Engineering Failure Analysis 16 (2009) 2202–2208

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Engineering Failure Analysis

journal homepage: www.elsevier .com/locate /engfai lanal

Failure analysis of the draft tube connecting bolts of a Francis-typehydroelectric power plant

Fernando Casanova *

Universidad del Valle, School of Mechanical Engineering, Cali, Colombia

a r t i c l e i n f o

Article history:Received 5 February 2009Accepted 1 March 2009Available online 9 March 2009

Keywords:Dynamic stress analysisFatiguePower plantVibration

1350-6307/$ - see front matter � 2009 Elsevier Ltddoi:10.1016/j.engfailanal.2009.03.003

* Tel.: +57 2 3212262; fax: +57 2 3334899.E-mail address: bando1271@yahoo.es.

a b s t r a c t

In this paper, the failure of the bolts that fasten the draft tube of a 95-MW Francis turbine ispresented. The fracture of the bolts is especially frequent when the machine operates atpartial load. Fracture surface analysis and stress measurements under several power levelswere done. Stress was also measured, while pressure relief in the spiral case was per-formed. Finally, stress measurements were conducted while pressured air was injectedat the stay vanes and the machine was operating at partial load. With the strain measured,stress on the bolts was calculated.

The fracture surface analysis showed that fatigue is the failure mechanism. Stress mea-surements revealed that strong vibration and broad stress variation is present when themachine operates at powers below 80 MW. Air injection effectively decreases vibrationand stress fluctuation, but pressure relief in the spiral case did not show any beneficialeffect.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

The variable demand on the energy market requires great flexibility in operating hydraulic turbines; therefore, turbinesare frequently operated over an extended range of regimes far from the full load and the best efficiency point. Francis tur-bines operating at partial load present pressure fluctuations that are due to the Von Carman Vortex in the vanes and VortexRope in the draft tube [1,2]. These phenomena generate strong vibrations and noise that may produce failures on themechanical elements of the machine.

Several methods have been proposed to decrease the vortex and the vibrations in these kinds of turbines. The most effec-tive of them is air injection over the stay vanes [3], or air admission into the draft tube [1]. Blommaert et al. [4] reportedimprovement in the stability of a 90-kW turbine model using an active control with a rotating valve exciter. Susan-Resiga[5] presented a method to decrease the vortex rope by using a jet issued from the crow tip. The method was evaluated witha numerical model of a turbine where the jet proved effective. Later, Susan-Resiga et al. [6] presented the development of atest rig where an artificial vortex rope was generated. Experimental tests on the rig showed that a jet injected axially at theconical diffuser inlet effectively suppresses the vortex.

This paper shows the failure analysis of the bolts fastening the draft tube of a 95-MW Francis-type hydroelectric powerplant. The plant has three units located in line and the draft tube of each unit is made up of two parts: the part near theturbine is formed by two uncovered cones made of low carbon steel wall 18 mm thick and the other part, which is far fromthe turbine, is embedded in concrete. The two uncovered cones are joined to each other by bolts and in the same manner arejoined to an upper and a lower flange embedded in concrete. Between the lower cone and the lower flange, a seal ring and a

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F. Casanova / Engineering Failure Analysis 16 (2009) 2202–2208 2203

ring with wedge form in the cross section are assembled to compress the cone against the flanges and to guarantee fixationand sealing. Fig. 1, shows the actual draft tube including sketches of the draft tube assembly with the upper and lowerflanges. The failure consists of very frequent fracture of the upper bolts, which increases the time lost in repairs and reducesplant reliability. Failures become more frequent when the plant has to operate at partial load. The analysis was conducted onunit two (where failures are most frequent) located between units one and three.

Visual examination of the system and strain measurements on an upper and a lower bolt during plant operation underseveral conditions were done. The aim is to understand the bolt failure mechanism and find the operating conditions whichcause the failure. We also evaluated the effect on the bolt stress of air injection over the vanes and the pressure relief in thespiral case. The actual bolts are usually preloaded between 70% and 90% of the yield strength. The experimental bolts werepreloaded at 80% of the yield strength, which is within that range.

2. Methodology

Visual examination of fracture surfaces to identify the failure mechanism was performed. Also, strain measurements onan upper and a lower bolt were carried out. The bolts were located at the position where the failures are most frequent. Thestrain was measured by using an FLA-3-11 TML strain gage on the upper bolt and an FLA-10-11 TML strain gage on the lowerbolt. Each strain gage was connected in quarter Wheatstone Bridge to a DC-104R TML dynamic strain recorder and the datawas registered at 1000 data per second. With the strain, stress was calculated and fatigue strength analysis was done.

