Fa of Hp Steam Superheater at a Fertilizer Plant

7/25/2019 Fa of Hp Steam Superheater at a Fertilizer Plant

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FAILURE ANALYSIS OF HP STEAM SUPERHEATER

AT A FERTILIZER PLANT

A. El-Batahgy*, H. Abu-Zamil** and H. Assaad**

* Head of Manufacturing Technology Department, Central Metallurgical

R & D Institute, Cairo, Egypt

** Inspection Sector, Abu-Qir Fertilizers & Chemical Industries Company,Alexandria, Egypt

Abstract

After 21 years of operation, both header and tubes of superheater were faileddue to creep damage accelerated by long-term overheating. Creep damage was

accumulated as a result of metallurgical instability including decomposition of

pearlite into ferrite and spheroidized carbide particles that caused severe

embrittlement and initiation of voids, intergranular micro-cracks, and separation

of grains. Such metallurgical instability could be accelerated with overheating

that in turn reduce creep lifetime.

Long-term overheating was evident from existence of adhesive, thick scale

layer with little or no detectable change in the tube wall thickness. Long-term

overheating of header was occurred as a result of plugging of 10 tubes, locatedbeside each other in the same zone.

Ruptured header was replaced with new one while ruptured tube was plugged

and boiler was set into normal operation as a temporary solution. Then, it was

decided that non-failed superheater' tubes will be subjected to detailed

investigations including non-destructive and destructive tests to evaluate its

condition during the nearest normal shut down. Based on such investigations, it

will be decided if other non-failed tubes of the superheater are still safe to be

used under the working conditions or if it will be about to be replaced with newtubes.

In order to decrease the possibility of such failure in future, the operation

conditions should be restrictly controlled to avoid overeating and then, to

increase the lifetime of superheater tubes. Besides, periodic maintenance for

checking and evaluating of superheater condition using different non-

destructive inspection techniques then, carrying out necessary repair works is of

considerable importance. During such periodic maintenance, removal of scale

from boiler tubes is recommended in order to avoid overheating of tubes. In thisregard, chemical or mechanical cleaning of tubes can be applied.


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1. Background

The subject HP steam superheater has been set into service 21 years ago. In this

superheater, the heat of flue gases (collected from both boiler and reformer) is

utilized for superheating the saturated steam leaving the steam drum after beingproduced from the boiler. The high pressure steam produced serves primarily

for driving the turbines for power generation. The superheater includes a total of

162 U-bends seamless tubes (54 rows x 3 tubes) fixed in a horizontal positionwhere length of superheater tube bundle is 7160mm. Direction of the heating

flue gases is a perpendicular to the axial direction of tubes, with its flow from

steam outlet header side to steam inlet header side.

Superheater tubes were made from 15Mo3 steel (DIN Standard) with 38mm

nominal diameter and 4.0mm wall thickness. Both inlet and outlet headers weremade from 15Mo3 steel (DIN Standard) with 298.5mm nominal diameter and

27.5mm wall thickness. Inlet and outlet temperatures of superheater steam are

328C and 410C respectively while working pressure of superheater tubes is

120bar. Temperature of flue gases outside superheater tubes is approximately

615C.

During the last two years, 10 tubes have been plugged after being leaked. All of

plugged tubes were located just beside each other, within the first 4 rows of

tubes, and its rupture zone was confined to steam outlet header side.

About 9 months later after plugging these tubes, steam outlet header has failed

due to through thickness cracking of its zone at which 10 tubes were previously

plugged. Superheater was shut down and repair has been carried out. In this

regard, the header cracked zone, that included 4 rows of plugged tubes was

removed and replaced with new part having same diameter, thickness and steel

type of header. Since tubes of the new header part will not be used again, the

new header part was made without holes or nozzles for tubes connection. Then,

superheater has been reset into normal operation.

Recently, catastrophic failure of another superheater tube took place, just beside

tubes' supporter zone that is located about 4-5m away from outlet header and

away enough from circumferential tube weld. This fractured tube was located in

the fifth row, just beside outlet header' part that was replaced 9 months ago.

Sample from the failed tube including ruptured zone together with header part

that was previously cracked and removed 9 months ago were subjected to

failure analysis investigations.


