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Central Engineering Services Reliance Industries Limited, Jamnagar
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Summer Internship project on
Cooling water corrosion related problems of Heat Exchanger
Reliance Industries Limited Refinery Division-Jamnagar
Submitted by:- Shashank Saraf B Tech. 3rd Year
Department of Metallurgical and Materials Engineering Indian Institute of Technology, Kharagpur
Mentored by
Mr. Amish Jani Mr. P.D. Shende General Manager Vice-President CES-Inspection CES-Inspection
Central Engineering Services Reliance Industries Limited, Jamnagar
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ACKNOWLEDGEMENT I’m grateful to the department of Metallurgical and Materials Engineering, IIT Kharagpur for
providing me an opportunity to do a summer internship at a premier organization like Reliance
Industries Ltd. (RIL) at their Jamnagar Site.
I wish to extend my heartiest thanks to my mentor Mr. Amish Jani of CES-Inspection department,
RIL Jamnagar, for the stepwise guidance that they have provided. I also greatly appreciate the
freedom that he gave me to pursue my own ideas. I also want to thank Mr. Keyur Kothiyar, Mr.
Pankaj Godara, Ms. Jahanvee Upadhyay and Mr. Vikas Verma for their constant support and help
in the project. The encouragement, support and faith of the whole CES department enabled me to
complete my project work efficiently in the stipulated time interval. I am also indebted to Mr.
Shobhan Mehta, learning center, RIL Jamnagar for his continuous support to avail all the
facilities during my training period.
Working in RIL was an enriching experience, which will help me in my future academic
aspirations.
Central Engineering Services Reliance Industries Limited, Jamnagar
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ACCEPTANCE CERTIFICATE This is to certify that SHASHANK SARAF, student of B tech., Metallurgical and Materials
Engineering Department of IIT Kharagpur has done the project on ‘Cooling water corrosion
related problems of cooling tower 5 Heat Exchanger’ under my guidance. The duration of project
has been from 15th May’09 till 14th July’09.
He has successfully been able to complete the project in the stipulated duration and report his
study. The work done will be useful in the further understanding of the subject.
Date: Signature
Amish Jani
General Manager
Central Engineering Services-Inspection
Reliance Industries Limited,
Refinery Division-Jamnagar
Central Engineering Services Reliance Industries Limited, Jamnagar
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TABLE OF CONTENTS
Acknowledgement ii
Acceptance Certificate iii
1. Executive Summary 5
2. An Introduction 6
2.1 Cooling water system 6
2.1.1 Heat Exchangers 7
2.1.2 Types of heat exchanger 7
2.1.3 Design criteria 8
2.2 Cooling Tower 10
3. Problems in cooling water system 11
3.1 Corrosion 11
3.2 Fouling 11
3.3 Scaling 12
4. Corrosion 13
4.1 Forms of corrosion 13
4.2 Causes of Corrosion 17
4.3 Corrosion Control Methods for Cooling water System 20
5. Case Study 23
5.1 Corrosion problems related to Cooling tower 5 23
5.2 Graphs 26
5.3 Analysis 26
5.4 Comparison studies between water supplies of cooling tower 5 and 3 31
6. Recommendations 35
7. References 36
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1. Executive Summary
The objective of this project is to study the cooling water related corrosion problems in heat
exchangers. Heat exchangers and cooling towers form an intrinsic part of any petroleum refinery,
therefore it becomes necessary to ensure that corrosion rate is below 0.7 mpy. Currently this
corrosion rate is a matter of concern in cooling tower 5’s related heat exchangers. This project
aims at studying of corrosion related problems and reducing corrosion rate.
This project involves studying of all parameters that affect corrosion rate. Usage of
chromates as corrosion inhibitors faces regulation problems, which are actually very good
corrosion inhibitors. Phosphates, organophosphates are used here as corrosion inhibitors, but due
to large concentration of iron in cooling water, phosphates have to be reduced to avoid
precipitation of iron phosphate which promotes under-deposit corrosion. Also, make-up water
contains very less quantities of calcium, which reduces the efficiency of formation of calcium
phosphate passive layer.
Pitting corrosion is caused by strong ions like chlorides and sulphates. In cooling tower 5 the
concentration of chlorides ions are generally higher than other cooling towers leading to more
pitting corrosion.
To reduce corrosion, ion exchange technique should be used to replace chlorides with
hydroxide ions which also increase pH value that sometimes goes down because of hydrogen
sulphide leak. There should be online monitoring of iron; whenever it goes higher than 2.5 ppm,
measures should be taken to reduce level of iron. All theses measures will lead to low corrosion
rate, less frequent retubing of tube bundles and will eventually reduce the maintenance cost of
heat exchangers.
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2. An Introduction
5.1 Cooling Water System Water cooling is a method of heat removal from components. As opposed to air cooling, water is
used as the heat transmitter. Water cooling is commonly used for cooling internal combustion
engines in automobiles and large electrical generators. Other uses include cooling the barrels of
machine guns, cooling of lubricant oil in pumps; for cooling purposes in heat exchangers;
cooling products from tanks or columns, and recently, cooling of various major components
inside high-end personal computers. The main mechanism for water cooling is convective heat
transfer.
There are three types of cooling water systems: once through, open circulating and closed
circulating.
Once through systems use cooling water on a one-time basis prior to discharge. These
systems uses large amount of water to remove heat from the process streams. Once through
systems have the advantage that evaporation does not take place and the amount of dissolved
solids remains the same as the supplied water. Any potential for scale formation is results from
the increase in temperature. Corrosion in these systems is primarily the result of relatively low pH
values and dissolved oxygen and the presence of corrosive contaminants that may be present in
water.
