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Vapour side corrosion in a ME desalination plant at Jebel Dhana
DESALINATION ELSEVIER
Desalination 155 (2003) 67-78
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Vapour side corrosion in a ME desalination plant at Jebel Dhana M.E. El-Dahshan* , S. Al Malek and E. Al Katheeri Research Centre, Abu Dhabi Water and Electricity Authority (AD WEA), PO Box 54111, Abu Dhabi, UAE Tel. +971(2) 506-1383; Fax +971 (2) 562-7447; email: [email protected]
Received 8 July 2002; accepted 18 September 2002
Abstract Jebel Dhana, from the Abu Dhabi Water and Electricity Authority (ADWEA), operates two 2 MGD multi-effect thermo-compression distillers commissioned in 1996. Tube failures were first reported approximately 16 months after commissioning. The initial tube ftilures were from the upper rows, but more recent failures were apparently randomly distributed throughout the tube bundle. Ten tubes were removed and their corrosion morphology was examined macroscopically and microscopically. The corrosion products were analysed by X-ray, EDAX, and EPMA techniques. The 90/10 copper nickel alloy tubes were found to have suffered from vapour side corrosion; however, there was no sign of any copper oxide deposition, as has often been seen in failed tubes from MSF distillers. The failures took two forms: corrosion fatigue cracking, and pitting corrosion. The corrosion fatigue was in the form of a circumferential crack at a flat portion of the tube in the centre of a tube span. It was believed that the flat surface had developed as a result of fretting against a neighbouring tube, and had been worn away to the point of failure. Unfortunately, the position of the tubes in the tube bundle had not been identified during their removal, but it is suspected that the damaged tube had been in the top row, with the flattened portion directly below one of the two vaponr inlet pipes. The other tubes sampled showed pitting corrosion. Some of the pitting had perforated the tube walls whilst others showed pitting on the outer surface only. Some of the pits contained corrosion products whilst others were free of them. Thii pitting corrosion is attributed to the formation of carbonic acid in the presence of oxygen (air). A weak acid is formed due to the dissolution of CO, in the condensing water vapour. The gas concentration varies within the tube bundle, due to the vapour phase, allowing the gas concentration to increase in specific areas of the tube bundle, causing the non-uniformity of attack. There was no immediate tie-up between the pitting corrosion and the two steam inlet pipes due to the unknown position of the sampled tubes within the tube bundle. It is recommended that the oxygen level be reduced by ensuring that the distiller vessels are as air-tight as possible, thus reducing the tendency towards pitting corrosion. Keywords:
Copper nickel alloys; Pitting corrosion; Fatigue corrosion cracking; Fretting; ME distillers; Vapour side corrosion
*Corresponding author.
001 l-9164/03/$-
See front matter Q 2003 Elsevier Science B.V. All rights reserved
PII: SO0 11-9 164(03)00240-6
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1. Introduction
The basic element of a cell in a ME evaporation unit is a condenser cell, which is a multicomponent structure comprising tube plates, tube support plates and tubes. These convert the vapour into distillate. During operation, all parts of the condenser suffer corrosion in a variety of ways, the rate and type depending on the materials from which they are manufactured [l-15]. Most modern desalination plants now operate within the designed corrosion allowance and achieve their design life with normal operation and maintenance. Unfortunately, vapour space conditions are less well controlled. Numerous cases have been reported [ 16-191 where severe corrosion of specific areas has occurred which is in excess of the design corrosion allowance. While some of these units, built by various manufacturers, have been acid dosed, the majority have operated with non-acidic antisealants. Some authors suggested that while the type and location of the failures can be similar [ 15-181, the causes may widely differ [ 19-261. Unless this situation can be corrected, the life of a distillation plant is likely to be decided by the extent of vapour side corrosion. The first step to correct this situation is a thorough understanding of the failure mechanism of the present materials of construction and the correct diagnosis of the type of failure, together with the successful deciphering of the fine features of attack. The failure of the condenser tubes at Jebel Dhana ME distillers is described, and an analysis of the type of failure, correlating it to material, design and working conditions is given. 2. Case description During a scheduled maintenance, it was found that the leaking tubes were from the final condenser section of the distiller. This section
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consists of a conventional condenser with seawater flowing through the tubes and the vapour from the distiller condensing on the outside ofthe tubes. The vapour is generated in the final effect of the multi-effect distiller. In this particular distiller a large thermocompressor is used to increase the efficiency of the distiller, but some residual heat has to be rejected to keep the system in equilibrium. It is this extra waste heat, in the form of vapour, which is condensed in the problem condenser. A general layout of the condenser is shown in Fig. 1, with a diagram of the tube plate in Fig. 2, and the tube fitting system in Fig. 3. The lowpressure vapour, at approximately 49OC, enters through two inlet pipes on the top of the vessel. Non-condensable gases and some vapour are withdrawn from the under cooler section on the side of the bundle and are taken to the ejector system. Some technical information of the plants is presented in Table 1. It was stated that the initial tube failures had appeared after approximately 16 months of operation and were located in the top tube row. Table 1 Technical information about the Jebel Dhanadesalination units Description
Value
Nominal capacity, MGD
2
No of similar units on site
2
Year of commissioning Steam pressure, bar Steam flow, t/h Vapour temp. in 1st effect, “C Vapour temp. in condenser, “C Vapour pressure in condenser, m bar
1996 23 49.5-53 62 49 120
Seawater temp., “C Distillate flow, t/h
30 365-370
Distillate conductivity, ps.cm Performance ratio
5-10 7
M.E. El-Dahshan et al. /Desalination 155 (2003) 67-78
-
69
_~~I.-
Fig. 1. General arrangement of condenser vessels.
Later failures were randomly positioned throughout the bundle with an indication that the next group of failures would be near the vent exhaust position, but more recently they have been randomly positioned in the bundle. Unlike the main evaporative effects, a failure of a condenser tube allowed the seawater to leak into the vapour side of the tube bundle and resulted in an increase in the condensate (distillate) conductivity. In this specific design, all vented vapour from the evaporative effects is vented to the condenser, and thereafter directly to the ejector system. There were no flow control valves or flow limiting devices in the pipe between the condenser and the ejector system, and the ejectors seemed to be operating satisfactorily. Fig. 2. Tube sheet layout.
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I
._,_i
i
/ 16
b”‘.”
EIanumaLEHDlH
Fig. 3. Details of tubes and expansion allowances.
