GB2081598A

GB2081598A - Oil storage tank

Info

Publication number: GB2081598A
Application number: GB8119244A
Authority: GB
Original assignee: Motherwell Bridge and Engineering Co Ltd
Current assignee: Motherwell Bridge and Engineering Co Ltd
Priority date: 1980-07-02
Filing date: 1981-06-22
Publication date: 1982-02-24
Also published as: GB2081598B

Abstract

In removal of water/saline and dissolved gases/vapours from crude oil, the crude is elevated by pipe 16 to a collector 18 with inverted weir outlet 19 and permitted to flow along a flash-release tray 20 which is at a shallow angle to the horizontal and on which degassing occurs. Cascade trays 21, 22 transfer the degassed crude to the stored oil body at a shallow angle which prevents turbulence on entry into the stored oil body. Gravitational oil/water separation occurs and the separated water is removed by draw-off 26. The crude treatment is confined to within a housing 24 with lateral apertures 25 which is vertically orientated and adjacent side wall 11 of the tank 10 diametrically disposed relative to draw-off 26. Housing 24 and trays 20, 21, 22, are carried by tank side wall 11 and floor 13 and the roof structure 12 is unloaded. <IMAGE>

Description

SPECIFICATION Oil storage tank This invention relates to oil storage tanks and to methods of treating crude oil.

Crude oil as delivered from an oilwell contains oil together with impurities in the form of water or saline and dissolved gases and/or vapours and in order to produce 'still' oil for storage in atmospheric-pressure containers, or for processing, it is necessary to treat the crude oil for removal of these impurities.

A known form of treating crude oil for removal of its water or saline content is to slow the flow to allow sufficient residence time within a storage container for a gravity separation to take place. A known form of treating crude oil for removal of its dissolved gas and/or vapour content is to flow the crude oil through a series of pressure vessels at successively reduced pressures (above atmospheric) each of which has internal flow surfaces at which flash release of the dissolved gas and/or vapour occurs.

It is an object of the present invention to provide an improved method of treating crude oil for removal of water or saline and dissolved gases and/or vapours, and apparatus for carrying out the method.

Accordingly, the present invention comprises a method of treating crude oil for removal of water or saline and dissolved gases and/or vapours, which comprises elevating the crude oil which is to be treated to a discharge station within a storage tank operating at a pressure in excess of atmospheric pressure, and containing a stored body of oil, collecting said crude oil at the discharge station and confining it to flow in film form along a predetermined path so as to enter the stored body of oil at a relatively shallow angle, said path having a length above the stored oil body sufficient to permit substantially complete flash-release of dissolved gases and/or vapours from the oil film and said shallow angle being sufficiently small as to prevent substantial turbulence as the flowing film enters the stored oil body whereby gravitational separation of the water/saline content of the flowing film occurs in the stored oil body.

The present invention also provides an oil storage tank comprising means for elevating crude oil to be treated to a discharge station adjacent the roof of the tank, means for collecting crude oil at the discharge station and causing it to form a film, and a treatment surface extending from the collecting means towards the floor of the tank and defining a predetermined path over which the oil film is confined to flow, said treatment surface having a first portion above the maximum level of stored oil of sufficient length to operate as a flash-release surface and a second portion inclined at a relatively shallow angle to the horizontal to prevent turbulence as the flowing film thereon enters the stored oil body in the tank.

Preferably, said storage tank has a cylindrical side wall and said treatment surface first portion is a tray extending in the direction of a chord within the storage tank side wall. Conveniently, said treatment surface second portion is a further tray.

Preferably said treatment surface is located within an apertured housing extending vertically within the tank and acting as a baffle in relation to the stored oil body.

Preferably also a water-draw off arrangement is located diametrically opposite the apertured housing and the apertures of the latter are located in the lateral side walls so that the flow path between the housing and the water draw off arrangement is maximised.

An embodiment of the present invention will now be described by way of example with reference to the accompanying schematic drawing.

In the drawing an oil storage tank 10 has a cylindrical side wall 11, a fixed roof 12 and a floor 13. Oil in crude form is supplied to the tank 10 at port 14 and treated oil is removed from the tank at port 15, the treatment within the tank being in accordance with the present invention.

Within the tank 10 and in communication with port 14 there is an elevator 16 which discharges crude oil at a station 17 adjacent the tank roof 12. Associated with station 17 there is a collector box 18 with an inclined floor at the lower end of which there is an inverted weir 19 which causes crude oil to flow in film form onto and along a first tray 20. This tray 20 extends along a chord with respect to the cylindrical side wall 11 and is of sufficient length to permit substantially complete degassing of the oil by the time the crude oil has reached the end 20A of tray 20. At end 20A the oil film is transferred to cascade along the first of several further trays 21, 22 which extend along a folded path towards the tank floor 13.Each of these trays is inclined at a shallow angle to the horizontal (i.e. to the surface of the stored oil body) so as to prevent turbulence when the flowing oil film enters the stored oil body. Conveniently this angle is less than 60% within the range 30-45 is particularly suitable; but less than 300 is preferred. It will be noted that in the illustrated embodiment trays 21,22 are inclined at an angle of about 350whereas tray 20 is inclined at a very much shallower angle, approximately 5-10O.

