KR970005076B1 - Method for manufacturing a membrane for a gas tank - Google Patents

Abstract

The membrane structure is a ring knot membrane having a ring knot separated from a rumple in the part of a board member. The membrane structure includes four rumples(10) having a front indented part(11) in the straight line towards an anchor bolt fixing hole(9) from an outer edge(1) of the board member(a), and the ring knot(2) being apart from the front indented part(11) at a predetermined distance and having a ring-shaped outer surface(5) and a ring-shaped inner surface(4) symmetrical to the ring-shaped outer surface(5).

Description

Membrane Structure for LNG Tank and Manufacturing Method

1 to 5 show a conventional membrane structure,

1 is a perspective view showing a membrane structure of a single wrinkled honeycomb structured type.

2 is a perspective view showing a membrane structure of a combination of triangular and square wrinkles.

3 is a perspective view showing a single triangular pleated membrane structure.

4 is a perspective view showing a single triangular pleated membrane structure.

5 is a perspective view of a bi-flat corrugated membrane structure.

6 is a perspective view showing a ring joint membrane structure.

Figure 7 is a perspective view of the separated ring joint membrane structure of the present invention.

8 is a plan view of FIG.

9 is a finite element network of ring-node membranes.

10 is a boundary condition diagram of a membrane.

11 shows the equivalent von mouse stresses of ring-node membranes.

FIG. 12 is the equivalent von mouse stress at the top of the separated ring-node membrane type 1 according to the present invention.

Figure 13 is an equivalent von mouse stress at the neutral plane of the separated ring-node membrane type 1 according to the present invention.

14 is an equivalent von mouse stress on the top surface of a separate ring-node membrane type 2 according to the present invention.

Figure 15 is an equivalent von mouse stress at the neutral plane of the separated ring-node membrane type 2 according to the present invention.

FIG. 16 is an equivalent von mouse stress on the top surface of an isolated ring-node membrane type 3 according to the present invention.

FIG. 17 is the equivalent von mouse stress at the neutral plane of the separated ring-node membrane type 3 according to the present invention.

18 is an equivalent von mouse stress on the top surface of an isolated ring-node membrane type 4 according to the present invention.

19 is an equivalent von mouse stress at the neutral plane of the separated ring-node membrane type 4 according to the present invention.

20 is coordinates.

FIG. 21 is a stress distribution along line AB of FIG. 20 of a ring-node membrane.

22 is a stress distribution diagram along line AB of FIG. 20 of the separated ring-node membrane type 1 according to the present invention.

23 is a stress distribution diagram along line AB of FIG. 20 of the separated ring-node membrane type 2 according to the present invention.

FIG. 24 is a stress distribution diagram along line AB of FIG. 20 of the separated ring-node membrane type 3 according to the present invention. FIG.

25 is a stress distribution diagram along line AB of FIG. 20 of the separated ring-node membrane type 4 according to the present invention.

FIG. 26 is a stress distribution along the AC line of FIG. 20 of a ring-node membrane. FIG.

FIG. 27 is a stress distribution diagram along the AC line of FIG. 20 of the separated ring-node membrane type 1 according to the present invention. FIG.

FIG. 28 is a stress distribution along the AC line of FIG. 20 of the separated ring-node membrane type 2 according to the present invention. FIG.

FIG. 29 is a stress distribution diagram along the AC line of FIG. 20 of the separated ring-node membrane type 3 according to the present invention. FIG.

30 is a stress distribution along the AC line of FIG. 20 of the separated ring-node membrane type 4 according to the present invention.

* Explanation of symbols for main parts of the drawings

A: Plate 1: Sheet edge

2: ring node 3: ring node top

4: inside ring node 5: outside ring node

6: ring node end 9: anchor bolt fixing hole

10: corrugation 11: shear bend

The present invention relates to an improved structure and manufacturing method of a membrane for a liquefied natural gas storage tank, and more particularly to a ring knot membrane in which a ring node and a pleat are combined with each other in a sheet.

