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PT PUPUK SRIWIDJAJA (PERSERO)REALIBILITY AND QUALITY ASSURANCE DEPARTMENT FIELD TECHNICAL INSPECTION – II

MEMO TO FILE

NUMBER : 001/MTF/ITL-PII/2010

AUXILIARY BOILER2A – 101BUPUSRI - II

BY:

MAULIDIN BASTIAN

On behalf of Failure Analysis team of:MAULIDIN BASTIAN/MARTHA INDRIYATI /DIKDIK YULIANA / BHARATA

andFIELD TECHNICAL INSPECTION COORDINATOR-I

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PT PUPUK SRIWIDJAJA (PERSERO)REALIBILITY AND QUALITY ASSURANCE DEPARTMENT FIELD TECHNICAL INSPECTION – II

MEMO TO FILE

NUMBER : 001/MTF/ITL-PII/2010

AUXILIARY BOILER2A – 101BUPUSRI - II

Prepared By, Agreed By, To be known by,

Maulidin Bastian M. Toni A. Muksin

PAGE3OF31

CONTENTS

A. ABSTRACT............................................................................................................................5

B. INTRODUCTION....................................................................................................................5

C. OBJECTIVE............................................................................................................................6

D. CHRONOLOGIES..................................................................................................................6

E. LITERATURE REVIEW........................................................................................................8

1. AUXILIARY HEATER DESIGN.............................................................................................8

2. STRESS RUPTURE OR SHORT-TERM OVERHEATING.............................................11

3. METALLURGICAL AFFECTS.............................................................................................11

4. CHEMICAL ANALYSIS........................................................................................................13

5. MECHANICAL PROPERTIES.............................................................................................13

F. METHODOLOGY.................................................................................................................16

G. RESULTS AND DISCUSSION..........................................................................................18

1. START UP AND FAILURE PROCESS..............................................................................18

2. MECHANICAL PROPERTIES.............................................................................................20

3. PHYSICAL PROPERTIES...................................................................................................22

4. METALLURGICAL EFFECTS.............................................................................................25

5. THERMAL CRACKS.............................................................................................................28

H. CONCLUSION.....................................................................................................................29

I. Bibliography........................................................................................................................30

PAGE4OF31

LIST OF FIGURES

Figure 1. Tube impose a reaction force as in inside Primary Reformer Furnace at Auxiliary

Heater 2A-101BU

Figure 2.Variation of fluid temperature and tube-wall temperature as water is heated through

the boiling point with low, moderate, high, and very high heat fluxes (rates of heat

transfer)(Steam, 1972)

Figure 3.Thin-lip rupture in a boiler tube that was caused by rapid overheating. This rupture

exhibits a “cobra” appearance as a result of lateral bending under the reaction

force imposed by escaping steam. The tube was a 64-mm outside-diameter × 6.4-

mm (0.250-in.) wall thickness boiler tube made of 1.25Cr-0.5Mo steel (ASME SA-

213, grade T-11).

Figure 4. A compilation of ultimate tensile strength versus Brinell hardness number for

selected metals based on handbook data.

Figure 5. Location of failure in Auxiliary Boiler

Figure 6. Evidences that found on header which can be blockaging media for steam flow

(welding debris, rod, plate, etc)

Figure 7. Relationship between Ultimate and Yield Strength with Temperature

Figure 8. The thickness measurement in failure tube

Figure 9. Cobra lip in Auxiliary Boiler 101BU Pusri 2 tube failure

Figure 10. The crack phenomenon which shown after failure

Figure 11. (a) Microstructure of crack lip ; (b) macrostructure of crack lip

Figure 12. The place of SA 106 Gr.B in Fe-C Diagram

Figure 13. Crack happened in weld joint of bottom header at no.1 from East and no.3 from

South

Figure 14. The mechanism of thermal crack

LIST OF TABLES

Table 1. Material SA 106 Gr.B specification

Table 2. Chemical composition of material SA 106 Gr.B

Table 3. Chronologies of Auxiliary Boiler tube failure

Table 4. Tensile strength properties of SA 106 Gr.B

Table 5. Hardness Measurement for each samples

Table 6. Strength Properties of SA 106 Gr.B material

PAGE5OF31

Table 7. Conversion of Hardness vs Tensile Strength for tube after failure in each location