Trays 20, 21 and 22 together define a predetermined path along which the oil film is confined to flow and they are located within a vertically extending housing 24 which acts as a baffle with respect to the stored oil body. The stored oil body is permitted to enter the housing 24 through apertures 25 in the form of slots located in the end walls of housing 24 but not adjacent the ends of the trays 20, 21,22 and the degassed oil meets this oil body within housing 24 so that any disturbance arising is confined to within housing 24. Within the stored oil body, because of the absence of turbulence the water/saline content separates out under gravity, escaping from housing 24 beneath the side wall which is mounted on feet 24A and is removed from the tank through an inverted weir arrangement 26 leading to an outlet port 26B and vented as at 26A.

Treated oil, i.e. degassed and dewatered is collected from or below the surface of the oil body in the tank by a pipe 27 leading to port 15 and having a floating head.

It will be appreciated that in the treatment of crude oil according to the present invention degassing occurs at an elevated level within the tank which operates at a pressure slightly above atmospheric pressure (by about 2 psi) so that collected gases can be expelled through gas outlet port 28 formed in a vented vapour dome 29 which also incorporates a demister 30 whereas dewatering occurs at a lower level dependent upon the volume of oil already stored in the tank. The crude oil under treatment is flowed in film form firstly to effect the degassing and then into the stored oil body. This arrangement controls the spread of water/saline droplets and permits rapid dewatering. The path along which the oil film moves may be defined by one or more chutes or cascades or spirals provided the relative angles of inclination conform to that required by the invention.

Various advantages arise from the described embodiment. For example because tray 20 extends as a chord housing 24 is confined to one vertical portion of the tank wall 11 and pipe 27 and arrangement 26 can be located diametrically opposite housing 24 to prevent any interference arising between the various liquid flows. It is particularly important to note that apertures or slots 25 are located in the end walls of housing 24 and consequently are almost diametrically remote from inverted weir arrangement 26 so that two semicircular flow paths occur between housing 24 and arrangement 26 which further contributes to dewatering of the oil due to the maximisation of the path length. Furthermore apertures or slots 25 are not located adjacent the end of a tray so that there is no tendency for the oil film to pass through apertures 25 whilst free falling from one tray to another. Indeed to control this transfer between trays a guard, such as 31 may be used. The arrangement is also functional independently of the level of stored oil in the tank and is mechanically simple in that the loading on the tank shell imposed by the presence of the treatment equipment is carried by the cylindrical wall 11 and the floor 13 with practically no loading on the domed roof 12.

Claims

1. A method of treating crude oil for removal of water or saline and dissolved gases and/or vapours, which comprises elevating the crude oil which is to be treated to a discharge station within a storage tank operating at a pressure in excess of atmospheric pressure and containing a stored body of oil, collecting said crude oil at the discharge station and confining it to flow in film form along a predetermined path so as to enter the stored body of oil at a relatively shallow angle, said path having a length above the stored oil body sufficient to permit substantially complete flash-release of dissolved gases and/or vapours from the oil film and said shallow angle being sufficiently small as to prevent substantial turbulence as the flowing film enters the stored oil body whereby gravitational separation of the waterisaline content of the flowing film occurs in the stored oil body.

2. The method according to claim 1, wherein said predetermined path is folded vertically and confined within a treatment station located adjacent the wall of the storage tank and the separated water/saline content is removed from the tank at a location spaced from the treatment station by the maximum extent.

3. The method according to claim 1 and substantially as hereinbefore described.

4. An oil storage tank comprising means for elevating crude oil to be treated to a discharge station adjacent the roof of the tank, means for collecting crude oil at the discharge station and causing it to form a film, and a treatment surface extending from the collecting means towards the floor of the tank and defining a predetermined path over which the oil film is confined to flow, said treatment surface having a first portion above the maximum level of stored oil of sufficient length to operate as a flashrelease surface and a second portion inclined at a relatively shallow angle to the horizontal to prevent turbulence as the flowing film thereon enters the stored oil body in the tank.

5. An oil storage tank according to claim 4, comprising a cylindrical side wall, and said treatment surface first portion is a tray extending in the direction of a chord within the storage tank side wall.

6. An oil storage tank according to claim 5, wherein said treatment surface second portion is a furthertray.

7. An oil storage tank according to claim 4, 5 or 6, wherein said treatment surface is located within an apertured housing extending vertically within the tank and acting as a baffle in relation to the stored oil body.

8. An oil storage tank according to claim 7, wherein a water-draw off arrangement is located diametrically opposite the apertured housing and the apertures of the latter are located in the lateral side walls so that the flow path between the housing and the water draw off arrangement is maximised.

9. An oil storage tank according to claim 4, and substantially as hereinbefore described with reference to the accompanying drawings.