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Evaluation of high temperature steels for CCGT gas turbines. OMMI, 2008, Volume 5, Issue 3, Dec. www/ommi.co.uk

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EVALUATION OF HIGH TEMPERATURE STEEL ALLOYS APPLIED TO HEAT RECOVERY STEAM GENERATORS FROM COMBINED CYCLE

GAS TURBINES

Jorge Pinto Fernandes 1* ; Eduardo Dias Lopes 1,2 1ISEL, Instituto Superior de Engenharia de Lisboa, Rua Conselheiro Emídio Navarro, 1959-007 Lisboa, Portugal

*Now with ETD Ltd., 6 Axis Centre, Cleeve Road, Leatherhead, Surrey, KT22 7ND, UK

2ISQ, Instituto da Soldadura e Qualidade, Avenida Cavaco Silva, nº 33, TAGUSPARK, 2780-994 Oeiras, Portugal

Jorge Pinto Fernandes, studied at ISEL, Instituto Superior de Engenharia de Lisboa. During this period, he worked in a small metallurgical construction company and later, gained experience at Energias de Portugal with problems relating to maintenance of power plant. Returning to ISEL, he commenced R&D activities associated with the Integrity and Maintenance Division and also the Research and Development Division of ISQ, Instituto de Soldadura e Qualidade. One of his interests was tubing and piping design issues. He recently graduated in Mechanical Engineering and joined ETD, European Technology Development, as a Project Engineer involved with projects related to power plant maintenance issues. [email protected]

Eduardo Manuel Dias Lopes, 60 years old, has a background in Materials Science and Chemical Engineering. Since 1992, he has been Director of Research and Development Division, at ISQ, Instituto de Soldadura e Qualidade. He is an Engineer and Researcher with about 35 years of experience. He has been responsible for the Management of Research Programs at ISQ, in areas related to Materials Performance, Plant Integrity and Environmental Sciences for more than 20 years. For the last twenty years, he was responsible for EU projects as coordinator and he is frequently contracted by EC as an expert for Project Evaluation. In 1991, he became the Invited Professor on Materials, Environmental Sciences and Structural Integrity in Mechanical Engineering Course at ISEL, Instituto Superior de Engenharia de Lisboa. He has more than 90 papers and communications in Technical and Scientific Meetings. [email protected]

Abstract

Mankind is seeking new solutions to produce power energy with great efficiency; an essential step for having more power per ton of consumed fuel or gas and related gas emissions. With this approach, steam pressure and temperature in fossil-fired power plants have been continuously increased to improve thermal efficiency. So, the future Gas Turbine Combined Cycle conditions will increase and design will be improved, pushing the Heat Recovery Steam Generators to operate at extreme conditions, thereby reducing the consumption of gas and lowering the CO2 and NOx emissions. On the other hand, in the highly competitive HRSG marketplace as it is


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today, designers, manufacturers and suppliers face the dilemma of furnishing the best equipment and, at the same time, remaining competitive. Owners are more concerned about initial cost, delivery and reliable operation. With all these demands, the HRSG has to increase its operation parameters and push its capacity to the limit. The HRSG is one of the few components tailored for each specific application, hence its technology is always improving. The simplest changes in design would directly affect the variables of the cycle. With the raising of the operating temperatures requiring superheater tubes and headers capable of allowing higher metal temperatures, the HRSG boiler designer faces the necessity for new steels to operate at higher temperature with a low cost. Because the HRSG is an important component for the GTCC, the new materials are the actual challenge for steelmakers and designers. Using the new operating conditions such as higher ranges of temperature, pressure and cyclic service, we will briefly analyse, in this paper, the new steel alloys present in the market or in development, analysing and defining the behaviour and impact on damage mechanisms and estimating the Remaining Life Assessment, RLA, in-service including in this issue the Risk Analysis.

Key Words: Gas turbine combined cycle power plant; heat recovery steam generators; heat resistant steel alloy; damage mechanism; remaining life assessment; risk analysis; creep strength and fatigue; finite element analysis.

1. Introduction

In the 21st century, mankind is facing new difficulties, new obstacles that create problems to their development. The needs of Power Energy are growing all over the world, [1]. Furthermore, the growing importance of the protection for the environment is making the process even more difficult but not impossible. So, mankind is facing the “critical challenge of providing abundant and cheap electricity to the world to meet the needs of a growing global population while at the same time preserving environmental values”, [2]. Then, in the near future, thermal power plant design must take into account lower fuel cost and CO2 emissions per KWh produced, through further improvements by elevating steam conditions to higher ranges of temperature. To achieve this goal, boilers and the turbine will be operating at higher metal temperatures pushing materials to the maximum capability and exposing them to time dependent damage mechanisms. New materials are needed to fulfil Mankind Progress and Development.

