Failure analysis and retrofitting of superheater tubes in utility boiler

Engineering Failure Analysis 50 (2015) 20–28

Contents lists available at ScienceDirect

Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal

Failure analysis and retrofitting of superheater tubes in utility boiler H. Shokouhmand a, B. Ghadimi a,⇑, R. Espanani b a b

School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran Iran Power Development Company (IPDC), Tehran, Iran

a r t i c l e

i n f o

Article history: Received 8 July 2014 Received in revised form 6 January 2015 Accepted 8 January 2015 Available online 29 January 2015 Keywords: Superheater Failure analysis Attemperator Retrofitting

a b s t r a c t The extreme spray water mass flow rate deviation was observed to occur in the middle temperature superheater of Sahand 2  325 MW Power Plant utility boiler, which severely affected its economic performance and safe operation. Boilers operating in these conditions led to failure in superheater tubes at the same place for two consecutive times in a three year span. Thus, the failure analysis of superheater tubes by investigating the visual inspection, chemical, scale and creep analysis was carried out. The brittle failure occurred in the superheater tubes after the fuel was changed from natural gas to heavy oil. Failure analysis showed that tubes were suffering from long term overheating which was instigated by high spray water flow rate. In order to rectify the boiler operating conditions, some modifications were applied in the boiler unit 1 and operating parameters on this boiler were compared with boiler unit 2. The results showed that the 8.33% reduction in heating surface area corresponds to 52.84 and 17.80% reduction in spray water mass flow rate for capacities equal to 300 and 260 MW, respectively. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction High temperature operation for a prolonged duration leads to significant damage in structural component, majorly caused by the occurrence of surface oxidation and decarburization processes, and temperature sensitive plastic deformation. Failures in superheater tubes may emerge from several causes, including design, material composition, and improper thermal operating conditions. Therefore, it is crucially necessary not only to identify these failures but also to determine their main cause when failures do occur. Almost 35% of common failures in utility boilers are caused by long term overheating (creep) in superheaters, reheaters and wall tubes [1–4]. Failure investigations on water tube boilers in Iran power plants have recently been reported [5–8]. Fatah et al. [6] investigate a failure in a boiler tube of low alloy steel, SA-210 Grade A-1. In their study, SEM images and XRD results showed the presence of NaFeO2 which is the main product of caustic attack. In this situation the evaporation at the waterside of the partially filled tube leads to locally caustic concentration which dissolves the protective magnetite layer and subsequently attacked the bare metal. In this paper, failure analysis on superheater of Sahand 2  325 MW Power Plant was studied. Boilers of Sahand Power Plant of Iran were a tangentially fired heavy oil and natural gas furnace. In the following, the main cause of superheater tubes failure is determined, the retrofitting for the boiler unit 1 is explained, and the results are reported. ⇑ Corresponding author. E-mail address: [email protected] (B. Ghadimi). http://dx.doi.org/10.1016/j.engfailanal.2015.01.003 1350-6307/Ó 2015 Elsevier Ltd. All rights reserved.

