Materials and Design 31 (2010) 613–619

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Materials and Design

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Short Communication

Effect of flow induced corrosion and erosion on failure of a tubular heat exchanger

Khalil Ranjbar *

Materials Department, Faculty of Engineering, Shahid Chamran University, Ahvaz 61355, Iran

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

Article history:Received 27 August 2008Accepted 16 June 2009Available online 21 June 2009

0261-3069/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.matdes.2009.06.025

* Tel.: +98 611 3330010 19(x)5682; fax: +98 611 3E-mail address: K_ranjbar@scu.ac.ir

Failure of a tubular heat exchanger made up of copper based alloys in a steam power plant was investi-gated. Tubes were made initially from yellow brass, and there after replaced by Cu–5Ni alloys. Circulatingwater from cooling tower was used as coolant, to condense the hot steam inside the steel shell. The veloc-ity of cooling water was found to be very low, causing accumulation of deposits, reduction of diameterand some times complete blockage of tubes. The causes of failure were investigated performing varioustests such as chemical analysis, XRD, optical, and SEM examination. Study revealed that tube materialsuffering from extensive dealloying and impingement attack.

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1. Introduction

The shell-and-tube type heat exchanger is a device in whichheat transfer takes place between two fluid streams through a con-ducting wall, while being kept physically separate. Shell-and-tubeheat exchangers are generally built of a bundle of round tubesmounted in a cylindrical shell. They are designed and classifiedin accordance with the widely used tubular exchanger manufac-turers association (TEMA) standards [1]. The prime concerns indesigning a heat exchanger are, resistance to corrosion and erosion,taken into account by proper materials selection for the particularapplication [2].

Copper alloys are quite corrosion resistant in many atmosphericand non-oxidizing aqueous environments [2–5]. The major alloy ofthe copper is brass, suffering from dealloying/dezincification andimpingement attack. Cu–Zn alloys containing more than 15% zincare susceptible to this kind of corrosion, where removal of Zn re-sults in a porous and weak layer of copper and copper oxide [5].Admiralty and Naval brasses have better resistance to erosionand dealloying. Bronzes and in particular Al-bronzes are still moreresistant to impingement attacks. But Copper–Nickel alloys (10–30 wt% Ni) are the most corrosion and antifouling resistant ofcommercial Cu-alloys [2,4]. This is attributed to the protectivecharacteristics of their corrosion product films mainly copper oxi-des formed on the surface of the alloy in an aqueous environment[4–6].

Coolant and its velocity play an important role in failure andperformance of a heat exchanger. Cooling tower water is the maincoolant in the power plants. Generally this water is not useddirectly but simply passes through the condensers and heatexchangers and returns to the tower. That is why it is also called

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336642.

circulating water. Cooling water may contain many types of slit,sand, mud, or other finely divided particles that may settle onthe heat exchanger surfaces and act as an insulator layer. The con-centrations of these components rise as the water is circulatedthrough the cooling systems. However the deposition of theseextraneous materials upon the heat transfer surface is called foul-ing. Various types of fouling are identified in the literature [7–9]and classified as per their origin of formation. Earlier Studies havepointed out that cooling water velocity through the tubes is a cru-cial factor for fouling [7,8,10–12]. Rapidly moving cooling waterstrip away the protective films from tube surface, but low flowrates less than 0.5 m/s [13] or even less than 1 m/s [14] causethe suspended fluent to drop out and deposit within the tubes.Some types of fouling can be controlled or minimized by using highflow velocity, provided proper materials are selected to avoid ero-sion. This can also help to minimize biological and corrosion foul-ing. Calcium compounds are the main constituents of fouling,forming tenacious layers in many industrial systems, including cir-culating water in power plants. In recent years, different aspects ofscale formation and in particular calcium carbonate have beenstudied [11,12,15–17].

In this study, various parameters effecting the normal operationof a tubular heat exchanger including quality of cooling water andits velocity, materials selection and the nature of deposited scales,were investigated. In particular, the effect of flow induced corro-sion and erosion on failure of tubes is discussed.