2. Gas turbine combined cycle.

The development of the technology applied to Combined Cycle Gas Turbines, with the lower pollutants released by this cycle, makes this type of power plant a popular commercial choice in the past years but, unfortunately, the input consumption of Natural Gas (NG) grows as well, [3]. Regarding the importance of NG, new solutions to improve thermal efficiency are currently being studied. The design of the Combined Cycle Gas Turbine, CCGT, power plant is inherently complex due to existence of the different power cycles, the Joules Bryton and Rankine Cycle, which are coupled through the Heat Recovery Steam Generator – HRSG. The recovery boiler is the connection between the gas turbine cycle, the Joules Bryton and the steam cycle, the Rankine Cycle. Therefore, any change in the design of the HRSG boiler directly affects the cycle efficiency, its power generation, the global cost, the maintenance procedures and many other variables, [4].


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Figure 1 – Configuration of a Gas Turbine Combined Cycle, [5].

3. Heat recovery steam generators

A HRSG Boiler is a recovery boiler that uses the heat from a gas turbine exhaust flow. The heat flow from these exhausted gases is used to produce superheated steam in the HRSG and after this process the superheated steam is used in conventional steam turbines, Figure 1. With this solution, depending on the displacement of the gas turbine, generator and steam turbine, coupling a gas turbine and a steam turbine in a single shaft is going to provide the generator with a higher kinetic energy in the generator shaft, thereby producing in this way an additional fraction of electric energy. The Heat Recovery Steam Generator of this study is a normal triple pressure HRSG, Figure 2.

Figure 2 – General representation of the study triple pressure heat recovery steam generator.

In Figure 2, a horizontal HRSG with 5 sections is shown where each section has 3 modules divided in 3 lines, A, B and C. The last two modules in the line C are different from the modules of the section 4A and 4B and section 5A and 5B.

Regarding the past investigations on this boiler type, Benson Technology have been making improvements and, in principle, when applied to some conventional and HRSG, bring new improvements to the Combined Cycle Power Plant. This technology, created in 1922 by Mark Benson, and today, as developed by Siemens Power Generation, produced the once-through HRSG where a once-through evaporator could be operated at both subcritical and supercritical pressure.


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The HRSG, with sliding pressure, can make the transition from subcritical to supercritical steam temperature increasing the process efficiency, with a considerable decrease in fuel costs per KWh produced, [6]. This way, the HRSG tubing and piping will be more “potentially damaged” through a wider range of pressures and temperatures. The result will be a larger risk of high temperature damage due to creep, corrosion fatigue and due to stresses generated by temperature gradients resulting from different thermal expansion.

4. Geometry and design conditions The component chosen for this study is a High Pressure Superheater Upper Outlet Manifold from a Heat Recovery Steam Generator installed in Israel. This component is chosen for this work because it is normally the part exposed to highest temperature and pressure conditions and is exposed to the most severe transients in gas flow and temperature. This is the final piping, where the steam flows, to come out of the HRSG Boiler and go to the Conventional Steam Turbine, (Figure 3).

Figure 3 – View of the HPSH Upper Outlet Manifold.

The following Table 1 is a summary of the basic design conditions, geometry and initial materials used in this component.

Table 1 – Design conditions calculated using the ASME section I.

Design Conditions I.S. Units U.S.Units

Design Pressure 15.4 MPa

2234 PSIG

Design Temperature 584 ºC 1083 ºF

Manifold Outside Diameter

323.9 mm 12.75in

Manifold Nominal Thickness

48 mm 1.89 in

Manifold Minimum Thickness

29.50 mm 1.16 in

Pipe Outside Diameter 114.3 mm 4.5 in

Pipe Minimum Wall Thickness

14.98 mm 0.59 in

Pipe Average Wall Thickness

17.12 mm 0.67 in

Pipe Material SA-335 P91 Plate Material SA-387 GR22 CL2


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All components were designed to meet the minimum thickness requirements in

accordance with ASME Section I Boiler & Pressure Vessel Code, Rules for Construction of Power Boilers. The “Diagonal Efficiency” and “Longitudinal Efficiency” was based on the Vicente Maneta calculation method for the drum, [7].

