HOT SECTION CREEP LIFE ESTIMATION OF A MEDIUM SIZE GAS TURBINE ENGINE
Musa, G. H.1, Ramsden K. W., Alhassan, Y. andSikiru G. S.
1Department of Mechanical Engineering, Kano University of Science & Technology
Wudil- Kano, Nigeria
email:
ABSTRACT
The LMP is the method used to estimate the engine creep life, the creep life of the blade at mean was found to be 29 years at ISA clean condition and reduce to 17 years at 42⁰C of the Northern part of Nigerian temperature for a period of one year with a percentage decrease of 41% due to effect of ambient temperature and sandy environment.
Due to compressor fouling the firing temperature has to be increase to maintain constant power output which results to a reduction in engine creep life. To maintain a constant power output TET has increased to 1615 K. The result shows that an increase in TET will leads to an increase in metal temperature from 1049K to 1127K which result in high stress. The turbine blade can also be affected by the gas flow around the blades, inlet and outlet cooling temperature, and cooling effectiveness of blade.
KEYWORDS:
Creep Life, Turbine Entry Temperature (TET), Larson Miller Parameter (LMP), Low Pressure Turbine (LPT), Gas Turbine
SIGNIFICANCE:
The significance of the study is to calculate the blade creep life of the HP Turbine at 42⁰Cand to maintain a constant power output of 26.7MW engine which results to a reduction in engine creep life.
1.0 INTRODUCTION
During the operation of an industrial GT, GT components undergo various types of degradation due to high temperatures and mechanical loading. These conditions make the components to have failure mechanisms such as creep (Weber et al, 2005). Creep is one of the most common failure mechanisms that reduce the components creep life. The creep effect depends on the operating conditions of the engine, mode of operation and the design parameters. The Larson Miller Parameter (LMP) is the method used to estimate the HPT rotor blade creep life for different ambient temperature for a period of one year. Even with the advanced cooling technology the performance degradation results to an increase in TET and the metal temperature as well since they are directly proportional to each other. This result to high stress due to increase in aerodynamic load and leading to early creep life failure.
Creep is the plastic deformation which increases over time due to influence of stresses and temperatures. Creep normally occurs at temperature above the absolute melting temperature of turbine materials (Viswanathan, 1998; Reimer, 1997).
Figure 1: Three Stages of Creep Failure Mechanisms [40]
2.0 ENGINE TECHNICAL SPECIFICATION
The engine consists of 19- stages compressor with a pressure ratio of 23.1:1 and has an annular combustor with externally mounted fuel nozzles. A 2-stage air cooled HPT which drives the compressor and auxiliary drive gear box, and a 6-stage aerodynamically coupled low pressure power turbine which is driven the gas generator (
Figure 1: Medium Size Twin Shaft Engine(
The engine model of 27.6MW GT engine was simulated using TURBOMACH software at DP, off design point for both the clean engine and degraded engine. The design point calculations have been carried out and presented in Table 1.
Table 1 : Design Point Results of 27.6MW Engine
S/N / Engine Parameters / Values1 / Power Output (MW) / 27.6
2 / Fuel Flow (kg/s) / 1.63
3 / SFC (mg/kWs) / 59.01
4 / Exhaust Temperature (⁰F) / 965
5 / Thermal Efficiency / 39.4
6 / Inlet Mass Flow (kg/s) / 80.5
7 / TET (K) / 1505
8 / Ambient pressure (KPa) / 101.325
9 / Ambient temperature (K) / 288
2.1 EFFECT OF INCREASING TET ON AMBIENT TEMPERATURE
An increase in ambient temperature to a target temperature of 42⁰C will lead to an increase in TET. The design point TET was 1505K at ISA condition and increases to 1615K at 42⁰C in order to recover the power output which gives a percentage increase of 6.8%. Since the graph is linear it shows that TET is directly proportional to the ambient temperature, which results to a decrease in creep life of the blade as the temperature on the hot section increases.
Figure 2: Effects of TET on Ambient Temperature
2.2 EFFECT OF FOULING ON FUEL FLOW
The result shows the effects of increasing TET to restore power will increase the fuel flow. In order to achieve the power output of 27.6MW, the TET of the fouled engine has to be increased to 1615K. However, fuel flow increases from 1.63kg/s to 1.67kg/s with a percentage increase of approximately 3% due to compressor fouling and the power output has been restored from 21.51MW to 27.6MW.
3.0 BLADE GEOMETRY SPECIFICATION
The HP turbine is a two stage turbine, for the design process the first stage of the blade was considered.