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2. Failure of Steam Outlet' Header

2.1. Non Destructive Examination

The failed header zone was carefully examined before sectioning it fordestructive investigations. General and enlarged views of both ruptured and

non-ruptured samples of failed header are shown in Fig. 1. Visual investigation

showed through-thickness, longitudinal cracking with wider width at header'outer surface particularly beside seam weld of nozzles with header. This

indicates that cracking was started beside seam weld of tubes' nozzles on

header' outer surface then, propagated through base metal. No thinning or

bulging was observed at header' ruptured zone. The results of wall thickness

measurements showed no remarkable difference in wall thickness of both

ruptured (30.8mm) and non-rupture (31.9mm) zones. It should be reported thatno indications for corrosion or other excessive wastage were observed.

Generally, spite of high operation temperature, visual inspection of ruptured

header showed brittle fracture surface. Except header' ruptured zone, dye

penetrant test showed no indications for other surface cracks either around or

away from ruptured zone.

2.2. Destructive Examination

Specimens from ruptured and non-ruptured zones of header were cut out and

prepared for chemical analysis, metallographic investigations, and hardnessmeasurements. Results of chemical analysis of ruptured header together with

the specified range for DIN 15Mo3 steel type are shown in Table 1. It is clear

that chemical composition of the ruptured header lies within the specified range

of type 15Mo3 steel.

As-polished stereoscopic and etched optical photographs of a cross section

taken at front of cracked or ruptured zone of header are shown in Fig. 2. The

most important notice is the non-continuous cracks indicating multiple cracks

initiation sites. Optical microscopic investigation confirmed that cracks wereinitiated at header outer surface. Higher magnifications of a cross section taken

at front of cracked or ruptured zone of header indicated that cracks were

propagated through grain boundaries voids.

Optical micrographs with different magnifications of a cross section taken from

header' ruptured zone are shown in Fig. 3 while those of a cross section of non-

ruptured zone are shown in Fig. 4. It is clear that no difference in

microstructures of both ruptured and non-ruptured zones was obtained.

Microstructure of both zones exhibited decomposition of pearlite into ferrite andspheroidal carbides. Another important notice is the formation of isolated voids


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at outer surface, mid-wall thickness and inner surface of both ruptured and non-

ruptured zones. Formation of voids has been confirmed by hardness

measurements where lower hardness value (~113HV) was obtained at voids

compared with about 153HV for matrix.

Change or deterioration in microstructure has been confirmed using scanning

electron microscopy as shown in Fig. 5. Scanning electron microscopic

examination has confirmed decomposition of pearlite into ferrite andspheroidized carbide particles in addition to formation of voids particularly, on

grain boundaries of both ruptured and non-ruptured zones.

Survey of hardness measurements through header' wall thickness of both

ruptured and non-ruptured zones was carried out and the results are shown in

Table 2. The given values are the average of five readings. Results indicatedalmost no difference in hardness values of both inner and outer surfaces of

header. However, higher hardness value was obtained at ruptured zone (160HV)

compared with 137HV for non-ruptured zone.

It can be deduced that the microstructure obtained at both ruptured and non-

ruptured zones is completely different from the original microstructure of

15Mo3 steel that is expected to be ferrite-pearlite structure with lamellar

pearlite phase.

In order to help in identification of failure mechanism, header' rupture surfacewas examined using scanning electron microscope as shown in Fig. 6. The most

important notice is the existence of large amount of voids, separation of grains

and the brittle-fracture mode.

3. Failure of Tube

3.1. Non Destructive Examination

The superheater' failed tube zone was carefully examined before sectioning it

for destructive investigations. General views of ruptured tube before and afterremoving it out of superheater are shown in Fig. 7. Fracture zone of the

superheater tube was located just beside the supporter of the tubes (Fig. 7-a),

about 4-5m away from outlet header. Visual examination showed tube bulging

at fracture initiation zone and fish-mouth shape rupture with no indication for

corrosion or other excessive wastage. Thickness measurements indicated that

wall thickness was reduced from 4.1mm at non-bulged zone to about 3.3mm at

bulged zone.

Another important notice is the existence of adhesive scale layer, with someexfoliation, on both external and internal surfaces of fracture zone (Fig. 7-c, d).


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Except fractured or ruptured zone, dye penetrant examination revealed no

surface cracks around or away from ruptured zone.

