Numerical analysis of the heat transfer for packing design of cryogenic gate valve Si Pom Kim 1,a, Rock Won Jeon 1,b, Il Ju Hwang 1,c, Jae Hoon Lee 1,d, Won Heaop Shin 2,e 1 Department of Mechanical Engineering 2 NK CO., LTD. Dong-A University Busan, 604-714 KOREA a spkim@dau.ac.kr, b rockwoni@naver.com, c hij10@hanmail.net, d dhxowkd31019@naver.com, e s31934@naver.com Abstract: The packing, among the components comprising the gate valve, is used to sustain the air-tightness and the study on change of shape or pattern has been carried out to maximize the functions, but the study on changing the location or the size of the packing in a bid to prevent the freezing has rarely been implemented. Thus, this study is intended to evaluate the thermal strain of packing by heat transfer from territory of extremely low temperature as well as the temperature distribution to the upper part of the packing using numerical analysis method. Key-Words: Liquefied Natural Gas, Cryogenic, Packing, Heat transfer, Numerical analysis 1 Introduction A recent date, the demand of crude oil and NG gas carriers are increasing due to growing energy demand of world countries, and especially, because the natural gas almost doesn't exhaust carbon dioxide and sulphurous acid oxide, etc. in case of combustion of natural gas, it is getting attention as alternative energy of petroleum. But because it is difficult to transport or store natural gas in a gas state due to very big volume, the equipment transporting and storing the liquefied natural gas gradually increases, and as the application weight of natural gas gradually increases, the demand of equipment controlling this and transportation equipment is increasing[1][2]. Out of the valves that are installed in large quantities, gate valve is structured in a way that when the handle is spun, the stem moves in a vertical manner, pushing the disk downwards to either seal fluid and vapor or allow them to pass through. The body of a valve that is used under cryogenic temperatures is made from stainless steel and so as to prevent the freezing of nuts and bolts, the bonnet and the stem is longer than normal valves [3]. Among the parts that comprise a gate valve, in order to maximize the capability of packing which is used to maintain air tightness, researches are focused on the outer shape or pattern but not on its capability to contain cold air or the location and the size of such. Most of the nations around the world are giving much attention to secure energy for economic development, but what is as much important is energy saving. On top of that, as researches to improve the capability of valves are under way, this paper intends to take a look into the heat transfer and the efficiency according to the location and the length of a packing under cryogenic temperatures. The LNG gate valve used under cryogenic temperatures is a concise model and was used as a basic data for the design of a packing not intended for airtightness but for controlling internal temperature, using the ANSYS, a numerical analysis program to conduct thermal analysis. 2 Theory The finite element equation on three-dimensional load is the same as the equation (1)[4]. (1) ISBN: 978-1-61804-084-8 232
The stress and strain is the same as the equation (2), and here, D is a stiffness matrix, and ε is a strain vector (2) In addition, the relationship equation between strain and displacement can be shown like the equation (3). Here, is a case that there is an elastic body in the boundary, is when the displacement was given to the boundary, and is a stress vector on the boundary. The equation (7) was obtained by substituting the equation (6) for the equation (5), and it is as follows. (7) The finite element equation is the same as the equation(8), and the stiffness matrix K and external force vector f can be respectively shown like the equation(9) and equation(10). (8) (3) In order to numerically analyze the finite element method, if the equation (3) is made as a Weak form, it is the same as the equation (4). (10) (9) (4) If the theorem of Green is used, it can be shown like the equation (5), and e is a virtual strain vector and Γ means a boundary. 3 Numerical Analysis Its numerical analysis was carried out by using ANSYS, which is a commercial program. To take a closer look at the packing of a gate valve, the complex upper part of the valve and the part where the actual fluid flow occurs were simplified, and the part that could have an effect on the packing was turned into a simpler model and then applied for analysis. The gate valve model that was used for the numerical analysis comprised of upper valve, stem, bonnet, packing and gasket are shown in the Fig. 1. The size of the simplified valve used for the numerical analysis is shown in Table. 1. (5) The stress vector is shown like the equation (6) to substitute a boundary condition. (6) Fig. 1 Analysis model of gate valve ISBN: 978-1-61804-084-8 233
