Life Prediction of Service Exposed Tubes in a Boiler of a Thermal Power ...

Life Prediction of Service Exposed Tubes in a Boiler of a Thermal Power Plant

A.K. Ray, Υ. N. Tiwari , P.K. Roy, G. Das* and S. Chaudhur i

MTE Division, * MTC Division National Metallurgical Laboratory, Jamshedpur-831007. India

Abstract

Boiler tubes in power plants have finite life because of prolonged exposure to high temperature, stress and

aggressive environment. Platen superheater and reheater tubes, made of 2.25Cr-l Mo steels, service-exposed

for 148,900 hours in a 120 MW boiler of a thermal power plant, underwent detailed metallurgical assessment

to analyze their remaining safety life. The investigation included hot tensile, hardness measurement,

dimensional measurement, microscopy and a few accelerated creep tests. Analysis revealed that

experimentally determined yield strength and ultimate tensile strength, as well as estimated 10,000- 100,000

hours rupture strength as obtained from experimental data in the temperature range of 520°C to 580°C,

exhibited a decreasing trend with increasing temperature. Microstructural study did not reveal any significant

degradation in terms of creep cavities, cracks, graphitization etc. In general, analysis of tensile and stress

rupture data revealed that, although there was degradation of the tubes due to the prolonged service exposure

in terms of the ultimate tensile strength values, stress rupture plots showed that the service exposed

superheater and reheater tubes could remain in service for a length of more than ten years at the operating

hoop stress level 40 MPa / 540 °C, provided that no localised damage in the form of cracks or dents

developed. It is recommended that a similar health check should be carried out after 50,000 hours of service

exposure at 540 °C.

Key w o r d s : Service exposed, superheater, reheater, boilers, stress rupture test, tensile properties,

residual life.

1. INTRODUCTION

Remaining life assessment (RLA) of aged power plant components in the present highly competitive

industrial scenario has become very popular both for economy and safety reasons. In India it has become all

the more important since out of total thermal power capacity of 60,000 MW, about 12,000 MW capacities are

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Vol. 14, Νos. 2-3, 2003 Service Exposed Reheater Tubes

over 25 years old and RLA has become an imperative task. In real life situations, both premature retirement

and life extension (in relation to design life) can be encountered. The decision for retiring a component is not

purely technical but also one of economics and safety. This has attracted a few multinationals to RLA studies

and the broad purpose is:

• Evaluation of parts regarding the degree of aging and remaining life

• Replacement of parts that failed

• Improvement in the construction of components through material substitution, shape alteration and

reinforcement

• Development of an on-line diagnosis system to monitor aging of the components and to assure proper

functioning of the unit within a fixed period of time.

Boiler components used in power plants have finite life because of prolonged exposure to high

temperature, stress and aggressive environment. However past experience has shown that for a variety of

reasons these may have significant remaining life beyond the design specification. This is best estimated by

conducting a systematic life assessment exercise during a planned shutdown. In most cases damage

accumulation starts from the outer surface. It manifests itself as surface cracks. Therefore careful visual

examination and non-destructive tests (e.g. dye penetration tests, MP1, etc.) carried out on the outer surface

can give a fair idea about the health of the component. In addition use of ultra sonic flaw detector can also

detect nucleation of defects within the material.

It is widely known that carbon and Cr-Mo steels are extensively used as high temperature components in

power plants /1-7/. Even though most of these components have a specific design life of 20 years, many of

them are known to have survived much longer. In view of the increasing cost of setting up a new plant, there

is now considerable interest in life extension of the existing units. In order to arrive at a quantitative estimate

of the remaining life of such ageing components, it is necessary to have some creep and stress rupture data.

The present work thus incorporates determination of tensile properties in the temperature range of room

temperature (25°C) to 600°C, creep rupture properties in the temperature range of 550°C to 700°C and

microstructura! study to assess the condition of the service-exposed reheater and platen superheater tubes for

their continued service.

