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Macroscopic structural behavior on the small-scale PHE prototype under high-temperature test condition

Kee-Nam Songa, S-D Honga, H-Y Leea, H-Y Parkb*aaKorea Atomic Energy Research Institute, Rep. of Korea

bAD solution, Rep. of Korea

Abstract

PHE (Process Heat Exchanger) is a key component to transfer the high temperature heat generated from a VHTR (Very High Temperature Reactor) to the chemical reaction for massive production of hydrogen. Korea Atomic Energy Research Institute established a small-scale gas loop for the performance test of VHTR components and manufactured a PHE prototype in order to test in the small-scale gas loop. In this study, in order to investigate the macroscopic structural characteristics and behaviour of the PHE prototype under the test condition of the small-scale gas loop, we carried out high-temperature structural analysis modelling, thermal analysis, and an elastic/elastic-plastic structural analysis for the PHE prototype under the loop test conditions as a precedent study prior to the performance test in the small-scale gas loop. The results obtained in this study will be compared with the performance test results in the near future.

© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of [name organizer]

Keywords:Process heat exchanger, High-temperature structural analysis, Very high temperature reactor,

1. Introduction

Hydrogen is considered a promising future energy solution because it is clean, abundant and storable and has a high energy density. One of the major challenges in establishing a hydrogen economy is how to produce massive quantities of hydrogen in a clean, safe, and economical way. Among the various hydrogen production methods, hydrogen production using the high temperature heat from nuclear energy has been the focus of recent research [1]. Researches demonstrating the massive production of hydrogen using a VHTR

a Corresponding author. Tel.:+82-42-868-2254; fax:+82-42-868-0266. E-mail address:[email protected].

doi:10.1016/j.proeng.2011.04.011

Procedia Engineering 10 (2011) 48–56

1877-7058 © 2011 Published by Elsevier Ltd. Selection and peer-review under responsibility of ICM11

Open access under CC BY-NC-ND license.

Open access under CC BY-NC-ND license.

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Kee-Nam Song et al. / Procedia Engineering 10 (2011) 48–56 49

(Very High Temperature Reactor) designed for operation up to 950 have been actively carried out worldwide, including in the USA, Japan, France, and the Republic of Korea (ROK) [1,2,3].

The nuclear hydrogen program in the ROK is strongly considered to produce hydrogen through Sulfur-Iodine (SI) water-split hydrogen production processes. An intermediate loop that transports the nuclear heat to the hydrogen production process is necessitated for the nuclear hydrogen program as shown in Fig. 1. In the intermediate loop, whereas the HGD (Hot Gas Duct) provides a route of high temperature gas from the nuclear reactor to the IHX (Intermediate Heat Exchanger), the PHE (Process Heat Exchanger) is a component that utilizes the nuclear heat from the nuclear reactor to provide hydrogen. PHE is used in various processes such as nuclear steam reformation, nuclear methanol, nuclear steel, nuclear oil refinery, and nuclear steam [1, 4]. The PHE of the SO3 decomposer, which generates processed gases, such as H2O, O2, SO2, and SO3 at very high temperatures is a key component in the nuclear hydrogen program in ROK [5, 6].

Recently, KAERI (Korea Atomic Energy Research Institute) established a small-scale gas loop for the performance test of VHTR components and also manufactured a PHE prototype in order to be tested under the small-scale gas loop conditions. In this study, in order to investigate the macroscopic structural characteristics and behavior of the PHE prototype under the test conditions of the small-scale gas loop, FE (finite element) modeling, thermal analysis, and elastic/elastic-plastic structural analysis on the PHE prototype are carried out.

Fig. 1 Nuclear hydrogen system

2. FE modeling

2.1. Structure of a PHE prototype

A schematic view of the inside of the PHE prototype is illustrated in Fig. 2. The PHE prototype is designed as a hybrid concept [7] to meet the design pressure requirements between a nuclear system and hydrogen

50 Kee-Nam Song et al. / Procedia Engineering 10 (2011) 48–56

production system. That is to say, the hot helium gas channel has a compact semicircular shape, similar to a printed circuit heat exchanger, and is designed to withstand the high pressure difference between loops, while the sulfuric acid gas channel has a plate fin shape with sufficient space to install and replace the catalysts for sulfur trioxide decomposition.

All parts of the PHE prototype are made from Hastelloy-X of very high temperature material. Grooves of 1.0 mm diameter are machined into the flow plate for the primary coolant (helium gas). Waved channels are bent into the flow plate for the secondary coolant (SO3 gas). Twenty flow plates for the primary and secondary coolants are stacked in turn, and are bonded along the edge of the flow plate using a solid-state diffusion bonding method. After stacking and bonding the flow plates, the outside of the PHE is covered with a 3.0 mm thick Hastelloy-X plate. A schematic view of the PHE prototype with attached pipelines is shown in Fig. 3.

