SABP-A-021

Best Practice SABP-A-021 6 September 2008 Corrosion Control in Desalination Plants

Document Responsibility: Materials and Corrosion Control Standards Committee

Saudi Aramco DeskTop Standards Table of Contents 1 Scope and Purpose................................................ 2 2 Conflicts and Deviations......................................... 2 3 References............................................................. 2 4 Definitions and Abbreviations................................. 2 5 Process Considerations......................................... 3 5.1 Introduction...................................................... 3 5.2 Process Descriptions....................................... 3 5.2.1 Multiple Stage Flash Evaporation......... 3 5.2.2 Multiple Effect Distillation...................... 4 5.2.3 Reverse Osmosis.................................. 4 5.3 Corrosive Species........................................... 5 6 Damage Types....................................................... 5 6.1 Damage Mechanisms...................................... 5 6.1.1 Pitting.................................................... 5 6.1.2 Crevice Corrosion................................. 5 6.1.3 Stress Corrosion Cracking.................... 6 6.1.4 Galvanic Corrosion............................... 6 6.1.5 Microbially Influenced Corrosion........... 6 6.1.6 Underdeposit Corrosion........................ 6 6.2 Damage Locations.......................................... 6 6.2.1 Multiple Stage Flash Evaporation......... 6 6.2.2 Multiple Effect Distillation...................... 7 6.2.3 Reverse Osmosis................................. 8 7 Corrosion Control Options...................................... 8 7.1 Dechlorination................................................. 8 7.2 Venting............................................................ 8 7.3 Materials Selection.......................................... 9 7.3.1 Metallic Materials.................................. 9 7.3.2 Non-Metallic Materials........................ 10 7.4 Chemical Inhibition........................................ 10 7.5 Cathodic Protection....................................... 11 8 Corrosion Monitoring............................................ 11 8.1 Inspection...................................................... 11 8.2 Stream Analysis............................................ 11

Previous Issue: New Next Planned Update: TBD Page 1 of 13 For additional information, contact Isaias, Nicos Philippou on 966-3-8760249

CopyrightSaudi Aramco 2008. All rights reserved.

Document Responsibility: Materials and Corrosion Control SABP-A-021 Issue Date: 6 September 2008 Next Planned Update: TBD Corrosion Control in Desalination Plants

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1 Scope and Purpose

This SABP provides guidelines that will improve the integrity of desalination plants through a fundamental understanding of the damage mechanisms, process parameters, inspection techniques, corrosion monitoring, analytical needs and corrosion control options.

It is based on current industry experiences and recent integrity assessments of desalination plants in Saudi Aramco by an inter-departmental and multidisciplinary team of experts. It is meant for internal use only.

2 Conflicts and Deviations

If there is a conflict between this Best Practice and other standards and specifications, please contact the Coordinator of ME&CCD/CSD.

3 References

3.1 Saudi Aramco References

Saudi Aramco Engineering Procedure

SAEP-1135 On-Stream Inspection Administration

Saudi Aramco Engineering Standards

SAES-L-132 Material Selection for Piping Systems SAES-W-010 Welding Requirements for Pressure Vessels SAES-W-011 Welding Requirements for On-Plot Piping

3.2 Industry Codes and Standards

American Petroleum Institute

API RP 570 Inspection, Repair, Alteration and Rerating of In-Service Piping Systems

American Water Works Association

AWWA M46 Reverse Osmosis and Nanofiltration

4 Definitions and Abbreviations

ASME American Society of Mechanical Engineers CO2 Carbon Dioxide


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MED Multiple Effect Distillation mpy Mils per Year MSFE Multiple Stage Flash Evaporator ORP Oxidation Reduction Potential (REDOX Potential) PFD Process Flow Diagram PREN Pitting Resistance Number RO Reverse Osmosis SCC Stress Corrosion Cracking SS Stainless Steel TDS Total Dissolved Solids

5 Process Considerations

5.1 Introduction

The primary function of a desalination plant is to reduce the TDS content of water, rendering it fit for potable or process applications. There are several different desalination processes, but the three that are widely used within Saudi Aramco plants are MED, MSFE and RO.

5.2 Process Descriptions

5.2.1 Multiple Stage Flash Evaporation

Figure 1 shows a schematic of a Single Effect Multiple Stage Flash Evaporator. The process principle of flash evaporation is that the maximum amount of energy that can be stored in water at its boiling point decreases as the water pressure is reduced. Therefore, when hot brine at its boiling point flows into a vessel operating at a lower pressure, the excess energy forms steam by flashing the hot brine. This reduces the temperature of the brine to its boiling point at the operating pressure. A multistage flash evaporator utilizes this principle to produce distillate and it has the following two main sections:

1) Heat Input

External energy is added to the recirculated brine to increase its temperature to 121C (250F) by using live steam in a shell and tube heat exchanger. Note that 121C is the highest allowable operating temperature and usually multi stage flash evaporators are designed to operate at 100C to reduce the scaling tendency of the water.