1.1. Material and history of the service exposed boiler tubes:

The material specifications with service condition and history of operation of the service exposed

superheater and reheater tubes of the boiler are given in Table 1.

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A.K. Ray et al. Journal of the Mechanical Behavior of Materials

Table 1

Material specification, dimension and service condition of the service exposed tubes

Material Platen Superheater Outlet Reheater Outlet

Material Specif icat ion BS 3059/622/50 SE BS3059 /622 SI

Design Steam Pressure at Outlet 151 kg/ cm 2 33.044 k g / c m 2

Operat ing Steam Pressure outlet 133.6 k g / c m 2 28.0 kg/cm 2

Operat ing Temperature 540 °C 540 °C

Design Temperature 570 °C 570 °C

Steam f low 393.000 kg/hour 361 .400 kg/hour

Outer Diameter ( O D ) 50.8 mm 50.8 mm

Thickness 10.97 mm 3.25 mm

Service- Exposed (Running) Hours 148,900hours 148,900 hours

1.2 Dimension and visual examination of the service exposed tubes:

The dimensional measurement (see Table 1) carried out on these tubes did not show any appreciable

damage in O.D. (outer diameter) or wall thickness, and the cross sections of the service exposed tubes were

found to be of uniform thickness. Dimensions of the outer diameter ( O D ) were measured at two mutually

perpendicular directions along the length of the tube at an interval of 150 mm. There was no evidence of any

localised attack on the outer and inner surfaces of the tube.

2. E X P E R I M E N T A L P R O C E D U R E S

Chemical analysis as revealed in Table 2 shows that the materials in the present investigation are

basically 2.25Cr-1 M o steels conforming to the grades specified in Table 1.

Table 2

Chemical analysis of the service exposed boiler tubes

SI. No Type of Material Wt % of Elements present SI. No Type of Material

C Mn Si S Ρ C r M o

1 Platen Superheater

Outlet

0.12 1.14 0.08 0.045 0.03 2.23 0.9

3 Reheater Outlet 0.15 0.62 0.21 0.034 0.033 2 .49 1.05

4. BS 3059/622 0.15

max

0.6

max

0.5

max

0.05

max

0.03

max

2.6

max

1.13 max

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Vol. 14, Nos. 2-3, 2003 Service Exposed Reheater Tubes

Optical metallographic examinations (Figs. 1-2) were carried out on the service exposed tubes. The

average hardness values (VHN) of these tubes are shown in Table 3.

Fig. 1: Optical micrograph of the service-exposed reheater outlet tube at X 500 revealed ferritic and bainitic

structure with no evidence of creep cavitation damage, oxide scale deposition at inner and outer

surface or spheroidization of the tube. The ferrite grains are dispersed with carbides

Fig. 2: A typical optical micrograph of service-exposed platen superheater outlet tube at X 500 showing a

ferrite bainitic structure with no evidence of spheroidization and creep cavitation damage. The

ferrite grains are dispersed with carbides

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Table 3

Hardness values of the service exposed- reheater and superheater tubes

SI.No Type of material Hardness Value

(VHN)

1 Platen superheater outlet tube 168

3 Reheater outlet tube 173

Tensile tests of the service exposed tubes were performed at room temperature, 25°C, 500°C, 575°C and

600°C, using a digitally controlled 8562 Instron servo-electric testing system, equipped with a 3-zone split

furnace with PID control. Standard tensile specimens were made from the service-exposed materials as per

ASTM E8-79 specification. Tensile tests were carried out on the base metal only from the longitudinal

direction of the service-exposed tubes. During tensile testing, constant test temperature within ±2°C and a

constant displacement rate of ± 0.2 mm/min were maintained. The variation of the Yield Strength (0 .2%

Proof Stress) and Ultimate Tensile Strength (UTS) with temperature of testing is shown in Figs. 3a and 3b.

Fig. 3c shows the variation of % EL (Elongation) with test temperature.