3. Failed tube examinations
The failed tubes were examined visually as well as through the use of an optical microscope. The tube material was analysed by X-ray to compare the nominal compositions with standard specifications. Sections of samples from the failed tubes were mounted and ground using emery paper beginning with coarse and finishing with fine. This was then followed by polishing with diamond paste and examination under an optical microscope. The composition of the corrosion products as well as the base metal were determined by energy dispersive x-ray analysis (EDAX). This technique aims to identify elements such as iron, copper, nickel, chlorine, bromine, sulphur, etc. 4. Results The failed tubes showed two different types of corrosion, namely pitting corrosion and corrosion
Fig. 4. The general features of pitting corrosion distillers tubes, at low magnification (0.2~).
of
fatigue cracking. Fig. 4 illustrates the general features of the first type, which is clearly a form of pitting corrosion. Some of the pits penetrate the entire thickness of the tube wall, as shown in Fig. 5, whilst other pits have penetrated most of the tube wall, as shown in Fig. 6. In some cases the pitting was not so severe, causing only surface roughness. Some of pits are free from corrosion products, as shown in Fig. 6; other pits contained corrosion products as shown in Fig. 7. The composition of the corrosion products was determined by EDAX and electron probe microanalysis (EPMA). Fig. 8 shows the elemental distribution of the corrosion products inside one of the pits analysed by EDAX. There
M.E. El-Dahshan et al. /Desalination 155 (2003) 67-78
Fig. 5. Pitting has perforated the whole tube wall.
j
Fig. 6. Cross-section of a part of the corroded tube showing the deep penetration of the pitting.
Fig. 7. Cross-section corrosion products.
of one of the pits containing
is evidence of the presence of chlorine in the corrosion product deposits, particularly outside the pits.
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The second type of damage or failure involved circumferential cracks which appeared in a number of tubes thought to be from the upper edge of the tube bundle. One particular tube sample cut from the parent tube was approximately 1020 mm long, 16 mm in outer diameter and a l-mm wall thickness. The mark of a tube support plate was 20 mm from one end of the tube section. The cracks were located on a flattened portion of the tube wall. The shape of the flat part is shown in Fig. 9, and at higher magnification in Fig. 10. The most prominent feature of this particular sample was the flat portion on one side of the tube. This portion started 245 mm from the tube support plate mark, was tapered over a length of 480 mm from the original curved outer surface, and was followed by a normal cylindrical section of 275 mm to the end of the sample tube. The flat section had thinned to such an extent that a small square piece of the tube, exactly in the centre of the flat section, had become detached, while the tube wall beside this missing portion was paper thin. Adjacent to the missing section, a small crack was apparent. At its maximum width the flat surface was 10 mm wide. At this point, the tube diameter was 14.6 mm from flat to outer surface, while from side to side at the centre of the flattened section it was 16.3 mm - slightly more than the nominal tube diameter. These measurements would indicate that the tube was slightly oval at its midpoint. There was a secondary flat section at approximately 60” to the main flat section. This started 370 mm from the tube support plate mark, and extended for approximately 220 mm, being 6 mm wide at its maximum. On the opposite side of the tube from the main flat section, there was some pitting observed. This started 50 mm from the tube support plate and extended for 50 mm, followed by a relatively undamaged portion of 340 mm and a further pitted section extending to 330 mm. The remainder of the tube was clear of pitting.
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Fig, 8. EDAX of the failed condenser tube showing the composition of the corrosion products at the bottom of the pit.
Fig. 9. The shape of the flattened tubes, showing as well the fatigue cracks, particularly in the middle tube.
X-ray analysis indicated that the tubes were not 70/30 or 90/l 0, the standard copper-nickel tubes, but that the combination is 84.2% Cu, 11.2% Ni, 1.3% Fe, 0.78% Mn, and 0.84% Al.
5. Discussion A consideration of the materials used for the condenser tubes of these distillers reveals that these are of standard quality, commonly used for
Fig. 10. Close-up of the corrosion-fatigue crack (of the circumferential type).
such purposes. The actual alloy would appear to be between the common 90/10 and 70/30 alloys. As such its appropriateness has been repeatedly confirmed by experience all over the world. The
M.E. El-Dahshan et al. /Desalination 155 (2003) 67-78
failure of the Cu-10% Ni alloy tubes of the condenser cannot therefore be attributed to the choice of improper metals alone. The tube failures took place on the vapour side, which is of particular interest. The failures have been proven to be of two distinct types, namely pitting corrosion and corrosion fatigue cracking. The failure was not in any way due to droplet erosion. If this type of attack was operative, it whould have been confined to the first 2-3 tube rows. These would act as a protective wall for the inner tubes while letting the vapour pass with as little pressure loss as possible [ 11. Since the tube failures were random in the bundles and not restricted to the top tube rows, erosion, and/or impingement is excluded. The more common type of corrosion observed on the failed tubes was pitting corrosion. This is an extremely localised attack that can result in holes in metal. These holes can be small or large in diameter, but in most cases, they are relatively small. The pits reported in this condenser are in various forms: either isolated, or so close together that they look like a rough surface. Pitting takes place in two steps, initiation and propagation. In order to initiate pitting, a local cell has to be formed. It is believed in this case that the lack of homogeneity at the metalcorrosive interface caused the formation of the local cell. The local cell formation could be due to a local breakdown of the passive (protective) film on the metal, resulting in small anodic sites. Tuthill and Horbberg [26,27] investigated the vapour side tube corrosion, suggesting that the passive film will be dissolved if the pH drops to around 5.5. The surrounding passive surface acts as a cathode and a cell with unfavourable area rates, even though the same metal or alloy is involved. This results in severe pitting. For these reasons, stainless steel and Cu-Ni alloys, depending on passivity for corrosion resistance, are especially susceptible to pitting attack.