5. Mechanical components

As mentioned earlier, the superheater manifold is the most loaded element in a heat recovery steam generator. It is therefore designed to be thermal and pressure resistant. Components of HPSH manifold are as shown in Figure 4 where:

1 – Alignment Plates; 2 – 12” SCH Pipe; 3 – Manifold support lug; 4 – Manifold end plug; 5 – Radiographic hole & plug; 6 – Bending 4” SCH Pipe; 7 – Direct 4” SCH Pipe;

Figure 4 – Trimetric view of the HPSH Upper Outlet Manifold.

6. Steel alloys for power applications Damage mechanisms like creep, thermal fatigue and steam oxidation, are complicated and influenced by material type, microstructure, external loading and environment, and, if possible, a complete set of material data is preferred. In this way, here are some material parameters that must be inserted in the present study to be able to reach a more realistic result.

The parameters that we will need for the Finite Element Analysis are shown in Table 2 for a range of materials.


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Table 2 – Mechanical properties (minimum value) for new steel grades.

YT (σc)

TS (σb)

Thermal Expansion

Thermal Conductivity

MPa MPa x10-6/K W/m.K P22 at 600 ºC 205a 415a 14.3c 33.0c

P23 at 600 ºC 400a 510a 13.9c 33.1c

P24 415b 585b 13.7c 36.9c

P91 415a 585a 12.9c 30.0c

E911 at 600 ºC 285d 620b 12.0d 27.0d

P92 at 600 ºC 368d 620b 12.0d 27.0d

P122 at 584 ºC 265.5e 415e 14.3e 33.0e

a ASME Section II, Materials, Metric Version, [ 8] b Bendick, B.; Gabrel, J [ 9] c V&M The T23/T24 Book, 2000, 600 ºC, [10]; d Starr F. in [11]; e Extrapolated data.

7. Finite element analysis To be able to make a valid comparison between the various steel alloys, it was necessary to use a finite element analysis program where the geometry and working parameters etc. are defined. Some important aspects of the finite element analysis model used in the study are given below. . The 12 SCH Pipe and 4 SCH Pipe (straight and bending) geometry was designed using

the Finite Element Modelling program SolidWorks version 2007; The analysis was made using the general-purpose finite element program ANSYS

Workbench V11 Service pack 1; The steel alloy parameters are introduced in the finite element program and the data

were retrieved from reviews of creep rupture strength studies or from the ASME Boiler and Pressure Vessel Code section II;

The Manifold is finely meshed in order to pick up any significant stress concentration and deformation.

8. General explanation of fatigue analysis 8.1. Maximum, Middle, and Minimum Principal From Mechanics of Materials, an element or an infinitesimal volume of material at one arbitrary point inside or outside the solid body can be rotated to a certain degree where shear stresses are zero and normal stresses remain, showing the maximum and minimum stress. These three normal stresses are called principal stresses because they show the maximum and minimum stress. The Ansys Workbench (Finite Element Modelling Software) [12] identifies the principal stresses as σ1 – Maximum stress, σ2 – Middle stress and σ3 – Minimum stress (Figure 5). The principal stresses are always ordered such that σ1 > σ2 > σ3 [12].


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Figure 5 - Finite element with the displacement of maximum, minimum and middle stress where the

shear stress is equal zero reference.

Generally, the Equivalent Stress or Von Mises stress cannot be used to determine the number of cycles in a fatigue analysis because this stress is always positive and generally the stress range is variable for a fatigue analysis, (Figure 6). So with the objective to make a correct calculation, it is needed to focus in the range between maximum and minimum tensile stress or between a maximum tensile stress and maximum compressive stress [13].

Figure 6 - Test parameters involved in fatigue cycle stress FEM.

8.2. Effects of Principal Test Parameters and Mean Stress There are many variables in a fatigue test procedure that affect the fatigue life, such as stress, temperature, environment, specimen size, specimen surface condition, and stress concentration [13]. During this work, some of these variables, such as the interaction of the body structure and the environment, specimen surface condition, will not be analysed. Regarding the temperature, even if this is very important for the Temperature-Stress interaction, thermal fatigue is not included in this work and the Finite Element Simulation was done with the stated temperature of 584 ºC defined in the project.


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Figure 7 - Fatigue amplitude load where the mean stress is equal zero.

