Accepted Manuscript
High operating steam pressure and localized overheating of a primary super‐
heater tube
J. Ahmad, M.M. Rahman, M.H.A. Zuhairi, S. Ramesh, M.A. Hassan, J.
Purbolaksono
PII: S1350-6307(12)00174-4
DOI: http://dx.doi.org/10.1016/j.engfailanal.2012.08.012
Reference: EFA 1822
To appear in: Engineering Failure Analysis
Received Date: 16 March 2012
Revised Date: 23 July 2012
Accepted Date: 10 August 2012
Please cite this article as: Ahmad, J., Rahman, M.M., Zuhairi, M.H.A., Ramesh, S., Hassan, M.A., Purbolaksono,
J., High operating steam pressure and localized overheating of a primary superheater tube, Engineering Failure
Analysis (2012), doi: http://dx.doi.org/10.1016/j.engfailanal.2012.08.012
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High operating steam pressure and localized overheating of a primary
superheater tube
J. Ahmada, M.M. Rahmanb, , M.H.A. Zuhairib, S. Rameshc, M.A. Hassanc, J.
Purbolaksonoc, *
aKapar Energy Ventures Sdn Bhd, Jalan Tok Muda, Kapar 42200, Malaysia
b Department of Mechanical Engineering, Universiti Tenaga Nasional, Jalan
IKRAM-UNITEN, Kajang 43000, Malaysia
c Centre of Advanced Manufacturing and Materials Processing, Department of
Engineering Design and Manufacture, Faculty of Engineering, University of
Malaya, 50603 Kuala Lumpur
Abstract
A primary superheater tube was found to be rupture due to a combination of a
high operating steam pressure and localized overheating as a result of the
concentrated high temperature flue gas flows. It was reported that the failure
region was considerably free from the clinkers but massive slaggings were found
to cover its surroundings. The investigation was carried out to confirm the main
root cause of failure. A brief discussion on mineral contents in coals to fly-ash
and deposit formation is presented.
Keywords: Coal-fired boiler; Steam pressure; Low ash fusion temperature;
Localized overheating.
* Corresponding author: J. Purbolaksono; E-mail: [email protected]; [email protected]
; Tel.: +603-89796815
1. Introduction
Common boiler sites which likely have tube failures due to short-term
overheating are superheaters and reheaters. Severe short-term overheating
caused by an unusual operating condition occurs when the tube temperature
significantly rises above design limits for a brief period. Therefore, immediate
investigation on unusual occurrences preceding failure may be extremely
important in identifying the cause of failure. The efforts should be thoroughly
addressed to the operating procedures and system design.
This paper presents failure evaluation on a primary superheater tube of a power
plant. In situ hardness measurements on the selected region of the failed primary
superheater tube were carried out. The average hardness was then used for
estimating the average operating temperature prior to failure. Creep analysis was
conducted to confirm whether or not the creep damage contributed to the failure.
Microstructures of the as-received tube were observed through microscopic
examinations. Similar incidents occurred at the different units of the power plant
in 2007 were reported by Purbolaksono et al. [1] and Rahman et al. [2]. The first
failure [1] showed that a primary superheater tube failed with a wide open burst
after running at around 28,194 h following the firing of the low quality coal
causing heavy clinker formations. At around the same time, another failure [2]
occurred due to the similar root cause but it was also in combination with the
coal-ash corrosion attack.
2. Operational background
On 26 February 2011, one of the units in the power plant was forced to shut
down due to a rupture in one of the primary superheater SA213-T12 tubes. It was
reported the superheater tube had been in service for around 48,000 h.
According to the operational records, the operating steam temperature and
pressure for primary superheater are maintained at around 430-460oC and 176.5
bar (17.65 MPa), respectively. The superheater tubes have outer diameter of 45
mm, the thickness of 4.6 mm and a standard hardness of 163 HV. Due to limited
coal supply in the market during some periods prior to failure, it was reported that
several types of coals with different ash fusion temperatures as listed in Table 1
were used in the last 45 days prior to failure. It can be seen from Table 1, there
are three types of coal (Types A, D, and G) having lower ash fusion temperatures
in comparison to the average furnace flame temperature of 1400oC. Thus, it
would likely trigger the formations of heavy clinker during the firing.
