Failure Analysis of Heat Exchanger Tubes

This article discusses the failure analysis of heat exchanger tubes, focusing on the factors contributing to failures such as fouling, corrosion, and erosion. It explains the impact of these factors on heat transfer and pressure drop in heat exchangers. The article also explores different types of heat exchanger failures and their root causes. A comprehensive understanding of these issues is crucial for maintaining the efficiency and integrity of heat exchanger systems.

Failure analysis of heat exchanger tubes
Introduction
A heat exchanger (HE) is a device built for efficient heat transfer from one fluid to another,
whether the fluids are separated by a solid wall so that they never mix, or the fluids are directly
contacted [1]. The heavy turbulence and counterflow principle enable efficient heat transfer. HE
are widely used in refrigeration, air conditioning, space heating (SH), for domestic hot water
(DHW), power production, and chemical processing (Fig. 1). Some examples are intercoolers,
pre-heaters, boilers and condensers in power plants [2]. A typical HE is the shell and tube heat
exchanger which consists of a series of finned tubes, through which one of the fluids runs. The
second fluid runs over the finned tubes to be heated or cooled. During HE operation, high
temperature and high-pressure water or steam are flowing at high velocity inside tubes or plate
systems. In tubes of HE, local wall thinning may result from erosion/corrosion.
Therefore, it is important to evaluate the strength and ductility for wall-thinned tubes, assess the
risk of failures to maintain the integrity of the secondary tubing systems. Another type of HE is
the plate heat exchanger, which can be done with brazed (Fig. 2) or gasket plates. It directs flow
through baffles so that the fluids are separated by plates with very large surface area [3]. This
plate type arrangement can be more efficient than the shell and tube. The beginning of using first
heat exchangers for SH and DHW in district heating substations is early 1980s (1990s in
Lithuania). A pioneer is this matter was Swedish company Alfa Laval. A survey of Lithuanian
district heating revealed that in 2005 approximately 95% of all heat exchangers were brazed
plate type. Although the HE is usually designed for a normal life of more than 10 years, their
actual service life, however varies from 2-3 to 6-8 years, depending on the service conditions and

of course on the quality of heat transfer media. The type of scale differs from industry to
industry, depending on the mineral content of the available water.
Despite the enormous costs associated with failure and fouling; only very limited research has
been done on this subject. Reliable knowledge of fouling economics is important when
evaluating the cost efficiency of various mitigation strategies. The total failure and fouling
related cost can be broken down into three main areas: - capital expenditure, which includes
excess surface area (10-50%, with the average about 35%), costs for stronger foundations,
previsions for extra space, increased installation costs; - extra fuel costs, which arise if fouling
leads to extra fuel burning in furnaces or boilers or if more secondary energy such as electricity
or process steam is needed to overcome the effects of fouling; - production losses during planned
and unplanned plant shutdowns due to failure and fouling. This paper presents the results of an
investigation the failure of steels tubes or plates in heat exchangers used in district heating and
industry.
The material of the tubes and plates has suffered corrosion, localized overheating, probably as a
result of local heat flux impingement phenomenon, caused by heat water steaming. The aim of
this paper is to identify which are the major factors that contribute to water main failures. In this
paper, we explain the impact of fouling and corrosion on heat transfer and pressure drop in HE.
The studies included microstructural examinations of cracked and uncracked tubes; fracture
surface investigations and estimation of creep rupture strength, etc.
Background
There is a high degree of uncertainty associated with all the factors contributing to HE element’s
failure and fouling, and especially corrosion rates because of large spatial and temporal

variability [2]. This requires a detailed uncertainly analysis to quantify the probability of HE
failures at a given time in order to plan maintenance and repair strategies [4]. Reduced efficiency
of the HE due to fouling represents an increase in fuel consumption with repercussions not only
in cost but also in the conservation of the energy resources. This study was performed to evaluate
the fracture behavior, failure and fouling mode and allowable limit of carbon steel straight tubes
with damage and local wall thinning. Maximum moment of tubes was evaluated using σ f, Rm
and σ adm, where σ f is the flow stress, Rm is the ultimate tensile strength and σ adm is
admissible stress.
Regardless of the tube material, the most effective way to ensure that tubes achieve their full life
expectancy and heat transfer efficiency is to keep them clean each time the tube deposits,
sedimentation and bio-fouling are removed, the surfaces are returned almost to bare metal,
providing the most effective heat transfer and the tube itself with a new life cycle [5].
Very negative occurrences are hydraulic shocks of heat transfer media which are closely related
with exploitation of all system. Frequent hydraulic shocks may deform plates of HE (Fig. 3),
which causes leakage of the media. According to the current standard, the main criterion for the
tube-line estimation is the condition of static strength. Stresses in a pipe wall σ should not exceed
the admissible value σ adm for the pipe material.
For the HE tube-lines, the value of circular tensile stresses σ y caused by the water service
pressure p (σ y = tpR, where = 2RD is the internal diameter of a tube, t is a wall thickness), and
the value of σ adm is established from the ultimate strength of the material and safety criteria,
which is chosen with respect to the type and service conditions of the tube line. Criterion (1) is
the basic one in design calculations and, particularly, in selecting the material of tubes and their
dimensions. Its applications for the tube-lines that have been operating for a long time require

some additional data, in order to take into consideration the temporal variation of the calculation
parameters as compared with their original values. Firstly, the degradation of material can cause
the decrease of the strength characteristics of material, that is, a corresponding decrease of σ adm
value. The degradation level can be established by laboratory testing or can be approximately
evaluated by correlation dependences of the material characteristics and its hardness [6]. For a
cylindrical tube under biaxial stress state caused by inner pressure p we can write
Literature Review
Types of heat exchanger failures
Hcat exchangers usually provide a long service life with little or no maintenance because they do
not contain any moving parts. However, there are four types of heat exchanger failures that can
occur, and can usually be prevented: mechanical, chemically induced corrosion, combination of
mechanical and chemically induced corrosion, and scale, mud and algae fouling. This article
provides the plant engineer with a detailed look at the problems that can develop and describes
the corrective actions that should be taken to prevent them.
Failure is a physical indication indicating the poor performance of the system. Different types of
heat exchanger failure have been analyzed based on literature studies and have been grouped
according to the type of damage and its effect on the heat exchanger’s characteristics, as follows:
• Crack and leak – one of the most dangerous failures caused by the separation of a material in
two or more pieces due to the effect of stress [1].
• Blockage – a type of failure which is due to substance deposition on the pipe surface resulting
in pipe blockage over time [1].

