Available online 7 January 2005

meantime, the key technology in the domestic manufacturing of titanium condensers was also studied.

* Corresponding author. Tel./fax: +86 21 6425 3810.

E-mail address: hxu@ecust.edu.cn (H. Xu).

Engineering Failure Analysis 12 (2005) 432439

www.elsevier.com/locate/engfailanal1350-6307/$ - see front matter 2004 Elsevier Ltd. All rights reserved.0. Introduction

There is a total of 12 large-scale titanium condensers in the oxidation sections of the two PTA (p-

phthalic acid) production lines at Yangzi Petrochemical Co., Ltd. in China. After about 10 years of oper-

ation, tube leakage failure occurred in most of the titanium condensers and severely aected the stable

operation of the circulation water system in the plant. Leakage also caused unexpected corrosion and

damage in the condenser shell, expansion section, tube sheet, equipment of the circulation water system,

etc.

The present paper introduces the results of the analysis on the fractured heat exchanger tubes of two HE-302 condensers in 1999 and 2000 and provides suggestions concerned with structural improvement. In theAbstract

The failure of titanium condensers used in Yangzi Petrochemical Co. Ltd. was analyzed by experiment and calcu-

lation. It was concluded that uid-induced vibration in the tube bundle resulted in the fatigue damage of titanium tubes

in condensers. Structure adjustments to reduce vibration were presented to the design of the titanium condensers. Also

the critical manufacturing technology was studied by tests and analysis.

2004 Elsevier Ltd. All rights reserved.

Keywords: Titanium Condenser; Fatigue; Vibration; Tube failuresFailure analysis and critical manufacturingtechnology research on titanium condensers

Qiu-Lin Ma, Hong Xu *, Zhi-Wen Wang, Feng Hou, Lin-Yun Xu

Research Institute of Process Equipment and Pressure Vessels, East China University of Science and Technology,

P.O. Box 402, Shanghai 200237, PR China

Received 5 January 2004; accepted 28 February 2004doi:10.1016/j.engfailanal.2004.10.005

thickness.

etrate through the nozzle area of the bae plate have vibrations.2. Vibration reduction measures and structural improvement

There are various types of vibration reduction technologies for heat exchangers [2,4,5]. Some are ex-1.2. Analysis of causes of fatigue fracture

1.2.1. Type of vibration that may occur in condensers and rules for determining whether vibration occurs

(1) Karman vortex shedding: Karman vortex shedding might occur when fv/f1P 0.5. Here, fv is the Kar-man vortex frequency, and f1 is the rst intrinsic frequency of the heat exchange tube [3]. The intrinsicfrequency of the tube is related to the uid density in the tube, the mass of the tube material, the size of

the tube, the span length of the tube, and the tube array number.

(2) Turbulence excitation: Turbulence excitation might occur when f0/f1P 0.5. Here, f0 is the main fre-quency of the turbulence excitation.

(3) Fluid elastic instability: Fluid elastic instability might occur when vP vc. Here, v is the transverse owspeed and vc the critical transverse ow speed of the tube bundle.

1.2.2. Results of analysis on invalid titanium condenser

All of the vibration calculation and assessment results are listed in Table 1. As can be seen from the re-

sults, the titanium condensers in use have the possibility of causing vibration, but only the tubes that pen-1. Analysis of failure causes

1.1. Determination of failure type

In the two HE-302 condensers used as the samples for the analysis, fractured sections of tubes are at

a distance of 160165 mm from the lower opening of the tube and are close to the upper surface of the

tubesheet (see Fig. 1). Fig. 2 shows the macroscopic appearance of the fractured section. As can be seenfrom the picture, fractures occurred in circles along the heat exchanger tube and have no branches.

Fractured sections are smooth and clean and have no obvious plastic deformation. There are no other

cracks in the vicinity of the fractured section. Neither the surface nor the inner wall of the titanium tube

has obvious corrosion. The preliminary judgment is that the fracture of the titanium tube is caused by

fatigue.

