report on Failure analysis of shell and tube heat exchangers

College of Engineering Pune

Abstract:

Heat exchanger is the equipment which is used to reduce temperature of one process

fluid, which is desired to cool, by transferring heat to another fluid which is desired to heat

with or without inter-mixing the fluid or changing the physical state of the fluid.

There are various causes by which Heat exchanger may fail. As they are used at wide

range of temperatures hence there is possibility that crack may induce in it and can cause

catastrophic failure. Heat exchanger may fail by corrosion of tubes. Vibrations and fatigue

are the examples of most dangerous causes that can cause failure. The study of ‘Failure

analysis of Shell and tube Heat Exchangers’ will help to understand methods to analyze

various types of failure in Shell and tube Heat Exchangers .

Chapter 1 deals with the introductory aspects of Heat Exchangers and its various

types. Various operations performed by Heat Exchangers and types of services in which

Heat Exchangers are used are also explained in this chapter.

The common failures of Heat Exchangers are described in Chapter 2. It includes

maintenance of Heat Exchangers and Fouling in Heat Exchangers. Also it describes types of

failure such as Stress Corrosion cracking, wear failure, creep in metals and stress rupture. It

also deals with causes of failure in Heat Exchangers such as vibration, tube breaking,

unexpected wall thinning of Heat Exchangers tubes, fracture in weld portion and tube

bending.

There are various methods of failure analysis of Heat Exchangers. It includes failure

analysis based on Thermodynamics point of view, based on fracture mechanics point of

view, vibration analysis, optimization of design, and reliability of design. All these aspects

are included in chapter 3.

The remedies for failures and conclusions on design improvement of Heat

Exchangers in order to minimize chances of failures with improved reliability are described

in chapter 4.


Contents:

1. Introduction of Heat Exchangers ………………………….................... 4

1.1Heat Exchangers……………………………………………... 4

1.2 Components of Heat Exchangers ………………………… 4

1.3 Types of Heat Exchanger based on ………………………... 5

1.3.1 Nature of process …………………………… 5

1.3.2 Relative direction of fluid motion ………….. 7

1.3.3 Design and construction features …………… 9

1.3.4 Physical state of fluids ……………………… 11

1.4 Advantages and disadvantages of Heat Exchangers………… 11

2. Failures in Heat exchangers ………………………………….................. 12

2.1 Operational problems in Heat Exchangers ………………… 12

2.2 Fouling in Heat Exchangers ……………………………….. 13

2.3 Types of failures ……………………………………………. 14

2.3.1 Stress corrosion cracking………………... 14

2.3.2 Wear failures …………………………… 15

2.3.3 Fatigue failures …………………………. 16

2.3.4 Creep in metals …………………………. 17

2.3.5 Corrosive failures ………………………. 18

2.4 Causes Of failures ………………………………………….. 20

2.4.1 Vibration ……………………………….. 20

2.4.2 Corrosion ………………………………. 20

2.4.3 Over heating of tubes ………………….. 21

2.4.4 Tube breaking ………………………….. 21

2.4.5 Fracture in weld portion ………………... 21

2.4.6 Tube bending …………………………… 21

3. Analysis of failures of Heat Exchangers ………………………………. 22

3.1 Checking design from heat transfer point of view ………… 22

3.2 Improving life of tube from fracture mechanics point of view

and reducing induced vibrations ……………………………

24

3.3 Cost optimization of shell and tube heat exchanger.............. 26

3.3.1 Particle swarm optimization (PSO) algorithm……….. 27

4. Conclusions ……………………………………………………………..

References ………………………………………………………………

29

30


List of Figures:

1.1 Schematic diagram of shell and tube heat exchanger …………………………. 4

1.2 Direct Contact Heat Exchangers ……………………………………………… 5

1.3 Distributions of temperature along tube axis ………………………………….. 8

1.4 Counter Flow Heat Exchange ……………………………………………………….. 8

1.5 Cross flow heat exchange …………………………………………………….. 9

1.6 One shell pass and two tubes pass heat exchanger ……………………………. 10

2.1 schematic of stress corrosion cracking ………………………………………... 14

2.2 Wear Failures ………………………………………………………………….. 15

2.3 Graph-1 S-N curve ………………………………………………..................... 16

2.4 Variation in stress ……………………………………………………………... 17

2.5 Strain-time curve ……………………………………………………………… 18

3.1 Effect of crack growth on Sigmoidal curve (log-log scale) …………………… 24


CHAPTER-1

Introduction of Heat Exchangers

1.1 Heat Exchangers

A heat exchanger may be defined as equipment which transfers the energy from the

hot fluid to a cold fluid or vice versa, with maximum rate and minimum investment and

running cost. The heat exchanger is used to reduce the temperature of one process fluid,

which is desirable to heat without inter-mixing the fluids or changing the physical state of

the fluids [1-3].

