Material selection of desalination plants
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Material selection of desalination plants

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Material selection of desalination plants

Material selection of desalination plants

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    Material selection of desalination plants Material selection of desalination plants Document Transcript

    • Contents1 Introduction .......................................................................................................................................... 32 Corrosion in desalination plants ........................................................................................................... 33 Vapor-Space Corrosion ......................................................................................................................... 34 Corrosion in Flash Champers ................................................................................................................ 4 4.1 The interesting features of the corrosion of flash chamber (mild steel) are as follows: .............. 4 4.2 The major causes of the corrosion damage are: .......................................................................... 51 Abstract ...................................................................................................................................................... 55 Historical review ................................................................................................................................... 66 Operational experience analysis ........................................................................................................... 77 Development of the technology and future prospects ...................................................................... 118 Corrosion in desalination plant ........................................................................................................... 14 8.1 Type of corrosion ........................................................................................................................ 14 8.1.1 Cavitations and Impingement ............................................................................................. 14 8.1.2 Crevice Corrosion ................................................................................................................ 15 8.1.3 Erosion Corrosion ................................................................................................................ 16 8.2 ........................................................................................................................................................... 16 8.2.1 Environmental Cracking ...................................................................................................... 179 Copper alloy ........................................................................................................................................ 18 9.1 Introduction ................................................................................................................................ 18 9.1.1 Specifications, Properties and Availability .......................................................................... 18 9.1.2 Resistance to Corrosion and Bio-Fouling ............................................................................ 21 9.1.3 Sea Water Intakes ............................................................................................................... 2410 Materials for Heat Exchanger Tubes ............................................................................................... 25 10.1 Introduction ................................................................................................................................ 25
    • 10.2 Design Requirements .................................................................................................................. 26 10.3 The Model ................................................................................................................................... 26 10.4 The Selection ............................................................................................................................... 28 10.5 Results ......................................................................................................................................... 30 10.6 PostScript .................................................................................................................................... 3111 Materials in Seawater Reverse Osmosis (SWRO) Plants ................................................................. 32 11.1 Use of Superior Materials ........................................................................................................... 34 11.1.1 Flash Chambers ................................................................................................................... 34 11.1.2 Heat Exchangers.................................................................................................................. 34 11.1.3 Relevance of Corrosion Research in the Material Selection ............................................... 34 11.1.4 Venting System ................................................................................................................... 35 11.1.5 Pumps.................................................................................................................................. 36 11.1.6 Pipings ................................................................................................................................. 3712 Conclusion ....................................................................................................................................... 37
    • 1 Introduction To build desalination plant needs a huge study of material metals, composites and non-metals which not just meet the requirements of the design and operation but also to adapt thenature of the environment that surround the plants. One important subject to think aboutduring selecting the proper material for constructing for specific unit or component is corrosioncharacteristics. Desalination power plant usually exposed to different and varied types ofenvironment such as seawater, seawater-air and salt-air aerosols, corrosive gases, very fast orextremely slow moving liquids, particulates contained in high velocity fluids or deposit-formingliquids all of them create a number of corrosion related problems.2 Corrosion in desalination plants Seawater desalination plant involve a lot of corrosion because of the operation inenvironment that doesn’t forgive that consist of seawater, seawater-air andsalt-air aerosols, corrosive gases, very fast or slow moving liquids, particulates containedin high velocity fluids or deposit forming liquids.3 Vapor-Space Corrosion In MSF plants vapor space conditions are less well controlled and severe corrosion hasbeen observed in both acid and additive dosed plants at rates well in excess of thedesigned corrosion allowance. Apart from water vapor which is always present,incondensable gases evolved from the flashing brine will be present. These gases aremainly CO2, O2, and N2. In some cases H2S and NH3 also would be present if seawater
    • feed to the plant is polluted with decomposing organic materials.4 Corrosion in Flash ChampersFigure 1, Corrosion of a flash chamber (3rd stage). One of the most familiar construction materials for flash chapmers is the carbon steel. It isused as such or cladded with stainless steel or Cu-Ni in early or all the stages. Epoxy coating hasalso been used. Flash chambers are subjected to severe corrosion and potential metal failures.The role of oxygen in the corrosion of metals of construction in MSF plants in general andevaporators in particular is quite complex.4.1 The interesting features of the corrosion of flash chamber (mild steel) are as follows:Corrosion is maximum in the middle of the stages where the combined effects oftwo competing factors e.g. oxygen leakage and temperature are optimum.Corrosion is usually most severe on the interstage walls and often one wall ismuch more attacked than the other.
