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: judha@um.edu.my; j.purbolaksono@gmail.com

; 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.