R. GUO et al.: ANALYSIS OF THE REASONS FOR THE LEAKAGE OF A BOILER WATER-WALL TUBE ...
221–226
ANALYSIS OF THE REASONS FOR THE LEAKAGE OF A BOILER
WATER-WALL TUBE IN A POWER PLANT
ANALIZA VZROKOV PU[^ANJA STEN CEVI IZMENJEV ALNIKOV
TOPLOTE V TERMOELEKTRARNI
Ruidong Guo
1
, Song Xue
1,2*
, Tao Yang
1
, Xuedong Liu
1
1
College of Manufacturing Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, China
2
Key Laboratory of Testing Technology for Manufacturing Process, Ministry of Education, Mianyang 621010, China
Prejem rokopisa – received: 2019-08-27; sprejem za objavo – accepted for publication: 2019-11-08
doi:10.17222/mit.2019.205
In the present research, the leakage causes of a boiler water-wall tube, with an outside diameter of 60 mm and a thickness of
6.75 mm, that transmits the water medium have been studied. Leakage occurred on the fire-facing side of the tube and close to
the boiler. Various techniques including visual inspection, chemical composition analysis, Vickers hardness test, tube wall
thickness measurements, mechanical performance testing, optical microscopy and energy-dispersive spectroscopy were carried
out to verify the probable cause of failure. Deposits formed on the inner wall of the tube, which caused local heat-transfer
performance of the tube to decrease, formed an overheating characteristic in the microstructure, and softened the tube wall and
made the tube wall thinner near the leakage points. The softening of the tube wall reduces the measured hardness at the edge of
the leakage point. In addition, due to the localized wall thinning and softening, the tube cannot withstand the stress flowing
liquids and gases, with the result that the tube was bulging and deforming. Local overheating caused the degeneration of the
initial ferrite–pearlite microstructure. In the microstructure near the leakage point of the water-wall tube, carbides were
spheroidized and coalesced. According to the investigation, it was concluded that the local heat-transfer performance of the
water-wall tube decreased and the tube performance deteriorated due to the deposits formed on the inner surface. The
combination of localized overheating and tube wall thinning led to the tube’s premature bulging and failure while running.
Keywords: boiler water-wall tube, overheating, deposits, leakage
V ~lanku avtorji opisujejo raziskavo vzrokov pu{~anja sten cevi izmenjevalnikov toplote. Cevi, po katerih se pretaka voda, imajo
zunanji premer 60 mm in debelino sten 6,75 mm. Netesnosti so se zgodile na ogrevani strani cevi in v bli`ini izmenjevalnika. Za
analizo odpovedi so avtorji uporabili razli~ne tehnike, kot so: vizualni pregled, analiza kemijske sestave, meritve trdote po
Vickersu, meritve debeline sten cevi, dolo~itev mehanskih lastnosti (natezni oz. trgalni preizkus), opti~na mikroskopija in
energijska disperzijska spektroskopija. Usedline, nastale na notranjih stenah cevi, so povzro~ile spremembo prenosa toplote in
pri{lo je do pregrevanja sten cevi ter sprememb v mikrostrukturi. Pri{lo je do meh~anja materiala in v okolici mest, kjer je pri{lo
do pu{~anja, so bile stene cevi stanj{ane. Meh~anje sten cevi je zni`alo izmerjeno trdoto materiala na robovih lukenj in razpok
(netesnosti). Dodatno, zaradi lokalnega tanj{anja debeline sten cevi in meh~anja, cevi niso zmogle prena{ati obremenitve
(napetosti) pretakajo~e se kapljevine in plinov, zaradi ~esar so se cevi napihovale (nabrekale) in deformirale. Lokalno
pregrevanje sten cevi je povzro~ilo spremembo feritno-perlitne mikrostrukture. V bli`ini lukenj in razpok na po{kodovanih
stenah cevi, kjer so cevi pu{~ale, je pri{lo v mikrostrukturi do sferoidizacije (okrogljenja) in zdru`evanja in rasti karbidov. Na
osnovi raziskave avtorji v zaklju~ku ugotavljajo, da se je zaradi usedlin na notranjih stenah cevi lokalno zmanj{al prenos toplote.
Kombinacija lokalnega pregrevanja in tanj{anja sten cevi je privedla do njihove prehitre deformacije in odpovedi.