Fig. 2 shows a strain gage on an experimental bolt. It was necessary to drill a radial and an axial hole to guide the wire out.Given that it was also necessary to perform machining where the strain gages were placed to keep the strain gage from beingdamaged when the bolts were assembled. The machined section has a diameter equal to the root diameter of the thread;therefore, the stress on the position measured is approximately the same as the nominal stress at the root of the thread.

The experimental bolts were made of AISI 4140 steel. The material was quenched from 840 �C and tempered at 540 �C.This is the same material of the actual bolts used in the draft tube. The chemical composition, the yield strength (Sy), andthe tensile strength are shown in Table 1.

Stress measurements where done in two phases. In phase one, measurements where conducted with units one and threeshut down and in the phase two, measurements where conducted with units one and three operating at 95 MW. In each ofthose phases, three sets of tests were carried out. In the first set, stress was measured with the machine working at 10, 20, 30,40, 50, 55, 60, 65, 70, 80, 90, and 100 MW. The second set was measured with the machine working at 60 MW and openingthe pressure relief valve (PRV) at 5%, 10%, and 15%. The PRV is currently used to prevent overpressure on the spiral case whenthe flow from the spiral case to the runner is suddenly stopped. In the third set, measurements were done while the unit wasworking at 60 MW and air was injected on a single point over the stay vanes. The air was taken from a storage tank where theair was compressed at 5.45 MPa.

Fig. 1. Draft tube with the upper and lower flange assemble.

Fig. 2. Strain gage on the bolt.

Table 1Chemical composition and mechanical properties of actual and experimental bolts.

%C %Mn %Si %Cr %Mo Sy (Mpa) Sut (Mpa)

0.38–0.43 0.75–1.00 0.20–0.35 0.80–1.10 0.15–0.25 798 964

2204 F. Casanova / Engineering Failure Analysis 16 (2009) 2202–2208

During the test series, the headwater level was at 1147.18 m and had a variation of 0.02 m. The tail water level began at1028.36 m and finished at 1032.4 m. The density of the water in the test place is 994 kg/m3. The hydraulic head at the tur-bine was 115.27 ± 0.1 m, calculated by taking into count the head loss.

Before the stress measurements, the strain gages were installed on the bolts and the zero-stress condition was set. Then,unit two was turned off and the water was removed from the draft tube. After that, the bolts were installed and the preloadwas applied with a pneumatic torque wrench.

3. Results and analysis

3.1. Visual examination

Visual examination clearly revealed that the fracture was by the fatigue mechanism as can be noted in Fig. 3. Althoughmost fractures have occurred at the threaded area of the bolt (Fig. 3a), some of them have occurred near the head of the bolt(Fig. 3b). The crack propagated normally to the main axis of the bolt and started at the root of the thread or at the changing ofsection between the head and the round section, due to the stress concentration. In general, the fracture surfaces presentfeatures of high cycle fatigue because of their large area of crack propagation and small area of brittle fracture. No prior plas-tic deformation was observed.

Fatigue failures occur due to fluctuating stress, which in this case is produced by the vibration of the draft tube which inturn is due to its interior vortices.

3.2. Stress measurements

Fig. 4a and b show the results of average stress and its standard deviation as a function of the power for the upper bolt.The stress values on Fig. 4a were obtained while units one and three were stopped and those on Fig. 4b were obtained whileunits one and three were operating at 95 MW. Under both tested conditions, the average stress has a maximum at 40 MW. It

Fig. 3. (a) Fracture surface at the shoulder and (b) fracture surface at the threaded area.

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100105110115120125130

0 20 40 60 80 100

0 20 40 60 80 1000 20 40 60 80 100

0 20 40 60 80 100Power (MW)

Stre

ss (M

Pa)

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c d

Fig. 4. Stress on the bolts as a function of the power: (a) and (b) upper bolt, (c) and (d) lower bolt; (a) and (c) units one and three stopped, (b) and (d) unitsone and three working at 95 MW.

F. Casanova / Engineering Failure Analysis 16 (2009) 2202–2208 2205

can be observed that the variation of the stress is high for power below 80 MW and the highest variation is around 40 MW,that is to say the stress varies with the highest amplitude with respect to the average value. This stress oscillation is respon-sible for fatigue failures and; therefore, the operation of the machine at power under 80 MW, increases the risk of failure onthe bolts.

The variation of the stress decreases only for power above 80 MW. In general, the average stress was higher when ma-chines one and three were working at 95 MW. This is probably due to the interaction with the vibration from the other ma-chines or to the higher tail water level when the three machines are working.

