A method of nitrogen injection for insulation of an LNG carrier

Abstract

The application relates to the technical field of liquefied natural gas (LNG) ship, in particular to a nitrogen injection method for an insulation layer of an LNG ship, which comprises the following steps: injecting a part of nitrogen into a cargo hold of the LNG ship for inertization; performing vacuum extraction on a secondary insulation layer of the cargo hold; when the pressure difference between the primary insulation layer and the secondary insulation layer is greater than a first set value, performing vacuum extraction on the primary insulation layer; when the primary insulation layer and the secondary insulation layer are vacuum extracted to corresponding first and second target values, stopping; when the oxygen content at the top of the cargo hold is lower than a second set value, starting nitrogen injection on the primary insulation layer; when the pressure difference between the primary insulation layer and the secondary insulation layer is greater than a third set value, performing nitrogen injection on the secondary insulation layer; the primary insulation layer and the secondary insulation layer always maintain a set pressure difference until the primary insulation layer and the secondary insulation layer return to normal pressure, and the nitrogen injection on the primary insulation layer, the secondary insulation layer and the cargo hold is completed. The nitrogen injection method for the insulation layer of the LNG ship aims to solve the problem that the operation process of the traditional nitrogen injection method for the insulation layer of the LNG ship is relatively complicated.

Description

Technical Field

[0001] The present invention relates to the technical field of liquefied natural gas ships, and in particular to a nitrogen injection method for an insulation layer of an LNG ship. Background Art

[0002] Before the gas trial, the primary and secondary insulation layers must be deoxygenated and nitrogen injected for nitridation. This critical task requires experienced personnel. Improper operation, resulting in the inhalation of air, increases the number of pumping and injection cycles, resulting in a waste of manpower and material resources. Therefore, a cost-effective and safe method is needed to meet the requirements of the trial operation.

[0003] In the current LNG ship insulation layer nitrogen injection method, such as Figure 1 As shown, the operation process is relatively complicated, which is a test for the operator's physical strength and energy. In order to ensure the smooth operation of the vaporizer, the opening of the valve V3 needs to be continuously adjusted during the nitrogen injection process of the insulation layer. Once a problem occurs in the liquid supply, the operator fails to discover it in time, causing the insulation layer with a large negative pressure to suck air back from the valve V3. This will cause the oxygen content in the space of the insulation layer to fail to meet the standard after two injections, thereby increasing the number of injections. In addition, the valve V3 is always kept at a certain opening, which also causes a certain amount of nitrogen waste, thereby increasing production costs and operational risks.

[0004] Therefore, the inventors provide a method for injecting nitrogen into the insulation layer of an LNG ship. Summary of the Invention

[0005] (1) Technical problems to be solved

[0006] The embodiment of the present invention provides a method for injecting nitrogen into the insulation layer of an LNG ship, which solves the technical problem that the conventional method for injecting nitrogen into the insulation layer of an LNG ship has a relatively complicated operation process.

[0007] (2) Technical solution

[0008] The present invention provides a method for injecting nitrogen into an insulation layer of an LNG ship, comprising the following steps:

[0009] Inject part of the vaporized nitrogen into the cargo hold of the LNG carrier for inerting;

[0010] Then, the secondary insulation layer of the cargo hold is vacuumed, and when the pressure difference between the secondary insulation layer and the primary insulation layer is greater than a first set value, the primary insulation layer is vacuumed;

[0011] When the primary insulating layer and the secondary insulating layer are evacuated to the corresponding first target value and second target value, the vacuum pump is stopped and the corresponding valves are closed;

[0012] when the oxygen content of the top of the cargo tank is lower than a second set value, another part of the vaporized nitrogen is used to inject nitrogen into the main insulation layer; when the pressure difference between the main insulation layer and the secondary insulation layer is greater than a third set value, the secondary insulation layer is injected with nitrogen;

[0013] The main insulation layer and the secondary insulation layer maintain a set pressure difference until the main insulation layer, the secondary insulation layer and the cargo tank are restored to normal pressure, and the injection of nitrogen into the main insulation layer, the secondary insulation layer and the cargo tank is completed.

[0014] Further, before the part of the vaporized nitrogen is injected into the cargo tank of the LNG ship for inertization, the cargo tank is subjected to dry airization in advance to reduce the cargo tank dew point to at least -25℃.

[0015] Further, after the dry airization of the cargo tank, the pressure of the cargo tank is reduced to below 4.5kPa.

[0016] Further, the first set value is 150mbar.

[0017] Further, the difference between the first target value and the second target value is 20-50mbar.

[0018] Further, the first target value is -750mbar.

[0019] Further, the pressure difference between the main insulation layer and the cargo tank is -800mbar.

[0020] Further, the second target value is -780mbar.

