Investigation on the Failure of Al-Wajh Multieffect Desal Condenser Tubes

8/13/2019 Investigation on the Failure of Al-Wajh Multieffect Desal Condenser Tubes

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INVESTIGATIONS ON THE FAILURE OF AL-WAJH MULTIEFFECTDESAL CONDENSER TUBES 1

T.L.Prakash and Anees U.Malik

Research & Development Center

Saline Water Conversion Corporation

P.O.Box: 8328, Al-Jubail 31951, Kingdom of Saudi Arabia

ABSTRACT

Amongst the SWCC seawater desalination plants, Al-Wajh desalination plant at the west coast

is unique in the sense that it is the only plant that operates on multieffect evaporation principle.

This plant has been in operation for more than 14 years. This paper covers the results of two

case studies regarding the desal effect tube failures in the plant.

Case - I

The Aluminum Brass tubes of the first and second rows of the tube bundle of the 2nd effect

chamber were found leaking. In all 31 numbers of tubes had failed and the R&D Center

received two failed tubes for investigation. The outer surface of the tubes contained patches of

visually thick scales and pits. Both the tube surfaces contained small and big size (approx. 15

mm dia) holes. Substantial thinning of the metal near the edges of the holes was noticed.

The results of the investigation pointed out that tubes had failed mainly due to erosion -

corrosion. The erosion - corrosion occurred when gases, vapors or liquid impinge on metal

surface at higher velocities. Non uniform spray distribution of seawater on the tubes was found

responsible for the observed localized perforation of the tubes. It was recommended that during

plant maintenance, thorough cleaning of seawater distribution trays including holes and

nozzles should be carried out in order to ensure uniform spray pattern on the tubes during

operation.

Case - I I

The investigation on the failure of Aluminum Brass tubes of reheat type desal unit D-03 and D-

04 was car ried out at R&D Center. The tube of D-03 was located on top few rows below the

distribution tray, whereas the tube of desal D-04 was situated at the bottom most row and the

1 Presented in Second Acquired Experience Symposium on Desalination Plants O&M, SWCC, Al-Jubail,Sept.29-Oct.3, 1997.

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failed region was facing the bottom of evaporator . The tube of desal unit D-03 contained a

relatively large perforation at the center and that of D-04 had a long rupture initiated from one

end. The results of the investigation indicated that basically both the tubes were failed by

erosion - cor rosion a lthough mode of failures were different. In D-03 tube, the failure was

ascribed to liquid impingement attack. The desal D-04 failure occurred due to erosion -

corrosion attack carried out effectively by non condensable gases or vapor pocket at the bottom

section of the evaporator chamber. It was recommended that in order to avoid impingement

corrosion attack, the existing tubes be replaced with impingement resistant compatible tubes

90/10 Cu Ni or Fe/Mn modified Cupronickel tubes. Improving the venting system of the flash

chamber could control the erosion - corrosion of bottom row tubes.

INTRODUCTION

Saline Water Conversion Corporation (SWCC) desalination plant at Al-Wajh produces

potable water from seawater through multieffect evaporator modules. This plant is operating

successfully since 14 years. The condenser tubes of this plant were constructed out of

aluminum brass alloy. It is a cost effective material generally recommended for use in

multieffect desalination plant. This alloy known to exhibit good seawater corrosion resistance

due to the development of a protective oxide layer. Unfortunately, its application is limited by

the velocity of liquid impinging on it. The impingement corrosion is normally expected when

the velocity of falling seawater exceeds 2 m/sec [1]. When the velocity exceeds 2 m/sec, the

protective oxide layer on the surface of the alloy is disrupted and carried away by the high

velocity water, thus exposing metal for localized attack. The nature of the protective oxide

deposits play an important role. If humid condition exists during outage of the plants, the

protective oxide deposits shall be of hydrated copper oxide [2] which is less adherent than the

unhydrated copper oxide and could be easily disrupted even if the flow velocity is far less than

2 m/s. Another factor that has to be taken into account while using this alloy for such

application is its susceptibility to erosion corrosion. The erosion corrosion occurs when gases,

vapors or liquids impinge on metal surface. In most of the cases it is found that seawater

containing small percentage of sulfur compounds [3], dissolved ammonia or chlorides attack

the metal surface and locally removes the protective films thereby contributing to the formation

of concentration cell and localized pitting of anodic sites.