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2) Heat Removal

Internal heat is removed from the flashed steam in the last stage by using raw seawater in the tube condenser to reduce the temperature to 38C (100F). The overall operating temperature range of 121-38C (250-100F) determines the efficiency of this process. The greater the temperature difference, the more product water is produced. These temperatures are limited by natural conditions. The lower temperature is determined by the temperature of the raw seawater. The higher temperature is set by the solubility of calcium sulfate in the recirculated brine that is in contact with the heat transfer surfaces in the brine heater.

5.2.2 Multiple Effect Distillation

Figure 2 shows a schematic drawing of a typical MED unit. In an MED, water vapor is produced in the first effect (vessel) by spraying feedwater onto a hot tube bundle. The heat source for the tube bundle is steam from a low pressure boiler. In giving up its latent heat to the feedwater, the steam condenses to give high purity distillate, which is collected and is product water. The vapor produced in the first effect is passed to a tube bundle in the second effect where its heat of condensation is used to evaporate more water from the solution at a lower temperature and pressure. This second quantity of water vapor then passes to a third effect at lower temperature and pressure than the second and the process is repeated, producing additional vapor. The optimum number of effects is determined by the overall temperature difference available, the temperature and pressure differences required per effect to maintain a satisfactory production rate, and the additional costs of adding each effect.

5.2.3 Reverse Osmosis

Reverse Osmosis (RO) is a membrane based desalination technique. The principle of RO is that when two solutions of differing salt concentrations are separated by a semi-permeable membrane, the natural tendency is for water to flow from the dilute to the more concentrated solution. The difference in head between the two solutions is the osmotic pressure. On applying a pressure to overcome the osmotic pressure, the flow direction is reversed, hence producing water with a lower salt concentration. Industrially, RO membranes are usually fabricated in a spiral wound configuration. The individual membrane elements of around 1meter in length are then arranged in series in a long pressure vessel (with typically 5-8 membranes per vessel).


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5.3 Corrosive Species

CO2 carbon dioxide is formed in thermal desalination systems as a result of the thermal decomposition of bicarbonates found in seawater.

Oxygen reacts with carbon steel to give general corrosion. Reacts synergistically with ammonia to increase corrosion of copper alloys.

Chlorides experience has shown that > 500 ppb chloride levels can accelerate the pitting corrosion of austenitic stainless steel.

Bromine bromine is another of the halogens. Bromides are naturally present in seawater and at the temperatures found in thermal desalination systems can decompose to give gaseous bromine. Like chlorine bromine can cause aggressive pitting in stainless steels.

6 Damage Types

6.1 Damage Mechanisms

6.1.1 Pitting

Pitting is a form of extremely localized corrosion that leads to the formation of small holes (pits) in the metal. The presence of chlorides in seawater significantly aggravates the conditions for formation and growth of pits through an autocatalytic process. The pits become loaded with cations through anodic dissociation. Chloride ions become concentrated in the pits to maintain charge neutrality, encouraging the reaction of positive metal ions with water to form hydroxide corrosion products and hydrogen ions. The pits are now weakly acidic, which accelerates the process. Bromides in seawater can also initiate pitting. Stagnant water conditions favor pitting, which means that proper drain down and lay-up procedures have to be followed in order to avoid corrosion occurring during equipment downtime.

6.1.2 Crevice Corrosion

Crevice corrosion occurs in restricted regions, such as at a bolted joint. Crevice corrosion is initiated by a depletion of the dissolved oxygen in the restricted region. As the supply of oxygen within the crevice is depleted, because of cathodic oxygen reduction, the metal surface within the crevice becomes anodic, the anodic current is balanced by cathodic oxygen reduction from the region adjacent to the crevice. The ensuing reactions within the crevice are the same as those described for pitting corrosion: halide ions migrate to the crevice, where they are then


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hydrolyzed to form metal hydroxides and hydrochloric acid.

6.1.3 Stress Corrosion Cracking (SCC)

Stress corrosion cracking can be defined as a failure mechanism by which high aspect ratio flaws (cracks) are initiated and grow under the combined effect of a tensile stress together with the chemical or electro-chemical effects of the environment to which it is exposed. Austenitic stainless steels are prone to SCC in the presence of chlorides, especially at higher temperatures (>50C). SCC is very unlikely in RO plants due to lower temperatures, but can be encountered in the higher temperature parts of thermal desalination equipment.

