WO1996017226A2

WO1996017226A2 - Separating and metering the flow of a multi-phase fluid

Info

Publication number: WO1996017226A2
Application number: PCT/GB1995/002797
Authority: WO
Inventors: John Mcintosh
Original assignee: John Mcintosh
Priority date: 1994-12-01
Filing date: 1995-11-30
Publication date: 1996-06-06
Also published as: AU3987695A; GB9424254D0; WO1996017226A3

Abstract

The invention teaches apparatus for and method of analysing, and particularly metering, multi-phase fluid. This is achieved by separating or forming homogenous slugs or bubbles (4) in fluid in a separator which pass successively into metering means. A sampler (31) may be attached to the apparatus for sampling fluid (either liquid or gas/vapour) at various points in the apparatus, including on either the separator or the metering means.

Description

SEPARATING AND METERING THE FLOW OF A MULTI-PHASE FLUID

The present invention relates to a method and apparatus for separating the flow of a multi-phase fluid for the purposes of measuring mass or volume fluid flow. The invention also and similarly relates to flow meters with an associated separator.

It is often important to quantify the fluid flow of various components within a multi-phase fluid.

Various methods exist for measuring multi-phase flow. These include mixing or homogenising the component phases and then measuring the flow of the mixture. Alternatively, the multi-phase fluid may be taken directly into volume displacement devices such as sliding vane displacers or double helical rotor displacers . A third method measures the mean velocity and phase fraction for each component phase. These methods have low accuracy at high gas (or vapour) to liquid ratios and/or involve expensive equipment.

A more traditional method involves measuring multi- phase flow by separating the multi-phase fluid into individual component phases followed by flow measuring in separate metering lines. However, in the past this method has also been associated with expensive equipment.

The present invention offers improvements over existing methods by being capable, with the use of relatively low cost equipment, of separating and using a single metering line, handling a wide range of gas (or vapour) to liquid ratios, and of handling the various flow regimes which can occur with multi-phase fluid flow.

According to a first aspect of the present invention there is provided a separator for separating homogenous slugs or bubbles from a multi-phase fluid, comprising a duct having a fluid inlet and a fluid outlet, wherein the fluid outlet communicates with a metering means .

At least part of the length of the duct may be inclined upwardly relative to the direction of fluid flow. The duct may also include a horizontal section, preferably upstream of the inclined section.

The duct inlet may communicate with a feed pipeline, wherein the inlet has a substantially larger flow area than the pipeline, such that the formation and growth of slugs or large bubbles is encouraged as the multi- phase fluid flows into the inlet.

The length of the duct is advantageously sufficient to allow for slug growth throughout passage of the fluid through the duct. Along the duct may be provided one or more tapping outlets for taking samples of gas or vapour, liquid mixtures, high density fluid and single phase or homogenized fluid. One or more of said tapping outlets may comprise a collecting chamber for flushing purposes.

The separator may comprise a means for trapping sand particles or other debris . This may be integral with a tapping point for taking samples of liquid mixtures, said tapping point being shaped to provide a sand trap. One end, or both ends, of the duct may be domed.

According to a second aspect of the invention, there is provided a metering means comprising a metering passage and measuring devices for measuring the pressure and temperature of fluid within the metering passage.

The metering means may further comprise gamma ray densiometers for measuring density and for sensing slug fronts and ends, enabling measurement of slug velocity and length.

The metering passage may include means for measuring the differential pressure. This may be in the form of a venturi or the duct may comprise one or more elbow meters. At least one elbow meter preferably indicates the mean density of fluid passing therethrough.

According to a third aspect of the present invention there is provided a sampler comprising a body assembly, upper and lower diaphragms, the body assembly and diaphragms together forming a chamber space for holding a sample, the sampler further comprising heating means and temperature measuring means . The heating means may be used for dewaxing purposes . Preferably the sampler also comprises pressure sensing means for measuring the pressure of the sample in the chamber space.

Preferably the chamber space is associated with an inlet and outlet, wherein the inlet and outlet are inclined to assist in clearing of previous samples and/or flushing. Also, the diaphragms may be sloped or inclined at a slight angle to assist the venting of gas and draining of liquid respectively from the chamber space. Preferably the internal surfaces of the body assembly and two diaphragms which define the chamber space are coated with a non-stick surface, such as polytetrafluoroethylene (PTFE) .

According to a fourth aspect of the present invention, there is provided apparatus for analysing components of a multi-phase fluid comprising a separator, metering means and a sampler. The separator, metering means and sampler may be as described herein.