Open recirculating systems reuse water by recycling it across a cooling tower. In cooling
tower, conduction and evaporation remove heat from the cooling water so that water can return to
the system to repeat the process. With evaporation comes the need of replenish the water removed
from the system (makeup water). Because of evaporation that takes place, the concentration of
dissolved solids in the recirculating water increases. This creates a numbers of potential
problems. These problems are generally related to corrosion, scale or fouling which can occur
within the cooling system. Makeup water is also added to replace water that is removed from the
system (blowdown) either in order to keep a desired level of concentration of total dissolved
solids in the system and loss of the water in the pump glands or drift.
Closed recirculation systems recirculate a fixed volume of water in a closed loop. The heat
removed from the heat exchanger surface is absorbed by the cooling water. The resulting higher
temperature water is then cooled by circulating the water back through another exchanger, which
is cooled by another means. The only makeup to a closed water system is to replace the amount
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loss via leaks. Due to lack of evaporation, the potential for scaling is very low, unless very
high hardness water is used as makeup.
2.1.1 Heat Exchangers
Heat exchangers are commonly used to transfer heat from steam, water, or gases, to gases, or
liquids. They are widely used in space heating, refrigeration, air conditioning, power plants,
chemical plants, petrochemical plants, petroleum refineries, and natural gas processing. Some of
the criteria for selecting materials used for heat exchangers are corrosion resistance, strength,
heat conduction, and cost. To meet corrosion requirements, tubing must be resistant to general
corrosion, pitting, stress-corrosion cracking (SCC), selective leaching or dealloying, and oxygen
cell attack in service.
2.1.2 Types of heat exchangers
a.) Shell and tube heat exchanger
b.) Plate heat exchanger
c.) Regenerative heat exchanger
d.) Adiabatic wheel heat exchanger
e.) Plate fin heat exchanger
f.) Fluid heat exchangers
g.) Waste heat recovery units
h.) Dynamic scraped surface heat exchanger
i.) Phase-change heat exchangers
j.) Direct contact heat exchangers
k.) HVAC air coils
l.) Spiral heat exchangers
Shell and tube heat exchanger is most widely used in refineries.
Shell and tube heat exchanger Shell and tube heat exchangers consist of a series of tubes. One
set of these tubes contains the fluid that must be either heated or cooled. The second fluid runs
over the tubes that are being heated or cooled so that it can either provide the heat or absorb the
heat required. A set of tubes is called the tube bundle and can be made up of several types of
tubes: plain, longitudinally finned, etc.
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2.1.3. Design Criteria
There are several thermal design features that are to be taken into account when designing the
tubes in the shell and tube heat exchangers. These include:
a.) Tube diameter: Using a small tube diameter makes the heat exchanger both economical
and compact. However, it is more likely for the heat exchanger to foul up faster and the
small size makes mechanical cleaning of the fouling difficult. To prevail over the fouling
and cleaning problems, larger tube diameters can be used. Thus to determine the tube
diameter, the available space, cost and the fouling nature of the fluids must be
considered.
b.) Tube thickness: The thickness of the wall of the tubes is usually determined to ensure:
a. There is enough room for corrosion
b. That flow-induced vibration has resistance
c. Axial strength
d. Ability to easily stock spare parts cost. Sometimes the wall thickness is
determined by the maximum pressure differential across the wall.
c.) Tube length: heat exchangers are usually cheaper when they have a smaller shell
diameter and a long tube length.
d.) Tube pitch: when designing the tubes, it is practical to ensure that the tube pitch (i.e., the
centre-centre distance of adjoining tubes) is not less than 1.25 times the tubes' outside
diameter. A larger tube pitch leads to a larger overall shell diameter which leads to a
more expensive heat exchanger.
e.) Tube corrugation: this type of tubes, mainly used for the inner tubes, increases the
turbulence of the fluids and the effect is very important in the heat transfer giving a better
performance.
f.) Tube Layout: refers to how tubes are positioned within the shell. There are four main
types of tube layout, which are, triangular (30°), rotated triangular (60°), square (90°) and
rotated square (45°).
g.) Baffle Design: baffles are used in shell and tube heat exchangers to direct fluid across the
tube bundle. They run perpendicularly to the shell and hold the bundle, preventing the
tubes from sagging over a long length. They can also prevent the tubes from vibrating.
The semicircular segmental baffles are oriented at 180 degrees to the adjacent baffles
forcing the fluid to flow upward and downwards between the tube bundles. Baffles must
be spaced with consideration for the conversion of pressure drop and heat transfer.
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Having baffles spaced too closely causes a greater pressure drop because of flow
redirection. Consequently having the baffles spaced too far apart means that there may be
cooler spots in the corners between baffles. It is also important to ensure the baffles are
spaced close enough that the tubes do not sag. The other main type of baffle is the disc
and donut baffle. This type of baffle forces the fluid to pass around each side of the disk
then through the donut baffle generating a different type of fluid flow.
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5.2 Cooling tower Cooling towers are heat removal devices used to transfer process waste heat to the atmosphere.
Cooling towers may either use the evaporation of water to remove process heat and cool the
working fluid to near the wet-bulb air temperature or rely solely on air to cool the working fluid
to near the dry-bulb air temperature. Common applications include cooling the circulating water
used in oil refineries, chemical plants, power stations and building cooling.
