Report: Failure Analysis of Heat Exchanger Tubes and Plates

This report provides a detailed analysis of heat exchanger failures, focusing on the causes and mechanisms behind tube and plate degradation. It explores various failure types, including mechanical failures, corrosion, and fouling, and investigates the factors contributing to these issues. The report delves into the impact of fouling and corrosion on heat transfer and pressure drop, examining the root causes of each failure mechanism. It covers topics such as corrosion rates influenced by fluid parameters, erosion caused by total dissolved solids, and the impact of design parameters on fouling. The study includes microstructural examinations, fracture surface investigations, and evaluations of creep rupture strength. The report also highlights the importance of maintaining tube cleanliness and addresses the influence of hydraulic shocks on plate integrity. Ultimately, the analysis aims to identify the major factors leading to water main failures in heat exchangers and provides insights into preventative measures and maintenance strategies to enhance their operational lifespan and efficiency.

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.