Table 2: Blade Geometry Specification at the Mean Height (Eshati et al, 2010)
Geometrical Parameter at Mean Height / Value / UnitStage loading / 1.48
Stage Reaction / 50%
Flow Coefficient / 0.40
Height to Chord Ratio / 1.46
LE/TE Radius / 0.3889 / m
Inlet Annulus Area / 0.1038 / m2
Blade Chord / 2.909 / cm
Stagger Angle / 35 / ⁰C
Rotational Speed / 10050 / rpm
Density of Material / 8180 / Kg/m3
Blade thickness / 0.002 / m
3.1 THERMAL MODEL
To calculate the blade section metal temperature, it is necessary to treat the blade as individual where the metal temperature is assumed to be constant at both span and chord wise. The cooling effectiveness is assumed to be the same for all the blade section, and the coolant inlet temperature for the blade section is taken from the exit of the compressor (Haslam, 2011). The section metal temperature can be calculated using the formula below.
/ (1)Where;
The section gas temperature is the TET at the design point which is 1505K, the section coolant inlet temperature is the compressor outlet temperature which is equal to 745.1K while is constant and has a value of 0.60
=
The same procedures were followed to calculate the metal temperature at other firing temperature.
Figure 3: Effects of TET on Metal Temperature
From the Figure 3 above it shows that an increase in firing temperature from 1505K to 1615K of the target temperature will lead to an increase in metal temperature from 1049K to 1127K for a period of one year to maintain constant power. Thus, leading to compressor fouling which reduces the blade creep life. The metal temperature has to be increased by 6.9% in order to recover the power output of the engine. Since the graph is linear it shows that TET is directly proportional to metal temperature, which results to a decrease in blade creep life.
3.2 STRESS MODEL
To calculate the creep life of a blade, the stresses from the root to tip were considered. To perform the stress model calculation, there are some parameters to be used such as pressure, rotational speed and temperature and were generated using the Cranfield University software called TURBOMATCH. It is assumed that the axial velocity remains constant along the span of the blade and the forces on the blade act at the blade section centre of gravity (Haslam, 2011).The centrifugal force was calculated at each section.
/ (2)Where;
In order to calculate the average cross section area, section height and distance CG to rotation axis from root to mean and mean to tip in Table 3is used together with the formula;
/ (3)/ (4)
/ (5)
Table 3: Blade Specification for the Area and Radius(Eshati et al, 2010)
TDS / ½ H / RDSMean Blade Cross Section Area (m2) / 5.96 / 6.71 / 7.94
Mean Blade Radius (m) / 0.41 / 0.39 / 0.37
Figure 4: Centrifugal Stress at Root
Since the shroud centrifugal force is zero now the centrifugal force at each section was calculated, example CF from mean to tip can be calculated using the values below; where the density of a material = 8180 kg/m3 (Eshatiet al, 2010), Section height = 0.02122 m, Average cross section TDS = , Angular velocity = 1052.43 rad/s and distance CG to rotation axis = 0.39953 m. The same procedure was applied from root to mean.
The centrifugal stress at each section can be calculated using the formula as follows;
/ (6)Where;
The assumed blade material is NIMONIC ALLOY 90, before calculating creep life of a blade using Larson Miller Parameter (LMP), a centrifugal stress were calculated at each section. The centrifugal stress from mean to tip was found to be 4.87 kN and the cross section area of the corresponding section is 6.71 presented in Table 3.The same procedure was applied from root to mean
3.3 HP TURBINE BLADES – ESTIMATION OF CREEP LIFE
The creep life of the blade at each section can be calculated once the blade section stress and the Tmsec are known. To find the LMP at each section the formula can be represented as follows.
/ (7)Where;
The constant C is equal to 20 for an industrial applications but it can vary according to conditions stated (Haslam, 2011).At ISA design point clean condition when TET = 1505K, the section metal temperature is 1049K and the stress at the mean blade was found to be 72.53MPa. Using a graph the LMP can be obtained at blade stress = 72.53MPa and the result found to be LMP = 26.65.
/ (8)
Assuming a factor of safety 60% (Haslam, 2011) at all temperatures, therefore the value of has changed to 17.4 years. The temperature across each stage is assumed to be the same. Since the stress at shroud is zero and the creep life was found to be very small, so it is assumed to be negligible. The same procedure was followed at the root.
3.4 EFFECTS OF CREEP LIFE ON METAL TEMPERATURE
An increase in metal temperature from 1049K to 1127K will lead to a decrease in creep life of the blade at mean from 29 years at ISA to 17 years at 42⁰C of the Northern part of Nigerian temperature for a period of one year. This shows about 41% decrease in creep life of the blade in order to maintain a constant power there is need to increase the TET which will also decrease the blade creep life to about 6 months.
Figure 5: Effects of Creep Life on Metal Temperature
3.5 EFFECTS OF CREEP LIFE ON FIRING TEMPERATURE
An increase in firing temperature from 1505K to 1615K will lead to a decrease in creep life of the blade at mean from 29 years at ISA to 17 years at 42⁰C of the Northern part of Nigerian temperature for a period of one year. This shows about 41% decrease in creep life of the blade. The blade creep life decreases proportionally with an increase in TET, the results is the same since the metal temperature is proportional to the turbine entry temperature.