    • Corrosion product is usually black magnetic oxide, Fe3O4.The corrosion product is separated from the metal by a void. In case the disturbedsheets of corrosion products several mm in thickness fall away, an even metalsurface is left behind.In some plants, blockage of demisters by corrosion products has caused plantshut down.4.2 The major causes of the corrosion damage are:High velocity of the brine flow affecting floor.Violent brine flashing (impingement) and collapsing of the flashing vapors(cavitation) on the walls.High chloride contents of brine.High Cu content of recirculating brine.1 AbstractThe operational experience on the first generation of large MSF desalination plant has demonstratedthat the original expected life of these units has been largely exceeded. Several contracts for therehabilitation and upgrading of desalination units installed 20 years ago have been recently awarded,aiming at extending the life of these units by a further 15 years. Developments in materials technologyhave resulted in the adoption of nobler materials, and it is expected that the second generation of largeMSF desalination plants installed in the last 10 years will last for more than 30 years with minimummaintenance and minor overhauling. On this basis, it is assumed that a 40- to 50-year design life is areasonable target which can be obtained if the material selection is optimized in respect of theoperating conditions. The gradual emergence of the MED process in the market portion previouslybelonging to MSF technology suggests that an evaluation of the operating conditions and material
    • selection for MED plants can also grant an expected life of 50 years. The sharp influence of materialselection on plant cost has previously been demonstrated; therefore, the choice of “where” and “how”to invest in upgrading materials and the evaluation of the financial revenue, in term of extension ofplant life and reduction of maintenance, are key technical aspects for the future of desalination. Bycomparing the various operating conditions occurring in the desalination units and the impact oncorrosion/erosion of the materials used, this paper aims at giving guidelines that will allow materialselection to be optimized with respect to the plant costs.15 Historical reviewThe first generation of desalination plants installed in the Gulf from the 1960s through the 1980s usemainly carbon steel as a material for the evaporator shell and internals. Carbon steel is relativelyinexpensive, readily available and possesses engineering properties that have been understood andused for decades. Another feature of carbon steel that is largely understood is its tendency to corrode,and allowance has been made for this in the past by increasing the thickness and hence the weight ofthe components that are subject to a corrosive environment. Some significant changes have occurred inthe material selection specified for the second generation of desalination plants designed andconstructed one decade later due to the deeper understanding of the operating conditions occurringinside the evaporator and their consequences on the material selected. Some of the most significantchanges are summarized in Table 1.1 http://www.desline.com/articoli/4071.pdf
    • Table 1Change in material specifications and operational experienceComponent First-generation Second-generation Reasons specification specificationVent baffles Carbon steel Stainless steel Understanding of Typically AISI 316L corrosion induced by high concentration of CO2, O2 bromamine and incondensable gasesSupport plates Carbon steel Stainless steel Ditto Typical AISI 316LDeaerator Carbon steel Stainless steel Understanding of Typical corrosion induced by AISI 317 LN high oxygen and chloramine concentrationShell Carbon steel painted Stainless steel Maintenance AISI 316L reduction Cost effectInternals Carbon steel painted Stainless steel Ditto AISI 316LMake-up spray Carbon steel Duplex steel Understanding of thepipe Stainless steel DIN 1.4462 erosion phenomena induced by flashing inside the pipeThe development of stainless steels continues as an understanding of corrosion mechanisms and theassociated kinetics is gained. This has resulted in a wide range of alloys under th umbrella title of“stainless steels” being readily available. Specific grades of stainless steel may now be applied to counterparticular types of corrosion and/or erosion. Development of corrosion-resistant materials, however, isnot confined to stainless steel. A notable material finding application, particularly for tubing, is titanium.The erosion and corrosion resistance of titanium is well known in the power industry, and its applicationto desalination has resulted in significant reduction in tube weights as a thinner wall thickness is usedfor what is already a lighter material than steel.6 Operational experience analysisThe corrosion mechanism for carbon steel that is most often encountered in desalination plants is thatof general corrosion, whereby metal is removed from the surface of the exposed material, resulting in ageneral thinning. This is not the case with stainless steel where corrosion usually takes the form ofpitting. This results in very little metal loss but raises the possibility of localized penetration. Much of the
    • development of stainless steel is associated with establishing resistance to pitting in high chlorideenvironments. The adoption of stainless steel instead of carbon steel for evaporator and de-aeratorshell has caused, along with a general upgrading of the material, a reduction in the weights of theEvaporator, which is indicated in Fig. 1.Figure 2 Overall weight against capacityIt is difficult, however, to distinguish between the contribution given to the weight reduction by theadoption of stainless steel and the elimination of corrosion allowances and the refinement in thestructural design which permits lower thickness. The engineering properties of the selected materialscould also result in design changes, for instance tube support spacing.As can be seen from the graph indicated in Fig. 2
    • Figure 3Installation cost trend line during the last two decades.The price per installed gallon has decreased drastically in the last two decades, largely as a result ofeconomy of scale as unit outputs have increased significantly. However, over this period upgrading ofmaterial selection has also taken place, and market prices have not been substantially affected. As withall commodities, the unit price tends to fall with increased production and it would be expected thatcosts will fall if the demand for the materials Increases. Also in this regard its is difficult to evaluate thecontribution to the price reduction given bythe enhanced commercial competition or by the decrease in the cost of stainless steel; in any casethe second generation of desalination plants is expected to achieve a lifetime of 40 years withminor overhauling. Maintenance and overhauling of the desalination units also largely benefit from theadoption of stainless steel material, solid or clad for the evaporator shell. In particular, long and delicateoverhauling periods for touch-up or restoration of the paint in the evaporator stages can be actuallyavoided as indicated in Table 2.
