates, where creep rupture of hot gas path 
components is the primary life limiter and is the mechanism   
that determines the hot gas path maintenance interval impact. 
With ash-bearing heavy fuels, corrosion and deposits are 
the primary influence and a different relationship with firing 
temperature exists.
Steam/Water Injection
Water or steam injection for emissions control or power 
augmentation can affect part life and maintenance intervals   
even when the water or steam meets GE specifications. This 
relates to the effect of the added water on the hot gas transport 
properties. Higher gas conductivity, in particular, increases the   
B/E-class
Max IGV (open)
Min IG
V IG
Vs close max to mi n
at constant T
F
IG Vs close max to mi n
at constant T
X
Heat Recover y
Simple Cycle
Base Load
Peak Loa d
2500
2000
1500
1000
1200
1000
800
600
°F
°C
FiringT emp .
% Load60 80100 120
40
20
Figure 12 . Firing temperature and load relationship – heat recovery vs. simple 
cycle operation
B/E-class:  Ap = e (0.018* T
f)
F-class: 
Ap = e (0.023* T
f)
Ap 
  =  Peak fire severity factor
 T
f =  Peak firing temperature adder (in °F)
Figure 11  . Peak fire severity factors - natural gas and light distillates
GE Power & Water | GER-3620M (00015001200140018
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heat transfer to the buckets and nozzles and can lead to higher 
metal temperature and reduced part life.
Part life reduction from steam or water injection is directly  
affected by the way the turbine is controlled. The control system   
on most base load applications reduces firing temperature as 
water or steam is injected. This is known as dry control curve 
operation, which counters the effect of the higher heat transfer 
on the gas side and results in no net effect on bucket life. This 
is the standard configuration for all gas turbines, both with and 
without water or steam injection. On some installations, however, 
the control system is designed to keep firing temperature constant 
with water or steam injection. This is known as wet control curve 
operation, which results in additional unit output but decreases 
parts life as previously described. Units controlled in this way   
are generally in peaking applications where annual operating  
hours are low or where operators have determined that reduced 
parts lives are justified by the power advantage. Figure 13 
illustrates the wet and dry control curve and the performance 
differences that result from these two different modes of control.
An additional factor associated with water or steam injection 
relates to the higher aerodynamic loading on the turbine 
components that results from the injected flow increasing the 
cycle pressure ratio. This additional loading can increase the 
downstream deflection rate of the second- and third-stage nozzles, 
which would reduce the repair interval for these components. 
However, the introduction of high creep strength stage two and 
three nozzle (S2N/S3N) alloys, such as GTD-222™ and GTD-241™, 
has reduced this factor in comparison to previously applied 
materials such as FSX-414 and N-155. Water injection for NOx abatement should be performed according 
to the control schedule implemented in the controls system.   
Forcing operation of the water injection system at high loads   
can lead to combustion and HGP hardware damage due to   
thermal shock.
Cyclic Effects and Fast Starts
In the previous discussion, operating factors that affect the   
hours-based maintenance criteria were described. For the   
starts-based maintenance criteria, operating factors associated 
with the cyclic effects induced during startup, operation, and 
shutdown of the turbine must be considered. Operating conditions 
other than the standard startup and shutdown sequence can 
potentially reduce the cyclic life of the gas turbine components 
and may require more frequent maintenance including part 
refurbishment and/or replacement.
Fast starts are common deviations from the standard startup 
sequence. GE has introduced a number of different fast start 
systems, each applicable to particular gas turbine models. Fast 
starts may include any combination of Anticipated Start Purge,   
fast acceleration (light-off to FSNL), and fast loading. Some fast 
start methods do not affect inspection interval maintenance 
factors. Fast starts that do affect maintenance factors are   
referred to as peaking-fast starts or simply peaking starts. 
