RU2770334C2 - Inerting device for liquefied gas storage tank of vessel for transportation of liquefied gas - Google Patents

Abstract

FIELD: oil and gas industry.SUBSTANCE: group of inventions relates to an inerting device for a liquefied gas storage tank of a liquefied gas carrier. Device consists of tank (1) for storage of liquefied gas, isolated by main insulation space, forming inner shell around tank, and auxiliary insulation space, forming an outer shell around the reservoir. Each of the spaces includes an insulator and is intended for filling with an inert gas. Reservoir (1) is located between two sealed partitions (2c) and above bottom (2a) of the reservoir. Drain box (20) is located under the reservoir. Box is connected to channel (22) passing between the bottom of the tank and the outer shell so that fluids can drain under the action of gravity from the bottom of the tank into the drain box. Drain box is in fluid communication with channel and auxiliary space, and means for supply and/or pumping out of nitrogen from auxiliary space includes nitrogen supply line (30), which is connected to the drain box so that nitrogen supplied to this space and/or pumped out from this space circulates through the drain box.EFFECT: inerting of auxiliary space of tank of any type without special crossing of its load-carrying structure.10 cl, 10 dwg

Description

FIELD OF TECHNOLOGY TO WHICH THE INVENTION RELATES

The present invention relates to an inerting device for a liquefied gas storage tank of an liquefied gas carrier, as well as to a ship containing at least one such device. The invention can be applied in particular to the inertia of a membrane tank used for the storage of liquefied natural gas (LNG).

PRIOR ART

The prior art includes documents FR-A1-3 014 197 and FR-A1-2 317 159.

An liquefied natural gas carrier includes one or more sealed and insulated storage tanks for liquefied natural gas. Each tank includes a tank wall having, in series in the thickness direction from the inside to the outside of the tank, a main sealing membrane for contact with liquefied natural gas, a main thermal barrier, an auxiliary sealing membrane, an auxiliary thermal barrier, and a supporting structure defining the overall shape. tank.

The primary and secondary sealing membranes define between them a primary space which contains the primary sealing barrier and which is intended to be filled with an inert gas, typically nitrogen. The inert gas is also supplied to the auxiliary space formed by the auxiliary sealing membrane.

The supporting structure of the tank is formed, on the one hand, by sealed transverse partitions, called "cofferdam", located on at least two opposite sides of the tank, and the bottom of the tank, which is located under the tank and forms a passage with it, in which fluids can flow e.g. water condensate or LNG in the event of a leak. These fluids typically drain by gravity into a drain box prior to being pumped out.

The sealing membranes of such a tank may have leaks causing LNG to flow from inside the tank towards the primary and secondary thermal barriers. However, when the combustible gas is in the presence of an oxidizing gas, and the concentration of the combustible gas is in the concentration range between its lower explosion limit (LEL) and the upper explosion limit (UEL), and the oxidizing gas is in the appropriate concentration range, the combustible gas may ignite and explode. .

Inertia of the main and auxiliary spaces, i.e. placing them in an inert atmosphere avoids accidents. Thus, combustible and oxidizing gases that may be present in thermal barriers are diluted so that they do not reach explosive conditions.

In the existing technology, each of the main and auxiliary spaces is connected to a nitrogen supply line, which allows nitrogen to be supplied to the space, as well as pumping nitrogen out of it. Therefore, there is a bi-directional line that is connected to the space on one or two fluid communication holes.

There are several tank placement technologies. The tank can be completely "sinked" and covered with a double deck of the vessel. In another configuration, the inner and outer shells of the tank protrude from the double deck of the ship to form vapor and liquid domes in the form of two columns or chimneys designed to pass through the cargo transfer equipment for pumping the liquid phase and vapor phase of the liquefied gas contained in the tank.

Thanks to this technology, it is easier to access the main and auxiliary spaces in the domes and connect them to the nitrogen supply lines. This does not apply to the first technology, in which the nitrogen supply lines must cross a double jumper to connect to the spaces to be inert. The problem also applies to the pilot operated safety valves that the tanks are equipped with.

The tank is equipped with such valves to prevent overpressure in the main and auxiliary spaces. In the first technology described above, the valves must traverse the double upper deck to connect to the spaces.

As a general rule, intersections between the double deck and the supporting structure of the tank should be avoided as they can lead to leaks and significant additional design and construction costs for the tank.

The present invention provides a simple, effective and cost effective improvement that allows the ancillary space of any type of tank to be inert without specifically intersecting its supporting structure.

