Cryogenic storage tank with a pump well structure for reducing dead zone liquid level and its operation method

Abstract

This invention relates to a cryogenic storage tank with a dead-zone level reduction pump well structure and its operation method. The storage tank includes a tank body; at least one first pump well structure is disposed within the tank body. The first pump well structure includes a first pump well pipe. A first submersible pump, capable of being completely submerged below the liquid level, is disposed at the bottom of the first pump well pipe. A negative pressure protective cover is disposed at the inlet of the first pump well pipe. The negative pressure protective cover is used to create a negative pressure chamber to reduce the dead-zone liquid level and avoid vortex bubbles when the first submersible pump discharges liquid. The top of the negative pressure protective cover can be opened and closed to connect to the gas phase space. A gas pressure regulating device is connected to the tank body to adjust the operating pressure of the gas phase space. This invention features a unique negative pressure protective cover, and the outer side of the first submersible pump forms a negative pressure chamber to prevent the intake of vortices and bubbles. The gas pressure regulating device connected to the tank body expands the operating pressure range within the gas phase space, reduces the dead-zone liquid level height of the storage tank, increases the effective volume of the storage tank, and maximizes the extraction of residual liquid after tank cleaning.

Description

Technical Field

[0001] This invention relates to the field of cryogenic storage tank technology, and in particular to a cryogenic storage tank equipped with a pump well structure for reducing dead zone liquid level and its operation method. Background Technology

[0002] Atmospheric pressure storage tanks for cryogenic media such as LNG are the most critical core equipment in receiving terminals, peak-shaving stations, and other energy storage facilities, occupying an important position in the energy storage system. Based on structural differences, vertical flat-bottomed storage tanks can be divided into various types, including single-containment tanks, double-containment tanks, full-containment tanks, and membrane tanks. These cryogenic storage tanks can store cryogenic LNG, cryogenic ethylene, cryogenic ethane, cryogenic liquid ammonia, cryogenic propane, cryogenic propylene, cryogenic butane, and other liquids at near-atmospheric pressure.

[0003] Currently, the main type of LNG cryogenic storage tank is a large prestressed concrete fully enclosed tank with a capacity of 100,000 to 270,000 cubic meters. Due to the low construction cost per unit volume of the tank, significant economies of scale, and the fact that larger tank volumes can more efficiently improve land utilization and reduce the BOG evaporation rate, LNG storage tanks are continuously developing towards larger sizes.

[0004] According to regulations, large cryogenic full-containment storage tanks should not have openings in the tank walls and bottom, and pipelines connecting to the tank should generally enter and exit from the top. Due to the large diameter (exceeding 110m) and height of the tanks (up to approximately 50m), and the fact that the cryogenic medium inside the tank approaches its saturated vapor pressure under storage conditions, vacuum pumps cannot be used for material extraction; only cryogenic submersible pumps immersed below the liquid level can be used. For relevant pump information, please refer to "CN103615394B Vertical Cryogenic Tank Submersible Pump".

[0005] When starting a cryogenic submersible pump, the lowest liquid level in the storage tank must not be lower than the minimum operable liquid level required by the pump. Currently, the minimum operable liquid level of a cryogenic submersible pump, plus a certain safety margin, is typically around 1.2m to 2.4m. This means that there is a 1.2m to 2.4m high layer of liquid at the bottom of the tank that cannot be completely drained, forming a "dead zone" in the tank, accounting for approximately 3% to 6% of the tank volume. As the diameter of storage tanks continues to increase, the volume of the dead zone also increases, leading to increased costs and wasted resources.

[0006] Because the material in the dead zone cannot be discharged from the tank by the cryogenic submersible pump, when tank cleaning is required, this bottom material can only be discharged by vaporization, which is time-consuming, energy-intensive, and not conducive to tank shutdown and maintenance. The reasons why the tank cannot be completely cleaned are that the bottom valve of the pump well is too high, cavitation occurs, it is easy to suck in vortices causing surge, and the submersible pump needs to be completely submerged below the liquid level to achieve good heat dissipation.

[0007] The existing technology involves excavating a trench at the bottom of the storage tank to create a liquid accumulation pool. A submersible pump is then placed in this pool to lower the liquid level in the dead zone of the tank. However, this method has the following problems:

[0008] (1): After the bottom of the storage tank is grooved, the integrity of the concrete foundation supporting the storage tank needs to be destroyed, which is very bad for the stress of the foundation structure. Especially under earthquake conditions, there is a risk of the foundation cracking. It is necessary to eliminate the risk by increasing the amount of reinforcement in the foundation, which increases the cost of the storage tank foundation.

[0009] (2): After the storage tank is slotted, in order to ensure that the heat outside the tank is not transferred into the tank through the sump, the bottom and the surrounding area of ​​the sump after the slot is opened need to be covered with heat insulation material. The temperature difference stress is large, the construction is difficult, and the heat insulation effect is poor.

[0010] (3): After the storage tank is slotted at the bottom, two layers of metal material, namely a secondary bottom plate and an inner tank bottom plate, need to be laid on the upper surface of the insulation material. The slotted area is an irregular structure, which makes the sealing performance poor and the construction difficult when laying steel plates. Metal materials processed into irregular structures are also at risk of tearing and leakage when exposed to thermal expansion and contraction.

[0011] (4): The above structure has too high costs and high safety risks, and the final benefits may not be able to offset the investment.

[0012] The second existing technology uses a Venturi mixer, which requires a series of complex process systems such as a Venturi mixer, a material circulation tank, a cryogenic pump, control valves, and a pressurization unit. It has a complex structure, high cost, and poor reliability.