2. Experimental procedures

A diagrammatic sketch of the exchanger used in this study isshown in Fig. 1. It is a two pass counter flow type exchanger. Thecoolant was circulating water from cooling tower. Its compositionis given in Table 1. Hot steam with the temperature of 104–110 �Cwas supplied either from the heater or directly received from low

Coolant In

Coolant Out

Steel Shell

Tube sheet

Tubes Steam In

Condensate Out

Fig. 1. Schematic view of a heat exchanger is showing main parts and basicoperation, used for present study.

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pressure turbine (LPT), in order to make feed water for steamturbines. The condensate temperature was about 90–92 �C. The cir-culating water velocity was determined by taking into account thetotal number of tubes, their cross sectional area, and volume of thecirculating water per unit time supplied to the exchanger. Thiswater was fed to the exchanger by two pumps of 40 t/h capacitieseach. There was no flow meter attached to supply line.

Several failed tubes were visually examined. Some of the failedtubes were cut and sectioned to reveal the internal surfaces, whichwere covered with fouling deposits. Chemical analysis of depositsas well as tubes was done. Qualitative phase analysis of depositswas performed by X-ray diffraction (XRD). Under deposited sur-

Table 1Chemical composition and properties of the circulating water.

PH TH (+) Ca2+ (+) Cl� (�) Na+ (�)

L H L H L H L H L H

7.1 7.95 17 41 12 28 580 1450 340 925

H = highest, L = lowest, TH = total hardness, TDS = total dissolved solids, (+) = Meg/L, and

Table 2Chemical compositions of tubes used in heat exchangers.

Cu Zn Pb Sn P Mn

A 64.46 35.45 0.02 0.01 <0.005 <0.010B 64.7 35.21 0.02 0.01 0.005 <0.010C 92.51 0.09 0.01 0.0 0.005 0.596

A = Laton-68, B = Admiralty brass, and C = copper–5%nickel.

Fig. 2. Optical micrographs of failed exchanger tubes at magnification of 1000�, obtainedBoth tubes exhibited same recrystallized alpha grains with some annealing twins.

faces were examined by optical microscope as well as by scanningelectron microscope (SEM) to detect the extent of pitting, and deal-loying. Elemental analysis was also carried out via energy disper-sive X-ray spectroscopy (EDX).

In this study, three types of tube materials used for heatexchangers were thoroughly investigated. In the power plant,(Ramin Steam Power Plant, Ahvaz) these alloys were identified asfollows:

1. An alloy called Laton-68 (Russian recognition).2. An alloy named as yellow brass (suppose to be Admiralty brass).3. An alloy of cupronickel (Cu–5Ni) (it is used as condenser tubes

in the plant).

Tubes have 19 mm internal diameter, and 3.42 m length, insidea steel shell made of simple plain carbon steel (St-37 grade). Tubesheets were also made from the same steel.

3. Results and discussion

3.1. Tubes alloy/material

Chemical composition of alloys used in this study is given inTable 2. It shows that, the Admiralty brass and Laton-68 have samecomposition. They are supposed to have 1–1.5 wt% Sn. In fact

Mg2+ (�) SO2�4 (�) TDS Temperature (�C)

L H L H L H L H

72 161 731 1855 2065 5053 27 48

(�) = mg/L.

Fe Ni Mg Al As Sb

0.03 0.011 0.005 0.006 0.001 0.010.03 0.011 0.005 0.006 0.001 0.011.18 5.57 0.015 0.005 0.001 0.01

after normal grinding, polishing and etching: (a) Laton-68 and (b) Admiralty brass.

Fig. 3. Particulate/sedimentation fouling at the inlet (on tube sheet): (a) cross section of a tube sheet showing how different extraneous materials entering the tubes, (b)collection of extraneous materials on tube sheet surface, (c) uniform fouling on tube sheet, (d) corrosion of tube sheet in the form of candles of iron oxide entering tubes,shown by arrows, and (e) chemical composition of the points marked by arrows in (d), by EDX.