SUMMARY OF THE INVENTION

The invention provides an inerting device for a liquefied gas storage tank of an liquefied gas carrier, comprising:

a tank for storing liquefied gas, including in series in the direction from the inside to the outside of the tank the main sealing membrane designed for contact with liquefied gas, the main heat-insulating barrier, the auxiliary sealing membrane, the auxiliary heat-insulating barrier and the supporting structure that determines the overall shape of the tank, wherein said main and auxiliary sealing membranes defining between them the main space containing said heat-insulating barrier form an inner shell around the tank, and said auxiliary sealing membrane and said supporting structure defining an auxiliary space between them containing said auxiliary heat-insulating barrier form an outer shell around tank, and each of the spaces includes an insulator and is designed to be filled with an inert gas, and said tank for storing liquefied this gas is located between two hermetic baffles and above the bottom of the tank, while the aforementioned hermetic baffles and the aforementioned tank bottom are part of the aforementioned supporting structure,

a drain box located under the liquefied gas storage tank, the drain box being connected to a channel extending between the bottom of the tank and the outer shell so that fluids can flow under gravity from the bottom of the tank into the drain box,

means for supplying nitrogen to the main and auxiliary spaces and/or pumping nitrogen out of the main and auxiliary spaces,

characterized in that said drainage box is in fluid communication with said channel and said auxiliary space, and in that said means for supplying and/or pumping out nitrogen from said auxiliary space includes a nitrogen supply line which is connected to said drainage box so that the nitrogen supplied through the nitrogen supply line to the auxiliary space and/or pumped out of the auxiliary space circulates through the drain box.

Thus, the invention proposes to connect the nitrogen supply line to the auxiliary space via a drain box. The drain box is connected to a channel extending between the bottom of the tank and the outer shell to allow collection of fluids that may flow through this channel. The drain box is also connected to the ancillary space to provide fluid communication between the nitrogen supply line and the ancillary space. In this regard, the auxiliary space should not be equipped with holes intended for connection with the nitrogen supply line, which simplifies the design of the tank. The nitrogen supply line may have a dual function, namely, the function of supplying nitrogen to the auxiliary space for inerting it, as well as the function of supplying nitrogen to the drainage box. This prevents accidental explosions, both in the ancillary space and in the drain box, where LNG can enter in the event of a leak.

Supplying nitrogen to the ancillary space via a drain box also makes it easier to pump gas, such as methane, from the ancillary space. This gas is displaced from the auxiliary space by nitrogen, which, because of its lightness compared to methane, tends to displace the methane to the top of the tank, which usually has a fluid communication port.

The device according to the invention may include one or more of the following features, taken alone or in combination with each other:

said drainage box is connected to a nitrogen drainage and circulation pipeline leading to said channel and said ancillary space,

the device further comprises at least one safety valve, preferably controlled, which is directly connected to said nitrogen supply line; this means that the safety valve or valves that shut off the auxiliary space can be installed directly on the nitrogen supply line and not on the tank; therefore, there are fewer reservoir crossings, which is preferable for the reasons described above; in addition, the placement of the valve(s) in the nitrogen supply line has a clear advantage in terms of reaction speed compared to prior art technology, which will be explained in more detail below,

said at least one safety valve includes a main line including a main valve and a control line including a control valve, wherein at least one of said main and control lines is directly connected to said supply line nitrogen,

the device includes two controllable safety valves, and at least one of the mentioned main and control lines of each of these valves is directly connected to the mentioned nitrogen supply line,

the main and control lines of the valve or of each valve are directly connected to said nitrogen supply line,

the device further includes at least. one safety valve, preferably controlled, which is directly connected to said auxiliary space; this relief valve provides additional safety by preventing excessive pressure build-up in the auxiliary space,

at the upper end of the tank, the outer shell is completely above the inner shell,

alternatively, at the upper end of the tank, the inner and outer shells protrude from a double bridge covering the tank to form domes.

The present invention also relates to a liquefied gas carrier including at least one apparatus as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become better understood and other details, features and advantages of the present invention will become more apparent upon reading the following description, provided by way of example and with reference to the accompanying drawings, in which:

fig. 1 is a very schematic view of the inerting device of a tank for transporting liquefied gas,

fig. 2 is a schematic view of the drain box and tank section and illustrates an embodiment of the invention,

figures 3-5 are very schematic views of other embodiments of the invention for a tank of the same type,

figures 6 and 7 are very schematic views of other embodiments of the invention for a different type of tank,

fig. 8 is a schematic sectional view of an actuated relief valve, and

Figures 9 and 10 are very schematic views of a tank fitted with a pilot operated relief valve.

