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Experimental and computational failure analysis of a high pressure regulating valve in a chemical plant
Engineering Failure Analysis 70 (2016) 188–199
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Experimental and computational failure analysis of a high pressure regulating valve in a chemical plant Junzhen Yi a,b, Hongxiang Hu a,⁎, Yugui Zheng a, Yaming Zhang a a b
Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, PR China School of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, PR China
a r t i c l e
i n f o
Article history: Received 3 April 2016 Received in revised form 21 June 2016 Accepted 9 July 2016 Available online 30 August 2016 Keywords: Failure analysis Chemical plant Regulating valve Cavitation erosion Erosion
a b s t r a c t A high pressure regulating valve for producing ethylene vinyl acetate copolymer in a chemical plant failed after three years in service. The failure analysis of valve was performed by means of visual inspection, stereo microscopy, optical microscopy, scanning electron microscopy and computational fluid dynamics (CFD). The morphology analyses showed that the craters on the valve plug and cage, especially the end of the valve plug, were mainly caused by cavitation erosion, and the ditch damage resulted from the erosion due to impingement and scouring of the high velocity flow. The corrosivity of vinyl acetate was proved to have few effects on the materials damage. The failure reasons were further demonstrated by the simulation results, which presented the distribution of the sites suffering the high velocity flow erosion and possible cavitation erosion. The computed numerical results showed a good agreement with morphology analyses. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Ethylene vinyl acetate copolymer (EVA) is a thermoplastic and elastomeric material used in many fields such as insulating materials, biomedical devices, and hot melts adhesives due to its good mechanical and physical properties [1–3]. In industrial production, EVA is prepared from Ethylene and Vinyl acetate (VA) through a series of chemical processes under high temperature and pressure. Fig. 1 schematically illustrates the production process of EVA. A regulating valve locating in the outlet of the secondary reactor was found to be failure. It is designed to regulate the flow rate of mixed mediums comprised of EVA, Ethylene and VA, and its opening is approximately 2 mm during the normal operation. The inlet and outlet pressure of the regulating valve are 280 MPa and 30 MPa respectively and its operating temperature is about 270 °C. Owning to harsh working conditions, the regulating valve failed after three years due to the severe damage on the valve plug and cage. In this study, the regulating valve failure analysis was performed by Visual Inspection, Stereo Microscopy, Optical Microscopy, Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The computational fluid dynamics (CFD) method was also employed to prove the reason causing valve failure.
⁎ Corresponding author. E-mail address: [email protected] (H. Hu).
http://dx.doi.org/10.1016/j.engfailanal.2016.07.015 1350-6307/© 2016 Elsevier Ltd. All rights reserved.
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Fig. 1. A schematic of EVA preparation and locations of damaged regulating valve.
2. Experimental Morphology analyses were carried out on the damaged sites of valve components surfaces using stereo microscopy, metallurgical microscopy, SEM and EDS. Materials cut from the valve plug and cage were grounded, polished and etched for the detection of impurities and defects in the microstructure. Chemical compositions of the valve materials were analyzed by the X-Ray Fluorescence Spectrometer. The fluid mediums contacting the valve are EVA, ethylene and VA, where VA is the most probable one causing corrosion. The pH-value of VA is measured by pH-meter. To further figure out the damage mechanism, comparison experiments were conducted between the VA immersion test and distilled water immersion test. VA solution was collected from the practical environment. Materials cut from the valve plug and cage were machined to 10 × 20 × 3 mm, grounded to 800# with series of sand papers and immersed in VA and distilled water respectively at the room temperature (25 ± 0.5 °C). In addition to the physical examination, CFD method was also used to further clarify the failure mechanisms of the high pressure regulating valve. A finite element model of the valve was constructed with the opening of 2 mm, which is the normal opening, as shown in Fig. 2. To make the inlet and outlet flow fully developed, the length of inlet pipe was 5d (d is the diameter of valve seat inlet) and the outlet pipe length was 5D (D is the diameter of outlet). Different slices named by BZ0, BY15, BY60 and BY120 were employed to show the distribution of the fluid fields in multiple perspectives. The numerical simulation of fluid field in the valve was carried out by a commercial code, FLUENT. In the multiphase flow model, the liquid phase and the gas phase were assumed to completely mix with each other due to the trace amount of gas phase. A κ-ε turbulence model was used to describe the turbulence in the valve. The active force between fluid field and wall
Fig. 2. Three-dimensional fluid model and the positions of four sections.