Klju~ne besede: stene cevi izmenjevalnikov toplote, pregrevanje, usedline, netesnost
1 INTRODUCTION
Economic and social development is greatly affected
by power supply and distribution.
1,2
Thermal power
plants use combustibles (such as coal) as the fuel to pro-
duce electricity. The continued normal operation of the
equipment and components of the power plant is a
prerequisite for ensuring uninterrupted power supply.
3
In
a thermal power station, proper functioning of the boiler
water-wall tube, heat exchanger, steam turbine, gene-
rators, etc., is critical to maintaining an uninterrupted
electricity supply. As long as a single device of the
power-generation system fails, the whole power supply
will be affected. Boiler water wall tube failure is a major
cause of power plant shutdown, and similar incidents
have been reported in many power plants.
4–6
Modern thermal power plants consist of fuel systems,
combustion systems, steam and water systems, electrical
systems and control systems. Failure may occur in all
power-plants systems: economizers, water walls, re-
heaters and superheaters. The failure of the boiler water
wall tube is usually related to the tube’s manufacturing
method, and the equipment is exposed to the field and
the entire environment after manufacture including
conditions during shipment, normal operation and
shutdown.
7
According to the location of the failure, the
water-wall tube is the second largest failure location after
the superheater tubes in steam and water systems. How-
ever, in terms of the failure condition of the materials,
carbon steel tubes are statistically the most common
boiler tube material that causes failure. The inner and
Materiali in tehnologije / Materials and technology 54 (2020) 2, 221–226 221
UDK 620.1/.2:683.97:621.181.123 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(2)221(2020)
*Corresponding author's e-mail:
xuesong2004@126.com (Song Xue)

outer surfaces of the boiler tube were exposed to
different working environments (i.e., water side and
fire-facing side, respectively), which may cause micro-
structure degeneration to the tube.
8–10
According to
statistics, the damage to boiler tubes inflicts $5 billion
loss to power plants every year.
11
If the surface of the
tube wall surface is covered with oxides or deposits, the
metal temperature could gradually go up over the years
with the growth of the thickness of the cover products on
the tube.
12
Several investigations have been concerned
with reporting failure cases in thermal power plants,
failure causes range from those associated with high
temperature to others caused by corrosion factors.
13–15
P.
Munda et al.
16
investigated the evolution of different
microstructures during the short-term overheating failure
of a boiler water wall tube made of SA210 Grade C
steel. They found that overheating caused degeneration
of the initial ferrite-pearlite. D. N. Adnyana
17
analyzed
four leakage heat-exchange tubes in service for only one,
results of the failure analysis showed that the leaked
tubes of the heat exchanger had primarily experienced
stress-corrosion cracking (SCC) in the longitudinal
direction caused by the combined effects of tensile stress
and corrosion. A. Kumari et al.
18
investigated the typical
premature failure of water wall tubes of two thermal
power plant boilers with the same capacity, the same
operational parameters, but with different boiler-water
chemistry compositions, and the cause of corrosion was
identified and the proposed strategies for their elimi-
nation were given. M. Nurbanasari et al.
19
reported a
failure analysis on the water-cooling distribution pipe in
a 175-MW hydroelectric power plant, the results
strongly evidence that the leakage occurred on the base
metal due to the severe erosion-corrosion at the localized
area. A. D. Camilo et al.
20
have studied the causes of
failure of boiler water-cooled tube wall. It was found that
tensile residual stress from an inadequate stress-relief
treatment prior to service and high concentration of
dissolved oxygen in the feed water was the main reasons
for the premature failure. E. S. Kim.
21
analyzed the cause
of physical explosion of boiler tube, they found that the
rupture may be caused by overheating induced by in-
sufficient water on the boiler, and it could be accelerated
by the metal temperature increase. Because the boiler
water-wall tube tends to suffer from failure, which can
lead to power plant shutdown, this causes a certain eco-
nomic loss to the power plant and brings inconvenience
to the power consumers. Hence, investigating the root
cause of failure is critical to avoiding the recurrence of
this situation.
