FIELD OF THE INVENTION
The present invention relates to a novel method of radiant coils or more
particularly to recognize the level of damages in legs of the radiant coils through
Scanning Electron Microscopy and optical microscopy of the pyrolysis furnace
in the petrochemical plant.
BACKGROUND OF THE INVENTION
Ethylene (C2H4) is generated by cracking ethane (C2H6) in pyrolysis furnaces. The
process consists of passing a mixture of steam and ethane through the radiant
tube which are heated externally to 850-1100°C by burners and gas passes
through the coil at a high velocity with short residence time. Each furnace has
24 W-type radiant coils arranged in vertical plane located centrally along the
length of the radiant box. Single W-type coil has four legs and inside diameter of
four legs are about ~64 mm, 70 mm 76 mm and 83 mm. The separate legs are
joined by suitable swaging and welding process to form a single coil with nominal
total length of 44 m. First and second leg were made from ASTM A297 centrifugal
cast Fe-25Cr-35Ni austenitic steel modified with Nb. Third and fourth leg were
made from 35%Cr-45%Ni heat resistant steel. The ethane decomposes to
ethylene is represented by the formula (C2H6-> C2H4 + H2) and subsequently,
ethylene produces free carbon according to the reaction (C2H4 -> CH4 + C). This
free carbon (coke) adheres on the inner surface and acts a thermal insulator
which requires high tube wall temperature in order to maintain the same gas
temperature and also coke accelerates the carburization of tube. At intervals of
weeks or months, adherent coke supposed to be removed by shutting off
hydrocarbon feed and passing air-steam or steam through the coil. Such a
process is known as de-coking. During decoking, tubes are subjected to severe
thermal shock where temperature rises above normal and causes sagging. In
practice, need for decoking is based on gas temperature, flow rate and conversion
ratio. Frequent de-coking produces thermal shock and less frequent decoking
increases the rate of carbonization attack. Carbon deposited on the inner wall
will be absorbed by inner surface and inters into austenitic matrix where it

precipitates as M7C3 and M23C6 carbides, decreasing Cr concentration in the
matrix and reducing material strength and creep resistance. Primary Cr carbides
transform into intra and intergranular M23C6 precipitates. The coils from the
external surface undergo oxidizing and nitriding by contact with flue gases.
PRIOR ART SEARCH
US patent (US 8776340 B2) discloses a method for replacing tubes in a
hydrocarbon vapour reforming unit that comprises the activities of measuring
the expansion of the tube diameters, taking X-ray photographs of the tubes,
realizing replicas of the surfaces of the tubes. The tube is replaced when it fulfils
at least one of the following conditions; the diameter expansion is higher than
3%; the X-ray photograph includes at least one crack; the replica shows a
sufficient thermal ageing and/or creep deformation.
European patent (EP 2906501 A2) discloses the methods and apparatus of
measuring real time temperature conditions within a reformer and then used for
process control optimisation, overheat protection, and improved creep damage
and fatigue life prediction.
US patent (US 20050237519 A1) describes a method and apparatus for
accurately determining the interior profile of cylindrical objects such as reformer
catalyst tubes based on calculating the angle of a reflected light beam and the
deviation thereof. US 20060050092 A1 discloses a method of generating one of
a colorized 2D or 3D graphical display depicting inner circumferential diameter
of reformer tube based on LOTIS (Laser Optic Tube Inspection System) Laser
Profilometry (LP) technology to generate the continuous radius/diameter data
necessary to provide the 3D visualization method.
But a very high temperature is need to be maintained in pyrolysis furnace.
Tube metal temperature (TMT-peak circumferential) of leg 1 reaches 862-922°C,
leg 2 in the range of 922-958, leg 3 in the range of 958-988°C and leg 4 in the
range 988-1008°C for the start of run period. For the end of run operation, tube
metal temperature (TMT-peak circumferential) of leg 1 reaches 881-947°C, leg 2

