Material selection of desalination plants

Contents 1 Introduction .......................................................................................................................................... 3

2 Corrosion in desalination plants ........................................................................................................... 3

3 Vapor-Space Corrosion ......................................................................................................................... 3

4 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: .......................................................................... 5

1 Abstract ...................................................................................................................................................... 5

5 Historical review ................................................................................................................................... 6

6 Operational experience analysis ........................................................................................................... 7

7 Development of the technology and future prospects ...................................................................... 11

8 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 ...................................................................................................... 17

9 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 ............................................................................................................... 24

10 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 .................................................................................................................................... 31

11 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 ................................................................................................................................. 37

12 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 the

nature of the environment that surround the plants. One important subject to think about

during selecting the proper material for constructing for specific unit or component is corrosion

characteristics. Desalination power plant usually exposed to different and varied types of

environment such as seawater, seawater-air and salt-air aerosols, corrosive gases, very fast or

extremely slow moving liquids, particulates contained in high velocity fluids or deposit-forming

liquids 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 in

environment that doesn’t forgive that consist of seawater, seawater-air and

salt-air aerosols, corrosive gases, very fast or slow moving liquids, particulates contained

in high velocity fluids or deposit forming liquids.

3 Vapor-Space Corrosion

In MSF plants vapor space conditions are less well controlled and severe corrosion has

been observed in both acid and additive dosed plants at rates well in excess of the

designed corrosion allowance. Apart from water vapor which is always present,

incondensable gases evolved from the flashing brine will be present. These gases are

mainly 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 Champers

Figure 1, Corrosion of a flash chamber (3rd stage).

One of the most familiar construction materials for flash chapmers is the carbon steel. It is

used as such or cladded with stainless steel or Cu-Ni in early or all the stages. Epoxy coating has

also 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 and

evaporators 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 of

two competing factors e.g. oxygen leakage and temperature are optimum.

Corrosion is usually most severe on the interstage walls and often one wall is

much 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 disturbed

sheets of corrosion products several mm in thickness fall away, an even metal

surface is left behind.

In some plants, blockage of demisters by corrosion products has caused plant

shut 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 Abstract

The operational experience on the first generation of large MSF desalination plant has demonstrated

that the original expected life of these units has been largely exceeded. Several contracts for the

rehabilitation 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 technology

have resulted in the adoption of nobler materials, and it is expected that the second generation of large

MSF desalination plants installed in the last 10 years will last for more than 30 years with minimum

maintenance and minor overhauling. On this basis, it is assumed that a 40- to 50-year design life is a

reasonable target which can be obtained if the material selection is optimized in respect of the

operating conditions. The gradual emergence of the MED process in the market portion previously

belonging 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 material

selection 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 of

plant life and reduction of maintenance, are key technical aspects for the future of desalination. By

comparing the various operating conditions occurring in the desalination units and the impact on

corrosion/erosion of the materials used, this paper aims at giving guidelines that will allow material

selection to be optimized with respect to the plant costs.1

5 Historical review

The first generation of desalination plants installed in the Gulf from the 1960s through the 1980s use

mainly carbon steel as a material for the evaporator shell and internals. Carbon steel is relatively

inexpensive, readily available and possesses engineering properties that have been understood and

used 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 of

the components that are subject to a corrosive environment. Some significant changes have occurred in

the material selection specified for the second generation of desalination plants designed and

constructed one decade later due to the deeper understanding of the operating conditions occurring

inside the evaporator and their consequences on the material selected. Some of the most significant

changes are summarized in Table 1.

1 http://www.desline.com/articoli/4071.pdf

Table 1Change in material specifications and operational experience

Component First-generation

specification Second-generation

specification Reasons

Vent baffles Carbon steel Stainless steel

Typically AISI 316L Understanding of

corrosion induced by

high

concentration of CO2,

O2 bromamine and

incondensable gases Support plates Carbon steel Stainless steel

Typical AISI 316L Ditto

Deaerator Carbon steel Stainless steel

Typical

AISI 317 LN

Understanding of

corrosion induced by

high

oxygen and

chloramine

concentration Shell Carbon steel painted Stainless steel

AISI 316L Maintenance

reduction

Cost effect Internals Carbon steel painted Stainless steel

AISI 316L Ditto

Make-up spray

pipe

Carbon steel

Stainless steel

Duplex steel

DIN 1.4462

Understanding of the

erosion phenomena

induced by flashing

inside the pipe

The development of stainless steels continues as an understanding of corrosion mechanisms and the

associated 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 counter

particular types of corrosion and/or erosion. Development of corrosion-resistant materials, however, is

not 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 application

to desalination has resulted in significant reduction in tube weights as a thinner wall thickness is used

for what is already a lighter material than steel.

