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Sealing behavior of the HTR-10 pressure vessel flanges
Nuclear Engineering and Design 216 (2002) 247– 253 www.elsevier.com/locate/nucengdes
Technical note
Sealing behavior of the HTR-10 pressure vessel flanges Suyuan Yu *, Junjie Liu, Weidong Zuo, Shuyan He Institute of Nuclear Energy Technology, Tsinghua Uni6ersity, Beijing 100084, China Received 15 March 2001; received in revised form 2 July 2001; accepted 30 January 2002
Abstract The flanges of 10 MW high temperature gas-cooled reactor (HTR-10) pressure vessel play an important role in sealing the primary coolant of Helium. They are bolt-connected with a metallic O-ring and a welded V-ring. An elastic–plastic nonlinear analysis was performed to evaluate the stress and deformation of the contact flanges with the finite element software of MSC MARC 2000. The multi-step loading process was employed to simulate the processes of pre-tightening and pressurizing of the HTR-10 pressure vessel. The structural effects of the flanges on the opening and the shifting of the HTR-10 pressure vessel flanges at the O-ring position were studied to determine the flange height and the head closure thickness. The good sealing performance of the O-ring and the V-ring was verified both numerically and experimentally. The finite element model analysis results compared well with the hydraulic test of the HTR-10 pressure vessel. The results show that the flanges can meet the strength requirement and that the O-ring and the V-ring can effectively seal the HTR-10 pressure vessel during both pre-tightening and pressurizing. © 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Nuclear power must be retained as a viable option to meet present and future energy supply needs. To achieve this goal, the US Department of Energy (DOE) has proposed the development of Generation IV nuclear power systems that are economically competitive in many markets while making further advances in safety, waste minimization, and proliferation resistance. The high temperature gas-cooled reactor (HTGR) is one of the advanced power systems that can meet the * Corresponding author. Address: Neng Ke Lou (INET), 104 Tsinghua University, Beijing 100084, China. Tel.: + 8610-6278-4823; fax: +86-10-6277-1150. E-mail address: [email protected] (S. Yu).
requirements of the Generation IV nuclear power systems (DOE, 2000). In order to develop commercial HTGRs, the Chinese government set up a 10MW high temperature reactor (HTR-10) project in 1986. The objective of the HTR-10 is to test the inherent safety of the modular HTGR and to develop the ability to supply industrial process heat with a high temperature reactor. The key technologies for HTR-10 have been studied since 1986. The HTR-10 construction started in 1992 at the Institute of Nuclear Energy Technology (INET), Tsinghua University (Xu, 1999). It reached criticality on December 21, 2000. The pressure vessel is an important part of the HTR-10 that prevents the costly helium from leaking from the reactor core. Especially the pressure vessel has a relative large diameter and rela-
0029-5493/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 9 - 5 4 9 3 ( 0 2 ) 0 0 0 5 5 - 9
248
S. Yu et al. / Nuclear Engineering and Design 216 (2002) 247–253
tive small wall thickness. Therefore the pressure vessel flanges are the key structures that must very reliably seal as well as meet the reactor strength requirements during the pre-tightening and pressurizing conditions. The flange structure design is most important. Two structural factors affect the sealing behavior: the form of the sealing structure and the size of the flanges and the head closure. For the sealing form, double hollow O-rings have been widely used for flange sealing of most PWR and BWR reactor pressure vessels and many studies have been focused on such seals. Bazergui did the milestone tests studying the major parameters such as the stress, deformation and contact of several kinds of gaskets (George and Leon, 1988). Kobayashi (1999) analyzed O-ring deformation characteristics with finite element model and compared with an experiment. Bouzid and Michel (1999) pointed that the seal behavior relies very much on the contact stress. Qu (1997) experimentally analyzed the thermal sealing characteristics of the pressure vessel. The HTR-10 is a pebblebed HTGR and it is not necessary to open the closure head of the pressure vessel to charge the fuel because it is the on-line fuel of the pebbles and can continually charge and discharge fuel elements. Thus HTR-10 flanges can use a welded V-ring to seal the pressure vessel as well as a metallic O-ring, which is an unique sealing advantage for pebble bed HTGRs over other nuclear reactors. Another important factor affecting the sealing characteristics is the flange height and head closure thickness. Yu (2000) analytically designed the sealing structure for 200 MW low temperature nuclear heating reactor and carefully analyzed the effects of flange dimensions on sealing characteristic. But his analysis was only based on the linear elastic method and ignored the effect of the contact stress. However, the flange seal surface with the O-ring must have some contact areas where plastic deformation occurs due to the local contact stress. Such contact plastic deformation plays a significant role in the sealing behavior. In early studies, the flanges were considered as rigid (Pindera and Sze, 1970). With the development of the finite element method and enhanced computing power, a single finite element model can now
simultaneously consider both the closure head and the vessel shell, so researchers can pay more attention to the elastoplastic effects of the contact surfaces on the sealing behavior. Therefore, a complete non-linear analysis was used to study the effects of the flange sealing structure, the flange height and the head closure thickness on the sealing behavior of HTR-10 pressure vessel.