6.1.4 Galvanic Corrosion

When two or more different sorts of metal come into contact in the presence of an electrolyte a galvanic couple is set up as different metals have different electrode potentials This leads to the anodic metal corroding more quickly than it otherwise would; while the corrosion of the cathodic metal is retarded even to the point of stopping. The presence of electrolyte and a conducting path between the metals may cause corrosion where otherwise neither metal alone would have corroded.

6.1.5 Microbially Influenced Corrosion (MIC)

Microbially Influenced Corrosion is an underdeposit type corrosion mechanism. The presence of bacteria and other microorganisms on the surface of a material (a biofilm) can alter the kinetics of the corrosion process. The metabolites of certain species of bacteria can reduce the pH by up to three pH units. MIC normally produces corrosion pits, either as a result of the pH depression, or due to underdeposit corrosion (see section 6.1.6).

6.1.6 Underdeposit Corrosion

Underdeposit corrosion can occur under scale deposits, or under slime mass. Both scale and slime masses produce a differential aeration cell. The area under the deposit becomes anodic relative to the deposit free surrounding area, causing severe localized attack.

6.2 Damage Locations

6.2.1 Multiple Stage Flash Evaporation

Typical corrosion mechanisms and the locations that these mechanisms have been found in MSFE units are shown in Table 1. In general, the


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majority of the corrosion found in MSFE is concentrated in the higher temperature sections.

Table 1 Typical Corrosion Mechanisms and Locations

Location Material Corrosion Mechanisms Seawater Strainers 316 SS Pitting

Flash Chambers 316 SS 317 SS Pitting, MIC (heat rejection stages only), SCC Pitting, MIC (heat rejection stages only), SCC

Ejector condensers 90/10 Cu/Ni 316 L 254 SMO

Erosion Pitting Crevice Corrosion

Tube Sheets

90/10 Cu/Ni 316 SS 904L SS 254 SMO

Pitting Pitting, galvanic corrosion, SCC Crevice Corrosion Crevice Corrosion

Heat Recovery Tubes 90/10 Cu/Ni 70/30 Cu/Ni Titanium

Erosion, pitting Erosion, pitting Hydriding

Heat Rejection tubes 90/10 Cu/Ni 70/30 Cu/Ni Titanium

Erosion, pitting, underdeposit corrosion Erosion, pitting, underdeposit corrosion Hydriding

Evaporator box 316 SS 317 SS Pitting, SCC Pitting, SCC

6.2.2 Multiple Effect Distillation

Table 2 shows the damage mechanisms and locations that the damage has occurred for MEDs. This data was compiled from both operating experience in Saudi Aramco plants and from a review of the published literature.

Table 2 Materials Employed in MED Plants and Associated Damage Mechanisms

Location Material Corrosion Mechanisms Seawater Strainers 316 SS Pitting

Distillate Condenser 90/10 Cu/Ni Al-brass 76/22/2 Aluminum

Erosion Erosion Erosion

Vacuum Ejectors 90/10 Cu/Ni 316 SS Erosion Pitting

Thermocompressor 90/10 Cu/Ni Erosion

Tube Sheet

90/10 Cu/Ni 316 SS 904L SS 254 SMO

Pitting Pitting Crevice Corrosion Crevice Corrosion


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Location Material Corrosion Mechanisms

Evaporator Tubes

90/10 Cu/Ni 70/30 Cu/Ni Aluminum Titanium Al-5052

Erosion, pitting Erosion, pitting Erosion, pitting Hydriding Erosion, pitting

Evaporator box 316 SS 317 SS Pitting Pitting

Interconnecting Piping 316 SS 317 SS Pitting Pitting

6.2.3 Reverse Osmosis

With RO plants, all metallic piping is subject to corrosion. Welds are normally the most susceptible location within the piping systems, with pitting corrosion at weldments being very common.

7 Corrosion Control Options

7.1 Dechlorination (MSFE/MED/RO)

Dechlorination is important to prevent pitting corrosion and/or stress corrosion cracking in austenitic stainless steels. Most thermal plants use significant amounts of austenitic materials for the construction of the main shell of the evaporators, evaporator tube sheets and interstage ducting. Feedwater to most thermal plants is chlorinated to prevent biofouling of the strainers and feedwater canals. Dechlorination is performed by the addition of sodium bisulfite to the feedwater. Sodium bisulfite reacts extremely rapidly with free chlorine, producing sodium sulfate. It is important to inject the bisulfite sufficiently far upstream of the basket strainers to ensure that no damage occurs to these units. A good rule of thumb is that in turbulent flow conditions, six pipe diameters will ensure good mixing. In the case of RO plants, the most common membrane chemistry in Saudi Aramco is polyamide. Polyamide is chlorine sensitive, so dechlorination is also essential to prevent damage to the membrane materials.

