Failure Analysis of a Dome Roof

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Failure analysis of storage tank

F. Trebuna *, F. Simcak, J. Bocko

Department of Applied Mechanics and Mechatronics, Technical University of Kosice, 042 00 Kosice, Slovakia

Received 13 December 2007; accepted 27 December 2007Available online 20 January 2008

Abstract

This paper has studied failure of a hot water storage tank. During operation, one of the two tanks has been damaged bycollapse of its roof. Consequently, analytical, numerical and experimental analysis of possible failure reasons was per-formed. Extreme stresses in the structure during its operation were determined taking into account shape and geometryimperfections as well as corrosion influence. These values were verified numerically by the finite element method. Extensiveexperiments performed by strain gauge measurements on the second tank of the same design helped us determine time-dependent stresses in extremely loaded locations during the chosen regimes of operation. The results of analysis allowedus to assess the failure reason and to express the recommendations for further analysis of the non-damaged tank. 2008 Elsevier Ltd. All rights reserved.

Keywords: Tank failures; Finite element analysis; Stress analysis

1. Introduction

Two vertical non-pressure tanks of the same structure (approximately 30 years old) originally designed forheavy oil storage were used as storage tanks for hot water (temperature 6595 C). The tanks were recon-structed, equipped with the necessary technological devices and brought into operation ( Fig. 1). After twoweeks of operation the roof structure of the storage tank collapsed (Fig. 2). The aim of solution is to analyzethe loading of the storage tank during its operation, to judge the reasons for its failure and to propose thecorrective actions for safely operation of the second tank, which was designed for the same purposes and oper-

ation as the first one.

2. Basic facts about the tank and its operation

As mentioned above, the aim of the paper is to investigate the vertical cylindrical tank with the sphericalroof and basic dimensions as given in Fig. 3:

1350-6307/$ - see front matter 2008 Elsevier Ltd. All rights reserved.

doi:10.1016/j.engfailanal.2007.12.005

* Corresponding author. Tel.: +421 55 6022462; fax: +421 55 6334738.E-mail address: [email protected] (F. Trebuna).

Available online at www.sciencedirect.com

Engineering Failure Analysis 16 (2009) 2638

www.elsevier.com/locate/engfailanal
mailto:[email protected]:[email protected]


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Inside diameter D = 14 289 mmHeight of cylindrical part Hs = 9 208 mm

Roof height h = 1100 mmRoof radius (spherical) R = 24 500 mmThickness of cylindrical shell d1 = 6 to 8 mmThickness of roof shell d = 4 mmVolume 1475 m3 for water surface height H = 9108 mmMaterial S235 ReHmin = 235 MPa, Rmmin = 340 MPa

The roof of the tank consists of a steel shell with thickness of d = 4 mm, which is welded to the steel frame-work of the roof by intermittent chain, welds (Fig. 4). The framework of the roof is constructed from outerand internal rings, polygonal rings and radial ribs.

The bottom of the tank (Fig. 5) consists of a ring plate of the thickness d2 = 16 mm to which the

first steel band cylindrical wall (shell) of the thickness d1 = 8 mm and the bottom plate of tank arewelded.

The operational regimes of the storage tank are the following:

Filling up the tank with hot water. Operation of the storage tank based on regulation of water temperature by filling the tank with cold water

(discharging) or with water which is hotter than that contained in the tank (charging). During these pro-cesses, the height of the water level can change at the velocity determined by the technological equipmentof the tank.

Discharging of water from the tank (maintenance, unavailability time, etc.). Corresponding control equip-ment ensures all the above-mentioned functions.

Fig. 1. View of the tanks.

F. Trebuna et al. / Engineering Failure Analysis 16 (2009) 2638 27


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3. Visual inspection of the collapsed tank

On the basis of the visual inspection it was found that:

the steel framework of the roof was extensively deformed (Fig. 6); the welds between radial ribs [11] andouter rings were damaged (Fig. 7),

supporting elements of the roof steel structure were significantly corroded ( Fig. 8); maximum corrosiondepths of the profiles are given in Table 1.