in the range of 947-1000, leg 3 in the range of 1000-1066°C and leg 4 in the
range 1066-1100°C. This very high temperatures are considered to be close to
the yield strength of the metal alloys of the tubes and demands a heat resistant
high alloyed steel. However, even these tubes made of special super alloys get
damaged over time and any operation at a temperature 20°C above the design
temperature tends to reduce 50% of service life of the tubes. Moreover, any
failure of tube in the middle of processing leads to shutdown of the furnace.
Hence, there is a long felt need to overcome such drawbacks of art. The present
invention meets the long felt need by providing a method to examine the health
status of critical radiant coil based on metallography replication technique and
its further analysis in the scanning electron microscopy.
OBJECTS OF THE INVENTION
It is therefore the primary object of the present invention to provide a method of
assessment of the health condition of radiant coils used in the pyrolysis furnace
in petrochemical plants.
Another object of the present invention to proving method of assessing radiant
coils, which can correctly locates the damages in radiant coils at different heights
based on the microstructural features observed in optical microscopy and
scanning electron microscopy (SEM) analysis.
Further object of the present invention to proving method of assessing radiant
coils, which can determine different microstructural damage features and
classifies the microstructural changes in four different stages in legs 2, 3 and 4
separately based on analysis in scanning electron microscopy and optical
microscopy.
Yet another object of the present invention to proving method of assessing
radiant coils, which is simple yet rapid.

SUMMARY OF THE INVENTION
One or more drawbacks of conventional systems and process for a method for
assessing the radiant coils are overcome, and additional advantages are provided
through the method as claimed in the present disclosure. Additional features
and advantages are realized through the technicalities of the present disclosure.
Other embodiments and aspects of the disclosure are described in details herein
and are considered to be part of the claimed disclosure.
A method for assessing the radiant coils of pyrolysis furnace comprises the steps
of :
i) selection of the outer surface of the tube at different altitude depending
on the location of the tube damage or deformation observed;
ii) preparation of successive silicon carbide abrasive papers of different
grits by using of high speed grinder;
iii) further polishing of the surface by using portable polishing machine
with the diamond paste of different dimension and a suitable lubricant;
iv) the finished surface is thereafter subjected to electrolytic etching with
oxalic acid for revealing the microstructure and a replication was
carried out by using the cellulose acetate tapes and high purity
analytical reagent (AR) grade acetone;
v) bonding of the replicas to glass slide and gold is sputtered for improving
the reflectivity , resolution and attaining the mandatory electron beam
conductivity for SEM analysis;
Various objects, features, aspects, and advantages of the inventive subject
matter will become more apparent from the following detailed description of
preferred embodiments, alongwith the accompanying drawing figures.
It is to be understood that the aspects and embodiments of the disclosure
described above may be used in any combination with each other. Several of the
aspects and embodiments may be combined to form a further embodiment of the
disclosure.

The foregoing summary is illustrative only and is not intended to be in any way
limiting. In addition to the illustrative aspects, embodiments, and features
described above, further aspects, embodiments, and features will become
apparent by reference to the drawings and the following detailed description.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The illustrated embodiments of the subject matter will be best understood by
reference to the drawings, wherein like parts are designated by like numerals
throughout. The following description is intended only by way of example, and
simply illustrates certain selected embodiments of devices, systems, and
processes that are consistent with the subject matter as claimed herein, wherein:
Figure 1 illustrates Optical microstructure of new coil material of 25Cr-35Ni
steel.
Figure 2 illustrates SEM microstructure of new coil material of 25Cr-35Ni steel.
Figure 3 illustrates SEM microstructures of 25Cr-35Ni steel in leg 2 coil showing
four stages in damage level.
Figure 4 illustrates Optical microstructure of new coil material of 35Cr-45Ni
steel.
Figure 5 illustrates SEM microstructure of new coil material of 35Cr-45Ni steel.
Figure 6 illustrates SEM microstructure showing four stages in damage level of
35Cr-45Ni steel in leg 3 coil.
Figure 7 illustrates SEM microstructure showing four stages in damage level of
35Cr-45Ni steel in leg 4 coil.
The figures depict embodiments of the disclosure for purposes of illustration
only. One skilled in the art will readily recognize from the following description
that alternative embodiments of the methods illustrated herein may be employed
without departing from the principles of the disclosure described herein.