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3. Problems in cooling water systems
Cooling water systems are an integral part of process operations in many industries. For
continuous plant productivity, these systems require proper chemical treatment and preventive
maintenance. The four problems normally associated with cooling water systems - corrosion,
scale, fouling and microbiological contamination - contribute to problems with heat transfer
and, ultimately, the quality of the process. Corrosion in heat exchanger in cooling water system is
most significant and dangerous.
3.1 Corrosion Corrosion can be defined as the disintegration of a material into its constituent atoms due to
chemical reactions with its surroundings. In the most common use of the word, this means a
loss of electrons of metals reacting with water and oxygen. Weakening of iron due to
oxidation of the iron atoms is a well-known example of electrochemical corrosion. This is
commonly known as rusting. This type of damage typically produces oxide(s) and/or salt(s)
of the original metal. Corrosion can also refer to other materials than metals, such as ceramics
or polymers. Although in this context, the term degradation is more common.
3.2 Fouling
Fouling refers to the accumulation of unwanted material on solid surfaces, most often in
an aquatic environment. The fouling material can consist of either living organisms
(biofouling) or a non-living substance (inorganic or organic). Fouling is usually
distinguished from other surface-growth phenomena in that it occurs on a surface of a
component, system or plant performing a defined and useful function, and that the fouling
process impedes or interferes with this function.
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3.3 Scale
Scaling is the precipitation from solution of sparingly soluble salts on the surfaces of the
cooling water system. Deposition on the heat transfer surfaces, piping and cooling tower fill
surfaces can cause under deposit corrosion, increased pressure drop and loss of heat transfer
efficiency.
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4. Corrosion 4.1 Forms of Corrosion
a.) Uniform attack
Uniform attack is a form of electrochemical corrosion
that occurs with equal intensity of the entire surface
of the metal. Iron rusts when exposed to air and
water, and silver due to exposure to air.
b.) Crevice Corrosion
Another form of electrochemical corrosion is crevice
corrosion. Crevice corrosion is a consequence of
concentration differences of ions or dissolved gases in
an electrolytic solution. A solution became trapped
between a pipe and the flange on the left. The
stagnant liquid in the crevice eventually had a
lowered dissolved oxygen concentration and crevice
corrosion took over and destroyed the flange. In the absence of oxygen, the metal and/or it's
passive layer begin to oxidize. To prevent crevice corrosion, one should use welds rather than
rivets or bolted joints whenever possible. Also consider non absorbing gaskets. Remove
accumulated deposits frequently and design containment vessels to avoid stagnant areas as much
as possible.
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c.) Pitting
Pitting, just as it sounds, is used to describe the
formation of small pits on the surface of a metal or
alloy. Pitting is suspected to occur in much the same
way crevice corrosion does, but on a flat surface. A
small imperfection in the metal is thought to begin the
process, then a "snowball" effect takes place. Pitting
can go on undetected for extended periods of time, until
a failure occurs. A textbook example of pitting would
be to subject stainless steel to a chloride containing stream such as seawater. Pitting would
overrun the stainless steel in a matter of weeks due to it's very poor resistance to chlorides, which
are notorious for their ability to initiate pitting corrosion. Alloy blends with more than 2%
Molybdenum show better resistance to pitting attack. Titanium is usually the material of choice if
chlorides are the main corrosion concern. (Pd stabilized forms of Ti are also used for more
extreme cases).
d.) Intergranular Corrosion
Occurring along grain boundaries for some alloys,
intergranular corrosion can be a real danger in the right
environment. On the left, a piece of stainless steel
(especially susceptible to intergranular corrosion) has
seen severe corrosion just an inch from a weld. The
heating of some materials causes chromium carbide to
form from the chromium and the carbon in the metals.
This leaves a chromium deficient boundary just shy of the where the metal was heated for
welding. To avoid this problem, the material can be subjected to high temperatures to redissolve
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the chromium carbide particles. Low carbon materials can also be used to minimize the
formation of chromium carbide. Finally, the material can be alloyed with another material such
as Titanium which forms carbides more readily so that the chromium remains in place.
e.) Selective Leaching
When one element or constituent of a metal is
selectively corroded out of a material it is referred
to as selective leaching. The most common
example is the dezincification of brass. On the
right, nickel has be corroded out of a copper-nickel
alloy exposed to stagnant seawater. After leaching
has occurred, the mechanical properties of the metal are obviously impaired and some metal will
begin to crack.
f.) Erosion-Corrosion
Erosion-corrosion arises from a combination of
chemical attack and the physical abrasion as a
consequence of the fluid motion. Virtually all alloy
or metals are susceptible to some type of erosion-
corrosion as this type of corrosion is very dependent
on the fluid. Materials that rely on a passive layer
are especially sensitive to erosion-corrosion. Once
the passive layer has been removed, the bare metal surface is exposed to the corrosive material.
If the passive layer cannot be regenerated quickly enough, significant damage can be seen. Fluids
that contain suspended solids are often times responsible for erosion-corrosion. The best way to
limit erosion-corrosion is to design systems that will maintain a low fluid velocity and to
minimize sudden line size changes and elbows. The photo above shows erosion-corrosion of a
copper-nickel tube in a seawater surface. An imperfection on the tube surface probably causes an
eddy current which provided a perfect location for erosion-corrosion.
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g.) Stress Corrosion
Stress corrosion can result from the combination
of an applied tensile stress and a corrosive
environment. In fact, some materials only
become susceptible to corrosion in a given
environment once a tensile stress is applied.
