Engineering Failure Analysis 17 (2010) 1466–1474

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Engineering Failure Analysis

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Stability analysis of a gas turbine exhaust stack

M.Z. Hamzah a, J. Purbolaksono b,*, J.I. Inayat-Hussain b, N.F. Nordin a

a TNB Research Sdn Bhd, No. 1 Lorong Air Hitam, Kajang 43000, Malaysiab Department of Mechanical Engineering, Universiti Tenaga Nasional, Km 7 Jalan Kajang-Puchong, Kajang 43009, Selangor, Malaysia

a r t i c l e i n f o a b s t r a c t

Article history:Received 11 February 2010Received in revised form 25 May 2010Accepted 26 May 2010Available online 31 May 2010

Keywords:DeformationStress analysisGas turbineSite inspectionFinite element

1350-6307/$ - see front matter � 2010 Elsevier Ltddoi:10.1016/j.engfailanal.2010.05.008

* Corresponding author. Tel.: +60 3 89212213; faE-mail address: judha@uniten.edu.my (J. Purbola

Stability analysis, based on finite element method, was carried out on a gas turbine exhauststack following obvious appearances of significant gaps between its silencer casings andembracing rings. The finite element model was developed by referring to the correspond-ing technical drawing of the exhaust stack and the information obtained during the siteinspection. In the case of axial constraint at both ends of the exhaust stack, the originaldesign was found incapable of withstanding the thermal loading experienced during oper-ation of the gas turbine, leading to instability of the structure. A design modification of theexhaust stack was proposed to rectify this problem. The outcome of the finite elementanalysis indicated that the modified design of the exhaust stack had improved stabilityas compared to the original design when subjected to typical thermal loading of the gasturbine operation.

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1. Introduction

In August 2007, the exhaust stack of an open-cycle gas turbine, Siemens 135 MW V92.4 Ratio model, at Putrajaya PowerStation experienced excessive deformation due to buckling causing a significant gap between the silencer casing and theembracing ring at an elevation of 15 m. The orientation of the gap was along the circumferential direction with a maximumopening of around 125 mm. This condition may pose a threat to the overall structural integrity of the exhaust stack design.The exhaust stack is subjected to the residual heat which is commonly exhausted to atmosphere at about 550 �C. ThePutrajaya Power Station is a peaking plant serving as a load center for areas that comprise of Kuala Lumpur and its suburbs,and adjoining cities and towns in the state of Selangor. It has a two-shift cycle operating regime, with a daily operation of12–16 h, mainly to meet the load demand during peak hours and to stabilize the grid line voltage. There are five gas turbineunits in Putrajaya Power Station, i.e., two units of 110 MW General Electric Frame 9E model and three units of Siemens135 MW V92.4 Ratio model. The plenum barrier plate of the 110 MW General Electric Frame 9E model gas turbine had re-cently experienced failure due to crack [1]. The crack in the plenum barrier plate resulted from thermal fatigue, and a mod-ified design of the barrier plate was proposed to rectify this problem.

The gas turbine chimney system consists of four major components, i.e. the diffuser, elbow, stack and steel structure(Fig. 1). The exhaust stack is made up of a silencer casing and an upper casing. The lower transition connects the rectangularelbow to the circular silencer casing, while the upper transition connects the silencer casing to the upper part of the stackchimney. The upper silencer casing is supported by steel structure from which the stack is suspended. The silencer casing hasan internal diameter of 9 m and is made of 15Mo3 steel, which is equivalent to SA 204 steel. Chemical composition of 15Mo3steel grade is presented in Table 1. The silencer casing is embraced by a thick circular ring made of the same material (SA 204steel).

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x: +60 3 89212116.ksono).

Fig. 1. The gas turbine chimney system (left) and the schematic of the studied area (right).

Table 1Chemical composition of 15Mo3 steel grade (wt.%).

C Si Mn P (max) S (max) Mo

0.12–0.20 0.10–0.35 0.40–0.90 0.035 0.030 0.25–0.35

Ring 4

Ring 3

Ring 2

Ring 3

Ring 3

Fig. 2. Significant gaps between the silencer casing and the embracing ring.