Fig. 3 is a picture illustrating the microscopic appearance of the fractured section of the titanium heat

exchanger tube observed under a scanning electron microscope. Fatigue striations and a smooth fatigue

expansion area (expansion in the direction from the outer wall to the inner wall of the tube) can be clearlyseen, indicating that the fracture failure type is fatigue fracture [2].All of the condensers are tubular heat exchangers [1], in which six cases are named HE-301 and oth-

ers HE-302. The conventional method of arranging tubes in the notched area of the bae plate is

adopted. Each HE-302 condenser has 1880 heat exchange tubes, 25 mm in diameter and 1.25 mm in

Q.-L. Ma et al. / Engineering Failure Analysis 12 (2005) 432439 433plained as following.

Fig. 1. (a) Outline of condenser. (b) Position of fractured section.

434 Q.-L. Ma et al. / Engineering Failure Analysis 12 (2005) 432439

Fig. 2. Fractured section in titanium tube: (a) front face of fractured section; (b) side face of fractured section.

Q.-L. Ma et al. / Engineering Failure Analysis 12 (2005) 432439 435(1) Reduce the ow speed near the shell: Examples of such measures include an increase in the central dis-

tance of the tube arrangement and changes in the arrangement angle of the tube bundle.

(2) Increase the intrinsic frequency of the tube: Since the intrinsic frequency of a tube is in reverse propor-

tion to the square of the span of the tube, the most eective method for increasing the intrinsic fre-

quency of the tube is to reduce the maximum span of the tube in the absence of support. It is also

possible to change the material of the tube or to place the tube in a stretched stress state. These meth-ods, however, are not very eective.

(3) Change the form of the bae plate: For example, a bow-shaped or ring-shaped bae plate with no

tube arranged at the notch can be adopted. A rod-shaped or stripe-shaped support (also known as baf-

ing rod) can solve the vibration problem eectively.

(4) Increase the sizes of the connecting tubes at the entrance and exit to lower the ow rate at said

entrances and exits. It is also possible to arrange a current fending plane or ow guiding barrel (or

current divider).

Since the tube arrays in a titanium condenser in which vibration occurs are all in the notched area of the

bae plate, the vibration problem can be solved most eectively by adopting a bow-shaped bae plate with

no tube arranged at the notch. This method does not require a signicant change to the structure of the

titanium condenser or a complete modication design. This type of structure not only can eliminate the

Fig. 3. SEM photo of fractured face.

The characteristics of titanium must be taken into consideration when manufacturing a titanium tube

Table 1

Vibration calculation results of titanium condenser

Vibration mode Position Calculated parameter Tubes through notches of support plates

Calculated values Critical values Results

Vortex shedding Outlet fv1/f1 0.772 0.5 V

Inlet fv2/f1 0.865 0.5 V

Support plates fv3/f1 0.775 0.5 V

Turbulence excitation Outlet ft1/f1 0.264 0.5 No V

Inlet ft2/f1 0.296 0.5 No V

Support plates ft3/f1 0.258 0.5 No V

Fluidelastic instability Outlet v1 1.325 1.52 No V

Inlet v2 0.945 0.97 No V

Support plates v3 1.995 2.35 No V

V means vibration takes place.

436 Q.-L. Ma et al. / Engineering Failure Analysis 12 (2005) 432439heat exchanger [3]. Since titanium has a relatively low elastic modulus and high resilience, tube expansionbecomes much more dicult when connecting a titanium tube and a steel tubesheet. In addition, since tita-

nium has a low creep resistance, creep will occur even at normal temperatures. Titanium cannot be welded

with other metals.

Reference [6] uses tests to compare the interfacial strength between the titanium tube and tubesheet con-

nected with dierent methods. The connecting methods include uniform expansion (hydraulic expansion),

roller expansion, welding, and their combination. The test results (see Fig. 4) show that it is inappropriatevibration of the tubes, which are most likely to have problems because they are supported at every other

bae plate, but also has no inuence at all on the pressure loss.

Consequently, the NO-TUBEs-IN-WINDOWs structure with no tube arranged in the notched area of

the bae plate is adopted for both the newly ordered imported titanium condenser and the domestically

manufactured titanium condenser to realize the vibration-reduction objective. Since the eective area fortube arrangement is reduced, the diameter of the cylindrical body of the condenser is slightly increased

when the structure with no tube arranged in the notched area of bae plate is adopted.