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was processed by an enhancement wall equation, and the SIMPLEC algorithm was employed to improve the computing accuracy. The boundary conditions were set as pressure inlet (280 MPa) and pressure outlet (30 MPa) according to the practical conditions respectively. The fluid remains in superfluid status in the high operation pressure of approximately 280 MPa, where the fluid density is much closer to liquid and the viscosity is much closer to gas. Its density and viscosity were set as and respectively. For the convergent–divergent structure of regulating valve, cavitation erosion may occur in the places of the valve where the pressure fluctuated. Therefore, a cavitation model was applied to predict the sensitive zone of the valve to cavitation erosion. In addition, it is noticed that the simulation work is to qualitatively interpret the damage mechanisms. Therefore, small differences in parameters setting, such as the density and viscosity of the flow, have few effects on judging damage mechanism. 3. Results and discussion 3.1. Visual investigations Fig. 3 shows the visual morphologies of the damaged sites on the valve plug. It can be found that the shape of valve plug still remains intact, but local zones marked as A, B and C are severely damaged shown in Fig. 3a. The details of these local damaged zones are shown in Fig. 3b–d respectively. Different positions present different damage morphologies indicating different failure mechanisms. A ditch morphology is observed on the circumference of the conical part of the valve plug (Zone A in Fig. 3b) which results from erosion damage. While in zone B, a large area of materials is peeled uniformly in the circumferential direction, and some light craters locate at the boundary of this area, which may be caused by cavitation erosion (Fig. 3c). Completely different from the damage appearances aforementioned, round craters with different depth and sharp-edges distribute densely in zone C (Fig. 3d). Based on the different damage characteristics between the head (zone A) and end (zone C) of the valve plug, it can be deduced that the failure of the valve plug is caused by two different mechanisms at least. Similar damage morphologies were also reported by many references. Liu et al. [4] observed severe cavitation erosion in the belt eye of the valve used in the feedwater system of a nuclear power plant. Jin et al. [5] also reported that a regulating valve in coal liquefaction failed due to the combined effects of cavitation erosion and abrasion. Therefore, preliminary estimate can be made that erosion and cavitation erosion were included in the reasons causing material damage. Fig. 4 shows the digital images of the failed valve cage and the detail of the damaged zone. In Fig. 4a, severe “moat-like” material loss occurs next to the inlet of the valve cage. The damaged zone is coupled with the front end of zone B (shown in Fig. 3c),
Fig. 3. (a) General view of the failed valve plug, (b), (c) and (d) Detail of the damaged zone.
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Fig. 4. (a) General view of the failed valve cage, (b) Detail of the damaged zone.
which shows a similar circumferential cavitation erosion. It indicates that the damages in these positions of valve plug and valve cage were caused by the same reasons. However, cavitation erosion on the valve cage (shown in Fig. 4b) is much more severe than that on the valve plug (Fig. 3c), and results in the wall thinning of root of the outlet lip and consequently the fracture and removal of large material as shown in zone B in Fig. 4a.
3.2. Stereoscopy Fig. 5 shows the surface damaged morphologies of the valve plug along the flow direction using a stereo microscope. Fig. 5a is a magnification of the erosion morphology on the cone of the valve plug (Fig. 3b). It presents a typical ditch appearance with a length of approximately 4 mm caused by flow erosion [6,7]. Due to the erosion, severe material loss changes the size of plug head and enlarges the gap between the plug head and valve seat, which leads to the loss of the function of pressure regulating. Moreover, a distinct protuberant dividing line can be observed in the middle of the damaged zone which caused by the changing of the
Fig. 5. Stereoscopic views of the damaged surface on (a) the cone, (b and c) the head and (d) the end of the valve plug.
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valve openings under the operating conditions. Fig. 5b and c magnifies the erosion appearance in zone B marked in Fig. 3a. Small “river type” erosion traces and local craters can be observed, and the craters in the boundaries of zone B are similar to those at the end of the valve plug (shown in Fig. 5d), but with smaller size and depth. They are typical cavitation erosion morphologies [8,9]. The deep and sharp-edged craters locating in the end of the valve plug indicate severe cavitation erosion there. For the valve cage, the damage morphologies are the same as those on valve plug, but the material loss are more severe, as shown in Fig. 6. It indicates that the valve cage encountered the same damage and it was less resistant to the erosion and cavitation erosion. The erosion caused by fluid flushing occurs at the head of the valve cage (Fig. 6a), and then cavitation erosion dominates the places downstream the erosion sites (Fig. 6b–d), which is more severe than the erosion and mainly results in the wall thinning and fracture eventually. 3.3. Chemical compositions analysis The chemical compositions of the valve plug and cage were analyzed by an X-Ray Fluorescence Spectrometer, and the results are listed in Table 1. The standards of Deutsches Institut für Normung (DIN) were taken as reference to value materials' quality because the valve was imported from Germany. The results show that the chemical compositions of valve plug are in agreement with those specified in the DIN 17350 standard although the carbon and chromium content is slightly lower than the standard content. The small difference in the C content has few effects on the mechanical properties and the erosion resistance, and the effect of lower Cr content on corrosion resistance of valve plug to the contact mediums can be ignored, which will be analyzed in the later section. The element contents of the valve cage are entirely within the standard contents of DIN EN 10083-1. Therefore, it is reasonable to conclude that valve plug is eligible in the chemical composition as well as the valve cage. 3.4. Metallography It is well known that the microstructure determines the performances of the materials. Some literatures have proved that the presence of microstructural defects could accelerate the process of erosion and cavitation erosion [10,11]. Therefore, it is necessary to investigate the microstructure of valve materials, which is closely related to the resistance to erosion and cavitation erosion. Metallographic samples obtained from the damaged and undamaged zones were both examined with an optical microscope after metallography preparation and chemical etch. Fig. 7 shows the microstructures of different parts in the valve plug and valve cage. It can be observed that the microstructure is acicular tempered martensite for the valve plug (Fig. 7a, b) and tempered
Fig. 6. Stereoscopic views of the damaged surface on the valve cage along the flow direction.