7.2 Venting (MSFE/MED)

In thermal desalination equipment, the vent system serves to create sufficient vacuum for the water to evaporate. The vents also remove non-condensible gases such as carbon dioxide and oxygen. Inadequate venting has resulted in corrosion of copper alloy tubing in the hot stages of MSFEs. If vent problems are suspected, then operations engineers need to work with the original equipment manufacturers to resolve this issue. Enlarging vents without proper attention to the vacuum conditions needed to maintain evaporation will lead to reduced levels of water production.


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7.3 Materials Selection

7.3.1 Metallic Materials

The most common construction materials for thermal desalination plants are stainless steels and copper alloys.

Stainless steels can give good performance, but great care has to be taken with both the grade of stainless to be used and the surface preparation of the steels (especially welds) if pitting corrosion or SCC is to be avoided.

One method of ranking stainless steels for pitting resistance is by the Pitting Resistance Number (PREN). The PREN is an empirical means of comparing stainless steels pitting resistance based on the composition of the alloy. The most commonly used formula for calculating PREN is:

PREN = Cr + 3.3Mo + 16N

The PREN numbers for alloys often used in the fabrication of desalination equipment are shown in Table 3. For use in seawater, a material should have a PREN of >40.

Table 3 PREN Numbers and Compositions for Stainless Steels

Grade Type Cr Mo N PREN

Austenitics 1.4301 304 17.0-19.5 NS 0.11max 17.0-20.8 1.4311 304LN 17.0-19.5 NS 0.12-0.22 18.9-23.0 1.4401 316 16.5-18.5 2.0-2.5 0.11max 23.1-28.5 1.4406 316LN 16.5-18.5 2.0-2.5 0.12-0.22 25.0-30.3 1.4539 904L 19.0-21.0 4.0-5.0 0.15max 32.2-39.9 1.4547 254SMO 19.5-20.5 6.0-7.0 0.18-0.25 42.2-47.6

Duplex 1.4362 SAF 2304 22.0-24.0 0.1-0.6 0.05-0.20 23.1-29.2 1.4462 SAF 2205 21.0-23.0 2.5-3.5 0.10-0.22 30.8-38.1 1.4410 SAF 2507 24.0-26.0 3.0-4.0 0.24-0.35 37.7-46.5 1.4501 Zeron 100 24.0-26.0 3.0-4.0 0.2-0.3 37.1-44.0

From this table it can be seen that the lower grade austenitics such as 304 are not suitable for seawater use. 316 Stainless stell has been used with mixed success. As steel makers have improved control of their alloying processing, in recent years the trend has been to control chromium contents close to 2%. As the chromium content is lowered, so is the


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PREN and the material is more likely to corrode. In recent years Saudi Aramco has specified 317SS as the minimum level austenitic stainless steel for seawater applications, as this material contains more chromium than 316 SS and is therefore more corrosion resistant.

Copper alloys have given satisfactory corrosion performance in seawater, being almost immune to chloride corrosion. However, the copper alloys are more prone to erosion than stainless steels. Pollution of seawater with sulfides or compounds that can decompose to give ammonia has also caused corrosion problems with copper alloys. The copper alloys used in desalination applications are shown in Table 4.

Table 4 Chart of Copper Alloys Used in Desalination Plants

NOMINAL COMPOSITION 5 ALLOY Cu Zn Al Ni Fe Mn As

UNS Number

Aluminium brass 76 22 2 - - - 0.04 C68700

90-10 Cu-Ni Rem - - 10 1.5 1.0 - C70600

70-30 Cu-Ni Rem - - 30 0.6 1.0 - C71500

66-30-2-2 Cu-Ni Rem - - 30 2.0 2.0 - C71640

A further point to be considered when considering the use of copper alloys is the downstream use of the desalinated water. In particular, boilers are sensitive to the presence of copper in the feedwater. For 625 psig boilers, (the most common pressure in use in Saudi Aramco) ASME guidelines limit the copper content to 15 ppb. Unless the plant also has ion-exchange demineralizers to remove copper, copper alloy tubing should not be used in plants where the thermal desalination units are to supply boiler feedwater.

7.3.2 Non-Metallic Materials

Non metallic materials do not find much application in thermal desalination plants, because of the higher temperatures employed in this type of equipment. For RO plants, non-metallics are an attractive option. All low-pressure piping can be fabricated from FRP. While FRP piping is more expensive to purchase and install, the life cycle costs are usually lower than for stainless steels as FRP does not corrode.

