Analysis of the causes of waste heat boiler leakage in large-scale chemical plants

A waste heat boiler is a kind of high-temperature and high-pressure heat exchanger in a large chemical plant, and its safe operation is directly related to the production efficiency and safety risk of a chemical plant. In this paper, the leakage failure analysis of a waste heat boiler in an ethylene plant is carried out, and the analysis methods such as field observation, macro observation of fracture, metallographic analysis, micro observation of fracture, phase analysis, and energy spectrum analysis are adopted respectively. The results show that the direct cause of the leakage of the waste heat boiler is the thinning of the wall thickness of the heat exchange tube caused by alkali corrosion, which leads to the overload and fracture of the heat exchange tube. The pits on the outer surface of some heat exchange tubes are caused by mechanical reasons, which are related to this leakage and closure. The corrosion failure of heat exchange tubes is widespread, which has a great influence on safety production and serious consequences. Through this comprehensive analysis, the failure mechanism is clarified, which can provide a reference for the maintenance of similar equipment. At the same time, preventive measures are put forward from the aspects of technology, structure, and detection to reduce the occurrence of similar accidents.


Introduction
A waste heat boiler is an important equipment in large-scale chemical plants.It is a boiler that utilizes waste heat from the chemical production process to produce steam and belongs to a kind of hightemperature and high-pressure heat exchanger.For example, in the ethylene plant, the cracked gas out of the tube furnace (temperature up to 800 ~ 900℃) directly enters into the waste heat boiler and is cooled sharply; in 0.1 s, the temperature is down to 350 ~ 600℃, which not only prevents excessive cracking and reduces the ethylene yield, but also ensures the production of high-pressure steam more than 10 MPa, playing an important role in improving the thermal efficiency of the entire plant and economic benefits.The safe operation of waste heat boilers is related to the continuous production of large-scale chemical plants, and avoiding the failure of waste heat boilers is an important way to avoid major economic losses and safety risk exposure [1].The failure of waste heat boilers is generally caused by heat exchanger tube cracking or corrosion perforation [2][3][4][5][6][7].After analyzing the cause analysis of a large number of heat exchanger tube cracking statistics, it is found that it is mainly caused by alkali corrosion, alkali stress corrosion cracking, chloride stress corrosion cracking, intergranular corrosion, corrosion under the scale, and other reasons [8][9][10][11][12][13].
An enterprise recently found that a waste heat boiler ladle level in an ethylene production unit dropped rapidly, resulting in a low interlock shutdown of the cracker.The main parameters of the equipment are shown in Table 1.Observing the temperature trend of the waste boiler outlet at the site, it was determined that the waste boiler was leaking and that a large amount of steam was leaking into the cracked gas system due to the high pressure on the steam side.After stopping the furnace, the waste pot was disassembled and it was found that a heat exchanger tube was broken, and some heat exchanger tubes had craters on the outer surface.The location of burst pipe was located 30 mm to 50 mm above the lower tube plate folding plate, as shown in Figure 1.
Through macro-inspection, metallographic analysis, energy spectrum analysis physical analysis, and other technical means of fracture and the existence of pits in the two heat exchanger tube specimen analysis, waste heat boiler operating environment and boiler water composition were studied.In addition comprehensive analysis was conducted to determine the direct cause of the waste heat boiler leakage of alkali corrosion caused by the thinning of the wall thickness, which in turn caused the overload fracture.According to the failure, the mechanism gives the corresponding preventive assessment and solution measures.

Experimental equipment and methods
In the leakage of waste heat boiler, two typical samples are taken, one for the occurrence of burst heat exchanger tube part, labeled as 4 #, and the other for the surface with a clear pit of a whole heat exchanger tube.With the pit of the heat exchanger tube for macro-inspection and labeling (Figure 2), it is found that the heat exchanger tube of the first refractory plate 1 # and the second refractory plate 2 # are corroded to varying degrees, and the second refractory plate has a white substance attached, as shown in Figure 3.At the same time, pits of different sizes are distributed from the lower tube plate 800 mm~1000 mm (3# position) and from the lower tube plate 1650 mm~1700 mm (5# position), as shown in Figure 4.Because the white material removable amount at the 2# position (at the second baffle from bottom to top) is small, XRD phase analysis could not be carried out.So energy spectrum analysis is carried out, and cross-section metallography and scanning electron microscope analysis are carried out for 4# tube explosion.
Experimental instruments: the macroscopic observation is made by visual observation combined with a low magnification stereomicroscope, and the instrument is Stemi 508 low magnification stereomicroscope made by ZEISS, Germany.Microscopic observation and energy spectrum analysis are observed by scanning electron microscope, and the instrument is EVO 10 /AZteLiveOne30 environmental scanning electron microscope made by ZEISS, Germany.The metallographic analysis instrument adopts the AXIO Imager A2m metallographic microscope made by ZEISS, Germany.