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Following the formation of the pit, it will autocatallytically propagate, with the small hole acting as a crevice or stagnant area. In many cases, the rate of penetration increases with time as the hole gets deeper because the crevice becomes more and more effective. Accumulation of acidic-soluble (ions) or soluble-corrosion products, as shown in Fig. 7, are produced in the pit, also enhancing the concentration cell effect. Another reason for the intensity of pitting is that the pit site corrodes and acts as a sacrificial anode, thus protecting the surrounding area from corrosion. Usually the fewer the pits on a surface, the greater is the intensity of attack. Pitting corrosion failures are always associated with a corrosive agent. It is believed the agent in this case is oxygenated carbonic acid, which is formed through the re-dissolving and ionisation of carbon dioxide. On the vapour side there are a number of incondensable gases, mainly CO,, oxygen and nitrogen, and in some cases hydrogen sulphide and ammonia if seawater feed to the plant is polluted by decomposing organic materials. In this instance, H,S and NH, were not thought to be a problem, but additional CO, is produced by the breakdown of the bicarbonate in the seawater when it is heated as part of the distillation process. In considering the reasons for this particular tube corrosion problem, a fundamental difference between a multi-stage flash (MSF) distiller and a ME distiller should be taken into account. A MSF distiller is also a multi-stage deaerator. In each flashing stage, dissolved gas will be liberated and will travel with the vapour to the tube bundle where it can be vented to the ejector system. In addition, many MSF distillers are fitted with a separate deaerator, which reduces the gas concentration of the fresh seawater makeup (feed) directly. Even without this deaerator, make-up sprayed directly into the last stage will deaerate due to both the low absolute pressure in the stage and the considerable amount of scrubbing steam (around 50 t/h in a medium-sized
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distiller). This will carry the noncondensable gas to the vent extraction point and the ejectors. The deaerator will not extract the CO, from the bicarbonate due to heating as this occurs after the deaerator. While any residual oxygen (air) in an MSF plant will be liberated in the first stage and taken directly to the ejector, studies by Genthner and associates [28] show that the CO, release is more gradual down through the stages, but it is eventually vented off to the ejectors [29]. There are many documented cases of tube failures in MSF distillers due to the vapour side corrosion caused by stagnant pockets of 0, and CO, (gases). In most cases, these failures are due to poor tube bundle design which has allowed small pockets of gas, generally remote from the vent point, to remain and re-dissolve into the falling condensate, forming dilute carbonic acid as shown in the following reaction: CO, + H,O = H&O,
(1)
Shams El Din et al. [30] suggest that corrosion proceeds by means of the following steps: The ionization of copper atoms releases two electrons per atom and produces copper ions at the surface (anodic reaction). The free electrons are consumed in the reduction of oxygen (cathodic reaction) with the formation of 40H _ ions at cathodic sites. These unite with Cu’+ions to form CuO. The oxide readily dissolves in CO, loaded water in an acid-base reaction to release Cu ‘+ ions. The sequence of reaction is represented as Eqs. (2)-(5): 2Cu = 2Cu” + 4e-, anodic reaction
(2)
0, = 2H,O + 4e- = 40H-, cathodic reaction
(3)
2Cu2’+ 40H- = 2CuO +2H,O, oxide formation (4) 2CuO + 4H2C0, = 2Cu2++ 2H,O + 4HCO; oxide dissolution (5)
By comparison, the ME design at Jebel Dhana does not have a deaerator and does not send the non-condensables directly to the vent system. The make-up, with the full seawater dissolved gas concentration, is injected directly into each effect. As it has some feed heating, there will be CO, present from both the natural content in the seawater and from bicarbonate breakdown, although this latter will be considerably less than in an MSF distiller which operates over a large temperature range. There will therefore be quantities of 0 2 and CO, in each effect. These gases will eventually be vented down to concentrate in the condenser vessel before being finally vented off to the ejector system. As in the MSF distiller, there is the chance that pockets of stagnant gas in the condenser, generally beside the tube support plates or remote from the vent points, will increase the rate of corrosion in some areas, while other areas are relatively free from corrosion. In the case of acid-dosed distillers, most of the carbon dioxide containing compounds present in the seawater react with the acid to release the CO,, which is then extracted by a separate degassing column, In additive-dosed ME desalination plants, in addition to water vapours, noncondensable gases such as CO,, O,, Cl, and N, evolve by the injection of the fresh make-up into each effect. As in the MSF designs, if the evolved gases are trapped somewhere inside the system, due to the fact that in some cases the ventilation system is not completely effective, then pockets of stagnant gas are quite possible. In consequence, acidic vapour-side condensate will cause corrosion. The formation of the acid will result in mainly pitting corrosion, and to a lesser extent, the thinning of the tube. Herr-o and Port have stated that pitting is mainly related to the formation of carbonic acid, H,CO,, and this is assumed to be sufficiently strong to attack copper tubes [22].
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It is suspected that a significant number of the tube failures have occurred in the tube bank remote from the vent extraction point. It is noted that the problem is in a two-pass condenser. This design requires a water box division plate to separate the incoming and exiting seawater flows. This division plate precludes the installation of tubes in this central area. A number of tube rows have therefore to be left out and this creates a steam lane. The steam lane can create a zone of relatively high pressure vapour which can stop the vented gas in the non-vented side of the vessel from jumping across to the vent area, unless specific efforts are made to divert the steam from the non-vented side to the vent position. A solution is to have the vent take-offs to the ejectors from both sides of the condenser vessel. This is common practice in large multi-pass brine heaters. The presence of additional oxygen in the vessels may also be due to some leakage of air through the flanges, glands and the sight glasses of the cells, promoted by the vacuum inside the chambers. The extent of leakage will decide the magnitude of corrosion and account for differences in the behaviour of distillers of the same design, age and material. The second type of failure detected in the condenser tubes was concluded to be due to fretting of the tube against its neighbours This is supported by: ?? The maximum wear (thinning) had occurred in the centre of the tube span. ?? There was a secondary flat section at approximately 60” to the main flat which would have been caused by rubbing against another tube with the 60”tube pitching layout. ?? That the cracks were typical of a corrosion fatigue cracking, indicating that there had been some mechanical movement of the tube. As the tube wall was worn away, the stresses due to the internal pressure and the flexing of the tube would increase, eventually causing failure due to fatigue.
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The steam velocity at the inlet from the two steam pipes is of the order of 50 m/s. It is normal in heat exchange design to fit a wrapper plate at the steam inlet to avoid the steam from impinging directly against the top tube rows. The diagram of the condenser shell (Fig. 1) does not show any baffle plate. Metals and alloys will crack in the absence of corrosion if they are subject to high cyclic stresses for a number of cycles. The number of cycles for failure decreases as the stress is increased. Below certain stress levels the metal will last indefinitely. This level is termed the endurance limit of the material. When the material under such cyclic stress is exposed to a corrosive environment, the endurance limit of the material is sharply reduced. The main reason for this is that the corrosion causes pits, trenches or other types of notches which act as stress raisers. The corrosion fatigue in the present case refers to the cracks resulting from the combined effects of cyclic stress and corrosion. The source of cyclic stresses is the vibration of the tubes. The distance between the tube support plates is about normal for heat-exchanger design. The protective copper oxide film would have been broken down due to the mechanical wear against the adjacent tubes and to the corrosive conditions which caused the more wide-spread pitting corrosion. The cyclic stress generated would also contribute to failure as the protective coating was successively ruptured and reformed. Corrosion fatigue cracks propagate as ruptures around a tube. Unfortunately, no common industrial metal is immune to corrosion fatigue since some reduction of the metal’s resistance to cyclic stressing is observed if the metal is corroded, even mildly, by the environment in which stressing occurs. As cracks are frequently associated with pits, grooves, or other forms of stress concentrators, so it is believed that the formation and presence of pitting, as discussed before, will to a great extent, facilitate the formation of corrosion fatigue
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cracking, to the extent that it will fracture a thick wall of this ductile material. Corrosion fatigue cracking at mid-span regions has been reported in heat-exchanger tubes used under conditions of flow-induced vibration [3 1,321. The initiation of corrosion fatigue cracking would have started at some pit mechanical damage, and as the author mentioned, is commonly seen where sufficient cyclic stresses are operating in pitted regions. Numerous factors affect corrosion fatigue cracking significantly, but the main factors are stress and corrosiveness of the environment. The level of stress may be and generally is, much less than the yield strength of the metal. However, in general, higher stresses increase crack growth rate; likewise the corrosiveness of the environment. As mentioned before, crack initiation is always related to a localized site corrosion such as pitting. Such sites serve as stress concentrators, with the stress being high enough to rupture the protective coatings. It is also quite possible that the environment at a pit site is more corrosive due to the concentration of aggressive substances, and hence more likely to produce corrosion fatigue in the presence of cyclic stress. The pitting corro-sion present in this case will generally enhance the corrosion fatigue cracking. 6. Conclusions 1. X-ray analysis determined that the tubes are made of cupronic kel alloy containing 84%Ni- and 11.2% Cu, which contains appreciable amounts of iron and manganese, up to the recom-mended values. This is a satisfactory material for the work. 2. The condenser tubes have suffered from two distinct types of corrosion: pitting corrosion and corrosion fatigue. 3. The pitting corrosion is attributed to oxygenated carbonic acid, the source of which is the non-condensable gases inherent in the seawater. These non-condensable gases can re-
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dissolve in condensed water vapour to form a corrosive carbonic acid. 4. The normal copper oxide film formed on cupronickel tubes did not give the normal protection in this case as it was dissolved by the carbonic acid. 5. The EDAX analysis showed the presence of chlorine ions in the corrosion products. This could be due to earlier over-chlorination of the feed water to the desalination plant, but it may not have played a major role in the pitting corrosion. 6. Most of the gases come from the sea water make-up which is sprayed directly into the evaporative effects, and all this gas is passed to the condenser section prior to being evacuated to the ejector system. 7. The two-pass design creates a central steam lane which could result in non-condensable gases in the section remote from the vent point being unable to be satisfactorily extracted. 8. The fatigue cracks had originated at the flattened surface where the mechanical wear due to the fretting had worn the tube wall to virtual failure and had increased the stress on the remaining tube wall. 9. The corrosion fatigue cracks had occurred midway between the tube support plates. Corrosion fatigue requires a stress and a corrosive media. It is assumed that vibration, causing the stress, could have been initiated by the steam entry conditions. The corrosion rate could have been exacerbated by pollutants such as ammonia or hydrogen sulphide, in addition to the carbonic acid, although these additional pollutants were not detected. 10. Fatigue failures of heat-exchanger tubes are usually associated with design problems. When they occur, it is usually soon after commissioning, as in the present case where failure started in the early life of the plant. Vapour side corrosion of distiller tubes usually occurs after a lengthy period following the commissioning of
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77
the unit. This time is known as the induction period.