Stereoscopic examination confirmed exfoliation and cracking of thick adhesive

scale layer formed on outer surface of fracture zone (Fig. 8). It also confirmedexfoliation and cracking of thick adhesive scale layer formed on inner surface of

fracture zone (Fig. 9). Cracking and exfoliation of this scale layer could be

attributed to tube expansion and contraction due to thermal stresses. Generally,fracture zone exhibited thick-wall rupture indicating brittle fracture appearance

as shown in Fig. 9-d.

In order to identify the nature of scale layer formed on tube internal and external

surfaces, X-ray diffraction (XRD) technique has been used. The result of this

analysis showed that the formed scale layer consists mainly of iron oxide ormagnetite (Fe304) as shown in Fig. 10.

3.2. Destructive Examination

Specimens from both ruptured and non-ruptured zones were cut out and

prepared for chemical analysis, metallographic investigations, and hardness

measurements. Results of chemical analysis of the ruptured tube together with

the specified range for DIN 15Mo3 steel' type, are shown in Table 1. It is clear

that chemical composition of the ruptured tube lies within the specified range of

DIN 15Mo3 steel.

Regarding metallographic examinations, optical micrographs with different

magnifications of a cross section taken from rupture zone are shown in Fig. 11.

Optical micrographs with different magnifications of a cross section taken from

non-ruptured zone are shown in Fig. 12.

It is obvious that no difference in microstructures of both ruptured and non-

ruptured zones was obtained. Microstructure of both zones exhibited complete

decomposition of pearlite into ferrite and spheroidal carbides. Anotherimportant notice is the formation of isolated voids at outer surface, mid-wall

thickness and inner surface of both ruptured and non-ruptured zones. Formationof voids has been confirmed by hardness measurements where lower hardness

value (~111HV) was obtained at voids compared with about 150HV for matrix.

Change or deterioration in microstructure has been confirmed using scanning

electron microscopy as shown in Fig. 13. Scanning electron microscopic

examination has confirmed complete decomposition of pearlite into ferrite and

spheroidized carbide particles in addition to formation of grain boundary' voidsand grains separation at both ruptured and non-ruptured zones.


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Survey of hardness measurements through tube' wall thickness of both ruptured

and non-ruptured zones was carried out and the results are shown in Table 3.

The given values are the average of five readings. Results indicated almost no

difference in hardness values of both inner and outer surfaces of tube. However,

higher hardness value (152HV) was obtained at ruptured zone compared with143HV for non-ruptured zone.

It is obvious that the microstructure obtained for both ruptured and non-rupturedzones is again completely different from the original microstructure of 15Mo3

steel, which is expected to be ferrite-pearlite structure with lamellar pearlite

morphology.

In order to help in identification of failure mechanism, superheater tube' rupture

surface was examined using scanning electron microscope as shown in Fig. 14.The most important notice is the existence of large amount of voids, grains

separation and the brittle fracture mode.

4. Discussion

Results of chemical analysis of failed header and tube revealed that its chemical

composition is conformed to that of DIN 15Mo3 steel. Visual investigation of

ruptured header showed through-thickness, longitudinal cracking with wider

width on header' outer surface. It is noticed that cracking was started beside

seam weld of tubes' nozzles on header' outer surface then, propagated throughbase metal. Visual investigation showed also brittle fracture appearance where

no thinning or bulging was observed at header' ruptured zone.

Regarding ruptured tube, visual examination showed bulging at fracture

initiation zone and fish-mouth rupture shape with no remarkable thinning,

indicating brittle fracture. Adhesive thick scale layer, with some exfoliation and

cracking was observed on both internal and external surfaces of tube' fracture

zone.

Optical and scanning electron microscopic examinations showed decompositionof pearlite and forming of grain boundary voids and micro-cracks in addition to

grains separation.

These findings support creep damage accelerated by overheating as failure

mechanism. Creep damage is the result of permanent plastic deformation at

elevated temperatures and at stresses much less than the high temperature yield

stress. Creep damage due to overheating is evident from decomposition of

pearlite and forming of grain boundary voids and micro-cracks in addition to

grains separation. Severe deformation at grain boundaries, together with

diffusion processes can lead to the nucleation and coalescence of voids, causingseparation of grain boundaries.


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In other words, such deterioration in microstructure could result in weakening

of header and tube materials that in turn will result in increase in superheater

pressure above the maximum safe working pressure, which could lead finally to

the current failure or rupture. This means that the properties of the materials of

both header and tube have changed from its original values with respect to thedesign values due to the effect of the prolong overheating. Therefore, the

original design values are no longer valid and the materials of both header and

tube failed in a premature manner.