Once the stress cracks begin, they easily
propagate throughout the material, which in turn
allows additional corrosion and cracking to take place. The tensile stress is usually the result of
expansions and contractions that are caused by violent temperature changes or thermal cycles.
The best defense against stress corrosion is to limit the magnitude and/or frequency of the tensile
stress.
h.) Galvanic Corrosion
Galvanic corrosion is a little more difficult to keep
track of in the industrial world. You'll notice
below that simply adding a screw of the wrong
material can have severe consequences. Galvanic
corrosion occurs when two metals having different
composition are electrically coupled in the
presence of an electrolyte. The more reactive
metal will experience severe corrosion while the more noble metal will be quite well protected.
Perhaps the most infamous examples of this type of corrosion are combinations such as steel and
brass or copper and steel. Typically the steel will corrode the area near the brass or copper, even
in a water environment and especially in a seawater environment. Probably the most common
way of avoiding galvanic corrosion is to electrically attach a third, anodic metal to the other two.
This is referred to as cathodic protection.
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i.) Underdeposit Corrosion
This type of corrosion occurs at sites where
deposits allows a localized concentration of a
specific chemical, such as chloride or oxygen
to be notably different from the amount found
in the bulk water environment. This corrosion
mechanism is considered a secondary reaction,
whereas the primary reaction is uniform metal wastage, or general corrosion. However, this
secondary reaction can be more devastating and unpredictable.
4.2 Causes of Corrosion There are a number of variables which can influence corrosion rates, especially for mild steel
water systems. The following list provides some of the key variables which can influence mild
steel corrosion rates and their relative influence on corrosion:-
1.) Water Quality
2.) Temperature
3.) pH
4.) Oxidant
5.) Biomass or slime
6.) Chloride and Sulfates
7.) Calcium Hardness
8.) Metallurgy
Effect of pH: - The effect on Corrosion of the pH of water to which iron or steel is exposed is
influenced by temperature in the following manner.
The pH value is used to represent the acidity of a solution. First, consider the exposure of
iron to aerated water at room temperature (aerated water will contain dissolved oxygen).The
corrosion rate for iron as a function of pH is illustrated in figure
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In the range of pH 4 to pH 10, the corrosion rate of iron is relatively independent of the
pH of the solution. In this pH range, the corrosion rate is governed largely by the rate at which
oxygen reacts with absorbed atomic hydrogen, thereby depolarizing the surface and allowing the
reduction reaction to continue.
For pH values below 4.0, ferrous oxide (FeO) is soluble. Thus, the oxide dissolves as it
is formed rather than depositing on the metal surface to form a film. In the absence of the
protective oxide film, the metal surface is in direct contact with the acid solution, and the
corrosion reaction proceeds at a greater rate than it does at higher pH values.
It is also observed that hydrogen is produced in acid solutions below a pH of 4,
indicating that the corrosion rate no longer depends entirely on depolarization by oxygen, but on a
combination of the two factors (hydrogen evolution and depolarization).
For pH values above about pH 10, the corrosion rate is observed to fall as pH is
increased. This is believed to be due to an increase in the rate of the reaction of oxygen with
Fe(OH)2 (hydrated FeO) in the oxide layer to form the more protective Fe2O3 (note that this
effect is not observed in deaerated water at high temperatures).
Temperature: - The effect of temperature on atmospheric corrosion rates is complex in nature. An
increase in temperature will tend to stimulate corrosive attack by increasing the rate of
electrochemical reactions and diffusion processes. For a constant humidity, an increase in
temperature would therefore lead to a higher corrosion rate. Raising the temperature will,
however, generally lead to a decrease in relative humidity and more rapid evaporation of the
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surface electrolyte. By reducing the time of wetness in this manner, the overall corrosion rate
would tend to diminish.
For closed air spaces, such as indoor atmospheres, it has been pointed out that the increase
in relative humidity associated with a drop in temperature has an overriding effect on corrosion
rate. This implies that simple air conditioning, involving a decrease in temperature without
additional dehumidification, will accelerate atmospheric corrosion damage.
Chloride and sulphate ions: - Chloride ion contamination is detrimental in breaking down
passivity. Acid conditions may be established beneath deposits as aggressive ions segregate to
these shielded regions.
Suspended matter: - Mud, sand, silt, clay, dirt and other particles may enter a cooing water
system either as airborne contamination or as part of the system’s makeup water supply. In areas
of the system where sedimentation of these materials takes place, porous deposits are easily
formed and differential aeration cells are quickly established, which can cause more corrosion
damage than precipitated salts.
Microorganisms: - Microbial growth often presents very special problems. Hydrogen is
metabolized by many species, causing depolarization of the corrosion cell, similar to the
action caused by dissolved oxygen. Anaerobic bacteria form differential aeration cells and
accelerate local attack. Some species produce acidic compounds.
Metallurgy: - Metals are never absolutely flat, plane structures. All have surface flaws such as
scratches, crevices, etc in which the potential for electron loss and metal ion formation
increases, which make these areas anodic to the rest of metal. A stressed metal would
normally set up anodic sites at certain intergranular boundaries. Anodic site formation may result
from a number of causes detectable under macroscopic inspection. Inclusion of a non
homogenous metal or other metallic compound in the grain structure results in the formation of a
small galvanic cell in that area. Two adjacent grains of different density might create a corrosion
cell. Precipitation at metal grain boundaries will cause a corrosion cell to form, especially if the
precipitate is nobler than the metal itself.
An increase in the metal purity provides no grantee that corrosion will decrease. Aluminum
and iron may serve as examples of contrasting behavior. Aluminum’s resistance to corrosion
increases as its purity increases. The resistance of iron remains the same as its purity increases.
