GB2129696A - Mixing device - Google Patents

Abstract

The device, which can be used for homogenising crude oil flowing through pipelines (1), comprises two substantially concentric tubes (2, 3) interconnected at one end by pumping means (5), the inner tube (2) having an inlet (8) at the other end projecting from the outer tube (3), and the outer tube (3) being closed at this end (9) adjacent to the inlet (8) and having a plurality of apertures (4) spaced along its length, whereby fluid can be drawn into the inner tube (2) through the inlet (8) and pumped out through the apertures (4) in the outer tube (3). The apertures (4) may be positioned (as shown Fig. 2) such that the liquid is pumped therefrom in a direction transverse to the axis of the pipeline (1). <IMAGE>

Description

SPECIFICATION Mixing device The present invention relates to an apparatus and a method for mixing unevenly distributed nonhomogeneous liquids.

In many industries, especially the petroleum industry, it is necessary to ensure adequate mixing of non-homogeneous liquids in order to enable representative samples to be taken. This is particularly important for example in the case of wet crude oil being sampled during transfer operations such as tanker loading and unloading and in pipeline operations where the flow rate is continuous but variable.

Measurement ofthe water content of crude oilsfor fiscal and oil loss accounting has become extremely important with the increase in the cost of crude oil. On an amount of 100,000 tons of crude oil, underestimation of the water content by only 0.1% by volume means that 100 tons of water is costed as oil. Bulk water contents of crude oils are typically in the range 0.1% to 3% volume. Modern analytical tests can measure water in oil to an accuracy of 0.01% by volume and it is therefore necessary to ensure that the sampleforanalysis is sufficiently representative of the whole crude oil to be compatible with such high accuracy.

For many years representative samples have been obtained using an automatic sampling device on the transfer pipeline. Such devices take a small volume (grab) from the pipe-line (orfrom a by-pass pipe-line) at frequent intervals and accummulate these grabs in a receiver. The object isto collecttypically 10 litres of sample from each cargo or from the daily production from a field. This volume is chosen as being aboutthe largestthatcan be conveniently handled manually and transported or stored, andfrom which an aliquot can be obtained for analysis. In certain circumstances such as with high vapour pressure crude oils the volume collected may have to be as small as only 1 or 2 litres.It is fundamental to the concept of grab sampling that the water content of each grab is assumed to be representative ofthe water content of the oil and is thereby homogeneously distributed between successive grabs. From this, it follows that accurate sampling will only be possible when the pipeline contents have been properly mixed.

To collect a truly representative sample of nonhomogeneous liquids flowing through a pipeline, the sampling probe must be positioned at a point where the pipeline contents are thoroughly mixed. In the case of crude oil flowing through pipelines it is known that at flow velocities below 2.75 metres/sec some form of mixing is essential for representative sampling. Conventional mixing devices include Tee's, U bends, strainers, static mixers, motorised mixers, baffles and multiple pumped recycling units. In all these cases, the mixing devices are either located outside the main pipeline, orthey are big and numerous to achieve adequate mixing and hence cause obstruction to the main flow through the pipeline.Multiple pumped recycling units which reinjecttthe mixed liquids at several points along the pipeline require several pipe entries and the attendant valves, seals and jets. These devices also consume a considerable amount of mixing energyto drive the pumps.

It has now been found thatthese problems are mitigated by a simple unit which requires only one pipe entry, one pump, is relatively unobtrusive in size and pumps the liquid in a direction transverse to the axis of the pipeline.

Accordingly the present invention is a fluid mixing device comprising two substantially concentric tubes interconnected at one end by pumping means, the inner tube having an inlet at the other end projecting from the outertube, andthe outertube being closed at the end adjacent to the inlet and having a plurality of apertures spaced along its length, whereby fluid can be drawn into the innertube through the inlet and pumped out through the apertures in the outertube.

The mixing device may be adapted to be inserted in a main stream flow of liquid e.g. crude oil pipelines, or a tank containing the liquid to be mixed.