A shell and tube heat exchanger is a class of heat exchanger designs [6]. It is the

most common type of heat exchanger in oil refineries and other large chemical processes,

and is suited for higher pressure applications. As its name implies, this type of heat

exchanger consists of a shell (a large pressure vessel) with a bundle of tubes inside it. One

fluid runs through the tubes, and another fluid flows over the tubes (through the shell) to

transfer heat between the two fluids. The set of tubes is called a tube bundle, and may be

composed of several types of tubes: plain, longitudinally finned, etc. There can be many

variations on the shell and tube design. Typically, the ends of each tube are connected to

plenums (sometimes called water boxes) through holes in tube sheets. The tubes may be

straight or bent in the shape of a U, called U-tubes.

Condensers are used to cool the temperature of a process vapor to the point where it

will become a liquid by the transfer of heat to another fluid without inter-mixing the fluids.

Water or air is used to condense the vapor [2].

1.2 Components of Heat Exchangers

Fig 1.1 Schematic diagram of shell and tube heat exchanger [7]


1 Stationary Head-Channel 15 Floating Tubes sheet

2 Stationary Head-Bonnet 16 Floating Head Cover

3 Stationary Head Flange 17 Floating Head Flange

4 Channel Cover 18 Floating Head Baring Device

5 Stationary Head Nozzle 19 Split Shear Ring

6 Stationary Tube sheet 20 Slip-On Backing Flange

7 Tubes 21 Floating Head Cover-External

8 Shell 22 Floating Tube Sheet Skirt

9 Shell cover 23 Packing Box

10 Shell Flange 24 Packing

11 Shell flange- Read Head End 25 Packing Gland

12 Shell Nozzle 26 Lantern Ring

13 Shell Cover Flange 27 Tie rods and Spacers

14 Expansion joint 28 Transverse Baffles/support Plates

1.3 Types of Heat Exchanger

In order to meet the widely varying applications several types of heat exchangers

have been developed which are classified on the basis of nature of heat exchange process,

relative direction of fluid motion, design and constructional features and physical state of

fluids[3].

1.3.1 NATURE OF HEAT EXCHANGE PROCESS.

Heat exchangers on the basis of nature of heat exchange process are classified as:

i. Direct contact opened heat exchangers.

ii. Indirect contact heat exchangers.

a. Regenerators.

b. Recuperators

I. Direct Contact Heat Exchanger

In a direct contact heat exchanger, exchange of heat takes place by direct mixing of

hot and cold fluids and transfer of heat and mass takes place simultaneously. The use of

such units is made under conditions where mixing of two fluids is either harmless or

desirable.


Fig.1.2 Direct Contact Heat Exchangers [3]

II. Indirect Contact Heat Exchangers

In this type of heat exchangers, the heat transfer between two fluids could be carried

out by transmission through wall which separates the two fluids.

a. Regenerator

In a regenerator type of heat exchangers the hot and cold fluids pass alternatively

through a space containing solid particles (matrix), these particles providing alternatively a

sink and a source for heat flow. Example: IC Engine and Gas Turbine.

The performance of these regenerators is affected by the following parameters

1. Heat capacity of Regenerating Materials.

2. The rate of absorption

3. The release of heat.

Advantages of regenerators are:

1. Higher heat transfer coefficient.

2. Less weight per KW of the plant.

3. Minimum pressure loss

4. Quick response to load variations

5. Small bulk weight.

Disadvantages of regenerators are:

1. Costlier compared to recuperative heat exchangers.

2. Leakage is the main trouble ; therefore, perfect sealing is required,


b. Recuperators

Recuperator is the most important type of heat exchanger in which the following

fluids exchanging heat are on either side of dividing wall. These heat exchangers are used

when two fluids cannot be allowed to mix i.e., when the mixing is undesirable.

Examples: - 1. Oil Coolers, Intercoolers,

2. Automobile radiators.

Advantages of a recuperator are:

1. Easy construction

2. More economical

3. More surface area for heat transfer.

4. Much suitable for stationary plants.

1.3.2 Relative Direction of fluid motion

According to relative directions of two fluids streams the heat exchangers are

classified into following three categories.

i. Parallel flow or unidirectional flow.

ii. Counter flow.

iii. Cross flow

i. Parallel flow heat exchanger.

In parallel flow heat exchanger as the name suggest the two fluid streams (hot and cold)

travel in the same direction. The two streams enter at one end and leave at the other end.

The flow arrangements and variations of temperatures of the fluid stream in case parallel

flow heat exchangers are shown in fig. 1.3. It is evident from the figure that the temperature

difference between the hot and the cold fluid goes on decreasing from inlet to outlet. Since

this type of heat exchangers needs a large area of heat transfer it is rarely used in practice.

Example: oil coolers, oil heaters, water heaters


As the two fluids separated by a wall, this type of heat exchanger may be called

parallel flow recuperated or surface heat exchanger.