Generally, liquefied natural gas is generally stored in a large tank as a cryogenic liquid (Cryogenic liqukd) having a boiling point of -162 ° C under atmospheric pressure, and the storage tank is configured as a vertical cylindrical double structure. The inner tank membrane is made of material resistant to low temperature brittleness, and the outer tank side plate is made of PC (Prestressed concrete) or carbon steel, and insulation material is interposed between them to evaporate due to heat intrusion from outside the tank. Manufacture so that gas (Boil-off gas) is minimized.

Membrane in cryogenic storage tank refers to a special plate designed and manufactured so that the material can expand and contract freely with changes in temperature and load.It is installed on the side and bottom of the tank to prevent leakage of cryogenic liquid. It is primarily responsible for the sealing function, and absorbs the load generated by the liquid pressure or the self-weight while transferring the fatigue cyclic load due to the thermal load and delivers it to the single panel (Insulation panel).

As a conventional membrane, the corrugated plate known from Japanese Patent Application Publication No. 50-21008 shown in FIG. 1 has a continuous hexagonal (honeycomb) corrugation structure that must absorb expansion contraction due to temperature change. It is a corrugated plate which forms a Y-shaped intersection with each corrugation having an angle of 120 °. The single corrugated honeycomb structure of this structure has the disadvantage that stress analysis is difficult, fabrication by press working is difficult, and residual stress exists.

In addition, the non-curvature low temperature tank known from Japanese Patent Application Publication No. 60-14959 shown in FIG. 2 has a triangular wrinkle having a triangular cross section, a trapezoidal wrinkle orthogonal to the triangular wrinkle, and an orthogonal portion. The three sides of the trapezoidal wrinkles are placed on the top surface of the trapezoidal wrinkles, and the three sides of each of the three sides that face each other and the tops of the trapezoidal wrinkles are parallel to each other, and the three sides of the triangles are bent and formed without oversized area. The quadrilateral tetrahedral shaped plate body is provided, but the combination of the triangular and square wrinkles has a difficult structure in stress analysis, residual stress, expensive manufacturing cost, and difficulty in forming.

In addition, the expansion and contraction structure of the membrane known from Japanese Patent Application Publication No. 60-32079 shown in FIG. 3 has branched arrangements in which at least one gathering portion has wrinkles protruding from the membrane surface of the low temperature liquefied gas tank. As the stretched structure, the gathering portion is bent in a shape in which the upper portion is raised with respect to the flat plate portion, and the upper portion and the corrugation are joined to connect the bent surfaces of both sides and are welded, but the stress analysis is difficult. Stress concentration occurs, residual stresses exist, and there is difficulty in manufacturing.

In addition, the single triangular wrinkle membrane known from Japanese Patent Application Publication No. 62-12439 shown in FIG. 4 is a type of expansion joint in which the expansion joint crosses the same height at the intersection of the expansion joint. And the corners are divided by S-shaped bends, Although there are expansion joint methods such as mold spiral winding and mold with concave, stress analysis is difficult, stress concentration occurs, residual stress exists, and there is difficulty in manufacturing.

In addition, the membrane structure known from Japanese Patent Application Publication No. 62-46759 shown in Fig. 5 is a shape of a bending wave with gentle cross-sections on one side and the other side of the unit corrugation, and gentle bending on both ends of the longitudinal section. It is formed to form a surface connected to the flat plate portion, and has a surface shape protruding smoothly three-dimensionally with respect to the flat plate, and the unit membrane mutually welded, but there is a stress concentration phenomenon and residual stress , Rotational heat shrinkage occurred.

As described above, many kinds of conventional membranes are known, but the problems are classified in detail by type.

The first corrugated membrane (FIG. 2) having a structure in which a triangular fold and a trapezoidal corrugation cross each other has a problem that many stress concentrations occur at corners where the bottom and the wall meet and sharp curves of the corrugation.