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This paper presents two interesting case studies carried out at SWCC Research and

Development Center, Al-Jubail on the failure of multieffect condenser tubes of Al-Wajh

desalination plant.

1. CASE I

1.1. BACKGROUND

The failed tubes of the Desal unit were located at the first and second row of the tube bundle of

the 2nd effect chamber. In all 31 numbers of tubes have failed during service. These tubes were

just below the seawater distribution trays and were in service since inception. The unit was

manufactured by M/S Sasakura Engineering Company Ltd. Japan.

1.2. PHYSI CAL I NSPECTI ON

The tubes were of size 19 mm OD x 1.0 mm thickness. The outer surface of the tubes contained

patches of visibly thick scales and pits. Both the tube's surfaces contained small and big size

(approximately 15 mm dia) holes (Fig. 1) . The big size holes were characterized by undercut

groves with directional pattern moving away from the center of the holes. Substantial thinningof the metal near the edges of the holes were seen indicating erosion corrosion of the metal

occurred from the top surface during service. Plenty of surface pits were also noticed on the

tubes. The inner portion of tube surface contained uniform layer of greenish colored scale

deposits and were free from pitting.

1.3. CHEM I CAL COM POSI TI ON

The tube samples were analyzed for the chemical composition by X-ray Fluorescence apparatus

(Outokumpu model X-MET 880) and wet chemical methods. The chemical compositions of the

tube samples were found as: Zn-20.2, Al-2.2, As-0.005 and Cu-Bal. The result of the analysis

indicates that the material of the tube belonged to aluminum brass category that is designated

by ASTM B 111 (UNS C 68700).

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1.4. METALLOGRAPHY

The samples for metallography were prepared from the area where samples surface contained

pits. The general microstructure of the tubes as observed in the metallography of the transverse

sample showed recrystallized alloy structure. The microstructure revealed a typical single

alpha phase alloy structure of aluminum brass and had an average grain size of 25 m. The

surface of the tube had revealed uneven depressions caused due to pitting.

1.5. SCANNI NG EL ECTRON M I CROSCOPY (SEM )

The surface areas containing large hole and areas containing scale deposits have been examined

by SEM. Outer scale deposits near the holes have been analyzed by Energy Dispersive X-ray

Analysis (EDAX) technique during SEM. The deposits were found to be consisted of S, Cl,

Mg, Ca and O 2 indicating the presence of sulfides, chlorides and oxides. The SEM of the inner

surface deposits have revealed mainly oxides of copper in EDAX analysi s (Fig. 2) . The inner

surface scale deposits were uniformly thick. The outer scale deposits were nonuniform in

thickness, discontinuous and wavy in nature (Fig. 3) , suggesting aggravated corrosion of thesurface. It is therefore thought that the gradual thinning and subsequent perforation of the tubes

have taken place mainly due to metal corrosion from the outer surface agents during service.

1.6. DI SCUSSI ON

All the evidences gathered in the course of this investigation suggest that the tubes have failed

mainly due to erosion corrosion. The pits and the leak spots observed on the tube surface were

characterized typically of erosion corrosion. In multieffect horizontal falling film desalination

process, the velocity of seawater falling on aluminum brass tubes may not exceed 2 m/s since it

is gravity fed through spray distribution trays. The leak spots localized to 1st and 2nd row of

tubes below spray distribution tray suggest that nonuniform distribution pattern of spray on the

tubes might have been occurring due to clogging of holes in the distribution tray. The non

uniformity in spray may have an effect on the velocity and thereby causing erosion corrosion of

the tubes. Another factor that aggravated the erosion corrosion is the nature of protective film

developed during outages. The development of less adherent hydrated copper oxide film

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deposits was also responsible for the increased corrosion rate. This could be avoided by

keeping the environment clean and non-humid during outages.

1.7. CONCLUSIONS

i) The failure of tubes was most likely due to erosion corrosion.

ii) Non-uniform spray distribution pattern of seawater on the tubes was responsible for the

observed localized perforation of tubes.

iii) Development of non adherent protective oxide film during outages might have also

been responsible for the premature failure of the tubes.

1.8. RECOMMENDATIONS

i) Regular cleaning of the seawater distribution trays, holes/nozzles of trays and ensuring

uniform spray pattern on the tubes would help in limiting the erosion corrosion.

ii) During outages, the environment should be dry in the effect chamber in order to

develop non hydrated copper oxide which is more protective film than other forms of

copper oxides on the tube surface.