[0013] Therefore, the inventors propose a cryogenic storage tank with a pump well structure for reducing dead zone liquid level and its operation method to overcome the defects of the prior art. Summary of the Invention

[0014] The purpose of this invention is to provide a cryogenic storage tank with a pump well structure for reducing dead zone liquid level and its operation method. This invention features a unique negative pressure protective cover, with a negative pressure chamber formed on the outside of the first submersible pump. The negative pressure chamber establishes a stable liquid level around the submersible pump, preventing the intake of vortices and bubbles. A gas pressure regulating device is connected to the tank body to expand the operating pressure range in the gas phase space, achieving "avoiding cavitation" and "the first submersible pump being completely submerged below the liquid level," reducing the dead zone liquid level height of the cryogenic storage tank, increasing the effective volume of the tank, and maximizing the extraction of residual liquid after tank cleaning.

[0015] The objective of this invention is achieved as follows: a cryogenic storage tank equipped with a pump well structure for reducing dead zone liquid level includes a tank body; at least one first pump well structure is disposed within the tank body, the first pump well structure including a first pump well pipe that is sealed and inserted through the top of the tank body, a first submersible pump that can be completely submerged below the liquid level is disposed at the bottom of the first pump well pipe, a negative pressure protective cover that is sealed at the top and open at the bottom of the first pump well pipe is disposed at the bottom inlet, the negative pressure protective cover is used to establish a negative pressure chamber to reduce the dead zone liquid level and avoid vortex bubbles when the first submersible pump discharges liquid, a gas phase space is formed between the liquid level and the top of the tank body, the top of the negative pressure protective cover can be opened and closed to communicate with the gas phase space; a gas pressure regulating device is connected to the tank body to adjust the operating pressure in the gas phase space, the gas pressure regulating device can increase the operating pressure in the gas phase space so that the liquid static pressure at the first submersible pump is greater than the liquid saturated vapor pressure.

[0016] In a preferred embodiment of the present invention, a flow-guiding skirt structure is provided at the bottom end of the negative pressure protective cover. The flow-guiding skirt structure is used to absorb residual liquid, collect vortex bubbles, and stabilize the inlet flow field.

[0017] In a preferred embodiment of the present invention, the top end of the negative pressure protective cover is connected to the first end of the external connecting pipe, and the second end of the external connecting pipe passes through the tank body and returns to the gas phase space of the tank body; a first valve is provided on the external connecting pipe at a position outside the tank body, and the first valve is used to control the communication state between the negative pressure protective cover and the gas phase space.

[0018] In a preferred embodiment of the present invention, a plurality of second pump well structures are further provided in the storage tank body, each second pump well structure including a second pump well pipe, and a second submersible pump is provided at the bottom end of the second pump well pipe.

[0019] In a preferred embodiment of the present invention, during the tank cleaning operation, the operating pressure of the gas phase space inside the tank body is greater than or equal to the saturated vapor pressure of the cryogenic medium under the operating environment, and less than or equal to the upper limit of the pressure that the tank body can withstand.

[0020] In a preferred embodiment of the present invention, the interval between the inner wall of the negative pressure protective cover and the outer wall of the first pump well pipe is in the range of 1mm to 10000mm; the sum of the heights of the negative pressure protective cover and the drainage skirt structure is in the range of 500mm to 12000mm.

[0021] In a preferred embodiment of the present invention, a plurality of stiffening plates and stiffening plate supports are provided at intervals inside the negative pressure protective cover.

[0022] In a preferred embodiment of the present invention, the first pump well pipe and the negative pressure protective cover are connected to the top and inner wall of the storage tank body through a pump well support structure.

[0023] In a preferred embodiment of the present invention, the drainage skirt structure is connected to the bottom plate of the tank body through a bottom support structure.

[0024] The object of the present invention can also be achieved as follows: an operating method for a cryogenic storage tank equipped with a pump well structure for reducing dead zone liquid level includes the following steps:

[0025] Step a, Feeding operation: The top of the negative pressure protective cover is connected to the gas phase space inside the storage tank. The gas pressure inside the negative pressure protective cover is the same as the gas pressure in the gas phase space. The internal space of the negative pressure protective cover is filled with liquid. Control the pressure of the external pipeline connected to the first pump well pipe. The liquid level in the first pump well pipe is equal to the operating liquid level inside the storage tank. The first submersible pump is completely submerged below the liquid level.

[0026] Step b, Discharge Operation: The top of the negative pressure protective cover is disconnected from the gas phase space inside the storage tank. The first submersible pump is started to discharge liquid outwards, and the operating liquid level inside the storage tank continues to decrease. When the operating liquid level inside the storage tank decreases to near the cavitation liquid level of the pump, the air pressure regulating device is started to inject air into the storage tank, increasing the operating pressure of the gas phase space to the first pressure value. The pressure at the bottom inlet of the first pump well pipe is higher than the cavitation pressure of the pump, and the first submersible pump continues to discharge liquid outwards.

[0027] Step c, tank cleaning operation:

[0028] The pressure regulating device maintains the operating pressure of the gas phase space at the first pressure value, and the first submersible pump continuously discharges liquid. When the operating liquid level in the tank body drops to near the minimum cleaning liquid level, the cleaning is completed.