7.4 Chemical Inhibition

Corrosion inhibitors are not employed in desalination plants as very few corrosion inhibitors are suitable for potable water use. The only chemicals that


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are used in desalination plants are antiscalants, (for both RO and thermal plants) antifoams (thermal) and sodium sulfite for residual free chlorine scavenging (for both RO and thermal plants).

7.5 Cathodic Protection

Cathodic protection is not a common means of preventing corrosion in desalination plants. However, there have been occasions when sacrificial anodes have been used, usually to try to prevent pitting and crevice corrosion of the tubesheets, most notably when titanium tubes have been rolled into duplex stainless steel (904L) tubesheets. Aluminum sacrificial anodes have been successfully used to mitigate crevice corrosion at the tube/tubesheet interface. Iron anodes have been used in water boxes and in the water chambers of distillate condensers to prevent galvanic corrosion at the tube/tubesheet interface. Zinc anodes should not be used in equipment containing copper alloys, as the zinc can interfere with protective film development.

8 Corrosion Monitoring

Conventional corrosion monitoring techniques such as weight loss coupons and probes are seldom used in any desalination plants. As stated earlier in this document, most desalination plants rely heavily on the use of corrosion resistant alloys as materials of construction. Conventional monitoring methods are not suited to determining the corrosion mechanisms, such as SCC and pitting, that are the most likely modes of failure in a desalination plant.

8.1 Inspection

Equipment Visual inspection, random ultrasonic thickness (UT) and wet fluorescent magnetic particle testing (WFMPT) are commonly used to check for localized corrosion and environmental cracking.

Piping Various techniques such as random ultrasonic thickness (UT), UT shear wave (UTSW) of welds and radiography (RT) are used to detect metal loss, weld preferential corrosion and fine cracking.

8.2 Stream Analysis

Analysis of the feedwater for both thermal and RO plants is important when the water has been chlorinated to control micro and macrofouling. Austenitic stainless steels are prone to pitting and/or SCC in chlorinated water, while


polyamide RO membranes are irreversibly damaged by contact with chlorine. To prevent damage caused by chlorine, the feedwater is treated with sodium sulfite. On-line chlorine monitors have been tried to provide continuous, real-time data. However, on-line free chlorine meters have to be calibrated at frequent intervals, and the reagents that are used in these meters have a short shelf-life. For on-line monitoring experience has shown that ORP is a better method of determining if free chlorine is present.

Stream analysis is very important for the correct operation of Reverse Osmosis equipment. A fuller discussion of the parameters that should be monitored for RO operations purposes is beyond the scope of this document. Readers wishing to know more should read the document cited in the references section of this document.

Product Water

Recycle Pump

Cl2Injection

Heat Rejection

Stages

Tem

pera

ture

Sca

le

Top Brine Temperature

Heat Recovery Stages

TQ0

Q1

T

T

T

T1

T1-T0

Brine Heater

Rise

TTD

T0T0

Blowdown Pump

Seawater Pump Make-up Water TreatmentAcid Feed

DeaeratorDecarbonator

Process Flow Diagram

Figure 1 Schematic of a Single Effect Multiple Stage Flash Evaporator

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SteamSteam

Distillate

Brine

Reject seawater

Thermo compressor Vapor from 4th

effect

Seawater feed

Brine

1st effect 2nd effect 3rd effect 4th effect

Tube bundle

Vacuum SystemVent

Distillate condenser

Figure 2 Schematic of a Multiple Effect Distillation Unit Revision Summary 6 September 2008 New Saudi Aramco Best Practice.

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1 Scope and PurposeThis SABP provides guidelines that will improve the integrity of desalination plants through a fundamental understanding of the damage mechanisms, process parameters, inspection techniques, corrosion monitoring, analytical needs and corrosion control options.It is based on current industry experiences and recent integrity assessments of desalination plants in Saudi Aramco by an inter-departmental and multidisciplinary team of experts. It is meant for internal use only.2 Conflicts and DeviationsIf there is a conflict between this Best Practice and other standards and specifications, please contact the Coordinator of ME&CCD/CSD.3 References3.1 Saudi Aramco References

Saudi Aramco Engineering ProcedureSAEP-1135 On-Stream Inspection AdministrationSaudi Aramco Engineering StandardsSAES-L-132 Material Selection for Piping SystemsSAES-W-010 Welding Requirements for Pressure VesselsSAES-W-011 Welding Requirements for On-Plot Piping3.2 Industry Codes and Standards