For the device to function it is essential that the inlet extends beyond the closed end of the outer tube.

The apertures along the length ofthe outer tube are preferably disposed on diametrically opposite sides of the tube wall. The size or numberofthe apertures is preferably graded along the length ofthe outertube in relation to their respective positions relative to the walls ofthe pipeline, so that the output of liquid from the apertures at any given point is approximately proportional to the flow of liquid through the pipeline atthat point. Thus, the outputfrom the apertures is such that relatively more ofthe liquid emerges in the middle of the pipeline, ensuring that there is an approximately equal mixing flux per unit crosssectional area along the length of the outertube.

The pumping means for extracting liquid through the inlet may be an extractor pump, a gear pump or a centrifugal impeller pump. The pump is preferably placed atthe point of communication between the inner and outer tubes.

Itis preferableto have all the components such as the pump, pressure valves if any and the power source forthe pump integrally mounted in a housing on top of the two tubes. Such a design ensuresthatthere is minimum projection ofthe device external to the pipeline and a single pipe entry is all that is required.

Such a design also results in a reduced usage of mixing energy. Normally, the mixing energy required by conventional devices varies as the cube of the pipe diameter. In the present design the energy used is one-eighth (1/8) of that normally required.

The device is particularly suitable for mixing watercrude oil mixtures flowing through pipelines.

The present invention is specifically illustrated with reference to the accompanying drawings.

Figure lisa sectional view of the device when in position in a pipeline and Figure 2 is an expanded section of the pipeline showing jets of extracted liquid emerging from the apertures in the outertube.

In the drawings an innertube 2 is connected to and surrounded by an outer tube 3. A centrifugal impeller pump 5 is interposed in a communicating chamber between the two tubes. The inner tube has an inlet 8.

The outertube 3 has spaced apertures 4 along the walls thereofanda closed end 9 near but above the inlet 8. A motor 6 is provided to impel the pump 5. The device is inserted into pipe 1 through a pipe entry and a gate valve 7 separates the part of the device in the pipeline from the pump 5 and motor 6 mounted in top ofthetubes.

In operation,the device is inserted into the main flow of a non-homogeneous liquid eg crude oil flowing through a pipeline. When the motor 6 switched on, the impeller pump 5 causes the liquid in the pipeline to be extracted into the inner tube 2 through the inlet8.The extracted liquid is initiallyforced into the outertube 3 bythe pump and subsequently back into the mainflow in the pipeline through the apertures 4 in the form of jets. The jets in this case emerge substantially transversely to the axis of the pipe thereby enabling more efficient mixing.

Claims (8)

1. Afluid mixing device comprising two substantiallyconcentrictubes interconnected atone end by pumping means, the innertube having an inletatthe other end projecting from the outertube, and the outer tube being closed at the end adjacentto the inlet and having a plurality of apertures spaced along its length, whereby fluid can be drawn into the inner tube through the inletand pumped outthroughthe apertures in the outer tu be.

2. A device according to claim 1 wherein the apertures along the length of the outertube are disposed on diametrically opposite sides ofthetube wall.

3. A device according to claim 1 or 2 wherein the size or number of the apertures is graded along the length of the outertube in relation to their respective positions relative to the walls of the pipeline, so that there is an approximately equal mixing flux per unit cross-sectional area along the length ofthe outer tube.

4. A device according to any one ofthe preceding claims wherein the pumping meansforextracting liquid through the inlet is an extractor pump, a gear pump or a centrifugal impeller pump.

5. A device according to any one of the preceding claims wherein the pump is placedatthepointof communication between the inner and outer tubes.

6. A device according to any one ofthe preceding claims wherein all the componentsofthe device including the pump, pressure valves if any and the power source for the pump are integrally mounted in a housing on top of the two tubes.

7. A device according to any one of the preceding claims wherein the device is used for mixing water crude oil mixturesflowingthrough pipelines.

8. A mixing device according to any one ofthe preceding claims as herein illustrated and described with reference to the accompanying drawings.