EP1091812B1 - Method for cleaning an oil storage tank and device for implementing same - Google Patents

Abstract

The invention concerns a method for cleaning an oil storage tank (1) with a floating cover containing crude oil, which consists in partially emptying the tank (1) above the sludge (3) contained therein; sucking up the crude oil and re-injecting it into the tank (1) using diffuser spray nozzles (5) borne by the floating cover (2), each spray nozzle being driven in rotation by a motor. The injection with the spray nozzles causes the oil volume to swirl and the sludge (3) to be suspended again in the oil. The invention also concerns a device for implementing said method.

Description

The present invention relates to a process for resuspending a sediment which has, by deposition, formed a layer on the bottom of a floating roof tank containing crude oil and has a device for carrying out said process. .

It is known that the crude oil storage tanks are very generally cylindrical tanks with floating roof. The roof structure is therefore at a higher or lower level relative to the ground depending on the amount of crude oil in the tank. Of course, the periphery of the floating roof comprises sealing members to prevent oil from passing to the periphery of the roof. It has been found that in these storage tanks, a sediment consisting for the most part of the heaviest fractions of the stored crude oil, in particular of paraffinic compounds, and of inorganic elements such as water, is gradually deposited on the bottom of the tank. water, sand, rust particles or the like. This sediment, called "sludge" in Anglo-Saxon terminology, is likely to form on the bottom of the tank very thick layers of 1 or 2 meters or more and it is necessary to clean the tanks by removing this sediment. Indeed, if the sediment accumulates on the bottom of the tank and the floating roof descends to the bottom as the emptying of the tank, the risk is to have a contact between the floating roof and a point of the tank. deposition of the sediment, which unbalances the roof and blocks it in an oblique position in the side wall of the tank; However, the weight of a floating roof is several hundred tons, it is conceivable that the repair of such an incident entails considerable costs.

To allow complete emptying of the tank without the floating roof comes to rest on the bottom of the tank, there are provided in the floating roof sleeves in which support crutches are inserted. When the roof descends to the bottom, all support crutches are simultaneously placed on the bottom, which keeps the roof away from the bottom and allows to completely empty the tank of liquid crude oil it contains. To extract the sediment when the tank is emptied of its liquid oil, it has already been proposed to carry out a manual removal, the personnel entering the tank through the manholes it has at the base of its side wall. This operation is long, painful and expensive and the sediment, thus extracted from a tank, must be evacuated avoiding any pollution; it is then usually destroyed by incineration. Such a cleaning operation has a prohibitive overall cost.

It has already been proposed also to introduce, through a manhole, when the vessel is empty, a central stirrer which is disposed near the sediment layer and which can rotate about a vertical axis; such a stirrer uses the thixotropic nature of the sediment to try to resuspend it when reintroducing liquid oil into the tank, but it only has a localized influence in the central zone of the bottom of the tank and, moreover, the power, which is necessary for its rotation at an effective speed, leads to the creation of an emulsion consisting of sediment water and crude oil, which emulsion is highly undesirable for the use of stored crude oil. Such a cleaning device does not give satisfaction.

It has also been proposed to set up at the base of the side wall of the tanks stirring agitators regularly distributed around the periphery of the tank; these helico-agitators effectively cause a movement of the sediment from the periphery of the tank to the central zone of said tank but overall the volume of the sediment layer is not significantly reduced; on the other hand, the sediment layer is much more irregular in thickness, which means that the risk of an incident when the roof is lowered is far from being ruled out. Finally, the implementation of these helico-agitators is extremely expensive given the energy required for their training. Such a method is therefore unsatisfactory.

The invention is based on the fact that the sediment can be resuspended in the crude oil which overcomes it, provided that turbulence and / or vortices are generated throughout the volume of liquid oil that overcomes the sediment. and to send sufficiently powerful oil jets on the sediment so that it is gradually resuspended and maintained in this state by the turbulence imposed in the liquid crude oil. In this way, the layer of sediment disappears little by little and is found inside the liquid crude oil, which enhances the sediment since there is no longer any sedimentary material to evacuate out of the tank to be destroyed. According to the invention, it has therefore been proposed to partially empty the sedimented vessel to allow only a limited quantity of crude oil to remain above the sediment, a quantity restricted in the whole of the volume of which turbulence or vortices will be created. firstly, by a pumping suction carried out at the lower level of the liquid crude oil and, secondly, by a reinjection into the tank of the liquid oil pumped, the sucked and reinjected flow rate must be sufficiently large compared to the total volume remaining under the floating roof at the time of the cleaning operation. The reinjection into the tank is carried out by rotary lances arranged vertically under the floating roof, these lances being equipped with nozzles which send oil jets in the direction of the sediment layer; the rotation of the lances generates vortices having as axis the axis of the lances and one makes so that these vortices concern the totality of the tank. To do this, the crutches of the floating roof are replaced by the lances that reinject the crude oil, the bottom of the lances arriving near the middle plane of the upper face of the sediment layer.