6 Operational experience analysis

The corrosion mechanism for carbon steel that is most often encountered in desalination plants is that

of general corrosion, whereby metal is removed from the surface of the exposed material, resulting in a

general thinning. This is not the case with stainless steel where corrosion usually takes the form of

pitting. 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 chloride

environments. The adoption of stainless steel instead of carbon steel for evaporator and de-aerator

shell has caused, along with a general upgrading of the material, a reduction in the weights of the

Evaporator, which is indicated in Fig. 1.

Figure 2 Overall weight against capacity

It is difficult, however, to distinguish between the contribution given to the weight reduction by the

adoption of stainless steel and the elimination of corrosion allowances and the refinement in the

structural design which permits lower thickness. The engineering properties of the selected materials

could 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 of

economy of scale as unit outputs have increased significantly. However, over this period upgrading of

material selection has also taken place, and market prices have not been substantially affected. As with

all commodities, the unit price tends to fall with increased production and it would be expected that

costs will fall if the demand for the materials Increases. Also in this regard its is difficult to evaluate the

contribution to the price reduction given by

the enhanced commercial competition or by the decrease in the cost of stainless steel; in any case

the second generation of desalination plants is expected to achieve a lifetime of 40 years with

minor overhauling. Maintenance and overhauling of the desalination units also largely benefit from the

adoption of stainless steel material, solid or clad for the evaporator shell. In particular, long and delicate

overhauling periods for touch-up or restoration of the paint in the evaporator stages can be actually

avoided as indicated in Table 2.

Table 2Comparison of first and second generations of MSF

First generation Second generation

Carbon steel shell

painted Stainless

steel shell

Overhauling time

requested Long overhauling at

frequent intervals Routine

Type of action

required Blasting, patch-work,

priming and coating Routine

Cost involved High Routine

The possible causes for shortening the life of this generation of

plant or imposing heavy rehabilitation work lie in unforeseen

events such as erosion or impingement caused by debris or

foreign matters and in the presence of highly corrosive

components in the raw seawater as a result of pollution or

accidents.

Figs. 3 show the effect of tube-plate damage due to

impingement by debris and corrosion by hydrocarbons.

Only proper plant monitoring and operation can avoid these

kinds of incidents. Further investment in material upgrading

would not result in additional plant security or longevity.

Figure 4Effects of corrosion from hydrocarbons and

7 Development of the technology and future prospects

Further development of the material selection for MSF plants is related to the recent developments

in the process thermodynamics and the economic competition with MED, which is the emerging process

in the market. The MED process, in fact, allows a higher heat-exchange coefficient than MSF, and

consequently 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 substantially

changeover in the last 10 years, the MED process has room for further improving efficiency and reducing

costs 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 material

selection. Table 3 indicates the main differences of material selection as well as in the operating

conditions between the two processes. The different types of material can be, in part, related to the

different 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

for MED plants Typical material selection

for MSF plants Exchange tubes Titanium ASTM B338 Gr.

2;first 3 rows

Al brass remaining rows

Copper nickel 90/10 or

66/30 high- temp.stages,

heat recovery section

Al brass, low-temp. stages,

heat recovery

Section 66/30or titanium for

heat rejection section

Tube plates Stainless steel AISI 316 L Solid CuNi 90 /10 or

aluminum nickel bronze Shell Stainless steel AISI 316 L Solid or clad stainless steel Water boxes Not applicable Carbon steel Cu Ni 90 /10

clad Steam boxes Stainless steel Not applicable

Table 4Comparison of MSF and MED operating conditions

MED plant MSF plant

Temperature Max 65°C Up to 114°C Oxygen content Chloroamine and

bromoamine

presence

Up to 600 ppb in high-temp. stages

Possible because deareation takes

place in the

first stage and due to ejecto-

compression

Less than 50 ppb

With proper deareation bromoamine

is released from deareator

to vent condenser

Acid cleaning Frequent Rare

The reason for the quite high oxygen concentration in the MED process arises from the fact

that no separate deareation takes place and the oxygen is released in the first stages. The concentration of

