Heavy Loaded Parts of Petrochemical Equipment Destruction Cause
Investigation
A B LAPTEV1, S A NAPRIENKO1, R ZH AKHIYAROV2, A V GOLUBEV3
1FSUE "All-Russian Research Institute of Aviation Materials",
17, Radio str., Moscow, 105005,
RUSSIA
2OOO "Tehekspertiza",
12, office 1, Dauta Yultya st., Ufa, 450081,
RUSSIA
3FSBEI of Higher Education "Ufa State Oil Technical University",
1, Kosmonavtov st., Ufa, 450064,
RUSSIA
Abstract: - The problem of using specialized passivating metals and alloys lies in a rather narrow range of the
protecting film performance. With a slight change in operating conditions, the film is destroyed and an
avalanche-like process of local corrosion begins at the place of film breakdown. A sequence has been
developed for determining parts destruction causes, including a sequential analysis of operating conditions;
nature of the part destruction; corrosion products composition; phase inversion in the alloy during overheating
or mechanical stress in the part.
Key-Words: Strength of materials, petrochemical equipment, metal, alloy during overheating, mechanical stress
Received: March 27, 2021. Revised: November 4, 2021. Accepted: December 12, 2021.Published: January 5, 2022.
1 Introduction
Intensification of oil production and processing,
toughening of oil and hydrocarbon production and
processing regimes lead to widespread use of
materials with higher strength and anticorrosive
properties. The process of creating new materials is
moving towards the production of high alloys based
on iron, nickel, titanium and aluminum. An increase
in strength characteristics and, at the same time, high
resistance to corrosion destruction leads to the
creation of passivating multiphase alloys [1]. In such
alloys, the forming passive surface film creates a
sufficient barrier for corrosion dissolution of the
metal. The hardening phases formed in the course of
smelting or heat treatment significantly increase the
strength of the alloy [2]. Particularly critical parts
and structures are made of this type of materials:
compressor pump impellers, measuring equipment
in the fields, turbine shafts and blades in
hydrocarbons transport, furnace coils in
petrochemical industries. Breakdowns of such
critical units and parts in the petrochemical complex
lead not only to significant economic damage, but
also to disasters with human casualties and severe
environmental consequences. [3].
Modern methods of calculating mechanism’s
parts function are performed without taking into the
account the influence of external environment and
the duration of its impact. An isotropic medium with
a penny-shaped crack is considered in [4]. Research
of the dependencies of the crack radius, opening and
pressure distributions on the crack surface on time,
fluid viscosity, and fracture toughness of the
medium is performed in [5]. Investigation of crack
formation and fracture processes with porosity
and various types of loads taken into consideration
[6], [7] also do not allow to evaluate the long-term
operation of mechanism parts under environmental
influence.
The problem of using specialized
passivating metals and alloys lies in a rather narrow
range of the protective film performance. With a
slight change in the flow rate, pH and composition
of the medium, the ingress of an abrasive, a
decrease in the density of hydrocarbons and
cavitation, the use of chemicals or other factors,
the film is destroyed and an avalanche-like
process of local corrosion begins at the place of
breakdown of the film, the formation of pits,
delamination, cracks and, as a result,
destruction of the part.
A particularly important factor in
parts performance ensuring is the compliance
of the technology and composition in the
manufacture of the alloy, the quality of
the parts and, correspondingly, the
incoming inspection of materials and the final
inspection of finished parts.
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DOI: 10.37394/232011.2022.17.1
A. B. Laptev, S. A. Naprienko,
R. ZH. Akhiyarov, A. V. Golubev
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2 Research Results
As an example of assessing the discrepancy between
alloy composition norms and the lack of quality
control of manufacturing, one can consider
destruction causes analysis of the heat exchanger
tube [8].
In this work, we investigated the material of the
tube of the shell-and-tube heat exchanger. Within 90
days, after the installation and the start of operation
of the heat exchanger, numerous through-corrosive
damage appeared on the newly mounted 20 steel
tubes.
The main reason for the low corrosion resistance
of the pipe material has been established - these are
the so-called HCNI (easily hydrolyzed inclusions of
alkali metal compounds. The calculated average
density of such inclusions is 13 pcs / mm2. This is
almost an order of magnitude higher than
the regulated value (2 pcs / mm2) [9], [10], [11],
[12] and led to the destruction of the heat
exchanger tubes.
Insufficient control of the finished after
casting air fan part after destruction was
investigated in the work to assess the cause of the
destruction of the impeller blade, which is one of
the components of the inlet guide vane for
controlling the compressor power of the
refrigeration system [13]. During the operation the
impeller blade was destroyed (Fig. 1).
Fig. 1: External view of impeller blade destruction.
The impeller operates at a temperature not higher
than 35 ° C and a rotation speed of up to 9500 rpm.
In the area where the blades are connected to the
shaft attachment sleeve, each blade experiences
tensile and bending loads. The impeller is made of
silumin.
The research of fractography, relief and
roughness was carried out in a similar way [8].
