Analysis on the perforation of eccentric reducer behind the outlet valve of 1 × pump at the bottom of pressure reducing tower
In recent years, with the increase of sulfur-containing crude oil import and the deterioration of crude oil quality, the corrosion problem of atmospheric and vacuum distillation unit, as the first process of crude oil processing, has become increasingly serious. Corrosion not only shortens the service life of the equipment and causes huge waste of energy, but also leads to disastrous accidents [1,2]. Therefore, the safety and reliability of atmospheric and vacuum unit equipment is more prominent. The atmospheric and vacuum distillation unit mainly distills crude oil by physical methods to generate gasoline, diesel oil, residue and other components, which provide raw materials for hydrocracking, coking and other units. The operation of the unit directly affects the production of subsequent units. The crude oil contains sulfur and high molecular organic acid, which will cause different degrees of corrosion to the equipment pipeline in the distillation process. In recent years, corrosion failure of residual oil pipeline in vacuum system has occurred from time to time. Therefore, this paper takes the corrosion leakage of the 1 × pump pipeline at the bottom of the vacuum tower in a company’s atmospheric and vacuum unit as an example, analyzes the causes and mechanism of the corrosion failure of the pipeline system, puts forward protective measures to control the corrosion failure, and also provides reference for the corrosion prevention work of similar atmospheric and vacuum units.
The experimental material was taken from the 1 × pump pipeline at the bottom of 5.5 × 109 kg / a atmospheric and vacuum unit of a refinery. It was put into use in 2006. In July 2018, leakage occurred at the eccentric reducer behind the outlet valve of the pipeline. The leakage point is near the joint and flange connection weld, as shown in Figure 1. The material of eccentric reducer is 1Cr5Mo steel, working temperature is 369 ℃, working pressure is 1.6Mpa; working medium is vacuum residue, its basic parameters are: density is 982.3kg/m3, freezing point is 25 ℃, viscosity is 14.1 (100) mm2 / s, acidity (acid value) is 0.05mg KOH / g, total sulfur content is 1.68%, residual carbon is 8.95%.
Fig.1 Photographs of the eccentric reducer
A 30 mm × 30 mm block sample was taken from the eccentric reducer. According to the standard of GB / T 16597-1996, the material of the sample was analyzed by the AAS of analyst aa800. The transverse section metallographic samples were taken from the eccentric reducer, after pre grinding, polishing and etching, and then observed and analyzed under the mef4a metallographic microscope. The inner wall and cross section of eccentric reducer were observed and analyzed by using Fei inspect fSEM scanning electron microscope (SEM) equipped with Oxford energy spectrometer (EDS). The corrosion products on the inner wall of eccentric reducer were analyzed by panalytic x’pertpro X-ray diffraction (XRD) and jade 5.0 software.
Results and discussion
Macro and low power analysis of the failure eccentric reducer
No obvious abnormality is found on the outer wall of eccentric reducer. Due to the leakage of residual oil, the outer wall of the joint is full of residual oil, showing black oil, as shown in Figure 1. There are leakage points near the weld joint connecting the joint and flange. The outer and inner walls of the joint at the leakage point are relatively smooth, especially the inner wall at the leakage point is free of corrosion products, with traces of fluid medium erosion, as shown in Fig. 2a and B. Through macro and low power inspection of eccentric reducer, it is preliminarily confirmed that the perforation leakage of eccentric reducer is from the inner wall of the joint outward, and its property is erosion corrosion damage.
Fig.2 Photographs (a) and macro morphology (b) of the leakage site on the inner wall of the eccentric reducer
Chemical analysis of the material of the failed eccentric reducer
Take 30 mm × 30 mm block sample from eccentric reducer, and conduct chemical analysis with spectrometer according to GB / T 16597-1996 and other standards. The chemical composition (mass fraction,%) is: Cr 4.95, Mo 0.50, Mn 0.44, Ni 0.067, Cu 0.092, P 0.019, s 0.007, C 0.11, Si 0.40, Fe margin. The material of eccentric reducer is 12cr5moi (1Cr5Mo) steel , its chemical composition (mass fraction,%) is: Cr 4.00 ~ 6.00, Mo 0.45 ~ 0.60, Mn 0.30 ~ 0.60, Ni ≤ 0.60, Cu ≤ 0.20, P ≤ 0.025, s ≤ 0.015, C ≤ 0.15, Si ≤ 0.50, Fe margin. It shows that the chemical composition of eccentric reducer material meets the standard requirements of 12cr5moi (1Cr5Mo) steel.