According to a fifth aspect of the present invention, there is provided a method of analysing components of a multi-phase fluid, comprising the steps of forming homogenized slugs or large bubbles of one or more of the components within the fluid and successively passing the slugs or large bubbles into a metering means for the purpose of measuring mass or volume fluid flow.

The method preferably makes use of apparatus described herein.

According to a sixth aspect of the present invention there is provided a method of metering with an associated separator which forms slugs or large bubbles wherein the metering is by sensing each slug or large bubble front and end passing two fixed points to give slug velocity and length. Preferably, the density is sensed by one or both of the two fixed sensors, although other means may be employed.

The sensing of each slug or large bubble front and end may be by ultrasonic means or other beam means to give slug velocity and length. In some cases a mixture may exist between component phases which then pass through the apparatus in the mixed state, for example the gas may separate from the multi-phase fluid leaving the oil and water as a mixture. If partial mixing of oil and water is present it may be advantageous to promote mixing of oil and water leaving the gas to separate from the oil/water mixture. Mixing may be achieved by various means, for example by incorporating a restriction in the feed pipeline or inlet, or by incorporating a bend or bends in the inlet feed pipe. The bend or bends would have the advantage that they could be used to achieve desired levels of pipeline at the inlet and outlet from the metering means and associated separator. Another example arises where all three phases may be mixed as a homogenized fluid, which would then be metered. Each successive slug in the metering passage will have a degree of mixing at the front and from wall adherants from the previous slug and this may be allowed for in analysis.

According to a seventh aspect of the present invention, there is provided a method of metering where samples are allowed to enter a chamber with isolation means, wherein the sample average density is measured by differential pressure sensor means, followed by heating, thus raising the temperature and the pressure of the sample, thereby giving data such as to enable the calculation of the component fractions (oil, gas and water) and the density of one fraction (usually oil) with the other two densities by other means, including sampling of concentrated samples like gas and water.

Specific embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which:-

Figure 1 shows the separator with mitred junctions; Figure 2 shows the separator with curved junctions; Figure 3 shows an elongated outlet from the separator; Figure 4 shows the separator and a flow metering passage with two gamma ray densiometers and a venturi flow meter; Figure 5 shows the separator and a flow metering passage with two elbow meters and a sand trap; Figure 6 shows a separator with domed ends, a double bend inlet, collecting chambers for gas and high density fluid, a flow metering passage with two elbow meters, a sand trap and a sampler; Figure 7 shows a sampler to a larger scale; and Figures 8-13 show further embodiments of a separator in accordance with the invention, being of compact arrangement or construction.

Referring to Figure 1, the multi-phase fluid flowing as indicated by arrow 1 in the pipe line 2 enters an enlarged duct 3 preferably at the bottom. Duct 3 is preferably horizontal but can be inclined to suit the fluids being handled. The increase in area from pipe line 2 to duct 3 encourages the phases to separate and form into slugs 4 or large bubbles of the component phases. The length of duct 3 is made long enough to encourage the growth in size of the slugs 4. The fluids move from duct 3 to duct 5 which is inclined upwards in the flow direction. This upwards inclination can be at a constant angle or can be progressively changed in a regular or an irregular manner or can be stepped to encourage the growth in size of slugs 4. The slugs 4 of each phase exit preferably at the upper level preferably through an elongated opening 6 to suit the slug shape. Preferably the fluid is accelerated through a transitional shaped piece 7 into the first part of the metering passage duct 8 shown in Figures 3, 4, 5 and 6.

Provision is made to take gas (or vapour) samples at 9, liquid mixture samples at 10, highest density fluid samples (usually water) at 11, and single phase or homogenised samples at 12 or from other points in the metering section. It may be advantageous to incorporate collecting chambers at sample points 9 and 11 for flushing purposes. The liquid mixture sampling point 10 is shown shaped to encourage the trapping of particles like sand. The sand or the like can be flushed out or returned to the pipe line 13 at the sample return point 14 shown in Figures 4, 5 and 6.

Referring to Figure 2, duct 3 and duct 5 are connected by a curved junction 15. The duct 5 is terminated with a curved junction 16.

The end of ducts 3 and 5 can advantageously have domed ends and may incorporate baffles and wall attachment devices known to the art to advantageously direct the fluid.

The horizontal or near horizontal duct 3 allows the separator to form slugs over a wide range of gas (or vapour) to liquid ratios. Duct 3 could be omitted for high gas (or vapour) to liquid ratios whence pipe line 2 may preferably connect directly into the bottom or near bottom of duct 5 and the sampling points 9 and 10 incorporated near the entry to duct 5. In some embodiments it may be advantageous to promote the separation of gas from the liquid by the provision of a loop or curve in the feed pipe line 2 near the separator inlet. Desirably, the loop would be concave up when departing from the horizontal such that, under the influence of centrifugal force, the gas moves to the inner radius of the loop, while the heavier gas tends toward the outer radius. Further mixing of the liquids may be achieved by utilizing restrictions or the like in that part of the pipe (or loop) occupied by the liquid.