DETAILED DESCRIPTION

With reference to FIG. 1 schematically shows a tank 1 for storing liquefied gas. Each wall of the tank 1 includes a multilayer structure containing, in the direction from the inside to the outside of the tank 1, a supporting structure 2 that determines the overall shape of the tank 1, an auxiliary heat-insulating barrier 3, including insulating elements resting on the supporting structure 2, an auxiliary sealing membrane 4, the main heat-insulating barrier 5, which includes insulating elements based on the auxiliary sealing membrane 4, and the main sealing membrane 6, intended for contact with the liquefied combustible gas contained in the tank 1.

In the following description, the term "main space" refers to the space containing the main thermal barrier 5, and the term "auxiliary space" refers to the space containing the auxiliary thermal barrier 3. As shown in the drawings, the main space forms an inner shell, which is surrounded by an outer shell , formed by an auxiliary space, which is itself surrounded by a supporting structure 2.

The supporting structure 2 may in particular be a self-supporting metal sheet and/or may be formed by the hull or double hull of the vessel.

Thermal barriers 3, 5 include insulating solids and a gas phase. According to an embodiment, the thermal barriers 3, 5 consist of thermal insulation boxes, which are not shown. The boxes include a bottom panel and a cover panel made of, for example, plywood, and a plurality of dividers located between the bottom and cover panels. Between the separators there are compartments for placing a heat-insulating gasket. The heat insulating pad may be made of any material having suitable heat insulating properties. For example, the thermal insulation pad is selected from materials such as perlite, glass wool, polyurethane foam, polyethylene foam, PVC foam, aerogels, or other materials.

The main and auxiliary sealing membranes 4, 6 are gas and liquid tight. The supporting structure 2 is also sealed. In this regard, for the purposes of the present description and claims, the term "tight barrier" covers both sealing membranes 4, 6 and the supporting structure 2. Thus, the auxiliary thermal barrier 3 is located in a tight space that is isolated from ambient pressure, on one on the one hand, the first hermetic barrier, consisting of an auxiliary sealing membrane 4, and, on the other hand, the second hermetic barrier, consisting of a supporting structure 2.

Liquefied gas includes a chemical or a mixture of chemicals that liquefies at low temperature and that is in the vapor phase at normal temperature and pressure. The liquefied gas 3 may in particular be liquefied natural gas (LNG), i.e. a gas mixture comprising primarily methane and one or more other hydrocarbons such as ethane, propane, n-butane, isobutane, n-pentane, isopentane and nitrogen in a small proportion. Liquefied natural gas is stored at atmospheric pressure and at a temperature of approximately -162°C.

The liquefied gas may also be ethane or liquefied petroleum gas (LPG), i. e. mixture of hydrocarbons obtained during oil refining, including mainly propane and n-butane. The combustible gas may also be ethylene.

In order to prevent that due to leakage of liquefied natural gas through the sealing membranes 4, 6 and/or air through the supporting structure 2, the gas mixture is present in the walls of the tank 1 in explosive proportions, they are subjected to a method known to a person skilled in the art and described, for example, in the application WO-A1-2015/124536.

In the example shown, the inerting device is specifically designed to inert the ancillary space. The inerting device also includes inert gas supply equipment 11 for purging the thermal barrier 3 with inert gas. The supply equipment 11 includes a compressed inert gas generator 12 connected to an inert gas supply line 14 leading to an auxiliary space. The compressed inert gas generator 12 is connected to the conduit 14 by means of a valve 16 to adjust the flow and/or pressure of the supply of inert gas within the thermal barrier 3. The inert gas is preferably nitrogen, more specifically molecular nitrogen or a mixture including this gas.

The conduit 14 can also be equipped with an optional additional pump 13 for supplying inert gas from the inert gas generator 12.

The invention provides an improvement to this technology, consisting in the supply of molecular nitrogen to the auxiliary space of the tank through the drain box 20 of this tank (see Fig. 2).

A drain box 20 is located below the tank 1 and is connected to a channel 22 extending between the tank bottom 2a of the supporting structure 2 and the outer shell so that fluids can drain by gravity from the bottom of the tank into the drain box. Thus, the box 20 is connected to the end of the first conduit 24, the opposite end of which exits on the upper surface of the bottom of the tank. The duct 20 is also connected to the end of the second pipeline 26 for pumping the collected fluids out of the duct, and the second pipeline may be equipped with a valve 28.