Overheating failures are often classified as either short-term or long term. Short-

term overheating frequently exhibits a thin-lipped longitudinal rupture,

accompanied by noticeable tube bulging, absence of large amounts of thermally

formed magnetite, and violent rupture (sometimes bending the tube almost

double and causing secondary metal tearing) and these are not the features ofthe subject case. Long-term overheating usually occurs in superheaters as a

result of different factors including gradual accumulation of adhesive, thick

scale layer and excessive heat input. Internal and external thermal oxidation of

the tube metal is often observed at long-term failure region. In other words,

long-term failures by creep damage can occur with little or no detectable

changes in the tube wall thickness. Microstructural examination is an effective

means of confirming long-term overheating. The platelets of iron carbide in the

pearlite structure of carbon steels will thermally decompose to spheroidized iron

carbide (1-3).

It can be estimated that both header and tube' rupture zones were exposed to

long-term overheating (4-6). It is believed that overheating of header was

occurred as a result of plugging of 10 tubes which were located just beside each

other in the same zone that included the first 4 rows of tubes. Plugging of these

tubes during the last 2 years could lead to long-term overheating of outlet

header particularly, with higher temperature of flue gases on outlet header side

in comparison with inlet header side.

On the other hand, overheating of tube is related mainly to restricted flue gasesaround tubes' supporter and forming of thick adhesive scale layer (magnetite;

Fe3O4) on tube surface that in turn will lead to decrease in heat transfer rate. It isexpected that tendency for forming such scale layer is higher at tube outlet side

due to higher temperature than that of tube inlet side. Since magnetite has a

lower thermal conductivity than the steel tube, the net effect is an increase in

tube metal temperature that in turn accelerates creep damage (1).


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Conclusion and Recommendations

Based on the results of this investigation, it can be concluded that failure of the

subject superheater' header and tube is attributed mainly to creep damage

accelerated by long-term overheating.

It is believed that creep damage was occurred as a result of metallurgical

instability including decomposition of pearlite into ferrite and spheroidizedcarbide particles that caused severe embrittlement and initiation of voids,

intergranular micro-cracks, and separation of grains (typical characteristic of

creep failure). Such metallurgical instability could be accelerated with

increasing temperature (overheating) that in turn reduce creep lifetime.

In this failure case, long-term overheating of header was occurred as a result ofplugging of 10 tubes which were located just beside each other in the same zone

that included the first 4 rows of tubes. Plugging of these tubes during the last 2

years could lead to long-term overheating of outlet header particularly, with

higher temperature of flue gases on outlet header side in comparison with inlet

header side.

On the other hand, long-term overheating of tube could be related to restricted

flue gases around tubes' supporter and forming of adhesive magnetite scale

layer on tube internal surface that acts as a barrier to heat transfer. It is expected

that tendency for forming such scale layer is higher at tube outlet side due tohigher temperature than that of tube inlet side. Since adhesive magnetite scale

layer (up to 0.4mm) has a lower thermal conductivity than the steel tube, the net

effect is an increase in tube metal temperature that in turn accelerates creep

damage as a result of change or deterioration of microstructure.

As a temporary solution, ruptured tube was plugged and superheater was reset

into service. At the same time, decision was made to subject non-failed

superheater' tubes to detailed investigations including different non-destructive

and destructive tests to evaluate its condition during the nearest normal shutdown. Based on such investigations, it will be decided if superheater tubes are

still safe to be used under the working conditions or if it will be about to be

replaced with new tubes. In case of replacing tubes, header will also be replaced

with new one.

In order to decrease the possibility of such failure in future, the operation

conditions should be restrictly controlled to avoid overeating and then, to

increase the lifetime of superheater. Besides, periodic maintenance for checking

and evaluating of superheater condition using different non-destructiveinspection techniques then, carrying out necessary repair works is of


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considerable importance. During such periodic maintenance, removal of scale

from boiler tubes is recommended in order to avoid overheating of tubes. In this

regard, chemical or mechanical cleaning of tubes can be applied.

References

1. R. D. Port and H. M. Herro: The NALCO Guide to Boiler Failure Analysis,McGraw-hil, Inc., USA, 1991.

2. A. El-Batahgy: "Catastrophic Failure of Waste Heat Boiler", Materials

Performance, February 1997.