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Table 1 Chemical compositions (mass %) of the regulating valve. Elements
C
Si
Mn
P
S
Cr
Mo
V
Ni
DIN 17350 Valve plug DIN EN 10083-1 Valve cage
0.37–0.42 0.36 0.26–0.33 0.30
0.9–1.2 1.03 0.15–0.40 0.23
0.30–0.50 0.49 0.30–0.60 0.39
≤0.03 0.015 ≤0.035 0.008
≤0.03 0.008 ≤0.035 0.003
5.00–5.50 4.84 1.80–2.20 2.01
1.20–1.50 1.27 0.30–0.50 0.39
0.90–1.10 0.93 – –
– – 1.80–2.20 1.96
sorbite for the valve cage (Fig. 7c, d). The structure is uniform with no defects and no remarkable differences are detected between the damaged zone (Fig. 7a) and the undamaged zone (Fig. 7b) of the valve plug, as well as that of the valve cage. Therefore, the failure of valve is not resulted from the materials defects.
3.5. SEM morphologies Fig. 8 displays surface morphologies of the damaged sites obtained by SEM on the valve plug. The sites selected for examination are corresponding to the zones B, C of Fig. 3a, respectively. On the head of valve plug wall, ditch morphologies in parallel with the flow direction mixed with light craters dominate the material surface, as shown in Fig. 8a. It indicates that the damage is mainly caused by the mechanical erosion. While in Fig. 8b, on the end of the valve plug, very deep and sharp-edged craters are observed indicating severe cavitation erosion. Fig. 9 shows surface morphologies of the damaged valve cage obtained by SEM. The damage morphologies are similar to those on the valve plug, i.e., ditch morphologies and obvious craters. From the magnification of the erosion region shown in Fig. 9b, some discontinuous ditch in the same direction as fluid flow can be observed, which indicates that the erosion is caused by the flow flushing at high velocity. Moreover, the inside surfaces of the big cavitation erosion craters are densely covered by small craters, as shown in Fig. 9c. It can be deduced that the big and deep craters result from continuing cavitation erosion. In
Fig. 7. Metallographic structures of the valve materials: (a) damaged zone on valve plug; (b) undamaged zone on valve plug; (c) damaged zone on valve cage; and (d) undamaged zone on valve cage.
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Fig. 8. SEM micrographs of the damaged valve plug: (a) the head of valve plug wall; (b) the end of valve plug wall.
addition to the damage characteristics, some particle-like residues stick to the surface as shown in Fig. 9c. EDS was conducted on these residues and the results show that the elements of C and O dominate the chemical compositions of the residues. It indicates that the residues are probably the residuals of chemical products instead of the corrosion products. There are probably two
Fig. 9. SEM micrographs of the damaged valve cage: (a) the connection region of the erosion and cavitation erosion; (b) the magnification of the erosion region; and (c) the magnification of the cavitation erosion region.
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reasons resulting in the absent of corrosion products. One reason is that the corrosion products were washed away by the fluid, the other one is that corrosion did not occur at all. Further analysis on the probability of corrosion causing damage will be carried out in the following section. 3.6. Evaluation on corrosivity of VA Fig. 10 shows the macroscopic views of the valve materials after immersion in VA solution for 85 days and in distilled water for 10 days. It can be found that the former still remain metallic luster without the corrosion traces in Fig. 10a and b. While the latter present obvious pit corrosion as shown in Fig. 10c and d. It is noted that these two surfaces have been washed by the deruster and obvious rust layers have been removed. It can be deduced that VA's corrosivity to the valve material is much weaker than distilled water. The pH-value of VA is approximately 5.54. However, the weak acidity does not play an important role in the corrosion according to the comparison results. Water content in VA is a key aspect affecting the solution corrosivity, which was maintained in an extremely low amount in the practical conditions. Therefore, the corrosion to the valve caused by VA can be ignored due to its limited effect. 3.7. CFD simulation Fig. 11 shows the pressure distributions in the BZ0 and BY15 sections of the valve under the condition of valve opening of 2 mm. In the section of BZ0 (Fig. 11a), it can be seen that the high pressure region is located in the valve seat. Most of the parts in the valve cage show relatively low and uniform pressure due to the small valve opening. In the section of the valve tip crossed by the inlet of the valve seat, high pressure distributes averagely in the circumferential direction, as shown in Fig. 11b. No pressure fluctuation is observed in the section around the tip of valve plug where cavitation erosion is difficult to
Fig. 10. Macroscopic views of the (a) valve plug and (b) valve cage after immersion in VA solution for 85 days; (c) valve plug and (d) valve cage after immersion in distilled water for 10 days.
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Fig. 11. Pressure distributions in the BZ0 and BY15 sections of the valve.