The average stress and its standard deviation on the lower bolt are shown in Fig. 4c and d. The stress values on Fig. 4cwere obtained with units one and three stopped and the stress values on Fig. 4d were obtained with those units workingat 95 MW. It is observed that the average stress drastically decreases from Fig. 4c to Fig. 4d.

The tests with the units one and three stopped (Fig. 4c) were conducted before the tests with opening of PRV and air injec-tion, and finally the test with units one and three working at 95 MW (Fig. 4d); therefore, it can be concluded that the reduc-tion of the average stress is due to loosening of the bolt. In fact, because the tests in Fig. 4c were conducted by varying thepower, beginning at 10 and ending at 100 MW, the continuous decrease of the average stress between 10 MW and 60 MW isalso probably due to the loosening of the bolt. For the lower bolt, the average stress first registered is at 10 MW with ma-chines one and two stopped (shown in Fig. 4c). This average stress is much lower than the preload stress (approximately640 MW). This difference is also due to the loosening of the bolt during the start up, the preliminary tests, and the powerstabilization of the machine.

The decrease of average stress with increasing power in Fig. 4d is not as pronounced as that found in Fig. 4c. Rather, ittends to stabilize around 115 MPa.

Similar to the upper bolt, the stress variation on the lower bolt is low only at power greater than 80 MW and has the max-imum around 50 MW for both tested conditions.

The opening of the PRV did not yield any effect on the stress of the upper bolt as can be noted in Fig. 5a and b. It can beobserved again that the average stress and its variation are more critical when units one and three were working than whenthey were stopped.

Fig. 5c shows the average stress on the lower bolt and its standard variation as a function of the percentage opening of thePRV. Apparently, an important reduction on the average stress is produced by opening the PRV; however, said reduction isdue to the loosening of the bolt because the datum at 0% opening was taken from the first test when the stress was evaluatedas a function of the power. So the tests run with the PRV opened were conducted much later, when the bolt had been loos-ened. Fig. 5d shows again that the loosening of the bolt had stabilized around 115 MPa.

It is expected that the loosening of the lower bolts increases the instability of the draft tube and worsens the workingconditions of the upper bolts. Therefore, the lower assembly should be modified to avoid slipping of the wedge ring and loos-ening of the bolts. This can be achieved by decreasing the angle of the wedge.

The air injection showed a significant effect on the stresses on the bolts because it reduced its fluctuation, as shown inFig. 6. Although the average value of the stress was maintained, there was a reduction in the oscillation which involves a

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Fig. 5. Stress as a function of the % opening of the PRV: (a) and (b) upper bolt, (c) and (d) lower bolt; (a) and (c) with the units one and three stopped, (b) and(d) with the units one and three operating at 95 MW.

2206 F. Casanova / Engineering Failure Analysis 16 (2009) 2202–2208

decrease in fatigue damage on the bolts. The air injection when units one and three were working (Fig. 6b) revealed similarresults except in the average value for the lower bolt, which fell with respect to that obtained when those units werestopped, due to the loosening of the bolt.

Given the lack of an air flow meter in the air injection system, the air flow was calculated by using the Van Der Waals realgas equation, knowing the tank volume (2.95 m3), and the temperature (26 �C). The air flow calculated was 0.207 m3/s. The

Fig. 6. Stress with the air injection (a) units one and three stopped, and (b) units one and three working at 95 MW.

Fig. 7. Stress vs. time at power 50 MW.

F. Casanova / Engineering Failure Analysis 16 (2009) 2202–2208 2207

water discharge in the turbine is 60.8 m3/s; therefore, there is a relation between the air discharge and the water discharge of0.34%. This air discharge effectively reduced the stress fluctuation and, thus, the pressure pulsation, although it was lowerthan the relation of 0.5% suggested by Qian et al. [1].

3.3. Fatigue strength analysis

Initially the fatigue analysis was conducted with the stress on the bolt during the operation at 50 MW and units one andthree stopped. Fig. 7 shows the stress vs. time under those conditions. It is noted that there is cyclic stress with amplitude of31 MPa and a frequency of approximately 0.86 Hz. Within those cycles, there are other cycles with amplitude of 7 MPa andfrequency of 31.77 Hz. The low frequency of 0.86 Hz falls into the range of 0.25–0.35 times the runner rotating speed (3 Hz);hence, according to Escaler et al. [7], it is due to the vortex rope inside the draft tube.