H. Shokouhmand et al. / Engineering Failure Analysis 50 (2015) 20–28

21

2. Boiler operating backgrounds In order to achieve amelioration in the efficiency of heat transfer, HRSG is structured in multi-level temperatures. Water is provided from Boiler Feed Pump (BFP) in high pressure to HRSGs low temperature header which turns into high temperature superheated steam and fed to steam turbine through multi-stages of pipe lines. In the Sahand boilers, the pipe between the middle temperature superheater header and the high temperature superheater header is connected with two attemperator, which maintains constant temperature of superheated steam that is fed to the steam turbine (Fig. 1). The arrangement of Sahand boiler heating surface was designed according to the heavy oil as the main fuel and checked by natural gas as an auxiliary fuel. Sahand Power Plant is operating since 2004 and it was observed that superheaters and reheaters steam temperatures of Boiler unit 1 and 2, deviated the design value when firing heavy oil or natural gas. The steam temperature at the middle superheater outlet was found to be almost 40 °C higher than design value. Furthermore, high spray water is required in both fuels. Although, in the case of heavy oil the spray water found to be greater than design value, but it is within the allowable range of spray water (less than 5% of main flow). When the fuel was changed to natural gas the spray water is increased dramatically, and its mass flow rate reached to the 18% of the main flow (the main flow of boiler in the full load is 1197 t/h and the spray water in this situation reached to 190 t/h). Boiler operating in these conditions led to failure of superheater tubes in July 2006; the same failure was reported once again at the same place in May 2009, when the fuel was changed from natural gas to heavy oil. To address the issue of consecutive failures, visual inspection, microscopic examinations and chemical, scale and creep analysis utilizing available related data are carried out to evaluate the failure mechanism and its root cause. The results are reported in the following. 3. Failure analysis 3.1. Visual inspections Visual observation of the failed tubes shows that the tubes are bent at several parts (Fig. 2). The outer surface is covered by relatively thick sediment and brittle failure in the different parts of the tube can be observed (Fig. 3). In the case of brittle failures, the micro cracks propagate in the materials and the atoms are gradually separated by tearing along the fracture plane in a very fast way. The path the crack follows depends on the material’s structure and the transgranular and intergranular cleavage are important. In this case, it seems that the changing the fuel from gas to heavy oil is provided a crack propagation conditions. 3.2. Chemical analysis In order to determine the chemical composition of the tubes, a chemical analysis was performed. The results, as shown in Table 1, indicated that the chemical composition was complied with the 12Cr1MoV original composition of the tube.

Fig. 1. Schematics of Sahand superheater system with two attemperator.

Fig. 2. The image of failed tube.

22

H. Shokouhmand et al. / Engineering Failure Analysis 50 (2015) 20–28

Fig. 3. (a) The outer surface of damage tube, (b) brittle failure of the damaged tube.

Table 1 Chemical composition of material.

Rupture tube Nominal 12Cr1MoV [9]

C

Si

Mn

Cr

Mo

V

Ni

S

P

0.109 0.08–0.15

0.223 0.17–0.37

0.556 0.4–0.7

1.01 0.9–1.2

0.274 0.25–0.35

0.232 0.15–0.3

0.0545 60.3

0.0069 60.035

0.0247 60.035

3.3. Scale analysis The outer surface of superheater tubes was covered by a thick layer of scale. The bulk chemical compositional analysis of scale was carried out using X-ray fluorescence (XRF) and the presence of highly harmful elements like V, S and Na were observed (Table 2). Vanadium compounds present in the combustion gases undergo fluxing reaction with oxide and/or sulfate scales on metal causing accelerated corrosion. Therefore, one can conclude that the heavy oil quality which is used in the burners is not suitable, and in the high temperature operation of tubes, hot corrosion may occur. Qualitative analysis of scale at the outer surface of tube was carried out by XRD and the result is depicted in Fig. 4. The presence of Na8V24O63V was confirmed from this analysis. 3.4. Working temperature The temperature of the metal surface adjacent to the oxide scale can be estimated. Generally, the growth of oxide scale in this grade of steel under the prevailing condition follows parabolic growth rate which is of the form [10]

Log X ¼ 7:1438 þ 2:1761  104 Tð20 þ log tÞ

ð1Þ

where X is the thickness of oxide scale in mils (1 mm = 40 mils), T is the temperature in R(R = F + 460) and t is the time of exposure in hours. In Eq. (1), T(20 + log t) is known as Larsen–Miller parameter. In the present work t is 33270 h and X is

Table 2 Chemical composition of scale.

(% wt)

Na2O

SO3

Fe2O3

MgO

V2O3

NiO

Cao

Al2O3

LOI

8.13

6.3

4.9

0.56

67.3

2.32

1.01

0.29

8.04

H. Shokouhmand et al. / Engineering Failure Analysis 50 (2015) 20–28

23

Fig. 4. X-ray diffraction pattern of the tube outer surface scale.

400 lm. By substituting these values in Eq. (1), the average operating temperature of tube was found equal to 596 °C, which is considered a high operating temperature for superheater tube, which are designed for operating in 540 °C. Due to this high operating temperature, superheater tubes are on the risk of long term overheating or creep rupture.

3.5. Microstructural degradation In order to describe microstructural evolution, classification of microstructural degradation has been provided. The nature of carbide spheroidization in 12Cr1MoV steel is shown in Fig. 5 [11]. The five levels of degradation are assigned based on the development of carbide spheroidization. The classification of mentioned levels is considered as follows: Level Level Level Level Level

1: 2: 3: 4: 5:

having having having having having

no spheroidization. slight spheroidization. medium spheroidization. complete spheroidization. serious spheroidization.