occur. The fact also proves that there are only ditch morphologies caused by the flow impingement erosion instead of craters morphologies caused by the cavitation erosion as shown in Fig. 3b. In contrast, the pressure in the BY60 section is not uniform and local high pressure occurs near the outlet of the valve cage facing the valve outlet (Fig. 12a). Bubbles are prior to be produced in the fluid with pressure fluctuation. They escape from the liquid fluid and flow with the steam when the environment pressure is lower than the equilibrium pressure between the air and the fluid. Finally, they will collapse after arriving in places where the environment pressure is higher than the equilibrium pressure of the two phases. Consequently, cavitation erosion occurs and the materials near the collapsing bubbles are removed, which is in accordance with the cavitation erosion morphologies shown in Fig. 12b. In addition, the level of cavitation erosion damage is not uniform in the circumferential directions of the valve plug due to the heterogeneous distribution of circumferential pressure, as shown in Fig. 3c and Fig. 12b. The most obvious cavitation erosion is located in the places near the end of the valve plug. Big pressure differences are observed in the section of BY120 as shown in Fig. 13a, which is prone to cause severe bubbles generation, collapse and cavitation erosion. It is in agreement with the characteristics of distributions of the cavitation erosion morphologies of the valve plug as shown in Fig. 13b-f. Not all the faces in the circumferential direction encountered the cavitation erosion like the one shown in Fig. 13b. Some faces have deeper and larger area craters (Fig. 13 c, d and f), while certain face has lighter and smaller area craters (Fig. 13e), which is dependent on the differences of the pressure shown in Fig. 13a. The regions with high pressure are more prone to encounter the cavitation erosion than those with low pressure, which contributes to the differences in damage degree. In addition, the space between the valve plug and valve cage is extremely narrow where the fluid is nearly stagnated. As a result, fluid erosion cannot occur there and cavitation erosion is responsible for the damage. Fig. 14 shows the velocity and trajectory distribution in the BZ0 section of the valve. It can be observed in the velocity contours (Fig. 14a) that the highest velocity appears near the tip of valve plug where the severe erosion resulting from the impact of the high velocity flow easily occurs. The sites on the valve facing high velocity are in agreement with those where flow erosion is located (Fig. 3b) indicating that the erosion is mainly caused by flow impingement. Moreover, after the fluid flows through the small gap between the valve plug and the valve seat, the open area increases suddenly resulting in negative pressure zones
Fig. 12. Pressure distribution in the BY60 section of the valve and the corresponding damaged zone of valve plug.
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Fig. 13. Pressure distributions in the BY120 section of the valve and the appearances in the corresponding damaged zones of valve plug.
and eddy (Fig. 14b). Correspondingly, erosion is easy to occur in the valve cage due to the flow impingement at multi impact angles of the turbulence [12]. In another view, the velocity and trajectory distribution in the BY60 section is revealed in Fig. 15 to better understand the damage mechanism. A high velocity region is observed near the outlet of the valve cage (Fig. 15a), which can prove that high velocity fluid erosion results in the materials remove at the corresponding place (Fig. 3c). The trajectory distribution shown in Fig. 15b also indicates that the flow inside is in a turbulent state suggesting high possibility of erosion.
4. Recommendations (1) Repair the failed valve plug with alloy coatings prepared by surface treatments, for example, Co-based alloy coatings prepared by laser cladding. (2) Replace the valve cage material with the materials with higher cavitation erosion resistance, such as NiTi alloy. (3) Control the water content of the fluid flowing through the valve strictly, avoiding the electrochemical corrosion of valve materials.
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Fig. 14. Velocity and trajectory distribution in the BZ0 section of the valve.
Fig. 15. Velocity and trajectory distributions in the BY60 section of the valve.
5. Conclusions Experimental examination and computational simulation were carried out to figure out the damage mechanism of the failed valve plug and valve cage. The following conclusions can be drawn: (1) Morphology analyses revealed that erosion dominates the damage on the head of the valve plug and the craters are for the end of the valve plug and the inlet of the valve cage. (2) The chemical compositions of valve plug and valve cage were in agreement with the material standard of Germany DIN 17350 and DIN EN 10083-1 respectively. Metallography analyses showed that no defects and remarkable differences were detected between the damaged zone and the undamaged zone of the valve plug and valve cage. (3) VA was barely corrosive to the valve materials due to its small water content, which is not the main reason causing valve failure. (4) Simulation results were well consistent with the damage sites and types of the valve, and explained the failure mechanisms combined with the physical analyses. The failure of the valve was caused by the combination effects of fluid erosion and cavitation erosion. Acknowledgements The authors would like to acknowledge the financial support of the National Natural Science Foundation of China (5157010762). References [1] P.M. Ma, L. Jiang, M. Hoch, W.F. Dong, M.Q. Chena, Reinforcement of transparent ethylene-co-vinyl acetate rubber by nanocrystalline cellulose, Eur. Polym. J. 66 (2015) 47–56. [2] F. Cavodeau, R. Sonnier, B. Otazaghine, J.M. Lopez-Cuesta, C. Delaite, Ethylene-vinyl acetate copolymer/aluminium trihydroxide composites: a new method to predict the barrier effect during cone calorimeter tests, Polym. Degrad. Stab. 120 (2015) 23–31. [3] A.M. Henderson, Ethylene-vinyl acetate (EVA) copolymers: a general review, IEEE. Electr. Insul. M. 9 (1993) 30–38.