In addition, the second double-row wrinkle membrane (figure 5) having a structure in which two parallel corrugations cross each other at right angles with a stainless steel plate has been subjected to rotational contraction due to the absorption action of the heat deflection of the parallel parallel wrinkles. Behavior occurs, and since the rotational shrinkage deformation acts symmetrically in opposite directions, wrinkle deformation is not a problem because it is designed to balance each other, but if thermal deformation is unbalanced by external conditions, Locally intense thermal stress concentrations can occur, presenting significant problems. However, welding, molding, and stress analysis are easier than the first corrugated membrane. Since the radius of curvature at the corners of the membrane is relatively large and gentle, there is less risk of local stress concentrations in this region than in the first corrugated membrane.

In addition, the third corrugated membrane welded on the top of the corrugation is formed by forming the end of the plate into a flanged bent shape, and then contacting the top portions in a butt-shaped form, that is, a membrane formed by welding and joining two sheets of the welded joint. It is easy to manufacture, but the stability of the quality can be secured only when the molding and welding technology is very excellent.

In addition, the fourth membrane (Non-corrugation membrane) without wrinkles in the membrane plate (non-corrugation membrane) is a membrane using an Invar (Invar) material having a very low coefficient of thermal expansion is mainly used in LNG carriers. Invar materials have very high heat shrinkage at cryogenic temperatures, but are limited in terms of high price and difficulty in purchasing them.

In order to improve the above-mentioned conventional problems of the present invention, the membrane structure according to an embodiment of the present invention is a straight line from the outer edge of the plate to the anchor bolt fixing hole and has four bent wrinkles having a curved shear bent portion And an annular outer surface formed at a predetermined distance from the shear bent portion and an annular inner surface symmetrical with the annular outer surface, and a ring node formed at a predetermined distance from the anchor bolt fixing hole.

In the above example, the heights raised from the four corrugations and the plate of the ring joint may be the same. Also in the above example, the cross section of the four pleats and ring nodes may be approximately semi-circular, or elliptical, polygonal in which the transverse diameter is narrower than the longitudinal diameter.

Furthermore, the unit board thickness may be thicker than the thickness in the panel board.

The present invention also proposes a method for manufacturing a membrane structure, which according to an example is directed in a straight line from the outer edge of the plate toward the anchor bolt fixing hole, the front end of the four cross-shaped corrugation and the annular inner and annular Press-processing a ring node composed of an outer surface, punching a hole for inserting the unit plate into the panel sheet, and equalizing the four corrugations of the unit sheet in the horizontal and vertical directions with respect to the hole of the panel sheet; Bending the pleats, and inserting the unit board into the punched holes of the panel board to fit the ends of each pleat, and welding the edges of the unit board to the panel board. The welding step is preferably an overlap welding method.

An object of the present invention is to provide a convenient and simple structure in manufacturing as a symmetrical form by combining the ring node and the pleats.

It is also an object of the present invention to provide a very practical and stable membrane by increasing the wrinkle curvature slightly decreases the height of the wrinkles when the pressure and temperature load act on the membrane.

It is also an object of the present invention to maintain a uniform stress distribution and low stress levels when pressure is applied to the membrane.