Fig. 1.3 Distributions of temperature along tube axis [1]

ii. Counter Flow Heat Exchanger

In a counter flow heat exchanger, the two fluids flow in opposite direction. The hot

and cold fluid enters the opposite ends. The flow arrangements and temperature distribution

for such a heat exchanger are shown in figure. The temperature difference between the

fluids remains more or less nearly constant. This type of heat exchanger due to counter flow

gives maximum rate of heat transfer for a given surface area. Hence such heat exchangers

are most favored for heating and cooling fluids

Fig. 1.4 Counter Flow Heat Exchange [2]

College Of Engineering Pune

ii. Cross Flow Heat Exchanger

In cross flow heat exchangers the two fluids (hot and cold) cross one another in space

usually at right angles. Fig.1.5 Shows schematic diagram of Cross flow heat exchangers

Fig.1.5 Cross flow heat exchange [1]

Referring to fig 1.5 hot fluid flow in the separate column and there is mixing in the

fluid streams. The cold fluid is perfectly mixed as its flow through the exchanger. The

temperature of this mixed fluid will be uniform across any section, and will vary only in the

directions of the flow. Example: cooling unit of refrigeration system.

Referring to figure, In this case each of the fluid follows a prescribed path and is

unmixed as it flows through heat exchanger. Hence the temperature of the fluid leaving the

heater section is not uniform.

Example: automobile radiator

1.3.3 DESIGN AND CONSTRUCTIONAL FEATURES

On the basis of design and constructional features, the heat exchangers are classified

as under

I. Concentric Tubes

In this type, two concentric tubes are used each carrying one of the fluids. The

direction flow may be parallel or counted as depicted in figure. The effectiveness of the heat

exchanger is increased by using swirling flow [4].

COLD FLUID (!N)

COLD FLUID (UOT)


II. Shell and Tube

In this type of heat exchanger one of the fluid flows through a bundle of tube

enclosed by a shell. The other fluid is forced through the shell and it flows over the outside

of surface of the tubes. Such an arrangement employed where reliability and heat transfer

effectiveness are important. With the use of multiple tubes heat transfer rate is amply

improved due to increased surface area.

III. Multiple Shell and Tube Pass

Fig. 1.6 One shell pass and two tubes pass heat exchanger [8]

Multiple shell and tube passes are used for enhancing the overall heat transfer.

Multiple shell passes is possible where the fluid flowing through the shell is re-routed. The

shell side fluid is forced to flow back and forth across the tubes in the by baffles. Multiple

tube pass exchangers are those which re-route the fluid through tubes opposite direction

IV. Compact Heat Exchanger

These are special purpose heat exchangers and have a very large transfer surface

area per unit volume of the exchanger. They are generally employed the convective heat

transfer co-efficient associated with one of the fluids is much smaller than that associated

with the other fluid.

Example: Plate - Fin, flattened fin tube exchangers.


1.3.4 PHYSICAL STATE OF FLUIDS

Depending upon the physical state of the fluids the heat exchangers are classified as

follows:

I. Condenser

II. Evaporators

I. Condenser

In a condenser, the condensing fluid remains at constant temperature throughout the

exchanger, while the temperature of colder fluid gradually increases from inlet to outlet.

The hot fluid losses latent part of heat and it is accepted by the cold fluid.

II. Evaporators

In this case, the boiling fluid remains at constant temperature while the temperature

of the hot fluid gradually decreases from inlet to outlet.

1.4 Advantages and disadvantages of Heat Exchangers

Advantages

1. Energy Savings

2. No Additional boilers are needed.

3. Condensation provides less space and safety operations.

Disadvantages

1. The use of heat exchange causes the flow restriction; hence, additional

pumps are required to correct the flow.

2. Friction losses.

3. Operation difficulties such as flange leakage.

4. Failure of heat exchanger.

5. Maintenance cost and operating cost.


CHAPTER-2

Failures in Heat exchangers

2.1 Operational problems in Heat Exchangers

Operational problems in heat exchangers may be broadly classified into three groups

[1-4].

I. Structural Problems

II. Performance Problems

III. Metallurgical problems

I. Structural Problems

Structural problems are the most serious; failure is often swift and irreversible.

Failures caused by flow - induced vibration of heat exchanger tubes over shadow all other

structural failures. Tube to tube sheet joints failure is also a frequent operational problem.

The other type of structural failure encountered in heat exchanger operation is

leakage from bolted joints. Leaks frequently occurred nozzle flanges due to moment loading

of the joint caused by thermal expansion of the interconnecting piping. In some cases, non-

temperature distribution in the tube sheet or cover in multiple pass design induces joint

leakage. Replacement of the leaking gaskets with one having more appropriate loading and

relaxation properties is usually the panacea for such structural problems.

