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IS YOUR GE CLASS B/E ROTOR APPROACHING “END OF LIFE”?
GE TIL 1576, 28 June 2007, Gas Turbine Rotor Inspections, discusses the requirement of GER 3620, Heavy Duty Turbine Operating and Maintenance
Considerations, that rotor inspections be performed at specific intervals recommended by GE. These intervals, unless otherwise specified, are 5,000 factored
starts or 200,000 factored hours. The revision of GER 3620 at that time was K, October 2004. Operating and maintenance discussions are generally
applicable to all GE heavy-duty gas turbines, Frames 3, 5, 6, 7, and 9.
GER-3620K, Firing Temperature, page 9, describes the relationship of firing temperature to maintenance factor.
“Each hour at Peak load firing temperature (+100 DegF) is the same as six hours of operation at Base load. This operation will result in a
maintenance factor of six.”
“It is important to recognize that a reduction in load does not always mean a reduction in firing temperature.” The text then explains that the
IGV temperature control and DLN turndown affect firing temperature by reducing airflow to main high exhaust temperature for combined
cycle and DLN stability.
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Key words are “does not always”.
The relationship between firing temperature and turbine exhaust temperature is nearly 1.66:1. For Base to Peak operation, an exhaust temperature increase
of 60 DegF results in a firing temperature increase of 100 DegF. Conversely, an exhaust temperature decrease of 60 DegF results in a firing temperature
decrease of 100 DegF.
Reference to firing temperature related to Peak load was introduced in GER-3620 Rev F, November 1998. The paragraph in Revision F continued to be
part of GER-3620 until Revision N in October 2017, a period of nineteen years.
GE indicated that an increase in maintenance interval could be expected from part load operation in GER-3620, Revision L, page 9, issued in November
2009. Experience has shown that operation at less than Base or Peak load has extended the on condition maintenance of gas turbines. Units that are
“Base” load but operate at less than the base rating, such as part load dispatch, have also shown extended life.
The GER provides calculation criteria for determining factored starts and hours on Page 34. The calculation is based on turbine operating procedures such
as hot starts, warm starts, cold starts trips, load condition and turning gear operation.
Hours Based Rotor Life Calculation, GER 3620K:
Maintenance Factor = Base Load Hours +2 Peak Load Hours + 2 Turning Gear Hours
Base Load Hours + Peak Load Hours
The typical MS6001B DLNI is not Peak capable and does not have a turning gear.
If the unit was operated at Peak load 100% of the time, the Maintenance Factor would be 2.0. The Rotor Life calculation allows a reduction in the
Maintenance Factor (for hot gas path parts, the Maintenance Factor is 6) as it applies to the rotor structure itself. Therefore, if the unit did not
operate in Peak Mode and did not have a Turning Gear, the Maintenance Factor would be 1 resulting in a Hours Based Rotor Life of 200,000 hours.
From the Maintenance Factor equation for Rotor Life Maintenance Interval, if the unit was operated at Peak 100% of the time with the Maintenance
Factor of 2.0, the Rotor Life Maintenance Interval would be 100,000 fired hours.
If the Peak load interval is reduced to 100,000 fired hours by 100% operation on Peak, operation at part load with the firing temperature 100 DegF
less than the design value should increase the interval. The Rotor Life Maintenance Interval would be 200,000 plus 100,000 fired hours or 300,000
fired hours. The Part Load Maintenance Factor based on 100 DegF less than design Firing Temperature would be:
Maintenance Factor = 200,000 = 200,000 = 0.667
Rotor Maintenance Interval 300,000
A Mid West utility operates an MS 6001B that has a plant condition that limits gas turbine output to an average of 84% of Base rating. The limit
is based on the ability of the steam Customer to accept the Base load steam generated. In 2013, the unit had reached over 50,000 hours without
a major inspection. The results of the 2013 inspection found very few problems with the compressor and hot section.
Data was retrieved from the data acquisition system over the span of one year, January 2021 to December 2021. The Compressor Discharge Exhaust
Temperature Control curve, TTRXP, was calculated using the Compressor Discharge pressure. The Exhaust Temperature TTXM was subtracted from
the CPD Exhaust Temperature Control Curve TTRXP. This allows a comparison of the turbine maintenance factor based on the 200,000 hour limit to
the maintenance factor adjusted for the hours the unit was operated at temperatures less than Base Load.
Results of the data for the year:
Yearly median Turbine Exhaust Temperature, 1015.63 DegF
Yearly median Compressor Discharge Exhaust Temperature Control, 1046.79 DegF.
Median DELTA Exhaust Temperature DegF = 30 DegF
Median DELTA Firing Temperature DegF = 50 DegF
The Part Load Maintenance factor for 50 DegF is = 0.667 + 0.333 (100-50)/100 = 0.667 + 0.167 = 0.834
Rotor Maintenance Interval = 200,000 = 200,000 = 239,808 fired hours
Maintenance Factor 0.834
Part Load Rotor Fired Hours Maintenance Interval of 239,808 fired hours is 10,192 hours less than the 250,000 hour design life of the rotor.