From Level 1 to 5, the lamellar in pearlite changes to a particle structure. Microstructure examinations on the ruptured and un-ruptured sections (120 cm away from the rupture area) of the tube are conducted, the result is illustrated in Fig. 6. An advanced stage of spheroidization can be seen from this figure, which indicates the probability of creep failure. To confirm the occurrence of overheating (creep phenomenon) in the middle superheater tubes, creep analysis was performed and results are discussed in the following.

Fig. 5. The nature of carbides spheroidization in 12Cr1MoV steel [11].

24

H. Shokouhmand et al. / Engineering Failure Analysis 50 (2015) 20–28

Fig. 6. (a) Microstructure of the rupture region, showing an advanced stage of spheroidization, (b) microstructure of the tube metal in the distance of 120 cm from rupture region (magnication of 500).

3.6. Creep analysis 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 rh developed in the tube can be determined as:

rh ¼ P

ðr þ t=2Þ t

ð2Þ

where P is operational internal pressure; r indicates outer radius and t represents wall thickness. Diagram of Larsen– Miller parameter with stress variation to rupture of 12Cr1MoV steel is utilized to determine the rupture time [12] (Fig. 7). According to Eq. (2) the operating hoop stress for the operating steam pressure of 17.45 MPa is equal to 65.43 MPa for ruptured tube with r = 19 5 mm and t = 6 mm. From Fig. 7 the Larson–Miller parameter is evaluated 21400 and corresponding rupture time for the tube metal temperature of 596 °C (869 K) would be 42267 h. To ensure the accuracy of the obtained results, creep rupture curve for 12Cr1MoV steel at 600 °C was considered in Fig. 8[13]. According to this figure, one can observe that rupture time is around the 45,000 h, which confirms the above calculations. Comparing the predicted time for creep rupture (42,267 h) and tube actual rupture time (33,270 h), indicates that the tube has spent 80% of the expected time for creep rupture before its failure. Hence, it can be concluded that the high temperature operation of tube metal affects its failure when the fuel was changed from gas to heavy oil. 4. Boiler retrofitting As it was mentioned earlier, superheater tubes suffer from long term overheating (creep phenomenon), which is occurred over a period of months or years, therefore the creep rupture could be originated by local overheating. During the normal operation, superheater tubes will experience increasing temperature and strain over the life of the tube until the creep life is expended. In order to evaluate the deviation of operating conditions from design conditions, the steam outlet temperature of middle superheater and the mass flow rate of spray water in several conditions were recorded. These results are reported

Fig. 7. Curve of stress vs. Larsen–Miller parameter of CrMoV steal [12].

25

H. Shokouhmand et al. / Engineering Failure Analysis 50 (2015) 20–28

Fig. 8. Creep–rupture curve for 12Cr1MoV steel tested at 600 °C [13].

Table 3 Operating results of Sahand Power plant boiler On March 2005. Fuel

Load (MW)

Steam temp. at SH outlet (°C)

Design value for steam temp. at SH outlet (°C)

Spray water (t/h)

Design value for spray water (t/h)