• Material removal – could occur due to material removal from the pipe surface due to the flow
in the fluid resulting in cracks and leaks [1].
3.1. Failure Mechanisms Failure mechanisms are the different processes which can lead a system
to failure. Five major mechanisms exist;
• Fouling – the deposition of solid particles inside the pipes, causing reduction of their cross-
section area eventually leading to blockage.
• Corrosion – the gradual destruction of the material surfaces by chemical reaction with their
environment which leads to material removal [3].
• Erosion – mechanical abrasion of the material’s surface which produces Total Dissolved Solids
(TDS) in the fluid and leads to material removal [4].
• Fatigue – the structural damage of a material caused by repeated loading: thermal and
mechanical stresses and leads to cracks and leaks.
• Vibration – a mechanical phenomenon which creates oscillations in the material at an
equilibrium point. It leads to cracks and other mechanical fractures which can result in leaks or
pressure drop changes.
Failure root causes
For the various mechanisms, root causes exist which lead to failure. The following are the root
cause for the mechanisms mentioned previously.
Fouling – root causes
Fouling caused by dirt or other particles, creates scale formation on the heat exchanger surface.
Consequences of this phenomenon can be very serious for heat transfer as it can result in

decreased flux and increased temperature of the fluid that is supposed to be cooled [5]. Three
types of root causes can be distinguished.
 Process parameters: Faster the flow rate, the smaller the probability of failure to occur;
higher the pH, the harder it gets to control rate of heat transfer; high temperature of the
cooled fluid disenables maintaining high process efficiency [6].
 Fouling phenomenon: To avoid failure, all manufactured components should be
appropriately cleaned at the end of production process to reduce scaling risk. Even if the
quality of the water doesn’t suggest fouling risk, exaggerated amount of the same water
cycles can be dangerous [5].
 Design parameters: The structure of the surface has an impact on the fouling process.
Heat exchangers are designed mostly with enhanced inner surface to improve their
thermal resistance and could accelerate the fouling process.
Research shows that textured surfaces allow faster and easier scale formation than smooth ones
[6]. What is more notable is that, not only is the velocity of fouling is different in case of smooth
and enhanced surfaces; the level of fouling on the smooth surface is far less in comparison with
its enhanced surface counterparts [7]. The appearances of dead areas also suggest bigger fouling
risk, due to unimpeded flux inhibition [8].
Corrosion – root causes
The overall corrosion rate is determined by the fluid parameters. Some parameters that enhance
the corrosion rate are: high temperature, low pH, high alkalinity, high conductivity, high rate of
total dissolved solids (TDS), high hardness, low-pressure of wet steam, changes in flow regime,
and high concentration of ions such as sulphate, nitrate, chloride, oxygen and iron. However,

oxygen can sometimes have a protective role by creating a passive film, and its breakdown layer
leads to pitting or crevice corrosion [9]. Elevated temperature leads to a lower rate of solubility
of oxygen and hence the protective film is more likely to breakdown. A low pH reflects the
presence of acid substances which are likely to attack and degrade the material rapidly. High
alkalinity also has a similar effect. Low-pressure of wet steam is one of the root causes for both
uniform corrosion and erosion corrosion. This leads to breakdown of unalloyed steel. The flow
medium causes a corrosive effect on the first baffle because of the water drops.
The damage of protection layer reveals the blank steel which does not have sufficient corrosion
resistance layer to avoid corrosion from wet steam [10]. An abrupt change in flow regime caused
due to change in critical Reynolds number, from laminar to turbulent flow, also causes a rapid
increase in corrosion rate. Turbulences can lead to cavitation which has catastrophic effects on
corrosion. They can also enhance the transport of corrosive agents by mechanically tearing away
corrosion products from the metal surface. An increase in the flow velocity facilitates oxygen
transport, reduces the thickness of the Prandtl boundary layer and decreases the polarisation
effects of particles which may react with the material [11].
Erosion –root causes
Erosion is a process that cannot be avoided. Some major factors are; Total Dissolved Solids
(TDS): the number TDS particles; sharp edges in the flow path, particle diameter, geometrical
parameters and physical characteristics of the particles all affect erosion rate [9] [11]. Fluid flow
characteristics: the type of material in contact with the fluid, velocity of the fluid, impact angle
[12][13] Surface finish: the manufacturing process such as welding and water jet cutting,
additive layer manufacturing, plasma laser powder welding could all add roughness to the

finished surface of the heat exchanger [14][15]. Reducing the roughness can decrease the
effective contact area; this implies a reduction in the erosion particle beam [15].
Fatigue – root causes
Four major root causes can lead to fatigue mechanisms.
Working parameters: The first root cause represents the working parameters and is divided into
two categories:
Thermal overload
Mechanical overstress- In the first case, the material is subjected to a very high temperature and
in the second case; it is subjected to very high stress.
Residual Stresses: Residual stress can be generated due to manufacturing processes. For
example, in the welding process materials undergo an increase in temperature and they re-appear
during the solidification of the melted material due to plastic deformation [16] [17]. With
dissimilar materials, them having different expansion coefficient, thermal conductivity and
melting point properties, the residual stress level is much higher. Nevertheless, residual stress
can be reduced with adequate post processes such as post weld heat treatment (PWHT) [18].
Residual stresses also appear during the material processing such as bending and rolling. If an
adequate post heat treatment is not performed, they can lead to fracture [19].
Coating process can also have an important impact on residual stress (e.g.: shot-blasting).
Metallurgical Transformation: The third root cause which can lead to fatigue is metallurgical
transformation during the welding process. This is very critical when joining steels as they might
undergo a microstructure change, leading to embrittlement of the material during transformation

