Gas Inlet Assembly for Oil Tank

Abstract

A gas inlet assembly for an oil tank connectable to a gas inlet pipe (12) for maintaining pressure and a non-explosive atmosphere during the withdrawal of oil from the oil tank (11), the gas inlet assembly comprising at least one inlet conduit (13) arranged to direct a unidirectional gas flow from the inlet pipe (12) towards a primary inlet nozzle (14), the primary inlet nozzle including a diffusing element (141, 141', 141'', 141''') configured to diffuse an incoming inert gas with a horizontal velocity component within the tank that is greater than its vertical velocity component.

Description

[Technical field]

[0001] One challenge in handling volatile fluids in large tanks, such as in oil tankers, is the volatilization of significant amounts of oil. On the one hand, such volatilization represents a loss of product and therefore an economic loss. On the other hand, it creates pollution and burdens on the environment. Last but not least, such volatilization represents a safety risk and may result in fire or explosion. [Background technology]

[0002] Therefore, systems have been designed to minimize or eliminate such volatilization. One such system is described in WO2007 / 086751, which reintroduces vapours volatilized from the oil back into the oil. This and other systems are primarily designed for in-transit situations where the tanks are sealed and in "steady state", or to handle gas emissions during cargo loading operations.

[0003] Certain challenges arise in situations where oil is drained from a tank and the atmosphere above the continuously decreasing oil level is replaced with an inert gas that is introduced to maintain a constant pressure and prevent explosion hazards, and the present invention is intended to minimize such challenges during oil drainage.

[0004] WO2013 162965 A1 teaches a method and device for handling liquid hydrocarbons loaded into and unloaded from tanks. This publication does not discuss addressing the problem of excessive volatilization of VOCs during unloading of tanks.

[0005] US2008 / 011219 A1 relates to a ship having improved ventilation means for oil tanks, but does not describe any mechanism for reducing volatilization during unloading of the tanks. Summary of the Invention [Problem to be solved by the invention]

[0006] It is an object of the present invention to provide a method and / or system that allows for the addition of inert gas to the tanks of an oil tanker during unloading in a manner that reduces the tendency for increased volatilization due to mechanically induced convection and turbulent mixing of the tank atmosphere during unloading. [Means for solving the problem]

[0007] The above object is achieved through the inert gas interface of the present invention as disclosed by claim 1.

[0008] Preferred embodiments of the invention are disclosed by the dependent claims.

[0009] Moderately conical as used herein is understood to mean a cone having an inclination angle of less than 25°, more preferably less than 15°, most preferably less than 5°.

[0010] As used herein, an "inverted cone" is understood to be a cone whose center point is lower than its periphery.

[0011] A negative vertical velocity component, as used herein, is understood to be an upward vertical velocity component.

[0012] A specific objective of the present invention is to increase the inlet area of ​​the tank inlet opening since this leads to lower velocities, less turbulence and therefore less volatilization. A simple lengthening of the inlet conduit would also lead to lower velocities but would be subject to disturbances or turbulence in the incoming flow and would be less than ideal in addressing the overall problem of reducing volatilization.

[0013] The inventors have found that a much better solution is to direct the flow from the extended inlet conduit vertically into a diffusion disk which creates a 360° diffusion of the inlet flow, ensuring that the major velocity components are horizontal from the diffusion disk into the tank.

[0014] The diffuser disk may be flat or moderately conical with a cone angle of a few degrees, or it may be "inverted" conical, i.e. the center of the diffuser disk is its lowest point. This latter embodiment is in fact the preferred embodiment, which makes the outward flow from the diffuser disk mainly horizontal, with a small vertical velocity component actually upwards at the circumference of the diffuser disk.

[0015] In yet another preferred embodiment, the diffusion disk has several small holes to allow a small "leak" flow downward through the disk. The holes are primarily to ensure that liquid does not collect on the disk, and are small enough that they do not significantly affect the general concept of approximately horizontal gas flow into the tank, but also contribute to increasing the overall inlet area.