Thanks are also due to Mr. I.D. McGregor for his help during the collection of the data, as well as his interest in the work.
7. Mitigation of the failed tubes
References
The pitting corrosion of the tubes would not have taken place if 0, had been completely excluded from the process. A make-up degasser would reduce the amount of oxygen injected into the distiller. Also, as all of the evaporator vessels are operating under vacuum conditions, it is important to reduce the air leakage rate into the distillers. In an ideal case, it is recommended that following every shutdown or overhaul, the system should be subjected to a general air leak test and any suspected areas of leaks should be tightened accordingly. To do this properly is time-consuming and increases the unavailability rate. An ultrasonic leak detector is useful in detecting small leaks when the plant is in service, and its procurement and application are strongly recommended. Monitoring of oxygen and carbon dioxide levels in vapour spaces and in the vent pipes is recommended, but may be difficult to carry out in practice. The remedial measure for the fatigue failures and fretting damage is the use of a mid-span antivibration device. An alternative is to fit baffle plates above the tubes at the vapour inlets to stop direct impingement against the top rows of tubes, but care has to be taken not to transfer the tube vibration problem to the tubes at the edges of these baffle plates.
Acknowledgments
The authors wish to express their thanks to Mr. Abdul Rehem, Operation Engineer at RAPC Jebel Al Dhanna station, who assisted in the collection of the data included in this report.
111A.M. Shams El-Din, Corrosion of condenser in multistage flash evaporation distillers, in: Industrial Corrosion and Corrosion Control Technology, H.M. Shalaby et al., eds., Kuwait Institute for Scientific Research, Kuwait, 1996. 121T.H. Rogers, Marine Corrosion, Wiley, New York, 1986. J.A. Ellor and GA. Gehring, Corrosion, 86 (1986) [31 (Houston), Paper No. 225. [41 A.M. Beccaria, G, Poggi, P. Traverso and M. Ghiazza, Corr. Sci., 32 (1991) 1263. [51 H.L. Logan, The Stress Corrosion of Metals, Wiley, New York, 1996, p. 156. 161 A.M. Shams El-Din, WSIA J., 1 l(2) (1984) 1. [71 W.E. Heaton, Br. Corr. J., 13 (1987) 57. PI T. Syderger and U. Lotz, J Electrochem. Sot., 129 (1982) 276. [9] J.M. Popplewell and E.A. Thiele, Corrosion, 80 (Houston) (1980), Paper No. 30. 1101A.M. Shams El-Din, Corrosion trouble shooting in MSF distiller, Manual WED, Abu Dhabi, UAE, 1985. 1111H.P. Hack, Mater. Perf., 22 (1983) 24. WI T.S. Lee, R.M. Kain and J.W. Oldfield, Mater. Pet-f., 23 (1984) 9. v31 W.S.W. Lee, J.W. Oldfield and B. Todd, Desalination, 44 (1983) 209. [I41 J.W. Oldfield and B. Todd, Desalination, 38 (1981) 232. v51 J.C. Drake, N. Miyamura and J.A. Mathews, Proc. 6th Intern. Conf. on Fresh Water from the Sea, 1 (1978) 245. [16] J.W. Oldtield and B. Todd, Desalination, 66 (1987) 171. [17] A.M. Shams El-Din and R.A. Mohammed, Desalination, 69 (1989)3 13; 99 (1994) 73. [18] E.A. Al Sum, Sh.A. Aziz, A. Radif, S.M. Said and 0. Heikal, Proc. IDA and WRPC World Conference, Vol. 1, Yokohama, 1993, p. 501. [ 191 N. Jawai, T. Ozeki, K. Ohi, M. Amemiya and H. Itoh, PWR, Vol. 19, ASME, 1992.
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[20] M.E. El-Dahshan, Principles of metallic corrosion and oxidation, Saline Water Conversion Corporation, Riyadh, Saudi Arabia, 2002. [21] M.E. El-Dahshan, Non-Ferrous Alloys, Scientific Publication and Press, King Saud University, Riyadh, Saudi Arabia, 1997. [22] H.M. Herro and R.D. Port, Nalco Guide to Cooling Water System Failure Analysis, McGraw-Hill, New York, 1993. [23] A Working Party Report, Illustrated Case Histories of Marine Corrosion, European Federation of Corrosion, Institute of Metals, London, 1990. [24] Aqua-Chem, Materials failure identification manual for sea water desalination plants, ORNL/TMdOOl, Oak Ridge National Laboratory, Milwaukee, WI, 1976. [25] N. Asrar, A.V. Malik, S. Ahmed, A.H. Al-Sheikh, F. Al-Ghamdi and M.A. Al-Thobiety, Desalination, 116 (1998) 135.
[26] Tuthil Associates, Investigation of vapour side corrosion in 1st three stages of UNE/4-6 desalination plants, Report to Italimpianti, 1995. [27] D. Homberg, Private communication to Tuthill, as reported in Ref. 26. [28] K. Genthner, A. Gregorzewski and A. Seifert, Desalination, 93 (1993) 207. [29] H. Glade and K. Genthner, The problem of predicting CO, releases in MSF Distillers, IDA World Congress, Abu Dhabi, 1995. [30] A.M. Shams El-Din, R.A. Mohamed and A.E. El Sati, Vapour side corrosion in MSF condensers, WED, UAE, 1997. [31] W. Glaeser and I.G.Wright, in: Metals Handbook, 9th ed., Vol. 13, Corrosion, ASM, Ohio, 1987, p. 142. [32] R.S. Hill, Sources of vibration in tube banks due to vortex shedding, Proc. Inst. Mech. Engrs., Vol. 200, No. C4, 1986.