Table 8. Thickness Measurement for failure tube

A. ABSTRACT

In allusion to the explosion of Auxiliary Boiler took place at PUSRI II in Turn Around

Plant August 2010 having a series of evidence collection and inspection jobs which

includes collecting operation condition and parameters, bent-tube, sampling the

explosion fracture, fin-tube apart from explosion fracture and fracture detection

medium and its hangover, etc., had been carried out firstly. Based on these jobs,

farther analysis and computation work has been done to the structural and materials

characteristics and the operation condition of the auxiliary boiler, including

composition, metallographic phases, complemented by chemical analyses of tube or

fireside deposits, tensile properties, impact energy, strain ageing characteristics and

fracture toughness of the fin-tube steels, the preview of leak detection medium and

it’s hangover in the boiler and also explosion energy, as appropriate,. The most

probable cause of the unexpected tube failure is the major factor causing unreliability

in boilers. Characterizing the degree of microstructural degradation can also help to

confirm and separate various potential high temperature tube damage modes. The

main medium reason which caused short-term overheating failure mode is abnormal

condition in presence of tube blockaging. This project is also investigating failure

mode which was happened in Primary Reformer Furnace at Auxiliary Heater(or

commonly, Auxiliary Boiler) 2A-101BU, PT Pupuk Sriwidjaja, Palembang-Indonesia,

fin-tube at start-up process and sharing ideas for improving the process and turn-

around plant.

B. INTRODUCTION

When water is boiled in a tube having uniform heat flux (rate of heat transfer) along

its length under conditions that produce a state of dynamic equilibrium, various

points along the tube will be in contact with sub-cooled water, boiling water, low-

quality steam, high-quality steam, and superheated steam. A temperature gradient

between the tube wall and the fluid within the tube provides the driving force for heat

transfer at any point.

PAGE6OF31

Port & Herro assigned a brief explanation of failures caused by short-term

overheating. Short-term overheating occurs when the tube temperature rises above

design limits for a brief period a boiler operation upset. Conditions leading to short-

term overheating, generally are partial or total tube plug gage and insufficient coolant

flow due to upset conditions and/or excessive fire-side heat input.

C. OBJECTIVE

This project is investigating failure mode which was happened in Primary Reformer

Furnace at Auxiliary Boiler 2A-101BU fin-tube at start-up process and sharing ideas

for improving the process and turn-around plant.

D. CHRONOLOGIES

A water leak was detected in Primary Reformer Furnace at Auxiliary Heater 2A-

101BU section on nine months after the boiler had been maintained. The failure was

located in farthest section from south side burner, clearly at MK “E” header coil.

BENT TUBE

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Figure 1. Tube impose a reaction force as in inside Primary Reformer Furnace at Auxiliary Heater 2A-101BU

The MK-E (coil) steam generating system is finned tube which having initial

properties as follows,

OD Wall Thickness SCH Material Type

48,26mm (1,9”) 0.508 cm (0,2”) 80 SA-106 Gr B Fin-Tube

Table 1. Material SA 106 Gr.B specification

The tube section was placed into the boiler that makes steam production around

5193 BTU/hr.ft2 as shown in Figure 1. The chemical composition of tube material is,

Material %C %P %Cr %Mo %S Application

SA106 Gr.B .[4]0,3

(mid-CS)

0.035

(max)0.4 0.15

0.035

(max)Boiler, etc

Table 2 Chemical composition of material SA 106 Gr.B

The fin tube is helically wound serrated with subsequently either annealed or

normalized and 112 tubes attached together. The coil can be operated at 1500 psig

(105 kg/cm2G) in inlet and also outlet with 325°C (598K) steam pressure, providing

an equivalent output in coil of 102,000,000 BTU per Hr.

On the failure time, operating parameters happened at;

Date and Time Explanations

August 12nd, 2010 Start-up of Ammonia Plant with 85% Gas Rate.

14.00 WIB Vent gas in PIC-8

Steam drum Steam outlet = 150 tonhr

BFW flow to steam drum (FRA-26) =195 tonhr

Steam drum Level (LRA-24) = 65 %.

HS Pressure in steam drum (PRC-18) = 80 kg/cm2.