the oxygen dissolved in the evaporating brine inside the MED shell is governed by Henry’s law, and it

tends to be in equilibrium with the oxygen pressure in the vapour side. The area surrounding the spray

nozzles can be considered as a flashing zone where flashing takes place as a result of a sudden pressure

difference. The oxygen concentration in the seawater at the spray nozzle entrance is in the range of 6 to 8

ppm, depending on the raw seawater temperature. The more the flashing and evaporating brine flows

towards 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 water

while the tube bundle provides a large, interfacial surface between the evaporating liquid and the gas so

that the equilibrium conditions are gradually reached on the bottom of the tube bundle. However, it is

likely that the superior parts of the MED shell and tube bundle, far from equilibrium, are subject to

oxygen concentration

in excess of 600 ppb. Fig. 4 shows the typical oxygen distribution in a multiple effect. The oxygen

distribution differs from stage to stage because each stage has a different operating temperature and

therefore a different Henry’s coefficient for oxygen concentration. The operational experience in several

MSF 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, have

been the reason for tube failure due to corrosion and excessive thinning, especially in the first

stages. Another area subject to pitting corrosion is the upper area in MSF deareator where a

moisture condensate rich in oxygen flows along the walls causing corrosion. Transferring this

experience from MSF to MED, it appears that the venting baffles as well as the upper shell and tube

bundle of MED plant could suffer from corrosion. Furthermore, hydrazine or similar ammonia

compounds inside the effect entrained inside the shell through the motive steam for the ejecto-compressor

can produce ammonia, which can stress corrosion crack the aluminium brass for the tube bundle. In

general, the low temperature and the adoption of titanium in the upper rows would prevent the risk of

stress corrosion cracking the aluminum brass tube to which they are subject in

the presence of ammonia-producing compounds. The adoption of titanium for all tube bundles or

titanium plate exchangers, which are currently used in the market, would completely solve the

problem 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 could

be reduced by the adoption of higher grade stainless steel such as DIN 1.4462 or 254 AVESTA. The

adoption of higher grades of stainless steel could possibly be an alternative to improve the quality of the

material selected for MED desalination plants and maintain costs at the desired levels. Cost reduction can

be achieved in this case by reducing the shell wall thickness thanks to the higher yield strength of duplex

steel. Fig. 5 shows the expected desalination plant weight against the production for a 2.5–5migd plant for

MSF and MED options with a conventional stainless-steel solution and a duplex-steel

Solution.

Figure 6 Weight comparison of MED and MSF desalination plants with 316L or duplex steel.

8 Corrosion in desalination plant

8.1 Typ2e of corrosion

8.1.1 Cavitations and Impingement

Cavitation occurs when a fluid's operational pressure drops below it's vapor pressure causing gas

pockets and bubbles to form and collapse. This can occur in what can be a rather explosive and dramatic

fashion. In fact, this can actually produce steam at the suction of a pump in a matter of minutes. When a

process 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 drops

This form of corrosion will eat out the volutes and impellers of centrifugal pumps with ultrapure water

as the fluid. It will eat valve seats. It will contribute to other forms of erosion corrosion, such as found in

elbows and tees. Cavitation should be designed out by reducing hydrodynamic pressure gradients and

designing to avoid pressure drops below the vapor pressure of the liquid and air ingress. The use of

resilient coatings and cathodic protection can also be considered as supplementary control methods.

Figure illustrate picture for cavitations dentition on pump casing

2 http://corrosion-doctors.org/Forms-cavitation/cavitation.htm

Figure 7Cavitation corrosion of a deaerator

8.1.2 Crevice Corrosion3

Crevice corrosion is a localized form of corrosion usually associated with a stagnant solution on the

micro-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 local

chemistry 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 crevice

3 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 up

between crevice (microenvironment) and the external surface (bulk environment). The

chronology of the aggravating factors leading to a full blown crevice is illustrated here. The cathodic oxygen reduction reaction cannot be sustained in the crevice area, giving it an anodic character in 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 the formation of an acidic micro-environment, together with a high chloride ion concentration.