3. V.J. Colangelo, F.A. Heiser: Analysis of Metallurgical Failures, 2nd. ed.,

New York, NY: Wiley, 1987.

4. R. D. Barer, B. F. Peters: Why Metals Fail, 6

th

ed., Gordon and Breach,Science Publishers, New York, 1991.

5. A. El-Batahgy and W. Metwaly: Failure Analysis of Boiler Water Wall-

Tubes at a Power Generation Plant, The 31stAnnual Convention of the

International Metallographic Society, 26 29 July 1998, Ottawa, Canada

6. Metals Handbook, Ninth Edition, Vol.11, Failure Analysis and Prevention,

ASM 1996.


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Table 1 Chemical analysis of ruptured header and tube' materials, together

with the specified range for DIN 15Mo3 steel (wt%).

ElementMaterial

C Si Mn P S Mo Fe

ruptured header 0.200 0.223 0.612 0.001 0.017 0.316 Bal.

ruptured tube 0.200 0.242 0.693 0.010 0.019 0.280 Bal.

DIN 15Mo3 steel

0.12~ 0.10~ 0.40~ 0.035 0.035 0.25~ Bal.

0.20 0.35 0.80 max. max. 0.35

Table 2. Results of hardness measurements of ruptured header.

HV (l0kgf)

Zone

Outer surface Mid-wall

thickness

Inner surface

Ruptured zone 155 163 162

Non-ruptured

zone

137 135 139

Table 3. Results of hardness measurements of superheater ruptured tube.

HV (l0kgf)

Zone

Outer surface Mid-wall

thickness

Inner surface

Ruptured zone 147 151 160

Non-ruptured

zone

143 140 146


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Fig. 1. General and enlarged views of both ruptured and non-ruptured

samples of failed header. Note through-thickness cracking of

header' rupture zone


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Fig. 2. As-polished stereoscopic (a) and etched optical photographs (b, c, d)

of a cross section taken at front of header' ruptured zone showingmultiple cracks initiated sites close to outer surface and propagated

through grain boundaries voids.


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Fig. 3 Optical micrographs with different magnifications of a cross

section taken from header' ruptured zone showing partial

decomposition of pearlite into ferrite and spheroidal carbides

with voids.


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Fig. 4 Optical micrographs with different magnifications of a cross

section taken from header' non-ruptured zone showing partial

decomposition of pearlite into ferrite and spheroidal carbides

with voids


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Fig. 5 Scanning electron micrographs of a cross section taken from

header' ruptured zone showing decomposition of pearlite into

ferrite and spheroidal carbide particles in addition to voids


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Fig. 6 Scanning electron micrographs of header' fracture surface

showing large amount of voids, grain boundary cracking

and brittle fracture mode.


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Fig. 7 General view of ruptured tube before (a) and after (b, c, d) removing it

out of superheater showing bulging with fish-mouth shape rupture and

scale layer on both inner and outer surfaces of fracture zone.


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Fig. 8. Stereoscopic photographs of outer surface of fracture zone

showing exfoliation and cracking of thick scale layer

formed on outer surface.


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Fig. 9. Stereoscopic photographs of (a, b, c) inner surface of fracturezone showing exfoliation and cracking of thick scale layer

formed on inner surface, and (d) fracture surface showing

thick-wall rupture or brittle fracture appearance.


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Fig. 10 Results of X-ray diffraction (XRD) analysis of scale layer

formed on superheater' tube surface. Note that magnetite

(Fe304) is the main component of this scale layer.


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Fig. 11 Optical micrographs of a cross section taken from rupture zone of

superheater tube showing complete decomposition of pearlite into

ferrite and spheroidal carbides with voids through tube thickness.


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Fig. 12 Optical micrographs of a cross section taken from non-rupture zone

of superheater tube showing complete decomposition of pearlite into

ferrite and spheroidal carbides with voids through tube thickness.


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Fig. 13 Scanning electron micrographs of a cross section taken from tube

rupture zone showing complete decomposition of pearlite into

ferrite and spheroidal carbide particles in addition to voids

and grains separation.


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Fig. 14 Scanning electron micrographs of tube rupture surface showing

large amount of voids, grains separation and the brittle fracture mode.

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Fa of Hp Steam Superheater at a Fertilizer Plant

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