The effect of peaking-fast starts on the maintenance interval 
depends on the gas turbine model, the unit configuration, and   
the particular start characteristics. For example, simple cycle   
7F.03 units with fast start capability can perform a peaking start 
in which the unit is brought from light-off to full load in less than   
15 minutes. Conversely, simple cycle 6B and other smaller frame 
units can perform conventional starts that are less than 15   
minutes without affecting any maintenance factors. For units   
that have peaking-fast start capability, Figure 14  shows 
conservative peaking-start factors that may apply.
Because the peaking-fast start factors can vary by unit and 
by system, the baseline factors may not apply to all units. For 
example, the latest 7F.03 peaking-fast start system has the start 
factors shown in Figure 15 . For comparison, the 7F.03 nominal 
fast start that does not affect maintenance is also listed. Consult 
applicable unit-specific documentation or your GE service 
representative to verify the start factors that apply. 
Exhaust Temperature °F
Compressor Discharge Pressure  (psig)
Dry ControlWet Control
The Wet Control Cur ve 
Maintains Constant T
F
St eam Injection for 25 pmm NOx
3% Steam Inj.
T F  = 2020°F  (1104°C)
Load Ratio = 1.10
3% Steam Inj.
T F  = 1994°F  (1090°C)
Load Ratio = 1.08
0% Steam Inj.
T F  = 2020°F  (1104°C)
Load Ratio = 1.0
Figure 13 . Exhaust temperature control curve – dry vs. wet control 7E.03 
						

							11
Starts-Based Combustion InspectionAs = 4.0 for B/E-class
As = 2.0 for F-class
Starts-Based Hot Gas Path Inspection P
s = 3.5 for B/E-class
P
s = 1.2 for F-class
Starts-Based Rotor Inspection F
s = 2.0 for F-class*
* See Figure 22 for details
Figure 14  . Peaking-fast start factors
7F .03 Starts-Based Combustion Inspection
As = 1.0 for 7F nominal fast start
As = 1.0 for 7F peaking-fast start
7F  .03 Starts-Based Hot Gas Path Inspection
P
s = Not applicable for 7F nominal fast start 
(counted as normal starts)
P
s = 0.5 for 7F peaking-fast start
7F  .03 Starts-Based Rotor Inspection
F
s = 1.0 for 7F nominal fast start
F
s = 2.0 for 7F peaking-fast start*
* See Figure 23 for details
Figure 15  . 7F.03 fast start factors
Hot Gas Path Parts
Figure 16 illustrates the firing temperature changes occurring 
over a normal startup and shutdown cycle. Light-off, acceleration, 
loading, unloading, and shutdown all produce gas and metal 
temperature changes. For rapid changes in gas temperature,   
the edges of the bucket or nozzle respond more quickly than   
the thicker bulk section, as pictured in Figure 17 . These gradients, 
in turn, produce thermal stresses that, when cycled, can eventually 
lead to cracking. 
Figure 18 describes the temperature/strain history of a 7E.03   
stage 1 bucket during a normal startup and shutdown cycle.   
Light-off and acceleration produce transient compressive strains  in the bucket as the fast responding leading edge heats up more 
quickly than the thicker bulk section of the airfoil. At full load 
conditions, the bucket reaches its maximum metal temperature 
and a compressive strain is produced from the normal steady 
 
state temperature gradients that exist in the cooled part. At 
shutdown, the conditions reverse and the faster responding   
edges cool more quickly than the bulk section, which results   
in a tensile strain at the leading edge.