II. Performance Problems

The excessive tube fouling usually causes performance problems. Deposition of foul

ants on the inside of the tube surface reduces the available flow area and increase the skin

friction, causing an increase in pressure loss and decrease in heat transfer. Uneven rates of

fouling of tubes usually occur in units with low flow velocity design. Uneven fouling may

occur on the shell side of the tubes due to a poor baffling scheme. This leads to a flow

misdistribution. Highly non-uniform fouling on severely modifies the metal temperature

profile in some tubes resulting in large tubes - to tube sheet joint leads.


Thermal stresses in the internal of the heat exchanger can cause serious degradation

of heat duty. The most obvious example is failure of welds joining pass partition plates to

each other and to the channel.

III. Metallurgical problems

Stress corrosion, galvanic corrosion, and erosion are the most frequently reported

metallurgical problems. Care in the selection of material can eliminate most of these

problems, where the galvanic action cannot be completely eliminated. The use of waster

anode is recommended.

2.2 Fouling in Heat Exchangers

In a heat exchanger during normal operations the tube surface gets covered by

deposits of ash, soot, and dirt and scale etc. This phenomenon of rust formation and

deposition fluid impurity is called fouling.

Fouling Processes

I. Precipitation or crystallization fouling

II. Sedimentation or particulate fouling

III. Chemical reaction fouling or polymerization

IV. Corrosion fouling

V. Biological fouling

VI. Freeze fouling

Parameter affecting fouling

I. Velocity

II. Temperature

III. Water chemist

IV. Tube material

Prevention of fouling

The following methods may be used to keep fouling minimum

I. Design of heat exchanger

II. Treatment of process system

III. By using clean system


2.3 Types of failures

2.3.1 Stress corrosion cracking

Stress corrosion cracking is a failure mechanism that is caused by environment,

susceptible material, and tensile stress. Temperature is a significant environmental factor

affecting cracking.

For stress corrosion cracking to takes place all three conditions must be met

simultaneously. The component needs to be in a particular crack promoting environment,

the component must be made of a susceptible material, and there must be tensile stresses

above some minimum threshold value. An externally applied load is not required as the

tensile stresses may be due to residual stresses in the material. The threshold stresses are

commonly below the yield stress of the material.

Stress Corrosion Cracking Failures:

Stress corrosion cracking is an insidious type of failure as it can occur without an

externally applied load or at loads significantly below yield stress. Thus, catastrophic failure

can occur without significant deformation or obvious deterioration of the component.

Pitting is commonly associated with stress corrosion cracking phenomenon.

Fig 2.1 schematic of stress corrosion cracking [6]

Aluminum and stainless steel are well known for stress corrosion cracking

problems. However, all metals are susceptible to stress corrosion cracking in the right

environment.


Controlling Stress Corrosion Cracking

There are several methods to prevent stress corrosion cracking. One common

method is proper selection of the appropriate material. A second method is to remove the

chemical species that promotes cracking. Another method is to change the manufacturing

process or design to reduce the tensile stresses. AMC can provide engineering expertise to

prevent or reduce the likelihood of stress corrosion cracking in your components.

2.3.2 Wear failures

Wear may be defined as damage to a solid surface caused by the removal or

displacement of material by the mechanical action of a contacting solid, liquid, or gas. It

may cause significant surface damage and the damage is usually thought of as gradual

deterioration. While the terminology of wear is unresolved, the following categories are

commonly used.

Fig.2.2 Wear Failures [6]

Adhesive wear

Adhesive wear has been commonly identified by the terms galling, or seizing

Abrasive wear

Abrasive wear, or abrasion, is caused by the displacement of material from a solid

surface due to hard particles or protuberances sliding along the surface

Erosive wear


Erosion, or erosive wear, is the loss of material from a solid surface due to relative

motion in contact with a fluid that contains solid particles. More than one mechanism can be

responsible for the wear observed on a particular part.

2.3.3 Fatigue failures

Metal fatigue is caused by repeated cycling of the load. It is a progressive localized

damage due to fluctuating stresses and strains on the material. Metal fatigue cracks initiate

and propagate in regions where the strain is most severe.

The process of fatigue consists of three stages:

Initial crack initiation

Progressive crack growth across

Final sudden fracture of the remaining cross section

Fig 2.3 Graph-1 S-N curve [5]

Schematic of S-N Curve, showing increase in fatigue life with decreasing stresses

Stress Ratio

The most commonly used stress ratio is R, the ratio of the minimum stress to the

maximum stress (Smin/Smax).

If the stresses are fully reversed, then R = -1.

If the stresses are partially reversed, R = a negative number less than 1.

If the stress is cycled between a maximum stress and no load, R = zero.

Cycles to Failure N

Stress S


If the stress is cycled between two tensile stresses, R = a positive number less than 1.

Variations in the stress ratios can significantly affect fatigue life. The presence of a

mean stress component has a substantial effect on fatigue failure. When a tensile mean

stress is added to the alternating stresses, a component will fail at lower alternating stress

than it does under a fully reversed stress.