8.1.3 Erosion Corrosion4

8.2

Erosion corrosion is acceleration in the rate of corrosion attack in metal due to the relative motion of a

corrosive fluid and a metal surface. The increased turbulence caused by pitting on the internal surfaces

of a tube can result in rapidly increasing erosion rates and eventually a leak. Erosion corrosion can also

be aggravated by faulty workmanship. For example, burrs left at cut tube ends can upset smooth water

flow, cause localized turbulence and high flow velocities, resulting in erosion corrosion. A combination

of erosion and corrosion can lead to extremely high pitting rates

4 http://corrosion-doctors.org/Forms-Erosion/erosion.htm

8.2.1 Environmental Cracking5

Environmental cracking refers to a corrosion cracking caused by a combination of conditions that can

specifically result in one of the following form of corrosion damage:

Stress Corrosion Cracking (SCC)

Corrosion fatigue

Hydrogen embrittlement

Stresses 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 be

tensile (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

Pressure

5 http://corrosion-doctors.org/Forms-EC/stresses.htm

9 Copper alloy

9.1 Introduction

Copper, the most noble of the metals in common use, has excellent resistance to corrosion in the

atmosphere and in fresh water. In sea-water, the copper nickel alloys have superior resistance to

corrosion 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 worms

such as the teredo, was discovered to prevent biofouling by weed and molluscs. This meant that ships

could stay at sea for long periods without cleaning. Nelson’s successful blockade tactics and subsequent

victory at Trafalgar was partly due to the superior speed of his clean-hulled ship. The addition of nickel

to copper improves its strength and durability and also the resistance to corrosion, erosion and

cavitation in all natural waters including sea-water and brackish, treated or polluted waters. The alloys

also show excellent resistance to stress-corrosion cracking and corrosion fatigue. The added advantage

of resistance to bio-fouling, gives a material ideal for application in marine and chemical environments

for 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 publication

is to discuss typical applications for copper-nickel alloys and the reasons for their selection. The two

main 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 in

wrought and cast forms.

9.1.1 Specifications, Properties and Availability

The copper-nickel alloys are single phased throughout the full range of compositions and many

standard alloys exist within this range, usually with small additions of other elements for special

purposes. The two most popular of the copper rich alloys contain 10 or 30% of nickel. Some manganese

is invariably present in the commercial alloys as a deoxidant and desulphurizer; it improves working

characteristics and additionally contributes to corrosion resistance in seawater. Other elements which

may be present singly or in combination are:

Iron, added (up to about 2% ) to the alloys required for marine applications. It confers

resistance 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-resistant

materials 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 more

provides higher strength. It is used in a newly-developed 30% nickel casting alloy (IN-768)*. A

low-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% nickel

alloys (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 in

conjunction with either chromium or niobium.

* INCO Designation

Tin confers an improved resistance to atmospheric tarnishing and at the 2% level is used with

9% nickel to produce the alloy C72500 †. This has useful spring properties and is used in the

electronics industry. It is not recommended for marine applications.

Table 1 – Application Standards for various Wrought and Cast Products

Table 5 Application Standards for various Wrought and Cast Products

Table 2 – Availability of Wrought Copper-Nickel Alloys

Table 6 Availability of Wrought Copper-Nickel Alloys

Table 7 90-10 copper-nickel-iron alloy. Mechanical properties

9.1.2 Resistance to Corrosion and Bio-Fouling

The 90/10 and 70/30 alloys have excellent resistance to sea-water corrosion and bio-fouling

with some variations in the performance of the alloys under different conditions as shown in

Table 5 and Table 6, for instance, the 90/10 alloy has the better bio-fouling resistance. In Table

5 the corrosion resistance of the 90/10 and 70/30 alloys in heat exchangers and condensers is

compared and in Table 6 the relative resistance of various alloys to fouling in quiet sea-water. If

water velocity is accelerated above 1 m/sec, any slight bio-fouling on metal with good fouling

resistance will be easily detached and swept away. On a material that does not have this good

fouling 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. 1

which also shows the typical service design speeds for certain items of common equipment in

contact with sea-water. The excellent corrosion resistance of 70/30 and 90/10 copper nickel

alloys and their suitability for many applications can be seen. Some materials with apparently

better 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 also

be 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 oxygen

at a joint or under attached bio-fouling. Table 7 shows the good tolerance of the copper-nickel

alloys to this type of attack, giving these alloys advantages over other materials of equal

corrosion resistance.

The copper-nickel alloys have good corrosion resistance in the quiescent or stagnant conditions

which may occur during the commissioning or overhaul of plant. Where plant is not being used

at design speeds some other materials may fail.

