RIL summer intern report

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


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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.


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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


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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


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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


25

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.


26

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.


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.


28

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.


29

-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.


30

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.


31

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


32

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


33

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

.


34

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.


35

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.


36

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.

Documents

RIL summer intern report