American Petroleum InstituteAPI RP 570 Inspection, Repair, Alteration and Rerating of In-Service Piping SystemsAmerican Water Works AssociationAWWA M46 Reverse Osmosis and Nanofiltration4 Definitions and Abbreviations5 Process Considerations5.1 Introduction

The primary function of a desalination plant is to reduce the TDS content of water, rendering it fit for potable or process applications. There are several different desalination processes, but the three that are widely used within Saudi Aramco plants are MED, MSFE and RO.5.2 Process Descriptions5.2.1 Multiple Stage Flash Evaporation

Figure 1 shows a schematic of a Single Effect Multiple Stage Flash Evaporator. The process principle of flash evaporation is that the maximum amount of energy that can be stored in water at its boiling point decreases as the water pressure is reduced. Therefore, when hot brine at its boiling point flows into a vessel operating at a lower pressure, the excess energy forms steam by flashing the hot brine. This reduces the temperature of the brine to its boiling point at the operating pressure. A multistage flash evaporator utilizes this principle to produce distillate and it has the following two main sections:1) Heat Input

External energy is added to the recirculated brine to increase its temperature to 121C (250F) by using live steam in a shell and tube heat exchanger. Note that 121C is the highest allowable operating temperature and usually multi stage flash evaporators are designed to operate at 100C to reduce the scaling tendency of the water.2) Heat Removal

Internal heat is removed from the flashed steam in the last stage by using raw seawater in the tube condenser to reduce the temperature to 38C (100F). The overall operating temperature range of 121-38C (250-100F) determines the efficiency of this process. The greater the temperature difference, the more product water is produced. These temperatures are limited by natural conditions. The lower temperature is determined by the temperature of the raw seawater. The higher temperature is set by the solubility of calcium sulfate in the recirculated brine that is in contact with the heat transfer surfaces in the brine heater.5.2.2 Multiple Effect Distillation

Figure 2 shows a schematic drawing of a typical MED unit. In an MED, water vapor is produced in the first effect (vessel) by spraying feedwater onto a hot tube bundle. The heat source for the tube bundle is steam from a low pressure boiler. In giving up its latent heat to the feedwater, the steam condenses to give high purity distillate, which is collected and is product water. The vapor produced in the first effect is passed to a tube bundle in the second effect where its heat of condensation is used to evaporate more water from the solution at a lower temperature and pressure. This second quantity of water vapor then passes to a third effect at lower temperature and pressure than the second and the process is repeated, producing additional vapor. The optimum number of effects is determined by the overall temperature difference available, the temperature and pressure differences required per effect to maintain a satisfactory production rate, and the additional costs of adding each effect.5.2.3 Reverse Osmosis

Reverse Osmosis (RO) is a membrane based desalination technique. The principle of RO is that when two solutions of differing salt concentrations are separated by a semi-permeable membrane, the natural tendency is for water to flow from the dilute to the more concentrated solution. The difference in head between the two solutions is the osmotic pressure. On applying a pressure to overcome the osmotic pressure, the flow direction is reversed, hence producing water with a lower salt concentration. Industrially, RO membranes are usually fabricated in a spiral wound configuration. The individual membrane elements of around 1meter in length are then arranged in series in a long pressure vessel (with typically 5-8 membranes per vessel).5.3 Corrosive Species

CO2 carbon dioxide is formed in thermal desalination systems as a result of the thermal decomposition of bicarbonates found in seawater.Oxygen reacts with carbon steel to give general corrosion. Reacts synergistically with ammonia to increase corrosion of copper alloys.Chlorides experience has shown that > 500 ppb chloride levels can accelerate the pitting corrosion of austenitic stainless steel.Bromine bromine is another of the halogens. Bromides are naturally present in seawater and at the temperatures found in thermal desalination systems can decompose to give gaseous bromine. Like chlorine bromine can cause aggressive pitting in stainless steels.6 Damage Types6.1 Damage Mechanisms6.1.1 Pitting

Pitting is a form of extremely localized corrosion that leads to the formation of small holes (pits) in the metal. The presence of chlorides in seawater significantly aggravates the conditions for formation and growth of pits through an autocatalytic process. The pits become loaded with cations through anodic dissociation. Chloride ions become concentrated in the pits to maintain charge neutrality, encouraging the reaction of positive metal ions with water to form hydroxide corrosion products and hydrogen ions. The pits are now weakly acidic, which accelerates the process. Bromides in seawater can also initiate pitting. Stagnant water conditions favor pitting, which means that proper drain down and lay-up procedures have to be followed in order to avoid corrosion occurring during equipment downtime.6.1.2 Crevice Corrosion