Failure of the boiler water-wall tube material is
SA-210 Grade C, which is medium-carbon manganese
steel, and is extensively used in the boiler water-wall
tube of power boilers due to its excellent mechanical
properties at elevated temperatures. The diameter and
thickness of the tubes are 60 mm and 6.75 mm, res-
pectively. The boiler water-wall tube runs at a pressure
of 11 MPa and at a temperature of 340 °C, and this infor-
mation was provided when the failure occurred. The
referred boilers use pulverized coal to produce steam for
the continuous electricity generation. The leakage point
is located on the fire-side of the main burner and close to
the boiler, which belongs to a relatively high heat load
area. It was reported that the boiler started service in
January 2013 and failed in February 2019, but the design
life of boiler tube is 20 years. After 6 years of service,
leakage of the failed tubes was reported in an overhaul,
which caused the power station to stop generating
electricity. Several mandatory preventive maintenance
practices were carried out every 6 months before failure
to facilitate inspection.
In this present investigation, the causes of leakage
reasons in boiler water-wall tube was studied by different
microscopic and analytical techniques to identify the
cause of failure. Moreover, some suggestions to prevent
similar failures have been made.
2 EXPERIMENTAL PART
The failure analysis was performed on the leakage
tube section provide by the power plant, especially the
leakage and bulging section of the tube. In order to
detect the surface morphology of the inner wall and other
subsequent detailed analyses, the leakage tube was cut
along the cross section and some microstructure/hard-
ness samples were collected from different locations.
The metallography samples were prepared using stan-
dard metallographic techniques, and the samples were
etched with a 2 % natal solution to display the micro-
structural constituents. Metallographic examinations of
the leakage specimens and normal specimens were
carried out with a metallographic WMJ-9950 micro-
scope. In order to determine the main mechanism of the
failure, the microstructure was analyzed by an optical
and a scanning electron microscope equipped with an
energy-dispersive X-ray analysis facility. The hardness
near the leakage area and the normal area is determined.
Average hardness data were obtained from six
measurement on the microstructure samples conducted
on the Vickers hardness tester (MHVS-50, China) with a
30-kg load and a dwell time of 15 s. The tensile tests
were concluded on a UTM5305 microcomputer-
controlled electronic universal testing machine. In
addition, the chemical composition and microstructures
of the samples were analyzed at 500 mm from the
leakage point to verify the material of the failure tube.
The material’s chemical composition of the boiler
water-wall tube was determined by spectral analysis, and
the thickness of the tube wall was measured with a
micrometer.
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222 Materiali in tehnologije / Materials and technology 54 (2020) 2, 221–226

3 RESULTS AND DISCUSSION
3.1 Visual examination
Visual inspection of the damaged water-wall tubes
was used to check for signs of corrosion, leakage points
and deformation etc., and the evidence was photo-
graphed. The position of the leakage area on the boiler
water wall tube was shown in Figure 1a. An enlarged
observation of the leakage area, the clear macro mor-
phology of the leakage point was shown in Figure 1b.
The oxidation layer on the surface of the water-wall tube
was observed, but blackening and abnormal thickness
were not found. To detect the condition of the tube’s
inner surface, the tube was radially sectioned, and a
macroscopic view of the cross-section is shown in
Figure 2a. The leakage area presents an inverted drop
shape, and the thickness of leakage point positon is
thinned (the thinnest part is 2 mm). Deposits attached to
the inner surface of the leakage area can be observed.
Diameters measured at different locations of the tube
wall are shown in Table 1. The variation in dimensions
at the leakage area indicates a significant bulging and
deformation occurred during the tube failure and the
maximum diameter of the tube wall bulge position
reaches 63 mm, which exceeds the original diameter of
60 mm. When the deposits are viewed from the inner
wall, the internal thread characteristic can be observed.
The color of the inner tube wall is reddish brown and the
exfoliated metal covering the inner wall of the pipe, as
shown in Figure 2b.
Table 1: Dimensions at different locations of the leak tube
Location
Outer
diameter/mm
Wall
thickness/mm
At leakage point 63 2
Near leakage point 61.5 6.6
Away leakage point 60.5 6.8
Initial specifications 60 6.75
3.2 Material chemical composition analysis
The material chemical composition of the failed
water-wall tubes was analyzed with a FOUNDERY-
MASTERPRO emission spectrometer with an analytical
accuracy of ± 0.01 w/%. The chemical composition of
the tube is obtained in Table 2. The chemical compo-
sition of the tube met the designated standard specifi-
cation of DL/T991-2006
22
. Therefore, compared with the
standard chemical composition of steel, there is no
significant difference in the chemical composition of the
tube.