Thermal mechanical fatigue testing has found that the number   
of cycles that a part can withstand before cracking occurs is 
strongly influenced by the total strain range and the maximum 
metal temperature. Any operating condition that significantly 
increases the strain range and/or the maximum metal temperature 
over the normal cycle conditions will reduce the fatigue life and 
increase the starts-based maintenance factor. For example,   
Time
Startup Shutdown
Temperatur e
Base Load
Acceleration
Light-Off
Warm-UpFired Shutdown
Full Speed
No Load
Full Speed
No Load Unload Ramp
Trip
Load Ramp
Figure 16 . Turbine start/stop cycle – firing temperature changes
Cold
Hot
Figure 17 . Second stage bucket transient temperature distribution
GE Power & Water | GER-3620M (00015001200140018
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Figure 19 compares a normal operating cycle with one that 
includes a trip from full load. The significant increase in the   
strain range for a trip cycle results in a life effect that equates 
to eight normal start/stop cycles, as shown. Trips from part 
load will have a reduced effect because of the lower metal  temperatures at the initiation of the trip event. Figure 20 
 
illustrates that while a trip from between 80% and 100% load   
has an 8:1 trip severity factor, a trip from full speed no load (FSNL) 
has a trip severity factor of 2:1. Similarly, overfiring of the unit 
during peak load operation leads to increased component   
0
Key Parameters
Fired 
Shutdown•Max Strain Range
• Max Metal Temperature
FSNL
Light Off
& Warm-up Acceleration Base Load
Metal Temperature
T
MAX
% Strain
MAX
Figure 18 . Bucket low cycle fatigue (LCF)
+
-
Normal Startup/Shutdown
Temperature
MAX
Strain 
~ % 
+
-
Strain 
~ % 
Leading Edge Te
mperature/Strain
T
MAX
Normal Start & Trip
1 Trip Cycle = 8 Normal Shutdown Cycles
Temperature
MAX
TMAX
TeTempmpereratatururee
MAMAXX
Figure 19 . Low cycle fatigue life sensitivities – first stage bucket 
						

							13
metal temperatures. As a result, a trip from peak load has 
a trip severity factor of 10:1. 
Trips are to be assessed in addition to the regular startup/shutdown 
cycles as starts adders. As such, in the factored starts equation  
of  Figure 43 , one is subtracted from the severity factor so that the 
net result of the formula ( Figure 43) is the same as that dictated   
by the increased strain range. For example, a startup and trip   
from base load would count as eight total cycles (one cycle for 
startup to base load plus 8-1=7 cycles for trip from base load),   
just as indicated by the 8:1 maintenance factor.
Similarly to trips from load, peaking-fast starts will affect the 
starts-based maintenance interval. Like trips, the effects of   
a peaking-fast start on the machine are considered separate   
from a normal cycle and their effects must be tabulated in   
addition to the normal start/stop cycle. However, there is no   
-1 applied to these factors, so a 7F.03 peaking-fast start during  
 a base load cycle would have a total effect of 1.5 cycles.   
Refer to Appendix A  for factored starts examples, and consult 
unit-specific documentation to determine if an alternative   
hot gas path peaking-fast start factor applies. 
While the factors described above will decrease the starts-based 
maintenance interval, part load operating cycles allow for an 
extension of the maintenance interval. Figure 21 can be used in 
considering this type of operation. For example, two operating 
cycles to maximum load levels of less than 60% would equate   
to one start to a load greater than 60% or, stated another way, 
would have a maintenance factor of 0.5.  Factored starts calculations are based upon the maximum load 
achieved during operation. Therefore, if a unit is operated at part 
load for three weeks, and then ramped up to base load for the last 
ten minutes, then the unit’s total operation would be described as a 
base load start/stop cycle.
Rotor Parts
The maintenance and refurbishment requirements of the rotor 
structure, like the hot gas path components, are affected by 
 
the cyclic effects of startup, operation, and shutdown, as well as 
loading and off-load characteristics. Maintenance factors specific 
to the operating profile and rotor design must be incorporated into 
the operator’s maintenance planning. Disassembly and inspection 
of all rotor components is required when the accumulated rotor 
starts or hours reach the inspection limit. (See Figure 44  and  
Figure 45  in the Inspection Intervals section.)
The thermal condition when the startup sequence is initiated   
is a major factor in determining the rotor maintenance interval 
and individual rotor component life. Rotors that are cold when   
the startup commences experience transient thermal stresses 
as the turbine is brought on line. Large rotors with their longer 
thermal time constants develop higher thermal stresses than 
smaller rotors undergoing the same startup time sequence. High 
thermal stresses reduce thermal mechanical fatigue life and the 
inspection interval.