2 CASE I I

2.1. BACKGROUND

The failed tubes were a part of the condenser tubes used in the reheat type multieffect desal

units namely D-03 and D-04. The tube of desal D-03 was located on top row below the

distributor tray, whereas, the tube of desal D-04 was situated at the bottom most row and the

failed region was facing the bottom of the evaporator.

2.2 PHYSI CAL I NSPECTI ON

The cut portions of failed tubes were inspected visually on receipt. The desal unit D-03 tubewas of size 25 mm outer dia, 82 mm length and 1 mm thickness. At the center it had contained

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a perforation of approximately 18 mm length and 10 mm width. Few perforations of small

diameter were also noticed along the line near the main perforation. The desal D-04 unit tube

was of size 35 mm outer dia, 720 mm length and 1 mm thickness. One end of the tube was

ruptured and the cracked length of the rupture was of approximately 360 mm. The photographs

of the close-up view of tube failures were show n in Figure 4 .

The D-03 tube had a uniform dark yellow color external appearance with a rough surface. The

internal surface appearance was brownish yellow and had fairly smooth surface. Both external

and internal surfaces were free from corrosion deposits. The thickness of the tube was almost

constant all along the length of the tube except at the place near perforation. Few depressions

on the surface were also noticed near the perforation at the center portion of the tube (Fig. 4a) .

Tube D-04 had brownish yellow external appearance. The external surface was smooth unliketube D-03, the internal surface appearance was similar to the external surface and were devoid

of any corrosion deposits. The ruptured edges were considerably thin and ragged because of the

tearing of wasted zone as shown in Figu re 4b .

2.3. CHEM I CAL COM POSI TI ON

The tube samples were analyzed for the chemical composition by wet chemical method and

Energy Dispersion Spectroscopy. The chemical compositions of the tube materials were :

Tube D-03 : Zn-19.8, Al-2.3, Fe-0.02, As-0.04 and Cu-Bal., Tube D-04 : Zn-19.6, Al-2.3, Fe-

0.02, As-0.04 and Cu-Bal. The results of the analysis indicate that the material of the tubes

belonged to aluminum brass class of alloy designated by ASTM B 111 (UNS C 68700).

2.4. METALLOGRAPHY

The metallography of the tubes were carried out on the cut cross section of the tubes. Standardmetallographical procedures have been adopted for the purpose. The microstructure showed

fine grained structure for D-04 tube when compared to D-03 tube. The microstructures were of

the type typical of a single phase recrystallized alloy with twining inside the grain boundaries.

The grain size of D-03 tube sample was approximately 15 m where as it was around 10 m for

D-04 tube samp le (Fig. 5 ).

2.5. SCANNI NG EL ECTRON M I CROSCOPY (SEM )

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The SEM examination at the failed portion of the D-03 tube revealed material loss regions due

to liquid impingement. The surface of the material was uneven and contained oxides layer

network at the troughs of the surface irregularity. These oxide particles were seen as bright

spots (Fig. 6a) in the SEM fractograph. These particles were analyzed by EDAX attachment of

SEM and they found to be the oxides of copper. The SEM examination around perforation

showed deep pit marks those were responsible for perforation (Fig. 6b) . The pit surface wall

was examined by EDAX and found that it contained no corrosion products, thus indicating

that the pitting was caused mainly due to impingement of the falling liquid.

The D-04 tube fracture surface exhibited material wasting at the ruptured place and it was

devoid of any corrosion products. The edges of fracture surfaces were thin and striated

grooves have been seen pointing towards the rupture edge and decorated with oxide particles.The erosion marks noticed at the place near the rupture were responsible for material thinning

(Fig. 7) .

2.6. DI SCUSSI ON

The observations made in this investigation suggest that the failure of both tubes were

characterized typically of erosion/impingement corrosion. It is particularly severe when they

were in the zone of top two rows of the evaporation chamber [4]. The D-03 tube, in fact, was

below the spray distribution trays of the evaporation chamber and one can therefore expect

impingement corrosion occurring from the falling seawater. The force of impingement might

have been enhanced due to increased velocity of flow resulting from reduction in the cross

section area of distribution tray nozzle openings. This is possible due to local obstruction by

foreign particles. The surface of the failed tube had also contained much surface depression

along the straight line containing the perforation which were presumably below distribution

tray nozzle. As this tube (failed D-03 tube) was in the top row, it confronts higher impingement

effect of liquid when compared to the one at the bottom and hence lead to perforation.