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[4] C.B. Liu, S.L. Jiang, Y.G. Zheng, Experimental and computational failure analysis of a valve in a nuclear power plant, Eng. Fail. Anal. 22 (2012) 1–10. [5] H.Z. Jin, Z.J. Zheng, G.F. Ou, L.T. Zhang, J. Rao, G.P. Shu, et al., Failure analysis of a high pressure differential regulating valve in coal liquefaction, Eng. Fail. Anal. 55 (2015) 115–130. [6] X.L. Ji, S.Z. Yang, J.H. Zhao, C.Y. Yan, L.F. Jiang, Effect of heat treatment on slurry erosion wear resistance of amorphous Ni-P electrodeposits, Tribol. T. 55 (2015) 86–90. [7] J.F. Santa, L.A. Espitia, J.A. Blanco, S.A. Romo, A. Toro, Slurry and cavitation erosion resistance of thermal spray coatings, Wear 267 (2009) 160–167. [8] J. Lin, Z. Wang, P. Lin, J. Cheng, X. Zhang, S. Hong, Effects of post annealing on the microstructure, mechanical properties and cavitation erosion behavior of arcsprayed FeNiCrBSiNbW coatings, Mater. Design. 65 (2015) 1035–1040. [9] Z.B. Zheng, Y.G. Zheng, W.H. Sun, J.Q. Wang, Effect of heat treatment on the structure, cavitation erosion and erosion–corrosion behavior of Fe-based amorphous coatings, Tribol. Int. 90 (2015) 393–403. [10] Y.P. Wu, P.H. Lin, C.L. Chu, Z.H. Wang, M. Cao, J.H. Hu, Cavitation erosion characteristics of a Fe–Cr–Si–B–Mn coating fabricated by high velocity oxy-fuel (HVOF) thermal spray, Mater. Lett. 61 (2007) 1867–1872. [11] W.J. Tomlinson, N. Kalitsounakis, G. Vekinis, Cavitation erosion of aluminas, Ceram. Int. 25 (1999) 331–338. [12] R. Li, M. Pellegrini, H. Ninokata, M. Mori, A numerical study on turbulence attenuation model for liquid droplet impingement erosion, Ann. Nucl. Energy 38 (2011) 1279–1287.
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Experimental and computational failure analysis of a high pressure regulating valve in a chemical plant Junzhen Yi a,b, Hongxiang Hu a,⁎, Yugui Zheng a, Yaming Zhang a a b
Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, PR China School of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, PR China
a r t i c l e
i n f o
Article history: Received 3 April 2016 Received in revised form 21 June 2016 Accepted 9 July 2016 Available online 30 August 2016 Keywords: Failure analysis Chemical plant Regulating valve Cavitation erosion Erosion
a b s t r a c t A high pressure regulating valve for producing ethylene vinyl acetate copolymer in a chemical plant failed after three years in service. The failure analysis of valve was performed by means of visual inspection, stereo microscopy, optical microscopy, scanning electron microscopy and computational fluid dynamics (CFD). The morphology analyses showed that the craters on the valve plug and cage, especially the end of the valve plug, were mainly caused by cavitation erosion, and the ditch damage resulted from the erosion due to impingement and scouring of the high velocity flow. The corrosivity of vinyl acetate was proved to have few effects on the materials damage. The failure reasons were further demonstrated by the simulation results, which presented the distribution of the sites suffering the high velocity flow erosion and possible cavitation erosion. The computed numerical results showed a good agreement with morphology analyses. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Ethylene vinyl acetate copolymer (EVA) is a thermoplastic and elastomeric material used in many fields such as insulating materials, biomedical devices, and hot melts adhesives due to its good mechanical and physical properties [1–3]. In industrial production, EVA is prepared from Ethylene and Vinyl acetate (VA) through a series of chemical processes under high temperature and pressure. Fig. 1 schematically illustrates the production process of EVA. A regulating valve locating in the outlet of the secondary reactor was found to be failure. It is designed to regulate the flow rate of mixed mediums comprised of EVA, Ethylene and VA, and its opening is approximately 2 mm during the normal operation. The inlet and outlet pressure of the regulating valve are 280 MPa and 30 MPa respectively and its operating temperature is about 270 °C. Owning to harsh working conditions, the regulating valve failed after three years due to the severe damage on the valve plug and cage. In this study, the regulating valve failure analysis was performed by Visual Inspection, Stereo Microscopy, Optical Microscopy, Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The computational fluid dynamics (CFD) method was also employed to prove the reason causing valve failure.
⁎ Corresponding author. E-mail address: [email protected] (H. Hu).
http://dx.doi.org/10.1016/j.engfailanal.2016.07.015 1350-6307/© 2016 Elsevier Ltd. All rights reserved.
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Fig. 1. A schematic of EVA preparation and locations of damaged regulating valve.
2. Experimental Morphology analyses were carried out on the damaged sites of valve components surfaces using stereo microscopy, metallurgical microscopy, SEM and EDS. Materials cut from the valve plug and cage were grounded, polished and etched for the detection of impurities and defects in the microstructure. Chemical compositions of the valve materials were analyzed by the X-Ray Fluorescence Spectrometer. The fluid mediums contacting the valve are EVA, ethylene and VA, where VA is the most probable one causing corrosion. The pH-value of VA is measured by pH-meter. To further figure out the damage mechanism, comparison experiments were conducted between the VA immersion test and distilled water immersion test. VA solution was collected from the practical environment. Materials cut from the valve plug and cage were machined to 10 × 20 × 3 mm, grounded to 800# with series of sand papers and immersed in VA and distilled water respectively at the room temperature (25 ± 0.5 °C). In addition to the physical examination, CFD method was also used to further clarify the failure mechanisms of the high pressure regulating valve. A finite element model of the valve was constructed with the opening of 2 mm, which is the normal opening, as shown in Fig. 2. To make the inlet and outlet flow fully developed, the length of inlet pipe was 5d (d is the diameter of valve seat inlet) and the outlet pipe length was 5D (D is the diameter of outlet). Different slices named by BZ0, BY15, BY60 and BY120 were employed to show the distribution of the fluid fields in multiple perspectives. The numerical simulation of fluid field in the valve was carried out by a commercial code, FLUENT. In the multiphase flow model, the liquid phase and the gas phase were assumed to completely mix with each other due to the trace amount of gas phase. A κ-ε turbulence model was used to describe the turbulence in the valve. The active force between fluid field and wall
Fig. 2. Three-dimensional fluid model and the positions of four sections.