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Laton-68 is a commercial name given to Admiralty brass. In otherwords, they are simply yellow brass with 35% Zn, and have noalloying additives to suppress dezincification. Their compositionswere matching with alloy C27000. They also have the same micro-structure as shown in Fig. 2. Theoretically speaking such brassesare susceptible to dealloying and impingement attack. The otheralloy used for tubes, was cupronickel (Cu–5 wt%Ni). This alloyhas replaced yellow brass since it has already been used as con-denser tubes in the same power plant. Although, it has also got bet-ter corrosion, dealloying and biological fouling resistance ascompared to yellow brass, but Cu–10Ni or Cu–30Ni could be a bet-ter choice.

3.2. Different types of fouling

The type of exchanger used in this study is a Steam to Watertype exchanger. Circulating water from cooling tower was passed

through a filter which separate coarser particulates, and then fedinto exchanger, by two pumps. Investigation showed that, this fil-ter is often not functioning well and big size particulates reachedtube sheet. This is shown in Fig. 3a and b. Depending on sizes ofthese foreign materials, they can either enter tubes, or accumulateon tube sheet. Circulating water also contained many types of slit,mud, sand or other finely divided solid particles. In addition, highamounts of total dissolved solids (TDS) and other depositing spe-cies such as Mg, Na, and Ca were detected as shown in Table 1.As a result, uniform particulate fouling has formed on tube sheetas shown in Fig. 3c. The other factor contributing to particulatefouling was corrosion of tube sheet in the form of iron oxide flakesor candles which taken off from the surface and entered the tubes,as shown in Fig. 3d. This was further confirmed by EDX elementalanalysis shown in Fig. 3e. Particulate fouling was also observed in-side the tubes. In the present study, low velocity circulating waterintensified the sedimentation inside tubes.

Fig. 4. Different views of fouling deposition inside the tubes: (a) complete clogging and reduction of internal diameter, (b) sectioned tube, and showing deposition inside theinternal surface, and (c) randomly selected tubes, in tube bundle and away from tube sheet inlet. It clearly shows reduction in internal diameter due to deposition.

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Different views of sedimentation inside the tube surfaces arepresented in Fig. 4. It is clear that accumulation of deposits is verysignificant, causing partial or complete clogging of tubes. An anal-ysis of deposit inside the tube surface by SEM–EDX showed thatcompounds of calcium carbonate and calcium sulphate were themain constituents, and other scale forming species of Magnesium,Silicon, and Iron were also present. Calcium carbonate, calcium sul-phate have inverse solubility with increasing temperature, lead to

Fig. 5. Morphology of main sediment inside the tube surface: (a) calcium carbonate cryanalysis of point marked by a circle in (a).

crystallization of these salts on tube surface. In addition to temper-ature, the fouling was also affected by low circulating water veloc-ity. The detection of iron in the deposits was due to the corrosion oftube sheet. Chemical analysis of deposit inside tube surface and themorphology of calcium carbonate crystals are shown in Fig. 5.Results shown in Figs. 3–5, revealed that, particulate as well ascrystallization fouling were occurred in combination inexchangers.

stals, (b) calcium carbonate at higher magnification, and (c) EDX spectrum and the

Fig. 6. Flow induced erosion due to change in velocity of circulating water. (a) Flowinduced erosion inside tube surface. Arrows indicating the locations where tubethickness is reduced and finally got holed, (b) horse-shoe appearance, and (c) sameeffect at higher magnification.

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3.3. Erosion

Fig. 6 shows the flow induced erosion effects inside the tubes.Failure of tubes due to extensive thinning at some locations(Fig. 6a) indicated that velocity of fluid inside the tubes has chan-

Fig. 7. Dezincification inside tube surface: (a) dezincification and partly removed depoporosities in (c) can be seen.

ged and speeded up, causing severe erosion. Erosion corrosionexhibited a directional pattern, and resulted in a pattern of horse-shoe-shaped grooves/pits, as shown in Fig. 6b. When this form ofcorrosion occurs in an exchanger tubes, it indicates that the fluidvelocity is rapid and turbulent. In other words, some of the tubesin a bundle become plugged, (as in Fig. 4) causing velocity to be in-creased in other tubes. Partially plugged tube also reduced internaldiameter, and increased the velocity with in the same tube. Thisfact has also been proved by other studies [8,10]. High flow ratelocally swept away the deposits and under deposits, led to severerosion and thinning of tubes. Both the Yellow brass and Cu–5Nitubes showed sever erosion, indicating that, tube material has tobe replaced with more flow resistant ones such as Cu–30Ni alloy.