The documents WO 85 04122 A , US 4828625 , EP 0495243 and US Patent 5087294 describe methods for cleaning an oil storage tank in which at least one tube is installed in the floating roof in place of a roof support leg, said tube being rotated about its axis and having at least one nozzle whose jet is directed towards the bottom of the tank. As for the document WO 9633820 , it discloses a device for removing a liquid containing a solid material suspended from a tank, which device comprises rotatably driven tubes provided with nozzles through which the liquid is ejected to resuspend deposited material.

The subject of the present invention is therefore a process for resuspending a sediment which has deposited, by deposition, a layer on the bottom of a floating roof tank containing crude oil, according to claim 1.

The choice of the number n that defines the global hourly flow rate sucked back into the tank is a function of the characteristics of the tank to be cleaned by resuspension of the sediment, the physicochemical characteristics of the sediment itself and the immobilization time that we accept to clean the tank.

The present invention also relates to a device for implementing the method as defined above according to claim 4.

Advantageously, each lance has three nozzle levels, a nozzle of the median level generating a substantially horizontal jet whereas the nozzles of the other two levels generate jets whose axis is inclined by 50 to 70 ° with respect to the axis of the nozzle. lance that carries the nozzle. It can be provided that each nozzle level has two diametrically opposed nozzles. Preferably, the lower level nozzle (s) is (are) disposed at a level between 20 and 50 cm above the average level of the sediment layer.

It can advantageously be provided that the (or) pump (s), which sucks (s) the crude into the tank is (are) volumetric (s); it may be, for example, screw pumps. This type of pump provides a given flow rate and the pressure that is obtained at the outlet of the nozzles is, accordingly, a function of the outlet section of all the nozzles fed by the pump. The volume involved in the turbulence around a lance is obviously all the more important as the ejection pressure of the jet is higher. It is ensured that these volumes, which are substantially cylinders having as their axis the axis of the lances, overlap with the volumes relating to the adjacent lances so that no part of the liquid oil contained in the tank remains outside the turbulence. set created according to the invention.

Of course, the stirring of the sediment also causes the resuspension of the inorganic elements contained in the sediment. There is therefore provided a suction filter pumps to remove these inorganic elements, the parts that pass through the filter is, in any case, ground by the pump screws. When the cleaning operation according to the invention is complete, there is in the tank, on the one hand, a filtered homogeneous liquid which is crude oil directly available for the tanker and, on the other hand, on the bottom of the tank a very weak layer constituted by the heavier inorganic solid particles initially contained in the sediment. Generally, the operator of the tank empties the liquid content of the tank by transferring it to another tank containing a similar crude, this emptying being, of course, made only after replacing the crutches of the floating roof. the place of the lances used for the implementation of the method according to the invention. After this mixture with the contents of another tank has been carried out, the refining of the crude used for cleaning according to the invention can take place.

According to a preferred embodiment, the device according to the invention comprises at least two pumps each preceded by at least one filter and each followed by a distributor, each output channel of which delivers on an enclosure which supplies p lances, p being the same. integer for all speakers in the device.

To better understand the object of the invention will now be described by way of purely illustrative and non-limiting example embodiment shown in the accompanying drawing.

• la figure 1 représente schématiquement un dispositif pour la mise en oeuvre du procédé selon l'invention ;
• la figure 2 représente schématiquement en coupe une cuve de pétrole brut à toit flottant dans laquelle on a mis en place une lance pour la réinjection du pétrole brut pompé comme indiqué sur la figure 1, les autres lances n'étant pas figurées ;
• la figure 3 représente une vue en plan du toit flottant de la cuve selon la figure 2, cette vue faisant apparaître les manchons de béquille prévus sur ledit toit flottant et, en pointillé, les cercles délimitant les volumes cylindriques de turbulence générés par chacune des lances insérée dans lesdits manchons de béquille.