Roof shell thickness was mostly in the range 3.43.7 mm, with the local minimum 2.83.1 mm.

Fig. 2. Collapsed roof of the tank.

Fig. 3. Basic dimensions of the tank.

28 F. Trebuna et al. / Engineering Failure Analysis 16 (2009) 2638


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Fig. 5. Bottom of the tank.

Fig. 6. Deformation of the roof frame.

Fig. 4. Steel framework of the roof. (1) radial ribs U 100, (2) polygonal rings L 70 70 8, (3) polygonal rings L 65 65 6, (4) internal

ring U 140, (5) outer ring U 180.



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4. Loading of the storage tank

The storage tank is loaded during operation by:

self-weight of the structure,

Fig. 8. Corroded elements of the supporting structure.

Table 1Decrease of the thicknesses of carrying elements in the roof

Profile Thickness decreaseL65 x 65 x 6 6.5%L70 x 79 x 8 1.5%U 100 6.5%U 140 11.5%U 180 7.5%

Fig. 7. Fractured weld between the radial rib and the outer ring.



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hydrostatic pressure of the contained liquid and by pressure (or under pressure) above the liquid surface inaccordance with working conditions defined by operating manuals,

climatic loading (snow, wind).

At this point, it is necessary to note that during operation of the storage tank the pressure of 1.5 kPa pro-

duced by steam above the liquid surface was ensured by the control system of the tank.

5. Analytical computation and numerical verification of the tank

The storage tank can be considered a non-pressure tank loaded mostly by hydrostatic pressure of the con-tained liquid, over- or under pressure of 0.51.5 kPa, respectively. During analytical solution check calcula-tions were carried out in accordance with the standard procedures [16]. Comparison of the computationalresults with the real structure shows the following anomalies in the tank:

incorrectly chosen dimensions of the ring plate at the bottom of the tank (Fig. 5) that increase negativeinfluences at the junction of the vertical wall and the bottom of the tank (according to experience, the rec-ommended dimensions are d2 = 6mm, b = min. 500 mm, [3,7,12]),

considerable imperfections in the geometry of the first steel band of the cylindrical wall (Fig. 9) mismatchin the thickness direction, considerable deviation from the vertical direction (up to 100 mm), non-constantradius of curvature,

according to empirical dependencies the stiffeners of the roof should consist of five polygonal rings and thecross-sections should have bigger dimensions.

Numerical procedures were applied to assess the strength of the cylindrical shell and the tank bottom takinginto account the conditions of continuity between the cylindrical shell and the tank bottom. The computationmethodology does not consider the elasticity of the foundation in the first approximation, the fact that has tobe considered for a real structure.

The bending moment for such hard boundary conditions would induce high stress levels with a consider-

able degree of plastic deformation. The high level of shear force probably led the designer to the proposal ofthe bottom ring plate 16 mm thick in order to ensure the stress level at the bottom of the tank at 170 MPa.With such values of internal force quantities, the radial displacement of the cylindrical shell would reach

Fig. 9. Imperfections in the geometry of the first steel band of the cylindrical wall.



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the value exceeding 5 mm. Stress in the circumferential direction would reach the value of approximately160 MPa.

Taking into consideration the elastic foundation and relations between the shell and the bottom of the tankthat corresponds to sufficient protrusion of the ring plate of approximately 50 mm to the outside, the bendingmoment decreases to 1450 Nmm mm1 and the stress is 144 MPa. Stress at the bottom part does not exceed

the value of MPa units.Radial deformation of the shell in the junction with the bottom of the tank is as a rule a few tenths of a mm.Stress in the shell in its junction to the bottom of the tank is approximately 145 MPa with the equivalent stressnot exceeding 135 MPa.