3.4. Dezincification

Different forms of dezincification marks are presented in Fig. 7.In the present study, tubes made of yellow brass, showed severlocalized corrosion typical of plug type dezincification. Dealloyingwas not observed in tubes made of alloy Cu–5%Ni. Investigationshowed that, the alloy type (yellow brass) and working conditionwere suitable for dezincification to occur. It is because Cu–Zn al-loys containing more than 15% zinc are susceptible to this kindof corrosion [5]. On the other hand, circulating water was high insalts content, and was operating above room temperature. Tubeswere also covered with thick layer of deposits with areas of differ-ential aeration and concentration cells which promoted plugformation. In this process, Zinc is selectively leached out fromCu-alloys and resulted in a porous and weak layer of copper andcopper oxide. This feature was observed in present study and isshown in Fig. 7. Dezincification eventually penetrated the metal,leading to further weakening and leakage of the structure as shownin Fig. 6a.

The dezincified plugs were consisted of a relatively porous massof copper, and the plug area was very well distinguished by its

sits, (b) plug dezincification), (c) and (d) uniform dezincification. Cracks in (a) and

Fig. 8. Variation of zinc content within a single dezincified plug taken by EDX, inside plug marked by a solid circle (d), and away from the plug (unaffected area), marked byhollow circle (s). EDX elemental analyses and the respective spectra clearly show the difference in zinc content.

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pinkish color (see Fig. 7). This was further proved by more detailedSEM and EDX studies on a single dezincified plug shown in Fig. 8.The figure revealed that, the amount of Zn content inside the plugand away from it (unaffected area), was very different. It was alsonoticed that, zinc oxide crystals formed as by-product of dezincifi-cation process and its adjacent area were associated with porosityand cracks. This fact is shown in Fig. 9. Thus, as far as dezincifica-tion is concerned, proper material for tubes is not selected.

3.5. Velocity

The resistance against corrosion in copper base alloys is due toprotective surface film. High velocity can destroy surface protec-tive film. On the other hand, low velocity leads to build-up of sed-iments and sites for under deposit corrosion. Thus, cooling watervelocity inside tubes is a crucial factor. That is why a critical designvelocity [5,13] as well as minimum flow rate is defined for a copperalloy, handling fluids. The investigation showed that cooling watervelocity was at the most 0.6 m/s, when two pumps used to supplythe water to exchanger. Velocity reduced to 0.4 m/s when only one

pump was in operation. These low velocities resulted in heavydeposition upon the tube surfaces (see Fig. 4), thereby cloggingor reducing tube diameter. Thus, velocity locally increased, de-stroyed surface protective film and led to sever erosion insidethe tubes. Even in Cu–Ni alloys with better erosion resistance, toavoid stagnant conditions a minimum flow rate of 0.5 m/s is ad-vised [13]. Therefore, flow induced erosion was one of the mainfailure mechanisms in these tubes.

4. Conclusions

1. Circulating water contained a lot of big size extraneous materi-als, TDS, and calcium compounds, led to extensive Particulateand crystallization fouling inside tubes.

2. Proper material selection was not done for tubes. Yellow brasstubes suffered dezincification and erosion. Even Cu–5%Ni alloyfaced sever erosion.

3. The velocity of circulating water was too low, causing settle-ment of deposits inside tubes. Flow induced erosion and thin-ning of tubes was the main cause of failure.

Fig. 9. ZnO crystals formed on release of Zn ions after dezincification: (a) a single dezincified plug shown by a ring, (b) magnified plug in (a), showing ZnO crystals along withcracks and porous mass, and (c) crystals of ZnO inside the plug, and its EDX spectrum and elemental analysis at a point marked by a circle.

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Acknowledgment

The financial support by Ramin Steam Power Plant Ahvaz, isappreciated.

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