Fig. 2. Inside of the process heat exchanger Fig. 3. Process heat exchanger prototype

Fig. 4. Parts of a process heat exchanger prototype


2.2. FE modeling

Figure 4 shows each part of the PHE prototype based on a 3-D CAD modeling. Based on Fig. 4, FE modeling using I-DEAS/TMG Ver. 6.1 [8] is carried out, and thermal and thermal expansion/structural analyses are conducted using ABAQUS Ver. 6.8 [9]. For the sake of simplicity and understanding of the overall behavior of the PHE prototype, the FE model is composed of 546,764 2-D linear quadrilateral shell elements and 911,012 3-D linear solid elements made of 830,304 brick elements, 80,348 wedge elements, and 360 tetrahedron elements.

In/Out-flow of primary coolant The inflow of the primary coolant into the PHE prototype is shown in Fig. 5(a). The outflow of the primary

coolant from the PHE prototype, after the heat is transferred to the secondary coolant, is shown in Fig. 5(b).

(a) In-flow (b) Out-flow

Fig. 5. In/Out-flow of primary coolant

In/Out-flow of secondary coolant The inflow of the secondary coolant into the PHE prototype is shown in Fig. 6(a). The outflow of the

secondary coolant from the PHE prototype, after the heat is received from the primary coolant, is shown in Fig. 6(b).

(a) In-flow (b) Out-flow

Fig. 6. In/Out-flow of secondary coolant

Boundary condition for thermal analysis Figure 7 shows the boundary conditions of the primary/secondary flow plates for a thermal analysis under a test

condition of 850 .


Fig. 7. Boundary condition of primary/secondary coolant

3. Analysis

3.1. Thermal analysis

Figure 8 shows the thermal analysis results of the outside surface of the PHE prototype under the fixed test conditions of the small-scale gas loop. According to Fig. 8(a), the temperature distribution is nearly symmetrical along the vertical axis, and the maximum temperature of the outside surface is about 837.15 .

Fig. 8. Temperature distribution of the PHE outside surface

A high temperature structural analysis was performed based on the thermal analysis results. The material properties of Hastelloy-X are from Ref. 10. The most important aspect of properly predicting the stress and deformation of the PHE prototype under high temperature is to determine how to impose the displacement


constraint condition at each end of the primary/secondary flow pipelines. In this study, after a search of the various boundary conditions [11-13], we chose a spring constraint condition for each pipeline end as shown in Fig. 9. We also assumed a 1.0 m length for each pipeline in order to apply the spring stiffness in the high-temperature structural model of the PHE prototype.

Fig. 9. Boundary condition with spring stiffness

3.2. Structural analysis

To investigate the macroscopic structural characteristics and behavior of the PHE prototype under the test conditions of the small-scale gas loop, an elastic structural analysis was first carried out. Next, an elastic-plastic structural analysis was conducted to understand the characteristics and behavior of the PHE prototype in detail.

Elastic structural analysis Figure 10(a) shows the stress distribution under temperature/pressure at the pressure boundary of the PHE

prototype. Considering the stress on the PHE pressure boundary, a maximum local stress of 1,009.8 MPa occurs on the outer surface of the PHE prototype near the connection between the secondary out-flow pipeline and the PHE, as shown in Fig. 10(a). Also, the maximum stress along the thickness through stress linearization is about 937.4 MPa, which far exceeds the yield stress of the material (291 MPa at 500 ) [10]. Judging from these structural analysis results, a measure to strengthen the structural integrity of the PHE prototype should be ascertained.

Figure 10(b) shows the stress distribution under temperature at the pressure boundary of the PHE prototype. Considering the stress on the PHE pressure boundary, a maximum local stress of 1,011.3 MPa occurs on the outer surface of the PHE prototype near the connection between the secondary out-flow pipeline and the PHE prototype as shown in Fig. 10(b). The stress levels and distribution under temperature are nearly the same as those under temperature and pressure when compared with Fig. 10 (a) and (b). Also, the linearized maximum stress along the thickness is about 938.8 MPa, which far exceeds the yield stress of the material (291 MPa at 500 ). Judging from these structural analysis results, a measure to strengthen the structural integrity of the PHE prototype should be discovered.

Looking at Fig. 10, a very high stress level of over 1,400 MPa occurs near the end of the primary in-flow


pipeline even if there is not a pressure boundary of the PHE prototype. It is assumed that this high stress level is attributed to an insufficient spring stiffness boundary condition at the end of the primary pipeline along with an excessive thickness of the pipeline.