The results are based on one year’s data and was done to evaluate the effect on maintenance interval of continuous operation at less than Base Load
conditions. The unit has been operating for over twenty five years.
The next step would be to obtain data back to initial operation and repeat the calculation to verify that the overall operation has been at firing
temperatures below the limit of the Compressor Discharge Pressure and Megawatt temperature control curves. “Local knowledge” is that this year
has been representative of prior year’s operation.
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In June 2023, timers were installed in the Mark V to record the fired hours from 1.0 to 15 DegF below Base Reference, 15 to 30 DegF below Base Reference and more than 30 DegF below Base Reference.
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Results of data taken In March of 2024:
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FIRED HOURS FIRED HOURS % OF TOTAL MED Tx Med Tf
Total Fired Hours 6840 hours
Fired hours 1.0 to 15 DegF below Base 2419.9 hours 35.4 % 7.5 12
Fired hours 15 to 30 DegF below base 3375.6 hours 49.4 % 22.5 36
Fired hours more than 30 DegF below base 336.9 hours 4.9% 30 48
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Calculating the Part Load Maintenance interval using the median firing temperatures, the Part Load Maintenance factor for 12 Tf DegF is:
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0.667 + (1-0.667) (100 - 12/100) = 0.667 + (0.333)(0.88) =0.667 + 0.266 = 0.933
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Part Load Rotor Maintenance Interval = 200,000 = 200,000 = 214362 fired hours
Maintenance Factor 0.933
The Part Load Maintenance factor for 36 Tf DegF is = 0.880 Part Load Rotor Maintenance interval = 227272 hours
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The Part Load Maintenance factor for 48 Tf DegF is = 0.807 Part Load Rotor Maintenance interval = 247831 hours
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Applying the percent fired hours to the three data points:
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DATA POINT Tx % OF PL HOURS MAINT FAC 200 000 /FACTOR FACTOR - 200000 % * HOURS
1-15 DegF 35.4% 0.933 214362 hours 14362 hours 5051 hours
15-30 DegF 49.4% 0.880 227272 hours 27272 hours 13457 hours
30+ DegF 4.9% 0.807 247831 hours 47831 hours 1864 Hours
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Total part Load hours +20372 hours. Part Load Rotor Maintenance Interval 200,000 + 20372 = 220,372 hours.
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The original data collected in 2022 estimated a Part Load Rotor Maintenance Interval of 239952 fired hours. The data collected in 2024 estimates a Part Load Rotor Maintenance Interval of 220372 fired hours, revising the interval down by 18580 hours.
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The Part Load data being collected provides a more accurate record of Part Load operation that can be applied to the Maintenance Interval calculation.
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As shown above, GE’s GER 3620 Revision L, points out that an increase in maintenance interval could be expected from part load operation. Experience has shown that operation at less than Base or Peak load has extended the on condition maintenance of Gas Turbines. Units that are “Base” load but operate at less than the Base rating have also shown extended life. GE could be approached to allow an increase in Part Load Maintenance Intervals.
The Part Load data can be applied to other Maintenance Intervals, such as Combustion, Hot Gas Path and Major Inspections.
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The basic calculation of Maintenance Interval as given in GER-3620P, February 2021, is:
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Maintenance Interval = Baseline Combustion Inspection
Maintenance Factor
Maintenance Factor = Factored Hours
Actual Hours
Factored Hours = (K * Af *Ap *tI)
Where: K = Injection factor,
Af = Fuel Factor
Ap = Load Factor
ti = Operating hours at Load
K, Injection Factor, normally 1.0 for most Non-DLN I units. DLN I units are generally not injected.
Af, Fuel Factor, is 1.0 for Gas Fuel
Ap Load Factor and ti, Operating Hours, is made up of Part Load, Base, and Peak.
Peak Hours are recorded by a Peak Load timer in the Speedtronic, DLN I units are normally not Peak rated.
Part Load hours are recorded by the Part Load Timers in the Speedtronic.
The calculation procedure given above for Part Load Operation could be applied to the calculation of Part Load, Ap.
Sample calculation for Combustion Inspection:
Maintenance Factor Part Load = Combustion Inspection hours – Combustion Inspection hours * % Tx hours
Maintenance Factor
Maintenance Factor Part load 1-15 Tx DegF= (12000/0.933) – 12000 * 0.354 = +305 hours
Maintenance Factor Part load 15-30 Tx DegF= (12000/0.880) – 12000 * 0.494 = +1408 hours
Maintenance Factor Part load +30 Tx DegF= (12000/0.807) – 12000 * 0.049 = +728 hours
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Total Part Load Hours for Combustion Inspection = 12000 +2441 hours = 14441 hours
C.L. Williams
March 2024