The stresses in circumferential direction are approximately 81 MPa so that the structure can operate safely.Analysis of welds and imperfections in the welded joints clearly shows critical locations and unsuitability of

some types of welds in the manufacture of storage tanks. Analysis of the effects of imperfections pointed out ahighly considerable increase in the stress level.

In transition parts between the elements of the roof structure and the shell there were internal force quan-tities determined from the conditions of deformation continuity. Shell deformations (of the roof and cylindri-cal wall) as well as deformations of the roof rings (internal and outer) were taken into account duringcomputations. Based on the assessed internal force quantities it was possible to determine stresses both in

the shells and rings. In this computation, apart from the self-weight of the structure and the weight of snowthe pressure (or under pressure) above the liquid surface was taken into consideration. The computations con-firmed that with under pressure 1.5 kPa the strength conditions for the nominal values of the geometry of thecarrying elements were fulfilled. In combination with snow loading pressure, stresses already reached criticalvalues. The stability limits were reached also with the nominal geometry and the above mentioned under pres-sure in combination with the snow. The decreased thickness of the carrying elements along with the abovementioned combination of the loading could cause plastic deformation or damage of the structure.

As there were found certain serious breaches of the conception of the roof design, special attention wasgiven to determination of the critical loading of the roof not as an isotropic shell, but as a shell stiffenedby radial ribs and polygonal rings [4,8,13].

Strength computation clearly documented that the critical location was between the second and third

polygonal ring (Fig. 6) where the value of the critical under pressure was the lowest. Nominal dimensionsof the roof structure with under pressure 1.5 kPa as well as the snow loading resulted in the loss of stabilitydue to the failure of several radial ribs.

As the character of the failure indicated that it was caused by under pressure above the surface of the liquid,the effect of the decrease of the liquid level on the pressure above the surface was investigated. It was foundthat the decrease of the liquid level even by 10 mm (with maximum height of the level) induced under pressure1.5 kPa. This statement applies on the assumption that the volume above the liquid surface is without access ofair. Graphical illustration of the change of the pressure Dp above the surface with the decrease of the level DHfor individual volumes of air above the surface and for the initial pressure above the surface p0 = 101.5 kPa isshown in Fig. 10.

The levels of stresses and deformations in the shell were verified by the finite element method. Fig. 11 gives,as an example, a field of von Mises stresses in the junction of the roof to the cylindrical shell with under pres-sure 1.5 kPa.

The results of the numerical analysis confirmed the conclusions of the analytical computation.

6. Experimental stress analysis of the cylindrical shell and the roof of the tank

Experimental stress analysis of the cylindrical shell and of the roof of the tank was performed on the secondnon-damaged tank that had an identical geometry and dimensions as the original one. The methods of straingauges were used for the measurement.

Positions of extreme loadings were chosen as locations for verification the junction of the stiffener ribs tothe outer ring of the roof from the lower side, location in the lower part of the roof shell between the stiffenerribs, outer and first polygonal rings and locations on the cylindrical shell. In the locations of the measurement

on the cylindrical shell the following imperfections could be found slight overhang of the bottom ring plate,



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conical ness of the first new steel band of the cylindrical wall, shifting between steel bands of the cylindricalwall in the thickness direction, inappropriate positioning of the bottom ring plate, etc.

Application of the strain gauges to the empty structure allowed to measure deformation increments duringfilling of the tank according to the instructions of the operator with respect to possible operational states ormaintenance of the tanks and subsequent operation of the tank after connecting tensometric strain gauges tothe tensometric apparatus and subsequent balancing of bridges.

The proposed procedure is based on preconditions that the regulation of operational parameters is accurateenough and the data given by operator correspond to real values.

The location of the applied strain gauges and their labelling is given in Fig. 12.In Fig. 13 are shown applied insulated strain gauges in along one meridian direction in lower part of the

tank.Fig. 14 gives the measurement and evaluation chain for determination of stresses from measured values of

time-dependent changes of the strains.