hardening (martensite/bainite) [16] [17]. Moreover, the welding process can also have an impact
on the grain size which might increase significantly.
Porosity / Micro cracks: The fourth root cause of fatigue is porosity/micro cracks. The welding
process itself leads to the formation of micro cracks and porosity, taking place mainly in two
areas: weld deposit and heat affected zone (HAZ). The former one can undergo hydrogen attack
leading to cracking mainly in the centre line or in the interface of columnar grains resulting in
gas entrapment during solidification [20]. On the other hand, if PWHT is not done properly,
micro cracks or even cracks can be created. This is referred to as reheat cracking. Moreover,
lamellar tearing in the edge of the HAZ and cold cracking can occur during service life because
of lower ductility after welding [20].
Vibration – root causes Vibration is a failure mechanism that leads to crack formation and
propagation as the component is unable to withstand the stress acting on it and leads to the
removal of the material [21]. For example, destroying the protective film that keeps the material
from corrosion, triggers localised corrosion [22]. Vibration triggers problems between elements
that are next to each other and are localized in assembly zones such as tube sheets or baffles [22]
[23]. It is normally induced by two different situations;
1. The working environment of the heat exchanger
2. The fluid flow conditions, for e.g., turbulent flow produces turbulent pressure pulsations inside
the cavities [23].
Mechanical-These failures can take seven different forms: metal erosion, steam or water
hammer, vibration, thermal fatigue, freeze-up, thermal expansion and loss of cooling water.

Metal Erosion-Excessive fluid velocity on either the shell or tube side of the heat exchanger can
cause damaging erosion as metal wears from the tubing. Any corrosion already present is
accelerated as erosion removes the tube's protective films, exposing fresh metal to further attack.
Most metal erosion problems occur inside the tubes. The U- bend of U-type heat exchangers and
the tube entrances are the areas most prone to erosion. Figure 1 shows the metal loss in a U- bend
caused by high temperature water flashing to steam. Tube entrance areas experience severe metal
loss when high-velocity fluid from a nozzle is divided into much smaller streams upon entering
the heat exchanger. Stream dividing results in excessive turbulence with very high localized
velocities. High velocity and turbulence produce a horseshoe erosion pattern at the tube entrance.
Fig. 2.
Maximum recommended velocity in the tubes and entrance nozzle is a function of many
variables, including tube material, fluid handled, and temperature. Materials such as steel,
stainless steel, and copper-nickel withstand higher tube velocities than copper. Copper is
normally limited to 7.5 fps; the other materials can handle 10 or 11 fps. If water is flowing
through copper tubing, the velocity should be less than 7.5 fps when it contains suspended solids
or is softened. Erosion problems on the outside of tubes usually result from impingement of wet,
high-velocity gases, such as steam. Wet gas impingement is controlled by oversizing inlet
nozzles, or by placing impingement baffles in the inlet nozzle. Steam or Water, Hammer --
Pressure surges or shock waves caused by the sudden and rapid acceleration or deceleration of a
liquid can cause steam or water hammer.
The resulting pressure surges have been measured at levels up to 20,000 psi, which is high
enough to rupture or collapse the tubing in a heat exchanger. For example, 3/4 in. x 20 BWG
light drawn copper tubing has a burst pressure of 2100 psi and a collapse pressure of 600 psi.

Damaging pressure surges can result from a cooling water flow interruption. The stagnant
cooling water is heated enough to generate steam, and the resumption of the flow causes a
sudden condensing of the steam and produces a damaging pressure surge, or water hammer.
Cooling water flow should always be started before heat is applied to the exchanger. Fluid flow
control valves that open or close suddenly also produce water hammer. Modulating control
valves are preferable to on-off types.
Vacuum breaker vents must be provided if condensable are handled in either the shell or tubes:
they prevent steam hammer damage resulting from condensate accumulation. Figure 3 shows
typical tube damage caused by steam hammer. In this case, condensate accumulated in the shell
and rapidly accelerated producing a high-pressure shock wave that collapsed the tube and caused
the tear holes. Properly sized steam traps with return lines pitched to a condensate receiver or
condensate return pump should be installed to prevent this type of damage.
Vibration
Excessive vibration from equipment such as air compressors or refrigeration machines can cause
tube failures in the form of a fatigue stress crack or erosion of tubing at the point of contact with
baffles. Heat exchangers should be isolated from this type of vibration. Shell-side fluid velocities
in excess of 4 fps can induce damaging vibrations in the tubes causing a cutting action at support
points with baffles.
Velocity-induced vibrations can also cause fatigue failures by work hardening the tubing at
baffle contact points or in U-bend areas until a fatigue crack appears. Thermal Fatigue --Tubing,
particularly in the U- bend area, can fail because of fatigue resulting from accumulated stresses
associated with repeated thermal cycling. This problem is greatly aggravated as the temperature

difference across the length of the U-bend tube increases. Figure 5 shows an example of thermal
fatigue. The temperature difference causes tube flexing, which produces a stress that acts
additively until the tensile strength of the material is exceeded and it cracks. The crack usually
runs radially around the tube, and many times results in a total break. In other cases, the crack
occurs only halfway through the tube and then runs longitudinally along it. Freeze-Up -- These
failures are most common in evaporators or condensers; however, they can occur in any heat
exchanger in which temperatures drop below the freezing point of either fluid in the unit.
Freeze-up results from failure to provide thermal protection, a malfunction of the thermal
protection control system or protective heating device, improper drainage of the unit for winter
shutdown, or inadequate concentration of antifreeze solutions. For example, assume a chiller has
improper settings or malfunctioning controls that cool the water to a point below its freezing
point ice forms and exerts tremendous pressure in the tubing, which causes it to rupture or
collapse. Collapse in an evaporator, Fig. 6, usually occurs near the tube sheet where the tube is
not protected by an inner splint. Freeze-up failure in a condenser tube is shown in Fig. 7. In this
case, cooling water was circulating inside the tube, refrigerant was condensed on the externally
finned surface, and the unit was not properly drained for winter shutdown.
The tube distortion indicates that it was exposed to excessive pressure caused by the freezing
water. This type of failure is also caused by the sudden release of refrigerant pressure from the
condenser. The sudden release caused by a line break or relief valve discharge suddenly drops
the pressure below the boiling point of the refrigerant. Boiling extracts heat from water in the
tubes and the liquid freezes. Thermal Expansion-These failures are must common in steam
heated exchangers; however, they can occur in any type in which fluid being heated is valved off
without provisions to absorb thermal expansion.