[0016] The invention will now be explained in more detail with reference to the non-limiting embodiments shown in the drawings. [Brief description of the drawings]

[0017] [Figure 1] 1 is a schematic cross-sectional side view of a tank in which the present invention is contained; FIG. [Figure 2a-2c] 1A-1C are top cross-sectional views of three variations of a vertical inlet conduit according to the present invention; [Figure 3a-3d] 4 is a schematic cross-sectional side view of a variation of an inlet gas supply nozzle in accordance with the present invention; FIG. [Figure 4] FIG. 4 is a schematic cross-sectional side view of an embodiment of the invention different from the embodiment of FIGS. 3a to 3d. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Figure 1 is a schematic simplified cross-sectional side view of a tank 11 with an inert gas interface according to the invention. A supply pipe 12 for gas leads into the tank 11 and branches into a number of vertical inlet conduits 13, usually two or more, each of which is equipped with a primary inlet nozzle 14 of a particular design. Both inlet nozzles shown in Figure 1 are primary inlet nozzles, the design and nature of which will be explained in more detail below. Usually, the inlet conduit (13) is disposed vertically between the supply pipe (12) and the inlet nozzle (14). The presence of a secondary inlet nozzle is optional. Figure 1 further shows a discharge pipe 15 for a volatile liquid, usually oil.

[0019] When draining the oil from the tank through the discharge pipe 15, inert gas is introduced into the tank through the supply pipe 12, the inlet conduit 13 and the inlet nozzle 14 to avoid low pressure in the tank. It is advantageous to set a certain overpressure in the tank to avoid excessive volatilization from the oil surface during draining. At the same time, it is important to avoid mechanically induced convection between the oil and the tank atmosphere or turbulent mixing at the oil surface, both of which would lead to increased volatilization. One factor that contributes to avoiding turbulence in the tank is the cross-sectional dimension of the inlet conduit 13, which is very large, usually larger than the cross-sectional area of ​​the supply pipe 12, thereby allowing a slow movement of the inlet gas for all relevant gas velocities.

[0020] Figures 2a, 2b and 2c are top cross-sectional views of three variations of the inlet conduit 13. In the embodiment shown by Figure 2a, the inlet conduit is divided into eight parallel inlet sections by partitions 131, which ensures that the inlet gas flow is not only slow but also parallel, which means that there is little or no turbulence in the flow. Thus, the entire gas flow reaching the inlet nozzle 14 is parallel, laminar and moves relatively slowly.

[0021] Figure 2b shows a different configuration of the inlet conduit 13' consisting of a number of parallel perforations 132 passing through an otherwise compact pipe element. The high number of perforations ensures extreme directional control of the incoming gas. The disadvantage compared to the embodiment of Figure 2a is that a larger part of the cross-sectional area is occupied by solid material and that the volume available for gas flow is smaller.

[0022] In the embodiment shown by Figure 2c, the inlet conduit 13'' is divided into a grid pattern by partitions 133. This provides better directional control than Figure 2a and the cross-sectional area occupied by solid material is less than in the embodiment of Figure 2b, thus making more volume available for gas flow.

[0023] Overall, all the variations shown in FIGS. 2 a , 2 b and 2 c allow a high degree of directional control of the inlet gas, i.e., laminar flow of the gas reaching the inlet nozzle 14 .

[0024] Attention is now directed to Figures 3a-3d, which show four embodiments of the primary inlet nozzle 14 in more detail. The primary inlet nozzle of Figure 3a includes a diffusing element or diffusing disk 141 connected to the lower end of the inlet conduit 13 and having an inverted conical shape, i.e. a cone shape with a central point as its lowest point. The inverted conical diffusing disk 141 is attached to a rod 142 that extends at least partially through the inlet conduit 13. The inverted conical shape of the disk 141 ensures that the inert gas entering through the pipe stub 141 is pushed radially outward and slightly upward, i.e. with a vertical velocity component defined herein as negative, as it leaves the periphery of the disk. The diffusing disk 141 of Figure 3a further shows several small holes 143 that prevent liquid from accumulating on the diffusing disk. In Figures 3a-3d, the internal structure of the inlet conduit 13, of the kind shown in Figures 2a, 2b and 2c, which ensures a generally parallel flow, is omitted.