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In this work, the stability of the exhaust stack model is examined based on the information obtained from the site inspec-tion and from the results of stress analysis using the finite element software package of ANSYS [2]. It is suspected that theoriginal design of the exhaust stack is incapable of withstanding the operating thermal loading. A modified design of the ex-haust stack is proposed and its stability is compared to that of the original design. The finite element method is a useful toolto support the investigations of failure problems provided that the operating parameters are well specified into the model-ing. The finite element method had been extensively used in the past to support the investigation of failures in Malaysianpower plants as reported in [1,3–6].

2. Site inspection

Findings from the site inspection revealed several gaps (cracking) in both the horizontal and vertical directions, originat-ing from the weldments between the silencer casing and the embracing ring (Fig. 2). The schematic of the radial deformationof Weldment 2, Weldment 3, Weldment 4 and Weldment 5 is illustrated in Fig. 3. It is seen in this figure that the most severedeformation occurred at Weldment 3, with a maximum opening of approximately 125 mm.

The site inspection also revealed another important finding at the lower silencer casing. Even though the exhaust stackwas designed to have a clearance of 75 mm for downward expansion as shown in Fig. 4, the lower silencer casing which con-nects to the lower transition casing was however found to have firmly rested on the guide bar, which is welded to the mainsteel structure. This condition may trigger buckling problem, which causes deformation or expansion of the exhaust stack inthe radial direction if the structure has insufficient rigidity to withstand thermal loading. The upper silencer casing and thelugs, shown in Fig. 1, were found in normal condition. These findings suggested that the evaluation of the structural stabilityof the current exhaust stack design should be undertaken by performing stress analysis utilizing operational thermal loadingdata.

Similar checks on the lower silencer casing of the other units with regard to the sufficiency of clearance for thermalexpansion were also carried out. However, those units were found to have the acceptable clearances for expansion. Sincethe exhaust stacks of the units were found in a good condition, the possibility of causing significant gap due to thermal fa-tigue can be disregarded. This observation indicated that the condition of the silencer casing resting on the guide bar of thefractured unit may have caused a buckling problem which resulted in excessive deformation of the exhaust stack in the ra-dial direction. It is also believed, if the lower silence casings of the other units are already resting on the guide bar, then thesimilar excessive radial deformation due to buckling is expected.

casing

LEGEND

deformedshapeRing 1 Ring 2 Ring 3

Ring 4 Ring 5 Ring 6

Fig. 3. Radial deformation at the weldments.

Fig. 4. Clearance between the lower silencer casing and the guide bar.

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3. Finite element analysis

The model of the exhaust stack for finite element analysis is made to be a close representative of the actual structure. It isessential to have better understanding on the design of the component prior to the establishment of the finite element mod-el. The approximated geometry of the model obtained from the corresponding drawing and information on the constraintconditions from the site inspection are required to generate the finite element models. The stabilities of the original andmodified models under the operational thermal loading are then evaluated by finite element method. For simplification

Fig. 5. Technical drawing of the original design of the exhaust stack.

Fig. 6. Technical drawing of the modified design of the exhaust stack.

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of the analysis, the residual stresses developed in weld joints are omitted and the perfect welding is considered. The com-ponents are modeled as two-dimensional axisymmetric solid.

3.1. Geometry of models

Fig. 5 shows the technical drawing of the studied area of the original exhaust stack design. The original design wasdiscretized with 61,500 elements overall. Meanwhile, the modified design as shown in Fig. 6 was discretized with 55,900

Fig. 7. Displacement constraints on the exhaust stack.

531.806

533.479

535.151

536.824

538.497

540.169

541.842

543.515

545.187

546.860

532.052

533.686

535.320

536.954

538.588

540.222

541.856

543.490

545.124

546.758

Original Design

Modified Design

Original Design Modified Design

Fig. 8. Temperature distribution of the original and modified designs of the exhaust stack.

