KR820001396B1 - Liquefied gas tank - Google Patents

Abstract

A liquefied gas, e.g. natural gas, tank which can be filled without thermal roll-over comprises a thermally insulated vessel (11) with a liquid filling line ending in the vessel upper half, a draft tube(21) in the vessel vertically from within the upper to within the lower quarter, and a liquid discharge pipe from near the vessel bottom. The filling line discharges liquid within the confines of the draft tube, which has a cross-sectional area at least 10 times that of the filling line. The draft tube upper end is in communication with the ullage part of the vessel, and its lower end with the liquid-holding part.

Description

Liquefied gas storage tanks

1 is a partial cutaway side view of a spherical tank arrangement incorporating various features of the present invention.

FIG. 2 is an enlarged cross-sectional view of the tank shown in FIG. 1 generally taken along line 2-2 of FIG.

3 is an enlarged cross-sectional view taken generally along line 3-3 of FIG.

The present invention generally relates to a liquefied gas filling tank having a normal boiling point of about 0 ° C. or less. In particular, the present invention relates to a system for filling a large closed tank with a low temperature liquid.

In order to significantly reduce its volume for the purpose of transporting natural gas from the world's production areas across the ocean to the consumer, it has become commonplace to liquefy it by dropping the temperature appropriately at the normal boiling point of atmospheric pressure, resulting in ships or barges. It is economical to transport. Liquefied natural gas (LNG), which has a boiling point of about -260 ° C (-161 ° C), consists mainly of methane, but also contains small amounts of other liquefied gases. Although economic interest is currently focused on LNG transportation, the problems associated with the transport and handling of large quantities of liquefied gas apply equally to liquefied gases such as ammonia, ethylene, propane, butane and chlorine.

Avoiding impacts when filling large closed tanks with low temperature liquids is a problem. Thermal shock occurs when the density of liquefied gas delivered into the tank is very different from the density of liquefied gas already filled in the tank. For example, if the liquid to be conveyed is slightly warmed so that its density is less than the density of the liquid already in the tank and supplied to fill from the bottom of the tank, the lighter liquid tends to rise to the top without the occurrence of any material mixing and thus generally the tank This results in fragmentation or imbalance in the liquid at rest. Similar possibilities exist when cooler liquid is supplied to the warmer liquid top surface. In large closed tanks, liquefied gas tends to layer so that separate chunks or areas of warmer liquid are trapped in this location and condense under the cooler and denser liquid as a result of the high pressure atmosphere at the bottom of the tank due to liquid pressure. Such hot lumps or layers soar as they unwind and at the same time the upper cold layer sinks like a vortex, which is called thermal shock.

This thermal shock phenomenon is undesirable because it is accompanied by the rapid scattering of a large amount of steam. This is because recovery by the steam recovery system is impossible because a large amount of increase occurs in a short time. In order to prevent the pressure from cracking or breaking beyond the design pressure of the tank, it is necessary to quickly release the pressure into the atmosphere. Not only does this result in loss of the product, but it can also lead to risks such as the flammability of LNG.

Previously, to solve this problem, valve devices have been used to inject liquid into the top or bottom of the tank. Unless there is equipment to continuously monitor the conditions in the tank, trained guesses are made as to whether the operator will inject top or bottom to reduce the possibility of thermal shock in the tank.

The present invention minimizes the potential for thermal shock in a liquefied gas tank regardless of whether the concentration of the liquid already in the tank is greater than or less than the concentration of the liquid currently being supplied, so no guesswork or adjustment is required. Before the liquid supplied is mixed with the raw liquid, it is adapted to the conditions in the tank, and as a result, the liquefied gas is relatively calmly injected into the large sealed tank.

Several advantages of the present invention will become apparent from the following detailed description of the cold tank arrangement and from the accompanying drawings.

As an example, a large spherical tank of the type designed as part of a general type of LNG carrier such as that shown in US Pat. No. 3,680,323 is shown. Such a tank, for example, has a diameter of 120 feet and is designed to hold about 25,000 m 3 of low temperature liquids, such as liquefied LNG. The body 11 is designed to fail in the hull through a skirt 13 or a peripheral support ring which merges with the tank around its equator. The body 11 is made of aluminum of various thicknesses from about 1 to 3/8 "to 7", and a columnar dome 15 is installed thereon.

The metal container and the dome are insulated by a suitable insulator 16 (eg polyurethane foam). In the illustrated tank, the heat insulator 16 is exposed outside the metal wall. However, it is advisable to install an insulator in the metal tank, but to install a liquid and vapor spill prevention film on the inner surface of the insulator to prevent the cargo liquid spilling into the insulator.