On this drawing :

• Figure 1 schematically shows a device for carrying out the method according to the invention;
• FIG. 2 is a diagrammatic sectional view of a floating roof crude oil tank in which a lance has been set up for the reinjection of the pumped crude oil as shown in FIG. 1, the other lances not being shown;
• FIG. 3 represents a plan view of the floating roof of the tank according to FIG. 2, this view showing the kickstand sleeves provided on said floating roof and, in dashed line, the circles delimiting the cylindrical turbulence volumes generated by each of the spears; inserted into said crutch sleeves.

Referring to the drawing, it is seen that 1 has been designated as a whole a crude oil storage tank; the tank 1 consists of a cylindrical side wall 1a, the base 1 b is the bottom of the tank. The tank is equipped with a floating roof 2 whose periphery comprises unrepresented sealing members. The tank 1 is intended for the storage of crude oil, the roof 2 floating on the crude. Conventionally, such a tank may have a diameter of about 50 m and a height of 15 m. In such a tank, the crude gradually settles and gives rise to a sediment 3 which constitutes a layer on the bottom of tank 1b , layer that can reach several meters thick; the surface of the layer is relatively irregular. The sediment is usually of a thyxotropic nature.

To clean a tank of this type and remove the layer of sediment 3, the vessel is partially emptied; if a layer of sediment with an average thickness of 1 m is assumed, about 3 m of crude oil is left in the tank above the sediment, that is, the roof 2 is about 4 m above the bottom of the tank. In the example described, the floating roof 2 is equipped with 72 crutches which cross the roof by crutch sleeves designated by 4 in Figure 3. These crutches have a length of between 1.8 and 2.2 m and come into support on the tank bottom when the floating roof drops sufficiently low.

When the floating roof was brought 2 to 4 m above the bottom of the tank, the crutches are removed one by one and replaced by lances 5; each lance 5 is a tube that is pivotally placed in a crutch sleeve. The lance 5 is rotated about its axis by an explosion-proof electric motor 6 located at the top of the lance, above the floating roof 2. The crude oil supply of the lance 5 is carried out by a pipe 7. The lance 5 comprises three levels of nozzles for discharging the crude oil brought into the tube which constitutes the lance through the pipe 7. These nozzles direct the exit jets. Each level has two diametrically opposed nozzles on the lance. The two nozzles of the lower level provide jets 8a whose axes are 60 ° with the axis of the lance 5, said jets being oriented towards the tank bottom; the two nozzles of the upper level provide jets 8c whose axes make with the axis of the lance 5 an angle of 60 °, these jets being directed towards the floating roof 2; the two nozzles of the median level form jets 8b whose axes are substantially horizontal. The lower level nozzles are approximately 30 cm above the mean plane of the sediment layer 3; the mid-level nozzles are approximately 60 cm above the lower level and the upper level nozzles are also about 60 cm above the mid-level nozzles. The base of the lances 5 is closed. The motors 6 drive the lances 5 at two revolutions / minute.

The pumping in the tank 1 is carried out by means of two pumps 8; the pumps 8 are volumetric screw pumps which each discharge 400 m ³ / h. The pumps 8 are arranged in parallel and, upstream of each pump, a filter 9 has been arranged. The two filters 9 are connected to the outlet of a tank of 2000 liters whose inlet is connected by a pipe 11 to the lower part of the side wall 1a of the vessel 1. the filters 9 are intended to remove inorganic elements found in the pumped oil. Each of the two pumps 8 delivers on a distributor 12. Each distributor 12 has three outputs each of which delivers on a chamber 13 of 200 liters; each enclosure 13 has six outputs equipped with a valve 14 and each connected to a pipe 7 for feeding two lances 5.

When the pumps 8 suck the crude into the tank 1 and push it back into the lances 5, a turbulent and swirling volume is created around each lance, which is approximately delimited by the dotted circles in FIG. nozzle outlet sections, which are small enough that the jet ejection pressure is sufficient for the radius of the vortex volume created around the lance allows this vortex volume to intersect the vortex volumes of the adjacent lances. In this way, all of the liquid crude oil that overcomes the sediment is set in motion, the nozzles of the lower levels of the lances 5 produce rotating jets that attack the upper surface of the sediment 3, which, being thyxotropic, is recovering. slowly suspended in the crude oil that overcomes it. Since this crude oil is brewed in all its volume, the sediment can not be redeposited and is distributed throughout the volume of crude oil that overcomes the sediment layer 3.

It can be seen that after a period of operation of between 5 and 7 days, the sediment has practically disappeared over the entire surface of the bottom of the tank 1b , the only ones remaining on said bottom of the elements. inorganic compounds initially contained in the sediment. To identify the end of the operation, a measure of crude oil density is generally used, the cleaning being stopped when the density reaches the value that has been predetermined by calculation from initial samples of crude and sediment and relative volumes of sediment and crude oil held in the tank for the cleaning operation.