190
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was processed by an enhancement wall equation, and the SIMPLEC algorithm was employed to improve the computing accuracy. The boundary conditions were set as pressure inlet (280 MPa) and pressure outlet (30 MPa) according to the practical conditions respectively. The fluid remains in superfluid status in the high operation pressure of approximately 280 MPa, where the fluid density is much closer to liquid and the viscosity is much closer to gas. Its density and viscosity were set as and respectively. For the convergent–divergent structure of regulating valve, cavitation erosion may occur in the places of the valve where the pressure fluctuated. Therefore, a cavitation model was applied to predict the sensitive zone of the valve to cavitation erosion. In addition, it is noticed that the simulation work is to qualitatively interpret the damage mechanisms. Therefore, small differences in parameters setting, such as the density and viscosity of the flow, have few effects on judging damage mechanism. 3. Results and discussion 3.1. Visual investigations Fig. 3 shows the visual morphologies of the damaged sites on the valve plug. It can be found that the shape of valve plug still remains intact, but local zones marked as A, B and C are severely damaged shown in Fig. 3a. The details of these local damaged zones are shown in Fig. 3b–d respectively. Different positions present different damage morphologies indicating different failure mechanisms. A ditch morphology is observed on the circumference of the conical part of the valve plug (Zone A in Fig. 3b) which results from erosion damage. While in zone B, a large area of materials is peeled uniformly in the circumferential direction, and some light craters locate at the boundary of this area, which may be caused by cavitation erosion (Fig. 3c). Completely different from the damage appearances aforementioned, round craters with different depth and sharp-edges distribute densely in zone C (Fig. 3d). Based on the different damage characteristics between the head (zone A) and end (zone C) of the valve plug, it can be deduced that the failure of the valve plug is caused by two different mechanisms at least. Similar damage morphologies were also reported by many references. Liu et al. [4] observed severe cavitation erosion in the belt eye of the valve used in the feedwater system of a nuclear power plant. Jin et al. [5] also reported that a regulating valve in coal liquefaction failed due to the combined effects of cavitation erosion and abrasion. Therefore, preliminary estimate can be made that erosion and cavitation erosion were included in the reasons causing material damage. Fig. 4 shows the digital images of the failed valve cage and the detail of the damaged zone. In Fig. 4a, severe “moat-like” material loss occurs next to the inlet of the valve cage. The damaged zone is coupled with the front end of zone B (shown in Fig. 3c),
Fig. 3. (a) General view of the failed valve plug, (b), (c) and (d) Detail of the damaged zone.
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Fig. 4. (a) General view of the failed valve cage, (b) Detail of the damaged zone.
which shows a similar circumferential cavitation erosion. It indicates that the damages in these positions of valve plug and valve cage were caused by the same reasons. However, cavitation erosion on the valve cage (shown in Fig. 4b) is much more severe than that on the valve plug (Fig. 3c), and results in the wall thinning of root of the outlet lip and consequently the fracture and removal of large material as shown in zone B in Fig. 4a.
3.2. Stereoscopy Fig. 5 shows the surface damaged morphologies of the valve plug along the flow direction using a stereo microscope. Fig. 5a is a magnification of the erosion morphology on the cone of the valve plug (Fig. 3b). It presents a typical ditch appearance with a length of approximately 4 mm caused by flow erosion [6,7]. Due to the erosion, severe material loss changes the size of plug head and enlarges the gap between the plug head and valve seat, which leads to the loss of the function of pressure regulating. Moreover, a distinct protuberant dividing line can be observed in the middle of the damaged zone which caused by the changing of the
Fig. 5. Stereoscopic views of the damaged surface on (a) the cone, (b and c) the head and (d) the end of the valve plug.
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valve openings under the operating conditions. Fig. 5b and c magnifies the erosion appearance in zone B marked in Fig. 3a. Small “river type” erosion traces and local craters can be observed, and the craters in the boundaries of zone B are similar to those at the end of the valve plug (shown in Fig. 5d), but with smaller size and depth. They are typical cavitation erosion morphologies [8,9]. The deep and sharp-edged craters locating in the end of the valve plug indicate severe cavitation erosion there. For the valve cage, the damage morphologies are the same as those on valve plug, but the material loss are more severe, as shown in Fig. 6. It indicates that the valve cage encountered the same damage and it was less resistant to the erosion and cavitation erosion. The erosion caused by fluid flushing occurs at the head of the valve cage (Fig. 6a), and then cavitation erosion dominates the places downstream the erosion sites (Fig. 6b–d), which is more severe than the erosion and mainly results in the wall thinning and fracture eventually. 3.3. Chemical compositions analysis The chemical compositions of the valve plug and cage were analyzed by an X-Ray Fluorescence Spectrometer, and the results are listed in Table 1. The standards of Deutsches Institut für Normung (DIN) were taken as reference to value materials' quality because the valve was imported from Germany. The results show that the chemical compositions of valve plug are in agreement with those specified in the DIN 17350 standard although the carbon and chromium content is slightly lower than the standard content. The small difference in the C content has few effects on the mechanical properties and the erosion resistance, and the effect of lower Cr content on corrosion resistance of valve plug to the contact mediums can be ignored, which will be analyzed in the later section. The element contents of the valve cage are entirely within the standard contents of DIN EN 10083-1. Therefore, it is reasonable to conclude that valve plug is eligible in the chemical composition as well as the valve cage. 3.4. Metallography It is well known that the microstructure determines the performances of the materials. Some literatures have proved that the presence of microstructural defects could accelerate the process of erosion and cavitation erosion [10,11]. Therefore, it is necessary to investigate the microstructure of valve materials, which is closely related to the resistance to erosion and cavitation erosion. Metallographic samples obtained from the damaged and undamaged zones were both examined with an optical microscope after metallography preparation and chemical etch. Fig. 7 shows the microstructures of different parts in the valve plug and valve cage. It can be observed that the microstructure is acicular tempered martensite for the valve plug (Fig. 7a, b) and tempered
Fig. 6. Stereoscopic views of the damaged surface on the valve cage along the flow direction.