3. Selection of connecting method between titanium tube and tubesheet during domestic production0

Joint typeC+A+B C+A C A+B

49.2651.4050.11

12.838.20

A

60

50

40

3020

10

Push

out F

orce

/ kN

Fig. 4. Pushout force test results for dierent joint types [6].

to only use the expanding connection method to endure the thermal stress or other load of the tube array in a

titanium-tube array heat exchanger. Roller expansion can reduce the remaining axial stress generated in the

tube during uniform expansion to improve the stress in the tube, so that the pushout force can be signicantly

increased. A very good connecting eect can be realized by welding. If uniform expansion or uniform expan-

sion and roller expansion is applied at that time, all of the fractured positions can be kept outside the tubesheet.Based on the aforementioned research results, a tube-to-tubesheet connecting method of strength weld-

ing plus bonding expansion is selected for the HE-302 titanium condenser studied in the present paper dur-

ing repair and domestic manufacturing.

4. Bonding expansion simulated test and pushout test

Q.-L. Ma et al. / Engineering Failure Analysis 12 (2005) 432439 4374.1. Bonding expansion simulated test

In order to investigate the eect of bonding expansion and to nd an appropriate tube expansion pres-

sure, a tube expansion test is carried out using a simulation test device.

The simulation test device has the same material and specications as the HE-302 condenser. The tube is

made of titanium (TA2). The thickness of the tubesheet is the same as that of the condenser. The material is

carbon steel. As shown in Fig. 5, the device has seven heat exchange tubes.

The bonding expansion width of each tubesheet is 180 mm, and the tube expansion test is carried out

under dierent tube expansion pressures. An airtight test is carried out after expansion. In the meantime,many simulated rods are prepared and dissected after expansion. The tube expansion rate and the tube wall

thinning rate are measured and calculated; the smooth transition state between the expansion connected

part and the non-expanded part is observed. The expansion connection parameters are nally determined

through two types of test methods. When carrying out the simulation test, although all 14 expanded holes

pass the 0.1-MPa airtight test, some expanded holes are found to have leakage after the sample is retested

after being set for 48 h. This proves that titanium material has a creep property at ambient temperature and

also proves that expansion connection alone cannot be used as the strength connection method for a tita-

nium tube with a steel tubesheet. This method can only be applied to bonding expansion.

4.2. Pushout test of heat exchange tube and tubesheet

As can be seen from the aforementioned analysis, the optimum connection method for a titanium heat

exchange tube and tubesheet is strength welding plus bonding expansion. A tube-to-tubesheet test is carriedFig. 5. Experiment device modelling the condenser HE302.

out in order to investigate the reliability of this combined connection method and to nd rational manu-

facturing parameters.

Heat exchange tubes are made of TA1, 25.4 mm in diameter and 1.25 mm in thickness. The composite

tubesheet is made of TA2 of 10 mm thick and 16MnR of 80 mm thick.

Table 2 and Fig. 6 show the test results. The following conclusions can be drawn from the test results.

(1) Even if strength expansion is adopted, its pushout is still relatively small. Also, the pushout result is

complete pushout. This explains why the method that only performs expansion but no welding is

not suitable for connecting a titanium heat exchange tube with a steel tubesheet.

(2) A relatively high pushout strength can be obtained using the connection method that only performs

welding but no expansion. However, if welding is only performed once or twice, the tube always breaks

at the weld seam. This means that the weld seam is the weak part in the method that only performs

welding but no expansion.

Table 2

Pushout test results

No Expansion situation Welding conditions Pushout force/kN Fractured status

1 Expanded 21.5 Pushout

2 Expanded Welded 45.5 Tube fractured outside of the tubesheet

3 Expanded Welded 46.5 Tube fractured outside of the tubesheet

4 Expanded Welded 45.0 Tube fractured outside of the tubesheet

5 Welded 43.5 Fractured in the weld

6 Expanded Welded 1 lled with

/2 lines + Welded 248.0 Fractured in the heat aected zone of the weld

7 Expanded Welded 1 + Welded 2 45.0 Tube fractured outside of the tubesheet

8 Welded 1 lled with

/2 lines + Welded 236.0 Fractured in the weld

9 Welded 1 + Welded 2

lled with /2 lines + Welded 345.0 Tube fractured outside of the tubesheet

10 Welded 1 + Welded 2 lled

with /2 lines + Welded 345.0 Tube fractured outside of the tubesheet

11 Expanded Welded 45.5 Tube fractured outside of the tubesheet

438 Q.-L. Ma et al. / Engineering Failure Analysis 12 (2005) 432439Fig. 6. Device photos of modelling tube-to-tubesheet after pushout experiment: (a) composite titanium side; (b) carbon steel tubesheetside.