[0025] Figure 3b also shows the primary inlet nozzle 14'. The only difference in Figure 3b compared to Figure 3a is the design of the diffuser disk 141' which extends flat and horizontally from the attachment point. This design ensures that the gas entering through the inlet conduit 13 is pushed outwards and predominantly horizontally, i.e. with no vertical velocity component at the periphery of the diffuser disk.

[0026] Reference is now made to Figure 3c, which shows the primary inlet nozzle 14''. The only difference in Figure 3c compared to Figures 3a and 3b is the design of the diffuser disk 141'' which has a moderately conical shape with the uppermost point at the center. This design ensures that the gas entering through the inlet conduit 13 is pushed outwards and slightly downwards, i.e. with a limited vertical velocity component at the periphery of the diffuser disk.

[0027] Reference is now made to Figure 3d, which shows the primary inlet nozzle 14'''. The only difference in Figure 3d compared to Figures 3a-3c is the design of the diffuser disk 141''', which has a curved shape with its concave surface facing upwards and its lowest point at the centre. This design, rather like the design of Figure 3a, causes the gas entering through the inlet conduit 13 to be pushed outwards and slightly upwards, i.e. with a slight negative vertical velocity component at the periphery of the diffuser disk.

[0028] Common to all embodiments 3a-3d is the fact that the horizontal velocity component is greater, even in absolute value, than the vertical velocity component when the gas flow leaves the primary inlet nozzle 14. In addition, the overall linear velocity is relatively small due to the fact that the incoming inert gas diffuses around the entire circumference of the diffusion disk, i.e., 360°.

[0029] A flat, and especially an inverted cone-shaped curved diffusion disk should preferably be provided with small drainage holes, such as holes 143 in FIG. 3a, to prevent liquid from accumulating thereon. These holes should be small enough so as not to change the overall properties of the nozzle (or interface). That is, the amount of inert gas flowing through such holes should be much less than the flow rate of inert gas flowing around the periphery of the diffusion disk. Typically, the flow rate through holes 143 etc. should constitute less than 10% by volume of the inert gas flow rate, more preferably less than 5% by volume.

[0030] The structure described above and shown in Figures 1-3 ensures minimal turbulence around the inlet opening and no turbulence on the surface of the oil, thereby achieving the desired objective of minimizing volatilization during oil extraction.

[0031] FIG. 4 shows an embodiment of the inert gas interface that differs from the previous ones mainly in that it shows a primary nozzle including a diffusion disk 141''' as shown in FIG. 3d, arranged below the inlet conduit 113, and a secondary inlet nozzle 16. The inert gas supply pipe 12 branches into a first pipe stub 12a connected to the inlet conduit 113 and a second pipe stub 12b connected to the secondary inlet nozzle. The second pipe stub 12b is bent twice in the illustrated embodiment and shows a section 12c which runs coaxially through the inlet conduit 113 and serves as a holder for the diffusion disk 141''' before terminating in the secondary inlet nozzle 16.

[0032] A switching valve member 17 is arranged to hold only one of the pipe stubs 12a and 12b open at a time, i.e. when one pipe stub 12a or 12b is available for gas supply the other is not available.

[0033] When the valve member 17 is in the horizontal position as shown in Figure 4, the supplied gas enters the primary nozzle through the pipe stub 12a, flows through the wide inlet conduit 113 connected to the primary nozzle, and is finally diffused by the diffusion disc 141''' in the same manner as described with respect to Figures 1 and 3a-3d.