    • Table 2Comparison of first and second generations of MSF First generation Second generation Carbon steel shell Stainless painted steel shellOverhauling time Long overhauling at Routinerequested frequent intervalsType of action Blasting, patch-work, Routinerequired priming and coatingCost involved High Routine Figure 4Effects of corrosion from hydrocarbons andThe possible causes for shortening the life of this generation ofplant or imposing heavy rehabilitation work lie in unforeseenevents such as erosion or impingement caused by debris orforeign matters and in the presence of highly corrosivecomponents in the raw seawater as a result of pollution oraccidents.Figs. 3 show the effect of tube-plate damage due toimpingement by debris and corrosion by hydrocarbons.Only proper plant monitoring and operation can avoid thesekinds of incidents. Further investment in material upgradingwould not result in additional plant security or longevity.
    • 7 Development of the technology and future prospectsFurther development of the material selection for MSF plants is related to the recent developmentsin the process thermodynamics and the economic competition with MED, which is the emerging processin the market. The MED process, in fact, allows a higher heat-exchange coefficient than MSF, andconsequently the heat exchange surface and the weight of this kind of evaporator are lower than for MSF.Furthermore, different from MSF where the process pattern and thermodynamics did not substantiallychangeover in the last 10 years, the MED process has room for further improving efficiency and reducingcosts by increasing operating temperature (still set at very low levels) and modifying operational patterns.Different operating conditions and process configurations result in a drastically different materialselection. Table 3 indicates the main differences of material selection as well as in the operatingconditions between the two processes. The different types of material can be, in part, related to thedifferent operating conditions in the plant. In this respect Table 4 summarizes the main differences.Table 3Comparison of MED and MSF material selection Typical material selection Typical material selection for MED plants for MSF plantsExchange tubes Titanium ASTM B338 Gr. Copper nickel 90/10 or 2;first 3 rows 66/30 high- temp.stages, Al brass remaining rows heat recovery section Al brass, low-temp. stages, heat recovery Section 66/30or titanium for heat rejection sectionTube plates Stainless steel AISI 316 L Solid CuNi 90 /10 or aluminum nickel bronzeShell Stainless steel AISI 316 L Solid or clad stainless steelWater boxes Not applicable Carbon steel Cu Ni 90 /10 cladSteam boxes Stainless steel Not applicableTable 4Comparison of MSF and MED operating conditions MED plant MSF plantTemperature Max 65°C Up to 114°COxygen content Chloroamine and Up to 600 ppb in high-temp. stages Less than 50 ppbbromoamine Possible because deareation takes With proper deareation bromoaminepresence place in the is released from deareator first stage and due to ejecto- to vent condenser compressionAcid cleaning Frequent RareThe reason for the quite high oxygen concentration in the MED process arises from the factthat no separate deareation takes place and the oxygen is released in the first stages. The concentration ofthe oxygen dissolved in the evaporating brine inside the MED shell is governed by Henry’s law, and ittends to be in equilibrium with the oxygen pressure in the vapour side. The area surrounding the spraynozzles can be considered as a flashing zone where flashing takes place as a result of a sudden pressuredifference. The oxygen concentration in the seawater at the spray nozzle entrance is in the range of 6 to 8ppm, depending on the raw seawater temperature. The more the flashing and evaporating brine flowstowards the bottom of the tube bundle, the more the oxygen concentration approaches the
    • equilibrium value. The vapour generated on the tube bundle acts as stripping steam for the makeup waterwhile the tube bundle provides a large, interfacial surface between the evaporating liquid and the gas sothat the equilibrium conditions are gradually reached on the bottom of the tube bundle. However, it islikely that the superior parts of the MED shell and tube bundle, far from equilibrium, are subject tooxygen concentrationin excess of 600 ppb. Fig. 4 shows the typical oxygen distribution in a multiple effect. The oxygendistribution differs from stage to stage because each stage has a different operating temperature andtherefore a different Henry’s coefficient for oxygen concentration. The operational experience in severalMSF desalination plants in the Gulf area has proven that the poor deareation and improper venting,resulting in the accumulation of stagnant pockets of oxygen underneath the vent channel, havebeen the reason for tube failure due to corrosion and excessive thinning, especially in the firststages. Another area subject to pitting corrosion is the upper area in MSF deareator where amoisture condensate rich in oxygen flows along the walls causing corrosion. Transferring thisexperience from MSF to MED, it appears that the venting baffles as well as the upper shell and tubebundle of MED plant could suffer from corrosion. Furthermore, hydrazine or similar ammoniacompounds inside the effect entrained inside the shell through the motive steam for the ejecto-compressorcan produce ammonia, which can stress corrosion crack the aluminium brass for the tube bundle. Ingeneral, the low temperature and the adoption of titanium in the upper rows would prevent the risk ofstress corrosion cracking the aluminum brass tube to which they are subject inthe presence of ammonia-producing compounds. The adoption of titanium for all tube bundles ortitanium plate exchangers, which are currently used in the market, would completely solve theproblem of stress-corrosion cracking.Figure 5 Typical oxygen distribution in a multiple effect
    • Oxygen concentration in excess of 500 ppb is a risk for pitting and stress corrosion cracking, which couldbe reduced by the adoption of higher grade stainless steel such as DIN 1.4462 or 254 AVESTA. Theadoption of higher grades of stainless steel could possibly be an alternative to improve the quality of thematerial selected for MED desalination plants and maintain costs at the desired levels. Cost reduction canbe achieved in this case by reducing the shell wall thickness thanks to the higher yield strength of duplexsteel. Fig. 5 shows the expected desalination plant weight against the production for a 2.5–5migd plant forMSF and MED options with a conventional stainless-steel solution and a duplex-steelSolution.Figure 6 Weight comparison of MED and MSF desalination plants with 316L or duplex steel.