PAGE8OF31

Discharge Pressure 104-JA Pump to Steam Drum = 95

kg/cm2.

Four (4 Ea) Main burner in Auxiliary Boiler was firing at 75%

average load.

Flue Gas Temperature = 650°C

101B Riser Temperature = 760°C

20.30 WIB Increasing of Gas Supply Rate from 80 to 85%

21.30 WIB Auxiliary Boiler blown up suddenly and fire was detected from

Main Burner No 1 & 2 ( From Bottom Side)

PIC-21 data shown the changes of pressure from -0.9mmHg

(Normal Pressure) to +1.5mmHg(Positive Pressure)

I.D Fan 101-BJT Speed increased from normal condition

(4200RPM) to 5400 RPM

Fuel Gas from Battery Limit was being cut to shut down the

plant

August 13th, 2010 1 tube was failure and bending in MK-E Header Coil. Failure

location was detected at Tube no.2 from East and 4th Row

from South

1 crack tube was found in weld joint tube-top Header and

located at no.1 from East and no.4 from south

1 leak was found in weld joint of bottom header and located at

no.1 from East and no.3 from South

Tube located at no.1, 2 and 3 from East and no.4 from South (

3Ea) was being plugged

Tube located at no.2 from East and no.3 from South ( 1Ea)

was being plugged too.

August 14th, 2010 Fail Tube was delivered to NDT-Lab for investigating the

failure mode.

Table 3. Chronologies of Auxiliary Boiler tube failure

E. LITERATURE REVIEW

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1. AUXILIARY HEATER DESIGN

The design of a steam-generating unit balances the heat input from the combustion

of a fossil fuel with the formation and superheating of steam. The heat absorbed is

converted into steam at its saturation temperature, a function of the operating boiler

pressure. Within the convection passes, the flue-gas temperature is further reduced

by the superheating or reheating of steam in superheaters and re-heaters. To extract

more heat and to improve overall thermal efficiency, an economizer preheats the

boiler feedwater to a temperature close to its boiling point. The flue gas travels

through an air preheater, which heats the combustion air, then makes its way up the

stack.

The formula for steady-state heat transfer is,

QA0

=U 0∆T❑

…. Formula 1

where QA0

(in Btuhrft 2) is the heat flux per unit area, U0 (in

Btuhrft 2° F) is the overall

heat-transfer coefficient, and ΔT (in 0F) is the temperature difference that drives the

heat flow.

There are unintended conditions that drive the system to another analysis which

using the equation of individual thermal resistances for heat flow. And the system will

run from flue gas to steam and also individual temperature gradient, the net effect

will be an increase in tube metal temperature. In simplify, decreases in steam-side

heat-transfer coefficient caused by reduced flow, will lead to tube metal temperature

increases.

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Figure 2.Variation of fluid temperature and tube-wall temperature as water is heated through the boiling point with low, moderate, high, and very high heat fluxes (rates of heat transfer) (Steam, 1972)

Figure 2 indicates the effects that different heat fluxes have on tube-wall

temperature. In the region where sub-cooled water contacts the tube (at left, Fig. 2),

the resistance of the fluid film is relatively low; therefore, a small temperature

difference sustains heat transfer at all heat-flux levels. However, the resistance of a

vapor film in steam of low quality is relatively high; therefore, at the onset of film

boiling, a large temperature difference between the tube wall and the bulk fluid is

required to sustain a high heat flux across the film. The effect of the onset of film

boiling on tube-wall temperature appears as sharp breaks in the curves for

moderate, high, and very high heat fluxes in Fig. 2. With increasing heat flux, the

onset of unstable film boiling, also known as departure from nucleate boiling (DNB),

occurs at lower steam qualities, and tube-wall temperatures reach higher peak

values before stable film boiling, which requires a lower temperature difference to

sustain a given heat flux, is established.

PAGE11OF31

2. STRESS RUPTURE OR SHORT-TERM OVERHEATING

Short term overheating failure is one in which a single incident or a small number of

incidents exposes the tube steel to an excessively high temperature (hundreds of

degrees above normal) to the point where deformation or yielding occurs. The

conditions such as loss of coolant flow and excessive boiler gas temperature are

happened frequently in this failure.