3. Visual Inspection
Heavy clinkers were found to almost entirely cover the primary superheater
region as depicted in Fig. 1. However, during on-site inspection, the failure region
was observed to be considerably free from the clinkers but massive slagging was
found to cover its surroundings. It is plausible evidence of having localized hot
flue gas flow causing overheating in the failed region.
Visual appearance of the failed tube shows opening burst with a longitudinal fish-
mouth rupture as shown in Fig. 2. There was no obvious evidence of the active
corrosion on both internal and external surfaces of the tube. There were no signs
of the localized wall thinning of the failed tube and at adjacent tubes. These
findings indicate common sign of experiencing short-term overheating.
4. Creep and stress analysis
A standard hardness of SA213-T12 was utilized to determine the Larsen-Miller
Parameter. Correlation between hardness (HV) and the Larsen-Miller parameter
for SA213-T12 in the as-normalized condition may be expressed as [3]
PHVHardness 012603.0453.595)( (1)
The operating metal temperature under the normal service condition may be
determined by using Eq. (1) as follows:
- 163 = 595.453 – 0.012603 (T (20 + log (48,000))
- T = 1390 Rankine = 499oC.
To evaluate whether or not creep damage contributed to the failure of the
superheater tube, it is necessary to conduct creep analysis. The operating hoop
stress h developed in the tube may be determined as
t
tr
Ph
)2
(
(2)
where P is operational internal pressure; r and t are outer radius and wall
thickness, respectively.
Diagram of Larsen-Miller parameter with stress variation to rupture of SA213-T12
steel (ASTM) [4] is utilized to determine the rupture time. The operating hoop
stress for the operating steam pressure of 14.1 MPa is equal to 77.2 MPa
(=11.24 ksi). Referring to the minimum curve [4] for conservative calculation,
Larsen-Miller parameter is found to be 35,800. Hence, the rupture time for the
tube metal temperature of 499oC is 562,941 h. This calculation confirms that the
creep damage is not expected under normal operating temperature. However, in
nature, for a prolonged operation, the operating hoop stress is considerably high
and relatively close to the maximum allowable stress as listed in Section 2, Part
D of The ASME Boiler and Pressure Vessel Code [5] (see Table 2). If a linear
interpolation for temperature versus stress in Table 2 is taken, hence the
maximum allowable stress at 499oC is around 85.2 MPa.
The localized overheating was identified to likely occur in the failed region as
evidenced by an advanced stage of spheroidization as depicted in Fig. 3a. Jones
[6] stated that the failure temperature may be indicated by using microstructural
evidence. If the pearlite has spheroidized, then the rupture has almost certainly
occurred at higher temperature operation above 600oC. Spheroidization in ferritic
tube structures would usually commence as the carbon tube metal temperature
is around 600oC. Meanwhile, the microstructure of the tube metal that was
covered by clinkers shows normal ferrite and pearlite as shown in Fig. 3b. In
principle, Fig. 3 illustrates the expected microstructures in the failed region and
its surroundings. Further, it can be referred to Table 2 that at more than 600oC,
the present operating hoop stress has significantly exceeded the maximum
allowable stress. This phenomenon might have begun in the last several
hundreds of hours prior to failure. Purbolaksono et al. [7] also reported a failed
reheater tube due to the higher temperature exposure and operating stress
exceeding the maximum allowable stress values for a quite prolonged period of
time. Therefore, it is worth to note that the plant operators should carefully
monitor the operating steam pressure that likely leads to excessive operating
hoop stress in water tube boiler.