    • 8 Corrosion in desalination plant8.1 Typ2e of corrosion8.1.1 Cavitations and ImpingementCavitation occurs when a fluids operational pressure drops below its vapor pressure causing gaspockets and bubbles to form and collapse. This can occur in what can be a rather explosive and dramaticfashion. In fact, this can actually produce steam at the suction of a pump in a matter of minutes. When aprocess fluid is supposed to be water in the 20-35°C range, this is entirely unacceptable. Additionally,this condition can form an airlock, which prevents any incoming fluid from offering cooling effects,further exacerbating the problem. The locations where this is most likely to occur, such as:  At the suction of a pump, especially if operating near the net positive suction head required (NPSHR)  At the discharge of a valve or regulator, especially when operating in a near-closed position  At other geometry-affected flow areas such as pipe elbows and expansions  Also, by processes incurring sudden expansion, which can lead to dramatic pressure dropsThis form of corrosion will eat out the volutes and impellers of centrifugal pumps with ultrapure wateras the fluid. It will eat valve seats. It will contribute to other forms of erosion corrosion, such as found inelbows and tees. Cavitation should be designed out by reducing hydrodynamic pressure gradients anddesigning to avoid pressure drops below the vapor pressure of the liquid and air ingress. The use ofresilient coatings and cathodic protection can also be considered as supplementary control methods.Figure illustrate picture for cavitations dentition on pump casing2 http://corrosion-doctors.org/Forms-cavitation/cavitation.htm
    • Figure 7Cavitation corrosion of a deaerator8.1.2 Crevice Corrosion3Crevice corrosion is a localized form of corrosion usually associated with a stagnant solution on themicro-environmental level. Such stagnant microenvironments tend to occur in crevices (shielded areas)such as those formed under gaskets, washers, insulation material, fastener heads, surface deposits,disbonded coatings, threads, lap joints and clamps. Crevice corrosion is initiated by changes in localchemistry within the crevice: a. Depletion of inhibitor in the crevice b. Depletion of oxygen in the crevice c. A shift to acid conditions in the crevice d. Build-up of aggressive ion species (e.g. chloride) in the crevice3 http://corrosion-doctors.org/Forms-crevice/Crevice.htm
    • Figure 8Full blown crevice in an otherwise very seawater resistant material.As oxygen diffusion into the crevice is restricted, a differential aeration cell tends to be set upbetween crevice (microenvironment) and the external surface (bulk environment). Thechronology of the aggravating factors leading to a full blown crevice is illustrated here. Thecathodic oxygen reduction reaction cannot be sustained in the crevice area, giving it an anodic characterin the concentration cell. This anodic imbalance can lead to the creation of highly corrosive micro-environmental conditions in the crevice, conducive to further metal dissolution. This results in theformation of an acidic micro-environment, together with a high chloride ion concentration.8.1.3 Erosion Corrosion4 8.2Erosion corrosion is acceleration in the rate of corrosion attack in metal due to the relative motion of acorrosive fluid and a metal surface. The increased turbulence caused by pitting on the internal surfacesof a tube can result in rapidly increasing erosion rates and eventually a leak. Erosion corrosion can alsobe aggravated by faulty workmanship. For example, burrs left at cut tube ends can upset smooth waterflow, cause localized turbulence and high flow velocities, resulting in erosion corrosion. A combinationof erosion and corrosion can lead to extremely high pitting rates4 http://corrosion-doctors.org/Forms-Erosion/erosion.htm
    • 8.2.1 Environmental Cracking5Environmental cracking refers to a corrosion cracking caused by a combination of conditions that canspecifically result in one of the following form of corrosion damage:  Stress Corrosion Cracking (SCC)  Corrosion fatigue  Hydrogen embrittlementStresses that cause environmental cracking arise from residual cold work, welding, grinding,thermal treatment, or may be externally applied during service and, to be effective, must betensile (as opposed to compressive).  Stress definition or stress variables o Mean stress o Maximum stress o Minimum stress o Constant load/constant strain o Strain rate o Plane stress/plane strain o Modes I, II, or III o Biaxial o Cyclic frequency o Wave shape  Stress origin o Intentional o Residual  Shearing, punching, cutting  Bending, crimping, riveting  Welding  Machining  Grinding o Produced by reacted products o Applied  Quenching  Thermal cycling  Thermal expansion  Vibration  Rotation  Bolting  Dead load  Pressure5 http://corrosion-doctors.org/Forms-EC/stresses.htm
    • 9 Copper alloy9.1 IntroductionCopper, the most noble of the metals in common use, has excellent resistance to corrosion in theatmosphere and in fresh water. In sea-water, the copper nickel alloys have superior resistance tocorrosion coupled with excellent anti-fouling properties. Copper cladding of wooden hulled warships,introduced by the Royal Navy in the 18th century to prevent damage by wood-boring insects and wormssuch as the teredo, was discovered to prevent biofouling by weed and molluscs. This meant that shipscould stay at sea for long periods without cleaning. Nelson’s successful blockade tactics and subsequentvictory at Trafalgar was partly due to the superior speed of his clean-hulled ship. The addition of nickelto copper improves its strength and durability and also the resistance to corrosion, erosion andcavitation in all natural waters including sea-water and brackish, treated or polluted waters. The alloysalso show excellent resistance to stress-corrosion cracking and corrosion fatigue. The added advantageof resistance to bio-fouling, gives a material ideal for application in marine and chemical environmentsfor ship and boat hulls, desalination plant, heat exchange equipment, sea-water and hydraulic pipelines,oil rigs and platforms, fish farming cages, sea-water intake screens, etc. The