Viswanathan stated abnormal conditions that driven to failure as:

Internal blockage of tube

Loss of coolant circulation or low water level

Loss of coolant due to an upstream tube failure

Overfiring or uneven firing or boiler fuel burners

The first three produce starvation and the tube can be blocked by erection or repair

debris, tools, steel shot, pre-boiler oxides, deposits from carryover or spray water, or

loose pieces of internal non-pressure part hardware such as scrap plates, bolts and

nuts which driven the worst condition like approach the furnace-gas temperature.

3. METALLURGICAL AFFECTS

In general, short-term overheating involves considerable tube deformation in the

form of metal elongation and reduction in wall area or cross section. Such failures

indicate wall thinning and local bulging precede the actual fracture, because the

strength of the material is reduced at the higher temperature.

Hence, for metallographic explanation, a fishmouth appearance with thin-edge

fracture surfaces and considerable swelling is typical for a ferritic steel tube that has

failed before its temperature has exceeded the upper critical temperature ( Ac3 ¿.

Figure if, however, the tube temperature was high enough to transform the iron in the

steel from ferrite to austenite, there will be no noticeable “necking down,” or

reduction in wall thickness, of the fracture edges. Microstructure of the steel will be

performed to confirm that the tube temperature prior to failure was high enough to

transform the ferrite to austenite.

PAGE12OF31

French is explained about specific indications on failure caused by rapid overheating.

The thin-lip, common shapes which happens in short-term ruptures are usually

transgranular tensile fractures occurring at metal temperatures from 650 to 870 °C

(1200 to 1600 °F). These elevated-temperature tensile fractures exhibit macroscopic

and microscopic features that are characteristics of the tube alloy and the

temperature at which rupture occurred. A tensile fracture results from rapid

overheating to a temperature considerably above the safe working temperature for

the tube material and is accompanied by considerable swelling of the tube in the

regions adjacent to the rupture that have been exposed to the highest temperatures.

As shown in Fig. 3, steam escaping at high velocity through the rupture will

sometimes impose a reaction force on the tube that is sufficient to bend it laterally.

Figure 3.Thin-lip rupture in a boiler tube that was caused by rapid overheating. This rupture exhibits a “cobra” appearance as a result of lateral bending under the reaction force imposed by escaping steam. The tube was a 64-mm outside-diameter × 6.4-mm (0.250-in.) wall thickness boiler tube made of 1.25Cr-0.5Mo steel (ASME SA-213, grade T-11).

PAGE13OF31

Changes in tube ID and OD measurements can be indicators of overheating.

Increases of 5% or more are indicative of short-term overheating. Also, significant

microstructural changes in carbon steel will occur when the steel is overheated, and

these changes can be used to estimate the metal temperature at failure.

4. CHEMICAL ANALYSIS

Chemical analysis should be done prior to before-after exposed to overheating and

also make carbon equivalent calculation to predict phase happening.

Formula 2

If the relative amounts of ferrite and martensite can be determined by microstructural

analysis, and if the alloy composition is known, a Fe-C equilibrium phase diagram

can be used to estimate the metal temperature at the time the tube burst.

Chemical analysis is subjected to the analysis tool for checking before-after material

condition. If there is any chemical intrusion during failure, physically it will appear as

deposit. This analysis also should be done for assuring external effect which come to

form of tissue layer or deposit on tube or fireside.

5. MECHANICAL PROPERTIES

Chemical analyses of tube or fireside deposits will continue to tensile properties

checking, impact energy, strain ageing characteristics and fracture toughness of the

fin-tube steels, the preview of leak detection medium and it’s hangover in the boiler

and also explosion energy,

An approximate relationship between the hardness and the tensile strength (of steel) is,

TS(Mpa)=3.55*HB (HB=<175) else 3.38*HB (HB>175) … Formula 3

TS(Psi)=515*HB (HB=<175) else 490*HB (HB>175) … Formula 4

whereHB is the Brinnell Hardness of the material, as measured with a standard

indenter and a 3000 kgf load.

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Table 4. Tensile strength properties of SA 106 Gr.B

PAGE15OF31

Figure 4. A compilation of ultimate tensile strength versus Brinell hardness number for selected metals based on handbook data.