Figure 6: Effects of Creep Life on Firing Temperature
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3.6 COST OF INCREASING FUEL FLOW
Compressor fouling has leads to an increase in fuel flow. The price of the fuel gas was obtained from NYMEX Natural Gas Features website for a period of 6 years from January 2006 to December 2011, but the analysis will focused on the year 2006 since it is the year that can give a clear effect of changing the fuel price.
During the period of 2006 price have gone down from $11.43/MMBTU to $8.32/MMBTU and the average being about $7.23/MMBTU. The following are some of the information used in calculating the fuel cost;
- Natural gas density (NGD) = 1.14 kg/m3 (Nafoora Chemical Laboratory)
- Natural gas lower heat value = 37456 BTU/m3 (NCL)
- Engine operating hour = 8760 hours
- Fuel flow increase = 0.042 kg/s
Since the quantity of the fuel has been found it is now easier to find the volume of the fuel using the formula as follows.
/ (9)3.6.1 YEARLY EXTRA FUEL COST
The following formulas are used in calculating the yearly extra fuel cost;
Yearly / (10)/ (11)
= 7.32 = $5332
3.7 COST OF COMPRESSOR FOULING AT MINIMUM FUEL GAS PRICE
The minimum fuel gas price was approximately $8.32/MMBTU in December due to decrease in price of crude oil and can be achieved as follows.
3.7.1 YEARLY EXTRA FUEL COST
If the unit works for one year the cost of fuel price during the operation will increase as follows.
Yearly increase fuel flow = 0.037×8760×37456×60=728.41 MBTU
Yearly increase cost of fuel = 8.32×728.41=$6060
3.8 COST OF FOULING AT MAXIMUM FUEL GAS PRICE
The maximum fuel gas price was approximately $11.43/MMBTU in November and can be achieved as follows.
3.8.1 YEARLY EXTRA FUEL PRICE
If the unit works for one year the cost of fuel price during the operation will increase as follows.
Yearly increase in fuel flow = 0.037×8760×37456×60=728.41 MBTU
Yearly increase in cost of fuel = 11.43×728.41=$8325
Table 4: Effect of Changing Fuel Price
Operating Hours / NYMEX Natural Gas PriceMinimum $8.32/MMBTU / Maximum $11.43/MMBTU
8760 / $6060.37 / $8325.73
17520 / $12120.74 / $16651.46
26280 / $18181.11 / $24977.18
The operating cost increases from $6060.37 to $8325.73 due to the effect of changing the fuel price at 8760 operating hours which is equivalent to one year of operation without compressor cleaning. The result also shows that an increase in fuel price has great effect on operating a gas turbine engines. The operating cost will increase significantly if there is no compressor cleaning.
4.0 CONCLUSION
During the operation of an industrial Gas Turbine, GT components undergo various types of degradation due to high temperatures and mechanical loading. These conditions make the components to have failure mechanisms. The LMP is the method used to calculate the engine creep life. The creep life of the blade at mean was estimated to be 29 years at ISA and reduced to 17 years at target temperature of 42⁰C with a percentage decrease of 41% due to effect of Ta and sandy environment.
To maintain a constant power output TET has increased from 1505K to 1615 K. The result shows that an increase in TET leads to an increase in metal temperature from 1049K to 1127K which results in high stress. The turbine blade can also be affected by the gas flow around the blades, inlet and outlet cooling temperature, and cooling effectiveness of blade.
5.0 REFERENCES
Eshati, S., Abdul Ghafir, M. F., Li, Y. G., and Laskaridis, P., (2010), “Impact of Operating
Conditions and Design Parameters on Gas Turbine Hot Section Creep Life” School of Engineering, Cranfield University, Bedford MK43 0Al, UK. Proceedings of ASME Turbo Expo 2010: Power for Land, Sea and Air. GT2010.
Haslam, A. S., Cookson, R. A., (2011), “Mechanical Design of Turbomachinery” Department
of power and Propulsion, School of Engineering, Cranfield university.
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LM2500+ accessed on 18 Nov.2011 by 10:00 pm.
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Contract Settlement Price” accessed on 6th December 2011 by 10:00 pm.
Reimer, M. M., (1997), “Stress and Strain” in Edward, P. J. (ed.) Rules of Thumb for
Mechanical Engineers, Gulf Publishing Company.
Viswanathan, R., (1998), “Damage Mechanisms & Life Assessment of High-Temperature
Components” ASME international, Metals Park, Ohio.
Weber, B., Jin, H., Pistor, R. And Lowden, P. (2005), “Application of an Integrated
Engineering Approach for LM1600 Blade Life On-line Assessment”, 16th symposium on industrial application of gas turbines (IAGT), 12-14, October, Canada.
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