If the mean stress is zero, σM = 0, the stress range between the minimum stress (σmin) and maximum stress (σmax) or tension-compression cycle simply has the opposite sign (Figure 7). However, in reality, the mean stress is different from zero, σM ≠ 0, and the amplitude varies. During this case, the fatigue limit depends on the ratio between minimum and maximum stress.

max

min

σσ=Ratio (1)

When the Ratio becomes more positive, that is equivalent to saying that mean stress is becoming higher and the endurance limit is becoming greater. Viswanathan [13] states that designing components against the specific R ratio to be encountered in service is therefore very critical.

2minmax σσσ −=A (2)

Regarding equation 2, we can see that for each value of σM, calculated in similar way as σA, there is a different value of alternating stress (σA), as shown in equation 2. Generally there are two ways for representing data shown in Figures 8 and 9 using the Goodman diagram and the modified Goodman diagram including Gerber parabola and Soderberg as described in [13]. In those diagrams, it is possible to see that the maximum alternating stress or maximum stress range can be tolerated when σM approaches zero. A general relationship among σA, σ-1, σM, and the ultimate strength σb is defined in reference [2], where it is shown that:

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−= −

x

b

MA σ

σσσ 11 (3)

where x=1 for the Goodman and Soderberg approaches, x=2 for the Gerber parabola approach, and σb is replaced by the yield strength σc for the Soderberg approach (equations 3a, b and c).

⎥⎦

⎤⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−= −

b

MA σ

σσσ 11 (3.a) Goodman


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⎥⎦

⎤⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−= −

c

MA σ

σσσ 11 (3.b) Soderberg

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−= −

2

1 1b

MA σ

σσσ (3.c) Gerber

The effects of cyclic loads caused by temperature variation in the material can be very strong when we talk about fatigue or more properly about thermal fatigue, giving a reduction in the fatigue limit and a lowering of fatigue strength.

Figure 8 - Goodman diagram, graphical method for presenting the combined effects of alternating

stress and mean stress on fatigue life [ 13].

Figure 9 - Modified Goodman diagram showing a comparison between the Gerber and Soderberg

law [13].


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8.3 Principal Stress and Mean Stress Correction Theory Stress solutions allow you to predict safety factors, stresses, strains, and displacements given the model and material of a part or an entire assembly and for a particular structural loading environment. For a Fatigue Finite Element Simulation, it is crucial to define an analysis type or fatigue method for handling mean stress effects. Regarding the Ansys Workbench, the choices are the SN Curves, the Goodman, Soderberg, Gerber Laws and the Mean Stress Curves. The Goodman, Soderberg, and Gerber options use static material properties along with S-N data to account for any mean stress while Mean-Stress Curves use experimental fatigue data to account for mean stress. Another point that has to be taken into consideration is that these laws are used in a uniaxial fatigue analysis. Nowadays, the most used, common and simple theory applied in the industry is the Goodman Law. So, during the fatigue simulation, we will use the Goodman Law where it is necessary to calculate the σA, σM, σ-1 and the σb that is defined in ASME Boiler and Pressure Vessel Code, Section II, Part D for the steel alloy P91. For other steels, we will resort to simple interpolation of data from Laboratory experimental information. A typical curve is represented in Figure 10, but a Log scale will be used to establish a comparison between the steel alloys.

2minmax σσσ −=A (4)

2minmax σσσ +=M (5)

Figure 10 - S-N Curve.

For the calculations, we will need the principal stress so is important to write the Goodman expression in terms of σ-1:


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b

M

A

b

MA

x

b

MA

σσ

σσσσσσ

σσσσ

−=⇔⎥

⎦

⎤⎢⎣

⎡−=⇔

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−= −−−

111 111 (6)

One important issue about the fatigue simulation is related to the number of cycles obtained from the finite element simulation. If information about the alloy steels is obtained, like the materials constant “a” and “b”, then is possible to define the number of cycles using the expression

( )bfNa=−1σ (7)

It is possible, from the equation 7, to define the number of cycles easily by means of equation 8.

feee Nba logloglog 1 +=−σ

⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜

⎝

⎛ −

=b

a

f

ee

eN

1logσ

(8)

9. Cycling Fatigue Analysis for P91 The engineering components of power equipment in service are frequently subjected to cyclic loading, which can result in failure due to the fatigue damage. Regarding to this type of equipment, a complete description and a total number of each type of operating cycle must be defined in order to evaluate the cycle life of a HRSG unit. This level of detail is not easy to define; it depends on the experience of the technical staff or in the judgement regarding the operation of the equipment. In addition, Lewis R. Douglas et al, [14], have defined various types of cycles as given below.