Heavy oil Natural gas Natural gas

319 242 321

426 478 469

415 462 450

33.4 110 137

21 107.7 110.2

in Table 3, and compared with design values. As it is observed from Table 3, for all cases, spray water rate ascends with increasing the steam temperature at the middle superheater outlet, which was concluded with respect to the design values. The most common reasons for increasing the tube temperature are deposits, scale, restricted flow and inappropriate design. Experimental results did not show any deposits in the inner side of tubes. Scale was observed on the external surface of tubes which slightly reduces metals temperature. Hence, only two factors remain to consider. Usually, when boiler is designed to burn with two very different types of fuel, especially when the characteristics of fuels were not fully clarified, boiler manufacturer used to hold more conservatively view point in design. Thus, the settings of superheater heating surface are quite difficult to determine. As it was mentioned earlier, the boiler was designed for mainly firing heavy oil, thus, it will cause the heat absorbed in superheater heating surface larger than the design for firing with natural gas. This phenomenon can be explained by studying the natural gas and heavy oil emissivity. Heavy oil and natural gas in a normal burner supply a flame with emissivity of 0.25–0.6 and 0.15–0.25, respectively [14]. This means that the transferred heat in a typical furnace tube burning natural gas may be 25–30% less than the same furnace burning heavy oil. Therefore, when the boiler is firing with natural gas, the combustion product was passing through the superheater with higher temperature than the case of heavy oil. Increasing the heat absorption in superheater tubes leads to higher steam temperature at middle superheater outlet and more spray water which is well known as ‘‘over spraying’’. Over spraying in superheaters results in long term overheating failures due to the metal temperature increase. When the over spraying occurs in superheaters, the steam flow through the preceding sections will be reduced to the extent of the given span. This reduced flow, increases the metal temperature at each point along the length of the tube. If the slope of this increase is more than the allowable limit of metal operating temperature, then it can lead to long term over heating failure. According to the rendered descriptions, it seems that the design of the boiler characteristics such as the superheater tube diameter, transverse and longitudinal pitch of tube bundles, and heat transfer area are not sufficiently correct. Due to the high cost of re-designing the boiler heating surface parameters, boiler modifications would be tried with minimal changes in boiler structures. Thus, the reduction of spray water mass flow rate was performed utilizing a reduction in the middle superheater heat transfer area. In this case, in order to rectify over spraying, some modifications for the heating surface of middle superheater in the boiler unit 1 were done. Modifications were carried out according to the Table 4 with removing 1 pipe at the outer part of superheater tubes (Fig. 9). In the following section, the measured values of the steam temperature at the middle superheater outlet and the mass flow rate of the spray water in the boiler unit 1 (modified boiler) and the boiler unit 2 of the Sahand Power Plant at the same operating condition are reported and compared.

26

H. Shokouhmand et al. / Engineering Failure Analysis 50 (2015) 20–28

Table 4 Modification of middle superheater structure.

Tube diameter Tube wall thickness Transverse pitch Transverse row no. Longitudinal pitch Tube loop no. Tube set no. Arranged heating surface Gas flow area Steam flow area

Unit

Original design

After modification

mm mm mm – Set – – m2 m2 m2

51 6.5 228 56 102 6 336 1248 118 0.3809

51 6.5 228 56 102 5 280 1040 118 0.3174

Fig. 9. Tube arrangement in the middle superheater of Sahand boiler (the removed tube was identified by red line). (For interpretation of the references to color in this Fig. 9 legend, the reader is referred to the web version of this article.)

5. Results and discussions The long term overheating (creep) of the metal surfaces of the middle superheater of Sahand Power Plant occurred twice. Investigations showed that the over spraying is the one of the reasons of this failure. Moreover, in the constant load of the boiler, high spray water reduces the main steam flow rate within the middle temperature superheater. Therefore, by

Fig. 10. Comparison of water spraying mass flow rate in boiler unit 1 and 2 at the same condition.

27

H. Shokouhmand et al. / Engineering Failure Analysis 50 (2015) 20–28 Table 5 Comparison of spray water mass flow rate between boilers unit 1(retrofit boiler) and 2 of Sahand Power Plant. Gross power  300 (MW)

Gross power  260 (MW)

Average spray water in boiler 1 (t/h)

Average spray water in boiler 2 (t/h)

Percent of reduction (%)

Average spray water in boiler 1 (t/h)

Average spray water in boiler 2 (t/h)

Percent of reduction (%)

34.12

72.35

52.84

58.57

71.26

17.80

decreasing the flow through the middle temperature superheater, the tube and the steam outlet temperatures of middle temperature superheater increase, and consequently, the amount of required spray water ascends. Due to reduce the over spraying in Sahand boilers, heating surface of middle superheater of boiler unit 1 was reduced as much as 8.33%, by removing one set of tube. However, boiler unit 2 remained intact. In Fig. 10 the amount of water spraying mass flow rate for Boiler unit 1 and 2 are compared, using the heavy oil in the burners. From this figure one can observe that the mass flow rate of spray water in the full load for boiler unit 1 was reduced with respect to boiler unit 2. Furthermore, when the boilers load are less than its maximum load, they spray water mass flow rate is almost equal for both boilers. In Table 5, the comparison of spray water mass flow rate between boilers unit 1(retrofit boiler) and 2 of Sahand Power Plant are reported. As it was observed from this table, the average reductions of spray water mass flow rate in the modified boiler in the load of 300 MW and 260 MW are equal to 52.84% and 17.80%, respectively. However, the amount of spray water mass flow rate exceeds the design value (50 t/h), when the boiler is not operates in its full load. 6. Conclusion In this study, the failure analysis of Sahand 2  325 MW superheater tubes was investigated. And the following results were obtained: (1) (2) (3) (4) (5) (6) (7) (8)