[0029] As described above, the cryogenic storage tank and its operation method with a pump well structure for reducing dead zone liquid level of the present invention have the following beneficial effects:

[0030] In this invention, a pressure regulating device is connected to the tank body, breaking the limitation of the conventional operating pressure fluctuation range of 4-7 kPa within the cryogenic storage tank. By expanding the operating pressure range within the gas phase space through the pressure regulating device, it achieves "avoiding cavitation" and "the first submersible pump being completely submerged below the liquid level," reducing the dead zone liquid level height of the cryogenic storage tank, increasing the effective volume of the tank, and maximizing the extraction of residual liquid after tank cleaning. A unique negative pressure protective cover is set up, and the outer side of the first submersible pump forms a negative pressure chamber. Through the negative pressure chamber, a stable liquid level is established around the submersible pump, preventing the intake of vortices and bubbles. This invention does not require modification of the tank's foundation, does not require the addition of complex process equipment, does not change the performance parameters of the existing submersible pump, does not change the main structure of the tank bottom and inner tank wall, and makes full use of existing process equipment. It is applicable to all types of cryogenic storage tanks that use submersible pumps for liquid discharge, with a simple structure, low investment, and good results. Attached Figure Description

[0031] The accompanying drawings are intended only to illustrate and explain the present invention and do not limit the scope of the invention.

[0032] in:

[0033] Figure 1 : This is an internal structural diagram of a cryogenic storage tank with a pump well structure for reducing dead zone liquid level according to an embodiment of the present invention.

[0034] Figure 2 : This is a structural diagram of the first pump well structure and the second pump well structure of the present invention.

[0035] Figure 3 : This is a structural diagram of the first pump well structure of the present invention.

[0036] Figure 4 This is a schematic diagram of the feeding operation in Embodiment 1 of the present invention.

[0037] Figure 5 This is a schematic diagram of the material discharge operation in Embodiment 1 of the present invention.

[0038] In the picture:

[0039] 1. First pump well structure; 11. First pump well casing; 12. First submersible pump;

[0040] 2. Second pump well structure; 21. Second pump well casing; 22. Second submersible pump;

[0041] 3. Tank body; 30. Gas phase space;

[0042] 4. Negative pressure protective cover; 41. First valve; 42. External connecting pipe;

[0043] 5. Drainage skirt structure. Detailed Implementation

[0044] To provide a clearer understanding of the technical features, objectives, and effects of the present invention, specific embodiments of the present invention will now be described with reference to the accompanying drawings.

[0045] The specific embodiments of the present invention described herein are for illustrative purposes only and should not be construed as limiting the invention in any way. Under the teachings of this invention, those skilled in the art can conceive of any possible modifications based on the invention, all of which should be considered within the scope of the invention. It should be noted that when an element is referred to as being "set on" another element, it can be directly on the other element or there may be an intervening element. When an element is referred to as being "connected" to another element, it can be directly connected to the other element or there may be an intervening element. The terms "mounted," "connected," and "linked" should be interpreted broadly; for example, they can refer to mechanical or electrical connections, or internal communication between two elements, and can be direct or indirect connections through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms according to the specific circumstances. The terms "vertical," "horizontal," "upper," "lower," "left," "right," and similar expressions used herein are for illustrative purposes only and do not represent the only possible embodiments.

[0046] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0047] like Figures 1 to 5 As shown, the present invention provides a cryogenic storage tank with a pump well structure for reducing dead zone liquid level, including a storage tank body 3. The storage tank body 3 can be an existing cryogenic storage tank, that is, without changing the main structure of the storage tank, such as the support platform, insulation structure, inner tank bottom, inner tank wall (existing technology), etc.

[0048] At least one first pump well structure 1 is provided inside the storage tank body 3, such as Figure 3 , Figure 4 , Figure 5As shown, the first pump well structure 1 includes a first pump well pipe 11 that is sealed and inserted from the top of the storage tank body 3. A first submersible pump 12 that can be completely submerged below the liquid level is installed at the bottom of the first pump well pipe 11. A negative pressure protective cover 4 that is sealed at the top and open at the bottom is installed at the bottom inlet of the first pump well pipe 11. The negative pressure protective cover 4 is used to establish a negative pressure chamber to reduce the dead zone liquid level and avoid vortex bubbles when the first submersible pump 12 discharges liquid. A gas phase space 30 is formed inside the storage tank body 3 between the liquid level and the top of the tank. The top of the negative pressure protective cover 4 can be opened and closed to connect to the gas phase space 30 of the storage tank body 3.

[0049] The storage tank body 3 is connected to a gas pressure regulating device that can adjust the operating pressure in the gas phase space 30. The gas pressure regulating device can increase the operating pressure in the gas phase space 30 so that the liquid static pressure at the first submersible pump 12 is greater than the liquid saturated vapor pressure, thereby avoiding cavitation and ensuring that the first submersible pump 12 is completely submerged below the liquid level.

[0050] The first submersible pump 12 can use the existing submersible pump, that is, without changing the performance parameters of the existing submersible pump, and without forcibly lowering the minimum suction liquid level of the submersible pump itself. While meeting the minimum suction liquid level of 1.2 to 2.4 m of the existing submersible pump, it can still draw all liquids above 0.15 m to the outside of the storage tank body 3.

[0051] The pressure regulating device of the present invention can increase the operating pressure in the gas phase space 30 so that the liquid static pressure at the first submersible pump 12 is greater than the liquid saturated vapor pressure, thereby avoiding cavitation.

[0052] This invention reduces the intake of excessive vortices and bubbles during the tank cleaning process, preventing the submersible pump from vibrating violently due to gas intake. This invention can also be used to maximally empty residual liquid from storage tanks, shortening the cleaning cycle from one to two months to about one week compared to traditional vaporization cleaning methods.