For the condition mentioned above, the mean stress and the alternative stress are:

rm ¼ 686 MPa ð1Þra ¼ 31 MPa ð2Þ

The endurance limit (S0e) of the bolt material can be estimated by using the simplified equation [8]

S0e ¼ 0:45 � Sut ð3Þ

To predict the fatigue strength, Se, for the bolt, the endurance limit has to be multiplied by several modifying factors by de-sign, manufacturing, environmental, and stress concentration [9]. Se is given as:

Se ¼ kakbkckdkeS0e ð4Þ

For a machined thread, the surface finish factor, ka, is included in the stress concentration factor, kf, which has a value of 3.8.The size factor, kb, and the load factor, kc, for axial load has a value of 1 and, in this case, the temperature factor, kd, is also 1[10]. Factor ke is given by:

Ke ¼1kf

ð5Þ

The calculated value of Se is 114 MPa, and for Sut is 964 MPa. Using the modified Goodman approach given as:

ra

Seþ rm

Sut¼ 1

nð6Þ

where n is the safety factor. The calculated value of n = 0.98 means that fatigue crack can initiate at the root of the thread.The safety factor was calculated again for the data shown in Fig. 6b after the air injection where the mean stress was

685.5 MPa and the alternating stress was 7.5 MPa. Under these conditions, the safety factor is n = 1.28, meaning that withair injection, the bolt works at infinite life condition.

According to the above, it can be said that air injection is a way to improve the reliability of the draft tube and the unit ingeneral; however, since the air injection could decrease the turbine efficiency, a better solution is to change the joint designin order to decrease the alternating stress on the bold.

4. Conclusions

Frequent failures of the upper bolts of the draft tube were investigated by visual examination and stress measurementsunder several working conditions. Fracture surface revealed that the failure mechanism was high-cycle fatigue and thatcracks begin at the root of the thread in most cases.

2208 F. Casanova / Engineering Failure Analysis 16 (2009) 2202–2208

Stress measurements showed that at power below 80 MW, high stress variation is present, producing fatigue failure. Thestress condition was worst when units one and three were operating at 95 MW than when those units were stopped.

Air injection effectively decreases the vibration and, therefore, the alternating stress on the bolt. However, the pressurerelief in the spiral case through the opening of the PRV did not show any beneficial effect. Rather, this opening of the PRVstrongly decreases the efficiency of the machine because the water is ejected to the tail water.

The lower assembly does not guarantee the fixation of the draft tube with the lower flange. This produces loosening of thelower bolts. Therefore, the lower assembly should be improved to avoid slipping of the wedge ring and loosening of the bolts.

References

[1] Qian Z, Yang J, Huai W. Numerical simulation and analysis of pressure pulsation in Francis hydraulic turbine with air admission. J Hydrodyn Ser B2007;19(4):467–72.

[2] Nicolet C, Arpe J, Avellan F. Identification and modelling of pressure fluctuations of a Francis turbine scale model at part load operation. In: The 22ndIAHR symposium on hydraulic machinery and systems, Stockholm, Sweden, July 2; 2004.

[3] Gunter H, Zanutto JC, Ponge-Ferreira W. Hydraulic power plant machine dynamic diagnosis. Shock Vib 2006;13:409–27.[4] Blommaert G, Prenat JE, Avellan F, Boyer A. Active control of Francis turbine operation stability. In: Proceedings of the third ASME/JSME joint fluids

engineering conference, San Francisco, USA; 1999.[5] Susan-Resiga R. Jet control of the draft tube vortex rope in Francis turbines at partial discharge. In: The 23rd IAHR symposium, Yokohama; 2006.[6] Susan-Resiga R, Bosioc A, Milos T, Bernad S, Muntean S, Stuparu A, et al. Swirling flow apparatus and test rig for flow control in hydraulic turbines

discharge cone. In: The second IAHR international meeting of the workgroup on cavitation and dynamic problems in hydraulic machinery and systems,Timisora, Romania, October 24–26; 2007.

[7] Escaler X, Egusquiza E, Farhat M, Avellan F, Coussirat M. Detection of cavitation in hydraulic turbines. Mech Syst Signal Process 2006;20:983–1007.[8] Hamrock B, Jacobson B, Schmid S. Elementos de máquinas. México: McGraw-Hill; 2000. p. 268–83.[9] Topac MM, Gunal H, Kuralay NS. Fatigue failure prediction of a rear axle housing prototype by using finite element analysis. Eng Fail Anal

2009;16(5):1474–82.[10] Hamrock B, Jacobson B, Schmid S. Elementos de máquinas. México: McGraw-Hill; 2000. p. 692–3.