2. Contact analysis method The sealing of the HTR-10 pressure vessel mainly depends on the O-ring sealing. The opening displacement at the O-ring between the flange seal surfaces is compensated by the rebound of the pressed O-ring. If the rebounding distance of the O-ring is smaller than the opening of the flange seal surfaces, the O-ring will loss its sealing ability. In addition, if the relative radial shifting distance of the flange seal surfaces is too large, the shifting will damage the O-ring surface allowing primary coolant to leak into the inside of the welded V-ring. In either case, the sealing function of the O-ring will fail but the welded V-ring will provide the secondary sealing. This is the sealing principle designed into the HTR-10 pressure vessel. Fig. 1 shows the pressure vessel of 10 MW high temperature gas cooled reactor. It consists of the head closure, upper flange, lower flange, vessel shell and bottom closure. The HTR-10 vessel inner diameter is 4.2 m and the wall thickness is 0.08 m. Its height is 11.16 m. The design pressure is 3.5 MPa and the design temperature is 350 °C. The shell material is SA516-70 and the flange material is 15MnNi. In such a case, it is relative easy for such pressure vessel to deform and the successful flange sealing is very important. Fig. 2 illustrates its flange bolt connection and sealing structure. The upper and lower flanges are joined with 80 bolts (M80× 4). It is sealed with a welded V-ring and a metallic O-ring. An axisymmetric finite element model with 1369 elements and 1597 nodes was used to analyze the flange deformation when the HTR-10 pressure vessel is pre-tightened and pressurized. Some simplifications were made in forming the
S. Yu et al. / Nuclear Engineering and Design 216 (2002) 247–253
Fig. 1. The pressure vessel of 10 MW high temperature gas cooled reactor.
mesh. The flange width is decreased uniformly to simulate the flange stiffness reduction by the bolt holes with the volume removed from the flange is equal to the total volume of the bolt holes (ASME, 1973). The counterforce of the O-ring is a circumference linear load. The pre-tightening
Fig. 2. Flange connection of HTR-10 pressure vessel.
249
force of the bolts is equivalent to a circumference load along the circle of the bolt hole centers (Toshiyuki, 1999). The ideal elastoplastic model was used in the numerical analysis and the Von Mises rule was adopted as the yield criterion. The stainless steel cladding at the flange seal surface was defined as the contact surface with consideration of the deformation and contact stress. An increased bolt pre-tightening force increases the contact surface stress and the plastic deformation. A fine mesh at the contact surface was used accurately calculate the elastoplastic stresses and deformations. The numerical analysis had two stages with multi-step loading process: the first stage simulated the pretension with 20-step loading; while the second stage simulated the pressurization with 5 steps. The stress and deformation of the flanges were calculated at each step together with the opening and shifting of the flange seal surface. The analysis was calculated using the finite element software MARC 2000/MSC (MSC, 2000). The elements were all 4-node axisymmetric quadrangles. Several different calculations verified that the calculating precision was high enough to assure the accuracy of the contact stress and deformation calculation.
3. Sealing behavior of the pressure vessel flanges
3.1. Rebound of the O-ring As the primary sealing, the O-ring was studied first. The rebound distance of the O-ring is an important part of the HTR-10 pressure vessel seal. The hollow Inconel O-ring produced by the Advanced Company is used in the HTR-10 pressure vessel. The prescriptive compression ratio of the O-ring is 12% and the least rebound distance is 0.36 mm. The rebound distance used in the design was 10% less and the safety coefficient for the O-ring was 1.6. Thus the O-ring rebound distance 0.2 mm was the opening limit of the flange seal surface at the O-ring to maintain the seal function in all cases. Another important factor is to minimize the opening and the shifting of the flange seal surface at the O-ring position.