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Table 1 Chemical compositions (mass %) of the regulating valve. Elements
C
Si
Mn
P
S
Cr
Mo
V
Ni
DIN 17350 Valve plug DIN EN 10083-1 Valve cage
0.37–0.42 0.36 0.26–0.33 0.30
0.9–1.2 1.03 0.15–0.40 0.23
0.30–0.50 0.49 0.30–0.60 0.39
≤0.03 0.015 ≤0.035 0.008
≤0.03 0.008 ≤0.035 0.003
5.00–5.50 4.84 1.80–2.20 2.01
1.20–1.50 1.27 0.30–0.50 0.39
0.90–1.10 0.93 – –
– – 1.80–2.20 1.96
sorbite for the valve cage (Fig. 7c, d). The structure is uniform with no defects and no remarkable differences are detected between the damaged zone (Fig. 7a) and the undamaged zone (Fig. 7b) of the valve plug, as well as that of the valve cage. Therefore, the failure of valve is not resulted from the materials defects.
3.5. SEM morphologies Fig. 8 displays surface morphologies of the damaged sites obtained by SEM on the valve plug. The sites selected for examination are corresponding to the zones B, C of Fig. 3a, respectively. On the head of valve plug wall, ditch morphologies in parallel with the flow direction mixed with light craters dominate the material surface, as shown in Fig. 8a. It indicates that the damage is mainly caused by the mechanical erosion. While in Fig. 8b, on the end of the valve plug, very deep and sharp-edged craters are observed indicating severe cavitation erosion. Fig. 9 shows surface morphologies of the damaged valve cage obtained by SEM. The damage morphologies are similar to those on the valve plug, i.e., ditch morphologies and obvious craters. From the magnification of the erosion region shown in Fig. 9b, some discontinuous ditch in the same direction as fluid flow can be observed, which indicates that the erosion is caused by the flow flushing at high velocity. Moreover, the inside surfaces of the big cavitation erosion craters are densely covered by small craters, as shown in Fig. 9c. It can be deduced that the big and deep craters result from continuing cavitation erosion. In
Fig. 7. Metallographic structures of the valve materials: (a) damaged zone on valve plug; (b) undamaged zone on valve plug; (c) damaged zone on valve cage; and (d) undamaged zone on valve cage.
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Fig. 8. SEM micrographs of the damaged valve plug: (a) the head of valve plug wall; (b) the end of valve plug wall.
addition to the damage characteristics, some particle-like residues stick to the surface as shown in Fig. 9c. EDS was conducted on these residues and the results show that the elements of C and O dominate the chemical compositions of the residues. It indicates that the residues are probably the residuals of chemical products instead of the corrosion products. There are probably two
Fig. 9. SEM micrographs of the damaged valve cage: (a) the connection region of the erosion and cavitation erosion; (b) the magnification of the erosion region; and (c) the magnification of the cavitation erosion region.
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reasons resulting in the absent of corrosion products. One reason is that the corrosion products were washed away by the fluid, the other one is that corrosion did not occur at all. Further analysis on the probability of corrosion causing damage will be carried out in the following section. 3.6. Evaluation on corrosivity of VA Fig. 10 shows the macroscopic views of the valve materials after immersion in VA solution for 85 days and in distilled water for 10 days. It can be found that the former still remain metallic luster without the corrosion traces in Fig. 10a and b. While the latter present obvious pit corrosion as shown in Fig. 10c and d. It is noted that these two surfaces have been washed by the deruster and obvious rust layers have been removed. It can be deduced that VA's corrosivity to the valve material is much weaker than distilled water. The pH-value of VA is approximately 5.54. However, the weak acidity does not play an important role in the corrosion according to the comparison results. Water content in VA is a key aspect affecting the solution corrosivity, which was maintained in an extremely low amount in the practical conditions. Therefore, the corrosion to the valve caused by VA can be ignored due to its limited effect. 3.7. CFD simulation Fig. 11 shows the pressure distributions in the BZ0 and BY15 sections of the valve under the condition of valve opening of 2 mm. In the section of BZ0 (Fig. 11a), it can be seen that the high pressure region is located in the valve seat. Most of the parts in the valve cage show relatively low and uniform pressure due to the small valve opening. In the section of the valve tip crossed by the inlet of the valve seat, high pressure distributes averagely in the circumferential direction, as shown in Fig. 11b. No pressure fluctuation is observed in the section around the tip of valve plug where cavitation erosion is difficult to
Fig. 10. Macroscopic views of the (a) valve plug and (b) valve cage after immersion in VA solution for 85 days; (c) valve plug and (d) valve cage after immersion in distilled water for 10 days.