Accelerated stress rupture tests using constant load Mayes creep testing machines were carried out as per

ASTM 139/83 specification with flat specimens made from the longitudinal direction of the service-exposed

Tempera ture,t

Fig. 3a: Plot of yield strength (0.2% proof stress) with temperature for the service-exposed platen

superheater outlet and reheater oulet tubes .

153


Tempera tu re,°C

Fig. 3b : Plot of ultimate tensile strength (UTS) with temperature for the service-exposed platen superheater

outlet and reheater oulet tubes.

Temperature, °C

Fig. 3c: Variation % elongation with temperature for the service-exposed platen superheater outlet and

reheater oulet tubes.

154

A.K. Ray etat. Journal of the Mechanical Behavior of Materials

tubes. These tests were carried out in the temperature range of 550-700 °C and in the stress range of 36-129

MPa. The stress levels above the operating stress at each temperature were selected in such a way as to obtain

rupture within a reasonable span of time. The hoop stress o h acting on the service-exposed tubes was

calculated using the following formula to predict the remaining life:

where Ρ is the operating pressure in MPa, D is the mean diameter in mm and t is the thickness of the tube in

mm.. The operating hoop stress thus evaluated is ~ 40. MPa.

The stress rupture data have been plotted in terms of stress Vs LMP (Larson Miller Parameter) along with

ASME (minimum) data line /12/ for similar grade of steel, for the purpose of comparison (Fig. 4). For the

grade of steel in the present investigation /12/, Larson- Miller Parameter (LMP) = Τ (20 + log tr),

where Τ = Absolute temperature in Κ and /,. = Rupture time in hours

The life of the tube in hours was then estimated at various temperatures from the LMP value read from Fig.4.

Regression analysis of stress rupture data for service-exposed reheater and superheater tubes has been

carried out using a standard software package, in order to evaluate the long term rupture strength of the tubes

over the range of temperatures presently investigated.

Larson- Miller Parameter (LMP) =

oh = PD/2t (1)

Τ (20 + log tr) = a„ + a,(log S) + a2 (logS)2 + + a,„ (log S)1 ,111 (2)

re 0.100 5

ο Platen superheater outlet ° Reheater outlet

1 0 16000 17000 18000 19000 20000 21000 22000

LMP = Τ (20 + Log tr)

Fig. 4: Plot of stress versus LMP (Larson Miller Parameter) for the service-exposed tubes

155


where

S = Rupture strength in MPa

m = Order of polynomial

a„, α ι a^&a,,, are polynomial constants and are shown in Table 4.

Table 4

Polynomial constants from regression analysis

Type of

material

Order of

polynomial

Standard

deviation

a l a 2 a j

Platen

superheater

outlet tube

m =3, C = 20 99.11931 0.24684X105 -158.94635 -3.1560110 0.01588321

Reheater

outlet tube

m =3, C = 20 94.518 0.23047X10 s -63.305985 0.14590814 4.280 X 10 ^

Table 5 reveals the rupture strength, S, of the service-exposed tubes in the temperature range of 520 to

580°C, for various rupture times and at m = 3. It is clear from Table 5 that the estimated 10000 - 1 0 0 0 0 0 hr

rupture strengths at various temperatures showed a decreasing trend with increasing temperature. This is the

general trend observed in service-exposed materials /14-17/.

Table 5

Estimated rupture strength in MPa

Type of Material Temp,

°C

Order of

Polynomial

Time (tr) in hrs Type of Material Temp,

°C

Order of

Polynomial tr =10,000

hrs

tr = 30,000

hrs

tr =100,000

hrs

Reheater outlet tube 520 m=3,C=20 75.38 69.39 62.8 Reheater outlet tube

540 m=3,C=20 67.7 61.62 54.87

Reheater outlet tube

550 m=3,C=20 63.96 57.73 50.90


560 m=3,C=20 60.15 53.85 46.94


580 m=3,C=20 52.54 48.38 40.95

Platen superheater outlet

tube

520 m=3,C=20 37.04 34.56 31.85 Platen superheater outlet

tube 540 m=3,C=20 33.89 31.36 28.57


tube

550 m=3,C=20 32.32 29.75 26.93


tube

560 m=3,C=20 30.75 28.15 25.29


tube

580 m=3,C=20 27.60 24.94 22.02

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3. RESULTS AND DISCUSSION

3.1 Visual observation and metallography:

Dimensional measurement revealed that there was no change in outer diameter and thickness of the