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elements. It can be evidenced from Fig. 5, there are variations of the thickness of the sheet metal (as indicated in Details A, Band C) used to construct the exhaust stacks. This would influence the stability of the structure under high temperature load-ing when both ends (upper and lower parts) of the exhaust stacks are restrained axially. The modified design as shown inFig. 6 is proposed for comparison. Nine reinforcements (braces) with equal pitch are introduced into the model. However,prior to examining the stability of the modified design, slight modification is also made on the original design by introducingreinforcement (brace) at the junction as shown in Detail B of Fig. 5.

3.2. Material properties

The carbon molybdenum 15Mo3 steel is usually used in the construction of pressure vessels and equipment for service upto 600 �C. The material has an approximate thermal expansion coefficient of 1.3e�05 mm/mm �C, thermal conductivity of40 W/m �C, Young’s modulus of 210,000 MPa, Poisson’s ratio of 0.3 and tensile strength up to 600 MPa. These material prop-erties were used to define the material model of the structure for the thermal and structural analyses.

3.3. Boundary conditions

Since the residual gas flow has high velocity, the phenomenon of the contact condition between the flowing hot air andthe metal of the silencer internal surface was considered as forced convection heat transfer. The convection coefficient forthe hot exhausted air film of 1000 W/m2 �C is used in this study. It was also considered that the rate of the hot air flowwas constant. The contacts between the external surface of the silencer casings and the atmosphere were considered as freeconvection heat transfer, with ambient temperature of 40 �C and convection coefficient of 10 W/m2 �C. The rest of the heattransfers that occurred in this system were considered as conduction processes.

-.067252

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0.

-.067398

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-.052421

-.044932

-.037443

-.029955

-.022466

-.014977

-.007489

0.

Original Design

Modified Design

Original Design Modified Design

Fig. 9. y-displacement distribution of the original and modified designs of the exhaust stack.

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In order to carry out the structural analysis, different displacement constraints were used in the models according to theinformation determined from the site inspection. In the normal operation, the upper silencer casing/stack was suspended tothe steel structure and the lower casing would freely expand downward. In the case of the lower casing resting firmly on theguide bar, the component would have restraints in vertical and radial directions. The displacement constraint conditions onthe exhaust stack are illustrated in Fig. 7. The radial deformation due to possible buckling problem was then examined fordifferent constraint conditions on the original and modified designs of the exhaust stack.

4. Results and discussion

In the present work, thermal analysis was initially carried out to determine the temperature distribution on the silencercasings. Structural analysis under thermal loading with different displacement constraint conditions was then performed.Fig. 8 shows similar temperature distribution of the original and modified designs, indicating that both designs were exam-ined under the same thermal loading for structural analysis. Similar feature was also seen for the distribution of y-displace-ment of both designs when the lower silencer casing was allowed to freely expand downward. This condition representssufficient clearance for thermal expansion. The maximum value of y-displacement, shown in Fig. 9, is less than the specifiedclearance (=75 mm) as depicted in Fig. 4.

It is particularly essential to consider the stability of the exhaust stack in the event that the lower silencer casing is foundto be resting on the guide bar. Although the lower silencer casing of the original design was not constrained in the verticaldirection as shown in the leftmost diagram of Fig. 10, the maximum radial deformation (x-displacement) was however mea-sured to be more than 5 mm. This deformation might indicate that the original model of the exhaust stack likely leads tostructural instability under high temperature loading. Restraining the lower silencer casing in both the vertical and radialdirections resulted in excessive radial deformation due to buckling as shown in the second diagram from the left, Fig. 10.This evaluation was also carried out on the original model with a minor modification by introducing reinforcement (bracing)at the junction as shown in Detail B of Fig. 5. This reinforcement, however, resulted only in a slight reduction of the radialdeformation as indicated in the second diagram from the right, Fig. 10. These results suggest that necessary modification

0

.595E-03

.001189

.001784

.002378

.002973

.003567

.004162

.004756

.005351Original Design (free)

0

.111E-03

.221E-03

.332E-03

.443E-03

.554E-03

.664E-03

.775E-03

.886E-03

.996E-03Modified Design (restrained)

-1.899

-1.688

-1.477

-1.266

-1.055

-.843

-.632

-.421

-.211

.902E-03Original Design (restrained)

Original Design

Free

Original Design

Restrained

Modified Design

Restrained

-1.865

-1.657

-1.450

-1.243

-1.036

-.828

-.621

-.414

-.206

.902E-03Modified Design (restrained)

Modified Design

Restrained

Fig. 10. The x-displacement distribution of the original and modified designs of the exhaust stack.

