System and method of filling a tank

Abstract

A loading system and method for reducing evaporation of volatile organic compounds (VOC) or other gases during filling of a liquid product into a storage and / or transport tank (1), comprises a conduit (4) having a first end (13) which is fluidly connected to an inlet line (3) via which the liquid product is supplied to the conduit, and a second end (14) arranged inside the tank (1). The conduit is arranged at an acute inclination angle (α) with respect to a vertical axis (y) for the tank, whereby a liquid product entering the conduit first end will flow on the conduit inner surface, substantially parallel with the conduit longitudinal axis, until it exits the conduit second end, and not fall freely through the conduit. The liquid product may comprise liquid crude oil.

Description

[0001] System and method of filling a tank

[0002] Technical field of the invention

[0003] The present invention concerns a system for filling a tank, and a method of filling a tank. In particular, the invention concerns a loading system as defined by the preamble of claim 1, and a method as defined by the preamble of claim 12. The invention is particularly advantageous for filling a liquid product, such as crude oil, into a tank.

[0004] Background of the invention

[0005] Petroleum, such as unprocessed crude oil, is made up of different components stabilised at a specified pressure and a specified temperature. If these conditions are altered, either through a reduction in pressure or an increase in temperature, a proportion of the volatile components will separate out and gasify. These components consist of volatile organic compounds (VOC) such as, e.g., methane, propane, butane and ethane. Prior art systems for removing VOC gases are based on treating the already separated gases by providing a process arrangement for treatment of the exhaust gas subsequent to evaporation. Such systems are complex plants that require a lot of energy, as pressure and temperature are manipulated to return the VOC gases to a liquid state.

[0006] It is common knowledge that a relatively large quantity of volatile organic compounds evaporate during pumping of petroleum into large tanks. Under normal circumstances, a pressure in the region 1.05 to 1.07 bar is maintained both in storage and transport tanks. When loading e.g. a tanker vessel (tank ship), crude oil is normally pumped from a storage tank, through an inlet line to a position above the cargo tank, from where the oil is sent into the tank through a vertical drop line (also known as a “downcomer”) to the bottom of the tank. A drop line of this type may have a length of several tens of metres.

[0007] When the crude oil flows into the upper end portion of the vertical drop line, gravity will accelerate the liquid flowing down through the drop line, whereby a lower static pressure is created in the inlet line and the upper portion of the drop line. In these pipes, where the static pressure is lower than the vapour pressure, the evaporation of volatile organic compounds is significant, and these compounds will only to a small extent condense back to a liquid state upon resumption of normal tank pressure. The prior art includes WO 03 / 048028 Al, by the same inventor as in the present invention, describes a method and an arrangement for reducing the evaporation of volatile organic compounds (VOC) or other gases during filling of (e.g.) crude oil into a tank via an inlet line, where the crude oil is fed into the tank via a vertical loading column extending into the tank. The loading column has a significantly larger cross section than that of the inlet line. The vertical loading column is a part of the tank and is formed as a cylindrical silo, with a domed cover on top and an outlet at the bottom. In use, oil containing relatively volatile components is pumped into the upper end portion of the loading column, where it allegedly assumes a helical flow pattern down through the loading column. The advantages of this arrangement, as asserted in the prior art document, are based on the assumption that the atmosphere in the loading column becomes saturated with VOC gases whereby further gas evaporation from the oil is essentially stopped, and that these VOC gases are in motion during loading. However, it has been seen that - in operational use - the oil flow is turbulent and does not readily assume a helical flow pattern along the loading column wall. Substantial amounts of oil also tend to fall down to the bottom of the loading column and not adhere to the wall as envisaged. The turbulent flow and falling oil tend to generate gas bubbles that subsequently release VOC gases.

[0008] An object of the present invention is to provide a system and a method whereby the generation of VOC gases during filling of a liquid product (e.g. crude oil) is reduced, compared to the methods and systems of the prior art.

[0009] Summary of the invention

[0010] The invention is set forth and characterized in the main claim, while the dependent claims describe other characteristics of the invention.

[0011] It is thus provided a loading system for reducing evaporation of volatile organic compounds or other gases during filling of a liquid product into a storage and / or transport tank, characterized by a conduit having a first end which is fluidly connected to an inlet line via which the liquid product is supplied to the conduit, and a second end arranged inside the tank, and the conduit is arranged at an acute inclination angle with respect to a vertical axis for the tank, whereby a liquid product entering the conduit first end will flow on the conduit inner surface, substantially parallel with the conduit longitudinal axis, until it exits the conduit second end, and not fall freely through the conduit.