service exposed reheater and superheater outlet tubes. It seems that the tubes have not undergone any

appreciable deformat ion during actual operat ing condit ions. N o evidence of localised damage was observed

on either external or internal surfaces. The hardness level of service exposed tubes revealed (see Table 3) no

significant variation in hardness values with exposure lives.

The microstructure of the service exposed reheater and platen outlet tubes mainly consisted of ferritic

bainitic structure (Figs. 1-2). The ferrite grains are dispersed with carbides. N o evidence of graphit ization and

creep damage in the form of cavitation and decarburisat ion was observed in any of the service exposed tubes.

Therefore , it is clear that the service exposed reheater and platen superheater outlet tubes have had hardly any

appreciable degradat ion f rom the microstructural point of view.

3.2 Mechanical properties

Room temperature as well as high temperature tensile propert ies as obtained f rom exper iments are

reported in Fig. 3. It is evident f rom the results that 0 . 2 % proof stress (yield strength) and the U T S (ultimate

tensile strength) values for the service-exposed tubes showed a decreasing trend with increasing temperature .

However, % EL (Elongat ion) showed an increasing trend with temperature. This is the common trend

observed for materials tested at elevated temperature /12-17/ . Analysis of tensile data revealed that there is

some deterioration in ultimate tensile strength (see Fig. 3b) compared to. A S T M 213-T22 grade of steel due

to service exposure; however , these variations fall within the specified limits for similar grade of steels viz.

2.25Cr- I M o steels, as reported in literature /12/. The % E L (see Fig. 3c) of the service-exposed reheater

outlet tubes show that they were less ductile at high temperature compared to the service-exposed platen

superheater outlet tubes of similar composi t ion.

In the absence of discernible cavitation or flaws, stress rupture tests can be selectively used to assess the

condition of components . One of the most widely used techniques for life assessment of componen t s involves

removal of samples and conduct ing accelerated tests at temperatures above the service tempera ture /8/. An

estimate of the remaining life is then made by extrapolation of the results to the service temperature . Several

uncertainties relating to the validity and application of the technique have been resolved in recent research

projects /8/.

In the present investigation, long term rupture strengths were estimated with best fitted curves for third

order polynomial . For different orders of the polynomial , the average sum square error ( A S S E ) was est imated

f rom the fol lowing equation:

ASSE = ^ (Yexperimental ~ Ycslmiulcct) ^ ^

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Vol. 14, Mos. 2-3, 2003 Service Exposed Reheater Tubes

where η is the number of data points. The third order polynomial was selected for estimation of rupture

strength as there was no significant change in the average sum square error for higher orders.

Since microstructural examination did not show any major degradation, it is expected that the mechanical

properties will be within expected limits, which is also shown in the experimental data.

Short term stress rupture tests were also carried out on standard test specimens made from platen

superheater outlet and reheater outlet coils. The data obtained have been compared with the reported data on

2 . 2 5 C r l M o steel in Fig. 4. The lines in the figure represent the minimum and mean rupture strengths for 2.25

C r l M o steels from literature (the A S M E minimum data line). It is clear that, barring one point, all the data

fall on or above the minimum and mean A S M E data lines. It is noteworthy that at low stress levels the stress

rupture data merged with the minimum and mean A S M E data line, which is the common trend observed in

such steels. It is also interesting to note that most of the data seemed to fall on or above the A S M E minimum

and well within the scatter band of the A S M E mean data line (see Fig. 4). Therefore, as far as creep strength

is concerned, there is no appreciable degradation due to service exposure. This is also consistent with the

information collected from other destructive and non-destructive tests conducted on boiler components.