Reference numeral 23 refers to a peripheral guide which is disposed in space and which guides fluids into the drain box 20. For example, the guide is made of a polymerizable resin and can continuously extend around the entire periphery of the tank bottom 2a so that fluids flowing down the peripheral walls bearing structure 2, then fed into the drainage box. Alternatively, the guide can be made of metal profiles.

Reference numeral 30 refers to the auxiliary space nitrogen supply line, i. e. a line that provides both the supply of nitrogen to the auxiliary space and the pumping of nitrogen from this space, i.e. in other words, nitrogen circulation in the auxiliary space. Thus, the nitrogen supply line 30 is bi-directional and can therefore be considered to function as conduits 8 and 14 shown in FIG. one.

The nitrogen supply line 30 leads directly to the drain box 20 and its connecting conduit 24 to the channel 22 is also configured to be in fluid communication with the auxiliary space. In practice, the end of the conduit 24 located in the channel 22 may be extended and may have a first opening leading to the channel 22 and a second opening leading to the ancillary space. Alternatively, the ancillary space may be in fluid communication with the channel 22 through at least one opening provided in the outer shell and leading into the ancillary space.

Figures 3-7 illustrate other embodiments of the invention.

Figures 3-5 refer to a first type tank known as "NO96" and figures 6 and 7 refer to a second type tank known as "Mark".

The upper double deck 2b of the supporting structure 2 of the NO96 tank covers the entire tank, which is also isolated from the other tanks or the rest of the vessel by transverse sealed baffles 2c, called "cofferdam".

In the embodiment shown in FIG. 3, the auxiliary space of the tank 1 is equipped with two controlled safety valves 32 to prevent overpressure in the space. Although not shown, the main space of the tank 1 is also equipped with one or two actuated safety valves.

From a functional point of view, classes, shipyards and shipowners clearly recognize that the main cause of excess pressure in the auxiliary tank space is the failure of the control valve 28, 34 located on the nitrogen supply line 30. Another possible cause of overpressure is related to the heating phase of the spaces.

A problem with prior art technology relates to the distance between the overpressure source (nitrogen supply line 30) and the valves 32. A sudden high flow of nitrogen (valve 34 failure) can cause some delay due to the pressure drop so that the overpressure is not immediately detected by the valve. In this case, the valve will not open quickly enough and the auxiliary space near the nitrogen supply line will quickly overpressure.

The solution to this problem is to place at least one of the two valves 32 directly on the nitrogen supply line 30, which is identified as the only source of overpressure. Preferably, the valve(s) is located between the shutter 34 and the drain box 20. In the embodiment shown in FIG. 4, one valve 32 is installed in line 30 and the other is installed on the tank crossing the double bridge 2b. In the embodiment shown in FIG. 5, two valves 32 are installed in line 30.

The upper double deck 2b of the support structure 2 of the Mark tank is intersected by portions of the inner and outer shells of the tank, which form domes 33a, 33b. The double bridge 2b is interrupted in two places where the tank forms a protruding structure in the form of a column or chimney. The first column is a liquid dome 33a, which serves as a penetration point for various LNG pumping equipment, such as a loading line, an emergency pumping line, a discharge line associated with unloading pumps, a spray line, a supply line associated with a spray pump, etc. d. The second column is a vapor dome 33b which serves as a penetration point, for example a vapor collection line. The operation of this equipment is also known.

In figures 6 and 7, the tank may be equipped with a dedicated line for pumping out the inert gas contained in the auxiliary space (shown by arrow 35). The pumping line can be connected to a degassing column to pump the inert gas to the atmosphere. Thus line 30 of this tank is an inlet line and not bi-directional. Similar technology can be used to equip the tank shown in figures 3-5.

In the present invention, the relief valves 34 are of the controlled type, i. e. they include a main line 36 including a main valve and a control line 38 including a control valve (FIG. 8). Typically, the main line 36 includes an inlet and outlet, between which is the main valve 36a. The inlet and outlet of the main line is connected to a fluid circulation conduit, such as a nitrogen supply line 30 .

The control line 38 includes an inlet and outlet, between which is a control valve 38a. The control line inlet is connected to the above pipeline, and the outlet is connected to the main valve 36a.

Control valve 38a is actuated by high pressure P ₁ and back pressure P ₂ . The control pressure P ₀ in the control valve 38a is equal to atmospheric pressure.