[0053] The first submersible pump 12 of the present invention is completely submerged below the liquid level to avoid heat dissipation problems of the submersible pump.

[0054] This invention makes full use of existing process equipment, eliminating the need for additional large-scale process equipment such as booster compressors. The air pressure regulating device of this invention can use the BOG booster compressor (existing technology), vaporizer (existing technology), and related pipeline devices that are standard equipment in the receiving station, and can be used in conjunction with the storage tank body 3 of this invention, without the need for additional process equipment for the storage tank.

[0055] This invention is applicable to all types of cryogenic storage tanks that use top-discharge pumps, such as single-containment tanks, double-containment tanks, full-containment tanks, and diaphragm tanks. The improved structure can also be used for bottom-discharge storage tanks, such as crude oil storage tanks and refined oil storage tanks.

[0056] In this invention, a cryogenic storage tank with a pump well structure for reducing dead zone liquid level is provided. A pressure regulating device is connected to the tank body 3, breaking the 4-7 kPa normal operating pressure fluctuation limit within the cryogenic storage tank. The pressure regulating device expands the operating pressure range within the gas phase space 30, achieving "avoiding cavitation" and "the first submersible pump 12 being completely submerged below the liquid level," reducing the dead zone liquid level height of the cryogenic storage tank, increasing the effective volume of the tank, and maximizing the extraction of residual liquid after tank cleaning. A unique negative pressure protective cover 4 is provided, with the outer side of the first submersible pump 12 forming a negative pressure chamber. This negative pressure chamber establishes a stable liquid level around the submersible pump, preventing the intake of vortices and bubbles. This invention does not require modification of the tank's foundation, does not require the addition of complex process equipment, does not change the performance parameters of the existing submersible pump, and does not alter the tank bottom or inner tank wall structure. It makes full use of existing process equipment and is applicable to all types of cryogenic storage tanks using submersible pumps for liquid discharge. It has a simple structure, low investment, and good results.

[0057] Furthermore, such as Figure 1 , Figure 2 , Figure 3 As shown, a flow-guiding skirt structure 5 is installed at the bottom of the negative pressure protective cover 4. The flow-guiding skirt structure 5 is used to absorb residual liquid, collect vortex bubbles, and stabilize the inlet flow field. The lower end of the flow-guiding skirt structure 5 needs to be open to guide the LNG liquid to the inlet of the first pump well structure 1 in an orderly manner. The flow cross-sectional area of ​​the gap between the flow-guiding skirt structure 5 and the bottom plate of the storage tank body 3 needs to be larger than the cross-sectional area of ​​the pump well to ensure stable flow.

[0058] The three functions of the drainage skirt structure 5 are as follows: First, together with the negative pressure protective cover 4, it forms a liquid-filled space to ensure the stability of the flow field at the inlet of the submersible pump; second, it provides drainage, and the minimum liquid level can be adjusted by adjusting the height of the lower edge of the drainage skirt structure 5; third, it can collect gas in time before the first submersible pump 12 inhales vortices and bubbles, and temporarily guide the gas into the negative pressure chamber (when vortices and bubbles are inhaled, the vortices and bubbles are guided along the inner wall of the drainage skirt structure 5 to the area above the L3 liquid level in the negative pressure chamber), thus avoiding problems such as surge caused by the first submersible pump 12 inhaling vortices. Furthermore, a set of vortex breakers (existing technology) can be added to the lower edge of the drainage skirt structure 5 to further reduce the risk of inhaling vortices.

[0059] Furthermore, such as Figure 1 , Figure 2 , Figure 3 As shown, the top of the negative pressure protective cover 4 is connected to the first end of the external connecting pipe 42, and the second end of the external connecting pipe 42 passes through the tank body 3 and returns to the gas phase space 30 of the tank body; a first valve 41 is set on the external connecting pipe 42 at the position outside the tank body 3, and the first valve 41 is used to control the connection state between the negative pressure protective cover 4 and the gas phase space 30.

[0060] The first valve 41 is located on the outside of the tank body 3 for easy operation by workers. The first end of the external connecting pipe 42 is connected to the top of the negative pressure protective cover 4, and the second end of the external connecting pipe 42 passes through the tank body 3 and returns to the gas phase space 30 inside the tank body. In this way, the operation of closing and opening the valve on the outside of the tank body 3 by the worker can establish and break the pressure difference between the negative pressure protective cover and the gas phase space 30 inside the tank body 3.

[0061] When liquid enters the storage tank body 3, the first valve 41 opens to ensure that the LNG liquid fills the internal space of the negative pressure protection cover 4. When liquid exits the storage tank body 3, the first valve 41 needs to be closed to ensure that a negative pressure chamber is established in the annular space between the negative pressure protection cover 4 and the first pump well pipe 11.

[0062] If the liquid level inside the negative pressure protection cover 4 drops significantly due to the intake of excessive air bubbles and vortices, affecting the stability of the flow field at the inlet of the submersible pump, it can be operated from outside the tank. The gas affecting the vacuum degree of the negative pressure chamber can be removed by using the first valve 41 and the external connecting pipe 42 to maintain the pressure difference and compensate for the vacuum degree to the maximum extent.

[0063] Furthermore, such as Figure 1 , Figure 2 As shown, the storage tank body 3 is also equipped with multiple second pump well structures 2. Each second pump well structure 2 includes a second pump well pipe 21, and a second submersible pump 22 is installed at the bottom end of the second pump well pipe 21. The second pump well structure 2 adopts a traditional pump well structure, and the second submersible pump 22 and the first submersible pump 12 can be submersible pumps with the same parameters.