DETAILED DESCRIPTION OF THE PRESENT INVENTION
While the embodiments of the disclosure are subject to various modifications
and alternative forms, specific embodiment thereof have been shown by way the
figures and will be described below. It should be understood, however, that it is
not intended to limit the disclosure to the particular forms disclosed, but on the
contrary, the disclosure is to cover all modifications, equivalents and alternative
falling within the scope of the disclosure.
It is to be noted that a person skilled in the art would be motivated from the
present disclosure to arrive at a method for assessing the damage of the radiant
coils in the pyrolysis furnace. Such a method for evaluating the same may vary
based on configuration of one or more workpieces. However, such modifications
should be construed within the scope of the disclosure. Accordingly, the
drawings illustrate only those specific details that are pertinent to understand
the embodiments of the present disclosure, so as not to obscure the disclosure
with details that will be clear to those of ordinary skill in the art having benefit
of the description herein.
As used in the description herein and throughout the claims that follow, the
meaning of “a”, “an”, and “the” includes plural reference unless the context
clearly dictates otherwise. Also, as used in the description herein, the meaning
of “in” includes “in” and “on” unless the context clearly dictates otherwise.
The terms “comprises”, “comprising”, or any other variations thereof used in the
disclosure, are intended to cover a non-exclusive inclusion, such that a method,
system, assembly, coils, metallography that comprises a list of components does
not include only those components but may include other components not
expressly listed or inherent to such method, or assembly, or method. In other
words, one or more elements in a system or device proceeded by “comprises…..a”
does not, without more constraints, preclude the existence of other elements or
additional elements in the system, apparatus or device.

The present disclosure relates to a novel method for assessing the radiant coils
used in pyrolysis furnace in petrochemical plant through microstructural
features observed in in-situ metallographic method.
The radiant coils are made up of the heat resistant steels.
The assessment is particularly based on the optical images, SEM and
microscopic images of the different damages occurred in the four legs of radiant
coils. The images are obtained in the replication process through the in-situ
metallographic examination.
The method for assessing the health condition of radiant coils comprises the
steps of :
i) selection of the outer surface of the tube at different altitude depending
on the location of the tube damage or deformation observed;
ii) preparation of successive silicon carbide abrasive papers of different
grits such as 80, 120, 220 and 600 of flapper wheels by using of high
speed grinder;
iii) further polishing of the surface by using portable polishing machine
with the diamond paste of different dimension such as 9, 6, 3, 1 and
0.5 microns and a suitable lubricant;
iv) the finished surface is thereafter subjected to electrolytic etching with
10% oxalic acid for revealing the microstructure and a replication was
carried out by using the cellulose acetate tapes and high purity
analytical reagent (AR) grade acetone;
v) bonding of the replicas to glass slide and gold is sputtered for improving
the reflectivity , resolution and attaining the mandatory electron beam
conductivity for Scanning Electron Microscopy (SEM) analysis.

For conducting the analysis, metallographic polishing starts with initial rough
grinding operation at the selected location. This grinding is done by hand held
high speed grinding equipment with different grades (80, 120, 220 and 600 grit)
of lapper wheel. Grinding actually started with 80 grit and then 120, 220 and
finished with 600 grit. Then polishing is continued with diamond paste.
Diamond paste is an abrasive used during polishing. Diamond paste polishing
started with 9-microns size paste and then 6, 3, 1 and finished with 0.5-micron
paste. Final mirror like surface is prepared in the metal surface after this
polishing activity.
These are different grades of diamond paste (9, 6, 3, 1 and 0.5 micron) and
represents size of the diamond particles. Big size particles (i.e. 9 microns) are
used at initial stage, then subsequently gradual smaller sizes (6, 3 and 1) and
finished with smallest size (i.e. 0.5 micron) for better polishing activity.
The lubricant used is aerosol spray, which is used during the diamond paste
polishing process as it creates the required lubrication and without such spray
the polishing can not be carried out.
The high pure analytical reagent grade acetone comprises very low levels of other
impurities in the chemical solution and has substantially better replication
property compared to other grades.
Fig 1 & 2 both illustrate the optical and SEM images of the microstructure of 25
Cr–35Ni steel which has an austenite matrix with intergranular electric like
primary chromium rich carbides (M7C3 and/or M23C6 type) and niobium carbides
(MC type). The temperature is maintained at 850-1050°C during the operation
when the primary chromium carbides transformed to M23C6.
Due to precipitation of intragranular secondary M23C6 carbides with the
austenite steel, it become more strong and creep resistant.
In accordance to the another embodiment of the present invention, each furnace
has 24 W-type radiant coils arranged in vertical plane located centrally along the
length of the radiant box. Single W-type coil has four legs (each single leg has