Embodiments of the present invention are shown in FIGS. 7 and 8. First, the unit membrane plate material A is provided in a square shape. This square shape is a shape for convenience of description, and is usually circular. Since the membrane is in cryogenic conditions and subjected to pulsating fluid pressure loads, stress deformation behavior analysis is very important in terms of material selection and design. As the membrane material, stainless steel, aluminum alloy, Invar, 9% Ni steel, etc. are used, and in the case of SUS material, austenitic stainless steel having a thickness of 1.2 to 2.0 mm is most used. Four corrugations 10 are formed in a straight line toward the anchor bolt fixing hole 9 at approximately the center of the outer edge 2 of the plate material A of a predetermined size. Each of the four corrugations has the same cross section and has the same curvature. The curvature of the left side 12 and the right side 13 of each corrugation is the same and the front end of the corrugation is bent and connected to the plate. In this way, four wrinkles 10 having a predetermined curvature are formed in the plate A so that the cross-sectional neutral axis of the plate A is stretched so that no stress is generated. The four pleats 10 are formed by shear bending portions 11 at predetermined diameters from the anchor bolt fixing holes 9 so as not to cross each other so that the pleats do not directly cross each other. The ring joint 2 of the present invention is composed of an annular inner surface 4 and an annular outer surface 5 which is symmetrical with the annular inner surface 4 at predetermined intervals from the anchor bolt fixing hole 9 and the pleats 10 ) Is formed by pressing. It is important that the annular ring segment 2 is configured apart from the four corrugations 10 and the curvature of the annular inner surface 4 and the outer surface 5 is the same. The heights raised from the four corrugations and the plate of the ring node, that is, the height from the flat plate A to the top of the corrugation and the top of the ring node are preferably equal. The cross section of the four corrugations and ring nodes may be approximately semi-circular, or elliptical, or polygonal, with a transverse diameter narrower than the longitudinal diameter. Thus, the unit board according to the present invention is easy to manufacture by taking the symmetrical form as a whole.

The panel board, not shown, is made up of the unit boards. This panel board material is attached to the side wall and bottom wall of a tank. The panel board member is punched with holes for accommodating the unit board member, and is bent to form corrugations in the horizontal and vertical directions based on the center points of the holes. On the other hand, the above-mentioned unit board is preferably formed by press working in order to manufacture wrinkles and ring joints integrally. When the unit board is completed in this way, it is put in the hole of the panel board material and welded to the edge of the unit board material to fix the unit board and the panel board material. In addition, in order to reduce the cost of the plate, the plate thickness of the unit plate is preferably thicker than the plate thickness of the panel plate. For example, 2 mm stainless steel plate may be used for the unit plate material for press working, and 1.2 mm stainless steel plate may be used for the panel plate material for bending.

Therefore, the method of manufacturing the membrane according to the present invention includes four corrugations 10 that crosswise from the outer edge of the unit plate A toward the anchor bolt fixing hole 9, the annular inner surface 4 and the annular outer surface. Press-processing the ring node 2 composed of (5), and punching holes at regular intervals in the panel plate formed according to the size and shape of the tank at regular intervals; Bending and forming the panel board in the horizontal and vertical directions about the center point of the punched hole to form a wrinkle of the same shape as the unit board, and inserting the unit board into the hole of the panel board After aligning the ends of the unit plate is composed of the step of welding the edge portion to the panel plate. The welding step is preferably an overlap welding method.

In addition, the membrane plate washed for the membrane welding is welded to the edge and the edge portion to inspect all the membrane plate partially assembled into a set of membrane panel to be sent to the storage tank construction site to perform the installation work.

There are two types of welding used when installing the membrane plate in the storage tank, circumferential welding of anchor point using automatic welding machine and overlapping welding of membrane plate and plate. The joint of the single corrugated ring knot membrane steel sheet according to the present invention is joined by adopting the conventional Lap welding and Plug welding method, and the unit board and the panel board Are joined by overlap welding. As such, when the membrane structure is manufactured by overlap welding of the unit plate and the panel plate, the unit plate can be formed by pressing and the panel plate can be formed by bending. The characteristics of the membrane manufactured by the bonding method utilizing the advantages of the two processing methods are as follows.

① Bending of panel plate material can greatly alleviate residual stress and stress concentration phenomenon. When press working, residual stress occurs due to non-uniform changes in the processing thickness, especially in the processing part, so processing such as annealing should be performed, but bending processing does not require such additional processing.

② The mold to be pressed may be simpler and smaller. When press working to the panel board, the entire membrane must be mold-processed, but here, only the unit board is press-processed, which makes the process relatively easy.

③ 2mm stainless steel plate is used for the unit plate that needs to be press processed, and 1.2mm stainless steel plate is used for the panel plate that is bent, which opens up the possibility of saving significant material.