Fig 2.4 variation in stress [14]

Preventing Fatigue Failure

The most effective method of improving fatigue performance is improvements in

design:

1. Eliminate or reduce stress raisers by streamlining the part

2. Avoid sharp surface tears resulting from punching, stamping, shearing, or other

processes

3. Prevent the development of surface discontinuities during processing.

4. Reduce or eliminate tensile residual stresses caused by manufacturing.

5. Improve the details of fabrication and fastening procedures

2.3.4 Creep in metals

High temperature progressive deformation of a material at constant stress is called

creep [6]. High temperature is a relative term that is dependent on the materials being

evaluated. A typical creep curve is shown below:

Cycles to failures


Fig 2.5 strain-time curve [11]

In a creep test a constant load is applied to a tensile specimen maintained at a

constant temperature. Strain is then measured over a period of time. The slope of the curve,

identified in the above figure, is the strain rate of the test during stage II or the creep rate of

the material.

Primary creep, Stage I, is a period of decreasing creep rate. Primary creep is a

period of primarily transient creep. During this period deformation takes place and the

resistance to creep increases until stage II. Secondary creep, Stage II, is a period of roughly

constant creep rate. Stage II is referred to as steady state creep. Tertiary creep, Stage III,

occurs when there is a reduction in cross sectional area due to necking or effective reduction

in area due to internal void formation.

2.3.5 Corrosive failures

Corrosion is chemically induced damage to a material that results in deterioration of

the material and its properties [6]. This may result in failure of the component. Several

factors should be considered during a failure analysis to determine the affect corrosion

played in a failure. Examples are listed below:

1. Type of corrosion

2. Corrosion rate

3. The extent of the corrosion

4. Interaction between corrosion and other failure mechanisms

Corrosion is a normal, natural process. Corrosion can seldom be totally prevented,

but it can be minimized or controlled by proper choice of material, design, coatings, and


occasionally by changing the environment. Various types of metallic and nonmetallic

coatings are regularly used to protect metal parts from corrosion.

Stress corrosion cracking necessitates a tensile stress, which may be caused by

residual stresses and a specific environment to cause progressive fracture of a metal.

Aluminum and stainless steel are well known for stress corrosion cracking problems.

However, all metals are susceptible to stress corrosion cracking in the right environment.

Laboratory corrosion testing is frequently used in analysis but is difficult to

correlate with actual service conditions. Variations in service conditions are sometimes

difficult to duplicate in laboratory testing.

Pitting Corrosion

Pitting is a localized form of corrosive attack [6]. Pitting corrosion is typified by the

formation of holes or pits on the metal surface. Pitting can cause failure due to perforation

while the total corrosion, as measured by weight loss, might be rather minimal. The rate of

penetration may be 10 to 100 times that by general corrosion.

Pits may be rather small and difficult to detect. In some cases pits may be masked

due to general corrosion. Pitting may take some time to initiate and develop to an easily

viewable size.

Pitting occurs more readily in a stagnant environment. The aggressiveness of the

corroding will affect the rate of pitting. Some methods for reducing the effects of pitting

corrosion are listed below:

• Reduce the aggressiveness of the environment

• Use more pitting resistant materials Uniform Corrosion

Uniform or general corrosion is typified by the rusting of steel. Other examples of

uniform corrosion are the tarnishing of silver or the green patina associated with the

corrosion of copper. General corrosion is rather predictable. The life of components can be

estimated based on relatively simple immersion test results. Allowance for general

corrosion is relatively simple and commonly employed when designing a component for a

known environment.


Some common methods used to prevent or reduce general corrosion are listed below:

1. Coatings

2. Inhibitors

3. Cathodes protection

4. Proper materials selection

2.4 Causes Of failures

2.4.1 Vibration

Damage from the tube vibration has become an increasing phenomenon as heat

exchanger sizes and quantities of flow have increased [9-10]. The shell side flow baffle

configuration and unsupported tube span are of prime consideration mechanism of tube

vibration are follows.

1. Vortex shelling

The vortex shelling frequency of the fluid in cross flow over the tubes may coincide

with a natural frequency of tube and excite large resonant vibration amplitudes.

2. Fluid elastic coupling

Fluid flowing over tubes causes them to vibrate with a whirling motion. The

mechanism of fluid elastic coupling occurs .When a critical velocity exceed and the

vibration then become self exited and grows in amplitude .This mechanism frequently

occurs in process heat exchangers which suffer vibration damage.

3. Pressure fluctuation

Turbulent pressure fluctuations which develop in the wake of a cylinder or are

carried to the cylinder from upstream may provide a potential mechanism for tube vibration

.The tube respond to the portion of the energy spectrum that is close to their natural

frequency.