In steam-heated systems, cooling down or condensing residual steam in the shell after the steam
control valve closes continues to heat water or other fluids on the tube side. Continued heating
causes thermal expansion, which creates pressures far in excess of the strength of tube sheets,
cast heads, and other heat exchanger components. Cast iron parts usually fracture because of lack
of ductility: steel tube sheets become bowed or permanently distorted because the material's
yield point is exceeded. Figure 8 shows thermal expansion failure of a cast iron heat exchanger
head.
Relief valves are installed in the heated fluid system to prevent this kind of failure. It is also
advisable to provide some means to absorb fluid expansion For example; installing a tank in the
heated fluid system prevents periodic discharge of relief valves, which results in loss of system
fluid and places an undue burden on the valve. These devices are installed between the heat
exchanger and any shutoff or control valves. Loss of Cooling Water-Compressed air after coolers
and gas coolers should always have a supply of cooling fluid before a flow of hot gas is started.
High gas temperatures melt or warp the tubing if there is not an adequate supply of cooling fluid.
Figure 9 shows the tube warping those results without enough cooling fluid. Temperature
actuated modulating control valves should be used to regulate cooling liquid flow.
Chemically Induced Corrosion -- These failures result from the complex chemical interaction
between the materials of the heat exchanger and the fluids circulated through it. There are seven
types of chemically induced corrosion failures: general corrosion, pitting corrosion, stress
corrosion, dezincification, galvanic corrosion, crevice corrosion, and condensate grooving.
General Corrosion -- This type of corrosion is characterized by a relatively uniform attack over
the tube, tube sheet, or shell, and there may be no evidence that corrosion is taking place. Figure
10 shows a tube that became so thin it actually tore down the middle. Fairly stable aggressive

conditions generate this type of attack. Low pH (less than 7) combined with carbon dioxide or
oxygen produces the attack on copper.
The blue or bluish-green color on the tubes in Fig. 11 shows the results of carbon dioxide attack
on the inside of a copper tube. Various chemicals, such as acids also produce this type of metal
loss. Selecting a material with adequate corrosion resistance for its environment, along with
using proper treatment chemicals, maximizes heat exchanger life. Pitting Corrosion -- Localized
pitting is frequently encountered in ferrous and nonferrous metals. It results from the
electrochemical potential set up by differences in oxygen concentration within and outside the
pit, and is frequently referred to as a concentration cell. The oxygen-starved pit acts as an anode
and the unattacked metal surface as a cathode. A small number of pits may be present; however,
any one can cause a heat exchanger failure. Pitting corrosion is most likely to occur during
shutdown periods a when there is no flow and the environment is most suitable for the build-up
of concentration cells. The susceptibility to pitting corrosion is further enhanced by scratches,
dirt or scale deposits, surface defects, and breaks in protective scale layers breaks in metal
surface films, and grain boundary conditions. Figure 12 shows an oxygen pining attack on a
copper tube, and Fig. 13 illustrates a carbon dioxide pitting attack on a copper tube. Stress
Corrosion -- This form of corrosion attacks the grain boundaries in stressed areas. Heat
exchanger tubes usually have both avoidable and unavoidable residual stresses. These stresses
are the result of drawing or forming the tube during manufacture, forming U bends, or expanding
the tubes into tube sheets. Failures from this corrosion take the form of fine cracks, which follow
lines of stress and material grain boundaries. Figure 14 shows stress corrosion failure in
admiralty tubing. The corrodent that causes stress corrosion on stainless steel tubes is the
chloride ion, which is potentially present in any compound formulated with chlorine. All

naturally occurring waters contain the chloride ion in varying degrees. The chloride stress
corrosion phenomenon is not well understood, but it is known that the frequency of occurrence
rises with an increase in temperature and chloride ion concentration. Keeping tube wall
temperatures below 115 F (calculated with maximum, not average. fluid temperatures) prevents
stress corrosion cracking problems with chloride ion concentration up to 50 ppm. Figure 15
shows stress corrosion cracking in a type 304 stainless steel U bend.
The corrodent that causes stress corrosion cracking on copper or copper alloy tubes is ammonia.
Very small concentrations (1 ppm or less) can create a problem. The vacuum breaker used on
steam-heated exchangers draws in ammonia from any leak in the ammonia refrigeration
machine. The ammonia causes stress cracking problems, particularly in the inner U bends of the
heat exchanger tubes. Copper-nickel alloys have good resistance to stress corrosion cracking and
should be used in applications where low concentrations of ammonia are expected.
Dezincification -- This problem occurs in copper-zinc alloys containing less than 85 percent
copper when they are in contact with water having a high oxygen and carbon dioxide content, or
in stagnant solutions. The effect tends to accelerate as temperature increases or pH decreases
below 7. Dezincification creates a porous surface in which the zinc is chemically removed from
the alloy. The remaining copper has a sponge-like appearance. Dezincification is prevented by
using a brass with lower zinc content or a brass containing tin or arsenic to inhibit the chemical
action, or by controlling the environment causing the problem. Galvanic Corrosion -- This type
of corrosion occurs when dissimilar metals are joined in the presence of an electrolyte, such as
acidic water. Galvanic corrosion usually produces a higher rate of reaction on the less noble
metal. For example, if a simple galvanic cell composed of copper and steel is immersed in a