purpose of this publicationis to discuss typical applications for copper-nickel alloys and the reasons for their selection. The twomain alloys contain either 10 or 30% nickel, with iron and manganese additions as shown in Table 12,which lists typical international and national standards to which the materials may be ordered inwrought and cast forms.9.1.1 Specifications, Properties and AvailabilityThe copper-nickel alloys are single phased throughout the full range of compositions and manystandard alloys exist within this range, usually with small additions of other elements for specialpurposes. The two most popular of the copper rich alloys contain 10 or 30% of nickel. Some manganeseis invariably present in the commercial alloys as a deoxidant and desulphurizer; it improves workingcharacteristics and additionally contributes to corrosion resistance in seawater. Other elements whichmay be present singly or in combination are:Iron, added (up to about 2% ) to the alloys required for marine applications. It confersresistance to impingement attack by flowing sea-water. The initial development of the optimum
    • compositions of the copper-nickel-iron alloys in the 1930’s has been described by G. L. Bailey(see bibliography). This work was to meet naval requirements for improved corrosion-resistantmaterials for tubes, condensers and other applications involving contact with sea water.Throughout the publication the term “copper-nickel” refers in fact to copper-nickel-iron alloys.Chromium, can be used to replace some of the iron content and at one per cent or moreprovides higher strength. It is used in a newly-developed 30% nickel casting alloy (IN-768)*. Alow-chromium 16% nickel wrought alloy (C72200) † has been developed in the USA.Niobium, can be used as a hardening element in cast versions of both the 10% and 30% nickelalloys (in place of chromium). It also improves weldability of the cast alloys.Silicon, improves the casting characteristics of the copper-nickel alloys and is used inconjunction with either chromium or niobium.* INCO DesignationTin confers an improved resistance to atmospheric tarnishing and at the 2% level is used with9% nickel to produce the alloy C72500 †. This has useful spring properties and is used in theelectronics industry. It is not recommended for marine applications.
    • Table 1 – Application Standards for various Wrought and Cast ProductsTable 5 Application Standards for various Wrought and Cast ProductsTable 2 – Availability of Wrought Copper-Nickel AlloysTable 6 Availability of Wrought Copper-Nickel AlloysTable 7 90-10 copper-nickel-iron alloy. Mechanical properties
    • 9.1.2 Resistance to Corrosion and Bio-FoulingThe 90/10 and 70/30 alloys have excellent resistance to sea-water corrosion and bio-foulingwith some variations in the performance of the alloys under different conditions as shown inTable 5 and Table 6, for instance, the 90/10 alloy has the better bio-fouling resistance. In Table5 the corrosion resistance of the 90/10 and 70/30 alloys in heat exchangers and condensers iscompared and in Table 6 the relative resistance of various alloys to fouling in quiet sea-water. Ifwater velocity is accelerated above 1 m/sec, any slight bio-fouling on metal with good foulingresistance will be easily detached and swept away. On a material that does not have this goodfouling resistance, strongly adherent, marine organisms would continue to thrive and multiply.The effect of water velocity on fouling and corrosion rates of various metals is shown in Fig. 1which also shows the typical service design speeds for certain items of common equipment incontact with sea-water. The excellent corrosion resistance of 70/30 and 90/10 copper nickelalloys and their suitability for many applications can be seen. Some materials with apparentlybetter corrosion resistance may have disadvantages such as lack of resistance to bio-fouling,lack of availability in the forms required, or susceptibility to crevice corrosion. They may alsobe more expensive and therefore less cost-effective over the required service lifetime.Crevice corrosion can occur in components in sea-water when they are locally starved of oxygenat a joint or under attached bio-fouling. Table 7 shows the good tolerance of the copper-nickelalloys to this type of attack, giving these alloys advantages over other materials of equalcorrosion resistance.
    • The copper-nickel alloys have good corrosion resistance in the quiescent or stagnant conditionswhich may occur during the commissioning or overhaul of plant. Where plant is not being usedat design speeds some other materials may fail.The corrosion resistance of the alloys is due to the protective surface film formed when incontact with water. On initial immersion cuprous oxide is formed but complex changes occur insea water which research work is only now beginning to elucidate. At a flow rate of 0.6 m/s theequilibrium corrosion rate is an almost negligible 0.002 mm/year. Normally, design flow ratesof up to 3.5 m/s give a satisfactory safety factor for use in pipework systems. This figure makesallowance for the fact that local speeds may be higher at changes of direction, points ofdivergence, etc. If water velocity is excessive, it can cause vortices leading to impingementattack which can cause premature failure. Where surfaces in contact with water allow smoothflow, as in ships hulls, different design criteria apply.As mentioned, the fouling resistance is due to the copper ions at the surface, making itinhospitable to most marine organisms in slowly moving water. In static conditions there may besome deposition of chemical salts and biological slimes, possibly leading to some weaklyadherent fouling but such residues are easily detached from the metal’s corrosion resistantsurface, exposing a fresh, biocidally active surface.When first brought into use, care must be taken to allow copper-nickel alloys to form theirprotective corrosion resistant surface freely. Normally, this protective film will develop in six toeight weeks. Contact with other less noble metals or with cathodic protection systems must beavoided to ensure development of the corrosion resistant surface film and the non-foulingproperties.