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Pure iron is no more resistant to corrosion than cast iron or steel. In the case of aluminum,
corrosion protection depends upon the formation of oxide films, which is aided by increases in
purity. For iron the controlling factors are the corrosion reactions themselves.
4.3 Corrosion Control Methods for Cooling water System
In principle, damage to cooling systems can be checked in many ways:
1.) Proper material selection
2.) Cooling water system design
3.) Continuous water treatment system
4.) Use of inhibitors
5.) Ferrous Sulfate dosing
6.) Protective Coatings
7.) Cathodic protection
8.) Passivation
9.) Biological control
10.) Scale control
11.) Systematic cleaning
Material Selection: - Corrosion control by means of cooling water design should take into
account two possibilities
1.) The water treatment program
2.) Selecting corrosion-resistant material
Proper material selection involves selecting a material that can be exposed to the cooling water
without the danger of corrosion.
Cooling water system design: - In planning a heat exchanger cooled by natural water, the first
step is to obtain information about water
a.) Empty the cooling system when in standstill condition. Many more cases of corrosion
have taken place when the cooling system was at a standstill than in operation.
b.) Avoid protruding weldments or crevices.
c.) Design sealing in such a way that seals cannot lift on water side.
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d.) Narrow gaps should be avoided in the design stage. If this is not possible, the gap width
should preferably be large.
e.) The flow velocities should neither be too low or too high as practical. Velocities of 3-8
ft/s are the minimum desired.
f.) Beware of galvanic coupling
g.) Always maintain vents on top tube sheets.
Water treatment: - By water treatment either remove aggressive components to a great extent or
add specific chemicals to the water. In this way, both corrosion and fouling are avoided. Water
treatments are generally divided into three main groups:
a.) Chlorination/settling/filtration to remove turbidity and microorganisms.
b.) Water softening to remove water hardness
c.) Partial or full demineralization for the removal of hardness and all dissolved salts.
Corrosion inhibitors: - The use of corrosion inhibitors is one of the foremost methods of
controlling corrosion in a cooling water system. The main effect that corrosion inhibitors have in
aqueous ferrous system is to reduce the initial corrosion rate sufficiently to allow the gamma-iron
oxide passive film to form and in some cases to take part directly in film formation.
The effectiveness, or corrosion inhibition efficiency, of a corrosion inhibitor is a function of many
factors like: fluid composition, quantity of water, flow regime etc. Some of the mechanisms of its
effect are formation of a passivation layer (a thin film on the surface of the material that stops
access of the corrosive substance to the metal), inhibiting either the oxidation or reduction
part of the redox corrosion system (anodic and cathodic inhibitors), or scavenging the
dissolved oxygen.
Some corrosion inhibitors are hexamine, phenylenediamine, dimethylethanolamine, sodium
nitrite, cinnamaldehyde, condensation products of aldehydes and amines (imines), chromates,
nitrites, phosphates, hydrazine, ascorbic acid, and others. The suitability of any given chemical
for a task in hand depends on many factors, from the material of the system they have to act in, to
the nature of the substances they are added into and their operating temperature.
An example of an anodic inhibitor is chromate which forms a passivation layer on aluminum
and steel surfaces which prevents the oxidation of the metal. Unfortunately, chromate is
carcinogenic in humans; the toxicity of chromates was featured in the film Erin Brockovich.
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Like hydrazine, the use of chromate to protect metal surfaces has been limited; for instance it is
banned from some products.Nitrite is another anodic inhibitor. If anodic inhibitors are used at
too low concentration, they can actually aggravate pitting corrosion, as they form a
nonuniform layer with local anodes.
An example of a cathodic inhibitor is zinc oxide, which retards the corrosion by inhibiting the
reduction of water to hydrogen gas. As every oxidation requires a reduction to occur at the same
time it slows the oxidation of the metal. As an alternative to the reduction of water to form
hydrogen, oxygen or nitrate can be reduced. If oxidants such as oxygen are excluded, the rate of
the corrosion can be controlled by the rate of water reduction; this is the case in a closed
recirculating domestic central heating system, where the water in the radiators soon becomes
anaerobic. This is a very different situation to the corrosion in a car door where the water is
aerobic. For instance, cars suffer from the fact that water can enter the cavity inside the door and
become trapped there. The fact that the oxygen concentration is not uniform within the layer of
water in the door then creates a differental aeration cell leading to corrosion. A cathodic inhibitor
would be of little use in such a situation as even after inhibiting the reduction of water, the
reduction of dioxygen would still be able to occur. A better method of preventing corrosion in the
car door would be to improve the design to prevent water being trapped in the door and to
consider using an anodic inhibitor such as phosphate.
One very good example of a cathodic inhibitor is a volatile amine present in steam; these are used
in the boilers used to drive turbines to protect the pipework in which the condensed water passes.
Here the amine is moved by the steam in a steam distillation to the remote pipework. The amine
increases the pH thereby making proton reduction less favorable. It is also possible that with
correct choice, the amine can form a protective film on the steel surface and, at the same time, act
as an anodic inhibitor. An inhibitor that acts both in a cathodic and anodic manner is termed a
mixed inhibitor. Hydrazine and ascorbic acid (vitamin C) both help reduce the rate of corrosion in
boilers by removing the dissolved oxygen from the water. However, as hydrazine is a highly toxic
carcinogen, its use is being discouraged.