The corrosion resistance of the alloys is due to the protective surface film formed when in

contact with water. On initial immersion cuprous oxide is formed but complex changes occur in

sea water which research work is only now beginning to elucidate. At a flow rate of 0.6 m/s the

equilibrium corrosion rate is an almost negligible 0.002 mm/year. Normally, design flow rates

of up to 3.5 m/s give a satisfactory safety factor for use in pipework systems. This figure makes

allowance for the fact that local speeds may be higher at changes of direction, points of

divergence, etc. If water velocity is excessive, it can cause vortices leading to impingement

attack which can cause premature failure. Where surfaces in contact with water allow smooth

flow, as in ships hulls, different design criteria apply.

As mentioned, the fouling resistance is due to the copper ions at the surface, making it

inhospitable to most marine organisms in slowly moving water. In static conditions there may be

some deposition of chemical salts and biological slimes, possibly leading to some weakly

adherent fouling but such residues are easily detached from the metal’s corrosion resistant

surface, exposing a fresh, biocidally active surface.

When first brought into use, care must be taken to allow copper-nickel alloys to form their

protective corrosion resistant surface freely. Normally, this protective film will develop in six to

eight weeks. Contact with other less noble metals or with cathodic protection systems must be

avoided to ensure development of the corrosion resistant surface film and the non-fouling

properties.

Copper-nickel alloys do not suffer the stress-corrosion problems associated with some other

materials.

Table 8 Comparison or corrosion behaviour of CuNi10Fe and CuNi30Fe in seawater

Environmental

conditions Type of

corrosion

Service experience

CuNi10Fe CuNi30Fe

Clean seawater at

velocities up to 1 m/s Uniform, general 0.0025-

0.025 mm/a

0.0025-

0.025 mm/a

Clean seawater at

velocities up to 3.5 m/s * Impingement attack Satisfactory Satisfactory

Polluted seawater Accelerated general

and pitting Less resistant Preferred but not

immune

Entrained sand in seawater Accelerated general

and erosion Unsuitable,

exceptin

mild conditions

Use

CuNi30Fe2Mn2

Accumulated deposits on

surface Local attack Generally good Tendency to pit

Hot spots due to local

overheating Local attack by

denickelification Good Good but some

failures

in extreme

conditions Corrosion plus stress Stress corrosion Very resistant Very resistant

(Vapour side conditions)

Feedwater heaters working

under cyclic conditions Exfoliation attack Resistant Susceptible

Non-condensable gases † Local attack and

general thinning Highly Resistant Most resistant

Hydrogen sulphide in

desalination plant General attack Less Resistant Less Resistant

* 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 seawater

Arbitrary Rating Scale of

Fouling Resistance

90-100 Best Copper90/10 copper-nickel alloy

70-90 Good Brass and bronze

50 Fair 70/30 copper-nickel alloy,

aluminium bronzes,

zinc

10 Very Slight Nickel-copper alloy 400

0 Least Carbon and low alloy steels,

stainless steels,

nickel-chromium-high molybdenum

alloys

Titanium

Above 1 m/s (about 3 ft/sec or 1.8 knots) most fouling organisms have increasing difficulty in attaching

themselves and clinging to the surface unless already securely attached.

(INCO)

9.1.3 Sea Water Intakes

Sea water is frequently required in large quantities for cooling purposes. One of the problems

associated with sea water intakes in marine- or land-based installations is the occurrence of

gross marine fouling of the entry. This may be of soft growth, barnacles or bivalves. Not only

can this restrict the water flow but the marine fouling may be detached from time to time and

cause 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 a

detrimental effect on the installation and the environment near the outflow. Storage of bulk

chlorine can also be hazardous. Adequate control is possible during steady-state running

conditions 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 to

fouling. The intake pipes themselves may be of copper-nickel or large concrete piping may be

internally lined either by casting the concrete round a formed pipe or by attaching sheet inside

pipes by rivets or adhesive.

Figure 9Comparison of zinc anode protected steel

Figure 10Large diameter concrete intake pipe

10 Materials for Heat Exchanger Tubes

10.1 Introduction

Heat exchangers take heat from one fluid and pass it to a second. The fire-tube array of a steam

engine is a heat exchanger, taking heat from the hot combustion gases of the firebox and

transmitting it to the water in the boiler. The network of finned tubes in an air conditioner is a

heat exchanger, taking heat from the air of the room and dumping it into the working fluid of the

conditioner. The radiator in a car performs a similar function. A key element in all heat

exchangers is the tube wall or membrane which separates the two fluids. It is required to transmit

heat 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 best

materials for a conduction-limited exchanger, with substantial pressure difference between the

two fluids?