2.4.2 Corrosion

High temperature in the system can cause oxidation due to its cause corrosion [16].

Chemical reactions of hydrocarbon can also causes corrosion.


2.4.3 Over heating of tubes

In the shell and tube heat exchanger at the inlet (bottom of the shell) hydrocarbon is

in liquid state [14]. The inlet temperature of hydrocarbon is 217°c outlet temperature is

229°c.

The heating fluid hot oil called Therminol passes through the tubes. The inlet of hot

oil is at top of the bundle and outlet is at the bottom. The inlet temperature of the hot oil is

320°c and the outlet temperature is 270°c. If there is any obstruction or processing delay in

the production line it causes the shortage of hydrocarbon supply in to the heat exchanger.

During when the hot oil will be passing through the tubes this converts the top hydrocarbon

in bundle to vapor state. In the vapor state convective heat transfer (h) is less. This causes

the top 120 tube become overheat.

2.4.4 Tube breaking

The corrosion and erosion in the tube can cause tube brakeage

2.4.5 fracture in weld portion

The clearance between the shell and tube bundle can cause vibration in the tube

bundle. This causes the fracture formation in the tube sheet.

2.4.6 tube bending

The clearance between the shell and tube and overheating can cause the bending of

tubes.


CHAPTER-3

Analysis of failures of Heat Exchangers

3.1 Checking design from heat transfer point of view

Method to check whether design is safe under heat transfer point of view is

explained as follows [15]:

1. Calculate Logarithmic mean temperature distribution.

Hear,

LMTD = Logarithmic mean temperature distribution

Th1 = temperature of hot fluid at inlet

Th2 = temperature of hot fluid at outlet

Tc1 = temperature of cold fluid at inlet

Tc2 = temperature of cold fluid at outlet

For The multi pass cross flow heat exchanger,

Correction factor ‘F’ can be find out from heat transfer data book using Temperature

ratio 'P' and Capacity ratio 'R'

2. Now consider flow inside the tube and calculate Convective heat transfer coefficient

'hi'


Here,

Nu = Nusset number

Re = Reynold’s number

Pr = Prandlt’s number

…

3. Similarly calculate Convective heat transfer coefficient 'ho' by considering flow

outside the tube

… Where v = µ/ρ

Nu = 0.53(Gr*Pr) 0.25 … For 1 4< Gr*Pr>109)

4. Find out overall heat transfer coefficient by relation

1

1

5. Calculate heat transfer rate Q for hot fluid as well as for cold fluid by relations

And choose maximum value between them and consider it as Q.

6. Now use relation given below and find out area required for heat transfer

0.7 < Pr <160

Re > 104

(L/di)> 10


Q =U* A* LMTD

A =Q/ (U*LMTD)

7. Find out Actual area from relation

Actual area =A’=3.14*do*L

8. Now compare A & A’ and check whether

Actual area (A’)> area required for the heat transfer (A)

If yes then design is safe;

Otherwise change dimension or change the material as per need and repeat all the

above steps to check whether design is safe or not from heat transfer point of view.

3.2 Improving life of tubes from fracture mechanics point of view and

reducing induced vibrations

The stresses in the heat exchanger tubes are mostly created by water pressure

flowing inside the tubes and vapor pressure of the input/output steam/condensed in

the entrance/exit areas [5-6]. In addition, because of the high speed of the entrance

steam during entering into heaters, it can create vibration bending stresses in the

tubes located in the entrance area.

Fracture mechanics analysis including the fatigue crack growth behavior can

be used to predict the amount of bending stress on the tubes in service. Using a

correlation between cyclic crack growth rates (da/dN) versus stress intensity factor

range (ΔK), the applied bending stresses amplitude can be predicted.

Fig 3.1 Effect of crack growth on Sigmoidal curve (log-log scale) [5]


Crack growth curve is known as Sigmoidal curve.

A fatigue crack grows with each applied load cycle and therefore, crack growth per

unit cycle da/dN is an important parameter.

Paris law is most widely used and is stated as

Where Y is function of ratio of initial crack length to width of plate i.e. F

(a0/W) and C & m are material constants.

By substituting value of ΔK and integrating we get an expression for crack

propagation life,

If we consider infinite life cycles i.e. greater than or equal to 106 million revolutions,

(to be on safer side consider it to be >=108 million revolutions) we can find out (Δσ)

theoretical i.e. difference between σmax and σmin (bending stresses) of tubes from above

expression.

(Δσ) actual can be found out from experiments i.e. by determining higher and lover

value of bending stresses acting on tube by measuring deflections in tube.

Check whether (Δσ) theoretical > (Δσ) actual and σmax < σut .

If Δσ is very small, that means we will get reduced vibrations in pipe.

If yes, then design is safe according to fracture mechanics point of view and will

have an infinite life with reduced vibrations.