height of ~11 meters), inside diameter of four legs are about ~64 mm, 70 mm 76
mm and 83 mm and has the nominal total length of 44 meters. First and second
leg were made from ASTM A297 centrifugal cast Fe-25Cr-35Ni austenitic steel
modified with Nb. Third and fourth leg were made from 35%Cr-45%Ni heat
resistant steel. During the cracking process, minimum TMT temperature will be
at the inlet first leg (about 850 deg C) and reaches maximum at the outlet fourth
leg (about 1100 deg C).
The present invention also discloses various microstructural damage features or
precipitation such as very fine to medium sized in different level in austenite
matrix and also in grain boundaries.
Different shaped precipitates such as round, irregular shaped and needle shaped
precipitates was visible in the next damaged level (b) with significant increase in
the precipitation quantity. The carbide precipitates gets coarsened in dendritic
boundary and isolated creep pores was also observed in the image (c). Plenty of
creep pores was seen in the last damage level (d). Figure 4 and 5 illustrates
optical and SEM images of the typical microstructure of as-cast 35Cr-45Ni steel
showing an austenite matrix with eutectic-like primary chromium-rich carbides
(M7C3 and/or M23C6 types) and niobium carbides (MC type) in dendritic
boundary. Figure 6 illustrates SEM images of four different damage levels in
35Cr-45Ni steel in leg 3 coil.
The isolated precipitation was observed in the first stage of the damage level in
the first image and shows very fine to medium sized precipitate in austenite
matrix and in grain boundary.
Like Leg 3 coil, different damages levels are also seen in 35Cr-45Ni steel in leg 4
(as shown in Fig 7).
The carbide precipitates gets coarsened in dendritic boundary, isolated creep
pores and precipitates within the precipitates was also observed in the image (c).
Plenty of creep pores was seen in the fourth stage of damage level (d).

Different types of damages have been selected in different coil legs such as leg 2,
3 and 4.
The leg 2 of the coil shows stage I: fine and medium size carbide precipitation
in the matrix, stage II: considerable increase in the precipitation quantity, stage
III: isolated creep cavities and coarsening of dendritic carbides and Stage IV:
numerous creep cavities.
The leg 3 and 4 of the coil shows stage I: fine and medium size carbide
precipitation in the matrix, stage II: considerable increase in the precipitation
quantity, stage III: fine precipitation within coarse precipitates and isolated creep
cavities and Stage IV: numerous creep cavities.
The stage II and II classification are considered as good condition and may be
recommended for continuing the operation, whereas the stage III and IV are
recommended suitably for replacing the coil.
Each of the appended claims defines a separate invention, which for
infringement purposes is recognized as including equivalents to the various
elements or limitations specified in the claims. Depending on the context, all
references below to the “invention” may in some cases refer to certain specific
embodiments only. In other cases, it will be recognized that references to the
“invention” will refer to subject matter recited in one or more, but not necessarily
all, of the claims.
Groupings of alternative elements or embodiments of the invention disclosed
herein are not to be construed as limitations. Each group member can be referred
to and claimed individually or in any combination with other members of the
group or other elements found herein. One or more members of a group can be
included in, or deleted from, a group for reasons of convenience and/or
patentability. When any such inclusion or deletion occurs, the specification is
herein deemed to contain the group as modified thus fulfilling the written
description of all groups used in the appended claims.