In Fig. 2 shows a photograph of impeller blade
destruction surface in the secondary electron mode
(the contrast is determined by the topography). It can
be seen that the crack surface has a dark color and a
slate macro-relief, reflecting alloy structure. The
morphology of service fracture surface from the
source zone to the zone of static fracture formed
during crack opening is the same and is represented
by a brittle transcrystalline cleavage oriented along
the crystallographic planes of the α-phase.
(A)
(B)
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Fig. 2: Fracture surface of the impeller blade
(A, Spectral mapping of the fracture surface of
the impeller blade (B.
The focal zone is located on the edge of the blade
and contains metallurgical defects in the form of a
void (Fig. 2. Voids are a typical type of rejects in
the place of pouring the melt by centrifugal casting.
As for the inclusions, it can be concluded from the
results of X-ray spectral microanalysis that these are
aluminum oxides and residues of aluminum
fluorides.
The presence of aluminum fluorides in the focal
zone of destruction is due to the fact that in the
production of aluminum alloys, a fluoride-chloride
electrolyte is often used as an electrolyte in the
electrolytic refining of aluminum [14].
As a result of the impeller blade airfoil surface
roughness study, traces of machining were found
near the crack. Studies have shown that the surface
roughness of the material far from the crack is Ra -
40 µm. At the same time, the roughness near the
crack is Ra - 129 µm, that is, 3 times more.
The destruction of the impeller blade during
operation occurred due to the initiation and
propagation of a crack under the influence of an
external load. The cause of crack initiation is the
large heterogeneity of the cast material. The crack
originated near the pore (the void formed during the
casting of the product and the accumulation of
aluminum oxides and fluorides. This heterogeneity
is due to the peculiarities of metallurgical production
[15], [16].
To exclude voids and foreign inclusions, each
casting must be subjected to non-destructive testing
(X-ray, ultrasonic or current-vortex methods.
When operating the installations for the
preparation and processing of oil, the compositions
of the media significantly deviate from the
regulatory ones. As an example of this kind of
violations, the premature failure of the pipes of the
furnace coils and the occurrence of an emergency
and shutdown of the furnace unit of the pyrolysis
unit of hydrocarbon raw materials in the ethylene
production are considered [17].
The nature of the metal destruction of pyrolysis
furnaces pipes made of passivated heat-resistant
steel 06Kh16N15M2G2TFR is intergranular
corrosion cracks, which can be caused by alkaline
corrosion cracking (СC) of austenitic steels. In
alkalis, CС is caused by the passivation of the main
surface of the metal and the destruction of passive
protective films along the grain boundaries on the
carbon-containing phase of the alloy, where
impurities are most present. Alkaline CС develops
most intensively at a level of tensile stresses close to
the yield point [18].
The sensitivity of steel to sodium hydroxide is
based on the amphoteric nature of iron oxides, i.e.
iron oxides dissolve at both low and high pH values.
Substances with a high pH value, in particular
sodium hydroxide at high temperatures, dissolves
magnetite:
O2H+FeONa+2NaFeOОFе+4NaOН222243
After dissolving the protective film of magnetite,
sodium hydroxide can react directly with iron atoms
[19].
222
2HFeONaNaOHFe
To establish destruction causes, studies were carried
out on a 06Kh16N15M2G2TFR steel sample cut
from the tube of the radiant section of the pyrolysis
furnace.
The chemical composition was determined on a
JEOL JSM 840 scanning electron microscope
equipped with an INCA Energy 350 energy-
dispersive (EDS) spectrometer. X-ray structural
analysis of the samples was carried out on a DRON-
3M X-ray diffractometer. The microstructure was
investigated on an Axiotech optical microscope
(Karl Zeiss). The microstructure parameters were
determined using the Kslite program.
X-ray diffractometer determined the composition
of deposits on the inner surface of a pipe sample,
which is based on carbides, metal oxides penetrating
to a depth of 100 microns (Fig. 3A), which indicates
a significant destruction of the surface due to
corrosion and the formation of coke deposits. A
decrease in the amount of austenitiс phase on the
inner surface as compared to the outer one is shown.
The pipe is heated on the outer surface, that is, it is
most exposed to gas corrosion, but the development
of local corrosion and cracks occurs on the inner
surface of the pipe, that is, the environment inside
the chimney affects the steel structure and intense
corrosion.
Prepared thin section of the sample of the end
surface of the research pipe with a step of 10 μm
from the inner surface of the pipe to the outer one.
The results of experiments to determine the sodium
content in the near-surface zone of the metal are
shown in the graph (Fig. 3B).
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The results of determining the sodium content in the
near-surface zone of the metal are shown in the
graph (Fig. 3B).
Fig. 3 clearly shows that the surface in contact
with the vapor-hydrocarbon medium containing
alkali (NaOH) has a high sodium concentration.
Considering that the surface has been repeatedly
washed with water, and sodium is well hydrated,
there is little of it on the surface itself, and in the
metal itself sodium is presented in the form of a
bound and chemically stable compound. Thus, the
heat-resistant steel collapsed due to the high sodium
concentration in the heated environment. Reducing
the amount of supplied alkali will significantly
increase the overhaul period of the furnace
operation.