solution of sulfuric acid, the less noble steel corrodes quickly and the more noble copper is
virtually unattacked.
The galvanic chart shows the relative potential of materials to support this type of corrosion.
Metals grouped together have relatively little tendency to produce galvanic corrosion: when two
metals from substantially different groups are coupled together in an electrolyte, substantial
corrosion of the less noble metal results. Figure 16 shows an example of galvanic corrosion on a
steel baffle. In this case, the less noble steel baffle was sacrificed at its point of contact with the
copper tubing Crevice Corrosion -- This type of corrosion originates in and around hidden and
secluded areas, such as between baffles and tubes, Fig 17, or under loose scale or dirt. A
localized call develops and the resulting corrosion appears as a metal loss with local pits, often
giving the impression that erosion is taking place. This condition is in contrast to a vibration
failure in which the metal is sharply cut and there are no pits. Relatively stagnant conditions
must exist for crevice corrosion to occur. The attack can often be controlled by making sure that
velocities are adequate to prevent stagnation or the accumulation of solids. Condensate Grooving
-- This problem occurs on the outside of steam-to-water heat exchanger tubes particularly in the
U-bend area. It is recognized by an irregular groove, or channel cut in the tube as the condensate
drains from the tubing in the form of rivulets. A corrosion cell usually develops in the wetted
area because of the electrical potential difference between the dry and wet areas.
Figure 18 shows condensate grooving and the pitting attack. The condensate, which must be
aggressive for this grooving to occur, wears away the protective oxide film as it drains from the
tubing. Controlling condensate pH and dissolved gases and cleaning the tube bundle outside
surface to remove oils that prevent uniform wetting of the tube usually reduce the problem
potential. Combination Mechanical Failure and Chemically Induced Corrosion -- In many

instances heat exchanger failure is not from a single cause, but a combination of more than one
condition. For example, a pitting attack, galvanic attack, and crevice corrosion attack could occur
simultaneously at one or more locations.
Quite often, a mechanical problem combining with a corrosion problem produces a quicker
failure than either of them alone. There are two common types of combination mechanical and
corrosion failures; erosion-corrosion and corrosion-fatigue. Erosion-Corrosion -- Any corrosion
is greatly accelerated if the protective films are worn away by excessive velocity, suspended
solids, or mechanical vibration.
Erosion-corrosion is usually found in the entrance area of tubes, below the shell inlet nozzle, at
the point of contact with baffles and tubes, and inside the U-bend area of tubes, particularly the
tighter bends; Figure 19 shows an erosion-corrosion attack. The bright copper surface at the tube
entrance is erosion; the yellowish deposit inside the tube is corrosion. Corrosion-Fatigue -- In
this dual failure mode, stresses associated with fatigue are the result of externally applied
mechanical loads- such as vibrations from machinery, expansion or contraction because of
temperature cycles, or light water hammer.
In most environments where only corrosion occurs, corrosion products and films block or retard
further attack. However, in corrosion-fatigue, Fig. 20, cyclic stresses rupture the protected areas
and make them permeable; this action subjects open areas to accelerated corrosion. Improperly
supported tube bundles in domestic hot water storage tanks often suffer from this kind of failure.
Vibrations may not be very severe: however, resulting stresses act additively to the existing
internal tensile stresses. The stress level is raised beyond the strength of the material and
cracking or a fracture results. Scale, Mud, and Algae Funding- Various marine growths deposit a
film or coating on the surfaces of heat transfer tubes. The film acts as an insulator, restricting

heat flow and protecting corrodents. As a result of this insulating effect, tube wall temperatures
go up and corrosion increases.
Scale is the result of dissolved minerals precipitating out of heat transfer fluids. The solubility of
these minerals is altered by forces within the heat exchanger, such as changes in temperature or
chemical reactions. For example, when calcium bicarbonate, a common constituent of many
waters, is heated, carbon dioxide is released and the material is reduced to calcium carbonate, a
relatively insoluble compound that precipitates and coats heat transfer surfaces. Experience
shows that the rate of precipitation is reduced with increasing find velocity. Fluid velocity must
be matched to the tube material's ability to withstand the erosive effects of velocity. Suspended
solids are usually found in the form of sand iron, silt, or other visible particles in one or both of
the heat transfer fluids, if velocities are not high enough to keep them in suspension, particles
settle out, causing the same kinds of problems associated with scale from dissolved solids.
In addition, suspended solids are very abrasive to tubing and other heat exchanger parts. If
abrasive suspended solids are handled in the heat exchanger, fluid velocity must be kept low
enough to prevent erosion. Algae and other marine growths are a serious problem if they get in
the heat exchanger. In many cases, the environment in the heat exchanger is conducive to rapid
proliferation of the algae or other marine growths which restrict flow and impede heat transfer.
Chemical algicides such as chlorine are effective in controlling algae and other marine growths;
high fluid velocities also discourage their attachment and expansion.
Initial investigation techniques
As part of the initial experimental investigation, accelerated corrosion testing was carried out on
metal coupons that form the core of oil-to-air heat exchanger. On the other hand, a pulsed

thermography inspection was carried out on the air-to-air heat exchanger unit to determine
degradations occurring on its outer shell.
Corrosion studies
Corrosion is one of the most likely degradations that occur in heat exchangers. As a scenario, it
is more likely that salted solution in the form of moisture is flowing through the heat exchanger
when the aircraft flies near the sea resulting in corrosion. In order to perform accelerated
corrosion testing simulating the real life events, salted solution should be flown in the part and
dried out. Exposure of such parts to moisture will then create a saturated solution and will
corrode the part.
A study carried out by Stratmann et al [24] demonstrated that the maximum corrosion rate
occurred at the limit between wet and dry atmospheres. If corrosion removes some material, the
wall will be thin; hence the heat transfer is expected to be high. Alternately, when the salt forms
an insulation layer, the heat transfer is expected to be low. However, it is worth noticing that the
transitive layers on both sides (between the fluid and the metal sheet) might have a predominant
effect on the reduction of wall thickness.
Pulsed thermography inspection Thermography is a method which redefines the appearance of
an object in the term of a surface temperature map [25]. It is a useful non-destructive testing
(NDT) method as it is a non-contact technique; both small and large objects can be inspected;
defects and damages occurring in components can be detected with a limitation to the depth
location (few mm’s) within seconds of external heat being applied. This method is widely used
in many industries such as aerospace, automotive, manufacturing and construction [25]. In
pulsed thermography technique, an external stimulus such as flash lamp or heat gun is used to