bines welding and expansion, a three-layer welding-only method can be used. This method, however, is

sheet. The method that combines welding and expansion is the suitable connection method. However,adopting strength expansion has no obvious superiority. The optimum connection method is strength

welding plus bonding expansion.

References

[1] Qingde Nie. Chemical equipment design. Beijing: Chemical Industry Publishing House; 1993.

[2] Xiangchen Liu, Chenshu Zhang. Chemical equipment accident analysis and prevention. Beijing: Chemical Industry Publishing

House; 1990.

[3] Yaoqin Wang. Titanium chemical equipment design. Shanghai: Shanghai Science and Technology Publishing House; 1985.

[4] GB151-89, Steel tube shell-type heat exchanger The Chinese Standard Committee; 1989.

[5] Sandifer JB. Guidelines for ow-induced vibration prevention in heat exchangers. WRC Bulletion 1992;372(5):2035.time consuming, and it is dicult to guarantee a stable quality at the welded joint. Therefore, this

method is not recommended.

5. Conclusions and suggestions

After thoroughly analyzing the causes of failure of the HE-302 condenser, it is concluded that the frac-

ture of the titanium tube in the condenser is fatigue fracture caused by vibration of the tube array. Also,

only the tube array arranged in the notch area of the bae plate has vibration. Based on this, we provide

the following suggestions for the domestic manufacture of the HE-302 titanium condenser.

(1) Heat exchange tubes still use titanium material, but the wall thickness is increased to 1.5 mm to rein-force the rigidity of the heat exchange tube.

(2) Increase the thickness of the bae plate, reduce the shearing eect of the bae plate on the tube,

increase the damping of the system, reduce vibration, or change the form of the bae plate by adopt-

ing a baing rod. All these measures can reduce the vibration problem and improve the heat exchange

eect.

(3) A NO-TUBEs-IN-WINDOWs structure is adopted for condenser design to eliminate vibration of tita-

nium tubes.

(4) Hydraulic expansion connection is adopted in tube-to-tubesheet connections to realize a relativelygood connection eect.

(5) Current fending plates are arranged at the entrance of the condenser to reduce the impact of the con-

densing medium on the tube array.

(6) Neither the method that only performs expansion but no welding nor the method that only performs

welding but no expansion is suitable for connecting a titanium heat exchange tubes with steel tube-(3) When the connection method that combines welding and expansion is adopted, not only can a relatively

high pushout strength be obtained, but it is also possible to guarantee that the fractioned positions of the

tube are all kept outside the substrate of the tubesheet. Therefore, it is the optimum connection method.

(4) In the connection method that combines welding and expansion, strength expansion and bonding

expansion have little inuence on the test results. Therefore, adopting strength expansion has no obvi-ous superiority. The necessity of adopting strength expansion is insucient.

(5) In order to obtain the same pushout test results as those realized by the connection method that com-

Q.-L. Ma et al. / Engineering Failure Analysis 12 (2005) 432439 439[6] Jawad MH, Clarkin EJ. Evaluation of tube-to-tubesheet junctions. J Press Vessel Tech 1987;109:1926.

Failure analysis and critical manufacturing technology research on titanium condensersIntroductionAnalysis of failure causesDetermination of failure typeAnalysis of causes of fatigue fractureType of vibration that may occur in condensers and rules for determining whether vibration occursResults of analysis on invalid titanium condenser

Vibration reduction measures and structural improvementSelection of connecting method between titanium tube and tubesheet during domestic productionBonding expansion simulated test and pushout testBonding expansion simulated testPushout test of heat exchange tube and tubesheet

Conclusions and suggestionsReferences