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Vapour side corrosion in a ME desalination plant at Jebel Dhana M.E. El-Dahshan* , S. Al Malek and E. Al Katheeri Research Centre, Abu Dhabi Water and Electricity Authority (AD WEA), PO Box 54111, Abu Dhabi, UAE Tel. +971(2) 506-1383; Fax +971 (2) 562-7447; email: [email protected]
Received 8 July 2002; accepted 18 September 2002
Abstract Jebel Dhana, from the Abu Dhabi Water and Electricity Authority (ADWEA), operates two 2 MGD multi-effect thermo-compression distillers commissioned in 1996. Tube failures were first reported approximately 16 months after commissioning. The initial tube ftilures were from the upper rows, but more recent failures were apparently randomly distributed throughout the tube bundle. Ten tubes were removed and their corrosion morphology was examined macroscopically and microscopically. The corrosion products were analysed by X-ray, EDAX, and EPMA techniques. The 90/10 copper nickel alloy tubes were found to have suffered from vapour side corrosion; however, there was no sign of any copper oxide deposition, as has often been seen in failed tubes from MSF distillers. The failures took two forms: corrosion fatigue cracking, and pitting corrosion. The corrosion fatigue was in the form of a circumferential crack at a flat portion of the tube in the centre of a tube span. It was believed that the flat surface had developed as a result of fretting against a neighbouring tube, and had been worn away to the point of failure. Unfortunately, the position of the tubes in the tube bundle had not been identified during their removal, but it is suspected that the damaged tube had been in the top row, with the flattened portion directly below one of the two vaponr inlet pipes. The other tubes sampled showed pitting corrosion. Some of the pitting had perforated the tube walls whilst others showed pitting on the outer surface only. Some of the pits contained corrosion products whilst others were free of them. Thii pitting corrosion is attributed to the formation of carbonic acid in the presence of oxygen (air). A weak acid is formed due to the dissolution of CO, in the condensing water vapour. The gas concentration varies within the tube bundle, due to the vapour phase, allowing the gas concentration to increase in specific areas of the tube bundle, causing the non-uniformity of attack. There was no immediate tie-up between the pitting corrosion and the two steam inlet pipes due to the unknown position of the sampled tubes within the tube bundle. It is recommended that the oxygen level be reduced by ensuring that the distiller vessels are as air-tight as possible, thus reducing the tendency towards pitting corrosion. Keywords:
Copper nickel alloys; Pitting corrosion; Fatigue corrosion cracking; Fretting; ME distillers; Vapour side corrosion
*Corresponding author.
001 l-9164/03/$-
See front matter Q 2003 Elsevier Science B.V. All rights reserved
PII: SO0 11-9 164(03)00240-6
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1. Introduction
The basic element of a cell in a ME evaporation unit is a condenser cell, which is a multicomponent structure comprising tube plates, tube support plates and tubes. These convert the vapour into distillate. During operation, all parts of the condenser suffer corrosion in a variety of ways, the rate and type depending on the materials from which they are manufactured [l-15]. Most modern desalination plants now operate within the designed corrosion allowance and achieve their design life with normal operation and maintenance. Unfortunately, vapour space conditions are less well controlled. Numerous cases have been reported [ 16-191 where severe corrosion of specific areas has occurred which is in excess of the design corrosion allowance. While some of these units, built by various manufacturers, have been acid dosed, the majority have operated with non-acidic antisealants. Some authors suggested that while the type and location of the failures can be similar [ 15-181, the causes may widely differ [ 19-261. Unless this situation can be corrected, the life of a distillation plant is likely to be decided by the extent of vapour side corrosion. The first step to correct this situation is a thorough understanding of the failure mechanism of the present materials of construction and the correct diagnosis of the type of failure, together with the successful deciphering of the fine features of attack. The failure of the condenser tubes at Jebel Dhana ME distillers is described, and an analysis of the type of failure, correlating it to material, design and working conditions is given. 2. Case description During a scheduled maintenance, it was found that the leaking tubes were from the final condenser section of the distiller. This section
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consists of a conventional condenser with seawater flowing through the tubes and the vapour from the distiller condensing on the outside ofthe tubes. The vapour is generated in the final effect of the multi-effect distiller. In this particular distiller a large thermocompressor is used to increase the efficiency of the distiller, but some residual heat has to be rejected to keep the system in equilibrium. It is this extra waste heat, in the form of vapour, which is condensed in the problem condenser. A general layout of the condenser is shown in Fig. 1, with a diagram of the tube plate in Fig. 2, and the tube fitting system in Fig. 3. The lowpressure vapour, at approximately 49OC, enters through two inlet pipes on the top of the vessel. Non-condensable gases and some vapour are withdrawn from the under cooler section on the side of the bundle and are taken to the ejector system. Some technical information of the plants is presented in Table 1. It was stated that the initial tube failures had appeared after approximately 16 months of operation and were located in the top tube row. Table 1 Technical information about the Jebel Dhanadesalination units Description
Value
Nominal capacity, MGD
2
No of similar units on site
2
Year of commissioning Steam pressure, bar Steam flow, t/h Vapour temp. in 1st effect, “C Vapour temp. in condenser, “C Vapour pressure in condenser, m bar
1996 23 49.5-53 62 49 120
Seawater temp., “C Distillate flow, t/h
30 365-370
Distillate conductivity, ps.cm Performance ratio
5-10 7
M.E. El-Dahshan et al. /Desalination 155 (2003) 67-78
-
69
_~~I.-
Fig. 1. General arrangement of condenser vessels.
Later failures were randomly positioned throughout the bundle with an indication that the next group of failures would be near the vent exhaust position, but more recently they have been randomly positioned in the bundle. Unlike the main evaporative effects, a failure of a condenser tube allowed the seawater to leak into the vapour side of the tube bundle and resulted in an increase in the condensate (distillate) conductivity. In this specific design, all vented vapour from the evaporative effects is vented to the condenser, and thereafter directly to the ejector system. There were no flow control valves or flow limiting devices in the pipe between the condenser and the ejector system, and the ejectors seemed to be operating satisfactorily. Fig. 2. Tube sheet layout.
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I
._,_i
i
/ 16
b”‘.”
EIanumaLEHDlH
Fig. 3. Details of tubes and expansion allowances.
3. Failed tube examinations
The failed tubes were examined visually as well as through the use of an optical microscope. The tube material was analysed by X-ray to compare the nominal compositions with standard specifications. Sections of samples from the failed tubes were mounted and ground using emery paper beginning with coarse and finishing with fine. This was then followed by polishing with diamond paste and examination under an optical microscope. The composition of the corrosion products as well as the base metal were determined by energy dispersive x-ray analysis (EDAX). This technique aims to identify elements such as iron, copper, nickel, chlorine, bromine, sulphur, etc. 4. Results The failed tubes showed two different types of corrosion, namely pitting corrosion and corrosion
Fig. 4. The general features of pitting corrosion distillers tubes, at low magnification (0.2~).
of
fatigue cracking. Fig. 4 illustrates the general features of the first type, which is clearly a form of pitting corrosion. Some of the pits penetrate the entire thickness of the tube wall, as shown in Fig. 5, whilst other pits have penetrated most of the tube wall, as shown in Fig. 6. In some cases the pitting was not so severe, causing only surface roughness. Some of pits are free from corrosion products, as shown in Fig. 6; other pits contained corrosion products as shown in Fig. 7. The composition of the corrosion products was determined by EDAX and electron probe microanalysis (EPMA). Fig. 8 shows the elemental distribution of the corrosion products inside one of the pits analysed by EDAX. There
M.E. El-Dahshan et al. /Desalination 155 (2003) 67-78
Fig. 5. Pitting has perforated the whole tube wall.
j
Fig. 6. Cross-section of a part of the corroded tube showing the deep penetration of the pitting.