In the case of D-04 tube, the fracture contained thin rupture lip unlike D-03 tube failure. The

failure area of the tube was facing the bottom of the evaporator. The examination carried out on

the rupture area indicated that the material wastage at the failure area had lead to failure of the

tube. Rupture edges were thin and ragged because of the wasted material. The wastage appears

to be the effect of erosion corrosion. The erosion corrosion of copper alloys can occur due to

interaction of noncondensable gases, vapors or liquids pockets present in the bottom vaporspace of the chamber. These vapors attack copper alloy tubes and remove locally the protection

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film [5,6]. They contribute to the formation of concentration cell and subsequent corrosion of

anodic sites resulting in material wastage (thinning) or erosion corrosion. A good venting

system obviously would minimize such erosion corrosion problem of the tubes arising from

noncondensable gases accumulated at the vapor space of the evaporator chamber.

2.7 CONCLUSIONS

Desal D-03 Tube:

i). The failure of the tube was due to impingement attack of sea water on the tube surface.

ii). Perforation of tubes was due to material loss by pitting preceded by liquid

impingement.

iii). The impingement attack might have enhanced due to increased velocity of flow ofliquid from the distribution tray. It is likely that flow passage of the distribution tray

might have clogged or obstructed by foreign particles present in the liquid.

Desal D-04 Tube:

i). The failure of tube was most likely due to erosion corrosion.

ii). The erosive thinning of material had led to rupture of tube.

iii). The erosion-corrosion might have resulted from the reaction of metal with non-

condensable gas or vapor pockets present at the bottom most section of the evaporator

chamber.

2.8 RECOMMENDATIONS

i) To avoid impingement corrosion failure on the top row tubes, the top two to three rows

of the existing tubes may be replaced with impingement resistant compatible tubes viz.

90/10 copper nickel tubes (UNS C 70610) or iron or manganese modified copper-nickel

tubes (UNS 671640 or C 71630). These tubes have adequate impingement corrosionresistance.

ii) Alternatively, reduction in the liquid impingement attack can also be accomplished by

increasing the cross-section of flow passages of distribution tray. Increasing the cross-

section of flow passage will reduce the flow velocity, thus reducing the intensity of

impingement attack.

iii) The erosion corrosion of bottom row tubes can be controlled by improving the venting

system of the evaporation chamber. The pockets of non-vented non-condensable

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vapors/gases which were responsible for erosion corrosion should be avoided by

redesigning the venting ports.

REFERENCES

1. Tuthill, A.H., Guidelines for the use of copper alloys in seawater, Material

Performance, Sept. 1987, PP 12-22.

2. North,R.F and M.J.Prayor, Corr. Sci.,10,1970, p-297.

3. Davis,J.R., J.D.Destafani and G.M. Crankovic, Eds., Metals Hand

Book, Vol. 13, 9th Edition, 1987, Metals Park, Ohio, PP 7-37.

4. Narain, S and S.Asad, Corrosion Problems in Low Temperature Desalination Units,Acom, Avesta Sheffield, Sweden,2,1993, p-4.

5. Desalination Technology, King Abdulaziz University, Jeddah, Short Course, 18-30

March 1980, p-48.

6. Al-Sum, E.A., S.Aziz,. A. Al-Radif, M. Samir, and O. Heikal, Vapor side corrosion

of copper base condenser tubes of MSF desalination plants of Abu Dhabi,

Desalination, 97, 1994 , PP 109-119.

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Figure 1. Photographs of the failed portions of condenser tubes in the asreceived condition.

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Figure 4. Close up view of failed portions of the evaporator tubes. a) Tube D-03showing few surface depression marks. b) Tube D-04 showing rupture edgesdue to erosive thinning.

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Figure 5. Photomicrographs of failed condenser tubes a) Microstructureof tube D-03, 200 X. b) Microstructure of tube D-04, 200 X.

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Figure 6. SEM Photomicrographs of failed portion of condenser tube D-03.a) Note the decoration of oxide particlea at the surface irregularities.b) Photomicrograph at the perforation region.

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Figure 7. SEM Photomicrograph near the fracture edge of evaporator tubeD-04 showing erosion marks pointing towards the rupture edge.

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Investigation on the Failure of Al-Wajh Multieffect Desal Condenser Tubes

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