[0012] In one embodiment, the second end is fluidly connected to a tank-internal discharge line via a reducer, and the discharge line is arranged in the vicinity of a bottom portion of the tank. In one embodiment, the conduit first end is arranged outside and above the tank. In one embodiment, the conduit diameter is greater than the diameter of the inlet line, and the conduit comprises a reducer between the conduit upper end and the inlet line. In one embodiment, the reducer is eccentric with respect to the conduit longitudinal axis, whereby the inlet line outlet is close to the conduit inner wall, and the inlet line outlet is arranged on that side of the inclined conduit on which the liquid product will flow downwards in the conduit.

[0013] In one embodiment, the inlet line outlet is provided with a guide pipe which directs the liquid product flow towards the conduit inner wall.

[0014] In one embodiment, the conduit comprises one or more internal, longitudinal, guide fins.

[0015] In one embodiment, a balancing pipe is fluidly connected between the tank and the upper end of the conduit, and the balancing pipe comprises a check valve allowing gas to flow from the tank and into the conduit but not the other way.

[0016] The conduit is in one embodiment manufactured from standard piping that is commonly used for handling hydrocarbons, and the reducers are manufactured from rolled plate material.

[0017] In one embodiment, the inclination angle is in the region between and including 8° - 12°. The liquid product may comprise liquid crude oil.

[0018] It is also provided a method of reducing the evaporation of volatile organic compounds or other gases during filling of a liquid product into a storage and / or transport tank, characterized in that the liquid product is flowed into the tank via a conduit which is arranged at an acute inclination angle with respect to a vertical axis for the tank, whereby a liquid product entering an upper end of the conduit will flow on the conduit inner surface, substantially parallel with the conduit longitudinal axis, until it exits the conduit towards the tank bottom, and not fall freely through the conduit. In one embodiment of the method, the liquid product, upon entering the conduit, is directed towards a portion of the inner wall of the conduit. In one embodiment, the method is executed by the loading system according to the invention.

[0019] The inclined conduit provides smoother flow and reduces turbulence in the conduit, compared to the prior art vertical designs.

[0020] In the claimed invention, the conduit may be manufactured from pipes that are commonly used for handling hydrocarbons, and the reducers may be manufactured from rolled plate material. This is a simplification compared to the prior art vertical loading columns, which generally are manufactured accruing to pressure vessel standards. The invention therefore provided a time a cost reduction, as the shift from pressure vessel standards to general piping standard makes approval, production, and commissioning less strict. The rolled reducers make replace expensive dished ends, thereby significantly reducing manufacturing cost.

[0021] The invented system causes the crude oil to follow the conduit wall, which reduces turbulent flow and therefore reduces formation of gas bubbles that subsequently release VOC gases.

[0022] The method and loading system according to the invention are well suited for use during loading of ships and other large tanks when dealing with petroleum products, such as crude oil, in order to minimize VOC gas release by preventing low pressure from occurring during loading. The method and loading system according to the invention may also be used for reducing evaporation of gas during filling of single components such as propane, butane, ethane and CO2 and liquefied natural gas, as well as other liquid products.

[0023] Brief description of the drawings

[0024] These and other characteristics of the invention will become clear from the following description of embodiments of the invention, given as non-restrictive examples, with reference to the attached schematic drawings, wherein: Figure l is a sectional view of a loading system according to the invention installed in a tank;

[0025] Figure 2 is a top view corresponding to the view of figure 1;

[0026] Figure 3 is an enlarged view of the section marked “C” in figure 1, where the tank and upper part of a conduit is shown in a transparent view;

[0027] Figure 4a is an elevation transparent view of an embodiment of the conduit according to the invention, and figure 4b is a sectional view along the longitudinal section line A-A in figure 4a;

[0028] Figure 5 is a sectional view along the section line B-B in figure 1; and

[0029] Figure 6 is a sectional view illustrating an alternative embodiment of the loading system.

[0030] Detailed description of embodiments of the invention

[0031] The following description may use terms such as “horizontal”, “vertical”, “lateral”, “back and forth”, “up and down”, ’’upper”, “lowed’, “inner”, “outer”, “forward”, “rear”, “underneath”, “above”, etc. These terms generally refer to the views and orientations as shown in the drawings and that are associated with a normal use of the invention. The terms are used for the reader’s convenience only and shall not be limiting.