Figure 1 Schematic of a heat exchanger

10.2 Design Requirements

FUNCTION Heat Exchanger

OBJECTIVE Maximise heat flow per unit area, or per unit weight

CONSTRAINTS (a) Support pressure difference p

(b) Withstand chloride ions

(c) Operating temperature up to 150°C

(d) Low Cost

10.2.1.1 Table 1

10.3 The Model

First, a little background on heat flow. Heat transfer from one fluid, through a membrane to a second

fluid, involves convective transfer from fluid 1 into the tube wall, conduction through the wall, and

convection again to transfer it into fluid 2. The heat flux q into the tube wall by convection (in units of

W/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 surface

from 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 temperature

difference across it.

It is helpful to think of the thermal resistance at surface 1 as 1/h1; that of surface 2 is 1/h2; and

that 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 then

given by

(4)

where T is the difference in temperature between the two working fluids. When one of the

fluids is a gas, as in an air conditioner, heat transfer at the tube surface contributes most of the

resistance; 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 conduction

through 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: conduction

limited heat transfer. Then 1/h1 and 1/h2 are negligible when compared with t/, and the heat

transfer equation becomes

(5)

Consider, now, a heat exchanger with many tubes, each of radius r and wall thickness t with a

pressure difference p between the inside and outside. Our aim is to select a material to

maximise the total heat flow, while safely carrying the pressure difference p. The total heat

flow 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 to

support the pressure difference p. This requires that the stress in the wall remain below the

elastic limit (yield strength) el (times a safety factor, which need not be included in this

analysis):

(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 performance

index:

(9)

Four further considerations enter the selection. It is essential to choose a material that withstands

corrosion in the working fluids, which we take here to be water containing chloride ions (sea

water). Cost will naturally be of concern. The maximum service temperature must be adequate

and the material should be available as drawn tube.

10.4 The Selection

A preliminary selection using the Generic filter is shown in Figures 2-4. The first chart is of elastic limit

versus thermal conductivity, to allow us to maximise the value of M1. The second stage shows maximum

service temperature plotted as a bar-chart against resistance to sea-water, selecting materials with high

temperature resistance and high resistance to corrosion in sea-water. The last stage shows a bar chart of

material cost against available forms, selecting cheap materials that are available as sheet or tube.

Figure 2 A Chart of Elastic Limit versus Thermal Conductivity

Figure 3 A Bar-chart of Maximum Service Temperature versus Resistance to Sea-Water

Corrosion

Figure 4 Material Cost against Available Forms

The results of this selection are shown in Results Table 1, and they suggest that it may be worth

transferring the selection criteria to the coppers database to refine the search for a suitable

material.

10.5 Results

Material (ranked by M1) Comment

High Conductivity Coppers Have the best performance index, but relatively poor corrosion

resistance

Brasses Again, relatively poor corrosion resistance

Wrought Martensitic Stainless

Steel A good choice, but steel is more dense than copper

Aluminium 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) Comment

90/10 Aluminium bronze, cold wkd (wrought) The aluminium bronzes are cheap

92/8 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

(wrought)

The Nickel iron aluminium bronzes are more corrosion

resistant

Nickel iron aluminium bronze, hot wkd

(wrought)

10.5.1.2 Table 2 The Results of the Selection by expanding the coppers branch

10.6 PostScript

Conduction may limit heat flow in theory, but unspeakable things go on inside heat exchangers. Sea

water—often one of the working fluids—seethes with biofouling organisms which attach themselves to

tube walls and thrive, creating a layer of high thermal resistance and impeding fluid flow, like barnacles

on a boat. Some materials are more resistant to biofouling than others; copper-nickel alloys are

particularly 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—the

traditional winter pass-time of boat owners.

It is sometimes important to minimise the weight of heat exchangers. Repeating the calculation

to 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 different

index—the strength varies to the power 2 because the weight depends on the wall thickness, and

from 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 this

index within the coppers in the database—but if copper alloys were compared with stainless

steels at the Generic level, then it would be relevant.

11 Materials in Seawater Reverse Osmosis (SWRO) Plants6

In SWRO plants the operating environments are much less severe than in MSF plants. For

example, 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. Austenitic

stainless steels are the conventional materials used for high pressure piping leading to RO

membrane module, brine rejection pipe, product water outlet pipe and high pressure pumps. In

SWRO 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 corrosion

attack in case stagnant conditions are developed or deposits are formed in the piping

system due to operational problems. The performance of materials in different

SWRO plants is given in Table 4.