Table 2: Material chemical composition of the boiler water-wall tube
(weight percentage, %)
Content C Mn Si P S Fe
Standard
requirement
 0.35
0.29–
1.06
 0.10  0.035 0.035 Bal
Measured
value
0.252 0.75 0.24 0.016 0.0008 Bal
3.3 Mechanical properties test
3.3.1 Tensile properties
In order to verify the compliance of the tensile
properties, the tensile test was performed on a UTM5305
microcomputer-controlled electronic universal testing
machine. The tensile tests were carried out on six
R. GUO et al.: ANALYSIS OF THE REASONS FOR THE LEAKAGE OF A BOILER WATER-WALL TUBE ...
Materiali in tehnologije / Materials and technology 54 (2020) 2, 221–226 223
Figure 2: Close views of the tube at failed region shows: a) the mor-
phology leakage point cross-section, b) the morphology of inner wall
Figure 1: Photograph of failed water wall tube and the macro mor-
phology of leakage point: a) the position of the leakage point, b)
macro morphology of the leakage point

standard samples, three samples from the normal base
metal and three samples from failed tubes. The tensile
properties of the tube were tested with specimens ex-
tracted from the leakage tube and machined respectively
according to GB/T228.1–2010 Metallic Materials-Ten-
sile Testing-Part 1: Method of test at room temperature.
23
The yield strength (R
p0.2
), tensile strength (R
m
)a n d
percentage elongation after fracture (A
50mm
) are collated
in Table 3. The tensile test results conform to the
specified standard SA-210 Grade C steel. Therefore, the
degradation of the tensile properties is not considered.
Table 3: Mechanical properties of the different tubes
Items R
m
/MPa R
p0.2
/MPa A
50mm
/%
Standard value in GB/T228.1–
2010 and GB/T231.-2009
 485  275  30
Measured value 532 334 36
3.3.2 Hardness measurement
In order to check whether the hardness is qualified,
the hardness measurements were carried out from the
undamaged zone of the failed tube and a normal tube.
The Vickers hardness was measured using a hardness-
testing machine (MHVS-50, China) with a 30-kg load
and converted to the corresponding Brinell hardness
values. The hardness values of each measuring point are
shown in Table 4. The average hardness of the tube was
155.5 HB, which meets the standard requirements
GB713-2014.
24
However, the hardness close to leakage
points was lower than the other undamaged areas and the
normal area measured points. Due to the poor
heat-transfer performance of the deposits, this causes the
heat inside of the water-wall tube not be transmitted over
time. Local overheating over the working temperature
causes a material flow and softening of the water-wall
tube.
Table 4: Micro-hardness test results (unit: HB)
Measuring point 123456
Measuring values 162 142 161 140 163 165
Average value 155.5
Standard requirements 130-180
Note: Measured points 2 and 4 are close to a leakage point
3.4 Metallography
The microstructures of the failure tube and the
normal tube were examined in detail, the aim to being
know the microstructural constituent phases in the
microstructures of the failed tube and the normal tube.