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Fig. 11. Pressure distributions in the BZ0 and BY15 sections of the valve.
occur. The fact also proves that there are only ditch morphologies caused by the flow impingement erosion instead of craters morphologies caused by the cavitation erosion as shown in Fig. 3b. In contrast, the pressure in the BY60 section is not uniform and local high pressure occurs near the outlet of the valve cage facing the valve outlet (Fig. 12a). Bubbles are prior to be produced in the fluid with pressure fluctuation. They escape from the liquid fluid and flow with the steam when the environment pressure is lower than the equilibrium pressure between the air and the fluid. Finally, they will collapse after arriving in places where the environment pressure is higher than the equilibrium pressure of the two phases. Consequently, cavitation erosion occurs and the materials near the collapsing bubbles are removed, which is in accordance with the cavitation erosion morphologies shown in Fig. 12b. In addition, the level of cavitation erosion damage is not uniform in the circumferential directions of the valve plug due to the heterogeneous distribution of circumferential pressure, as shown in Fig. 3c and Fig. 12b. The most obvious cavitation erosion is located in the places near the end of the valve plug. Big pressure differences are observed in the section of BY120 as shown in Fig. 13a, which is prone to cause severe bubbles generation, collapse and cavitation erosion. It is in agreement with the characteristics of distributions of the cavitation erosion morphologies of the valve plug as shown in Fig. 13b-f. Not all the faces in the circumferential direction encountered the cavitation erosion like the one shown in Fig. 13b. Some faces have deeper and larger area craters (Fig. 13 c, d and f), while certain face has lighter and smaller area craters (Fig. 13e), which is dependent on the differences of the pressure shown in Fig. 13a. The regions with high pressure are more prone to encounter the cavitation erosion than those with low pressure, which contributes to the differences in damage degree. In addition, the space between the valve plug and valve cage is extremely narrow where the fluid is nearly stagnated. As a result, fluid erosion cannot occur there and cavitation erosion is responsible for the damage. Fig. 14 shows the velocity and trajectory distribution in the BZ0 section of the valve. It can be observed in the velocity contours (Fig. 14a) that the highest velocity appears near the tip of valve plug where the severe erosion resulting from the impact of the high velocity flow easily occurs. The sites on the valve facing high velocity are in agreement with those where flow erosion is located (Fig. 3b) indicating that the erosion is mainly caused by flow impingement. Moreover, after the fluid flows through the small gap between the valve plug and the valve seat, the open area increases suddenly resulting in negative pressure zones
Fig. 12. Pressure distribution in the BY60 section of the valve and the corresponding damaged zone of valve plug.
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Fig. 13. Pressure distributions in the BY120 section of the valve and the appearances in the corresponding damaged zones of valve plug.
and eddy (Fig. 14b). Correspondingly, erosion is easy to occur in the valve cage due to the flow impingement at multi impact angles of the turbulence [12]. In another view, the velocity and trajectory distribution in the BY60 section is revealed in Fig. 15 to better understand the damage mechanism. A high velocity region is observed near the outlet of the valve cage (Fig. 15a), which can prove that high velocity fluid erosion results in the materials remove at the corresponding place (Fig. 3c). The trajectory distribution shown in Fig. 15b also indicates that the flow inside is in a turbulent state suggesting high possibility of erosion.
4. Recommendations (1) Repair the failed valve plug with alloy coatings prepared by surface treatments, for example, Co-based alloy coatings prepared by laser cladding. (2) Replace the valve cage material with the materials with higher cavitation erosion resistance, such as NiTi alloy. (3) Control the water content of the fluid flowing through the valve strictly, avoiding the electrochemical corrosion of valve materials.
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Fig. 14. Velocity and trajectory distribution in the BZ0 section of the valve.
Fig. 15. Velocity and trajectory distributions in the BY60 section of the valve.
5. Conclusions Experimental examination and computational simulation were carried out to figure out the damage mechanism of the failed valve plug and valve cage. The following conclusions can be drawn: (1) Morphology analyses revealed that erosion dominates the damage on the head of the valve plug and the craters are for the end of the valve plug and the inlet of the valve cage. (2) The chemical compositions of valve plug and valve cage were in agreement with the material standard of Germany DIN 17350 and DIN EN 10083-1 respectively. Metallography analyses showed that no defects and remarkable differences were detected between the damaged zone and the undamaged zone of the valve plug and valve cage. (3) VA was barely corrosive to the valve materials due to its small water content, which is not the main reason causing valve failure. (4) Simulation results were well consistent with the damage sites and types of the valve, and explained the failure mechanisms combined with the physical analyses. The failure of the valve was caused by the combination effects of fluid erosion and cavitation erosion. Acknowledgements The authors would like to acknowledge the financial support of the National Natural Science Foundation of China (5157010762). References [1] P.M. Ma, L. Jiang, M. Hoch, W.F. Dong, M.Q. Chena, Reinforcement of transparent ethylene-co-vinyl acetate rubber by nanocrystalline cellulose, Eur. Polym. J. 66 (2015) 47–56. [2] F. Cavodeau, R. Sonnier, B. Otazaghine, J.M. Lopez-Cuesta, C. Delaite, Ethylene-vinyl acetate copolymer/aluminium trihydroxide composites: a new method to predict the barrier effect during cone calorimeter tests, Polym. Degrad. Stab. 120 (2015) 23–31. [3] A.M. Henderson, Ethylene-vinyl acetate (EVA) copolymers: a general review, IEEE. Electr. Insul. M. 9 (1993) 30–38.
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