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5. Case study 5.3 Corrosion problems related to Cooling Tower 5 heat exchangers. Coker cooling water exchangers are serviced by water from CT 5 (cooling tower). The number of
cooling water exchanger leaks in CT5 has been very high in the past.
Typical values of cooling water control parameter for month of March-07
Parameters Values (avg. value for March
’07)
a.)
b.)
c.)
d.)
e.)
f.)
g.)
h.)
i.)
pH
O-PO4 (total orthophosphonate),
Calcium Hardness, ppm as CaCO3
M-alkanity
Free chlorine
Chloride, ppm as Cl
Total Iron, ppm as Fe
Soluble Iron, ppm as Fe
Conductivity
7.2
11.4
136
30
0.25
448
2.07
0.9
2494
a.) Although the level of iron was within target range, occasional spikes resulting from
makeup water iron intrusion can raise the bulk water total iron >4ppm. Iron levels in this
range restrict bulk water-soluble orthophosphate levels because at total iron levels
>3ppm, iron phosphate precipitation becomes problematic.
b.) Bulk water and calcium conditions clearly present a competing scenario, which presents a
difficult water treatment chemistry choice. Since iron phosphate exchanger fouling and
resulting under-deposit, pitting-type corrosion has consistently been the major issue
resulting in excessive unit shutdown frequency; the only practical choice is to limit
orthophosphate levels in the bulk water. This reduces tenacity of the surface metal
passivation and therefore weakens the ability of the treatment program to restore
passivation after upset conditions have occurred.
c.) Desalination water and blowdown from CT-7 and CT-8 contain virtually zero calcium
and LTDS water makeup contains only ~15-20 ppm calcium. Therefore supplement
Central Engineering Services Reliance Industries Limited, Jamnagar
24
calcium must be added in the form of calcium chloride to achieve the minimum calcium
residual in the cooling water. So dozing additional amount of calcium chloride leads to
increase in the chloride level.
Based on number of failures in process critical heat exchangers, the following exchangers are
identified as critical.
a.) Compressor interstage cooler ME-RK372-S09A/B
b.) Absorber stripper feed cooler ME-RK372-S10 A/B
c.) Debutanizer overhead condenser ME-RK372-S11A/B
d.) Fractional Overhead condenser No.1 ME-RK371-S02
e.) Fractional Overhead condenser No.1 ME-RK371-S03
a, b, c are tube side cooling water exchanger and d, e are shell side cooling water exchanger.
Following is a table representing failures retubing and plugging operation done on these critical
exchangers.
Equipment
Metallurgy
Tubes CS,
Cu-Ni
Inspection findings,
Indicate Plugged (P), Retubed (RT), Metallurgy Upgrade
(MU), * for inspection only.
2001 2002 2003 2004 2005 2006
ME-
RK371-
S02
CS * * P RT
ME-
RK371-
S03
CS P P RT P
ME-
RK372-
S09A
CS RT P P RT
ME-
RK372-
S09B
CS * * RT RT
ME- CS * * * RT
Central Engineering Services Reliance Industries Limited, Jamnagar
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RK372-
S10A
ME-
RK372-
S10B
CS * * RT
ME-
RK372-
S11A
CS * RT RT RT P
ME-
RK372-
S11B
CS * RT(Partia
l)
RT
The corrosion inhibitor used in CT-5 is a passivating (anodic) inhibitor, which forms a
protective oxide film on metal surfaces. It is a good inhibitor because it can be used in
economical concentrations and the protective film is tenacious and tends to be rapidly repaired it
damaged. The test for O-PO4 an active ingredient in the corrosion inhibitor, should indicate a
level of 7-11 ppm soluble O-PO4 to ensure adequate corrosion protection.
The Deposit control agent (DCA used in CT-5 most likely contains a combination of
low molecular weight acrylate polymers, organophosphorus compounds and polyacrylic acid).
The DCA is a “threshold inhibitor” and delays or retards the rate of dissolved salt precipitation.
Crystals eventually form, depending on the degree of supersaturation and system retention time.
After stable crystals appear, their continued growth is retarded by adsorption of inhibitor.
A residual of 80-100 ppm should be maintained. A review of the available deposit analyses
confirms that more than 70% deposit is Fe2O3 in nature, because of this deposit there is
problem in heat transfer efficiency loss, under deposit corrosion and pitting corrosion. In
particular, the massive and expensive vertical heat exchangers (shell-side cooling water) have
been more problematic.
Central Engineering Services Reliance Industries Limited, Jamnagar
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0.00
200.00
400.00
600.00
800.00
1000.00
1200.00
4/1/
2006
7/1/
2006
10/1
/200
6
1/1/
2007
4/1/
2007
7/1/
2007
10/1
/200
7
1/1/
2008
4/1/
2008
7/1/
2008
10/1
/200
8
1/1/
2009
4/1/
2009
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
Chlorides(ppm)CR GEN
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
4/1/
2006
7/1/
2006
10/1
/200
6
1/1/
2007
4/1/
2007
7/1/
2007
10/1
/200
7
1/1/
2008
4/1/
2008
7/1/
2008
10/1
/200
8
1/1/
2009
4/1/
2009
0.001.002.003.004.005.006.007.008.009.0010.0011.0012.0013.0014.0015.0016.0017.0018.0019.0020.00
Silica (ppm)CR GEN
5.2 Graphs
1. Graph b/w Chloride ion concentration, General corrosion Rate and Time
Maximum allowable limit of chloride is <1000 ppm.
Corrosion rate should be less then 3 mpy (mills per year).