Though the concept of rotor maintenance factors is applicable 
to all gas turbine rotors, only F-class rotors will be discussed in 
detail. For all other rotors, reference unit-specific documentation 
to determine additional maintenance factors that may apply. 
10
8
6
4
2
0
aT – Tr ip Severity Factor
% Load02 0406080 100
Base
FSNL Note:
• For Trips During Star
tup Accel Assume aT=2
• For Trips fr om Peak Load Assume  a
T=10
F-class and 
B/E-class units with  Inlet Bleed Heat
Units without
Inlet Bleed Heat
Figure 20 . Maintenance factor – trips from load
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Maintenance Factor
% Load02 040608 0 100
Figure 21 . Maintenance factor – effect of start cycle maximum load level
GE Power & Water | GER-3620M (00015001200140018
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The rotor maintenance factor for a startup is a function of the 
downtime following a previous period of operation. As downtime 
increases, the rotor metal temperature approaches ambient 
conditions, and thermal fatigue during a subsequent startup 
increases. As such, cold starts are assigned a rotor maintenance 
factor of two and hot starts a rotor maintenance factor of less  
than one due to the lower thermal stress under hot conditions.   
This effect varies from one location in the rotor structure to 
another. The most limiting location determines the overall rotor 
maintenance factor.
Initial rotor thermal condition is not the only operating factor   
that influences rotor maintenance intervals and component   
life. Peaking-fast starts, where the turbine is ramped quickly 
to load, increase thermal gradients on the rotor. Trips from 
load, particularly trips followed by immediate restarts, and hot 
restarts reduce the rotor maintenance interval. Figure 22  lists 
recommended operating factors that should be used to determine 
the rotor’s overall maintenance factor for certain F-class rotors. 
F-class* Rotors
Rotor Maintenance Factors
Peaking-Fast  Start** Normal 
Start
Hot 1 Start Factor 
(0–1 Hr. Down) 4.0
2.0
Hot 2 Start Factor 
(1–4 Hrs. Down) 1.0
0.5
Warm 1 Start Factor 
(4–20 Hrs. Down) 1.8
0.9
Warm 2 Start Factor 
(20–40 Hrs. Down) 2.8
1.4
Cold Start Factor 
(>40 Hrs. Down) 4.0
2.0
Trip from Load Factor 4.04.0
*Other factors may apply to early 9F.03 units
**An F-class peaking-fast start is typically a start in which the 
unit is brought from light-off to full load in less than 15 minutes.
Figure 22 . O peration-related maintenance factors
The significance of each of these factors is dependent on the unit 
operation. There are three categories of operation that are typical 
of most gas turbine applications. These are peaking, cyclic, and 
continuous duty as described below:
•	Peaking units have a relatively high starting frequency and a low
number of hours per start. Operation follows a seasonal demand.
Peaking units will generally see a high percentage of warm and
cold starts.
•	 Cyclic units start daily with weekend shutdowns. Twelve to
sixteen hours per start is typical, which results in a warm rotor
condition for a large percentage of the starts. Cold starts are
generally seen only after a maintenance outage or following a
two-day weekend outage.
•	 Continuous duty applications see a high number of hours
per start. Most starts are cold because outages are generally
maintenance driven. While the percentage of cold starts is high,
the total number of starts is low. The rotor maintenance interval
on continuous duty units will be determined by operating hours
rather than starts.
Figure 23 lists operating profiles on the high end of each of   
these three general categories of gas turbine applications. These 
duty cycles have different combinations of hot, warm, and cold 
starts with each starting condition having a different effect on 
rotor maintenance interval as previously discussed. As a result, 
the starts-based rotor maintenance interval will depend on an 
application’s specific duty cycle. In the Rotor Inspection Interval 
section, a method will be described to determine a maintenance 
factor that is specific to the operation’s duty cycle. The application’s 
integrated maintenance factor uses the rotor maintenance factors 
described above in combination with the actual duty cycle of a 
specific application and can be used to determine rotor inspection 
intervals. In this calculation, the reference duty cycle that yields   
a starts-based maintenance factor equal to one is defined in   
Figure 24 . Duty cycles different from the Figure 24  definition,  
in particular duty cycles with more cold starts or a high number   
of trips, will have a maintenance factor greater than one. 