Cold Start up Cycle Internal pressure within the boiler is raised from zero and 24 ºC to the operating pressure

of 15.4 MPa (2234 Psig) and 584 ºC and then back to zero. This cycle is considered to be a full range pressure cycle. Operating pressure is somewhat less than the Maximum Allowable Working Pressure.

Warm Start up Cycle

Internal pressure within the boiler drops from normal operating pressure of 15.4 MPa to some lower pressure and then back to normal operating pressure. This is defined as the cycle for a warm start, occurs after a 6-8 hours shutdown and further assumed that for a warm start, the pressure will reduce by no more than 75% of normal operating pressure corresponding to 3.85 MPa or 1682.4 Psig and then return to normal operating pressure.


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Hot Start up Cycle Internal pressure within the boiler decreases from the normal operating pressure, to a lower pressure and back to normal operating pressure. A hot start occurs within an hour of the last shutdown. It is assumed that a hot start will reduce the pressure by no more than 10% of normal operating pressure and back.

Gas Turbine Floating Pressure Operation Gas Turbine load swing varies from 100% to 50% and then to 100%. The change in steam turbine load resulting from the change in GT output and steam flow causes the pressure within the boiler to drop and then increase. It is assumed that for this case, the pressure will reduce by no more than 50% of normal operating pressure and back.

Auxiliary Burners Floating Pressure Operation Burner load changes from 100% to 0% to 100%. These changes in steam flow cause the pressure within the high pressure section of the boiler to drop. In the case of a HRSG with the first module referred to as a Superheater, this will influence the stress of the High Pressure Superheater Manifold. It is assumed that, for this case, the pressure will reduce by no more than 10% of normal operating pressure and back. Generally, this type of load change is not considered significant. The described cycles are presented here for during the life time of a HRSG. In this study, special attention is given to three important types of cycles represented in Figure 11.

Figure 11 – General view of pressure range applied to the HPSH Manifold describing a Cycle.

Using Finite Element Analysis, a uniaxial simulation was developed, based in the HPSH Manifold, where submitting the geometry of this component to design operation values and using the steel alloy P91 gave the results, for the three different fatigue cycles, presented below.


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Figure 12 - Fatigue analysis of HPSH Manifold subjected to the cyclic loading defining an S-N curve.

Regarding the cyclic simulation of steel P91, it is possible to conclude that the applied stress is directly influenced the magnitude of the alternating stress or the range of the internal pressure. Comparing the cases of high and small pressure range for one million cycles (Figure 12), the only difference between these two operating cases is the magnitude of pressure decay between cycles. Therefore it can be seen that larger alternating stress intensity reduces the allowable stress for the same number of allowable cycles. Hence keeping the alternating stress intensity as small as possible is an important factor for increasing the high cycle fatigue life due to any direct loading. Although it is not analysed in this paper, thermal gradients between the inside and the outside of walls from a pipe are a second source of stress in the HRSG components. These typically occur in a transient operating condition in which the outside surface temperature is higher than the inside temperature. It’s this through thickness transient temperature gradient that produces the thermal stress. Meantime, fatigue is greatly influenced by the magnitude of the alternating stress range and the number of cycles as we can see during the analysis made in this paper. In the future, a transient analysis should be made to define clearly the importance of thermal stress and direct loading. Two factors that improve a component’s ability to withstand fatigue are material selection and construction details. These two factors should be heavily considered during the design of the HPSH Manifold. In future Finite Element Modelling, a complete exhaustive analysis should be taken into consideration applying the “Design by Rule Requirements” and “Design by Analysis Requirements” presented in the ASME Code, Section VIII, division II.

10 Remnant life assessment, (RLA).

RLA is a predictive maintenance approach for industrial equipments and other structures whose operating methods make them susceptible to limited life. In the specific case of energy production facilities like Conventional Thermal Power Plants, Gas Turbine Combined Cycle units, Cogeneration Power Plants and other industrial plants like Crude oil

Cold Start up Cycle

Hot Start up Cycle

Warm Start up Cycle


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Refineries, the RLA studies are associated with the possible exploration activities before the project lifetime is finished; increasing the operation time and pushing the installation to its operational and economical limit. In this work we will focus on the behaviour of steel alloys in the most important areas for a heat recovery steam generator.