The present invention has the simplest structure and is excellent enough to have a uniform stress distribution and minimum stress level compared to other conventional membranes. In particular, when the pressure and temperature loads are applied, the wrinkle curvature is slightly larger but the wrinkle height is reduced. It is very practical and stable.

Comparative example

The stress behavior analysis of the membrane structure has relied on experimental analysis since it is very difficult to theoretically analyze because of the complex three-dimensional shape of the membrane knot.

The present invention analyzes the stress deformation behavior of the membrane, which is the most important structure in cryogenic tanks, by using the finite element method, and compares and reviews the existing model to overcome the existing problems and develop a new type of membrane model and optimize the design of the model. To provide. Considering the geometric symmetry of the membrane plate, the model was divided into quarters and three-dimensional behavior analysis was performed. Membrane plates were analyzed by finite enzyme analysis considering the effects of the uniform distribution load acting perpendicular to the corrugation by the pressure of the storage liquid and the effect of thermal contraction and thermal expansion due to cryogenic liquid at -162 ℃.

9 shows a finite element network for finite element analysis of ring-node membranes, which consists of 1100 to 1300 axisymmetric quadrangular elements and 120 to 1400 element nodes. Because of the complexity, the quadrangular mesh configuration was reconstructed to match the shape characteristics of the structure. Particularly, the pleated ends and rings, where the pleats and ring nodes meet each other, were divided into finer elements as shown in FIG. Here, MARC (MARC User's Manual, 1993, Version K.5, MARC Analysis Research Co.) was used for the finite element analysis program, and the boundary condition that was closest to the load condition actually applied to the membrane plate was substituted.

The simulation data values for the material properties and boundary conditions used in the numerical analysis are given in Table 1, and the boundary conditions for performing an analytical model of a ring joint membrane are shown in FIG.

In FIG. 10, boundary 1 excludes the displacement in the Y-axis and rotational displacement in the X and Y-axis directions, and boundary 2 defines the displacement in the X-axis direction and the rotational displacement in the Y- and Z-axis directions. Assumed no. In addition, the flat plate (Boundary 3) except for the wrinkle section was analyzed in the absence of displacement in the Z-axis direction.

The model in which the computer simulation was performed is called ring-barrier membrane in the inventor's invention (see FIG. 6). In order to analyze the membrane more precisely, the thickness of the plate is in contact with the liquid (Layer 1). Numerical results were obtained by dividing the middle surface of (Layer 2) and the lower surface (Layer 3) into.

FIG. 11 shows the equivalent von Mises stress distribution on the ring-node membrane plate. Equivalent von Mice stress generated on the top of ring-node membrane was interpreted as 32Kg as shown in Figure 11 (a). In addition, the equivalent von Mice stress appeared in the middle surface of the ring node membrane was about 21Kg.

According to these calculations, the ring node type membrane has symmetrically uniform stress and shows excellent performance in the stress concentration. However, the ring node and the wrinkle area, which are indicated as the most vulnerable parts of the membrane, have a stress of 21-32 Kg. Occurred.

In this way, the compressive stress of about 21Kg is generated at the site where the top of the ring node meets the ring wrinkles, so it is necessary to separate them to prevent the compressive stress from occurring at the site where the top of the ring node meets the ring wrinkles. The design and analysis of the separated ring-knot corrugated membrane according to the invention were carried out.

The basic shape of the separate ring knot corrugated membrane according to the present invention is to reduce the stress concentration and stress level by separating the ring knuckle membrane developed by the present invention at a certain distance between the ring knot and the corrugation. As the main design parameters of the membrane, as shown in Table 2 and Fig. 8, the size of each of the size of the anchor bolt fixing a from the apex of the ring node and the distance between the apex of the ring node and the corrugation vertex separated from each other are b. The analysis was performed while varying.