[0064] In a traditional design, the submersible pump draws in LNG liquid from a bottom valve at the bottom of the pump well, where the dead zone level is typically no less than 1.2–2.4 m. As the impeller of the submersible pump rotates, LNG liquid is continuously pumped into the pump well pipe, raising the liquid level inside the pipe, and finally flowing out of the tank from the outlet at the top of the pipe, completing the liquid discharge operation.

[0065] At atmospheric pressure, LNG is a liquid methane at around -162°C, with a saturated vapor pressure of approximately 0.1 MPa(A). When the pressure drops below 0.1 MPa(A), the LNG liquid will boil and vaporize, and can no longer remain in a liquid state. Therefore, when extracting LNG from storage tanks, vacuum pumps cannot be used; only submersible pumps can be employed.

[0066] Submersible pumps (SPMPs) can extract LNG because their impellers provide initial kinetic energy to the LNG within the pump well. This kinetic energy propels the LNG to the top of the well and out of the tank, preventing the LNG level from falling below its saturated vapor pressure. However, even with this capability, SPMPs still face the risk of cavitation. Due to the high impeller speed and the high LNG velocity in contact with the blades, the LNG pressure in a localized area near the impeller can drop below its saturated vapor pressure, leading to severe cavitation and damaging the blade structure. To mitigate this risk, the minimum suction level of the SPMP needs to be maintained at a certain height, such as the 1.2–2.4 m mentioned earlier. This level is the minimum level required to prevent cavitation. When the operating level in the storage tank falls below this level, traditional SPMPs struggle to operate normally and cannot be started immediately, resulting in a significant dead zone.

[0067] In addition, the motor of the submersible pump generates heat during operation and requires continuous heat dissipation. Therefore, the entire submersible pump must generally be continuously submerged in LNG to maintain its heat dissipation performance. This also requires that the minimum liquid level be maintained at a certain height, generally above the top edge of the pump.

[0068] Furthermore, during the tank cleaning operation, the operating pressure of the gas phase space 30 inside the tank body is greater than or equal to the saturated vapor pressure of the cryogenic medium under the operating environment, and less than or equal to the upper limit of the pressure that the tank body 3 can withstand.

[0069] When it is necessary to drain the dead zone liquid level, the operating pressure P2 of the gas phase space 30 above the liquid level in the tank body 3 should always be maintained in the range of 10 to 29 kPa (depending on the different tank types). Considering the overall safety performance of the tank, the range of 10 to 15 kPa is preferred.

[0070] Furthermore, a certain space needs to be reserved between the inner wall of the negative pressure protective cover 4 and the outer wall of the first pump well pipe 11. The interval between the inner wall of the negative pressure protective cover 4 and the outer wall of the first pump well pipe 44 is 1mm to 10000mm, preferably between 50mm and 1000mm.

[0071] The upper end of the negative pressure protective cover 4 is sealed to the outer wall of the first pump well pipe 11 by welding or other means to ensure strength and airtightness.

[0072] While ensuring that the flow area is not less than the flow area required by the pump well, the lower edge of the drainage skirt structure 5 should be as close as possible to the minimum cleaning liquid level L1 required during cleaning. The preferred range of L1 liquid level is 150-1500mm.

[0073] The sum of the heights of the negative pressure protective cover 4 and the drainage skirt structure 5 is selected according to different design requirements and working conditions. The sum of the heights of the negative pressure protective cover and the drainage skirt structure is in the range of 500mm to 12000mm, preferably between 1000mm and 6000mm.

[0074] Furthermore, multiple stiffeners and stiffener supports are spaced apart inside the negative pressure protective cover 4 to prevent the negative pressure from causing instability of the negative pressure protective cover 4.

[0075] Furthermore, the first pump well pipe 11 and the negative pressure protective cover 4 are connected to the top and inner wall of the storage tank body through a pump well support structure. The pump well support structure is similar to the traditional pump well structure, with some limiting 6 degrees of freedom and others limiting 3 degrees of freedom.

[0076] Furthermore, the drainage skirt structure 5 is connected to the bottom plate of the tank body 3 through a bottom support structure to reduce the risk of vibration when liquid is sucked in.

[0077] Furthermore, the gas pressure regulating device regulates the operating pressure within the gas phase space 30 inside the storage tank body 3 in the following ways:

[0078] (1) Under normal operating conditions, the liquid in the storage tank body 3 (LNG storage tank) will evaporate naturally, which will automatically increase the operating pressure P2 of the gas phase space 30. When P2 is too high, a certain amount of gas can be drawn out of the tank through the gas pressure regulating device (BOG compressor) to reduce the pressure P2; by controlling the amount of gas drawn out, the pressure difference value can be quantitatively controlled.

[0079] (2) When it is necessary to quickly increase the pressure of the gas phase space 30, P2 can be quickly increased by replenishing gas into the tank through the external gas pressure regulating device (BOG compressor, vaporizer such as open shelf vaporizer ORV, submerged vaporizer SCV, intermediate medium vaporizer IFV, etc.).

[0080] Example 1

[0081] The cryogenic storage tank of the present invention, equipped with a pump well structure for reducing dead zone liquid level, is used as a prestressed concrete fully enclosed LNG storage tank. The tank body 3 contains three second pump well structures 2 and one first pump well structure 1. Alternatively, multiple first pump well structures 1 can be provided, allowing the remaining first pump well structures 1 to be activated as backups in case one fails.