Turning gear or ratchet operation after shutdown and before 
starting/restarting is a crucial part of normal operating procedure. 
After a shutdown, turning of the warm rotor is essential to avoid 
bow, or bend, in the rotor. Initiating a start with the rotor in a 
bowed condition could lead to high vibrations and excessive rubs.   
						

							15
Figure F-1 describes turning gear/ratchet scenarios and operation 
guidelines (See Appendix ). Relevant operating instructions and 
TILs should be adhered to where applicable. As a best practice, 
units should remain on turning gear or ratchet following a planned 
shutdown until wheelspace temperatures have stabilized at or   
near ambient temperature. If the unit is to see no further activity 
for 48 hours after cool-down is completed, then it may be taken   
off of turning gear.
Figure F-1 also provides guidelines for hot restarts. When an 
immediate restart is required, it is recommended that the rotor 
be placed on turning gear for one hour following a trip from load, 
trip from full speed no load, or normal shutdown. This will allow 
transient thermal stresses to subside before superimposing a 
startup transient. If the machine must be restarted in less than   
one hour, a start factor of 2 will apply. 
Longer periods of turning gear operation may be necessary prior to 
a cold start or hot restart if bow is detected. Vibration data taken 
while at crank speed can be used to confirm that rotor bow is at 
acceptable levels and the start sequence can be initiated. Users 
should reference the O&M Manual and appropriate TILs for specific 
instructions and information for their units.
Combustion Parts
A typical combustion system contains transition pieces, 
combustion liners, flow sleeves, head-end assemblies containing 
fuel nozzles and cartridges, end caps and end covers, and assorted 
other hardware including cross-fire tubes, spark plugs and flame 
detectors. In addition, there can be various fuel and air delivery 
components such as purge or check valves and flex hoses. GE 
provides several types of combustion systems including standard 
combustors, Multi-Nozzle Quiet Combustors (MNQC), Integrated 
Gasification Combined Cycle (IGCC) combustors, and Dry Low NO
x 
(DLN) combustors. Each of these combustion systems has unique 
operating characteristics and modes of operation with differing 
responses to operational variables affecting maintenance and 
refurbishment requirements.
DLN combustion systems use various combustion modes to reach 
base load operation. The system transfers from one combustion 
mode to the next when the combustion reference temperature 
increases to the required value, or transfer temperature, for the 
next mode. 
Peaking CyclicContinuous
Hot 2 Start 
(Down 1-4 Hr.) 3%
1%10%
Warm 1 Star t 
(Down 4-20 hr.) 10%
82% 5%
Warm 2 Star t   
(Down 20-40 Hr.) 37% 13%
5%
Cold Star t 
(Down >40 Hr.) 50%
4%80%
Hours/Start 416 400
Hours/Year 6004800 8200
Starts/Year 150
300 21
Percent Trips 3%1%20%
Tr i p s / Ye a r 5
3 4
Typical Maintenance 
Factor (Starts-Based) 1
 .7
1 .

0NA
•	 Operational Profile is Application Specific
•	 Inspection Interval is Application Specific
Figure 23 .  7F gas turbine typical operational profile
Baseline Unit
Cyclic Duty
6Star ts/Week
16 Hours/Start
4 Outage/Year Maintenance
50 Weeks/Year
4800 Hours/Year
300 Starts/Year
0 Tr i p s / Ye a r
1 Maintenance Factor
12 Cold Starts/Year (down >40 Hr.) 4%
39 Warm 2 Starts/Year (Down 20-40 Hr.) 13%
24 6 Warm 1 Starts/Year (Down 4-20 Hr.) 82%
3 Hot 2 Starts/Year (Down 1-4 Hr.) 1%
Baseline Unit Achieves Maintenance Factor = 1
Figure 24 . B aseline for starts-based maintenance factor definition
GE Power & Water | GER-3620M (00015001200140018
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•	Continuous mode operation is defined as operation in a
combustion mode for longer than what is required during normal
startup/shutdown.