A comparison of ultimate tensile strength with Brinell hardness is shown in Fig. 4 for

several metals. Many of the copper-base alloys have much higher UTS than

predicted and the gray irons tend to have a much lower UTS than that predicted. In

the case of gray iron, the graphite flakes serve as crack initiators in tension, whereas

these flakes are under compression during the hardness test, and work hardening

that occurs increases the hardness. Ductile irons also are limited in tensile strain, or

elongation, but not as severely as gray irons because the graphite is nodular rather

than flake. Desulfurized steel would behave in a manner similar to that of ductile vs.

gray irons, when compared with plain carbon steel.

To analyze tension characteristic of recently pipes in resist transversal tension

causing from leakage, the formula of Tension vs(P, Di, t) will be using as follows

σ tr=P Di2 t

… Formula 5

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F. METHODOLOGY

A 30.5 cm (1 ft) long piece of the failed tube was cut out for analysis and examined

metallographically. The failed piece was sectioned, and regions associated with

crack initiation were examined under the optical stereoscope and microscope.

Chemical analysis will be done using X-Met to reveal information for predicting

material deposit and at elevated temperature using carbon equivalent calculation for

predicting phase happening.

Mechanical properties calculations which are giving advantage to stress to failure

calculation are headed to hardness measurement at bottom side of lip, at lip,

opening crack and bare tube.

Samples tracking :

A = Inside tube (farthest location from failure)

B = Circumferential side (farthest location from failure)

C = Location of failure which is not breaking and perpendicular to alligator lip

D = Location of failure which have transition to deform plastically

E = Location of failure which deformed plastically

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Figure 4. Thin-lip rupture in a boiler tube that was caused by rapid overheating happened to fin tube Primary Reformer Furnace at Auxiliary Section (102B) PUSRI II Plant

Sample A

Sample B

Sample C

Sample D

Sample E

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PAGE19OF31

G. RESULTS AND DISCUSSION

1. START UP AND FAILURE PROCESS

In the MK “E” region of Auxiliary Boiler, 101-BU (Figure 6), the fin-tube was attached

to header and steam flow was delivered from down to upper side of auxiliary boiler.

In firing there was an accident where Gas Supply Rate increased from 80-85% and

Riser Temperature reached ±750°C. This temperature was fit enough for material

SA-106 Gr.B to be damaged and got short-term rupture. For the failure criteria itself,

the temperature between 650-800°C should make yielding phenomenon due to

mechanical properties degradation and phase changes on the fin-tube. The weakest

section will be failed after explosion to be more-yielding than creep and if the other

tube surround got rapid increasing of pressure and temperature, crack will happened

in joint between header and tube.

The failure location in the farthest section was found because of lack of steam

distribution on that side where it should be fitted with relevant steam condition in all

tubes. Figure 5 stated the location of failure as follow,

Figure 5. Location of failure in Auxiliary Boiler

By field, after the accident tube was examined

with Boroscope to reveal information inside tube

and header. The surprising evidences that found

on the results were blockaging media for steam

flow is located on the header and overfiring or

uneven firing on boiler fuel burners shown in the

figure 8 below. Viswanathan stated the

abnormal condition as major cause of failure

happened.

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(a) (b) (c)

(d) (e) (f)

Figure 6. Evidences that found on header which can be blockaging media for steam flow (welding debris, rod, plate, etc)

13 Agt 2010

Welding debris Rod Bent Rod

Plate

PAGE21OF31

Based on Figure 6 above, the unintended objects caused change of tube wall

temperature and it drive to temperature gradient and lack of heat transfer.

Consequently, decreases steam-side heat-transfer coeficient caused by reduced

flow will lead to tube metal temperature increases and the failure will be happened if

the tube could not sustain the changes.

2. MECHANICAL PROPERTIES

Hardness measurement has done using MicroVickers in NDT Lab PT Pupuk

Sriwidjaja for several samples. The data collections were being checked minimum 6

indentation for each locations.