The main valve 26a is an opening valve. The high pressure P ₁ acts on the underside of the bell 42. The pressure P ₁ also passes through the piston rod 43 into the upper chamber 44 of the valve, which exerts pressure on the piston 45. The surface of the piston is larger than the surface of the valve seat, which, under the action of the spring, holds the valve closed.

When the pressure P ₁ reaches the set pressure of the control valve, the opening starts. The pressure P ₂ increases in the control line and in the lower chamber 46 of the main valve. The pressure in the lower chamber is limited by the flow through port 47. When the flow through the control valve exceeds the capacity of the port, the pressure in chamber 46 rises, causing the main valve to open. When the pressure P ₁ decreases, the pilot valve closes and the pressure P ₂ is balanced by port 47. The spring then closes the main valve.

As shown in Figures 9 and 10, when a relief valve 32 is installed in line 30, both main line 36 and control line 38 may be connected to line 30 (Figure 9) or only one of them (Figure 10). In the case shown in FIG. 10, the main line 36 is connected to the line 30, while the control line 38 is connected to the auxiliary space. This excludes the effect of dynamic pressure, and only the static pressure in the volume is measured.

Therefore, the invention makes it possible to simplify the design of a tank for transporting liquefied gas by optimizing the inerting device and the safety valve associated with this device.

Claims (14)

1. An inerting device for an liquefied gas storage tank of an liquefied gas carrier, comprising a tank (1) for storing liquefied gas, including in series in the direction from the inside to the outside of the tank the main sealing membrane (6) intended for contact with liquefied gas, the main heat-insulating barrier (5), the auxiliary sealing membrane (4), an auxiliary heat-insulating barrier (3) and a supporting structure (2) that determines the overall shape of the tank (1), moreover, the said main (6) and auxiliary (4) sealing membranes, defining between them the main space containing the said heat-insulating barrier (5), form the inner shell around the tank (1) and the mentioned auxiliary sealing membrane (4) and the mentioned supporting structure (2), defining between them the auxiliary space containing the mentioned auxiliary heat-insulating barrier (3), form an outer shell around the liquefied gas storage tank, while each of the spaces is meant to be filled with inert gas and said reservoir (1) for storing liquefied gas is located between two sealed partitions (2c) and above the bottom (2a) of the tank, and said sealed partitions and said tank bottom are part of the mentioned supporting structure, a drain box (20) located under the tank for storing liquefied gas, and the drain box is connected to a channel (22) extending between the bottom of the tank and the outer shell so that fluids can drain under the action of gravity from the bottom of the tank into the drain box, means for supplying nitrogen to the main and auxiliary spaces and/or pumping nitrogen out of the main and auxiliary spaces, different in that said drainage box is in fluid communication with said channel and said ancillary space, and that said means for supplying and/or evacuating nitrogen from said auxiliary space includes a nitrogen supply line (30), which is connected to said drainage box so that nitrogen supplied through the nitrogen supply line to the auxiliary space and/or pumped out of the auxiliary space, circulates through the drainage box. 2. The device according to claim 1, in which said drainage box (20) is connected to a pipeline (24) for drainage and nitrogen circulation, which leads to said channel (22) and said ancillary space. 3. Device according to claim 1 or 2, further comprising at least one safety valve (32), preferably controlled, which is directly connected to said nitrogen supply line (30). 4. The device according to the previous paragraph, in which the said at least one safety valve (32) contains the main line (36), including the main valve (36a), and the control line (38), including the control valve (38a) , wherein at least one of said main and control lines is directly connected to said nitrogen supply line (30). 5. The device according to the previous paragraph, which includes two controlled safety valves (32), and at least one of the mentioned main (36) and control (38) lines of each of these valves is directly connected to the mentioned nitrogen supply line (30). 6. The device according to claim 4 or 5, in which the main (36) and control (38) lines of the valve (32) or each valve (32) are directly connected to the mentioned nitrogen supply line (30). 7. A device according to any one of the preceding claims, further comprising at least one safety valve (32), preferably controlled, which is directly connected to said auxiliary space. 8. A device according to any one of the preceding claims, in which, at the upper end of the reservoir (1), the outer shell is located completely above the inner shell. 9. The device according to any one of paragraphs. 1-7, in which at the upper end of the tank (1) the inner and outer shells protrude on a double bridge covering the tank to form domes (33a, 33b). 10. A vessel for the carriage of liquefied gas, including at least one inerting device according to any one of the previous paragraphs.

Images