The LNG supply pipe 17 penetrates the tank wall near the upper end of the dome 15 and includes a connecting portion 19 at the outer end, where it is connected to the cargo piping system on the vessel. The feed duct includes a 90 ° bend and continues down vertically below the dome, ending at about 57 feet around the equator of the sphere. There is a draft pipe 21 that becomes a large tower or 8.5 feet in diameter vertically on a ship's center line. As shown, the low tip of the liquid supply tube 17 extends about one foot below the top end of the draft tube 21, so that the LNG supplied is discharged into the top end of the draft tube 21. So it flows vertically down through the draft tube before mixing with the LNG in the tank.

The top end of the draft tube 21 is functionally open and connected to the inside of the top of the body (11). However, the shield plate 23 is provided for the purpose of the following description. The bottom of the draft tube 21 is suitably supported by a mechanical connection to the metal container and it extends to a distance of seven feet from the bottom of the tank. The remaining tubing appears within that range to continue to the draft tube 21, leaving an area between the outer surface of the draft tube and the inner surface of the spherical tank that can freely retain the LNG.

The central discharge pipe 25 is in a suitable position inside the draft pipe 21 to discharge the LNG from the body 11. The lower end of the discharge pipe 25 is attached to a submersible pump 27 driven by an electric motor. In order to cope with the heat shrinkage of the pipe that occurs when the temperature is lowered from room temperature to low temperature, an activity support 29 is provided for the inside and outside submersible pumps properly attached to the bottom of the spherical container, and the base is a vertical fixture (31). ). This activity support allows the free movement of the pump 27 in the vertical direction as the temperature drops and the outlet pipe 25 shrinks. The upper end of the discharge pipe 25 is bent at 90 °, and at the end there is a coupling 32 connected to the cargo discharge pipe of the entire ship through the wall of the dome 15.

Also connected to the draft pipe 21 is a pipe 33 which is connected to a lesser submersible pump 35 supported by the support 29. This pipe 33 is about 1-1 / 2 inch in diameter and has many bends so that the inherent elasticity of the pipe accommodates thermal expansion and contraction. This piping 33 passes through a round wall and ends at a valve 37 which is connected to a spray piping structure (not shown). Two conduits 39 have a spray nozzle 41 which is supported in the draft tube 21 and located on the outer surface of the draft tube 21 at an intermediate position between the equator and the top of the tank. The mother pipe 39 is connected to the pipe of FIG. 2 which connects to the spray piping structure of a ship through a round wall. Thus a small pump 35 can be used to pump LNG from the bottom of the tank to the top through the pipe 33. Each pump is mated through the spray piping to the piping 43 of the other tanks in the vessel and the submersible pump 35 is used to spray LNG from the bottom of one tank into a relatively empty cargo tank during navigation.

LNG is supplied to the tanks to allow the steam to liquefy again. A facility for condensation of steam is also built in the ship's cargo steam handling unit. A steam pipe 49 is thus provided in the body 11, which passes through the top surface of the dome 15, where a suitable coupling 51 is attached to be connected into the ship's cargo steam handling device. The bottom end of the steam pipe 49 is closed at both ends next to a suitable disc and ends in a short cross pipe 53 with a hole tissue 55 which completely fills both bottoms there. This cross-pipe inlet structure minimizes the amount of LNG carried with the steam being removed from the tank. This is because such liquids tend to form droplets at the perforated surface and fall down into the draft tube 21.

In consideration of the safety management surface, another discharge pipe 59 is connected to the safety valve 61 through the top of the side wall of the dome 15. As noted above, the tank is not installed to operate at high pressure, so the safety valve 61 opens if the pressure in the tank reaches 3 psig (1.2 atm in absolute pressure). When the safety valve 61 is opened, the discharged LNG is diverted through the conduit on the mast of the ship to be discharged to the atmosphere from the main deck of the ship is distributed to the atmosphere without jeopardizing the source. Of course, if the ship system is operating normally, the safety valve opening pressure is not reached.

In order to maintain the LNG in the tank even if there is heat inflow from the atmosphere, various facilities are provided to equilibrate near the boiling point of the cargo. The freezer, for example, balances the inflow of heat into the tank. However, if the inflow of heat is kept to a minimum by a valid insulation system, some evaporation of the LNG is allowed and the steam pipe 49, the coupling 51 and the steam treatment unit are allowed to vapor from the tank at a rate equivalent to that generated by evaporation. Used to remove Generally, the steam treatment device is connected to the inlet side of one or more compressors capable of maintaining steam pressure at about 1.8 psig. The introduced natural gas can be burned as fuel in the ship's propulsion boiler or re-liquefied through the vessel's liquefaction equipment and returned to individual tanks.