Fig. 11. Distribution of von Mises stresses with under pressure 1.5 kPa.

Fig. 10. Pressure changes caused by decrease of the level for individual air volumes above the surface and initial pressure above the surfacep0 = 101.5 kPa.



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After connection of the strain gauges to the measurement apparatus and setting up zero on bridges, themeasurement was carried out during gradual filling of the storage tank with hot water and due to the limited

capacity of the chemical water treatment plant as well as water intake this process took several tens of hours.

Fig. 12. Location and labelling of strain gauges.



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In the first part there was a storage tank filled up to the height 320 mm. After a longer pause the tank was filledup to the required level which was H= 9050 mm.

In the further stage, measurements were carried out during the existence of the steam pressure above thewater surface and during discharging of water the steam pressure above the water surface was changed.

Next, the measurement of the time-dependent changes of the measured strain values during accumulationof heat in the tank was carried out. Cooling of water in the tank with cold water is an inverse process and so itwas not necessary to keep a record of it.

From the strains, the time-dependent values of the stress changes were determined in the supervised loca-tions of the tank using the linear theory of elasticity.

Fig. 15 shows time-dependent values of stress increments taking into account the influence of the steam

charging above the water level from 0 to 1.5 kPa. Fig. 16 shows time-dependent values of the stress

Fig. 13. Strain gauges applied along the meridian direction of the bottom part of the tank.

Fig. 14. Measurement and evaluation chain.



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increments after balancing the measurement system and steam pressure 1.3 kPa. The steam charging wasinterrupted for 30 s, which with discharging of hot water at 16 t/h caused a significant decrease of stressin the roof shell.

Discharging of hot water in the amount 65 t/h caused significant decrease of steam pressure resulting inunder pressure and only the regulation system of the tank eliminated the danger of the lost stability of theroof. During discharging of water in the amount 26 t/h, steam pressure increased to 1.3 kPa. The graph alsoshows the stopping of steam charging during discharging of hot water in the amount 100 t/h that againresulted in the decrease of pressure and opening of the vacuum valve.

On the basis of experimental analysis of stresses in the cylindrical shell and the roof of the tank during sim-

ulation of operation regimes it could be stated that:

Fig. 15. Time-dependent values of stress increments for delivery of steam.

Fig. 16. Time-dependent values of stress increments under operating regimes with both, changes of steam extraction and changes of steam

delivery.



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Charging of the tank with water of temperature 65 C and the water level below 320 mm resulted in thestress 45 MPa due to the temperature and bending moment. After temperature alignment the stress did notexceed 14 MPa. Accordingly, the stress dynamics was significantly influenced by the temperature.

During charging of the tank and increasing of the temperature the stresses in some parts of the structureincreased monotonically, but in other locations they varied in both directions and so the change was not

monotonic. The last mentioned locations are in places, where there were changes in local bending momentsdue to the loss of stability or sudden change of the meridian line gradient (shape imperfection).The stress in the roof shell due to the hydrostatic pressure of water column at a height of 9050 mm

did not exceed the value 80 MPa. The stress in the lower part of the cylindrical shell caused by bendingmoment was 170 MPa and circumferential stress in the junction of the new and old wall reached the value150 MPa.

Very interesting is the fact that hydrostatic pressure caused a change in the orientation of the stress incre-ment in the location of breaking of the meridian line (the junction of the first and second steel band of thecylindrical wall).

The stresses measured during filling of the volume above the water surface by steam are shown in Fig. 15.Specified time-dependent values of stress increments clearly documented the influence of pressure on potentialnon-monotony in the course of stress increments.

Very interesting results of the measured stresses are shown in Fig. 16. These results show that the influ-ence on stresses was absolutely identical for the interruption of steam charging and for the increased dis-charging of hot water from the tank. Discharging at a higher level of water caused a higher level ofunder pressure that can be corrected only by the correct functioning of the overstress- or vacuum valve.This result can be considered to be very important and it affirms the conclusion that the failure was causedby under pressure.