Table 1 Condition of simple gate valve Stem diameter 30mm Packing diameter 40mm Length of stem 500mm Length of packing 200mm The convective heat transfer coefficient condition for thermal analysis used were 9 for the outside of cylindrical shape and 8 for globular shape. Inside the globular shape, assuming the effects of LNG, a temperature of -162c was set. Fig. 2 shows the creation of mesh used in this research. As for the grid system, a tetrahedron mesh was used and in the packing part where heat transfer was crucial, more mesh were concentrated. (a) body (b) packing and stem Fig. 2 Grid system and model of gate Valve The analysis condition was 3-d transient thermal, applying -162C as the inside temperature of the valve and convection condition of a cylinder and a globe on the outside. As the actual use environment of the valve considered in the model was the almost stagnant air around the vertical cylinder, by introducing appropriate heat transfer coefficient and conducting approximate calculation, the computation of the air heat transfer can be easily done. But in order to increase the accuracy and to predict phenomenon of condensation and defrosting, momentum and energy equation of the air was applied for the analysis. For the research on the differences of inside and outside temperature that have effects on the gate valve under cryogenic temperatures, after selecting a standard model, thermal-structure FSI analysis regarding the direction of the length, and the location and the material of the packing. 3.1 Numerical Analysis on the change in length The sizes of the packing for each condition are shown in table. 2. Regarding the effects of length, an interval of 10mm was selected for the thermal analysis with the fixed condition shown in table. 3. Numerical results for the change of length is shown in Fig. 3 is shown. Table 2 Variables of simple gate valve Variables Note 10mm 20mm 30mm 40mm 50mm 60mm 70mm 80mm 90mm 100mm Length 110mm 120mm 130mm 140mm 150mm 160mm 170mm 180mm 190mm 200mm Polyethylene Material Teflon Silicon Top Position Middle Bottom Table 3 Condition of length model Convective heat transfer Cylinder 8W/m 2 k coefficient Globe 9W/m 2 k Temperature Inside -162 Outside 22 Length of model without packing Constant Variable Length ISBN: 978-1-61804-084-8 234
research, an experiment research and a numerical analysis research on the deformation of a packing as its length changes. Fig. 3 Temperature variation according to the length Looking at the graph, the gradient is near zero from the point of 180mm. At the basic length of 200m, which is at the 90 % of the total length, it can be confirmed that the temperature is maintained. Therefore, based on the internal temperature of the packing, the optimal length for the temperature upper part of the valve to be maintained at room temperature is 180mm. Also, thermal-structure FSI analysis for the thermal strain of the valve due to heat transfer was conducted. Fig 4. shows the result value when the packing is 40mm long. Fig. 5 is a graph of the numerical analysis. 3.2 Numerical Analysis on the location of packing Regarding the effects of the location of packing, a numerical analysis regarding packing of same length and material located on the top, middle and bottom was conducted and the fixed condition is shown in table. 4. And Fig. 6, Fig. 7 are result of analysis. Table 4 condition of position model Convective heat Cylinder 8W/m 2 k Transfer coefficient Globe 9W/m 2 k Temperature Inside -162 Outside 22 Length of model without packing Constant Variable Position of packing (a) Analysis of top position Fig. 4 Elastic strain of case by 40mm (b) Analysis of middle position Fig. 5 Temperature variation according to the deformation Transferring the data into graph reveals that the deformation increases in line with the increase in length. Based on the analysis, at the 80mm and 150mm point, the deformation decreases. But as the length increases from 180mm to 190mm, the deformation greatly increases. For a more accurate (c) Analysis of bottom position Fig. 6 Analysis results of change with position ISBN: 978-1-61804-084-8 235