induce the energy on to the surface of component over a short period of time. The infrared
camera, which is controlled by a computer control unit, is used to record the response of emitted
energy from the component as it cools. For a homogeneous material, a transient heat flow
through an object is not interrupted and therefore the surface temperature shows an even cooling
profile. As soon that the sub-surface feature appears along the diffusion path, the heat flow
characteristic is changed. This break in transient heat flow shows up as hot spot or cold spot on
the components surface temperature profile [26].
Manufacturing Technologies used for Finned Tubes
Improving energy efficiency is one of the main pillars of energy security and improves the
competitiveness of the European economy [1, 2]. The growing demand for electricity and the
conditions of European Union directives impose modernization of the energy industry. This
makes it necessary to take action in terms of design, technology, manufacturing and operation of
power plants, as well as finding new sources of renewable energy [2]. One of the directions of
improving the efficiency of gas power plants is the use of finned tubes. Finned tubes with
continuous rib or serrated, depending on the operating parameters of the working medium and
the flue gas can be used as: heaters, economizers or super heaters. The fins greatly enhance the
heat transfer surfaces, allowing the full optimization of heating surfaces of the boiler, which is
achieved by reducing the dimensions of the boiler, and thus reducing its weight. The efficiency
of the heat exchanger tube depends on the thermal conductivity between the pipe wall and the
fluid and the surface area of the tube. For pipes of the finned heat exchanger surface can be
increased by 30 times compared to the finned tube is not, and thus significantly increases thermal
conductivity and the heat flux per unit increased by almost 300% as compared to compared to
smooth pipes, leading to an increase in overall efficiency of industrial boilers.

Manufacture heat exchangers with finned tubes, due to the increasing competition requires the
implementation of new technological solutions in the production area. The basic technology of
finned tubes should include rewinding the tape directly on the pipe, mechanical crimping wound
strip production process wrought with high frequency welding and arc welding, mostly MAG.
Tubes with tape or tape wound kneaded not be used in power because of the shared structure that
does not provide a sufficiently good conditions for heat transfer. In addition, over time the
quality of the connection service falls under the influence of thermal expansion and vibrations,
which results in deterioration of thermal efficiency [4]. In modern gas boilers are increasingly
being used finned tubes made in the technology of laser welding. The use of concentrated laser
beam power allows for a significant increase in connection speeds, ensuring the quality of the
connection required by the technical regulations and standards.
Finned Tubes Manufacture in Plastic Working Process
The oldest patented method of producing cross-finned tube with a full connection is a plastic
working. In the process of cold rolling formed monometallic and bimetallic tubes. For the
production of pipes is used triaxial angular contact rolling. Each roll contains the right amount of
disk utilities variable geometry part of the work, and on the number of fins per meter depends on
their thickness. In this technology can be divided into two types: depressing and grading method.
Depressing method involves inflicting deformation using the tool disk of increasing diameter and
height of the ribs is achieved by a gradual penetration of the surface pressure and the working
tools. This technology is used for the production of low finned tubes. In method grading with
strong clamps and tools for working with thicker walls of the tube base is obtained while
thinning the cross ribs and increase its diameter. This method is applicable to the production of
high-finned pipes. The materials used in the production of bimetallic pipes on the base pipe can

be continuously boiler, austenitic stainless steel, brass, copper and its alloys, while the external
fins of the tube is used in aluminum, copper and their alloys. Due to the differences in the
thermal properties of the materials used pipe can be applied to most chemically aggressive media
and the operation to a temperature of 200 ° C.
Another method of manufacture of finned tubes is to use the hot rolling process. This technology
involves winding the tape and clamping it in before the notched groove in the base pipe (Fig. 3a).
Difficulties occur in the production of stainless steel pipes. Willing stainless steel to strengthen is
much higher and the groove in the base pipe cause a reduction of its thickness, which makes the
production of pipes in this technology becomes uneconomic. In the case where the base pipe is
made of ordinary steel, the wall thickness does not significantly affect the price of the heat
exchanger. Finned tubes produced with this technology are applied to a temperature of 450° C in
the case of aluminum fins and 500° C in the case of stainless steel ribs [7, 8].
Another manufacturing technology finned tubes is applied to the base pipe and the webs of sheet
welding or soldering the ends of the tube webs (Fig. 3b). The configuration webs create a form
of a helical spring, which is tightly applied to the pipe acts as a heat sink. Dissolve the sheet in
this manner causes the tape is heavily creased, thereby increasing the contact area between the
base of the finned tube and the thermal surface and the air flowing over the turbulent motion. For
the production of such pipes can be used, almost all commercially available materials [9].
Finned Tubes Welding Technologies
Due to the low efficiency of cross-finned tube heat caused by the lack of a lasting connection
with a rib metal pipe, production technologies have been developed using welding techniques.
Currently, the production of finned tubes used shielded welding consumable electrode active gas

(MAG) and welding with a laser beam, which has contributed to a significant increase in the
efficiency of the production process. Another variation is to connect pipes with fins welded high-
frequency currents. The most commonly used in industrial manufacturing technology finned
tubes is an automatic welding consumable electrode active gas welding (MAG). Connecting the
ribs with the pipe is done using a fillet weld or by performing a joint front in the ribs. The first
method has a relatively low yield (linear welding speed 2 m / min). However, in the case of the
second technology cannot guarantee sufficiently high quality for use of connectors, due to the
incompatibility unacceptable welding, such as lack of fusion, adhesion, flooding and splashes.
Welding contact currents frequency (HF) is a variation of resistance welding which uses high-
frequency properties of the welded contact surface heating elements to melt temperature, and
combinations thereof by pressure (Fig. 6). In this method, it is important to fine-tune current
switch position. The typical non-compliance includes local burnout and flooding the surface of
objects and weld splatter of molten metal.
Innovative Technology for Laser Welding of Finned Tubes
Technological advances in modern production methods and the growing demand and
requirements of investors made people look for more efficient production technology finned
tubes. An alternative for the production of welded finned tubes with conventional bonding
technology can be laser welding. Currently, lasers are used for welding of boilers for power, for
reasons of the limited power of the beam, the difficulty of precise preparation of the connector,
the ability to cure both in a narrow heat-affected zone and the weld seams and the tendency for
hot cracking. However, the advantages of disk laser, in particular the stability of the welding
process, the increased tolerance of joint preparation, welding high energy density and a
significant increase in productivity are causing that these technologies are an important direction