The present disclosure provides a method for assessing the damages of radiant
coils in the pyrolysis furnace.
Equivalents:
With respect to the use of substantially any plural and/or singular terms herein,
those having skill in the art can translate from the plural to the singular and/or
from the singular to the plural as is appropriate to the context and/or
application. The various singular/plural permutations may be expressly set forth
herein for sake of clarity.
It will be understood by those within the art that, in general, terms used herein,
and especially in the appended claims (e.g., bodies of the appended claims) are
generally intended as “open” terms (e.g., the term “including” should be
interpreted as “including but not limited to”, the term “having” should be
interpreted as “having at least”, the term “includes” should be interpreted as
“includes but is not limited to”, etc.). It will be further understood by those within
the art that if a specific number of an introduced claim recitation is intended,
such an intent will be explicitly recited in the claim, and in the absence of such
recitation no such intent is present. For example, as an aid to understanding,
the following appended claims may contain usage of the introductory phrases
“at least one” and “one or more” to introduce claim recitations. However, the use
of such phrases should not be construed to imply that the introduction of a claim
recitation by the indefinite articles “a” or “an” limits any particular claim
containing such introduced claim recitation to inventions containing only one
such recitation, even when the same claim includes the introductory phrases
“one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a”
and/or “an” should typically be interpreted to mean “at least one” or “one or
more”); the same holds true for the use of definite articles used to introduce claim
recitations. In addition, eve it a specific number of an introduced claim recitation
is explicitly recited, those skilled in the art will recognize that such recitation
should typically be interpreted to mean at least the recited number (e.g., the bare

recitation of “two recitations”, without other modifiers, typically means at least
two recitations, or two or more recitations).
The above description does not provide specific details of manufacture or design
of the various components. Those of skill in the art are familiar with such details,
and unless departures from those techniques are set out, techniques, known,
related art or later developed designs and materials should be employed. Those
in the art are capable of choosing suitable manufacturing and design details.
The terminology used herein is for the purpose of describing particular
embodiments only and is not intended to be limiting of the present disclosure. It
will be appreciated that several of the above-disclosed and other features and
functions, or alternatives thereof, may be combined into other systems or
applications. Various presently unforeseen or unanticipated alternatives,
modifications, variations, or improvements therein may subsequently be made
by those skilled in the art without departing from the scope of the present
disclosure as encompassed by the following claims.
The claims, as originally presented and as they may be amended, encompass
variations, alternatives, modifications, improvements, equivalents, and
substantial equivalents of the embodiments and teachings disclosed herein,
including those that are presently unforeseen or unappreciated, and that, for
example, may arise from applicants/patentees and others.
While various aspects and embodiments have been disclosed herein, other
aspects and embodiments will be apparent to those skilled in the art. The various
aspects and embodiments disclosed herein are for purposes of illustration and
are not intended to be limiting, with the true scope and spirit being indicated by
the following claims.

WE CLAIM :
1. A method for assessing the radiant coils of pyrolysis furnace comprises the
steps of :
i) selection of the outer surface of the tube at different altitude depending
on the location of the tube damage or deformation observed;
ii) preparation of successive silicon carbide abrasive papers of different
grits by using of high speed grinder;
iii) further polishing of the surface by using portable polishing machine
with the diamond paste of different dimension and a suitable lubricant;
iv) the finished surface is thereafter subjected to electrolytic etching with
oxalic acid for revealing the microstructure and a replication was
carried out by using the cellulose acetate tapes and high purity
illustrates (AR) grade acetone;
v) bonding of the replicas to glass slide and gold is sputtered for improving
the reflectivity , resolution and attaining the mandatory electron beam
conductivity for Scanning Electron Microscopy (SEM) analysis.
2. The method for assessing the radiant coils as claimed in claim 1, wherein the
abrasive papers are of different grits or grinders such as 80, 120, 220 and 600
of flapper wheel.
3. The method for assessing the radiant coils as claimed in claim 1, wherein the
silicon carbide abrasive paper is prepared by using high speed grinder (kindly let
us have the speed), wherein the grinding started with 80 grit and speed is
increased gradually upto 600 grit.
4. The method for assessing the radiant coils as claimed in claim 1, wherein the
diamond paste is used of different dimension such as 9, 6, 3, 1 and 0.5 microns.
5. The method for assessing the radiant coils as claimed in claim 1, wherein the
big size diamond particles are used at initial stage and the size is subsequently
getting smaller and finished with the smallest size for better polishing activity.

6. The method for assessing the radiant coils as claimed in claim 1, wherein the
lubricant is selected from aerosol spray.
7. The method for assessing the radiant coils as claimed in claim 1, wherein
etching is facilitated with 10% oxalic acid for revealing the microstructure.
8. The method for assessing the radiant coils as claimed in claim 1, wherein the
high pure analytical reagent grade acetone comprises very low levels of other
impurities in the chemical solution and has substantially better replication
property compared to other grades.