[0082] In a cryogenic storage tank employing only a conventional pump-well structure, specifically the second pump-well structure 2, the gas phase space 30 enclosed by the tank body 3 (concrete outer tank) is filled with vaporized NG gas (natural gas, primarily methane) above the liquid level. The maximum operating pressure P2 (gauge pressure) within this gas phase space 30 is approximately 29 kPa (varies depending on the tank type), and is generally maintained between 4 and 7 kPa (varies depending on the storage tank). The method for maintaining the gas pressure is as follows: After the liquid in the storage tank body 3 evaporates naturally, P2 will gradually increase. Once P2 exceeds 7 kPa, the gas pressure regulating device (BOG compressor) is activated to draw BOG gas (Boil-Off Gas, abbreviated as BOG, refers to the gas that evaporates from a low-temperature liquid after being liquefied under pressure below its critical temperature, which is difficult to absolutely insulate from the environment and absorbs external heat) from the outside of the tank. Once P2 is detected to be below 4 kPa, BOG gas can be supplied into the tank through the gas pressure regulating device (BOG compressor or vaporizer, etc.) to maintain the operating pressure inside the tank in a stable range of 4 to 7 kPa.

[0083] The cryogenic storage tank of the present invention is equipped with a pump well structure for reducing dead zone liquid level. In addition to the conventional pump well structure, namely the second pump well structure 2, a first pump well structure 1 is also provided. A negative pressure protective cover 4 and a drainage skirt structure 5 are provided on the outside of the first pump well pipe 11 of the first pump well structure 1. In this embodiment, the interval between the inner wall of the negative pressure protective cover 4 and the outer wall of the first pump well pipe 44 is 200mm, and the sum of the heights of the negative pressure protective cover and the drainage skirt structure is 3000mm.

[0084] The inner wall of the negative pressure protective cover 4 and the outer wall of the first pump well pipe 11 form a negative pressure chamber. The pressure in the negative pressure chamber is P1, which is the saturated vapor pressure of 0.1 MPa (A); P2 is the operating pressure of the gas phase space 30, which is 0 to 29 kPa higher than P1.

[0085] L4 is the liquid level inside the first pump well pipe 11, L3 is the liquid level inside the negative pressure chamber (h3 is the distance from L3 to the bottom plate of the tank body 3), L2 is the operating liquid level inside the tank body 3 (h0 is the distance from L2 to the bottom plate of the tank body 3, h1 is the distance from L3 to L2 when L2 is lower than L3, i.e., h3 = h0 + h1), L2 is the lower surface of the bottom valve (existing technology) in a conventional pump well structure, and L1 is the lowest tank cleaning liquid level achievable by the present invention (lowest liquid level, h2 is the distance from L1 to the bottom plate of the tank body 3). L5 is the liquid level of the bottom valve of the submersible pumps (first submersible pump 12 and second submersible pump 22).

[0086] In this embodiment, the height of the first submersible pump 12 is approximately 2m; to avoid cavitation, the minimum liquid level to prevent pump cavitation must not be less than 1.5m.

[0087] To achieve both "avoiding cavitation" and "fully submerging the submersible pumps below the LNG level," the following conditions must be met:

[0088] (1) The L3 liquid level should be higher than the upper surface of the first submersible pump 12, i.e., h3>2m+h2, so that the submersible pump can be completely submerged below the LNG liquid level.

[0089] (2) The liquid level h3 of L3 should be higher than 1.5m+h2.

[0090] At the liquid level L1 at the pump well inlet, Bernoulli's equations can be established on the outside and inside of the negative pressure chamber, respectively, to obtain:

[0091] P2+ρg(h0-h2)+0.5*ρv2 2 =P1+ρg(h3-h2)+0.5*ρv1 2

[0092] The average flow velocity inside the negative pressure chamber is set to v1 = 0.454 m / s, while the flow velocity outside the negative pressure chamber, v2, can be approximated as 0. The density of LNG, ρ, is 480 kg / m³. 3 .thus:

[0093] △P=P2-P1=4708.8(h3-h0)+49.5

[0094] △P can be adjusted within the range of 0 to 29 kPa, thus the range of h1 = (h3 - h0) can be calculated. Simultaneously, L2 is set to the lowest cleaning level L1, i.e., h0 = h2 = 150 mm. Calculations show that when △P ≥ 10 kPa, h3 = 2.263 m. At this point, not only can the submersible pump be completely submerged below the LNG level and cavitation be avoided, but the L2 level can also be lowered to 0.15 m to achieve the goals of reducing the dead zone level and achieving extreme cleaning.

[0095] Under the above operating conditions, the LNG storage tank of the present invention can start and stop the cryogenic pump at any time when L2 is less than 1.5m, and is not affected by cavitation.

[0096] In this embodiment, the operating pressure P2 (gas gauge pressure) of the gas phase space 30 inside the storage tank body 3 is assumed to be P2 = 10 kPa, and the flow velocity V1 in the first pump well pipe 11 is assumed to be 0.454 m / s; the cavitation level of the first submersible pump 12 is set to 1.5 m. In this embodiment, the dead zone level is reduced from 1.5 m to 0.15 m. The operation method of the present invention will be described below through a complete feeding and discharging operation process, specifically including the following steps:

[0097] Step a, Feeding operation: The top of the negative pressure protective cover 4 is connected to the gas phase space 30 inside the storage tank body 3, so that the gas pressure inside the negative pressure protective cover 4 is the same as the gas pressure inside the gas phase space 30 inside the storage tank body 3, and the internal space of the negative pressure protective cover 4 is smoothly filled with liquid; control the pressure of the external pipeline connected to the first pump well pipe 11, so that the liquid level in the first pump well pipe 11 is basically equal to the operating liquid level inside the storage tank body 3, and the first submersible pump 12 is completely submerged below the liquid level;