Claims (8)

1. Method of placing back in suspension a sediment (3) that has, by deposit, formed a layer on the bottom of a tank (1) with a floating roof (2) containing crude oil, said floating roof (2) comprising a plurality of support foot holes (4) evenly distributed across its surface, said holes (4) making it possible solidly to attach the roof (2) to support feet that are capable of supporting the roof (2) by resting against the bottom of the tank (1b) when the level of crude in the tank is less than the length of the support feet, in which:

   - the level of crude in the tank (1) is raised to a given height from the highest level of the sediment (3) on the bottom of the tank (1b)

   - the crude in the tank (1) is suctioned by means of at least one pump (8) with an overall hourly pumping rate equal to V/n, V being the volume remaining in the tank (1) below the floating roof (2),

   - the pumped crude is reinserted into the tank (1), injecting it by means of needles (5) supported by the floating roof (2) and placed vertically in the tank (1) above the sediment (3), each of said needles (5) having, below the floating roof, a length that is less than said given distance, being driven in rotation about its own axis, being blocked at its bottom end and comprising a plurality of nozzles located at various heights in relation to the bottom of the tank (1b), the jets (8a) from the nozzles of the lower level being directed towards the bottom of the tank (1b), the outlet area of the nozzles of a single needle (5) being small enough so that, with the flow provided by the pump(s) (8), the outlet pressure of the jet of crude is sufficient for the swirling volume generated by one needle (5) to intersect with the swirling volumes pertaining to the adjacent needles (5),

   - the implementation of the method is continued until the density of the pumped crude becomes equal to a predetermined value that corresponds to almost completely placing all the sediment (3) back in suspension, characterised by the fact that said determined distance is selected to be comprised between around 2 and 5 metres, the number n is comprised between around 8 and 15 and that the needles (5) are driven in rotation at a speed comprised between 0.25 and 4 turns/minute.
2. Method according to claim 1, characterised by the fact that the nozzles of the lower level of each needle are placed at a height comprised between 20 and 50 cm above the mean height of the sediment layer (3).
3. Method according to one of the claims 1 or 2, characterised by the fact that each needle (5) comprises a plurality of nozzles distributed over the length of the needle, such that the swirling volume generated by one needle (5) affects the total height of the layer of liquid crude that remains in the tank (1).
4. Device with a tank (1) with a floating roof (2) for placing a sediment back in suspension for the implementation of a method according to one of the claims from 1 to 3, the sediment (3) having, by deposit, formed a layer over the bottom of the tank (1) containing crude oil, said floating roof (2) comprising a plurality of support foot holes (4) evenly distributed across its surface, said holes (4) making it possible solidly to attach the roof (2) to support feet that are capable of supporting the roof (2) by resting against the bottom of the tank (1b) when the level of crude in the tank is less than the length of the support feet, the device also comprising at least one pump (8) for suctioning the crude in the tank, needles (5) supported by the floating roof (2) and placed vertically in the tank (1) to reintroduce the pumped crude into the tank, each of said needles (5) being driven in rotation about its axis at a speed comprised between 0.25 and 4 turns/minute, being blocked at its bottom end and comprising a plurality of nozzles located at various heights in relation to the bottom of the tank (1b), the jets from the nozzles of the lower level being directed towards the bottom of the tank (1b), and the jets (8c) from the nozzles of the upper level being directed towards the floating roof (2), the needles (5) being placed, instead of the support foots, in the sleeves for the support foot holes (4) in the floating roof (2) of the tank (1).
5. Device according to claim 4, characterised by the fact that each needle (5) comprises three nozzle levels, one medium level nozzle generating a substantially horizontal jet (8b) while the other levels generate jets (8a, 8c) in which the axis is tilted by 50° to 70° in relation to the axis of the needle (5) that supports the nozzle.
6. Device according to claim 5, characterised by the fact that each nozzle level comprises two diametrically opposed nozzles.
7. Device according to one of the claims from 4 to 6, characterised by the fact that the pumps (8) that suction the crude in the tank (1) are volumetric.
8. Device according to claim 7, characterised by the fact that it comprises at least two pumps (8), each preceded by at least one filter (9) and each followed by a distributor (12), in which each outlet flows into an enclosure (13) that supplies p needles (5), p being the same integer for all the enclosures (13) of the device.

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