of development of the process of joining materials for energy. In company Energoinstal SA have
been attempts to weld pipes with fins on an innovative position for automatic laser welding,
consisting of a laser disk TruDisk 8002 Trumph of the system of distribution of the laser beam
into two welding stations equipped with systems on the market and the feed pipe welding and
automatic painting. Welding schematically shown in Figure 4a. Laser TruDisk 8002 is equipped
with two optical fiber length of 30 m, which in combination with two heads welding allows
welding alternating on each line position. This arrangement will maximize the use of the laser.
When welding on line 1, the second line it takes to prepare the next pipe. Tube transport system
enables the movement of the linear speed of 5 m / min (Fig. 4c). Welded pipe length range is
from 3 m to 24 m, while keeping runs from 50 mm to 250 mm.
An important factor in the possibility of welding finned tubes at speeds up to 200 rev / min
(linear welding speed about 24 rpm) the welding head positioning system (Fig. 4b). In
Energoinstal SA triaxial system was designed with continuously adjustable in each axis, which
resulted in the possibility of: adjusting the position of the laser head, automatic tacking strips
before welding and automatic cutting the tape at the end of the laser beam welding process. In
order to heat pipes before welding the induction heating system is designed to pipes for heating
the gas allows for the full control and the control of the preheating temperature. The system is
integrated with the entire line and allows the heating pipes with a diameter of φ 44, 5 and a wall
thickness of 5 mm and a temperature of 300°C, the linear velocity of the feed 5 rpm. The
advantage of the proposed solution over the currently used is the ability to apply it to high-
welded finned tubes (eight-fold increase in performance over MAG welding), while complying
with all the requirements of technical regulations. The tests were performed Energoinstal SA
laser welding steel pipes P235GH with a rib X2CrTi12 serrated steel beam with a capacity of 4

to 4.5 kW. Tube rotational speed is in the range from 100 to 250 rpm, welding performed in
argon at a flow rate of 5 l/min. Visual examination performed according to standard PN-EN ISO
17637 has shown that the joint is continuous along the entire length of the pipe (Fig. 8a). Weld is
characterized by a smooth grains and full penetration (Fig. 8b). On this basis, welded joint
qualified to the quality level B according to PN-EN ISO 13919.
Analysis of the microstructure of the welded joint (Fig. 8c) showed the presence of
ferriticmartensitic structures of narrow heat affected zone (50 - 100 μm) (Fig. 8c) and martensitic
in the joint (Fig. 8d). Measurements made by the Vickers hardness at a load of 9.8 N (HV1)
revealed no increase in the hardness of the connection above required by the standard PN-EN
15614-11 level of 380 HV (Fig. 9)
Complementing assess the quality of the connection tube-flat bar technology peel test was ribs.
The samples were prepared rib fragments of pipe joint, which cut production tubing. The tensile
test was performed on the company Cometech in brackets that allow attachment of the sample
perpendicular. The sample before stretching shown in Fig. 10a and Fig. 10b shows the samples
after static stretching. It was found that in all tested combinations rupture occurs outside the
terminal, indicating that the strength of the connection is greater than the strength of the ribs.
Thus, a combination of laser tube made of flat bar meets construction requirements.
The study of structural and hardness measurements have shown that laser welded joints tubes flat
bar (rib) meet the quality level B according to PN-EN ISO 13919, so the technology can be
classified according to with PN-EN ISO 15614-11.
In summary the use of welded finned tubes in the power equipment leads to savings in energy
and cost savings in the operation of industrial boilers, heat recovery condensing and its deliberate

use and minimizes energy losses by lowering the temperature of the flue gases. There are several
technologies for production of finned tubes for the energy industry. The most important of these
include the production of plastic working process, high frequency welding, arc welding, and
MAG welding process. These methods, despite its advantages, which include first of all a
combination of continuous tube-fin, which significantly increases the thermal efficiency,
characterized by low productivity and the possibility of the occurrence of imperfections such as:
incomplete fusion, no weld penetration, spatters, etc. In the company Energoinstal SA has
developed an innovative technology for high-performance laser welding finned tubes, which
ensures a level of quality welded joints PNEN ISO 13919, with an eightfold increase in
production efficiency. Made welded joints are characterized by continuous full penetration weld
the entire length of the pipe joint hardness does not exceed 380 HV, which indicates that this
technology can be qualified by the Society of Notified Bodies for use in the energy industry.
Experimental Study on Failure Analysis
Material and experiments
The chemical composition and mechanical characteristics of the HE materials are given in Table
2. Mechanical properties such as Brinell, Vickers hardness, tensile strength and impact toughness
were tested on the TШ-2 and micro hardness testers, 50 kN multi-functional hydraulic servo
machine and CIEM-30D testing machine, respectively. The diameter of the tensile specimens
was 5 mm. The size of the impact toughness specimens was 10x10x55 mm. Three specimens
were tested in each experiment and an average of the experimental data was taken as the result.
The tube specimens were sectioned perpendicular to the axis polished and etched by amount of
flux. The thickness was measured on the unfailed tube layer of scaling was noticed in the inner
side of both the cracked tubes. On the inner surface a thick, hard and sticky deposit noticed and

the tubes were found to be distorted and changed their dimensions. For HE stainless steel is
always used. Grade 254 SMO steel has a very good resistance to uniform corrosion in
environments containing halides and to stress corrosion cracking. Also this material has very
high resistance to pitting and crevice corrosion.
The corrosion resistance of stainless steels is a result of passive layer of oxidized chromium
contained in the steel [10]. The formation and stability of this layer mainly depends on the
chromium content, but these qualities can be increased by the presence of molybdenum and
nitrogen in the stainless steel; the environment of use also affects the corrosion resistance.
Stainless steels are susceptible to both various forms of local corrosion damage, wear and
general corrosion. It has been observed experimentally that resistance to pitting corrosion
follows the index which can be derived from chemical composition of stainless steel. The PRE
(Pitting Resistance Equivalent) index can be calculated as follows:
The higher is the index value, the better corrosion resistance. Table 3 presents the PRE values of
various stainless steel grades in HE. Tables 2 and 3 show chemical analysis and mechanical
properties for some steels used for HE. The microstructure is controlled by heat treatment in
order to achieve the best compromise between strength, ductility and toughness. 9% nickel steels
have even higher yield and tensile strength and are the ideal materials for large boilers.
Addition of nitrogen as in grade AISI 304 or grade AISI 316 steels increases yield and tensile
strength and material thickness may be reduced in vessels. Because of the high cost the use of
stainless steels is restricted and is mainly used when there is need also for high corrosion
resistance. Strong chlorine or salt-based waters are very aggressive to stainless steel (AISI 316).