[0098] Specifically, it includes:

[0099] When the storage tank body 3 is fed, the first valve 41 on the outside of the storage tank body 3 opens, and the top of the negative pressure protective cover 4 connects to the gas phase space 30 inside the storage tank body 3, ensuring that the LNG liquid fills the internal space of the negative pressure protective cover 4. At this time, the negative pressure chamber of the negative pressure protective cover 4 is filled with liquid, and the L3 liquid level reaches its maximum value (the maximum height of the negative pressure protective cover 4 in this embodiment is 3m, therefore L3 = 3m), while the L2 operating liquid level will be much higher than L3. In this embodiment, the maximum value of L2 is set to 30m; Figure 4 As shown, during the feeding operation, the top of the first pump well pipe 11 is connected to the gas phase space 30, so the liquid level L4 in the first pump well pipe 11 is approximately equal to L2, and the first submersible pump 12 is completely submerged below the liquid level. At this time, it can be started at any time to perform the liquid discharge operation.

[0100] Step b, Discharge operation: The first valve 41 on the outside of the storage tank body 3 is closed, the top of the negative pressure protective cover 4 is disconnected from the gas phase space 30 inside the storage tank body 3, the first submersible pump 12 is started to discharge liquid outward, and the operating liquid level inside the storage tank body 3 continues to decrease; when the operating liquid level inside the storage tank body 3 decreases to close to the cavitation liquid level of the pump, the air pressure regulating device is started to inject air into the storage tank body to increase the operating pressure of the gas phase space 30 to the first pressure value. The pressure at the bottom inlet of the first pump well pipe 11 is higher than the cavitation pressure of the pump, and the first submersible pump 12 continues to discharge liquid outward.

[0101] Specifically, it includes:

[0102] Before performing the liquid discharge operation, the first valve 41 on the outside of the storage tank body 3 must be closed in advance to ensure that the negative pressure chamber is filled with as much LNG liquid as possible.

[0103] like Figure 5 As shown, after starting the first submersible pump 12, the liquid level L2 will continuously decrease from a height of 30m. When the liquid level L2 decreases to near the pump's cavitation level (in this embodiment, the pump's cavitation level is 1.5m, at which point L2 is close to 4m), the pressure regulating device (BOG compressor or vaporizer) is activated to inject evaporated gas into the storage tank body 3, so that the operating pressure P2 of the gas phase space 30 is maintained at about 10kPa.

[0104] When the L2 liquid level continues to drop to about 0.887m, the L3 liquid level also begins to drop from a height of 3m, and the liquid level difference between L3 and L2 will always remain at 2.113m (2.113m + 0.887m = 3m).

[0105] If the first submersible pump 12 is kept running continuously without interruption, the liquid level L4 in the first pump well pipe 11 will always be full. Since the P2 pressure is always maintained at 10 kPa, the pressure at the bottom inlet of the first pump well pipe 11 is much higher than the pump's cavitation pressure, which meets the requirements of "avoiding cavitation" and "fully immersing the submersible pump below the LNG level". Therefore, the first submersible pump 12 can continue to work.

[0106] If the first submersible pump 12 is temporarily stopped at this time, the liquid level L4 in the first pump well pipe 11 will be consistent with the liquid level L3 in the negative pressure chamber. Since the minimum value of L3 is h3 = 2.263m, it can also meet the requirements of "avoiding cavitation" and "fully immersing the submersible pump below the LNG level". The pump can be started at any time to continue pumping the dead zone liquid level.

[0107] In contrast, in a traditional pump well structure, if the pump is stopped at this point, the L4 liquid level is likely to fall below the 1.5m cavitation level. Once the pump is stopped, it cannot be restarted, and the residual liquid cannot be extracted. This is one of the advantages of this invention over traditional pump wells.

[0108] Step c, tank cleaning operation:

[0109] The pressure regulating device maintains the operating pressure of the gas phase space at the first pressure value, and the first submersible pump 12 continuously discharges liquid. When the operating liquid level in the storage tank body 3 drops to near the minimum cleaning liquid level, the cleaning is completed.

[0110] Specifically, it includes:

[0111] When the L2 liquid level drops to approximately 0.15m (i.e., the L1 liquid level), h3 = 2.663m, which still meets the requirements of "avoiding cavitation" and "fully submerging the submersible pump below the LNG liquid level". At this point, tank cleaning is complete.

[0112] As can be seen from the above operations, the present invention can reduce the dead zone liquid level from 1.5m in the traditional pump well structure to 0.15m (0.15m is the minimum value allowed by the specification).

[0113] For a storage tank with a diameter of 84.2m, the effective volume will be increased by approximately 7513m³. 3 The volume will increase by 3.75%; for a 270,000 cubic meter storage tank with a diameter of approximately 110m, the effective volume will increase by approximately 12,823 m³. 3 The volume increased by 4.75%.

[0114] As described above, the cryogenic storage tank and its operation method with a pump well structure for reducing dead zone liquid level of the present invention have the following beneficial effects:

[0115] In this invention, a pressure regulating device is connected to the tank body, breaking the normal operating pressure fluctuation limit of 4-7 kPa within the cryogenic storage tank. The pressure regulating device expands the operating pressure range within the gas phase space, achieving "avoidance of cavitation" and "complete immersion of the first submersible pump below the liquid level," reducing the dead zone liquid level height of the cryogenic storage tank, increasing the effective volume of the tank, and maximizing the extraction of residual liquid after tank cleaning. A unique negative pressure protective cover is set up, with the outer side of the first submersible pump forming a negative pressure chamber. This negative pressure chamber establishes a stable liquid level around the submersible pump, preventing the intake of vortices and bubbles. This invention does not require modification of the tank's foundation, does not require the addition of complex process equipment, does not change the performance parameters of the existing submersible pump, and does not alter the main structure of the tank bottom or inner tank wall. It makes full use of existing process equipment and is applicable to all types of cryogenic storage tanks using submersible pumps for liquid discharge. It has a simple structure, low investment, and good results.