The data for hardness for each samples are shown in table as follows,

NoHardness (HV)

A B C D E1 160 159 155 177 2362 161 160 150 163 2333 159 160 157 154 2534 163 155 168 152 2315 155 157 149 163 2356 155 157 150 168 2417 148 1878 153

Average158,833

3 158 156,6 171,6 238,1667Table 5. Hardness Measurement for each samples

As data shown above, there are increasing of average hardness trend when it come

to failure location. The closer to alligator lip, the harder material it be. This was

proved that deformation was taking role in failure, not creep or diffusion in phases.

Dislocations along grains were multiplied as time of failure reached the limit.

From Formula 3 and Formula 4, conversion was done to compare the tube standard

ASTM SA 106 Gr.B with recently tube condition. For tensile and yield strength,

standards for related material are,

Tensile Strength 415 Mpa (min)

Yield Strength 240 Mpa (min)

Table 6. Strength Properties of SA 106 Gr.B material

PAGE22OF31

And after the failure, the data for strength are shown in Table 6 as follow,

Location HV HBTensile Strength

(Mpa)

A 158,8333 159 564,45

B 158 158 560,9

C 156,6 157 557,35

D 171,6 172 610,6

E 238,1667 227 767,26

Table 7. Conversion of Hardness vs Tensile Strength for tube after failure in each location

The strength were increasing when it comes to fail. Before break, the yield and

ultimate point would be reached in maximum limit. The relationship between Ultimate

and Yield Strength with Temperature is shown in Figure 7 below. The higher

temperature of service, the weaker material to withstand stress. So, the standard of

451 Mpa for ulitimate tensile strength of SA106 Gr.B will be lower than initial.

Figure 7. Relationship between Ultimate and Yield Strength with Temperature

What we are concerning here is the recently strength and standard of ASTM 106

GrB which are all values in above limits. The sample C,D,E are above the standard

because of deformation was happened when failure and it proved about short-term

overheating. Samples A & B is the location where no failure effect was affected in

that area. But, the comparison between these values with standard which having 415

PAGE23OF31

MPa, it will be high concern for the next maintenance. The writers suggest taking

the sample for predicting what maintenance will be in the Auxiliary Boiler 101-BU

PUSRI-2 Plant.

3. PHYSICAL PROPERTIES

Another condition to prove short-term overheating failure that happened in tube is

thickening. Changes in tube ID and OD measurements can be indicators of

overheating. Increases of 5% or more are indicative of short-term overheating. The

measurement has been done in several location to classify degree of deformation.

REGION STANDARD (ASTM SA 106 GR.B) t(MM) % OF REDUCTION

1

5.08 mm (0,2”)

1,522,5

Average 2 60%

2

332,52,521

Average 2.33 54.1%

33,54

Average 3.75 26.2 %Table 8. Thickness Measurement for failure tube

Reduction of thickness in large number shown in Table 8 fully proven about the role

of deformation is more concerning in failure which can make cobra lip. This would

happen if material have lack of strength to withstand internal pressure in tube. The

thicker tube means the tube have lower strength to handle large deformation.

The diameter increase at the deformed area was approximately 5 times (± Ø 10 in.)

from the respective initial diameter. The OD tube surface around the bulge had

cracks (also referred to as mud flat cracking or elephant hide) and exfoliated oxide

patches that resulted from the oxidation and deformation that occurred during

PAGE24OF31

overheating as a result of thermal stressed caused by tube expansion and

contraction.

For the fluid quality, after investigation from the inner tube, the fireside was clean

means no additional sulfide, phosphate, etc was grown during the operation.

Viswanathan also said that the fluid was having less failure factor than debris or solid

blockaging media. So, it was learned from the plant investigations that there had

been no operating problems with the water softening system and that untreated

water had been used in the boiler.

The deformed area had thinned by 60% (Fig. 8) with respect to the original wall

thickness (0.508 cm, or 0.2in.). Boiler tube ruptures are typically labeled as thin-

lipped (which are related to short-term overheating) to describe the amount of

thinning (±60%) and diameter increase more than 10% at the bulged area. There are

features of short-term overheating. However, the no presence of any scale, mud

crack and oxide in the innerside reveal that longterm overheating was not present.

The thick tubes in some places and high tensile stress are two conditions which are

serious to handle. So, with the condition of recent tubes which still attached in

Auxilary Boiler until now, we are suggesting to handle the operation and

maintenance with care.