Such mediums are usually e.g. seawater and swimming pool water. Chlorine concentration under
100mg/l grade AISI 316 steel is suitable. If the chlorine concentration is 100-400 mg/l, only the
grade SMO 254 steel is available.
Stress corrosion cracking (SCC) may occur if a material is subjected to tensile stress while in
contact with a corrosive medium, usually resulting in the formation of cracks. Tensile stress may
be caused by fabrication processes such as welding and bending. Typically, HE with their
possible plates thickness (<1.5 mm) are subjected to possible damaging temperature gradients
and transients due to cyclic operation during start-up and shut down (Fig. 5). These types of load
cause large strain ranges in the wall. Whenever bad chemical conditions are present, corrosion
might occur, especially during stand-still periods, when oxygen and water are present. This
happens in plates and severe pitting might be the result. Water used for HE should meet all
national (domestic) standards. Some of water features are more important than other from HE
maintenance point of view. Those agents (if are above allowed levels) are dangerous for HE. It
means that the chemical process can cause some damage to the HE plates (both materials AISI
304 and AISI 316) and also can interfere with the brazing material (cooper). There are two main
things which may help to avoid problems in operating HE. One of them is to create a better
chemical treatment of the water so that hardness will satisfy European norms. It is possible to
decrease hardness of the water by installing the magnetizers. Second thing which is very
important when there is a danger of HE stone creating is to ensure proper maintenance. All the
time the pressure drop on the HE should be examined and if it is rising it means that the
resistance is caused by the HE stone created inside. At that point the HE should be cleaned with
the compound recommended by the manufacturer.

Fig. 8 shows how the yield stress of carbon steel changes with temperature. It is to be noted how
the yield stress fall rapidly once the metal temperature has reached about 350°C. However, such
a high temperature arises only in some technological processes where the use of heat exchangers
is very rare. If the fouling problem only becomes apparent after a period of operation it is
possible to retrofit on-line cleaning equipment. An alternative to on-line cleaning is to stop
operations and to clean the HE either chemically or by mechanical means. Visual inspection of
one of the HE revealed that the tube-sheets contained deep grooves, while the oval tubes suffered
corrosion in the form of metal thinning, removed metal pieces and occasional formation of
blisters and was sagged at various locations. There is a wide variety of different materials of
construction for the fabrication of corrosion resistant HE.

As you can see on the attached pictures (Fig. 9) the heat exchanger is totally “silver”. No cooper
is left between the plates. In this case pressure inside the heat exchanger begins to separate the
plates’ one from another what can cause a leakage. For liquid systems associated with shell and
tube HE, tubes are often made of various copper base alloys, principally admiralty brass,
aluminium brass, cupro nickel (70/30) and aluminium bronze together with stainless steels,
Monel and other high quality corrosion resistant alloys. Mild steel tubes are also used for low
cost applications. In certain operating conditions the use of “duplex” or bimetal materials may be
required.
The principal types of evaluated temperature failures are creep and stress rupture, low-cycle or
high-cycle thermal fatigue, and combinations of these failure types. Identifying features of low-
cycle thermal fatigue failures are multiple initiation sites that join randomly to form the main
crack, transverse fractures and transgranular fracture. All of the tubes with inner fouling showed
a significant enhancement in the hardness of the layers (Fig. 10). Maximum hardness values
were found at the distance of 0.1-0.2 mm from tube’s surface. The reason for the enhanced
hardness was studied by Bott [2]. Bott showed that the coherency strain only plays a minor role
in the hardening of these alloyed super-lattice layers.
Zettler et al. [5] concluded that a difference in elastic modulus between two layer materials is
required for the increase in hardness for the fouling. It is very critical to control the inner
thickness of the individual layers in this multiplayer film and by so doing to control the thickness
of the superlattice period. However, cracks with similar characteristics, but distinguished by
intergranular fracture are caused by creep or stress-rupture phenomena [13]. The present
investigation disclosed both multiple initiation sites and transverse fractures. This indicates that
the likely failure cause of the cracking heater tube could be attributed to either creep or thermal

fatigue. However, intergranular fracture increases the possibility of creep damage as the main
cause of failure.
Creep is time dependent strain occurring under stress and may terminate in fracture by stress
rupture (creep rupture). The main variables determining the rate of deformation under creep
conditions are temperature, time, and stress. In general, the used cracking heater material is
mainly carbide precipitation strengthening alloy. To minimize such failure in future, periodic
inspection to monitor crack formation will be scheduled. Nondestructive tests including dye
penetration test for surface cracking and radiographic test for internal crack will be implemented.
Conclusions
1. On the inner wall side, the fracture was found to be initiating from some pits, and hard, non-
brittle region which caused layer wise separation and chipping off of the material. 2. The
degradation in microstructure is reflected by the formation of continuous chains of brittle hard
phase along the grain boundaries of microstructure. This effect is more pronounced on the inner
wall side of the tubes.
3. Fouling starts at areas that are subjected to minimum shearing forces, rear at the end of the
tubes and then spreads to the circumference due to particle transport to the already deposited
particles.
4. The investigation has indicated that the fouling attributed to the formation of thick scale of
magnetite with mud on the inner surface of the tube wall.
5. The failure in HE tubes could be avoided by: (a) limiting the temperature; (b) minimizing the
impurities and harmful chemical species of the heat transfer media to reduce the chemical attack

on the inner side; (c) removal of contaminants in the water to minimize carburisation and scaling
on the plates and tube walls.