•	 Extended mode operation is defined as operation in a
combustion mode at a firing temperature greater than the
transfer temperature to the next combustion mode.
The DLN combustion mode recommended for continuous mode 
operation is the premixed combustion mode (PM), as it achieves 
lowest possible emissions and maximum possible part life. 
Continuous and extended mode operation in non-PM combustion 
modes is not recommended due to its effect on combustion 
hardware life as shown in Figure 25 . The use of non-PM combustion 
modes has the following effects on maintenance:
•	 DLN-1/DLN-1+ extended lean-lean operation results in a
maintenance factor of 10 (excluding Frame 5 units where MF=2).
•	 DLN 2.0/DLN 2+ extended piloted premixed operation results in a
maintenance factor of 10.
•	 Continuous mode operation in lean-lean (L-L), sub-piloted
premixed (sPPM), or piloted premixed (PPM) modes is not
recommended as it will accelerate combustion hardware
degradation.
•	 In addition, cyclic operation between piloted premixed and
premixed modes leads to thermal loads on the combustion liner
and transition piece similar to the loads encountered during the
startup/shutdown cycle.
Figure 25 .  DLN combustion mode effect on combustion hardware life
Continuous mode operation of DLN 2.6/DLN 2.6+ combustors will 
not accelerate combustion hardware degradation. 
Another factor that can affect combustion system maintenance 
is acoustic dynamics. Acoustic dynamics are pressure oscillations 
generated by the combustion system, which, if high enough 
in magnitude, can lead to significant wear and cracking of 
combustion or hot gas path components. GE practice is to 
~85% TNH  FSNL  Full Loa d
Combustor Combustion mode effect on hardware life
DLN 1/1+  Primary  L-LPremixed
Extended L-L
DLN 2.0/2+  Diffusion  L-L/sPPM  PPM Premixed
Extended PPM
Severity
High
Low
tune the combustion system to levels of acoustic dynamics low 
enough to ensure that the maintenance practices described here 
are not compromised. In addition, GE encourages monitoring of 
combustion dynamics during turbine operation throughout the   
full range of ambient temperatures and loads.
Combustion disassembly is performed, during scheduled 
combustion inspections (CI). Inspection interval guidelines are   
included in Figure 39 . It is expected, and recommended, that 
intervals be modified based on specific experience. Replacement 
intervals are usually defined by a recommended number of 
combustion (or repair) intervals and are usually combustion 
component specific. In general, the replacement interval as a 
function of the number of combustion inspection intervals is 
reduced if the combustion inspection interval is extended. For 
example, a component having an 8,000-hour CI interval, and a 
six CI replacement interval, would have a replacement interval of 
four CI intervals if the inspection interval were increased to 12,000 
hours (to maintain a 48,000-hour replacement interval).
For combustion parts, the baseline operating conditions that result 
in a maintenance factor of one are fired startup and shutdown to 
base load on natural gas fuel without steam or water injection. 
Factors that increase the hours-based maintenance factor include 
peak load operation, distillate or heavy fuels, and steam or water 
injection. Factors that increase starts-based maintenance factor 
include peak load start/stop cycles, distillate or heavy fuels, steam 
or water injection, trips, and peaking-fast starts.
Casing Parts
Most GE gas turbines have inlet, compressor, compressor 
discharge, and turbine cases in addition to exhaust frames. Inner 
barrels are typically attached to the compressor discharge case. 
These cases provide the primary support for the bearings, rotor, 
and gas path hardware.
The exterior of all casings should be visually inspected for cracking, 
loose hardware, and casing slippage at each combustion, hot   
gas path, and major outage. The interior of all casings should   
be inspected whenever possible. The level of the outage 
determines which casing interiors are accessible for visual 
inspection. Borescope inspections are recommended for the   
inlet cases, compressor cases, and compressor discharge cases