2. Graph B/w Silica level, Corrosion rate and Failure Rate
Silica level should be below 150 ppm.
a.) During April-06 to april-08 silica level it remained low at an avg value of 6 ppm
b.) From June-08 onwards silica level is high and corrosion rate is moderate while
no of failure reduced.
Central Engineering Services Reliance Industries Limited, Jamnagar
27
0.00
20.00
40.00
60.00
80.00
100.00
120.00
140.00
160.00
4/1/
2006
7/1/
2006
10/1
/200
6
1/1/
2007
4/1/
2007
7/1/
2007
10/1
/200
7
1/1/
2008
4/1/
2008
7/1/
2008
10/1
/200
8
1/1/
2009
4/1/
2009
0.002.004.006.008.0010.0012.0014.0016.0018.0020.00
TSS (ppm)CR GEN
0
100
200
300
400
500
600
700
4/1/
2006
7/1/
2006
10/1
/200
6
1/1/
2007
4/1/
2007
7/1/
2007
10/1
/200
7
1/1/
2008
4/1/
2008
7/1/
2008
10/1
/200
8
1/1/
2009
4/1/
2009
0.002.004.006.008.0010.0012.0014.0016.0018.0020.00
ORP SUPPLYCR GEN
3. Graph b/w total suspended particles and corrosion rate
a.) During October-06 to October-07 TSS level was in between 15-25 ppm range.
b.) October-07 onwards, there is a high degree of correlation b/w TSS and corrosion
rate.
4. Graph of Oxygen reduction potential
According to Graph when ORP supply is low corrosion rate is high.
Central Engineering Services Reliance Industries Limited, Jamnagar
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0.00
2000.00
4000.00
6000.00
8000.00
10000.00
12000.00
14000.00
4/1/
2006
7/1/
2006
10/1
/200
6
1/1/
2007
4/1/
2007
7/1/
2007
10/1
/200
7
1/1/
2008
4/1/
2008
7/1/
2008
10/1
/200
8
1/1/
2009
4/1/
2009
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
P/C RATIOCR GEN
0.00
100.00
200.00
300.00
400.00
500.00
600.00
4/1/
2006
7/1/
2006
10/1
/200
6
1/1/
2007
4/1/
2007
7/1/
2007
10/1
/200
7
1/1/
2008
4/1/
2008
7/1/
2008
10/1
/200
8
1/1/
2009
4/1/
2009
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
18.00
20.00
COD (ppm)CR GEN
5. P/C ratio versus General Corrosion rate
a.) During October-06 to April-07 P/C ratio was relatively very high as compared to
other values.
b.) No of failures were very less during October-06 to April-07.
c.) After April-07 there were frequent failures in Heat-exchangers.
6. Graph B/w Chemical Oxygen Demand (COD) and corrosion Rate
COD should be below 250 ppm.
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-1.001.003.005.007.009.00
11.0013.0015.0017.0019.0021.0023.0025.00
4/1/
2006
7/1/
2006
10/1
/200
6
1/1/
2007
4/1/
2007
7/1/
2007
10/1
/200
7
1/1/
2008
4/1/
2008
7/1/
2008
10/1
/200
8
1/1/
2009
4/1/
2009
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
18.00
20.00
TP (ppm)CR GEN
0.00
0.50
1.00
1.50
2.00
2.50
4/1/
2006
7/1/
2006
10/1
/200
6
1/1/
2007
4/1/
2007
7/1/
2007
10/1
/200
7
1/1/
2008
4/1/
2008
7/1/
2008
10/1
/200
8
1/1/
2009
4/1/
2009
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
Zinc (ppm)CR GEN
7. Phosphate level and Corrosion rate
Phosphate level is almost constant within range of 10-15 ppm.
8. Graph of Zinc and corrosion rate
*Vertical black lines represent failures of exchangers of coker plant.
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5.3 Analysis
During April-06 to october-06 corrosion rate was very high (avg value 15 mpy). Large
number of failures were seen during this period.
Probable reasons
a.) Oxidation Reduction Potential (ORP) was very low during this period, so water lacks in
HOCl species, due to that more iron get reduced.
b.) Silica level was low as compared to other Cooling towers, leading to less formation of
passive layer.
During October-06 to april-07 pitting corrosion was the main factor while general corrosion
rate was very low.
Probable reasons
a.) Chlorides level (avg value 700ppm) was high which initiate localised corrosion, which is
required to increase ORP level as chlorine gas dozing forms both chloride ions and HOCl
species.
During October-07 to April-08 there were lots of failure.
Probable reasons
a.) As before october-07 there was a period of high pitting corrosion which might lead to
failures during this period.
b.) After april-08 silica level was increased to 25ppm from 10 ppm which leads to
stabilisation of corrosion rate and less failures.
After april-08 corrosion rate is in between 1 to 5 mpy(satisfying its designed values) but it is
following a similar curve as TSS.
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5.4 Comparison studies between water supplies of cooling tower 5 and 3
Comparision studies between cooling tower 5 and 3.
As both cooling towers uses LTDS water supply, so their water supply is quite similar.