[0021] Further, the second set value is 3%vol.

[0022] Further, the top of the cargo tank is the upper 1 / 3 space in the height direction thereof.

[0023] (3) Beneficial effects

[0024] In summary, the present application completes the nitrogenization of the top of the cargo tank in advance, uses it as a buffer tank, and simultaneously performs the nitrogenization of the cargo tank and the injection of nitrogen into the insulation layer, so that the liquid nitrogen is fully utilized, the injection of nitrogen into the insulation layer is performed under the condition of stable operation of the vaporizer, the production cost is reduced, the effect of reducing cost and increasing benefit is obtained, the test time is shortened, and the production capacity is improved. The method has both economy and safety. BRIEF DESCRIPTION OF DRAWINGS

[0025] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required to be used in the embodiments of the present application will be briefly introduced as follows. Obviously, the drawings described below only constitute some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0026] Figure 1 is a flow direction schematic diagram of nitrogen injection of an insulation layer of a conventional LNG ship;

[0027] Figure 2 is a flow direction schematic diagram of nitrogen injection of an insulation layer of a conventional LNG ship;

[0028] Figure 3 is a flow direction schematic diagram of nitrogen injection of an insulation layer of a conventional LNG ship.

[0029] In the drawings:

[0030] 1-cargo hold; 2-main insulation layer; 3-secondary insulation layer. DETAILED DESCRIPTION

[0031] The embodiments of the present application will be further described in detail below with reference to the drawings and embodiments. The detailed description of the following embodiments and the drawings are used to exemplarily illustrate the principles of the present application, but cannot be used to limit the scope of the present application, that is, the present application is not limited to the described embodiments, and covers any modification, replacement and improvement of parts, components and connection modes without departing from the spirit of the present application.

[0032] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The present application will be described in detail below with reference to the drawings and in combination with the embodiments.

[0033] Figure 2 is a flow direction schematic diagram of nitrogen injection of an insulation layer of a conventional LNG ship;

[0034] S100, a part of the vaporized nitrogen is injected into the cargo hold of the LNG ship for inertization.

[0035] Specifically, according to the design requirements, the dew point of all cargo holds is measured, and it is confirmed whether the dew point of the cargo hold meets the design requirements; according to the test requirements, the cargo hold pressure is reduced; a certain cargo hold is selected for inertization. The number of replacement inertization cabins can be 1 or 2, which can be determined according to the supply rate and supply amount of liquid nitrogen; since the insulation layer needs to be extracted and injected twice, in order to cooperate with the two nitrogen injection, four cargo holds cannot be replaced at the same time.

[0036] S200, then the cargo compartment of the secondary insulation layer vacuum, when the pressure difference between the primary insulation layer and the secondary insulation layer is greater than the first set value, the primary insulation layer is vacuumed.

[0037] Specifically, the initial vacuum to the secondary insulation layer can be pulled with the primary insulation layer pressure difference larger, can be selected to 150 mbar (i.e. the first set value), and then the primary insulation layer and vacuum. Throughout the process of vacuum, the need to control the opening of the relevant valve, so that the primary insulation layer, the pressure difference between the secondary insulation layer gradually reduced, until the vacuum is in place, the pressure difference between them in 20 ~ 50 mbar.

[0038] S300, when the primary insulation layer, the secondary insulation layer vacuum to the corresponding first target value, the second target value, stop vacuum pump and close the corresponding valve.

[0039] Specifically, the first target value is -750 mbar, and the second target value is -780 mbar.

[0040] S400, when the top of the cargo compartment oxygen content is lower than the second set value, the other part of the nitrogen gas vaporization begins to inject nitrogen into the primary insulation layer; when the pressure difference between the primary insulation layer and the secondary insulation layer is greater than the third set value, the secondary insulation layer is injected.

[0041] Specifically, the second set value is 3%vol, and the pressure of the cargo compartment is not greater than 4.5kpa during the vacuum of the insulation layer, and the inerting rate is also controlled. The best rate is when the insulation layer is vacuumed, as shown in Figure 3 The inerting of the cargo compartment 1 ensures that the top oxygen content meets the standard (i.e. the second set value), and then the insulation layer is injected with nitrogen. First, open valve V1 to inject nitrogen into the primary insulation layer 2. When the primary insulation layer 2 and the secondary insulation layer 3 are pulled apart by a certain pressure difference, the secondary insulation layer 3 is opened, and valve V2 is opened to inject nitrogen. It should be noted that the top of the cargo compartment is generally the upper 1 / 3 space in the height direction.

[0042] S500, the primary insulation layer and the secondary insulation layer always maintain a set pressure difference until the primary insulation layer and the secondary insulation layer return to normal pressure, and the nitrogen injection of the primary insulation layer, the secondary insulation layer and the cargo compartment is completed.