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S. Yu et al. / Nuclear Engineering and Design 216 (2002) 247–253
3.3. Stress strength of the welded d-ring
Fig. 3. Variation of flange opening and shift with flange height.
3.2. Influence of the structural size The structural size of the closure has the most significant effect on the sealing behavior, such as the opening and the shifting of flange seal surfaces. The closure head of the HTR-10 pressure vessel is semispherical. The flange width is 258 mm to meet the stress strength requirement and the economic limit. In this design, the flange height and the closure head thickness have the most influence on the opening and shifting of the flange seal surfaces. The contact elastoplastic analysis was performed to study this influence. Fig. 3 shows the relationship of the seal surface opening, shifting with the flange height. Increasing flange heights increases the flange stiffness that reduces the opening of flange seal surfaces at the O-ring. The shifting of the flange seal surfaces varies in a similar way. However, for the flange height greater than 700 mm, the influence is less significant. Increasing the closure head thickness increases the closure head stiffness that reduces both the opening and the shifting of the seal surfaces as in Fig. 4. However the mechanical behavior of the material deteriorates and the production cost increased with larger closure head thickness. Therefore, the closure head thickness cannot be too large. The flange height of the HTR-10 pressure vessel was chosen as 600 mm and its closure head thickness was chosen as 90 mm.
The pulse pneumatic fuel handling system makes it possible for the HTR-10 flanges to use a welded V-ring to seal the pressure vessel in addition to the metallic O-ring. This is a unique sealing advantage for the pebble bed HTGRs over BWRs and PWRs. As the secondary seal, the welded V-ring prevents the costly helium from leaking into the reactor cavity and provides a leak signal capability in case leakage occurs. The Vring material is 15MnNi with the yield stress of 360 Mpa (ASME, 1995). The safety coefficient is 1.6 for Class I components. The elastic limit is 225 MPa. During the pre-tension process, the relative displacement between the sealing surfaces is very small. In addition, the stress and distortion of the V-ring is axially symmetric. The largest equivalent von Mises stress is 150 MPa in the middle of the V-ring at the weld line (Fig. 5). The pressurization process increases the relative displacement between the sealing surfaces with the largest stress at the bottom edges of the V-ring where it is connected to the flange. The largest equivalent von Mises stress is 193 Mpa (Fig. 6). Both cases show that the V-ring is in the linear elasticity state and that the welded V-ring can maintain sealing in the HTR-10 pressure vessel in case the O-ring fails.
4. Comparison with hydraulic pressure test and other reactor pressure vessels The hydraulic pressure test was finished in 1997 before the welding of the V-ring (INET, 1998).
Fig. 4. Variation of flange opening and shift with head closure thickness.
S. Yu et al. / Nuclear Engineering and Design 216 (2002) 247–253
Fig. 5. Stress of the V-ring at the pre-tension.
One objective of the test was to verify the sealing ability of the O-ring and the flange seal surfaces. After the test successfully remained at the pressure of 5.6 MPa and the temperature of 25 °C for 10 min, the test pressure was remained at 3.5 MPa
251
with the temperature of 25 °C to measure the pressure vessel stress and deformation including the radial displacements of the outer flange surface and the axial displacements between the flange seal surfaces. The radial position of the axial displacement measurements between the sealing surfaces was around a circle with radius of 2338 mm. A finite element analysis was done in order to compare with the measurements. The analysis was at the pressure of 3.5 MPa for pretighting and the temperature of 25 °C. The experimental results are compared with the numerical results in Table 1. They well agree with each other which verifies the numerical contact analysis. To evaluate the sealing behavior, four typical PWR pressure vessels with double sealing O-rings are compared with the HTR-10 pressure vessel in Table 2. The HTR-10 sealing characteristics compare very well. The opening and shifting of the flange seal surfaces are relatively small. The rebound distance of the O-ring used in the HTR-10 pressure vessel is 0.2 mm which is an order of magnitude larger than the opening of the flange seal surfaces at the O-ring. Because the four typical PWR pressure vessels have good sealing behavior, the seal structure design for HTR-10 pressure vessel is reasonable.
5. Conclusions
Fig. 6. Stress of the V-ring at the pressurization.