Taking in consideration the data showed in Table 1, stress analysis was developed for the 12 SCH Pipe and in the 4 SCH pipe where the critical area is related with the fixed support (Figure 13), where a simple procedure developed in the simulation was fixing the manifold support lug next to manifold end plug and maintain the support free next to the exit steam section (blue axis).

Figure 13 – Finite element analysis consideration for the HPSH Manifold Geometry 12 Schedule

Pipe and 4 Schedule Pipe.

A methodology was also developed for gathering data (Figure 14) where some data points related with stress was analysed and shown in the Figures 15 and 16.

Figure 14 – Designed procedure for the 12 Schedule Pipe and 4 Schedule Pipe.


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Some considerations should be taken during the simulation where the equivalent stress is being used for design analysis with the main objective to determine the behaviour of the applied steel alloy used. The equivalent stress is related to the principal stresses by the following equation:

( ) ( ) ( ) 21

213

232

221

2 ⎥⎦

⎤⎢⎣

⎡ −+−+−= σσσσσσσ e

This equivalent stress, σe, (also called von Mises stress), is used in design work because it allows any arbitrary three-dimensional stress state to be represented as a single positive stress value making use of the maximum equivalent stress failure theory used to predict yielding in a ductile material. The equivalent stress is also used in the parametric extrapolation techniques for estimation of the long-term creep and rupture strength of materials based on short-duration laboratory tests.

The diagram shown in Figure 15 illustrates the equivalent stresses (or Von Mises Stresses) that are generated for the specific geometry of the High Pressure Super Heater Manifold, considering an internal pressure of 15.4 MPa (2234 PSIG) and a design metal temperature of 584 ºC (1083 ºF) for each of the materials considered. These data are related with the 12 SCH pipe where the steel alloy behaviour of the P91 is clearly the best in comparison with the 2.25% Cr steel. Figure 16 illustrates the stress distribution in the header for typical P91 material considering the cold start up cycle conditions.

Figure 15 – HPSH Manifold Geometry 12 Schedule Pipe Equivalent Stress propagation comparison

between 2 ¼ Cr Steel (P22, P23 and P24) and 9 Cr steel (P91).


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Figure 16 – Range of equivalent Stress for SA-335 P91 Grade.

Analysing the data from Figure 15, it is quite obvious that P91 in almost all the 12 SCH pipe sections shows a good behaviour. But in the sections where the stress is low, its behaviour is very similar to the obsolete P22. Meanwhile, between P22 to P23, there is a very high difference where P23 have a better behaviour than P22. The difference between P23 and P24 is very low, these two steel are very similar. Meanwhile, continuing the Figure 15 analysis, in the right side of the diagram, the stress grows between section 8 to 13 because in that area, the manifold is fixed in the right manifold support lug whereas the left side completely free. Even in the mentioned sections, the P91 shows a good behaviour but it is also true that is quite difficult to take a better conclusion because the developed methodology for analysing data is not appropriate for this kind of analysis. In future analysis, a more restricted methodology will be developed and the fixed support FEM analysis will be improved, as in [15 and 16]. A similar analysis, applying the same methodology was carried out for the 4 schedule pipe (Figure 17), where it is important to make a complete analysis for the straight 4 SCH pipe and another one for the direct 4 SCH pipe. Regarding the data collected, it is easy to see that a high stress concentration is present between the connection of 4 SCH pipe and 12 SCH pipe because of the transition of the weld metal and the base metal. Note that the welding characteristics imputed in the program are similar to thermanit filler metal defined in [17]. Meanwhile, the welding was not analysed in this paper but it brings implications in stress changes that should be studied in future research. In the straight 4 schedule pipe, the P91 again shows a good behaviour when compared with the old P22 and with the new 2.25 Cr steels P23 and P24. But, in the bending 4 schedule pipe, (bending with 18º), the P91 shows a high probability for high stress concentration associated with the bending. This is a very interesting situation where in this case the 9% Cr steel and the new 2% Cr steel show very low stress behaviour when compared with the old 2 ¼ Cr 1Mo (P22). After the bending, the P91 start to again show very good behaviour but in the end the new 2% chromium steel P24 start to show interesting values. The main conclusion is that the new grades like P23, P24 and P91 with improved mechanical properties at high temperature and pressure clearly are better than the old P22 material. A remaining question is related with the P23 and P24, where it is difficult to define which is the most suited for a particular situation. This is another question that should be analysed in future work to be reported, [15 and 16]. With these results, it is expected that the good creep strength of P91, P23 and P24 make it possible to reduce the wall thickness of the pipes and consequently improve the thermal fatigue behaviour.