CT 3 CT5
date
del
PO4
total
iron COD ORP
General
Corrosion
rate
del
PO4
total
iron COD ORP
General
Corrosion
rate
01-Feb-
09 1.25 1.95 224 503 1.31 1.35 1.72 193 518 1.50
02-Feb-
09 1.24 1.90 224 517 1.30 1.30 1.65 193 485 1.40
03-Feb-
09 1.22 1.86 224 525 1.30 1.29 1.57 193 442 1.40
04-Feb-
09 1.21 1.87 267 531 1.31 1.35 1.69 220 345 1.50
05-Feb-
09 1.19 1.89 267 544 1.32 1.27 1.53 220 410 1.70
06-Feb-
09 1.21 1.91 267 583 1.33 1.21 2.17 220 418 1.80
07-Feb-
09 1.17 1.86 295 603 1.32 1.15 2.65 247 442 2.10
08-Feb-
09 1.19 1.92 295 521 1.31 1.24 3.31 247 461 2.20
09-Feb-
09 1.27 1.97 295 475 1.32 1.32 3.73 247 433 2.30
10-Feb-
09 1.25 1.88 295 539 1.31 1.26 3.65 247 405 2.20
11-Feb-
09 1.24 1.82 173 587 1.30 1.26 3.55 239 420 2.10
12-Feb-
09 1.20 1.76 173 630 1.20 1.32 3.49 239 511 1.90
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13-Feb-
09 1.22 1.75 173 615 1.21 1.35 3.45 239 581 1.80
14-Feb-
09 1.21 1.74 147 587 1.22 1.35 3.39 197 475 1.80
15-Feb-
09 1.17 1.70 147 565 1.21 1.29 3.40 197 502 1.80
16-Feb-
09 1.22 1.70 147 529 1.20 1.25 3.49 197 529 1.80
17-Feb-
09 1.18 1.65 147 481 1.21 1.30 3.61 197 491 1.70
18-Feb-
09 1.20 1.72 267 441 1.22 1.24 3.35 257 421 1.60
19-Feb-
09 1.18 1.72 303 359 1.23 1.26 3.19 310 383 1.60
20-Feb-
09 1.26 1.92 345 275 1.24 1.20 3.12 351 321 1.50
Following are the curves between the difference in values of parameters between CT 5 and CT 3
a.) Curve between difference in total iron and general corrosion rate of CT 5 and CT 3 over a
period of time.
-1.00-0.500.000.501.001.502.002.503.003.504.004.50
2/1/092/15/09
3/1/093/15/09
3/29/094/12/09
4/26/09-0.20
0.00
0.20
0.40
0.60
0.80
1.00
1.20
total ironCr gen
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b.) Curve between difference in ORP and general corrosion rate of CT 5 and CT 3 over a
period of time
-250
-200
-150
-100
-50
0
50
100
150
200
250
2/1/092/15/09
3/1/093/15/09
3/29/094/12/09
4/26/09
-0.20
0.00
0.20
0.40
0.60
0.80
1.00
1.20
ORPCr gen
c.) Curve between difference in del PO4 and total iron rate of CT 5 and CT 3 over a period of
time.
-0.40
-0.20
0.00
0.20
0.40
0.60
0.80
1.00
2/1/092/15/09
3/1/093/15/09
3/29/094/12/09
4/26/09
-1.00-0.500.000.501.001.502.002.503.003.504.004.50
del PO4total iron
.
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d.) Curve between difference in del PO4 and total iron rate of CT 5 and CT 3 over a period of
time.
-60
-40
-20
0
20
40
60
80
2/1/093/1/09
4/1/09
-250
-200
-150
-100
-50
0
50
100
150
200
250
CODORP
Observations
a.) From the graph between diff. in iron and corrosion rate it can be established that
whenever iron increases in CT 5 corrosion rate increases in same proportion
b.) According to graph increase in Phosphate level in CT 5 as compared to CT 3 leads to
increase in corrosion rate.
c.) When there is increase in ORP level in CT 5 as compared to CT 3 corrosion rate
decreases.
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6. Recommendations
a.) Level of iron directly affect corrosion rate, occasional spikes resulting from makeup
water iron intrusion can raise the bulk water total iron >4ppm. Iron levels in this range
restrict bulk water-soluble orthophosphate levels because at total iron levels >3ppm, iron
phosphate precipitation becomes problematic. So, there should be online monitoring of
iron level in cooling water of CT 5, and whenever it increases beyond 2.5 ppm measures
should be taken to bring it down.
b.) Higher level of chlorides leads to high pitting corrosion in cooling tower 5, which is main
reason of failures in critical exchangers, so it’s very important to check the level of
chlorides. Chlorides came into system from cooling water and additionally from dozing
of calcium chloride (to increase calcium concentration) and chlorine dioxide (to increase
ORP). Chlorides level should remain below <400 ppm. It can be done by ion exchange
with hydroxide (OH-) ions which in turn increase pH.
c.) pH level should be in between 7.2-7.6, basicity would reduce corrosion.
d.) Exchangers of Coker plant are especially prone to H2S leak, so if there is any leak then
there will be drop in pH value and ORP. So, leaking exchanger should be identified and it
should be either bypassed or treat water at the outlet of leaking exchanger.
e.) More frequent back flushing should be scheduled to reduce "fouling" which is the build-
up of contamination (dirt) clinging to the inside walls of those tubes when non-treated
water is used such as river or lake water. Fouling will both impair flow and reduce heat
transfer and lead to under-deposit corrosion and pitting corrosion.
Central Engineering Services Reliance Industries Limited, Jamnagar
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7. References
1. Drew “Principles of industrial water treatment”
2. Eric C. Guyer, David L. Brownell “Handbook of Applied Thermal Design”
3. Corrosionist The Website of Corrosion Protection and Corrosion Prevention
4. The NALCO guide to cooling water system failure analysis.
5. Calcium phosphates in biological and industrial systems By Zahid Amjad.
6. A Discussion of the Effect of pH on the Solubility of Hydrogen Sulfide by John J.
Carroll.