Crevice corrosion occurs in restricted regions, such as at a bolted joint. Crevice corrosion is initiated by a depletion of the dissolved oxygen in the restricted region. As the supply of oxygen within the crevice is depleted, because of cathodic oxygen reduction, the metal surface within the crevice becomes anodic, the anodic current is balanced by cathodic oxygen reduction from the region adjacent to the crevice. The ensuing reactions within the crevice are the same as those described for pitting corrosion: halide ions migrate to the crevice, where they are then hydrolyzed to form metal hydroxides and hydrochloric acid.6.1.3 Stress Corrosion Cracking (SCC)

Stress corrosion cracking can be defined as a failure mechanism by which high aspect ratio flaws (cracks) are initiated and grow under the combined effect of a tensile stress together with the chemical or electro-chemical effects of the environment to which it is exposed. Austenitic stainless steels are prone to SCC in the presence of chlorides, especially at higher temperatures (>50C). SCC is very unlikely in RO plants due to lower temperatures, but can be encountered in the higher temperature parts of thermal desalination equipment.6.1.4 Galvanic Corrosion

When two or more different sorts of metal come into contact in the presence of an electrolyte a galvanic couple is set up as different metals have different electrode potentials This leads to the anodic metal corroding more quickly than it otherwise would; while the corrosion of the cathodic metal is retarded even to the point of stopping. The presence of electrolyte and a conducting path between the metals may cause corrosion where otherwise neither metal alone would have corroded.6.1.5 Microbially Influenced Corrosion (MIC)

Microbially Influenced Corrosion is an underdeposit type corrosion mechanism. The presence of bacteria and other microorganisms on the surface of a material (a biofilm) can alter the kinetics of the corrosion process. The metabolites of certain species of bacteria can reduce the pH by up to three pH units. MIC normally produces corrosion pits, either as a result of the pH depression, or due to underdeposit corrosion (see section 6.1.6).6.1.6 Underdeposit Corrosion

Underdeposit corrosion can occur under scale deposits, or under slime mass. Both scale and slime masses produce a differential aeration cell. The area under the deposit becomes anodic relative to the deposit free surrounding area, causing severe localized attack.6.2 Damage Locations6.2.1 Multiple Stage Flash Evaporation

Typical corrosion mechanisms and the locations that these mechanisms have been found in MSFE units are shown in Table 1. In general, the majority of the corrosion found in MSFE is concentrated in the higher temperature sections.6.2.2 Multiple Effect Distillation

Table 2 shows the damage mechanisms and locations that the damage has occurred for MEDs. This data was compiled from both operating experience in Saudi Aramco plants and from a review of the published literature.6.2.3 Reverse Osmosis

With RO plants, all metallic piping is subject to corrosion. Welds are normally the most susceptible location within the piping systems, with pitting corrosion at weldments being very common.7 Corrosion Control Options7.1 Dechlorination (MSFE/MED/RO)

Dechlorination is important to prevent pitting corrosion and/or stress corrosion cracking in austenitic stainless steels. Most thermal plants use significant amounts of austenitic materials for the construction of the main shell of the evaporators, evaporator tube sheets and interstage ducting. Feedwater to most thermal plants is chlorinated to prevent biofouling of the strainers and feedwater canals. Dechlorination is performed by the addition of sodium bisulfite to the feedwater. Sodium bisulfite reacts extremely rapidly with free chlorine, producing sodium sulfate. It is important to inject the bisulfite sufficiently far upstream of the basket strainers to ensure that no damage occurs to these units. A good rule of thumb is that in turbulent flow conditions, six pipe diameters will ensure good mixing. In the case of RO plants, the most common membrane chemistry in Saudi Aramco is polyamide. Polyamide is chlorine sensitive, so dechlorination is also essential to prevent damage to the membrane materials.7.2 Venting (MSFE/MED)

In thermal desalination equipment, the vent system serves to create sufficient vacuum for the water to evaporate. The vents also remove non-condensible gases such as carbon dioxide and oxygen. Inadequate venting has resulted in corrosion of copper alloy tubing in the hot stages of MSFEs. If vent problems are suspected, then operations engineers need to work with the original equipment manufacturers to resolve this issue. Enlarging vents without proper attention to the vacuum conditions needed to maintain evaporation will lead to reduced levels of water production.7.3 Materials Selection7.3.1 Metallic Materials