The metallographic samples of transverse section were taken from the eccentric reducer, which were pre ground, polished and etched, and then observed and analyzed under the microscope. The pipe wall near the hole of eccentric reducer is seriously thinned; the metallographic structure of the pipe wall is point like, spherical pearlite + ferrite , as shown in Figure 3.
Fig.3 Metallographic structure of the perforation site of the eccentric reducer: (a) metallographic structure, (b) magnified image of Fig.3a
The inner wall and section near the hole of eccentric reducers were observed and analyzed by SEM. The perforation of eccentric reducer results in a large amount of leakage of residual oil medium in the pipeline. The flushing of residual oil causes less corrosion products on the inner wall of the joint at the perforation of the joint, and there are traces of fluid scouring on the inner wall of the joint, as shown in FIG. 4A. There are different thickness corrosion products on the inner wall of the joint which is far away from the hole of eccentric reducer. The outer layer of these corrosion products is loose and easy to peel off, while the inner layer is denser than the outer layer, as shown in Figure 4B. Energy spectrum analysis shows that the corrosion products of the outer layer are mainly composed of Fe and s, as well as Al, Cr, Si, O, Na, etc., as shown in Fig. 4C. According to the Fe / S ratio, it is concluded that the main corrosion product of the outer layer of the inner wall of eccentric reducer is iron sulfide. When the loose outer layer corrosion product peels off, the exposed inner layer corrosion product is denser than the outer layer; energy spectrum analysis shows that the inner layer corrosion product is also mainly Fe sulfide, but the Cr content is significantly increased, as shown in Figure 4D.
Fig.4 Surface morphologies (a, b) and EDS analysis result of areas A (c) and B (d) in Fig.4a of the inner wall close to the perforation site of the eccentric reducer
The microstructure of the cross section of the eccentric reducer shows that the corrosion product on the inner wall of the reducer peels off seriously, but there is still residual in some parts, as shown in Figure 5. The corrosion product layer has obvious layered structure, the outer layer is loose, porous and seriously peeled off (bright color, see mark a); the inner layer is thick, dark gray and most of it has been separated from the joint pipe wall (see Mark b); there are many transverse and longitudinal cracks in the corrosion product layer; energy spectrum analysis shows that the corrosion product is mainly the mixed sulfide of Fe and Cr, and the higher the content of Cr is, as shown in Figure 5C And D.
Fig.5 Cross sectional morphology (a), magnified image of Fig.5a (b), and EDS analysis of areas A (c) and B (d) in Fig.5b of the eccentric reducer
Take some corrosion products on the inner wall of eccentric reducer and conduct XRD analysis, as shown in Figure 6. It can be seen from the figure that the corrosion product on the inner wall of eccentric reducer is FES.
Fig.6 XRD diffractogram of the corrosion product on the inner wall of the eccentric reducer
The results of the above inspection and analysis show that the perforation of the eccentric reducer results in a large amount of leakage of residual oil in the pipeline. Residual oil scour causes fluid scour on the pipe wall near the perforation. Although there are few corrosion products left on the pipe wall near the perforation site, the composition analysis shows that these corrosion products are iron rich sulfide. However, the remaining thick corrosion products can still be observed on the local surface of the inner wall of the joint far away from the perforation. The corrosion products are layered, the outer layer is FES, the inner layer is mixed sulfide of Fe and Cr, and the Cr content tends to increase from the outside to the inside. The outer layer of ferrous sulfide is loose and porous, while the inner layer of Fe Cr mixed sulfide layer is relatively dense, but most of it has been separated from the substrate. At the same time, there are some longitudinal and transverse cracks in the corrosion product layer. The macro and micro analysis of the inner wall of eccentric reducer shows that the corrosion products on the inner wall of the pipe are easy to peel off, while most of the corrosion products on the inner wall of the pipe at the perforation have peeled off under the scouring effect of residual oil fluid.
According to the above analysis, the interaction between high temperature sulfur corrosion and residual oil erosion is the main cause of eccentric reducer perforation. The results of vacuum residue analysis showed that the oleic acid value of the residue was lower (0.05 mg KOH / g) and the sulfur content was higher (1.68%). It is generally believed that [5,6], when the acid value of crude oil reaches 0.5 mg KOH / g, naphthenic acid corrosion can be caused. The higher the acid value is, the more serious the corrosion will be. The temperature range of naphthenic acid corrosion is about 230 ~ 400 ℃, and there are two peaks, the first peak is 270 ~ 280 ℃, and the second peak is 350 ~ 400 ℃. Because the oleic acid value of vacuum residue is low and the sulfur content is high, the corrosion in the pipe is mainly high temperature sulfur corrosion, and the effect of naphthenic acid corrosion is small. The corrosion products on the inner wall of the tube are mainly composed of FES in the outer layer and Fe Cr mixed sulfide in the inner layer.