The sealing components in the HTR-10 pressure vessel consist of a metallic O-ring and a welded V-ring. An elastoplastic nonlinear contact analysis between the two flanges was used to evaluate the stress and deformation of the flanges. The plastic deformation of the flange seal surfaces is large enough to affect the sealing behavior. The multi-step loading process was utilized to simulate the pre-tightening and pressurizing processes of the HTR-10 pressure vessel. Increased flange height and the closure head thickness both reduced the opening and the shifting of the flange seal surfaces, which favors the vessel seal. The O-ring can meet the sealing requirements for both the pre-tightening and the pressurization. The welded V-ring which acts as the secondary seal
Experimental result Numerical result
Bottom of the shell flange
0.351 0.33
0.43 0.37
−0.337
−0.28
0.45
0.483 −0.0014
−0.00164
Closure flange
Top of the shell flange
Top of the closure flange
Bottom of the closure flange
Distortion angle
Radial displacement of the flange outer surface (mm)
Table 1 Comparison of experimental results with numerical results
−0.00023
−0.000251
Shell flange
0.391
0.326
Axial displacement at a radius of 2238 (mm)
252 S. Yu et al. / Nuclear Engineering and Design 216 (2002) 247–253
S. Yu et al. / Nuclear Engineering and Design 216 (2002) 247–253
253
Table 2 Comparison of opening and shifting of flange seal surfaces Pressure vessel
HTR-10 HR-200 (Yu, 2000) PWR PWR at Daya Bay PWR at Qinshan (Qu, 1987)
Power (MW)
10 (t) 200 (t) 1300 (e) 900 (e) 300 (e)
works in the linear elasticity stress state in case the O-ring fails. The numerical results compared well with the results of a hydraulic pressure test to verify the numerical method. Similar to the typical BWRs, the HTR-10 pressure vessel also has quite safe sealing behavior during both the pretightening and pressurizing.
Acknowledgements The authors greatly appreciate the financial support from the Chinese 863 High Technology Project in energy field.
References ASME, 1973. Stress analysis of bolted flanges for pressure vessels. First International Conference on Pressure Vessel Technology: Part 1, New York. ASME, 1995. ASME CODE, Vol. III, Subsection 1, Appendix. Bouzid Abdel-Hakim, Michel Derenne, 1999. A simple method for analyzing the contact stress in bolted flange joints with non-linear gaskets. Analysis of bolted joints 1999, The 1999 ASME pressure vessels and piping conference Boston, Massachusetts, pp. 103 –111. DOE, 2000. Discussion on Goals for Generation IV Nuclear Power Systems — from a workshop held on May 1 –3,
Shift (mm)
0.155 0.20 0.39 0.74 1.00
Opening (mm) Pre-tightening
Pressurization
−0.008 0.034 0.048 0.053 0.045
0.016 0.010 0.015 0.056 0.156
Bethesda, Maryland, USA. George F., Leon P.E., 1988. An Overview of the US PVRC research program on bolted flanged connections, Pressure Vessel Technology Vol. 1 Design & Analysis, Pergamon Press. INET, December 1998. Hydraulic Test of HTR-10 Pressure Vessel. Tsinghua University. Kobayashi Takashi, 1999. Deformation characteristics of metal hollow O-ring. Analysis of bolted joints 1999, the 1999 ASME pressure vessels and piping conference Boston, Massachusetts, pp. 103 – 111. MSC, 2000. Marc 2000, User’s Guide. MSC Software Corporation, USA. Pindera J.T., Sze Y., 1970. Studies of physical and mathematical models of some flanged connections, proceedings of the fourth international conference on experimental stress analysis, Cambridge, pp. 396 – 408. Qu, Jiadi, 1987. Special research on sealing behaviour for reactor vessel of 300 Mwe nuclear power plant. Ch. J. Nuc. Sci. Eng. 7 (3 – 4), 193 – 201. Qu, Jiadi, 1997. Performance measurements for silver-plating O-ring made of GH169 under ambient and high temperatures. Pressure Vessel Technol. 14 (3), 3 – 8. Toshiyuki, S.A.W.A., 1999. Stress analysis and the new gasket factors of pipe flange connections with spiral wound gaskets subjected to internal pressure. Analysis of bolted joints 1999, the 1999 ASME pressure vessels and piping conference Boston, Massachusetts, pp. 63 – 71. Xu Yuanhui, 1999. High temperature gas-cooled reactor program in China. Proceedings of the fourth nuclear energy symposium March 15 – 16, Taoyuan, China, pp. 25 – 32. Yu, Suyuan, 2000. Analytical design for the flange sealing structure of LTNHR-200. Nucl. Power Eng. 21 (10), 222 – 224.