The specimens were selected from different sections of
the failed tube, then polished and etched with 2 % Nital
solution. The metallographic structure of the failed
specimens near the leakage point consisted of ferrite +
pearlite structure, and pearlite was distributed in a strip,
as shown in Figure 3a. The metallographic structure of
the normal specimens was shown in Figure 3b, which is
also a ferrite + pearlite structure. All the microstructures
of the test specimens conform to the standard require-
ments, and these test results are consistent with the test
results of the contrast samples, conforming to the
designated SA-210 Grade C steel. At the cross-section of
the leakage point, creep holes were observed on the inner
wall, as shown in Figures 3c and 3d. The pearlite
structure at the leakage point was seriously spheroidized
due to overheating, and the grains were elongated due to
plastic deformation (Figure 3e). Microstructure obser-
vations revealed that the grains were elongated, which
indicates that plastic deformation occurs at high
temperature. Overheating caused a degeneration of the
initial ferrite–pearlite microstructure. Spheroidization
and coalescence of the carbides occurred in the matrix of
the boiler tube near the leakage-point region. At the edge
R. GUO et al.: ANALYSIS OF THE REASONS FOR THE LEAKAGE OF A BOILER WATER-WALL TUBE ...
224 Materiali in tehnologije / Materials and technology 54 (2020) 2, 221–226
Figure 3: Microstructure obtained from different regions of boiler
water-wall tubes specimens: a) microstructure near leakage point, b)
microstructure of normal specimens, c) and d) creep holes, e) pearlite
structure spheroidized and grains were elongated, f) precipitated
carbides, g) deposits were attached to tube inner wall, and presented in
a layered structure, h) creep cracks in the outer wall near the leakage
point

of the leakage point, a severely spheroidized pearlite
microstructure and a large number of precipitated car-
bides were observed, shown in Figure 3f. According to
DL/T884-2004,25 the microstructure aging level near the
leakage point was graded at 4.5. These microstructural
changes were due to the atomic migration at high
temperatures. When the temperature is above 280 °C and
lower than the eutectoid temperature, the cementite in
the pearlite spheroidized. Changing the shape of the
layer of cementite to a sphere can reduce its internal
energy. In Figure 3g the deposits were attached to the
tube inner wall of the leakage point. Figure 3h shows
that the deposits mainly presented in a layered structure,
and the thickness of the tube thinned at the deposits’
covered zone. Due to the deposits formed on the inner
wall, which caused the local heat-transfer performance
of the water-wall tube to decrease and the tube’s overall
performance deteriorated, creep cracks formed in the
outer wall near the leakage point, as shown in Figure 3j.
3.5 Detection of components and phase structure of
inner wall deposits
The chemical composition of the deposits on the
inner wall is detected using energy-dispersive spectros-
copy (EDS). The deposits sample was selected from the
tube to determine the chemical composition. Figure 4a
shows the inner wall morphology at the leakage position,
with deposits present as the spallation state. The EDS
test results are presented in Figure 4b and the deposits
mainly contained Fe and O. Furthermore, the deposits
also contained a few Ca, S, and Mn elements, as shown
in Table 5.
Furthermore, the component analysis of the sedi-
ments separated and accumulated on the inner surface of
the pipe near the leakage point was carried out with
X-ray fluorescence spectrometry and the test results
were listed in Table 6. According to the test results, it
was found that the deposits consist of the compound
(CaSO
4
) and the corrosion products (Fe
2
O
3
), which
indicate that the failed tube suffered a corrosion attack.
An X-ray test result is consistent with the results of the
energy spectrum test.
Table 5: Energy spectrum test results of inter wall deposits (weight
percentage, %)
Element Fe O Ca Mn S
Measured values 66.74 28.66 2.08 0.78 1.74
Table 6: X-ray test results of inter wall deposits on fire-facing side
(%)
Element Fe 2O 3 CaO CaSO
4 CuO MnO2
Measured values 88.9 1.60 7.39 0.45 1.66
4 CONCLUSIONS
In this paper the leakage reasons analysis of a boiler
water-wall tube was performed. Based on the above
analysis and discussions, the following conclusions can
be drawn:
a) The deposits formed on the inner wall caused the
local heat-transfer performance to decrease, which
caused local overheating and softening.
b) The bugling brings local softening, which formed
local thinning.
Above all, the tube leakage at the location where the
deposits covered (at the bulge position).
5 RECOMMENDATIONS
Future actions to prevent failures are as follows:
a) Boiler sewage and water quality test should be
carried out regularly to prevent deposits from form-
ing on the inner tube wall. In addition, clean up the
deposits formed in the inner tube wall in time to
avoid local overheating of the tube.
b) Preventing the boiler tube from exceeding the work-
ing temperature and working pressure, and regular
cleaning with a boiler tube wall thickness inspection.
R. GUO et al.: ANALYSIS OF THE REASONS FOR THE LEAKAGE OF A BOILER WATER-WALL TUBE ...
Materiali in tehnologije / Materials and technology 54 (2020) 2, 221–226 225
Figure 4: SEM morphology and EDS test results: a) SEM morphology of deposits covering area in the tube inner wall, b) energy-dispersive
spectroscopy analysis of deposits

Acknowledgement
This work was partially supported by doctoral fund
of Southwest University of Science and Technology
(Grant No.13zx7153) and opening project of Key
Laboratory of Testing Technology for Manufacturing
Process, Ministry of Education, Southwest University of
Science and Technology (Granted No. 13zxzk04), PR
China.
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226 Materiali in tehnologije / Materials and technology 54 (2020) 2, 221–226