    • Copper-nickel alloys do not suffer the stress-corrosion problems associated with some othermaterials.Table 8 Comparison or corrosion behaviour of CuNi10Fe and CuNi30Fe in seawaterEnvironmental Type of Service experienceconditions corrosion CuNi10Fe CuNi30FeClean seawater at Uniform, general 0.0025- 0.0025-velocities up to 1 m/s 0.025 mm/a 0.025 mm/aClean seawater at Impingement attack Satisfactory Satisfactoryvelocities up to 3.5 m/s *Polluted seawater Accelerated general Less resistant Preferred but not and pitting immuneEntrained sand in seawater Accelerated general Unsuitable, Use and erosion exceptin CuNi30Fe2Mn2 mild conditionsAccumulated deposits on Local attack Generally good Tendency to pitsurfaceHot spots due to local Local attack by Good Good but someoverheating denickelification failures in extreme conditionsCorrosion plus stress Stress corrosion Very resistant Very resistant(Vapour side conditions)Feedwater heaters working Exfoliation attack Resistant Susceptibleunder cyclic conditionsNon-condensable gases † Local attack and Highly Resistant Most resistant general thinningHydrogen sulphide in General attack Less Resistant Less Resistantdesalination plant* Local velocities caused by obstructions can be very high.† lf concentration of CO2 is extremely high, stainless steel may be better cholce.‡ Attack will increase in concentration or temperature.Table 6 – Fouling resistance of various alloys in quiet seawaterArbitrary Rating Scale ofFouling Resistance90-100 Best Copper90/10 copper-nickel alloy70-90 Good Brass and bronze50 Fair 70/30 copper-nickel alloy, aluminium bronzes, zinc10 Very Slight Nickel-copper alloy 400
    • 0 Least Carbon and low alloy steels, stainless steels, nickel-chromium-high molybdenum alloys TitaniumAbove 1 m/s (about 3 ft/sec or 1.8 knots) most fouling organisms have increasing difficulty in attachingthemselves and clinging to the surface unless already securely attached.(INCO)9.1.3 Sea Water IntakesSea water is frequently required in large quantities for cooling purposes. One of the problemsassociated with sea water intakes in marine- or land-based installations is the occurrence ofgross marine fouling of the entry. This may be of soft growth, barnacles or bivalves. Not onlycan this restrict the water flow but the marine fouling may be detached from time to time andcause blockages in heat exchangers or severe mechanical damage to pumps and valves.Injection of chemicals such as chlorine can be effective against marine fouling organisms.However, additions must be closely controlled to be effective and even so, may have adetrimental effect on the installation and the environment near the outflow. Storage of bulkchlorine can also be hazardous. Adequate control is possible during steady-state runningconditions but this becomes difficult during downtime when flow ceases.An alternative is to make intakes and intake screens of 90/10 copper-nickel which is resistant tofouling. The intake pipes themselves may be of copper-nickel or large concrete piping may beinternally lined either by casting the concrete round a formed pipe or by attaching sheet insidepipes by rivets or adhesive.Figure 9Comparison of zinc anode protected steel
    • Figure 10Large diameter concrete intake pipe10 Materials for Heat Exchanger Tubes10.1 IntroductionHeat exchangers take heat from one fluid and pass it to a second. The fire-tube array of a steamengine is a heat exchanger, taking heat from the hot combustion gases of the firebox andtransmitting it to the water in the boiler. The network of finned tubes in an air conditioner is aheat exchanger, taking heat from the air of the room and dumping it into the working fluid of theconditioner. The radiator in a car performs a similar function. A key element in all heatexchangers is the tube wall or membrane which separates the two fluids. It is required to transmitheat and there is frequently a large pressure difference across it.What are the best materials for making heat exchangers? Or, more specifically, what are the bestmaterials for a conduction-limited exchanger, with substantial pressure difference between thetwo fluids?