3mm

3mm

2.5mm

3mm

4mm

3.5mm

2.5mm

1.5mm

2mm

2.5mm

1mm

2mm

PAGE25OF31

Figure 8. The thickness measurement in failure tube

1 2 3

PAGE26OF31

4. METALLURGICAL EFFECTS

In the middle and last region of the boiler (at MK E), tube was finned to enhance

efficiency of heat transfer at this location. The tube had a 20 cm (7,8 in.) long crack

within a 8,5 cm (3,34 in.) deformed area ( Fig. 9.).

Figure 9. Cobra lip in Auxiliary Boiler 101BU Pusri 2 tube failure

The crack started at the OD of the tube. Note that, the crack is wide-mouthed and

burned area formed oxide (Fig 10)

Figure 10. The crack phenomenon which shown after failure

Oxide in fin

Initial crack

Transgranular cleavage

PAGE27OF31

Ahead of the crack tip, there are a number of voids that have formed at the grain

boundaries around the crack as a result of graphitization and grain boundary sliding

(Fig 11)

(a) (b)Figure 11. (a) Microstructure of crack lip ; (b) macrostructure of crack lip

A normalized microstructure of this type of tube contains approximately 60% pearlite

if the carbon content is at the high end of the specification. In this case, pearlite is

still visible near the tube OD and has deformed. Graphitization, in the early stages of

transformation, the carbide plates in the pearlite transform into spheroids. The

spheroids subsequently coalesce to form graphite. Graphite nodules were identified

in this study, and the only carbides found were mostly at the grain boundary (Fig

11.a). Decarburization due to short-term overheating is thought to be responsible for

the appearance of graphite on the finned area of the sample where the crack

occurred. The deformed grains and appearance of graphite observed in the

microstructure at the crack occurred and likely the usual grains at far of failure tube

noted the supportive of short-term overheating in the boiler tube.

graphite

Initial pearlite

Deformed pearlite

Deformed grain

250µm100x Magnification

PAGE28OF31

Figure 12. The place of SA 106 Gr.B in Fe-C Diagram

Creep failures are typically expected in superheaters and temperatures. However,

creep may occur under other conditions as well, provided that there are high firing

rates on the fire side (OD) along with the heavy scaling on the water side (ID), as in

the case, and a loss of strength due to decarburization. But in this case, if creep is

not presented means the awareness still in first concern due to degradation of

mechanical properties in some of tubes.

It was confirmed that part of the shutdown procedure for this boiler could result in

burner operation after draining water from the system. The authors believe that hot

gas from the burner impinging on the surface of the tube without cooling water cause

the short-term overheating.

SA 106 Gr.B

PAGE29OF31

5. THERMAL CRACKS

In August 13th,2010, after the failure happened, one crack tube was found in weld

joint tube-top Header and located at no.1 from East and no.4 from south and also

one leak was found in weld joint of bottom header and located at no.1 from East and

no.3 from South. Figure 13 below is shown the phenomenon,

Figure 13. Crack happened in weld joint of bottom header at no.1 from East and no.3 from South

Mechanism of above failure is thermal shock crack, which is originally a special type

of corrosion fatigue or stress-induced corrosion caused by alternating heavy heating

and cooling over a temperature span of some hundred degrees.

The mechanism of crack is explained as concerted action of environment and

mechanical stress (variations) leads to cracking. At thermal stresses, the material

makes the protecting magnetite layer break in narrow cracks in a more or less

parallel arranged pattern.

Figure 14. The mechanism of thermal crack

PAGE30OF31

H. CONCLUSION

The observed bulging and cracking of the tube was caused by SHORT-term

overheat followed by a brief episode of excessive overheating.

Internal blockage of tube, Loss of coolant circulation or low water level, Loss of

coolant due to an upstream tube failure and overfiring or uneven firing or boiler

fuel burners are the major cause of short-term overheating.

The heavy scale, was not developed in the inner tube, means it was pure of

short-term overheating

I. SUGGESTIONS

PAGE31OF31

I. BIBLIOGRAPHY

G.F. Vander Voort. (1986). Failure Analysis and Prevention,. Vol 11, Metals Handbook, 9th ed. American Society for Metals, Metals Park , 715–727.

Institute, A. P. (2003). API 573 : Inspection of Fired Boilers and Heaters .

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