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Figure 17 – Finite Element Modelling equivalent stress analysis for the straight 4 Schedule Pipet and bending 4 Schedule Pipe.

Applying the fatigue analysis technique described in this paper, a general uniaxial fatigue analysis involving the HPSH Manifold was developed. The operation condition is identical to the Cycle analysis where, the major consideration goes to cold start up cycle. The FEA data is shown in Figure 18 where is possible to compare the mechanical behaviour of new 2%Cr Bainitic steels P23 and P24 and also the 9%Cr Martensitic steel P91.

Figure 18 - Logarithmic representation of Stress and Number of cycles for P91 and comparison with 2 ¼ Cr Steels P24 and P23 defining one SN curve or Wohler curve.

Analysing the above diagram, below 600 000 cycles, the P91 has a good behaviour compared with the 2Cr Bainitic P23 and P24 alloys. Meanwhile, above 600 000 cycles, the P91 have a low performance compared with P24 whose capacity to stand high number of


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cycles during a simulation about 584 ºC is very good but due to oxidation behaviour, the application of P24 is limited to the temperature of 575 ºC [18]. Figure 19 define the life range for the HPSH Manifold geometry, where the right manifold support lug is the element with the lower fatigue life in all the component because it is the restrained support. In future studies, a fixing system must be developed with the objective of improving the data analysis.

Figure 19 – Number of Cycles for SA-335 P91 Grade.

11. Risk analysis

Environmental restrictions, risk of accidents, world energy crisis, simulation of cost-benefits analysis, online monitoring, material behaviour, modelling tools etc are today the major elements of concern for a Maintenance Operator. Included in this is “Risk Based Inspection” which brings a new maintenance philosophy of power equipment involving the planned inspection only on the basis of the information obtained from a “Risk Analysis” study. Today, with the development of strong software and finite elements, it is possible to simulate almost every thing creating the operation environment preventing failure and identifying the most critical elements which have high probability of failing and high consequences for reducing the cost and shutdowns. Using for example, Ansys, CosmosWorks and Abacus, it is possible to run high quality complex simulation contributing directly to Risk Assessment Analysis, identifying critical elements and defining operation and maintenance procedures, simplifying and applying some complex procedures and utilization of complex codes.

12. Conclusions

The present paper is a contribution to the understanding of the use of high alloy

type steels, resistant to new imposed operating conditions in the new CCGT generation, for new operating conditions with improved efficiency.

It is also the first step for the development of complex finite element analysis simulation, where the contribution of Finite Element Modelling, Finite Element Simulation and Finite Element Analysis will be dealing with a great number of considerations and operations factor.

The new power generations systems based on CCGT will be, in the medium term, an alternative and a complementary solution to the coming generation of Hypercritical Coal Fired Power Plants.

Analysing the behaviour of the HPSH Manifold, where steel alloys have a strong contribution factor, an exercise was performed, simulating the behaviour of this element, by defining a stress and fatigue analysis.


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The 12” SCH pipe is a component with the higher equivalent stress, due to higher thickness and temperature.

Regarding the straight 4” SCH pipe, P91 shows a good behaviour but the new bainitic 2%Cr Steel P23 and P24 show a similar behaviour.

The bending 4” SCH pipe show interesting results. P22 shows the best performance among the other steels, but after the bending the results show the good behaviour of P91.

In case of 12” SCH pipe and 4” SCH operating at 584ºC and 15.4MPa, 9%Cr steel P91 is the best material solution, in the present imposed conditions, for cyclic operation conditions.

13. Future studies

The study will be continued into the Gas Turbine Combined Cycle and conventional thermal power plants leading to:

contributions of procedures of maintenance and design of high temperature and pressure power elements with crucial importance,

definition of all the characteristics involved as welding procedures, pre weld heat treatments,

effect of external parameters on the gas turbine flow and the steam flow inside of piping, tubing and conventional steam turbine,

refining of the characteristics of steel alloys and its behaviour relating with geometry design of the equipment,

development of application methods for logic and simple mathematical application that permit the calculation of remaining life from a component regarding to creep, fatigue and corrosion behaviour,

preparation and support of the gas turbine designer and boiler designer to apply geometry optimization for better cost-effectiveness equipments,

development of a model based (algorithm) on chemical composition to be linked with FE with the objective to create a dependence of the Finite Element software’s to the chemical steel alloys components,

integration of a multiaxial finite element simulation where the multiaxial fatigue and multiaxial creep will be added to thermal transient analysis.