    • Figure 1 Schematic of a heat exchanger10.2 Design RequirementsFUNCTION Heat ExchangerOBJECTIVE Maximise heat flow per unit area, or per unit weightCONSTRAINTS (a) Support pressure difference p (b) Withstand chloride ions (c) Operating temperature up to 150°C (d) Low Cost10.2.1.1 Table 110.3 The ModelFirst, a little background on heat flow. Heat transfer from one fluid, through a membrane to a secondfluid, involves convective transfer from fluid 1 into the tube wall, conduction through the wall, andconvection again to transfer it into fluid 2. The heat flux q into the tube wall by convection (in units ofW/m2) is described by the heat transfer equation: (1)
    • in which h1 is the heat transfer coefficient and T1 is the temperature drop across the surfacefrom fluid 1 into the wall. Conduction is described by the conduction (or Fourier) equation (2)where  is the thermal conductivity of the wall (thickness t) and T12 is the temperaturedifference across it.It is helpful to think of the thermal resistance at surface 1 as 1/h1; that of surface 2 is 1/h2; andthat of the wall itself is t/. Then continuity of heat flux requires that the total resistance 1/U is (3)where U is called the total heat transfer coefficient . The heat flux from fluid 1 to fluid 2 is thengiven by (4)where T is the difference in temperature between the two working fluids. When one of thefluids is a gas, as in an air conditioner, heat transfer at the tube surface contributes most of theresistance; then fins are used to increase the surface area across which heat can be transferred.But when both working fluids are liquid, convective heat transfer is rapid and conductionthrough the wall dominates the thermal resistance. In this case simple tube elements are used,with their wall as thin as possible to maximise /t. We will consider the second case: conductionlimited heat transfer. Then 1/h1 and 1/h2 are negligible when compared with t/, and the heattransfer equation becomes (5)Consider, now, a heat exchanger with many tubes, each of radius r and wall thickness t with apressure difference p between the inside and outside. Our aim is to select a material tomaximise the total heat flow, while safely carrying the pressure difference p. The total heatflow is (6)where A is the total surface area of tubing.
    • This is the objective function. The constraint is that the wall thickness must be sufficient tosupport the pressure difference p. This requires that the stress in the wall remain below theelastic limit (yield strength) el (times a safety factor, which need not be included in thisanalysis): (7)Eliminating t between the last two equations gives (8)The heat flow per unit area of tube wall, Q/A, is maximised by maximising the performanceindex: (9)Four further considerations enter the selection. It is essential to choose a material that withstandscorrosion in the working fluids, which we take here to be water containing chloride ions (seawater). Cost will naturally be of concern. The maximum service temperature must be adequateand the material should be available as drawn tube.10.4 The SelectionA preliminary selection using the Generic filter is shown in Figures 2-4. The first chart is of elastic limitversus thermal conductivity, to allow us to maximise the value of M1. The second stage shows maximumservice temperature plotted as a bar-chart against resistance to sea-water, selecting materials with hightemperature resistance and high resistance to corrosion in sea-water. The last stage shows a bar chart ofmaterial cost against available forms, selecting cheap materials that are available as sheet or tube.
    • Figure 2 A Chart of Elastic Limit versus Thermal ConductivityFigure 3 A Bar-chart of Maximum Service Temperature versus Resistance to Sea-WaterCorrosion
    • Figure 4 Material Cost against Available FormsThe results of this selection are shown in Results Table 1, and they suggest that it may be worthtransferring the selection criteria to the coppers database to refine the search for a suitablematerial.10.5 Results Material (ranked by M1) Comment Have the best performance index, but relatively poor corrosionHigh Conductivity Coppers resistanceBrasses Again, relatively poor corrosion resistanceWrought Martensitic Stainless A good choice, but steel is more dense than copperSteelAluminium Bronzes An economical and practical choice
    • 10.5.1.1 Table 1 The Results of the Selection using the Generic Filter Material (ranked by M1) Comment90/10 Aluminium bronze, cold wkd (wrought) The aluminium bronzes are cheap92/8 Aluminium bronze, hard (wrought)93/7 Aluminium bronze, hard (wrought)95/5 Aluminium bronze, 1/2 hard (wrought)95/5 Aluminium bronze, hard (wrought)Nickel iron aluminium bronze, as extruded The Nickel iron aluminium bronzes are more corrosion(wrought) resistantNickel iron aluminium bronze, hot wkd(wrought)10.5.1.2 Table 2 The Results of the Selection by expanding the coppers branch10.6 PostScriptConduction may limit heat flow in theory, but unspeakable things go on inside heat exchangers. Seawater—often one of the working fluids—seethes with biofouling organisms which attach themselves totube walls and thrive, creating a layer of high thermal resistance and impeding fluid flow, like barnacleson a boat. Some materials are more resistant to biofouling than others; copper-nickel alloys areparticularly good, probably because the organisms dislike copper salts, even in very low concentrations.Otherwise the problem must be tackled by adding chemical inhibitors to the fluids, or by scraping—thetraditional winter pass-time of boat owners.It is sometimes important to minimise the weight of heat exchangers. Repeating the calculationto seek materials for the lightest heat exchanger gives, instead of M, the index:
    • (10)where  is the density of the materials from which the tubes are made. This is quite a differentindex—the strength varies to the power 2 because the weight depends on the wall thickness, andfrom Eqn 7 we know that wall thickness varies as 1/strength.Of course, all copper alloys have roughly the same density, so there is little point applying thisindex within the coppers in the database—but if copper alloys were compared with stainlesssteels at the Generic level, then it would be relevant.11 Materials in Seawater Reverse Osmosis (SWRO) Plants6In SWRO plants the operating environments are much less severe than in MSF plants. Forexample, the operating temperatures are much lower (below 50o C and non-condensable gases(e.g. CO2 , O2 , H2S NH3, Br2 etc) are not involved during seawater conversion. Austeniticstainless steels are the conventional materials used for high pressure piping leading to ROmembrane module, brine rejection pipe, product water outlet pipe and high pressure pumps. InSWRO plants, high velocities of the feed water and design do not encourage crevice formation.However, high pressure pipings (close to weld or heat affected zone) headers, connectors,flanges, seals of pumps and membrane containment vessels are prone to crevice corrosionattack in case stagnant conditions are developed or deposits are formed in the pipingsystem due to operational problems. The performance of materials in differentSWRO plants is given in Table 4.6http://www.swcc.gov.sa/files/assets/Research/Technical%20Papers/Corrosion/RELEVANCE%20OF%20CORROSION%20RESEARCH%20IN%20THE%20MATERIALSELECTION%20FOR.pdf
    • 11.1 Use of Superior Materials11.1.1 Flash ChambersThe use of stainless steel cladding or Cu/Ni cladding on mild steel in all the stages orin the first few stages and that of bare CS in the remaining stages appear to work verywell in evaporators of the desalination plants. Keeping in view the existing materials,the evaporators are designed with a high corrosion allowance therefore, the chambersare not much affected by general or uniform corrosion. However, localized corrosionproblems are quite frequent and troublesome and are the cause of concernDuring shut down, slow moving or stagnant high chloride brine, crevices (formedby salt and by scaling) and D.O. produce most favourable environment for initiatingand propagating corrosion process. Use of high alloy stainless steels, Ni-base alloysor titanium as construction materials could perhaps be the ideal solution to avoid corrosionin flash chambers. Even considering these materials as the best propositionfor long term trouble free operational life of the plant, the exorbitant cost would notpermit their use as constructional material.Use of cement-concrete as construction material appears to be quite promisingdue to its low cost, strength and durability. Erosion-corrosion resistance of thecementitious material under conditions of high flow corrosive brine and thermalstresses and airtightness of the structures are the problems which are to be lookedinto.11.1.2 Heat ExchangersThe predominant cause of the failure of heat exchanger tubes is circulating waterflow conditions resulting in tube inlet erosion/corrosion. This tube inlet damage isalmost located in the first 150 mm of the tube inlets and often results in perforation.11.1.3 Relevance of Corrosion Research in the Material SelectionCu-Ni (90/10 or 70/30), modified alloy Cu-Ni-Fe-Mn (66/30/2/2) or Ti are thematerials used for heat exchanger tubes. The choice of the most suitable materialdepends upon the system (brine heater, heat recovery or heat rejection) to be considered.Titanium though costly has replaced Cu/Ni alloys for heat rejection and brineheater tubes due to its excellent erosion-corrosion resistance and heat transferproperties. Titanium has the tendency to undergo crevice corrosion specially at hightemperatures. Addition of precious metals to titanium though make it slightly moreexpensive but increases its resistance to crevice corrosion tremendously. Alloys likeTi-0.15pd, Ti-005u-0.05Niand Ti-0.05pd-.3 Cu have excellent resistance towardscrevice corrosion. Tables 5 and 6 provide some physical properties data relevant toheat transfer tubes including failure rates and cost. Conductive plastic compositescontaining high aspect ratio fillers (brasses, Al, Ni-plated mica, stainless steel fibres)have thermal conductivity many magnitude higher than the case polymer. Thesematerials could be easier to fabricate, stronger, immune to corrosion and erosion,
    • possessing good heat transfer properties and should be cheaper than the traditional7heat exchanger materials.811.1.4 Venting SystemConventional materials like CS cladded with SS 304, 316 or Cu-Ni which were previouslyemployed in ejector body, nozzle and condenser, pipings showed pitting, SCC,metal loss or erosion, are now replaced by more superior materials like Incoloy 825,254SM0 or FRP.7 Saricimen, H., et al, “Performance of Austenitic StainlessSteels in MSF DesalinationPlant Flash Chambers in the Arabian Gull” Desalination October,1990.8 Malik, A.U., and Kutty, Mayan, A., “Corrosion and Materials Selection inDesalination Plants” Proc. SWCC Operation and Maintenance Conference,April 27-29, 1992 p. 304.
    • 11.1.5 PumpsDischarge columns and diffusers of brine recycle and blow down pumps which areusually made of Ni-resist showed SCC, fatigue or erosion due to porosity, lack ofstress relieving operation and poor casting. Replacements with more expensivematerials like SS316 or Duplex SS 2205 appear to overcome most of the erosioncorrosionproblems.
    • 11.1.6 PipingsCement concrete (CC), reinforced cement concrete (RCC) and prestressed concrete(PC) which have been conventional material for product water transmission pipelines showed frequent failures mainly due to rebar corrosion. The replacement of these materialswith fusion bonded epoxy (FBE) or urethane (FBU) appears to minimize the risk of failure.Coating of rebars with FBE or FBU is though costlier but provide protection against corrosion.12 Conclusion All in all, you can see that desalination plant has to adapt the nature of the environment.Environment won’t forgive the material of desalination plant. Therefore, you need to select the materialthat can adapt the environment and resist the corrosion. Corrosion is the main part of material selection. Itattacks almost every part of desalination plant. It results from stagnancy, deposition, dealloying, galvaniccouplation, dealloying and vapor space attack. The local attack (pitting, crevice) can be avoided inmost of the cases by minimizing dissolved oxygen level of brine and incondensable gases, properflushing and keeping an inert atmosphere during shut down, mechanical or chemical cleaning ofdeposits and maintaining C.P.