There are many sulfides in crude oil, which are mainly in the form of mercaptan, sulfide, polysulfide, cyclosulfide, elemental sulfur, H2S, thiophene, etc. Thiophene sulfur is mostly contained in residual oil. Under heating condition, sulfide in residual oil will decompose to form H2S. Under high temperature, H2S and free sulfur in oil will react with 1Cr5Mo steel as follows to form sulfide of Fe and Cr:
Compared with Cr, Fe is easier to diffuse and react with corrosive medium to form FES, so the outer layer of corrosion product is Fe sulfide. With the external diffusion consumption of Fe, the Cr content in the surface layer of the tube increases relatively, and the sulfide of Cr can be formed under the FES. Due to the low chromium content of the alloy, only (Fe (Fe2-xCrx)S4) can be formed on the surface of the tube, but not single CR sulfide.
The sulfide formed blocks the direct contact of substrate with H2S and other corrosive media, and to a certain extent, slows down the corrosion. However, due to the large deviation of the stoichiometric ratio of sulfides, the higher defect concentration and the faster diffusion of elements in sulfides, the sulfidation corrosion rate of metals is generally faster. Although the formed sulfide corrosion product layer has a certain protective effect, its adhesion is poor. On the one hand, the growth of sulfide is mainly through the external diffusion growth of cation under the condition of pipe working temperature, so it is easy to form holes at the sulfide / substrate interface, reducing the binding force between the corrosion product and the substrate, and the corrosion product is easy to peel off; on the other hand, the sulfide PBR (the volume ratio of metal sulfide to the metal consumed for its formation) is larger (the PBR of FES is about 2.6-2.7, and the PBR of cr2s3 is about 3.1). Therefore, there is a large compressive stress in the formed sulfide corrosion product layer, and the corrosion product is easy to crack, thus losing the protective effect. In addition, the hydrogen energy generated by the above reaction is intermittently dissolved in the sulfide, which is easy to cause the loose and porous corrosion product layer .
The sulfurization rate of Cr is generally 1-2 orders of magnitude lower than that of Fe. Increasing the Cr content in the alloy can generally improve its sulfur corrosion resistance . Because 1Cr5Mo steel contains only about 5% Cr, the effect of Cr on improving its high temperature sulfur corrosion resistance is limited.
High temperature sulfur corrosion is the main form of bend corrosion. Although the acidity of residual oil is weak, naphthenic acid corrosion is not the main form of corrosion, but a small amount of naphthenic acid may react with high temperature sulfur corrosion products such as FES as follows:
The reaction (5) not only destroys the FeS corrosion product layer, but also further corrodes the metal with the H2S formed by the reaction.
The formation of FES corrosion product layer is loose and porous, and the sulfide layer is easy to crack due to the large stress in the sulfide layer. H2S and other corrosion media can diffuse corrosion metals along these cracks and holes, and the local peeling of corrosion products can make new metals directly exposed to the corrosion media, so that the corrosion develops in depth.
It is worth noting that due to the poor adhesion of sulfide corrosion product layer and the existence of large stress, it is more likely to crack and peel off under the impact of high-speed residual oil, thus accelerating the high-temperature sulfur corrosion of the pipe, and even leading to corrosion perforation in the local area of the pipe.
Conclusions and suggestions
- (1) The material of eccentric reducer meets the standard requirements of 1Cr5Mo steel; the metallographic structure of eccentric reducer is point, spherical pearlite + ferrite.
- (2) The inner surface of eccentric reducer has formed a layer of corrosion products, the outer layer is ferrous sulfide and the inner layer is iron chromium mixed sulfide, which can slow down the corrosion of the pipe to a certain extent, but the corrosion products of this layer are loose, porous, poor adhesion, easy to crack and peel off. The cracking and spalling of sulfide corrosion product layer make the metal pipe directly exposed to H2S and other corrosive media, so that the corrosion develops in depth.
- (3) The high-speed flow of bottomless residuum will accelerate the cracking and peeling of corrosion products, thus accelerating the corrosion of eccentric reducer.
- (4) The synergistic effect of high temperature sulfur corrosion and residual oil erosion is the main cause of corrosion thinning and perforation of eccentric reducer.
- (5) Strengthen the pre desulfurization treatment of crude oil to reduce the high temperature sulfur corrosion of residual oil pipeline.
- (6) Strengthen the monitoring of key parts of the device, upgrade materials or apply surface protective coating if necessary.
Source: China Eccentric Reducer Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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