The use of gas turbojet engines (GTE) in the
transport of hydrocarbons is becoming more and
more widespread. During their operation, the phase
composition of the alloys and the chemical
composition of the surface layers of the parts
change. Determining the terms of safe operation of
expensive high-loaded products, such as gas
turbines, is an important material science problem
[20].
Determination of the causes of damage to parts of
a high-pressure compressor (HPC) makes it possible
to ensure resource reliability, as well as to correct
the phase structure of newly created alloys [21], to
improve the technologies for the production of
critical GTE parts [22]. In the works on the study of
titanium alloys [23] the possibility of titanium
corrosion is not taken into account. Determination of
the destruction mechanism was carried out for
damage to the HPC disk of a gas turbine unit used
for pumping natural gas. The disc is made of VT8
titanium alloy. Service life of a part in the engine
before the start of destruction <30,000 h /.
The determination of the metal composition was
carried out on an Agilent 5100 ICP-OES, LECO CS-
600, LECO TS-600, RHEN 602 spectrometer;
fractography and microstructure were investigated
using a JSM-6490LV microscope. The study of the
phase structure was carried out on a Tecnai G2 F20
S-TWIN TMP.
Fig. 4 shows the locations of fatigue cracks
localization on the disk, which led to significant
costs for replacing it, determining the causes of
failure and, as a result, changing the manufacturing
technology.
The outer diameter of the disc is coated with
silver, which in some places is almost completely
erased and in these places on the surface of the
chamfer there are dark spots and cracks (Fig. 4).
Evaluation of the microstructure and composition
of the disc metal confirmed compliance with the
alloy requirements. No structural defects and
microinclusions were found in the sample.
The study of the fractographic features of the
alloys was carried out on samples cut along the
cracks (Fig. 4). It was found that corrosion
destruction and the formation of both cracks began
at the outer edge of the disk. The inner cavity of the
crack is covered with oxides. The morphology of the
fracture in the area where the corrosive environment
was exposed is represented by the facet microrelief,
which indicates the brittle nature of the fracture. A
static dolly testifies to the viscous nature of
destruction.
EDX found that the chemical composition on the
surface of the disk sample, as well as on the surface
of the cracks, contains silver, VT8 elements, as well
as oxygen and chlorine.
In order to determine the mechanisms of disk
destruction, a cyclic loading test was carried out: a
fatigue crack was grown on the sample, into which,
under a static load, a 3% sodium chloride solution
was supplied for 2 hours, then the sample was kept
in a desiccator to remove moisture and tested on a
pendulum impact machine.
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Fig. 4: Cracked disc rims
The fracture photograph shows the following zones
(Fig. 5 a): fatigue crack propagation in air; crack
development zone in the medium 3% sodium
chloride solution; static downhole in the air. In this
case, the first zone has characteristic fatigue grooves
(Fig. 5 b). Failure in the second zone is represented
by a brittle microrelief (Fig. 5c), similar to the
operational one. The third zone has a structure
similar to the first (Fig. 5 d).
Tests of VT8 alloy specimens for resistance to the
rate of crack growth in a corrosive environment
showed the alloy's tendency to embrittlement and
stress corrosion cracking in chloride-containing
media, with the formation of a microrelief (Figs. 5 c,
d), corresponding to a brittle one that is not typical
for fracture in air, but identical to operational
fracture (Fig. 4). Thus, the destruction occurred due
to the embrittlement of the metal in the process of
electrochemical corrosion in molten nickel
chlorides, intensified by galvanic corrosion in the
presence of a silver coating film.
3 Conclusion
In the process of creating equipment, the experience
of operating materials in conditions of aggressive
action of corrosive environments must be taken into
account and the reasons leading to the destruction of
materials must be thoroughly investigated. The
authors have developed a sequence for determining
the causes of the destruction of various parts:
1) Assessment of operating conditions;
2) Determination of the nature of the part
destruction by methods of X-ray diffraction, X-
ray phase analysis and electron emission
microscopy;
3) Determination of corrosion products
composition, diffusion of elements of the
corrosive medium into the depth of the metal
and alloying elements in the volume of the
metal;
4) The phase inversion in the alloy is
investigated to analyze possible technological
overheating or excessive mechanical loads of
the part.
5) Based on the analysis of the data obtained
and, if necessary, additional experiments on
modeling the processes during the operation of
the part, the reasons for the destruction of the
part are determined and recommendations are
given to prevent such destruction in the future.
Preliminary testing of materials with
simultaneous exposure to a variable load, a corrosive
environment and contact of materials in the mated
parts is necessary in order to ensure the equipment is
going to work in the long run and to prevent
breakdowns and accidents.
4 Appendix
Appendixes, if needed, appear before the
acknowledgment.
Acknowledgment:
«The work was performed using the equipment of
the center shared research facilities "Climate Tests"
of FSUE «VIAM».
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Contribution of Individual Authors to the
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The authors equally contributed in the
present research, at all stages from the
formulation of the
problem to the final findings and
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Conflicts of Interest
The authors have no conflicts of interest to declare .
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