Metallographic analysis.
The pits near the 3# position are sampled and the metallographic analysis of the cross-section shows that there are pits caused by mechanical damage in the crosssection, as shown in Figure 7.At the same time, as can be seen from Figure 8, the metallographic structure is ferrite+pearlite, there is an oxide layer on the outer surface of the tube, and some parts of it extend to the inner wall in the form of cracks.The inner surface is slightly decarbonized, there are oxides in some places, and the internal structure of the pit is deformed.

Phase analysis.
The XRD analysis of the attachments on the inner surface of position 3# shows that the results are Fe3O4, Fe, Al2O3, and FeO,as shown in Figure 9.

Analysis of pits on the outer surface of the heat exchange tube
After the white material at the baffle is scraped off, there is no obvious corrosion on the pipe wall.Because the white material in the 2# position (at the second baffle from bottom to top) is small, XRD phase analysis cannot be carried out, so energy spectrum analysis can only be carried out.The results of energy spectrum analysis show that white material contains C, O, P, Na, Fe, Ca, Mg, and Al.Because only the results of energy spectrum analysis can not accurately give the phase of white material, it is inferred from experience that white material may be the product of scale and corrosion under scale, and its components are probably calcium carbonate, magnesium hydroxide, magnesium carbonate, sodium ferrite (Na2FeO2), and so on.

Analysis of tube explosion 3.3.1 Macroscopic examination.
Macro-examination of the sample of tube explosion shows that the fracture is obviously plastic deformation, the wall thickness is seriously reduced, and the tube wall is corroded and thinned from the outside.

Metallographic analysis.
The metallographic analysis of the cross-section of the tube burst shows that the metallographic structure of the tube burst is ferrite+pearlite, there is an oxide layer on the outer surface of the tube, and the microstructure at the final fracture is seriously deformed, as shown in Figure 10.There is slight decarburization on the inner surface of the fracture area, and there are oxides in local positions.The analysis shows that the scaling occurs on the outer surface of the pipe section at the fracture position, which leads to the temperature increase in this area, resulting in a slight decarbonization of the inner surface structure.

Scanning electron microscope analysis.
The position of the explosion mouth is seriously worn, and some oxide layers, mainly iron oxides, remain on the outer wall of the explosion mouth, with Ca, Si, and P in some places.