[0032] The loading system according to the invention comprises a tank 1 and one or more conduits 4 extending into the tank. In the illustrated embodiment, the conduits are tubular members, also referred to as column lines, but the conduits may have other shapes. Figure 1 shows three such column lines, but the invention is not limited to this quantity. In one embodiment, the tank is arranged in a tanker ship and is configured for holding a liquid product, such as petroleum, for example crude oil. Reference number 16 indicates a portion of a bulkhead.

[0033] The conduit 4 comprises a first end 13 fluidly connected to an inlet line 3 and a second end 14 being arranged inside the tank - below the first end and preferably in the vicinity of the tank bottom. The inlet line is fluidly connected to a reservoir (not shown), for example a tank for crude oil or other liquid product. In the embodiment illustrated in figures 1 and 5, the second end 14 is fluidly connected to a tank-internal discharge line 5 via a concentric reducer 18, but this is optional: the second end 14 may be directly open into the tank cavity, as illustrated in figure 6. In either case the conduit 4 lower opening or discharge line opening is arranged relatively close to the tank bottom, such that the opening becomes submerged by inflowing the liquid product soon after filling commences, whereby gas evaporation (e.g. VOC gas evaporation) from that point onwards is virtually eliminated.

[0034] In the illustrated embodiment, the conduit first end 13, with its connection to the inlet line, is arranged outside and above the tank, for example on or above a deck of a tanker ship. As the conduit diameter is greater than the diameter of the inlet line and the discharge line, the conduit comprises reducers 17, 18 at each end. The conduit 4 is connected to a support structure, for example an upper portion of the tank or a structure above the tank (e.g. a deck structure 11), via a suspension unit 6 (see e.g. figure 3). The suspension unit thus supports the conduit in the vicinity of the first end 13.

[0035] The conduit 4 is extending into the tank 1 and towards the bottom of the tank, and is arranged at a downward and acute angle a with respect to a tank-vertical axis , as illustrated in figures 1 and 3. The conduit 4 is supported by stringers 12 (or other suitable support structure) inside the tank (see also figure 5).

[0036] Fluid flow in open channels (i.e. where the fluid has a free upper surface) may be characterized by the so-called Froude number, which expresses the relation between the inertia force and the gravitation force of bulk flow. The Froude number is defined as: where u is the local flow velocity (in m / s), g is the local gravitational acceleration (in m / s2), and A is a characteristic dimension (in m) of the channel.

[0037] The characteristic dimension for the conduit 4 may be defined as the wetted area inside the conduit (i.e. the internal surface area exposed to liquid product flow). The wetted area is proportional with the inclination angle a, and the Froude number is inversely proportional with the wetted area. Increasing the inclination angle a therefore lowers the Froude number, which is desirable. It has been found that a Froude number less than 0.45 is favourable. Pressure in the conduit will be balanced at Fr = 0.31, and the best effect is achieved at Fr < 0.18.

[0038] In a practical application, considering the limited space inside the tank, an angle of 15° has been found yield an optimal balance between Froude number and space requirement.

[0039] In order to provide a smooth transition in the liquid flow between the inlet line 3 (normally a pipe) and the conduit 4 (effectively a channel having a free upper surface), the reducer 17 is eccentric with respect to the conduit longitudinal axis, in the sense that the inlet line outlet is close to the conduit inner wall, as shown in figure 3. The inlet line outlet is thus arranged on that side of the inclined conduit on which the liquid product will flow (downwards in the conduit). This configuration reduces turbulent flow as the liquid flows into the conduit, which in turn reduces gas bubbles formation and - in the case of petroleum flow - subsequent VOC gas release. In the illustrated embodiment, the inlet line outlet is provided with a guide pipe 19 which directs the liquid flow towards the conduit inner wall. Referring to figure 4b, the conduit 4 may comprise an internal, longitudinal, guide fin 15, which further aligns the liquid flow and thus mitigates turbulent flow. It should be understood that more than one longitudinal guide fins may be provided, for example arranged in parallel in a portion of the conduit.