Since the LNG supply pipe 17 is generally terminated at the top of the draft pipe 21, the injected LNG is immediately exposed to the pressure condition in the tank. As a result, the inner region of the draft tube 21 is used as a single expansion chamber in which the LNG entering it is released and regulates itself in the vapor pressure of the cargo tank therein. Furthermore, there is sufficient time for the incoming LNG to reach thermal equilibrium because the incoming LNG flows down into the draft tube and mixes with the LNG there before it can enter the main portion outside the old tank of the draft tube. Thus, if the incoming LNG is warm, evaporation takes place immediately and the vapor generated can be moved up in the draft tube to cool the incoming LNG and immediately discharge it to be supplied to the offshore facility for reliquefaction. As the liquid slowly moves to the bottom of the draft tube 21, its density gradually changes and as a result it essentially has the same density as LNG at the tank bottom when it begins to flow up from the bottom of the draft tube. Thus, the possibility of thermal shock is essentially eliminated and relatively quiet or floating filling occurs from the presence of the incoming liquid near the bottom of the tank and simply moves the liquid in the tank upwards.

In order to achieve this objective it is believed that the inlet LNG enters the expansion chamber at a location within the vertical upper quarter of the tank or at the location within 10% of the tank. Thus when draft tube 21 is used to provide an expansion chamber tube it expands up at least at the same level as the discharge point of feed tube 17.

However, the feed pipe 17 ends at the vertical level in the upper half of the tank. So there is expansion time and thermal stability occurs before mixing. Similarly, the drift tube 21 extends below a distance within a quarter bottom of the vertical height of the tank and to a distance from the bottom of the tank within about 10% of the height of the vessel.

In the arrangement shown, feed tube 17 generally has a fixed inner diameter of about 14 ″ and discharges into draft tube 21 having an inner diameter of about 8.5 feet. Therefore, the area of the draft pipe 21 is more than 50 times the area of the supply pipe 17 and it is discharged from the supply pipe, thus there is no problem with the LNG. In order to achieve the desired object, the expansion chamber area is at least 10 times the supply pipe area, and usually 20 times or more is preferable.

Since the open top of the draft tube 21 is fluidly connected with steam in the outlet of the tank 11, the drop flow of LNG is well adapted to the pressure conditions in the tank. In addition, it is possible to reach sufficient thermal equilibrium with the LNG already in the tank during the time that the LNG in the draft tube 21 moves to the bottom. As a result, the density of LNG flowing from the bottom of the column is similar to the density of LNG in the reservoir, so that no bad phenomena such as circulation occur. Once a year it is necessary to empty the tank for physiologic examination therefrom, usually by injecting hot natural gas to evaporate all the LNG in the tank. At this time, the gas is injected into one of the supply pipe 17 or the steam pipe 49 and discharged to the other. In order not to rotate briefly between these pipes through the bottom of the draft pipe 21, a plurality of holes corresponding to a 6 ″ diameter area are drilled.

Although the present invention has been described in relation to the practice described in the figures. Hardly, one of ordinary skill in the art, within the scope of the invention as defined in the claims, should understand that changes or imitations can be made. In this regard, instead of freely discharging LNG into the large diameter vertical pipe, an expansion chamber may be provided which expands the supply pipe downward and subjects it to the pressure conditions in the vessel. In addition to such expansion chamber arrangements, the feed pipe itself may be extended to a location near the bottom of the tank to compensate for the pressure at the top of the vessel and to achieve thermal stability, as the influent liquid moves down through the liquid reservoir surrounding it in the vessel. The supply pipe itself can also be used as a medium for transverse heat conduction.

Likewise, LNG shipments have been mentioned intensively, but can also be applied to other low temperature liquids that are liquefied at low temperatures and shipped in bulk. For example, this type of injection device is particularly beneficial for liquefied gas shipments with ammonia or lower normal boiling points, so the purpose of the application is for general low temperature liquid utilization. Moreover, large closed tanks were considered for ordinary tanks that could hold at least 5000 cubic meta liquids. Features of the invention are defined in the claims.

Claims (1)

A body 11 through which liquid forming the liquid reservoir does not pass, an insulator 16 connected to the body so as to provide a thermal barrier between the low temperature part and the surroundings of the container, and the upper end of the container. A liquefied gas supply pipe 17 into the container, a connecting portion 19 connecting the supply pipe to the liquefied gas source, and a cross-sectional area of about 10 times or more of the supply pipe cross section, from the upper 1/4 position to the lower 1/4 position of the vessel. And a discharge pipe 25 extending vertically from the bottom of the container for emptying the liquefied gas to be added to the supply pipe installed in the container extending vertically and to supply the liquefied gas into the boundary of the draft pipe. Wherein, the draft pipe is a liquefied gas storage tank having an upper portion in fluid communication with the space portion of the container and a lower portion in fluid communication with the liquid holding portion of the container.

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