As the limit level of pressure and under pressure according to the instructions of the producer of the controland protecting system is adjusted during its manufacture, it could be expected that their values would be keptin the full range to avoid tank failures due to exceeding these limits.

Time-dependent increments of stresses showed that for the charging of the tank (charging of water withtemperature higher than the temperature of discharged water without changing the amount of water in the

tank) the highest value of stress increment did not exceed 60 MPa. A more detailed description of the resultscan be found in [9,10].

7. Conclusions

On the basis of visual inspection, diagnostics, analysis, modelling, stress computations as well as analysis ofload cycles modelled during experimental simulation the following facts can be stated:

The ring plate did not have the required width 500 mm that is recommended for all storage tanks with adiameter exceeding 12.5 m.

Although tanks with a diameter up to 15 m can be designed without a stiffening structure in the roof, the

reviewed tank roof had such a structure. However, in this structure according to conventional rules thereshould have been one more polygonal ring.

New and old steel bands of the cylindrical wall do not lie on one meridian line (imperfection) and their off-set is half of their thicknesses.

Faulty welds between the old and new steel bands of the cylindrical wall were also documented by speci-mens taken for determination of mechanical properties of the material.

The most important welds are those which connect the first steel band of the cylindrical wall to the circularplate. Problematic welds are in the junction of the roof shell to the outer ring of the roof (corrosion andleakages), the fact that extremely influences safe operation of the tank.

Geometry imperfections of the tank caused higher loading of the carrying elements which was also verifiedby experiment. Further operation of the tank is possible only in the emergency mode and requires period-

ical inspections and protection against corrosion, especially from the outer side of the tank.



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Experimental measurements confirmed all the results gained by analytical and numerical analysis. It wasshown that during the increased discharging of water from the tank (approximately over 50 t/h) therewas an extremely high decrease of steam pressure above the water surface that could cause under pressureabove the surface due to not properly functioning control system and thus result in the failure of the tankroof.

Acknowledgments

This work was supported by State Project No. 2003 SP 51/028 00 09/028 09 11 as well as by Project VEGANo. 1/2187/05.

References

[1] Girkmann K. Flachentragwerke. Wien: Springer-Verlag; 1956.[2] Kantorowitsch SB. Die Festigkeit der Apparate und Maschinen f}ur die chemische Industrie. Berlin: VEB Verlag; 1955.[3] Krupka V, Schneider P. Konstrukce aparatu. Brno: PC DIR Nakladatelstv; 1998.[4] Lizin VT, Pjatkin VA. Projektirovanije tenkostennych konstrukcij. Moskva: Masinostrojenie; 1976.[5] Long B, Gardner B. Guide to storage tanks and equipment. London: John Wiley and Sons; 2004.[6] Myers PE. Above ground storage tanks. New York: McGraw-Hill; 1997.[7] Ponomarjev SD. kol.: Rascety na procnost v masinostrojenii. Moskva: GNTIML; 1958.[8] STN 73 1401, Navrhovanie ocelovych konstrukci. Bratislava: UNM; 1998.[9] Trebuna F. A kol.: Akumulacne nadrze, analyza namahania, prciny havarie a urcenie zvyskovej zivotnosti po realizacii opatren.

Ciastkove zavery z analyzy pre realizatora. Januar: TU Kosice; 2005.[10] Trebuna F. a kol.: Akumulacne nadrze, analyza namahania, prciny havarie a urcenie zvyskovej zivotnosti po realizacii opatren.

Zaverecna sprava. Jul: TU Kosice; 2005.[11] Trebuna F, Bursak M. Medzne stavy kovy. Grafotlac presov 2002.[12] Trebuna F, Simcak F. Odolnost prvkov mechanickych sustav. Kosice: Emilena; 2004.[13] Volmir AS. Ustojcivost uprugich system. Moskva: GIFML; 1963.


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