Failure analysis of low-pressure steam superheater in a purification device

For a coal chemical enterprise purification unit in the low-pressure steam superheater in the use of frequent occurrence of heat exchanger tube bundle cracking, to find the cause of failure and ensure the safe and stable continuous operation of the device, the failure analysis work shall be carried out. In this paper, we have examined the cracked heat exchanger tube bundle in terms of macroscopic inspection, material chemical composition analysis, mechanical properties analysis scanning electron microscopy analysis, corrosion product analysis, metallurgical analysis, and many other tests, and also analyzed the internal and external media of the tube bundle, ultimately concluding the main reason for cracking of heat exchanger tube bundle for the alkali stress corrosion cracking. At the same time, to avoid similar problems, we hope that the failure analysis of the use and maintenance, inspection, and testing of the equipment provides a good solution to the failure of the equipment, and provides the best solution to the problem. It is hoped that this failure analysis will provide valuable experience for the use and maintenance of this type of equipment.


Introduction
The heat exchanger is one of the main pieces of equipment in a petrochemical plant, whose main function is to realize the heat transfer of materials between two or more fluids at different temperatures and improve energy utilization efficiency.The heat exchanger is mainly used to realize heat transfer through a heat exchanger tube bundle, which is usually thin in wall thickness and convenient for rapid heat transfer.Due to the large temperature difference between internal and external media, media, and scouring, the failure of heat exchanger tubes often occurs [1][2][3][4][5].The main failures are thinning leakage and cracking tube explosion.
Two low-pressure steam superheaters of water gas purification devices in a coal chemical enterprise frequently failed due to the leakage of heat exchanger tube bundles.The main task of the water gas purification device is to treat the crude water gas from the upstream gasification device, remove most of the CO 2 and all H 2 S from the shift gas, and adjust the hydrogen-carbon ratio to meet the feed requirements of the subsequent methanol synthesis device.The two heat exchangers are two heat exchangers used in the same position in two production series, and the design parameters and operating conditions of the equipment are the same.The structure of the failed heat exchanger is a U-tube heat exchanger, which is placed horizontally.The leaking heat exchange tubes are mainly distributed in the straight tube section of the U-bend, which is located near the low-pressure steam inlet of the shell side.The operating parameters of the heat exchanger are shown in Table 1.

MATMA-2023
Table1.Basic parameters of low-pressure steam superheater.In the process of use and inspection over the years, no obvious thinning and cracking of the heat exchanger body was found.However, the tube bundle frequently leaks and fails, and the user has upgraded the material of the heat exchanger tube bundle from 321 and 316L to S22053 several times in recent years, but the leakage has not been effectively alleviated.The history of heat exchanger maintenance, replacement, and brand upgrade is shown in Table 2.We analyzed the failure of the heat exchange tube to avoid the recurrence of similar situations and ensure the long-term operation of the device.
Table 2. Statistics on the operation of low-pressure steam superheater.

Materials
A cracked heat exchanger tube was selected from a low-pressure steam superheater (1146E106), as shown in Figure 1 below.The heat exchanger tube is made of S22053, with a design temperature of 290°C, a design pressure of 7.15 MPa, an operating temperature of 232 to 265°C, and an operating pressure of 6.14 MPa.The heat exchanger tube bundle has an outside diameter of 19 mm and a wall thickness of 2 mm.The medium of the tube process is transforming gas.As can be seen from Figure 1, the heat exchanger tube cracks along the axial expansion, with a total length of more than 14 cm.Cracks are relatively straight, with no obvious bifurcation and the outside of the visible yellow-brown rust spots.

Experimental methods and instruments
We take the cracked heat exchange tube from the low-pressure steam superheater (1146E106) as a "failed tube" and name it A. At the same time, we take a section of a heat exchange tube without cracks from the same heat exchanger as a "comparison tube" and name it B. We will carry out chemical composition analysis, metal tensile test, scanning electron microscope test, corrosion product analysis, and metallographic test on the two tubes respectively, and analyze the internal and external medium components of the tube bundle, to find out the reasons for the cracking of the heat exchange tube bundle.
Experimental instruments: SPECTROMAXx photoelectric direct reading spectrometer manufactured by Spike Analytical Instruments Company of Germany was used for chemical composition analysis.The metal tensile test adopts a WDW-200 electronic universal testing machine manufactured by Shanghai Songdun Instrument Manufacturing Company.The scanning electron microscope instrument adopts the EVO10/AZteLiveOne30 environmental scanning electron microscope made in ZEISS, Germany.The corrosion product analysis instrument adopts the SmartLabSE X-ray diffraction analyzer produced by Lifei, Japan.The metallographic analysis instrument adopts the AXIO Imager A2m metallographic microscope manufactured by ZEISS, Germany.

Analysis results of chemical composition of materials
The heat exchanger tube material for S22053, according to the quality certificate requirements, is to meet the material GB/T 20878-2007 "stainless steel and heat-resistant steel grades and chemical composition" requirements.Comparing the chemical composition of the material Tube B and Tube A, analysis results are shown in Table 3.The results of the chemical composition analysis of the material show that the content of the main chemical elements of the two heat exchanger tubes is within the standard requirements, and no abnormality is found.