[0040] Referring to figures 1 and 3, a balancing pipe 7 is fluidly connected between the tank and the upper end of the conduit 4. In the illustrated embodiment, the balancing pipe 7 is connected to the reducer 17 (such that it is not exposed to the liquid flowing into the conduit). A check valve 9 is arranged in the balancing pipe, such that gas can flow from the tank and into the conduit but not the other way. A first valve 10 (manually or remotely operated) is arranged in the balancing pipe between the check valve and the conduit. The first valve is open when the liquid product (e.g. crude oil) is being flowed into the conduit. The balancing pipe will serve to equalize the tank pressure and the pressure at the top of the conduit, which further reduces the gas formation (e.g. VOC gas formation) or / and recycles gas from the tank and back into the conduit. A bypass line 8 bypasses the check valve 9 and the first valve 10, and comprises a second valve 20 (manually or remotely operated). The second valve 20 is closed when the liquid product is being flowed into the conduit. The conduit 3 may conveniently be manufactured from standard piping that is commonly used for handling hydrocarbons, and the reducers 17,18 may be manufactured from rolled plate material.

[0041] In the embodiments described above, various features and details are shown in combination. The fact that several features are described with respect to a particular example should not be construed as implying that those features by necessity have to be included together in all embodiments of the invention. Conversely, features that are described with reference to different embodiments should not be construed as mutually exclusive. As a person skilled in the art readily will understand, embodiments that incorporate any subset of features described herein and that are not expressly interdependent have been contemplated by the inventor and are part of the intended disclosure. However, explicit description of all such embodiments would not contribute to the understanding of the principles of the invention, and consequently some permutations of features have been omitted for the sake of simplicity or brevity.

Claims

Claims1. A loading system for reducing evaporation of volatile organic compounds (VOC) or other gases during filling of a liquid product into a storage and / or transport tank (1), characterized by a conduit (4) having a first end (13) which is fluidly connected to an inlet line (3) via which the liquid product is supplied to the conduit, and a second end (14) arranged inside the tank (1), and the conduit (4) is arranged at an acute inclination angle (a) with respect to a vertical axis (y) for the tank (1), whereby a liquid product entering the conduit first end (13) will flow on the conduit (4) inner surface, substantially parallel with the conduit (4) longitudinal axis, until it exits the conduit second end (14), and not fall freely through the conduit (4).

2. The loading system of claim 1, wherein the second end (14) is fluidly connected to a tank-internal discharge line (5) via a reducer (18), and the discharge line (5) is arranged in the vicinity of a bottom portion of the tank (1).

3. The loading system of claim 1 or claim 2, wherein the conduit first end (13) is arranged outside and above the tank (1).

4. The loading system of any one of claims 1-3, wherein, the conduit (4) diameter is greater than the diameter of the inlet line (3), and the conduit (4) comprises a reducer (17) between the conduit (4) upper end and the inlet line (3).

5. The loading system of claim 4, wherein the reducer (17) is eccentric with respect to the conduit (4) longitudinal axis, whereby an inlet line (3) outlet is close to a conduit (4) inner wall, and the inlet line (3) outlet is arranged on that side of the inclined conduit (4) on which the liquid product will flow downwards in the conduit.

6. The loading system of claim 5, wherein the inlet line (3) outlet is provided with a guide pipe (19) which directs liquid product flow towards the conduit (4) inner wall.

7. The loading system of any one of claims 1-6, wherein the conduit (4) comprises one or more internal, longitudinal, guide fins (15).

8. The loading system of any one of claims 1-7, wherein a balancing pipe (7) is fluidly connected between the tank (1) and the upper end of the conduit (4), and thebalancing pipe (7) comprises a check valve (9) allowing gas to flow from the tank (1) and into the conduit (4) but not the other way.

9. The loading system of any one of claims 1-8, wherein the conduit (4) is manufactured from standard piping that is commonly used for handling hydrocarbons, and the reducers (17, 18) are manufactured from rolled plate material.

10. The loading system of any one of claims 1-9, wherein the inclination angle (a) is in the region between and including 8° - 15°.

11. The loading system of any one of claims 1-10, wherein the liquid product comprises liquid crude oil.

12. A method of reducing the evaporation of volatile organic compounds (VOC) or other gases during filling of a liquid product into a storage and / or transport tank (1), characterized in that the liquid product is flowed into the tank (1) via a conduit (4) which is arranged at an acute inclination angle (a) with respect to a vertical axis (y) for the tank, whereby liquid product entering an upper end of the conduit (4) will flow on the conduit inner surface, substantially parallel with the conduit longitudinal axis, until it exits the conduit towards the tank bottom, and not fall freely through the conduit.

13. The method of claim 12, wherein the liquid product, upon entering the conduit (4), is directed towards a portion of the inner wall of the conduit (4).

14. The method of any one of claims 12 and 13, wherein the method is executed by the loading system as defined by any one of claims 1-11.

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