Failed superheater tubes are bent at several parts. The outer surface of superheater tubes are covered with relatively thick sediment. Chemical analysis indicates that, the chemical composition experienced no changes. The XRF results showed the presence of highly harmful elements like V, S and Na, which is indicates that the heavy oil quality is not suitable. The average operating temperature of tube was found 596 °C, which is higher than designed value (540 °C). The average spray water mass flow rate is more than design value. Microstructure analysis showed an advanced stage of spheroidization of the rupture region. The tube has spent 80% of predicted time for creep rupture before its failure.

Based on these discussions, it was concluded that the superheater tubes suffer from creep phenomenon (long term overheating), which occurred because of high spray water and high heat absorption in the middle superheater. In order to rectify the overheating, some modifications for the middle temperature superheater were done to reduce both heat flux absorption and spray water mass flow rate. The results showed that the 8.33% of reduction in heating surface area leads to 52.84 and 17.80% reduction in spray water mass flow rate at the load of 300 and 260 MW, respectively. Acknowledgment This work was sponsored by the Iran Power Development Company (IPDC) under grant no. 91-231-1-68. The advice and financial support of the IPDC are gratefully acknowledged. References [1] Port RD, Herro HM. The NALCO guide to boiler failure analysis. New York: Nalco Chemical Company: McGraw-Hill Inc.,; 1991. [2] Rahmana MM, Purbolaksono J, Ahmad J. Root cause failure analysis of a division wall superheater tube of a coal-fired power station. Eng Fail Anal 2010;17:1490–4. [3] Xu Lijun, Khan JamilA, Chen Zhihang. Thermal load deviation model for superheater and reheater of a utility boiler. Appl Therm Eng 2000;20:545–58. [4] Jones DRH. Creep failures of overheated boiler, superheater and reformer tubes. Eng Fail Anal 2004;11:873–93. [5] Abbasfard Hamed, Ghanbari Mehdi, Ghasemi Amin, Ghader Sattar, Rafsanjani Hasan Hashemipour, Moradi Ali. Failure analysis and modeling of super heater tubes of a waste heat boiler thermally coupled in ammonia oxidation reactor. Eng Fail Anal 2012;26:285–92. [6] Daneshvar-Fatah Farhad, Mostafaei Amir, Hosseinzadeh-Taghani Reza, Nasirpouri Farzad. Caustic corrosion in a boiler waterside tube: root cause and mechanism. Eng Fail Anal 2013;28:69–77. [7] Rahimi Masoud, Khoshhal Abbas, Shariati Seyed Mehdi. CFD modeling of a boiler’s tubes rupture. Appl Therm Eng 2006;26:2192–200. [8] Poursaeidi E, Moharrami Ali, Amini M. Failure probability and remaining life assessment of reheater tubes. Int J Eng. Trans B: Appl 2013;26:543–52. [9] Chinese Standard: GB/T 17107–1997.

28

H. Shokouhmand et al. / Engineering Failure Analysis 50 (2015) 20–28

[10] Viswanathan R. Damage mechanism and life assessment of high temperature components. Metals Park, Ohio, USA: ASM International; 1989. pp. 228– 229. [11] DL/T 773–2001, 2002; Spheroidization evaluation standard of 12Cr1MoV steel used in power plant. [12] Kimura K, Kushima H, Abe F, Yagi K. Inherent creep strength and long term creep strength properties of ferritic steels. Mat Sci Eng A-Struct 1997;234– 236:1079–82. [13] Klueh RL. Elevated-Temperature Ferritic and Martensitic Steels and Their Application to Future Nuclear Reactors. Oak Ridge National Laboratory; 2004. [14] William Short, 1979; Fuel economy handbook, NIFES Ltd., (2nd ed.), Graham and Trotman.