Tensile test results of metallic materials
Two specimens were taken from each of the failed tubes A (no crack location) and the comparison tube B for metal tensile testing, named A1, A2, B1, and B2, respectively, and the results are shown in Table 4

Test results of scanning electron microscope (SEM)
The electron microscope test was carried out on the crack fracture of the failed heat exchange tube.
The crack section presents a block structure with granular dirt attached, and the result is shown in Figure 2 below.As can be seen from Figure 2, there are no obvious plastic cracks such as dimples on the fracture surface.

Corrosion product analysis results
The intergranular corrosion products are analyzed, and the main components are shown in Figure 3 below.It can be seen that the corrosion products are mainly Fe, S, and O, and a trace of Cl is found.

Metallographic experimental results
A metallographic test was carried out at the cracked part of the failed heat exchange tube.The results are shown in Figure 4 and Figure 5.As can be seen from Figure 4, the grain size of the heat exchange tube material is uniform, and the grain boundary is obvious.There are secondary cracks near the main cracks, mainly intergranular cracks, accompanied by transgranular cracks.It can be seen from Figure 5 that there are corrosion marks on the edge of the upper (outer wall) crack and plastic deformation caused by tearing on the edge of the lower (inner wall), so it can be judged that the crack is caused by the outer wall and extends to the inner wall.
The shell side medium is low-pressure steam.After verification with the factory technicians, the low-pressure steam has been treated with chemicals.The added medicine is trisodium phosphate, and the water quality is alkaline (pH value is 9.6 ~ 10.0).
This heat exchanger cracking failure parts were analyzed, and tensile testing of the chemical composition of the material aims to meet the standard requirements without abnormality.Scanning an electron microscope test on the fracture, you can see that the fracture has a clear block-like organizational structure, but cannot see the tough nest and other plastic tearing characteristics, thus judging that the crack is brittle cracking.Corrosion product analysis of the cracked surface scale can show that the corrosion products are mainly Fe, S, and O while containing a small number of Cl elements.Combined with the media analysis, it can be seen that the main composition of the pipe process media transform gas contains H 2 S and COS, while containing CO and CO 2 , etc., which should be the source of S and O elements, but the internal medium does not contain Cl elements.In the cracking of the tube bundle, the pressure of the tube journey is much higher than the pressure of the shell journey, which leads to the spraying of the internal change gas along the crack and contacting with the low-pressure steam, generating corrosion products in the cracked cross-section.The medium of the shell journey is the low-pressure steam, and after the dosing treatment, the Cl element should originate from the low-pressure steam.A metallographic test on the cracked tube bundle cross-section shows that the cracking is mainly along the crystal cracking, accompanied by a small amount of cracking through the crystal.At the same time, it is also seen that there are obvious corrosion traces on the outer wall of the cracked tube bundle section, while there are obvious traces of plastic deformation caused by tearing at the inner wall fracture, and it can be concluded that the cracking starts from the outer wall and expands to the inner wall, and the final tearing is caused by the fact that the strength of the section cannot satisfy the pressure of the tube bundle operation.
The tube bundle is cracked from the outer wall, and the contact medium of the outer wall of the tube bundle is low-pressure steam.After dosing treatment, the added medicine is trisodium phosphate, and the water quality is alkaline (pH value is 9.6 ~ 10.0).At the same time, metallographic examination shows that the cracking is mainly along the grain, which conforms to the characteristics of alkali stress corrosion cracking.The cracking part is the tube bundle at the lower part of the tube box, which belongs to the low-pressure steam inlet of the shell side, and the tube bundle will bear a large temperature difference, which will produce a certain temperature difference stress.At the same time, the baffle and other obstacles will produce alkali liquor enrichment at the shelter and the surface of the tube bundle, which will cause corrosion of the tube bundle.The alkali liquor concentrated and gathered at the corrosion place will cooperate with the temperature difference stress to cause alkali liquor corrosion cracking of the tube bundle.Finally, with the crack propagation, the strength of the pipe wall decreases, which cannot meet the high operating pressure of the pipe side, leading to cracking.

Conclusions and suggestions
The failure analysis of the heat exchanger of the water gas purification plant, through the material chemical composition analysis, metal material tensile test, scanning electron microscope test, corrosion product analysis, metallurgical test, and media composition analysis, etc., concluded that the stainless-steel tube bundles are cracked due to alkali stress corrosion cracking caused by alkali concentration of low-pressure steam after dosing treatment of the shell process.To avoid similar problems in subsequent use, it is recommended that the outer wall of the tube bundle can be regularly cleaned to reduce the concentration of alkali corrosion on the outer wall, or low-pressure steam should be added to improve or replace the drug, reducing its alkalinity or neutralizing it to neutral.

Figure 1 .
Figure 1.Macroscopic morphology of cracked heat exchanger tubes of low-pressure steam superheater.

Figure 2 .
Figure 2. Morphology of crack section of heat exchange tube.

Figure 3 .
Figure 3. Analysis results of scale composition in cross-section of heat exchange tube.

Figure 5 .
Figure 5. Metallography of cracks in fracture section of tube bundle.

Table 3 .
Chemical composition analysis results.

Table 4 .
below.Tensile test results of metal materials.