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needs to be further performed on the original design. The modified design, represented by the model in Fig. 6, was proposedfor consideration. The rightmost diagram in Fig. 10 showed that, despite the lower silencer casing resting on the guide bar,the modified design had much improved structural stability. The improved stability of the proposed design modification isalso evidenced from the uniform contour layers in the radial direction. Fig. 11 shows the three-dimensional full expansion ofthe x-displacement (radial deformation) distribution of the original and modified designs of the exhaust stack. It clearly illus-trates that the modified design remained stable under the operating thermal loading, as opposed to the original design,which is seen to have excessive deformation when subjected to the same thermal loading.

Regular monitoring of the clearance between the lower silencer casing and the guide bar on the other units needs to beundertaken to ensure sufficient margin is available. The reason for the lower silencer casing of the failed unit to move

-1.899

-1.688

-1.477

-1.266

-1.055

-.843

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-.421

-.211

.902E-03

0

.111E-03

.221E-03

.332E-03

.443E-03

.554E-03

.664E-03

.775E-03

.886E-03

.996E-03

Original Design Modified Design

Original Design

Modified Design

Fig. 11. Three-dimensional full expansion of the x-displacement distribution of the original and modified designs of the exhaust stack.

Fig. 12. In situ breakdown of the pearlite in which the platelets of pearlite have spheroidized, but the original pearlite colonies are still clearly defined, andseveral graphite nodules are also found at the grain boundaries.

Fig. 13. Time–temperature plot of the competing mechanism of spheroidization and graphitization in carbon and low-alloy steels [8].

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downward and rest on the guide bar may possibly be due to undetected cracks at the weldment regions, allowing the casingbudging downward. In order to prevent this from recurring, the welding process and erection procedures need to be givenparticular attention. The inspections during the erection of the exhaust stacks and necessary checking after the completion ofthe works should be duly undertaken in order to ensure compliance with the required specifications. It is however proposedin future that proper finite element analyses should be performed to examine the structural stability and integrity of thestack design prior to their construction and commissioning.

Sample of the silencer casing metal was also taken for the purpose of microscopic examination. The microstructure shownin Fig. 12 indicates in situ breakdown of the pearlite in which the platelets of pearlite have spheroidized, but the originalpearlite colonies were still clearly defined. Several graphite nodules were also found at the grain boundaries. The tempera-ture variation due to the nature of the gas turbine operation encourages development of graphitized microstructures. As re-ported by French [7], temperature cycling is one of the ways of transforming iron carbide to ferrite and graphite. Under theoperating temperature of 550 �C due to the residual heat of the gas turbine, the developments of spheroidization and graph-itization indicate competing processes. It agrees with the time–temperature plot (Fig. 13) of pearlite decomposition by thecompeting mechanism of spheroidization and graphitization in carbon and low-alloy steels as presented in [8].

5. Conclusions

Stress analysis using finite element method was successfully carried out to evaluate the stability of a gas turbine exhauststack, which was found to have experienced substantial deformation during operation. The corresponding technical drawingof this exhaust stack and the information obtained during the site inspection were utilized to develop the finite elementmodel. The results of the analysis revealed that, in the case of axial constraint at both ends of the exhaust stack, the originaldesign was incapable of withstanding the operating thermal loading, causing instability of the structure. A modification ofthe exhaust stack design showed improved stability against the typical operational thermal loading.

Acknowledgement

The authors wish to extend their gratitude to Putrajaya Power Station Malaysia for granting permission of utilizing theavailable facilities during the completion of this work.

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