14. Acknowledgements The authors thank:

Mr. Vicente Maneta from the Manufacturing and Design Division of Alstom Power, S.A.

(Setúbal Factory) for discussions and assistance with the ASME Code details of the analysis;

Professor Bin Li for the fatigue explanation; Mr. Pedro Nunes Navarro of ISEL, for assistance with the Finite Element Analysis; Rui Rodrigues, Carlos Silva, Sílvia Costa, Marisa Pereira for discussions and assistance; PTMN EDP Team for discussion, assistance and access to the relevant documents.

15. References

[1] ENERGY.EU; “Europe’s Energy Portal”; www.energy.eu;


20

[2]Viswanathan R., Henry J.F, Tanzosh J., Stanko G., Shingledecker J.P., Vitalis B., “U.S. Program on Materials Technology For Ultrasupercritical Coal-Fired Steam Power Plants”; Eighth Internacional Conference on Creep and Fatigue at Elevated Temperatures; July 22-26, 2007; San Antonio, Texas;

[3] Eurostat Statistical Book; “Gas and Electricity Market Statistics”; European Comission; Luxemburg; 2007 Edition Page 45;

[4] Valdés M., Rapún L. J.; “Optimization of heat recovery steam generators for combined cycle gas turbine power plants“; Elsevier Science Ltd.; 2001

[5] Pinto Fernandes J., Dias Lopes E.; “Damage Mechanism and Component Life Assessment applied to Heat Recovery Steam Generators”; Expo Mechanic 2008; ISEL; April 2008; [6] Siemens AG; “BENSON Boilers for Maximum Cost-Effectiveness in Power Plants”; Power Generation Group; July 2000; Germany.

[7] Maneta, V.; “Calculation of the Demonstration Drum for Renewal of Stamp ASME to be Manufactured in Accordance with ASME Sect. I – 1992 Edition”; Mague; Alverca – Portugal;

[8] American Society of Mechanical Engineers; “ASME Boiler and Pressure Vessel Code Section II Part D – Ferrous Material Specifications”; Metric Version; July 1, 2007 Edition; United States of America;

[9] Bendick, B.; Gabrel, J.; “New low alloy heat resistant ferritic steels T/P23 and T/P24 for power plant application”; International Journal of Pressure Vessel and Piping; 2007, (page 13-20);

[10] Arndt J., Haarmann K., Kottmann G., Vaillant J.C.; “The T23/T24 Book – New Grades for Waterwalls and Superheaters“; Vallourec & Mannesmann Tubes; 2nd Edition; October 2000;

[11] Starr, F.; “Potential Issues in the Cycling of Advanced Power Plants”; OMMI Vol.1, Issue 1; April 2002;

[12] “ANSYS Simulation 11.0”; Service pack version SP1; 20 September 2007; United States of America;

[13] Viswanathan R.; “Damage Mechanisms and Life Assessment of High-Temperature Components”; ASM International; 1st Edition; August 1, 1989; Carnes Publication Services, Inc; United States of America;

[14] Douglas L.R.; Schroeder J.E.; Eriksen V.L.; “Evaluation of Cyclic Operation of Heat Recovery Steam Generators – A Practical Approach“; ASME Turbo Expo 2000; May 8-11, 2000; Munich Germany;

[15] Pinto Fernandes J., Dias Lopes E.; “New Steel Alloys for the Design of Heat Recovery Steam Generator Components of Combined Cycle Gas Plants”; American Society of Mechanical Engineers ASME TURBO EXPO; June 8-12, 2009; Orlando, Florida USA (To be published);

[16] Pinto Fernandes J., Dias Lopes E.; “Evaluation, Simulation and Component Life Assessment of High Pressure Superheater applied to Heat Recovery Steam Generators”; ECCC Conference Creep & Fracture in High Temperature Components – Design & Life Assessment 2009; EMPA Materials Science & Technology; Switzerland, Zurich (To be published);

[17] Vaillant J.C., Vandenberghe B., Hahn B., Heuser H., Jochum C.; “T/P23, 24, 911 and 92: New grades for advanced coal-fired power plants – Properties and experience“; Elsevier Ltd; 2007;

[18] Robertson D.; “T23 / P23 and T24 / P24 Plant Experience + Fabrication, Welding, Heat Treatment, Oxidation & Damage Mechanisms”; European Technology Development Ltd.; 28-29 April 2008; London, UK;

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