5. Discussion
Findings from the investigation likely showed the main root causes of the failure
to be attributed to the operating procedures. It is essential to notice that the
primary superheater had been in service with a relatively high operating steam
pressure, resulting in the hoop stress in the tube being close to the maximum
allowable stress. For prolonged period of time the tubes could experience higher
operating metal temperature due to unexpected circumstances, thus the
operating hoop stress would have a high possibility to exceed the maximum
allowable stress. In particular, the nature of failure in the present case study is
triggered by the localized overheating as a result of uneven flue gas distributions.
The formations of clinkers that massively cover the primary superheater region
can cause the concentrated flue gas flow at the uncovered spots, producing a
higher convective film coefficient on fireside. Thus, a higher tube metal
temperature is expected.
In order to prevent similar problem due to massive clinkers in the future, the
consequence of using coals with low ash fusion temperatures below the average
furnace flame temperature must be avoided if possible. However, as reported by
Hatt [8], a high ash fusion temperature does not guarantee successful firing. The
boiler could still be occasionally subject to slagging deposits. Thus, it is important
to incorporate ash chemistry analysis into evaluation of past experiences of using
coals with higher ash fusion temperatures. The iron levels in the coal ash, and
several indexes such as base to acid ratio, slagging factor and iron loading
provided good bases for separating the coals that caused problems and those
that did not. In other words, the ash chemistry can provide information for
determining whether or not a coal can be successfully used at fossil-fired plants.
Liu et al. [9] stated that understanding of the transformation of mineral matter in
coals to fly-ash and deposit formation has improved knowledge and helped
industrial engineers in better handling ash-related problems. Nowadays, ash
fusibility tested in accordance to the standardized procedure or measured by
Thermo-Mechanical Analysis (TMA) has been widely used to compare and
predict slagging potential of various coals. The TMA measurements on coal
ashes are very sensitive to iron content and can be used to indicate iron related
slagging problems in pulverized-fired boilers. A review report in relation to the
removal of potential slagging elements from coals has recently been presented
by Izquierdo and Querol [10]. They reported a number of elements that are tightly
bound to fly ash from coal combustions. They presented an extensive figure at
the extent to which major and trace elements are leached from coal fly ash,
giving an insight into the factors underlying the leachability of elements and
addressing the causes of the mobility. The mode of occurrence of a given
element in the parent coal was found to play an important role in the leaching
behaviour of fly ash.
6. Conclusions
Findings from the investigation indicated that the failure mechanism of the
primary superhetar tube was a combination of a high operating stress exceeding
the allowable maximum stress and localized overheating as a result of the
concentrated high temperature flue gas flows. The main root causes were
attributed to the operating procedures of imposing relatively high steam pressure
and usage of coals with ash fusion temperatures lower than the furnace flame
temperature. The ash chemistry of coals also needs to be properly identified in
order to have successful firing.
Acknowledgements
The authors would like to thank Kapar Energy Ventures Sdn Bhd Malaysia for
permission of utilizing all the facilities and resources during this study.
References
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Table 1. Types of coal with different ash fusion temperature.
Coal Temperature, oC
Type A 1240
Type B 1550
Type C 1500
Type D 1325
Type E 1500
Type F 1500
Type G 1310
Table 2. The maximum allowable stress value for seamless tube SA213-T12 [5].
Temperature, oC 482.2 510.0 537.8 565.6 593.3
Max. allowable stress, MPa 96.53 77.91 49.64 31.03 19.31
Fig. 1. Massive clinkers covering the primary superheater area (left) and some clinkers
being removed from the site for close visual inspection (right).
Fig. 2. (a). Failed superheater tube (in dotted circle) in the location; (b). After being
removed from the location.
Figure
Fig. 3. (a). Microstructure of the rupture region, showing an advanced stage of
spheroidization; (b). Microstructure of the tube metal that was covered by clinkers,
showing normal ferrite and pearlite.
Research Highlights:
- High operating hoop stress exceeding the maximum allowable stress.
- Localized overheating due to formations of massive clinker.
- The ash chemistry of coals determines successful firing.