Fig. 7 Temperature variation according to the position (c) Silicon Fig. 8 Analysis results of material model Looking at (a), (b) and (c), it is better to locate the packing in the middle in order to increase the effect of such 3.3 Numerical Analysis on the material In a condition where a packing is full, numerical analysis applying boundary condition of table. 5 was conducted. Fig. 8 and Fig. 9 are result of analysis. Table 5 condition of material model Convective heat Cylinder 8W/m 2 k transfer coefficient Globe 9W/m 2 k Temperature Inside -162 Outside 22 Length of model without packing Constant Variable Material Polyethylene Teflon Silicon Fig. 9 Temperature variation according to the material The result revealed that when comparing the packing made from Teflon with the two packing of different materials, the Teflon one excelled and based on such, the aforementioned numerical analysis was re-conducted with the packing made from the three materials. The result of the numerical analysis on the material which applied the best combination of conditions such as the length and the location, is shown in Fig. 10. And Fig. 11 is result of analysis. (a) 180mm-middle-Teflon (a) Teflon (b) Polyethylene (b) 180mm-middle-Polyethylene ISBN: 978-1-61804-084-8 236
(c) 180mm-middle-Silicon Fig. 10 Analysis results of optimization model to the temperature difference rising from the contact between LNG of -162C and the air of room temperature could be found. 4. After analyzing the 20 models regarding length, 3 models regarding location and 3 models regarding materials confirmed that when the packing is at 90% length of the insert part of the packing, located at the middle and made from teflon showed the greatest temperature control capability. 5. Acknowledgment This work was supported by Technical Centre for High-Performance Valves from the Regional Innovation Centre (RIC) Program of the Ministry of Knowledge Economy (MKE). Fig. 11 Temperature variation according to the optimization model Based on the result, the length of the packing is the 90% of the length of the insert part of the packing which is also the 90% length of the packing used currently. And as for the location, the middle of the insert part with the material of Teflon was the best combination for the best packing. 4 Conclusion In this research, the heat transfer characteristic for the best packing of cryogenic gate valve used for LNG transfer was analyzed using the numerical analysis method. 1. Conducting numerical analysis on the internal temperature distribution and deformation according to the length of packing revealed that the efficiency gradually culminated from the 90% area. 2. Numerical analysis on the internal temperature distribution according to the location and the material of the packing showed that location and material have effects on the temperature distribution. 3. The thermal analysis under -162C revealed that the upper part of the stem maintains room temperature due to the room temperature itself and the convective heat transfer. At the middle part of the stem, deformation of the packing due References: [1] Eleftherakis, John G., Determining valve contaminant sensitivity effect using two contaminants, SAE TP 910960, 1991, pp459~464 [2] Thompson G, Askari, A.R., Air leak detection through ball plug valves by vibration monitoring, Nosie & Vibration control world wide, V17, pp140~143, 1986 [3] Merati, P., Macelt, M.J, 2001, Flow investigate around a v-sector ball valve, ASME Fluids engineering, Vol. 123, No.3, 2001, pp. 662-671. [4] Sang-Kyu Bae, Dong-Soo Kim, Hyun-sub Kim, A characteristic analysis of high pressure cryogenic ball valve for LNG, Mechanical engineering congress, 2006, pp. 3411-3416. [5] Young-Chul Park, Han-Seok Park, Si-pom Kim, Analysis method on Structural Safety Evaluation of Butterfly Valve of Piping for LNG carrier, Journal of the Korean Society of Manufacturing Process Engineers, Vol. 7, No. 4, 2008, pp. 76~81. [6] Dong-Kyoon Kim, Jeong-Hwan Kim, A Study on Structural Analysis of Globe Valve for LNG Carrier, Journal of the Korean Society of Marine Engineering, Vol.31, No.8, 2007, pp. 1013~1019. [7] Jae-Ung Cho, Moon-Sik Han, A Study on Flow Analysis at Ball Valve according to Opening and Shutting Angle, Journal of the Korean Society of Manufacturing Process Engineers, Vol. 10, No. 2, 2011, pp. 46~51. ISBN: 978-1-61804-084-8 237