Technical note
Sealing behavior of the HTR-10 pressure vessel flanges Suyuan Yu *, Junjie Liu, Weidong Zuo, Shuyan He Institute of Nuclear Energy Technology, Tsinghua Uni6ersity, Beijing 100084, China Received 15 March 2001; received in revised form 2 July 2001; accepted 30 January 2002
Abstract The flanges of 10 MW high temperature gas-cooled reactor (HTR-10) pressure vessel play an important role in sealing the primary coolant of Helium. They are bolt-connected with a metallic O-ring and a welded V-ring. An elastic–plastic nonlinear analysis was performed to evaluate the stress and deformation of the contact flanges with the finite element software of MSC MARC 2000. The multi-step loading process was employed to simulate the processes of pre-tightening and pressurizing of the HTR-10 pressure vessel. The structural effects of the flanges on the opening and the shifting of the HTR-10 pressure vessel flanges at the O-ring position were studied to determine the flange height and the head closure thickness. The good sealing performance of the O-ring and the V-ring was verified both numerically and experimentally. The finite element model analysis results compared well with the hydraulic test of the HTR-10 pressure vessel. The results show that the flanges can meet the strength requirement and that the O-ring and the V-ring can effectively seal the HTR-10 pressure vessel during both pre-tightening and pressurizing. © 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Nuclear power must be retained as a viable option to meet present and future energy supply needs. To achieve this goal, the US Department of Energy (DOE) has proposed the development of Generation IV nuclear power systems that are economically competitive in many markets while making further advances in safety, waste minimization, and proliferation resistance. The high temperature gas-cooled reactor (HTGR) is one of the advanced power systems that can meet the * Corresponding author. Address: Neng Ke Lou (INET), 104 Tsinghua University, Beijing 100084, China. Tel.: + 8610-6278-4823; fax: +86-10-6277-1150. E-mail address: [email protected] (S. Yu).
requirements of the Generation IV nuclear power systems (DOE, 2000). In order to develop commercial HTGRs, the Chinese government set up a 10MW high temperature reactor (HTR-10) project in 1986. The objective of the HTR-10 is to test the inherent safety of the modular HTGR and to develop the ability to supply industrial process heat with a high temperature reactor. The key technologies for HTR-10 have been studied since 1986. The HTR-10 construction started in 1992 at the Institute of Nuclear Energy Technology (INET), Tsinghua University (Xu, 1999). It reached criticality on December 21, 2000. The pressure vessel is an important part of the HTR-10 that prevents the costly helium from leaking from the reactor core. Especially the pressure vessel has a relative large diameter and rela-
0029-5493/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 9 - 5 4 9 3 ( 0 2 ) 0 0 0 5 5 - 9
248
S. Yu et al. / Nuclear Engineering and Design 216 (2002) 247–253
tive small wall thickness. Therefore the pressure vessel flanges are the key structures that must very reliably seal as well as meet the reactor strength requirements during the pre-tightening and pressurizing conditions. The flange structure design is most important. Two structural factors affect the sealing behavior: the form of the sealing structure and the size of the flanges and the head closure. For the sealing form, double hollow O-rings have been widely used for flange sealing of most PWR and BWR reactor pressure vessels and many studies have been focused on such seals. Bazergui did the milestone tests studying the major parameters such as the stress, deformation and contact of several kinds of gaskets (George and Leon, 1988). Kobayashi (1999) analyzed O-ring deformation characteristics with finite element model and compared with an experiment. Bouzid and Michel (1999) pointed that the seal behavior relies very much on the contact stress. Qu (1997) experimentally analyzed the thermal sealing characteristics of the pressure vessel. The HTR-10 is a pebblebed HTGR and it is not necessary to open the closure head of the pressure vessel to charge the fuel because it is the on-line fuel of the pebbles and can continually charge and discharge fuel elements. Thus HTR-10 flanges can use a welded V-ring to seal the pressure vessel as well as a metallic O-ring, which is an unique sealing advantage for pebble bed HTGRs over other nuclear reactors. Another important factor affecting the sealing characteristics is the flange height and head closure thickness. Yu (2000) analytically designed the sealing structure for 200 MW low temperature nuclear heating reactor and carefully analyzed the effects of flange dimensions on sealing characteristic. But his analysis was only based on the linear elastic method and ignored the effect of the contact stress. However, the flange seal surface with the O-ring must have some contact areas where plastic deformation occurs due to the local contact stress. Such contact plastic deformation plays a significant role in the sealing behavior. In early studies, the flanges were considered as rigid (Pindera and Sze, 1970). With the development of the finite element method and enhanced computing power, a single finite element model can now
simultaneously consider both the closure head and the vessel shell, so researchers can pay more attention to the elastoplastic effects of the contact surfaces on the sealing behavior. Therefore, a complete non-linear analysis was used to study the effects of the flange sealing structure, the flange height and the head closure thickness on the sealing behavior of HTR-10 pressure vessel.