(a) under temperature and pressure (b) under only temperature Fig. 10. Structural analysis results

Figure 11 shows the stress distribution under pressure (3.0 MPa for primary coolant; 0.1 MPa for secondary coolant) at the pressure boundary of the PHE prototype. Considering the stress on the PHE pressure boundary, a maximum local stress of 9.65MPa occurs on the inner surface edge of the PHE prototype top plate near the primary out-flow pipeline as shown in Fig. 11, which is sufficiently below the yield stress of the material (291MPa at 500 ). This is a very interesting observation because, from the viewpoint of structural integrity, pressure is far less influential than temperature. Also, the location of maximum stress under pressure is different than under temperature/pressure. That is to say, the location of maximum stress under temperature/pressure or temperature alone is near the inside connection between the secondary in-flow pipeline and the PHE prototype, as indicated in “A” in Fig. 11, while the location of maximum stress under pressure alone occurs on the inner surface edge of the top plate near the primary out-flow pipeline.

Fig. 11. Structural analysis results under pressure

For the sake of comparison under the above-mentioned conditions, stress linearization is also carried out along the thickness at “A” in Fig. 11. The linearized maximum stress along the thickness at “A” is about 0.67MPa, which is sufficiently below the yield stress of the material (291MPa at 500 ).

Also, temperature is far more influential on the structural integrity of the PHE prototype than pressure when we compare the structural analysis results in Figs. 10 and 11. Thus, thermal expansion and thermal stress are very important for the structural integrity evaluation of the PHE prototype in the small-scale gas loop.


Elastic-plastic structural analysis Figure 12 shows the bilinear stress-strain curve for an elastic-plastic structural analysis [10]. Figure 13(a)

shows the stress distribution under temperature/pressure at the pressure boundary of the PHE prototype. Considering the stress on the PHE pressure boundary, a maximum local stress of 393.7 MPa occurs on the inner surface of the PHE prototype near the connection between the secondary in-flow pipeline and the PHE prototype, which is significantly reduced when compared with the elastic structural analysis results in Fig. 10. The maximum stress along the thickness through stress linearization is then about 407.5 MPa, which far exceeds the yield stress of the material (291 MPa at 500 ) [10].

Equivalent plastic strain on the PHE pressure boundary is shown in Fig. 13(b). According to Fig. 13(b),most of the PHE pressure boundary remains in the elastic region, but a maximum equivalent plastic strain of 0.0355 occurs on the outer surface of the PHE prototype near the connection between the secondary out-flow pipeline and the PHE prototype. Therefore, a measure to modulate the creep-fatigue damage of the PHE prototype should be discovered, taking into consideration precedent researches [14, 15, and 16].

Fig. 12. Bilinear stress-strain curve for elastic-plastic analysis

(a) Stress distribution result (b) Equivalent plastic strain distribution results

Fig. 13. Structural analysis results under temperature and pressure

4. Conclusions

To discover the high-temperature structural integrity of the PHE prototype prior to the actual performance test, FE modeling, thermal analysis, and high-temperature elastic and elastic-plastic structural analyses were carried out under the test conditions of the small-scale gas loop established at KAERI. As a result of these


analyses, we drew the following conclusions. 1. Under temperature/pressure, the maximum stress at the pressure boundary of the PHE prototype from

the elastic structural analysis is much higher than the yield stress of Hastelloy-X. Thus, a way to strengthen the structural integrity of the PHE prototype should be discovered.

2. Under temperature/pressure, the maximum equivalent plastic strain at the pressure boundary of the PHE prototype from the elastic-plastic structural analysis is very high, even if most of the PHE pressure boundary remains in the elastic region. Therefore, a way to modulate the creep-fatigue damage of the PHE prototype should be ascertained.

3. Temperature is a far more influential on the structural integrity of the PHE prototype than pressure. Therefore, thermal expansion and thermal stress are very important for the structural integrity evaluation of the PHE prototype in the small-scale gas loop.

Acknowledgements

This work was supported by the Nuclear Research & Development Program of the Korea Science and Engineering Foundation (KOSEF) - a grant funded by the Korean government (MEST). (grant code: M20701010002-08M0101-00210)

References

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Production System 2008; R.O.K Patent # 10-0877574. [8] I-DEAS/TMG Analysis User Manual Version 6.1 2009. [9] ABAQUS Analysis User’s Manual, Version 6.8 2009. [10] Hastelloy-X Alloy website, www.haynesintl.com. [11] Song, K. N., Input Data for Thermal Analysis of a Small-sized Process Heat Exchanger Prototype, 2010; Calculation note No.

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of KSME 2010; 34 (10), p. 1453-1460. [14] Lee, H. Y. et al., Preliminary Application of the Draft Code Case for Alloy 617 for a High Temperature Component, Journal of

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Accepted for publication in Journal of Pressure Vessel Technology.

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