When the steam temperature is 540°C, the metal wall temperature is usually around 570°C. Therefore to

estimate remaining life it will be more appropriate to use actual operating design temperature, which is

570°C. Figure 5 represents the variation of temperature with the balance life in hours for the service exposed

platen superheater and reheater outlet tubes. It is evident that at 540°C and at 570°C, all these service-exposed

tubes have a remaining life of more than 100,000 hours, provided there are no cracks, dents, microstructural

degradation like creep cavitation damage due to micro void coalescence, evidence of

spheroidization/graphitization, decarburization etc. in all the materials due to long term service exposure.

580

570 Ο ο Ο-ι . 3 560 2 Κ ε iS? 550

540

Operating stress, 40 IVPa Ο Δ

Δ Platen superheater outlet tube ο Reheater outlet tube

10000 100000 Balance Life, hrs

Ο Δ

1000000

Fig. 5: Plot of temperature versus balance life of the service-exposed platen superheater ouMet and reheater

outlet tubes .

158


However, a similar health check is desired after 5 years of service exposure.

The hardness values of the service-exposed platen superheater and reheater tubes have been superimposed

on the mean data line in the hardness versus LMP plot (see Fig. 6), thus enabling us to substantiate that that

the hardness data lie on or above the mean data line for 2 .25Cr lMo steels as reported in literature /18,19/. A

similar behaviour was also observed from the stress versus LMP plot (Fig. 4), where most of the data lie on

or above the minimum and mean data line for 2 .25Cr lMo steel.. Since there was no appreciable change in

the hardness values of such steels even due to prolonged service exposure, this proves that the temperature of

the metal wall was not lower than the steam temperature.

LMP = Τ ( 20 + log tr )

Fig. 6: Plot of hardness versus LMP (Larson Miller Parameter) for the service exposed platen superheater

outlet and reheater outlet tubes .

So far as the remaining life at 540°C / 40 MPa is concerned, it is possible to obtain a minimum life of

>100,000 hours for the service-exposed reheater and platen superheater outlet tubes provided there is no

evidence of localised damage in the form of surface cracks, cavitation or dents. It is recommended that

another check for safety of the service exposed pipes in terms of residual life be carried out after expiry of

50,000 hours of service life for economical and safety reasons. Also, during shut down of the plant, NDT

(nondestructive) tests viz. dimensional (thickness and diameter) measurement, hardness measurement and in

situ metallography may be carried out to assess the condition of the materials for their future serviceability.

159


4. CONCLUSIONS

The above study leads to the following conclusions:

i) So far as the residual life at 540°C/ 40 MPa is concerned, it is possible to obtain a minimum life of about

100,000 hours for the service-exposed reheater and platen superheater outlet tubes provided there is no

evidence of localised damage in the form of surface cracks, cavitation or dents.

ii) Analysis of tensile data revealed that there is some deterioration in ultimate tensile strength of the service-

exposed reheater and platen superheater outlet tubes compared to those of the virgin tube reported in

literature, but these variations are within the specified limits for similar grade of steels.

iii) The service-exposed reheater and platen and reheater outlet tubes appear to be in a reasonably good state

of health. Another check for safety of the service exposed pipes in terms of residual life should be carried

out after expiry of 50,000 hours of service life for economical and safety reasons. Also, during shut down

of the plant, NDT (nondestructive) tests viz. dimensional (thickness and diameter) measurement, hardness

measurement and in situ metallography may be carried out to assess the condition of the materials for

their future serviceability.

ACKNOWLEDGEMENT

The authors are grateful to Mr. Santokh Singh, Mr. Hira Lai and Mr. Sashi Bhushan Singh for their

assistance in the stress rupture tests, and to the Director, National Metallurgical Laboratory, Jamshedpur,

India for his kind permission to publish this paper.

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Life Prediction of Service Exposed Tubes in a Boiler of a Thermal Power ...

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