2. Contact analysis method The sealing of the HTR-10 pressure vessel mainly depends on the O-ring sealing. The opening displacement at the O-ring between the flange seal surfaces is compensated by the rebound of the pressed O-ring. If the rebounding distance of the O-ring is smaller than the opening of the flange seal surfaces, the O-ring will loss its sealing ability. In addition, if the relative radial shifting distance of the flange seal surfaces is too large, the shifting will damage the O-ring surface allowing primary coolant to leak into the inside of the welded V-ring. In either case, the sealing function of the O-ring will fail but the welded V-ring will provide the secondary sealing. This is the sealing principle designed into the HTR-10 pressure vessel. Fig. 1 shows the pressure vessel of 10 MW high temperature gas cooled reactor. It consists of the head closure, upper flange, lower flange, vessel shell and bottom closure. The HTR-10 vessel inner diameter is 4.2 m and the wall thickness is 0.08 m. Its height is 11.16 m. The design pressure is 3.5 MPa and the design temperature is 350 °C. The shell material is SA516-70 and the flange material is 15MnNi. In such a case, it is relative easy for such pressure vessel to deform and the successful flange sealing is very important. Fig. 2 illustrates its flange bolt connection and sealing structure. The upper and lower flanges are joined with 80 bolts (M80× 4). It is sealed with a welded V-ring and a metallic O-ring. An axisymmetric finite element model with 1369 elements and 1597 nodes was used to analyze the flange deformation when the HTR-10 pressure vessel is pre-tightened and pressurized. Some simplifications were made in forming the
S. Yu et al. / Nuclear Engineering and Design 216 (2002) 247–253
Fig. 1. The pressure vessel of 10 MW high temperature gas cooled reactor.
mesh. The flange width is decreased uniformly to simulate the flange stiffness reduction by the bolt holes with the volume removed from the flange is equal to the total volume of the bolt holes (ASME, 1973). The counterforce of the O-ring is a circumference linear load. The pre-tightening
Fig. 2. Flange connection of HTR-10 pressure vessel.
249
force of the bolts is equivalent to a circumference load along the circle of the bolt hole centers (Toshiyuki, 1999). The ideal elastoplastic model was used in the numerical analysis and the Von Mises rule was adopted as the yield criterion. The stainless steel cladding at the flange seal surface was defined as the contact surface with consideration of the deformation and contact stress. An increased bolt pre-tightening force increases the contact surface stress and the plastic deformation. A fine mesh at the contact surface was used accurately calculate the elastoplastic stresses and deformations. The numerical analysis had two stages with multi-step loading process: the first stage simulated the pretension with 20-step loading; while the second stage simulated the pressurization with 5 steps. The stress and deformation of the flanges were calculated at each step together with the opening and shifting of the flange seal surface. The analysis was calculated using the finite element software MARC 2000/MSC (MSC, 2000). The elements were all 4-node axisymmetric quadrangles. Several different calculations verified that the calculating precision was high enough to assure the accuracy of the contact stress and deformation calculation.
3. Sealing behavior of the pressure vessel flanges
3.1. Rebound of the O-ring As the primary sealing, the O-ring was studied first. The rebound distance of the O-ring is an important part of the HTR-10 pressure vessel seal. The hollow Inconel O-ring produced by the Advanced Company is used in the HTR-10 pressure vessel. The prescriptive compression ratio of the O-ring is 12% and the least rebound distance is 0.36 mm. The rebound distance used in the design was 10% less and the safety coefficient for the O-ring was 1.6. Thus the O-ring rebound distance 0.2 mm was the opening limit of the flange seal surface at the O-ring to maintain the seal function in all cases. Another important factor is to minimize the opening and the shifting of the flange seal surface at the O-ring position.