The given boundary conditions applied to the isolated ring-knotted membranes of the present invention are the same as those used for the analysis of existing membranes and ring-knot corrugated membranes, and the materials related to the material properties and thickness of the material are the same as the ring-knuckle membranes. Interpreted The simulation results are presented in FIGS. 12-19 as significant von mouse stresses on the plane where the membrane cell contacts the liquid (Layer 1) and the neutral plane (Layer 2).

16, 17 and 12 to 19 it can be seen that the stress level of the membrane of the present invention improved significantly compared to the ring joint corrugated membrane, the stress concentration diagram according to the change of the design parameters It can be seen that it can be reduced.

21 to 25, the membrane of the present invention having a small radius a of the ring can be seen that the radius of curvature of the ring node is so small that a relatively large stress is applied to the inside of the ring node. In the case of the corrugated membrane type 4, the inner portion of the ring increases in size, but the stress concentration does not appear, but the stiffness decreases and the stress level increases. As a result of the analysis by changing the design variable a from the center of the ring, it was confirmed that the optimum stress level and stress distribution can be obtained when a is 100nm. The analysis was carried out by changing.

In FIGS. 21 and 23 to 25, the change of stress as the design variable b changes can be observed. As b increases, the level of stress decreases, and the stress concentration in the region where ring nodes and wrinkles meet is greatly increased. It was confirmed that it can be reduced. However, if the distance of b is too far, the decrease in stress level is very weak but the distribution of stress will not be even. The design variable b representing the ring top and the pleat top has a value of 120mm ~ 140mm. This area is expected to be good in terms of stress concentration.

In FIGS. 26 to 30, the stress distribution along the AC cross-section of FIG. 20 is shown. The stress distribution has the same shape for all models, and the change in the stress level according to the increase and decrease of the design variable b is constant. However, it slightly increases with the increase of b and then decreases from the point of more than 120mm.

Based on the results of the analysis, the optimum value should be found by not undersizing or oversizing the design variables. For the variable b, it is advantageous in that the level of stress decreases, but in case of exceeding a certain amount, It appears to be separated due to the nonuniformity of and the increase of the processing cost according to the size of the press working range.

Therefore, by optimizing a and b to reduce the stress level of the entire membrane and to make the stress distribution uniform, the stability of the membrane, particularly the fatigue life, can be improved.

Claims (8)

Four bending wrinkles (10) having a curved shear bend (11) in a straight line from the outer edge (1) of the plate (A) toward the anchor bolt fixing hole (9), and the shear bend (11) and Membrane structure for a liquefied natural gas storage tank, characterized in that it comprises an annular outer surface (5) and a ring node (2) consisting of an annular inner surface (4) symmetrical with the annular outer surface at regular intervals. The membrane structure according to claim 1, wherein the heights raised from the plate of the four corrugations (10) and the ring joint (2) are the same. 3. The membrane structure according to claim 1 or 2, wherein the cross section of the four corrugations and the ring nodes is approximately semicircular. The membrane structure according to claim 1 or 2, wherein the cross-sections of the four corrugations and the ring nodes are in the shape of an ellipse with a transverse diameter narrower than the longitudinal diameter. The membrane structure according to claim 1 or 2, wherein the cross section of the four corrugations and ring nodes is polygonal. The membrane structure according to claim 1 or 2, wherein the unit sheet has a thickness greater than that of the panel sheet. Pressing a ring node consisting of four cross-shaped corrugations formed by curved shear bends and straightly spaced annular inner and annular outer surfaces spaced apart from the shear bends in a straight line from the outer edge of the sheet toward the anchor bolt fixing hole. Punching holes for inserting the unit board into the panel board, bending the same corrugation as the four corrugations of the unit board in the horizontal and longitudinal directions of the panel board, and punching the unit board. And inserting the end of each corrugation into a hole to weld the edge of the unit board to the panel board, thereby manufacturing a membrane structure for a LNG storage tank. 8. The method of claim 7, wherein the welding step is an overlap welding method.