If not, then try to reduce vibrational bending stresses Δσ. The main cause of the

vibrational bending stresses in the tubes is the high speed input steam. Therefore,

such high stresses should be minimized using proper methods such as using of an

impingement plate under the entrance gate, or installing one or two baffle plates

supporting heat exchanger tubes in the input steam region. Use of each of these

techniques should be carefully accompanied with consideration of thermal and

mechanical design parameters of heat exchangers to prevent future problems.


3.3 Cost optimization of shell and tube heat exchanger

Total cost Ctot is taken as the objective function [12], which includes capital

investment (Ci), energy cost (Ce), annual operating cost (Co) and total discounted operating

cost (Cod).

Ctot = Ci + Cod … (1)

Let

a1 = numerical constant (V) H = annual operating time (h/yr)

a2 = numerical constant (V/m2) P = pumping power (W)

a3 = numerical constant n = number of tube passes

d0 = tube outside diameter (m) B = baffles spacing (m)

Ds = shell inside diameter (m)

Adopting Hall's correlation, the capital investment Ci is computed as a function of

the exchanger surface area.

Ci = a1 + a2Aa3

… (2)

Where, a1 = 8000, a2 = 259.2 and a3 = 0.93 for exchanger made with stainless steel

for both shell-and-tubes.

The total discounted operating cost related to pumping power to overcome friction

losses is computed from the following equation,

Co = PCeH … (3)

… (4)

Based on all above calculations, total cost is computed from equation (1). The

procedure is repeated computing new value of exchanger area (A), exchanger length (L),

total cost (Ctot) and a corresponding exchanger architecture meeting the specifications.


3.3.1 Particle swarm optimization (PSO) algorithm:

A basic variant of the PSO algorithm works by having a population (called a swarm)

of candidate solutions (called particles) [17-18]. These particles are moved around in the

search-space according to a few simple formulae. The movements of the particles are

guided by their own best known position in the search-space as well as the entire swarm's

best known position. When improved positions are being discovered these will then come to

guide the movements of the swarm. The process is repeated and by doing so it is hoped, but

not guaranteed, that a satisfactory solution will eventually be discovered.

Formally, let f: ℝn → ℝ be the cost function which must be minimized. The function

takes a candidate solution as argument in the form of a vector of real numbers and produces

a real number as output which indicates the objective function value of the given candidate

solution. The gradient of f is not known. The goal is to find a solution a for which f(a) ≤ f(b)

for all b in the search-space, which would mean a is the global minimum. Maximization can

be performed by considering the function h = -f instead.

Let S be the number of particles in the swarm, each having a position xi ∈ ℝn in the

search-space and a velocity vi ∈ ℝn. Let pi be the best known position of particle i and let g

be the best known position of the entire swarm. A basic PSO algorithm is then:

For each particle i = 1, ..., S do:

o Initialize the particle's position with a uniformly distributed random vector:

xi ~ U (blo, bup), where blo and bup are the lower and upper boundaries of the

search-space.

o Initialize the particle's best known position to its initial position: pi ← xi

o If (f(pi) < f(g)) update the swarm's best known position: g ← pi

o Initialize the particle's velocity: vi ~ U(-|bup-blo|, |bup-blo|)

Until a termination criterion is met (e.g. number of iterations performed, or a

solution with adequate objective function value is found), repeat:

o For each particle i = 1, ..., S do:

For each dimension d = 1, ..., n do:

Pick random numbers: rp, rg ~ U(0,1)

Update the particle's velocity: vi,d ← ω vi,d + φp rp (pi,d-xi,d) +

φg rg (gd-xi,d)


Update the particle's position: xi ← xi + vi

If (f(xi) < f(pi)) do:

Update the particle's best known position: pi ← xi

If (f(pi) < f(g)) update the swarm's best known position:

g ← pi

Now g holds the best found solution.

The parameters ω, φp, and φg are selected by the practitioner and control the behavior and

efficacy of the PSO method

Each time the optimization algorithm changes the values of the design variables do,

Ds and B in an attempt to minimize the objective function.

Present approach uses the following steps for optimal heat exchanger design:

Step 1: Assuming values of a set of design variables and estimating heat transfer area of the

heat exchanger based on the required heat duty and other design specification.

Step 2: Evaluation of the capital investment, operating cost and formulation of the objective

function.

Step 3: Utilization of the PSO algorithm to select a new set of values for the design

variables.

Step 4: Iteration of the previous steps until a minimum of the objective function is found.


CHAPTER-4

Conclusions

1. After checking design from heat transfer point of view, designer has freedom to

choose between to modify the design of heat exchanger or to change its material.

Due to constraints of design one should try changing material.

2. If design is safe from heat transfer point of view it will not fail at high temperatures.

3. The vibration of tube is one of the main reasons of failure which can be prevented by

using of an impingement plate under the entrance gate, or installing one or two

baffle plates (DTS plates) supporting heat exchanger tubes in the input steam region.