[0116] The above description is merely an illustrative embodiment of the present invention and is not intended to limit the scope of the invention. Any equivalent changes and modifications made by those skilled in the art without departing from the concept and principles of the present invention should fall within the scope of protection of the present invention.

Claims

1. A cryogenic storage tank equipped with a pump well structure for reducing dead zone liquid level, characterized in that, The system includes a storage tank body; at least one first pump well structure is provided within the storage tank body, the first pump well structure includes a first pump well pipe that is sealed and inserted through the top of the storage tank body, a first submersible pump that can be completely submerged below the liquid level is provided at the bottom of the first pump well pipe, a negative pressure protective cover that is sealed at the top and open at the bottom is provided at the bottom inlet of the first pump well pipe, the negative pressure protective cover is used to establish a negative pressure chamber to reduce the dead zone liquid level and avoid vortex bubbles when the first submersible pump discharges liquid, a gas phase space is formed between the liquid level and the top of the tank body, the top of the negative pressure protective cover can be opened and closed to communicate with the gas phase space; a gas pressure regulating device is connected to the storage tank body to adjust the operating pressure in the gas phase space, the gas pressure regulating device can increase the operating pressure in the gas phase space so that the liquid static pressure at the first submersible pump is greater than the liquid saturated vapor pressure; The bottom of the negative pressure protective cover is provided with a flow-guiding skirt structure, which is used to absorb residual liquid, collect vortex bubbles, and stabilize the inlet flow field. The drainage skirt structure is connected to the bottom plate of the storage tank body through a bottom support structure; The lower opening of the diversion skirt structure guides liquefied natural gas to the inlet of the first pump well structure; the flow cross-sectional area of ​​the gap between the diversion skirt structure and the bottom plate of the storage tank body is larger than the cross-sectional area of ​​the first pump well structure. The lower edge of the drainage skirt structure is close to the lowest cleaning liquid level during tank cleaning. The top of the negative pressure protective cover is connected to the first end of the external connecting pipe, and the second end of the external connecting pipe passes through the tank body and returns to the gas phase space of the tank body; a first valve is installed on the external connecting pipe at the outside of the tank body, and the first valve is used to control the communication state between the negative pressure protective cover and the gas phase space.

2. The cryogenic storage tank with a pump well structure for reducing dead zone liquid level as described in claim 1, characterized in that, The storage tank body is also provided with a plurality of second pump well structures, each of which includes a second pump well pipe and a second submersible pump is provided at the bottom end of the second pump well pipe.

3. The cryogenic storage tank with a pump well structure for reducing dead zone liquid level as described in claim 1, characterized in that, During tank cleaning operations, the operating pressure of the gas phase space inside the tank body is greater than or equal to the saturated vapor pressure of the cryogenic medium under the operating environment, and less than or equal to the upper limit of the pressure that the tank body can withstand.

4. The cryogenic storage tank with a pump well structure for reducing dead zone liquid level as described in claim 1, characterized in that, The distance between the inner wall of the negative pressure protective cover and the outer wall of the first pump well pipe is 1mm to 10000mm; the sum of the heights of the negative pressure protective cover and the drainage skirt structure is 500mm to 12000mm.

5. The cryogenic storage tank with a pump well structure for reducing dead zone liquid level as described in claim 1, characterized in that, The negative pressure protective cover is provided with multiple stiffening plates and stiffening plate supports at intervals.

6. The cryogenic storage tank with a pump well structure for reducing dead zone liquid level as described in claim 1, characterized in that, The first pump well pipe and the negative pressure protective cover are connected to the top and inner wall of the storage tank body through the pump well support structure.

7. An operating method for a cryogenic storage tank equipped with a pump well structure for reducing dead zone liquid level as described in any one of claims 1-6, characterized in that, Includes the following steps: Step a, Feeding operation: The top of the negative pressure protective cover is connected to the gas phase space inside the storage tank. The gas pressure inside the negative pressure protective cover is the same as the gas pressure in the gas phase space. The internal space of the negative pressure protective cover is filled with liquid. Control the pressure of the external pipeline connected to the first pump well pipe. The liquid level in the first pump well pipe is equal to the operating liquid level inside the storage tank. The first submersible pump is completely submerged below the liquid level. Step b, Discharge Operation: The top of the negative pressure protective cover is disconnected from the gas phase space inside the storage tank. The first submersible pump is started to discharge liquid outwards, and the operating liquid level inside the storage tank continues to decrease. When the operating liquid level inside the storage tank decreases to near the cavitation liquid level of the pump, the air pressure regulating device is started to inject air into the storage tank, increasing the operating pressure of the gas phase space to the first pressure value. The pressure at the bottom inlet of the first pump well pipe is higher than the cavitation pressure of the pump, and the first submersible pump continues to discharge liquid outwards. Step c, tank cleaning operation: The pressure regulating device maintains the operating pressure of the gas phase space at the first pressure value, and the first submersible pump continuously discharges liquid. When the operating liquid level in the tank body drops to near the minimum cleaning liquid level, the cleaning is completed.

Images