[0034] On the other hand, when the valve member 17 is switched to the vertical position, the supply gas enters the pipe stub 12b, which is connected to a vertically disposed secondary inlet nozzle 16, which does not have a diffusion disk or similar element. The secondary inlet nozzle (16) is designed to deliver gas at a relatively high velocity and with a predominantly downward vertical velocity component, the magnitude of which depends on the applied pressure and the selected dimensions. Typically, the secondary inlet nozzle (16) is arranged to deliver gas with a vertical velocity component of more than 3 m / s at a height of 3 meters below the nozzle.

[0035] Secondary inlet nozzles are not intended for use when draining oil from a tank, but rather for efficient purge of the tank atmosphere when the tank is already empty and there is no risk of evaporation of volatile fluids. This type of operation is usually required prior to tank inspection, repair work, etc., and is used to initially purge the explosive tank atmosphere, first with inert gas and then with breathable air.

[0036] As also shown by FIG. 4, the exterior surface of the secondary inlet nozzle 16 acts as a holder for a diffusion disk 242, thereby also fulfilling the role of the rod 142 shown in FIGS. 3a-3d.

[0037] Naturally, the switching valve 17 can also be replaced by two separate valves, one for each pipe stub 12a, 12b. The valves can be controlled not only manually, but also automatically or remotely.

[0038] Typically, a primary nozzle according to the invention is arranged to deliver gas at a vertical velocity of less than 0.2 m / s when measured at a height 3 metres below the nozzle.

[0039] In general, all embodiments of the present invention allow the layer of VOC gas just above the liquid (oil) to remain undisturbed by the introduced inert gas forming a gas layer above the VOC layer, which is a major factor in reducing volatilization of the liquid.

Claims

1. An oil tank gas inlet assembly connectable to a gas supply pipe (12) for maintaining pressure and a non-explosive atmosphere while extracting oil from an oil tank (11), comprising: at least one inlet conduit (13) arranged to send a unidirectional gas flow from the supply pipe (12) towards a primary inlet nozzle (14), the primary inlet nozzle (14) including diffusion elements (141, 141', 141'', 141''') configured to diffuse the incoming inert gas with a horizontal velocity component greater than the vertical velocity component within the tank; The oil tank gas inlet assembly, characterized in that the cross-sectional area of the inlet conduit (13) is larger than the cross-sectional area of the supply pipe (12).

2. The oil tank gas inlet assembly according to claim 1, wherein the diffusion elements (141, 141', 141'', 141''') are designed to diffuse the inert gas as a circular or conical flow of 360° from the primary inlet nozzle (14).

3. The oil tank gas inlet assembly according to claim 1 or 2, wherein the diffusion elements (141, 141', 141'', 141''') have a disk shape selected from the group consisting of a flat disk, a conical disk, and a curved disk (hereinafter, an inverted conical disk).

4. The oil tank gas inlet assembly according to claim 1, wherein the primary nozzle is arranged to supply gas with a vertical velocity of less than 0.2 m / s when measured at a height 3 meters below the nozzle.

5. The oil tank gas inlet assembly according to claim 1, wherein the inlet conduit (13) is arranged vertically between the supply pipe (12) and the inlet nozzle (14).

6. The oil tank gas inlet assembly according to claim 1, further comprising a secondary inlet nozzle (16), the secondary inlet nozzle (16) being designed to supply gas at high speed and mainly with a downward vertical velocity component.

7. The oil tank gas inlet assembly according to claim 6, wherein the secondary inlet nozzle (16) is arranged to supply gas with a vertical velocity component exceeding 3 m / s at a height 3 meters below the nozzle.

8. The oil tank gas inlet assembly according to claim 6 or 7, wherein the primary inlet nozzle (14) and the secondary inlet nozzle (16) are connected to a common supply pipe (12) and receive supply intermittently depending on the position of a switching valve (17).

9. The oil tank gas inlet assembly according to claim 8, wherein a pipe stub 12c connected to the secondary inlet nozzle (16) is disposed coaxially within the inlet conduit (113) connected to the primary nozzle (141''').

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