Fig. 7. Cross-section corrosion products.
of one of the pits containing
is evidence of the presence of chlorine in the corrosion product deposits, particularly outside the pits.
71
The second type of damage or failure involved circumferential cracks which appeared in a number of tubes thought to be from the upper edge of the tube bundle. One particular tube sample cut from the parent tube was approximately 1020 mm long, 16 mm in outer diameter and a l-mm wall thickness. The mark of a tube support plate was 20 mm from one end of the tube section. The cracks were located on a flattened portion of the tube wall. The shape of the flat part is shown in Fig. 9, and at higher magnification in Fig. 10. The most prominent feature of this particular sample was the flat portion on one side of the tube. This portion started 245 mm from the tube support plate mark, was tapered over a length of 480 mm from the original curved outer surface, and was followed by a normal cylindrical section of 275 mm to the end of the sample tube. The flat section had thinned to such an extent that a small square piece of the tube, exactly in the centre of the flat section, had become detached, while the tube wall beside this missing portion was paper thin. Adjacent to the missing section, a small crack was apparent. At its maximum width the flat surface was 10 mm wide. At this point, the tube diameter was 14.6 mm from flat to outer surface, while from side to side at the centre of the flattened section it was 16.3 mm - slightly more than the nominal tube diameter. These measurements would indicate that the tube was slightly oval at its midpoint. There was a secondary flat section at approximately 60” to the main flat section. This started 370 mm from the tube support plate mark, and extended for approximately 220 mm, being 6 mm wide at its maximum. On the opposite side of the tube from the main flat section, there was some pitting observed. This started 50 mm from the tube support plate and extended for 50 mm, followed by a relatively undamaged portion of 340 mm and a further pitted section extending to 330 mm. The remainder of the tube was clear of pitting.
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Fig, 8. EDAX of the failed condenser tube showing the composition of the corrosion products at the bottom of the pit.
Fig. 9. The shape of the flattened tubes, showing as well the fatigue cracks, particularly in the middle tube.
X-ray analysis indicated that the tubes were not 70/30 or 90/l 0, the standard copper-nickel tubes, but that the combination is 84.2% Cu, 11.2% Ni, 1.3% Fe, 0.78% Mn, and 0.84% Al.
5. Discussion A consideration of the materials used for the condenser tubes of these distillers reveals that these are of standard quality, commonly used for
Fig. 10. Close-up of the corrosion-fatigue crack (of the circumferential type).
such purposes. The actual alloy would appear to be between the common 90/10 and 70/30 alloys. As such its appropriateness has been repeatedly confirmed by experience all over the world. The
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failure of the Cu-10% Ni alloy tubes of the condenser cannot therefore be attributed to the choice of improper metals alone. The tube failures took place on the vapour side, which is of particular interest. The failures have been proven to be of two distinct types, namely pitting corrosion and corrosion fatigue cracking. The failure was not in any way due to droplet erosion. If this type of attack was operative, it whould have been confined to the first 2-3 tube rows. These would act as a protective wall for the inner tubes while letting the vapour pass with as little pressure loss as possible [ 11. Since the tube failures were random in the bundles and not restricted to the top tube rows, erosion, and/or impingement is excluded. The more common type of corrosion observed on the failed tubes was pitting corrosion. This is an extremely localised attack that can result in holes in metal. These holes can be small or large in diameter, but in most cases, they are relatively small. The pits reported in this condenser are in various forms: either isolated, or so close together that they look like a rough surface. Pitting takes place in two steps, initiation and propagation. In order to initiate pitting, a local cell has to be formed. It is believed in this case that the lack of homogeneity at the metalcorrosive interface caused the formation of the local cell. The local cell formation could be due to a local breakdown of the passive (protective) film on the metal, resulting in small anodic sites. Tuthill and Horbberg [26,27] investigated the vapour side tube corrosion, suggesting that the passive film will be dissolved if the pH drops to around 5.5. The surrounding passive surface acts as a cathode and a cell with unfavourable area rates, even though the same metal or alloy is involved. This results in severe pitting. For these reasons, stainless steel and Cu-Ni alloys, depending on passivity for corrosion resistance, are especially susceptible to pitting attack.
73
Following the formation of the pit, it will autocatallytically propagate, with the small hole acting as a crevice or stagnant area. In many cases, the rate of penetration increases with time as the hole gets deeper because the crevice becomes more and more effective. Accumulation of acidic-soluble (ions) or soluble-corrosion products, as shown in Fig. 7, are produced in the pit, also enhancing the concentration cell effect. Another reason for the intensity of pitting is that the pit site corrodes and acts as a sacrificial anode, thus protecting the surrounding area from corrosion. Usually the fewer the pits on a surface, the greater is the intensity of attack. Pitting corrosion failures are always associated with a corrosive agent. It is believed the agent in this case is oxygenated carbonic acid, which is formed through the re-dissolving and ionisation of carbon dioxide. On the vapour side there are a number of incondensable gases, mainly CO,, oxygen and nitrogen, and in some cases hydrogen sulphide and ammonia if seawater feed to the plant is polluted by decomposing organic materials. In this instance, H,S and NH, were not thought to be a problem, but additional CO, is produced by the breakdown of the bicarbonate in the seawater when it is heated as part of the distillation process. In considering the reasons for this particular tube corrosion problem, a fundamental difference between a multi-stage flash (MSF) distiller and a ME distiller should be taken into account. A MSF distiller is also a multi-stage deaerator. In each flashing stage, dissolved gas will be liberated and will travel with the vapour to the tube bundle where it can be vented to the ejector system. In addition, many MSF distillers are fitted with a separate deaerator, which reduces the gas concentration of the fresh seawater makeup (feed) directly. Even without this deaerator, make-up sprayed directly into the last stage will deaerate due to both the low absolute pressure in the stage and the considerable amount of scrubbing steam (around 50 t/h in a medium-sized
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distiller). This will carry the noncondensable gas to the vent extraction point and the ejectors. The deaerator will not extract the CO, from the bicarbonate due to heating as this occurs after the deaerator. While any residual oxygen (air) in an MSF plant will be liberated in the first stage and taken directly to the ejector, studies by Genthner and associates [28] show that the CO, release is more gradual down through the stages, but it is eventually vented off to the ejectors [29]. There are many documented cases of tube failures in MSF distillers due to the vapour side corrosion caused by stagnant pockets of 0, and CO, (gases). In most cases, these failures are due to poor tube bundle design which has allowed small pockets of gas, generally remote from the vent point, to remain and re-dissolve into the falling condensate, forming dilute carbonic acid as shown in the following reaction: CO, + H,O = H&O,
(1)
Shams El Din et al. [30] suggest that corrosion proceeds by means of the following steps: The ionization of copper atoms releases two electrons per atom and produces copper ions at the surface (anodic reaction). The free electrons are consumed in the reduction of oxygen (cathodic reaction) with the formation of 40H _ ions at cathodic sites. These unite with Cu’+ions to form CuO. The oxide readily dissolves in CO, loaded water in an acid-base reaction to release Cu ‘+ ions. The sequence of reaction is represented as Eqs. (2)-(5): 2Cu = 2Cu” + 4e-, anodic reaction
(2)
0, = 2H,O + 4e- = 40H-, cathodic reaction
(3)
2Cu2’+ 40H- = 2CuO +2H,O, oxide formation (4) 2CuO + 4H2C0, = 2Cu2++ 2H,O + 4HCO; oxide dissolution (5)