4. Life estimation from fracture mechanics point of view gives more accurate results as

it is not conservative method unlike life estimation from S-N curve.

5. If heat exchanger will fail in service, then the cost of each failure will be significant

as it can affect the production. Hence total cost incurred in operation of heat

exchanger must be optimized.

6. Particle swarm optimization (PSO) algorithm gives best way to optimize the cost.

Future scope:

The environment effect can be considered in case of life estimation from

fracture mechanics point of view. The modified Pascal law can be used to get more

accurate results.


References:

[1] Nag P. K. (2006), Heat and mass transfer, 2nd

ed., Tata McGraw Hills, 622-700

[2] Holman J.P.(2002), Heat transfer, 9th

ed., Tata McGraw Hills, 518-566

[3] Suryanarayana N.V. (2015), 2nd

ed., Engineering Heat Transfer, PRI PENERAM,

825-904

[4] Gawande S. H., A. A. Navale, L. G. Nandgaonkar, M. R. Sonawane, V. J., Ubarhande,

U. B., (2011) Design optimization of shell and tube Heat Exchangers by vibration

analysis, Modern Mechanical Engineering, 1, 6-11.

[5] P. Kumar (2009), Elements of Fracture Mechanics, 1st ed., Tata McGraw Hills, 188-200

[6] Ibrahim M Ghyayad, Z Abdel Hamid, N. Gomaa, Journal of Metallurgical Engineering

(ME) Volume 4, 2015, A case study: corrosion failure of tube Heat Exchangers

[7] Shi-Meng Hu, Sheng-Hui Wang, Zhen-Guo Yang, Case study in Engineering Failure

Analysis 3 (2015) 52-61, Failure analysis on unexpected thinning of H.E tube wall in

ammonia heat evaporators

[8] Nuclear Energy Agency Committee on the safety of Nuclear Installations, NEA/CSNI/R

(2015)11 April 2013, 06-Aug-2015, ICDE project report, Collection and analysis of

common causes of failure of Heat Exchangers

[9] Shrivasula Reddi Pilli, Jyothi Prakash A, Jayaraj K, Phanni Kummar S.V.S, Ramma

Murthy M.V., (National Institute of Ocean technology, Chennai, India), INDACON

2014, Vibration analysis of Shell and tube Heat Exchangers for low temperature

thermal desalination plant.

[10] Carlos Gamio, (08/09/1999), Shell and tube heat exchanger failure spur reliability

programs

[11] Jahangiri, M. R., (2011) Failure Analysis of Type 304 Stainless Steel Tubes in an

Energy Production Plant Heat Exchangers, J. Pressure Vessel Technol.133(6), 064503

(10 pages)

[12] V. K. Patel and R.V. Rao, “Design Optimization of Shell- and-Tube Heat Exchanger

Using Particle Swarm Optimization Technique,” Applied Thermal Engineering, Vol.

30, No. 11-12, 2001, pp. 1417-1425. doi:10.1016/j.applthermaleng.2010.03.001


[13] Sijini Josef, http://www.indiastudychannel.com/projects/6089-FAILUR-ANALYSIS-

OF-SHELL-AND-TUBE-HEAT-EXCHANGERS.aspx, ISC Technologies, Kochi –

India, Failure Analysis of Shell and Tube Heat exchangers. (2015)

[14]Usman, A. Nusair Khan, Failure analysis of heat exchanger tubes,

Engineering Failure Analysis, 15 (1–2) (2008) 118–128

[15]Shah R.K., Sekulic D.P. (2003) Fundamentals of Heat Exchanger

Design.‐Published by John Wiley & Sons, Inc., Hoboken, New Jersey, USA,‐93 p

[16] American Society of Mechanical Engineers, 2004, Rules for Inservice Inspection of

Nuclear Power Plant Components, ASME Boiler and Pressure Vessel Code Section

XI, ASME, New York.

[17] Wachel, J. C., 1995, “Displacement Method for Determining Acceptable Piping

Vibration Amplitudes,” ASMEJSME Pressure Vessels and Piping Conference, Paper

No. 72, Honolulu, ASME, New York, pp. 197–208.

[18] Dian Palupi Rini, Siti Mariyam Shamsuddin, Siti Sophiyati Yuhaniz, 2011, Particle

Swarm Optimization: Technique, System and Challenges, International Journal of

Computer Applications (0975 – 8887), Volume 14– No.1.

http://www.indiastudychannel.com/projects/6089-FAILUR-ANALYSIS-OF-SHELL-AND-TUBE-HEAT-EXCHANGERS.aspx

http://www.indiastudychannel.com/projects/6089-FAILUR-ANALYSIS-OF-SHELL-AND-TUBE-HEAT-EXCHANGERS.aspx

report on Failure analysis of shell and tube heat exchangers

Engineering

Transcript of report on Failure analysis of shell and tube heat exchangers