250
S. Yu et al. / Nuclear Engineering and Design 216 (2002) 247–253
3.3. Stress strength of the welded d-ring
Fig. 3. Variation of flange opening and shift with flange height.
3.2. Influence of the structural size The structural size of the closure has the most significant effect on the sealing behavior, such as the opening and the shifting of flange seal surfaces. The closure head of the HTR-10 pressure vessel is semispherical. The flange width is 258 mm to meet the stress strength requirement and the economic limit. In this design, the flange height and the closure head thickness have the most influence on the opening and shifting of the flange seal surfaces. The contact elastoplastic analysis was performed to study this influence. Fig. 3 shows the relationship of the seal surface opening, shifting with the flange height. Increasing flange heights increases the flange stiffness that reduces the opening of flange seal surfaces at the O-ring. The shifting of the flange seal surfaces varies in a similar way. However, for the flange height greater than 700 mm, the influence is less significant. Increasing the closure head thickness increases the closure head stiffness that reduces both the opening and the shifting of the seal surfaces as in Fig. 4. However the mechanical behavior of the material deteriorates and the production cost increased with larger closure head thickness. Therefore, the closure head thickness cannot be too large. The flange height of the HTR-10 pressure vessel was chosen as 600 mm and its closure head thickness was chosen as 90 mm.
The pulse pneumatic fuel handling system makes it possible for the HTR-10 flanges to use a welded V-ring to seal the pressure vessel in addition to the metallic O-ring. This is a unique sealing advantage for the pebble bed HTGRs over BWRs and PWRs. As the secondary seal, the welded V-ring prevents the costly helium from leaking into the reactor cavity and provides a leak signal capability in case leakage occurs. The Vring material is 15MnNi with the yield stress of 360 Mpa (ASME, 1995). The safety coefficient is 1.6 for Class I components. The elastic limit is 225 MPa. During the pre-tension process, the relative displacement between the sealing surfaces is very small. In addition, the stress and distortion of the V-ring is axially symmetric. The largest equivalent von Mises stress is 150 MPa in the middle of the V-ring at the weld line (Fig. 5). The pressurization process increases the relative displacement between the sealing surfaces with the largest stress at the bottom edges of the V-ring where it is connected to the flange. The largest equivalent von Mises stress is 193 Mpa (Fig. 6). Both cases show that the V-ring is in the linear elasticity state and that the welded V-ring can maintain sealing in the HTR-10 pressure vessel in case the O-ring fails.
4. Comparison with hydraulic pressure test and other reactor pressure vessels The hydraulic pressure test was finished in 1997 before the welding of the V-ring (INET, 1998).
Fig. 4. Variation of flange opening and shift with head closure thickness.
S. Yu et al. / Nuclear Engineering and Design 216 (2002) 247–253
Fig. 5. Stress of the V-ring at the pre-tension.
One objective of the test was to verify the sealing ability of the O-ring and the flange seal surfaces. After the test successfully remained at the pressure of 5.6 MPa and the temperature of 25 °C for 10 min, the test pressure was remained at 3.5 MPa
251
with the temperature of 25 °C to measure the pressure vessel stress and deformation including the radial displacements of the outer flange surface and the axial displacements between the flange seal surfaces. The radial position of the axial displacement measurements between the sealing surfaces was around a circle with radius of 2338 mm. A finite element analysis was done in order to compare with the measurements. The analysis was at the pressure of 3.5 MPa for pretighting and the temperature of 25 °C. The experimental results are compared with the numerical results in Table 1. They well agree with each other which verifies the numerical contact analysis. To evaluate the sealing behavior, four typical PWR pressure vessels with double sealing O-rings are compared with the HTR-10 pressure vessel in Table 2. The HTR-10 sealing characteristics compare very well. The opening and shifting of the flange seal surfaces are relatively small. The rebound distance of the O-ring used in the HTR-10 pressure vessel is 0.2 mm which is an order of magnitude larger than the opening of the flange seal surfaces at the O-ring. Because the four typical PWR pressure vessels have good sealing behavior, the seal structure design for HTR-10 pressure vessel is reasonable.
5. Conclusions
Fig. 6. Stress of the V-ring at the pressurization.