By comparison, the ME design at Jebel Dhana does not have a deaerator and does not send the non-condensables directly to the vent system. The make-up, with the full seawater dissolved gas concentration, is injected directly into each effect. As it has some feed heating, there will be CO, present from both the natural content in the seawater and from bicarbonate breakdown, although this latter will be considerably less than in an MSF distiller which operates over a large temperature range. There will therefore be quantities of 0 2 and CO, in each effect. These gases will eventually be vented down to concentrate in the condenser vessel before being finally vented off to the ejector system. As in the MSF distiller, there is the chance that pockets of stagnant gas in the condenser, generally beside the tube support plates or remote from the vent points, will increase the rate of corrosion in some areas, while other areas are relatively free from corrosion. In the case of acid-dosed distillers, most of the carbon dioxide containing compounds present in the seawater react with the acid to release the CO,, which is then extracted by a separate degassing column, In additive-dosed ME desalination plants, in addition to water vapours, noncondensable gases such as CO,, O,, Cl, and N, evolve by the injection of the fresh make-up into each effect. As in the MSF designs, if the evolved gases are trapped somewhere inside the system, due to the fact that in some cases the ventilation system is not completely effective, then pockets of stagnant gas are quite possible. In consequence, acidic vapour-side condensate will cause corrosion. The formation of the acid will result in mainly pitting corrosion, and to a lesser extent, the thinning of the tube. Herr-o and Port have stated that pitting is mainly related to the formation of carbonic acid, H,CO,, and this is assumed to be sufficiently strong to attack copper tubes [22].
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It is suspected that a significant number of the tube failures have occurred in the tube bank remote from the vent extraction point. It is noted that the problem is in a two-pass condenser. This design requires a water box division plate to separate the incoming and exiting seawater flows. This division plate precludes the installation of tubes in this central area. A number of tube rows have therefore to be left out and this creates a steam lane. The steam lane can create a zone of relatively high pressure vapour which can stop the vented gas in the non-vented side of the vessel from jumping across to the vent area, unless specific efforts are made to divert the steam from the non-vented side to the vent position. A solution is to have the vent take-offs to the ejectors from both sides of the condenser vessel. This is common practice in large multi-pass brine heaters. The presence of additional oxygen in the vessels may also be due to some leakage of air through the flanges, glands and the sight glasses of the cells, promoted by the vacuum inside the chambers. The extent of leakage will decide the magnitude of corrosion and account for differences in the behaviour of distillers of the same design, age and material. The second type of failure detected in the condenser tubes was concluded to be due to fretting of the tube against its neighbours This is supported by: ?? The maximum wear (thinning) had occurred in the centre of the tube span. ?? There was a secondary flat section at approximately 60” to the main flat which would have been caused by rubbing against another tube with the 60”tube pitching layout. ?? That the cracks were typical of a corrosion fatigue cracking, indicating that there had been some mechanical movement of the tube. As the tube wall was worn away, the stresses due to the internal pressure and the flexing of the tube would increase, eventually causing failure due to fatigue.
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The steam velocity at the inlet from the two steam pipes is of the order of 50 m/s. It is normal in heat exchange design to fit a wrapper plate at the steam inlet to avoid the steam from impinging directly against the top tube rows. The diagram of the condenser shell (Fig. 1) does not show any baffle plate. Metals and alloys will crack in the absence of corrosion if they are subject to high cyclic stresses for a number of cycles. The number of cycles for failure decreases as the stress is increased. Below certain stress levels the metal will last indefinitely. This level is termed the endurance limit of the material. When the material under such cyclic stress is exposed to a corrosive environment, the endurance limit of the material is sharply reduced. The main reason for this is that the corrosion causes pits, trenches or other types of notches which act as stress raisers. The corrosion fatigue in the present case refers to the cracks resulting from the combined effects of cyclic stress and corrosion. The source of cyclic stresses is the vibration of the tubes. The distance between the tube support plates is about normal for heat-exchanger design. The protective copper oxide film would have been broken down due to the mechanical wear against the adjacent tubes and to the corrosive conditions which caused the more wide-spread pitting corrosion. The cyclic stress generated would also contribute to failure as the protective coating was successively ruptured and reformed. Corrosion fatigue cracks propagate as ruptures around a tube. Unfortunately, no common industrial metal is immune to corrosion fatigue since some reduction of the metal’s resistance to cyclic stressing is observed if the metal is corroded, even mildly, by the environment in which stressing occurs. As cracks are frequently associated with pits, grooves, or other forms of stress concentrators, so it is believed that the formation and presence of pitting, as discussed before, will to a great extent, facilitate the formation of corrosion fatigue
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cracking, to the extent that it will fracture a thick wall of this ductile material. Corrosion fatigue cracking at mid-span regions has been reported in heat-exchanger tubes used under conditions of flow-induced vibration [3 1,321. The initiation of corrosion fatigue cracking would have started at some pit mechanical damage, and as the author mentioned, is commonly seen where sufficient cyclic stresses are operating in pitted regions. Numerous factors affect corrosion fatigue cracking significantly, but the main factors are stress and corrosiveness of the environment. The level of stress may be and generally is, much less than the yield strength of the metal. However, in general, higher stresses increase crack growth rate; likewise the corrosiveness of the environment. As mentioned before, crack initiation is always related to a localized site corrosion such as pitting. Such sites serve as stress concentrators, with the stress being high enough to rupture the protective coatings. It is also quite possible that the environment at a pit site is more corrosive due to the concentration of aggressive substances, and hence more likely to produce corrosion fatigue in the presence of cyclic stress. The pitting corro-sion present in this case will generally enhance the corrosion fatigue cracking. 6. Conclusions 1. X-ray analysis determined that the tubes are made of cupronic kel alloy containing 84%Ni- and 11.2% Cu, which contains appreciable amounts of iron and manganese, up to the recom-mended values. This is a satisfactory material for the work. 2. The condenser tubes have suffered from two distinct types of corrosion: pitting corrosion and corrosion fatigue. 3. The pitting corrosion is attributed to oxygenated carbonic acid, the source of which is the non-condensable gases inherent in the seawater. These non-condensable gases can re-
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dissolve in condensed water vapour to form a corrosive carbonic acid. 4. The normal copper oxide film formed on cupronickel tubes did not give the normal protection in this case as it was dissolved by the carbonic acid. 5. The EDAX analysis showed the presence of chlorine ions in the corrosion products. This could be due to earlier over-chlorination of the feed water to the desalination plant, but it may not have played a major role in the pitting corrosion. 6. Most of the gases come from the sea water make-up which is sprayed directly into the evaporative effects, and all this gas is passed to the condenser section prior to being evacuated to the ejector system. 7. The two-pass design creates a central steam lane which could result in non-condensable gases in the section remote from the vent point being unable to be satisfactorily extracted. 8. The fatigue cracks had originated at the flattened surface where the mechanical wear due to the fretting had worn the tube wall to virtual failure and had increased the stress on the remaining tube wall. 9. The corrosion fatigue cracks had occurred midway between the tube support plates. Corrosion fatigue requires a stress and a corrosive media. It is assumed that vibration, causing the stress, could have been initiated by the steam entry conditions. The corrosion rate could have been exacerbated by pollutants such as ammonia or hydrogen sulphide, in addition to the carbonic acid, although these additional pollutants were not detected. 10. Fatigue failures of heat-exchanger tubes are usually associated with design problems. When they occur, it is usually soon after commissioning, as in the present case where failure started in the early life of the plant. Vapour side corrosion of distiller tubes usually occurs after a lengthy period following the commissioning of
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77
the unit. This time is known as the induction period.