Discussion
Through the analysis of the test results, it is found that the pit damage at positions 3# and 5# is caused by mechanical external force, which is not directly related to the leakage of the waste heat boiler.There is obvious plastic deformation at the explosion of the 4# tube, and the thickness at the explosion mouth is serious, which can be judged as an overload fracture from the morphology.The normal structure of the material at the tube burst is ferrite plus pearlite.The inlet medium of the waste heat boiler shell side is 326℃ saturated water, which is heated by pyrolysis gas to generate 326℃ steam, which is discharged from the shell side outlet.Because the inlet and outlet temperatures of the medium are both 326℃, the inlet medium water will be converted into steam soon after absorbing heat, and the tube explosion position is near the water inlet, where the water will not be converted into steam quickly.Some impurities in the water will gradually concentrate because they cannot be discharged with the steam, and local scaling may occur on the surface of the heat exchange tube.Energy spectrum analysis also shows that the fracture contains Na+ and Ca2+ impurities.Most of the PH value of boiler water is maintained at 8.5~9.6.At the same time, sodium, dissolved silicon, and other components are also detected.These media together form an alkaline environment.Because of the continuous evaporation of boiler water, the media gradually accumulate and concentrate, eventually forming alkaline corrosion, resulting in the continuous thinning of the wall thickness of heat exchange tubes, which can not meet the strength requirements, and finally, overload fracture occurs.
Alkaline corrosion is a common corrosion in boiler systems.When scale and other attachments are attached to the metal surface of the boiler, serious pit corrosion will occur under the scale and attachments, and alkali corrosion is mostly manifested as local corrosion.In general, there is a magnetic Fe3O4 film on the surface of boiler system equipment after chemical cleaning and operating under normal working conditions.Under normal circumstances, the film is dense and plays a good role in protecting metals, and only when the Fe3O4 film is damaged will the equipment corrode.However, when there are attachments on the metal surface, due to the poor thermal conductivity of the attachments, the temperature of the metal under the attachments rises sharply, which makes the boiler feed water infiltrated under the attachments concentrate sharply, making the alkali concentration of boiler water under the attachments become very high, and at the same time, the temperature of the furnace tube rises sharply.When there is free NaOH in the boiler feed water, the alkali concentration under the attachment will become very high, thus alkaline corrosion will occur.
Under these attachments, under the joint action of local high temperature and local high concentration NaOH, both the passivation film of the furnace tube and the base metal are corroded and dissolved: Fe O + 4NaOH → 2NaFeO + Na FeO + 2H O Fe + 2NaOH → Na FeO +H Sodium ferrite, the corrosion product, can be hydrolyzed to produce sodium hydroxide and ferrous oxide (Na FeO +H O → 2NaOH + FeO), the former makes the corrosion continue, and the latter becomes the corrosion product.It can be seen that the alkali corrosion of the furnace tube is repeated in the occluded area where corrosion occurs, and there is no need to supplement alkaline substances (NaOH) from the outside, and the furnace tube eventually corrodes and leaks.

Conclusions and suggestions
The direct cause of leakage of the waste heat boiler is alkali corrosion of the heat exchange tube, which leads to wall thickness thinning and eventually fracture.The pits on the outer surface of the heat exchange tube found in the waste heat boiler are caused by mechanical damage and have no direct relationship with the leakage of the waste heat boiler.
The position of the tube burst of the heat exchange tube is near the water inlet, where the water will not quickly turn into steam, and some impurities in the water will gradually concentrate because they cannot be discharged with the steam.Local scaling may occur on the surface of the heat exchange tube, forming an alkaline environment, and finally, alkali corrosion will occur, resulting in the continuous thinning of the wall thickness of the heat exchange tube.
It is suggested to control the quality of boiler water in the waste heat boiler, reduce the content of impurities, and strictly control the PH value.It is necessary to check the thickness of heat exchange tubes during shutdown inspection, especially those located near the water inlet, to prevent the explosion of tubes and avoid causing greater economic losses and safety risks.

Figure 1 .
Figure 1.Leakage point of waste heat boiler.Table 1. Main Parameters of Waste Heat Boiler.

Figure 4 .
Figure 4. Damage morphology of the outer wall.The external damaged parts (3# position and 5# position) with external surface pits are cut open and observed.The pits near the 3# position are sampled and analyzed by cross-section metallography, and the attachments on the inner surface of the 3# position are analyzed by XRD.Because the white material removable amount at the 2# position (at the second baffle from bottom to top) is small, XRD phase analysis could not be carried out.So energy spectrum analysis is carried out, and cross-section metallography and scanning electron microscope analysis are carried out for 4# tube explosion.Experimental instruments: the macroscopic observation is made by visual observation combined with a low magnification stereomicroscope, and the instrument is Stemi 508 low magnification stereomicroscope made by ZEISS, Germany.Microscopic observation and energy spectrum analysis are observed by scanning electron microscope, and the instrument is EVO 10 /AZteLiveOne30

3. 1 . 1
Macroscopic inspection of internal and external surfaces.After dissecting the external damaged parts with external surface craters (3# position and 5# position), as shown in Figure5, it is found that there are also traces of damage on the inner surface of the 3# position, and no corrosion has occurred on the inner surface of the tube, while the locations of the damage traces on the inner and outer surfaces are almost one-to-one.No obvious corrosion and damage traces have been found on the inner surface of the 5# position, and Figure6is the morphology of the inner and outer surfaces of the 5# position after dissecting.

Figure 9 .
Figure 9. XRD analysis results of attachments on the inner surface of position 3#.Based on the above test and analysis results, it can be judged that the pit damage at 3# and 5# positions is caused by mechanical external force.

Figure 10 .
Figure 10.Metallographic structure of the explosive tube.

Table 1 .
Main Parameters of Waste Heat Boiler.