6

http://www.swcc.gov.sa/files/assets/Research/Technical%20Papers/Corrosion/RELEVANCE%20OF%20CORROSION%20RESEARCH%20IN%20THE%20MATERIALSELECTION%20FOR.pdf

11.1 Use of Superior Materials

11.1.1 Flash Chambers

The use of stainless steel cladding or Cu/Ni cladding on mild steel in all the stages or

in the first few stages and that of bare CS in the remaining stages appear to work very

well in evaporators of the desalination plants. Keeping in view the existing materials,

the evaporators are designed with a high corrosion allowance therefore, the chambers

are not much affected by general or uniform corrosion. However, localized corrosion

problems are quite frequent and troublesome and are the cause of concern

During shut down, slow moving or stagnant high chloride brine, crevices (formed

by salt and by scaling) and D.O. produce most favourable environment for initiating

and propagating corrosion process. Use of high alloy stainless steels, Ni-base alloys

or titanium as construction materials could perhaps be the ideal solution to avoid corrosion

in flash chambers. Even considering these materials as the best proposition

for long term trouble free operational life of the plant, the exorbitant cost would not

permit their use as constructional material.

Use of cement-concrete as construction material appears to be quite promising

due to its low cost, strength and durability. Erosion-corrosion resistance of the

cementitious material under conditions of high flow corrosive brine and thermal

stresses and airtightness of the structures are the problems which are to be looked

into.

11.1.2 Heat Exchangers

The predominant cause of the failure of heat exchanger tubes is circulating water

flow conditions resulting in tube inlet erosion/corrosion. This tube inlet damage is

almost 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 Selection

Cu-Ni (90/10 or 70/30), modified alloy Cu-Ni-Fe-Mn (66/30/2/2) or Ti are the

materials used for heat exchanger tubes. The choice of the most suitable material

depends 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 brine

heater tubes due to its excellent erosion-corrosion resistance and heat transfer

properties. Titanium has the tendency to undergo crevice corrosion specially at high

temperatures. Addition of precious metals to titanium though make it slightly more

expensive but increases its resistance to crevice corrosion tremendously. Alloys like

Ti-0.15pd, Ti-005u-0.05Niand Ti-0.05pd-.3 Cu have excellent resistance towards

crevice corrosion. Tables 5 and 6 provide some physical properties data relevant to

heat transfer tubes including failure rates and cost. Conductive plastic composites

containing high aspect ratio fillers (brasses, Al, Ni-plated mica, stainless steel fibres)

have thermal conductivity many magnitude higher than the case polymer. These

materials could be easier to fabricate, stronger, immune to corrosion and erosion,

possessing good heat transfer properties and should be cheaper than the traditional7

heat exchanger materials.8

11.1.4 Venting System

Conventional materials like CS cladded with SS 304, 316 or Cu-Ni which were previously

employed 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 Desalination

Plant Flash Chambers in the Arabian Gull” Desalination October,

1990. 8 Malik, A.U., and Kutty, Mayan, A., “Corrosion and Materials Selection in

Desalination Plants” Proc. SWCC Operation and Maintenance Conference,

April 27-29, 1992 p. 304.

11.1.5 Pumps

Discharge columns and diffusers of brine recycle and blow down pumps which are

usually made of Ni-resist showed SCC, fatigue or erosion due to porosity, lack of

stress relieving operation and poor casting. Replacements with more expensive

materials like SS316 or Duplex SS 2205 appear to overcome most of the erosioncorrosion

problems.

11.1.6 Pipings

Cement concrete (CC), reinforced cement concrete (RCC) and prestressed concrete

(PC) which have been conventional material for product water transmission pipe

lines showed frequent failures mainly due to rebar corrosion. The replacement of these materials

with 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 material

that can adapt the environment and resist the corrosion. Corrosion is the main part of material selection. It

attacks almost every part of desalination plant. It results from stagnancy, deposition, dealloying, galvanic

couplation, dealloying and vapor space attack. The local attack (pitting, crevice) can be avoided in

most of the cases by minimizing dissolved oxygen level of brine and incondensable gases, proper

flushing and keeping an inert atmosphere during shut down, mechanical or chemical cleaning of

deposits and maintaining C.P.