Thanks are also due to Mr. I.D. McGregor for his help during the collection of the data, as well as his interest in the work.
7. Mitigation of the failed tubes
References
The pitting corrosion of the tubes would not have taken place if 0, had been completely excluded from the process. A make-up degasser would reduce the amount of oxygen injected into the distiller. Also, as all of the evaporator vessels are operating under vacuum conditions, it is important to reduce the air leakage rate into the distillers. In an ideal case, it is recommended that following every shutdown or overhaul, the system should be subjected to a general air leak test and any suspected areas of leaks should be tightened accordingly. To do this properly is time-consuming and increases the unavailability rate. An ultrasonic leak detector is useful in detecting small leaks when the plant is in service, and its procurement and application are strongly recommended. Monitoring of oxygen and carbon dioxide levels in vapour spaces and in the vent pipes is recommended, but may be difficult to carry out in practice. The remedial measure for the fatigue failures and fretting damage is the use of a mid-span antivibration device. An alternative is to fit baffle plates above the tubes at the vapour inlets to stop direct impingement against the top rows of tubes, but care has to be taken not to transfer the tube vibration problem to the tubes at the edges of these baffle plates.
Acknowledgments
The authors wish to express their thanks to Mr. Abdul Rehem, Operation Engineer at RAPC Jebel Al Dhanna station, who assisted in the collection of the data included in this report.
111A.M. Shams El-Din, Corrosion of condenser in multistage flash evaporation distillers, in: Industrial Corrosion and Corrosion Control Technology, H.M. Shalaby et al., eds., Kuwait Institute for Scientific Research, Kuwait, 1996. 121T.H. Rogers, Marine Corrosion, Wiley, New York, 1986. J.A. Ellor and GA. Gehring, Corrosion, 86 (1986) [31 (Houston), Paper No. 225. [41 A.M. Beccaria, G, Poggi, P. Traverso and M. Ghiazza, Corr. Sci., 32 (1991) 1263. [51 H.L. Logan, The Stress Corrosion of Metals, Wiley, New York, 1996, p. 156. 161 A.M. Shams El-Din, WSIA J., 1 l(2) (1984) 1. [71 W.E. Heaton, Br. Corr. J., 13 (1987) 57. PI T. Syderger and U. Lotz, J Electrochem. Sot., 129 (1982) 276. [9] J.M. Popplewell and E.A. Thiele, Corrosion, 80 (Houston) (1980), Paper No. 30. 1101A.M. Shams El-Din, Corrosion trouble shooting in MSF distiller, Manual WED, Abu Dhabi, UAE, 1985. 1111H.P. Hack, Mater. Perf., 22 (1983) 24. WI T.S. Lee, R.M. Kain and J.W. Oldfield, Mater. Pet-f., 23 (1984) 9. v31 W.S.W. Lee, J.W. Oldfield and B. Todd, Desalination, 44 (1983) 209. [I41 J.W. Oldfield and B. Todd, Desalination, 38 (1981) 232. v51 J.C. Drake, N. Miyamura and J.A. Mathews, Proc. 6th Intern. Conf. on Fresh Water from the Sea, 1 (1978) 245. [16] J.W. Oldtield and B. Todd, Desalination, 66 (1987) 171. [17] A.M. Shams El-Din and R.A. Mohammed, Desalination, 69 (1989)3 13; 99 (1994) 73. [18] E.A. Al Sum, Sh.A. Aziz, A. Radif, S.M. Said and 0. Heikal, Proc. IDA and WRPC World Conference, Vol. 1, Yokohama, 1993, p. 501. [ 191 N. Jawai, T. Ozeki, K. Ohi, M. Amemiya and H. Itoh, PWR, Vol. 19, ASME, 1992.
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[20] M.E. El-Dahshan, Principles of metallic corrosion and oxidation, Saline Water Conversion Corporation, Riyadh, Saudi Arabia, 2002. [21] M.E. El-Dahshan, Non-Ferrous Alloys, Scientific Publication and Press, King Saud University, Riyadh, Saudi Arabia, 1997. [22] H.M. Herro and R.D. Port, Nalco Guide to Cooling Water System Failure Analysis, McGraw-Hill, New York, 1993. [23] A Working Party Report, Illustrated Case Histories of Marine Corrosion, European Federation of Corrosion, Institute of Metals, London, 1990. [24] Aqua-Chem, Materials failure identification manual for sea water desalination plants, ORNL/TMdOOl, Oak Ridge National Laboratory, Milwaukee, WI, 1976. [25] N. Asrar, A.V. Malik, S. Ahmed, A.H. Al-Sheikh, F. Al-Ghamdi and M.A. Al-Thobiety, Desalination, 116 (1998) 135.
[26] Tuthil Associates, Investigation of vapour side corrosion in 1st three stages of UNE/4-6 desalination plants, Report to Italimpianti, 1995. [27] D. Homberg, Private communication to Tuthill, as reported in Ref. 26. [28] K. Genthner, A. Gregorzewski and A. Seifert, Desalination, 93 (1993) 207. [29] H. Glade and K. Genthner, The problem of predicting CO, releases in MSF Distillers, IDA World Congress, Abu Dhabi, 1995. [30] A.M. Shams El-Din, R.A. Mohamed and A.E. El Sati, Vapour side corrosion in MSF condensers, WED, UAE, 1997. [31] W. Glaeser and I.G.Wright, in: Metals Handbook, 9th ed., Vol. 13, Corrosion, ASM, Ohio, 1987, p. 142. [32] R.S. Hill, Sources of vibration in tube banks due to vortex shedding, Proc. Inst. Mech. Engrs., Vol. 200, No. C4, 1986.