[0043] Specifically, during the nitrogen injection process, the cargo compartment pressure is not reduced, which indicates that the nitrogen gas from the vaporizer is being injected into the insulation layer. However, even if the cargo compartment pressure decreases, there is no need to worry about the insulation layer being sucked back into the air, because the top of the cargo compartment has been nitrogenized at this time, and the gas will not cause the oxygen content of the insulation layer to exceed the standard, as shown in Figure 3As shown, only by adjusting the valve V3 to ensure that the cargo hold pressure is flat or slightly increased, so that the cargo hold replacement of the amount of gas will be small, continue to inject nitrogen to the insulation layer. At the same time, the completion of the cargo hold inerting needs to be controlled before the end of the insulation layer nitrogen injection, so that the nitrogenation of the cargo hold has been completed, and the insulation layer injection is not completed. In order to continue to inject nitrogen, nitrogen will inevitably flow directly to the atmosphere, causing waste.

[0044] As an optional embodiment, before step S100, a part of the vaporized nitrogen is injected into the cargo hold of the LNG ship for inerting, further comprising: pre-drying the cargo hold to make the cargo hold dew point at least-25℃. Of course, the drier the cargo hold is replaced by dry air, the better the drying effect after subsequent inerting with nitrogen.

[0045] As an optional embodiment, after pre-drying the cargo hold, further comprising: reducing the pressure of the cargo hold to below 4.5kPa. In order to ensure that the pressure difference between the main insulation layer and the cargo hold is within-800mbar when the main insulation layer is subsequently pumped to-750mbar of large negative pressure, the safety of the insulation layer is ensured.

[0046] It should be clear that each embodiment in the specification is described in a progressive manner, and the same or similar parts between each embodiment can be referred to each other. Each embodiment focuses on the difference from other embodiments. The present application is not limited to the specific steps and structures described above and shown in the drawings. Moreover, for the sake of brevity, detailed description of known methods and techniques is omitted.

[0047] The above is only an embodiment of the present application, and is not limited to the present application. Without departing from the scope of the present application, the present application can have various modifications and changes for those skilled in the art. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application shall be included in the scope of the claims of the present application.

Claims

1. A method for injecting nitrogen into the insulation layer of an LNG ship, characterized in that: The method comprises the following steps: Inject part of the vaporized nitrogen into the cargo hold of the LNG carrier for inerting; Then, the secondary insulation layer of the cargo hold is vacuumed, and when the pressure difference between the secondary insulation layer and the primary insulation layer is greater than a first set value, the primary insulation layer is vacuumed; When the primary insulating layer and the secondary insulating layer are evacuated to the corresponding first target value and second target value, the vacuum pump is stopped and the corresponding valves are closed; When the oxygen content at the top of the cargo hold is lower than a second set value, another portion of the vaporized nitrogen begins to be injected into the primary insulation layer; when the pressure difference between the primary insulation layer and the secondary insulation layer is greater than a third set value, the secondary insulation layer is injected with nitrogen; The primary insulation layer and the secondary insulation layer always maintain a set pressure difference until the primary insulation layer and the secondary insulation layer return to normal pressure, and the nitrogen injection of the primary insulation layer, the secondary insulation layer and the cargo hold is completed.

2. The nitrogen injection method for the insulation layer of an LNG ship according to claim 1, characterized in that: Before injecting a portion of the vaporized nitrogen into the cargo hold of the LNG ship for inerting, the method further includes: pre-drying the cargo hold to reduce the dew point of the cargo hold to at least -25°C.

3. The nitrogen injection method for the insulation layer of an LNG ship according to claim 2, characterized in that: After the cargo hold is pre-dry-aired, the method further includes: reducing the pressure of the cargo hold to below 4.5 kPa.

4. The nitrogen injection method for the insulation layer of an LNG ship according to claim 1, characterized in that: The first set value is 150 mbar.

5. The nitrogen injection method for the insulation layer of an LNG ship according to claim 1, characterized in that: The difference between the first target value and the second target value is 20-50 mbar.

6. The nitrogen injection method for the insulation layer of an LNG ship according to claim 5, characterized in that: The first target value is -750 mbar.

7. The nitrogen injection method for the insulation layer of an LNG ship according to claim 6, characterized in that: The pressure difference between the main insulation layer and the cargo hold is -800 mbar.

8. The nitrogen injection method for the insulation layer of an LNG ship according to claim 5, characterized in that: The second target value is -780 mbar.

9. The nitrogen injection method for the insulation layer of an LNG ship according to claim 1, characterized in that: The second set value is 3% vol.

10. The nitrogen injection method for the insulation layer of an LNG ship according to claim 1, characterized in that: The top of the cargo hold is the upper 1 / 3 of the space in its height direction.

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