The sealing components in the HTR-10 pressure vessel consist of a metallic O-ring and a welded V-ring. An elastoplastic nonlinear contact analysis between the two flanges was used to evaluate the stress and deformation of the flanges. The plastic deformation of the flange seal surfaces is large enough to affect the sealing behavior. The multi-step loading process was utilized to simulate the pre-tightening and pressurizing processes of the HTR-10 pressure vessel. Increased flange height and the closure head thickness both reduced the opening and the shifting of the flange seal surfaces, which favors the vessel seal. The O-ring can meet the sealing requirements for both the pre-tightening and the pressurization. The welded V-ring which acts as the secondary seal
Experimental result Numerical result
Bottom of the shell flange
0.351 0.33
0.43 0.37
−0.337
−0.28
0.45
0.483 −0.0014
−0.00164
Closure flange
Top of the shell flange
Top of the closure flange
Bottom of the closure flange
Distortion angle
Radial displacement of the flange outer surface (mm)
Table 1 Comparison of experimental results with numerical results
−0.00023
−0.000251
Shell flange
0.391
0.326
Axial displacement at a radius of 2238 (mm)
252 S. Yu et al. / Nuclear Engineering and Design 216 (2002) 247–253
S. Yu et al. / Nuclear Engineering and Design 216 (2002) 247–253
253
Table 2 Comparison of opening and shifting of flange seal surfaces Pressure vessel
HTR-10 HR-200 (Yu, 2000) PWR PWR at Daya Bay PWR at Qinshan (Qu, 1987)
Power (MW)
10 (t) 200 (t) 1300 (e) 900 (e) 300 (e)
works in the linear elasticity stress state in case the O-ring fails. The numerical results compared well with the results of a hydraulic pressure test to verify the numerical method. Similar to the typical BWRs, the HTR-10 pressure vessel also has quite safe sealing behavior during both the pretightening and pressurizing.
Acknowledgements The authors greatly appreciate the financial support from the Chinese 863 High Technology Project in energy field.
References ASME, 1973. Stress analysis of bolted flanges for pressure vessels. First International Conference on Pressure Vessel Technology: Part 1, New York. ASME, 1995. ASME CODE, Vol. III, Subsection 1, Appendix. Bouzid Abdel-Hakim, Michel Derenne, 1999. A simple method for analyzing the contact stress in bolted flange joints with non-linear gaskets. Analysis of bolted joints 1999, The 1999 ASME pressure vessels and piping conference Boston, Massachusetts, pp. 103 –111. DOE, 2000. Discussion on Goals for Generation IV Nuclear Power Systems — from a workshop held on May 1 –3,
Shift (mm)
0.155 0.20 0.39 0.74 1.00
Opening (mm) Pre-tightening
Pressurization
−0.008 0.034 0.048 0.053 0.045
0.016 0.010 0.015 0.056 0.156
Bethesda, Maryland, USA. George F., Leon P.E., 1988. An Overview of the US PVRC research program on bolted flanged connections, Pressure Vessel Technology Vol. 1 Design & Analysis, Pergamon Press. INET, December 1998. Hydraulic Test of HTR-10 Pressure Vessel. Tsinghua University. Kobayashi Takashi, 1999. Deformation characteristics of metal hollow O-ring. Analysis of bolted joints 1999, the 1999 ASME pressure vessels and piping conference Boston, Massachusetts, pp. 103 – 111. MSC, 2000. Marc 2000, User’s Guide. MSC Software Corporation, USA. Pindera J.T., Sze Y., 1970. Studies of physical and mathematical models of some flanged connections, proceedings of the fourth international conference on experimental stress analysis, Cambridge, pp. 396 – 408. Qu, Jiadi, 1987. Special research on sealing behaviour for reactor vessel of 300 Mwe nuclear power plant. Ch. J. Nuc. Sci. Eng. 7 (3 – 4), 193 – 201. Qu, Jiadi, 1997. Performance measurements for silver-plating O-ring made of GH169 under ambient and high temperatures. Pressure Vessel Technol. 14 (3), 3 – 8. Toshiyuki, S.A.W.A., 1999. Stress analysis and the new gasket factors of pipe flange connections with spiral wound gaskets subjected to internal pressure. Analysis of bolted joints 1999, the 1999 ASME pressure vessels and piping conference Boston, Massachusetts, pp. 63 – 71. Xu Yuanhui, 1999. High temperature gas-cooled reactor program in China. Proceedings of the fourth nuclear energy symposium March 15 – 16, Taoyuan, China, pp. 25 – 32. Yu, Suyuan, 2000. Analytical design for the flange sealing structure of LTNHR-200. Nucl. Power Eng. 21 (10), 222 – 224.