The most common construction materials for thermal desalination plants are stainless steels and copper alloys.Stainless steels can give good performance, but great care has to be taken with both the grade of stainless to be used and the surface preparation of the steels (especially welds) if pitting corrosion or SCC is to be avoided.One method of ranking stainless steels for pitting resistance is by the Pitting Resistance Number (PREN). The PREN is an empirical means of comparing stainless steels pitting resistance based on the composition of the alloy. The most commonly used formula for calculating PREN is: PREN = Cr + 3.3Mo + 16NThe PREN numbers for alloys often used in the fabrication of desalination equipment are shown in Table 3. For use in seawater, a material should have a PREN of >40.From this table it can be seen that the lower grade austenitics such as 304 are not suitable for seawater use. 316 Stainless stell has been used with mixed success. As steel makers have improved control of their alloying processing, in recent years the trend has been to control chromium contents close to 2%. As the chromium content is lowered, so is the PREN and the material is more likely to corrode. In recent years Saudi Aramco has specified 317SS as the minimum level austenitic stainless steel for seawater applications, as this material contains more chromium than 316 SS and is therefore more corrosion resistant.Copper alloys have given satisfactory corrosion performance in seawater, being almost immune to chloride corrosion. However, the copper alloys are more prone to erosion than stainless steels. Pollution of seawater with sulfides or compounds that can decompose to give ammonia has also caused corrosion problems with copper alloys. The copper alloys used in desalination applications are shown in Table 4.A further point to be considered when considering the use of copper alloys is the downstream use of the desalinated water. In particular, boilers are sensitive to the presence of copper in the feedwater. For 625 psig boilers, (the most common pressure in use in Saudi Aramco) ASME guidelines limit the copper content to 15 ppb. Unless the plant also has ion-exchange demineralizers to remove copper, copper alloy tubing should not be used in plants where the thermal desalination units are to supply boiler feedwater.7.3.2 Non-Metallic Materials

Non metallic materials do not find much application in thermal desalination plants, because of the higher temperatures employed in this type of equipment. For RO plants, non-metallics are an attractive option. All low-pressure piping can be fabricated from FRP. While FRP piping is more expensive to purchase and install, the life cycle costs are usually lower than for stainless steels as FRP does not corrode.7.4 Chemical Inhibition

Corrosion inhibitors are not employed in desalination plants as very few corrosion inhibitors are suitable for potable water use. The only chemicals that are used in desalination plants are antiscalants, (for both RO and thermal plants) antifoams (thermal) and sodium sulfite for residual free chlorine scavenging (for both RO and thermal plants).7.5 Cathodic Protection

Cathodic protection is not a common means of preventing corrosion in desalination plants. However, there have been occasions when sacrificial anodes have been used, usually to try to prevent pitting and crevice corrosion of the tubesheets, most notably when titanium tubes have been rolled into duplex stainless steel (904L) tubesheets. Aluminum sacrificial anodes have been successfully used to mitigate crevice corrosion at the tube/tubesheet interface. Iron anodes have been used in water boxes and in the water chambers of distillate condensers to prevent galvanic corrosion at the tube/tubesheet interface. Zinc anodes should not be used in equipment containing copper alloys, as the zinc can interfere with protective film development.8 Corrosion MonitoringConventional corrosion monitoring techniques such as weight loss coupons and probes are seldom used in any desalination plants. As stated earlier in this document, most desalination plants rely heavily on the use of corrosion resistant alloys as materials of construction. Conventional monitoring methods are not suited to determining the corrosion mechanisms, such as SCC and pitting, that are the most likely modes of failure in a desalination plant.8.1 Inspection

EquipmentVisual inspection, random ultrasonic thickness (UT) and wet fluorescent magnetic particle testing (WFMPT) are commonly used to check for localized corrosion and environmental cracking. PipingVarious techniques such as random ultrasonic thickness (UT), UT shear wave (UTSW) of welds and radiography (RT) are used to detect metal loss, weld preferential corrosion and fine cracking.8.2 Stream Analysis

Analysis of the feedwater for both thermal and RO plants is important when the water has been chlorinated to control micro and macrofouling. Austenitic stainless steels are prone to pitting and/or SCC in chlorinated water, while polyamide RO membranes are irreversibly damaged by contact with chlorine. To prevent damage caused by chlorine, the feedwater is treated with sodium sulfite. On-line chlorine monitors have been tried to provide continuous, real-time data. However, on-line free chlorine meters have to be calibrated at frequent intervals, and the reagents that are used in these meters have a short shelf-life. For on-line monitoring experience has shown that ORP is a better method of determining if free chlorine is present.Stream analysis is very important for the correct operation of Reverse Osmosis equipment. A fuller discussion of the parameters that should be monitored for RO operations purposes is beyond the scope of this document. Readers wishing to know more should read the document cited in the references section of this document.

SABP-A-021

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Transcript of SABP-A-021