Referring to Figure 4, the separator is shown connected to a metering means including a metering passage comprising duct 8, venturi 17 and duct 18 which exits into pipe line 13. The metering passage comprising 8, 17 and 18 is made long enough to suit metering objectives and to maintain slug flow. The slugs are allowed to exit to the flow metering passage through an opening at or near the top of the second section of the enlarged duct. This opening is preferably elongated to suit the slug shape but the opening can have any shape and can have transitional shaping to blend the opening to the metering passage. Two gamma ray densiometers 19 and 20 are provided. Facilities (not shown) are also provided to measure pressure and temperature within the metering passage and to measure the venturi differential pressure. The gamma ray densiometers 19 and 20 sense the slug fronts and ends enabling slug velocity and length to be measured for each slug. Either densiometer gives the mean density.

The flow metering means can comprise various flow metering devices (described elsewhere) which can be used advantageously with the separator described herein. The venturi is used when the flow is single phase which would be indicated by the absence of slugs over a predetermined time interval. The venturi 17 can alternatively be in pipeline 2 or 13.

Referring to Figure 5, the separator is shown connected to a metering passage comprising duct 8, two elbow meters 21 and 22, and duct 23 and 24. The metering passage comprising 8, 21, 22, 23 and 24 is made long enough to suit metering objectives and to maintain slug flow. Facilities (not shown) are also provided to measure pressure and temperature within the metering passage and to measure the differential pressure between the inner and outer bend radii of each elbow meter 21 and 22. The elbow meters 21 and 22 sense the slug fronts and ends enabling slug velocity and length to be measured for each slug. Either elbow meter gives the mean density. When the flow is single phase, which would be indicated by the absence of slugs over a predetermined time interval, a sample could be taken at 12 to give the single phase density whence either elbow meter would give the velocity. Alternatively, the density for single phase operation may be from other means including sampling from a point in the metering passage.

It may be advantageous to incorporate a sand trap 25 which can be flushed out or returned to pipeline 13 at the sample return point 14 shown in Figs. 4, 5 and 6.

Referring to Figure 6, a separator is shown with domed ends 26 and 27, a double bend inlet 28 for optional mixing of liquids, a collection chamber 29 for gas at sample tapping point 9, a collection chamber 30 for high density fluid (usually water) at sample tapping point 11, a sampler with body assembly 31 and a return to pipe line 13 at 32. It may be advantageous to combine 14 and 32 return points.

The collecting chambers 29 and 30 can be separate chambers as shown or, alternatively, can be made part of the separator ducts 3 and 5 respectively.

Referring to Figure 7, a sampler is shown with a body assembly 31, two low stiffness diaphragms (referred to as floppies or floppy) 32 and 33 which with valves 35, 36, 37, 38, 39 and 40 form a chamber space 34 for samples. The chamber space 34 has heating means 41 and a temperature measuring means 42. The heating means 41 and the temperature measuring means 42 may be incorporated in the body assembly or in the floppies 32 and/or 33. Three differential pressure sensing means 43, 44 and 45 known to the art are incorporated. Pressure sensing means 43 measures the average density of the sample in the chamber space 34 by displacing the barrier fluid 46 between the floppy 33 and the pressure sensing means 43. Pressure sensing means 44 and 45 which are connected by barrier fluid 47 via pipe 48 measure the pressure differential relative to the system pressure due to the expansion of the sample by heating. The system pressure taken conveniently from sample point 11 acts on floppy 49 and the system pressure is thus transmitted to barrier fluid 50 which is in contact with pressure sensing means 44 and 45 through pipe 51. When a sample is admitted to chamber space 34 its density is monitored by differential pressure sensing means 43 until a constant reading indicates that the sample has displaced the fluid previously filling the chamber space 34. The density is recorded and then the energies applied to the heating means 41. The energy input, the temperature rise and the pressure rise are recorded. When the energy input to the heating means 41 is stopped, the temperature continues to be recorded thus giving data for cooling correction. The specific heats, the expansion coefficients for the sampler components and the sample fractions (oil, gas and water), the corrected temperature rise, the pressure rise and sample average density enable the fractions of oil, gas and water, and the density of one of the fractions (usually oil) to be calculated, with the density of the gas and the water being determined separately from concentrated samples taken from sample points 9 and 11 or from other means .

Various sampling and of flushing sequences are possible .../12

and the following give examples:

Fluid Sample Sample Valves Open Comment Point

Gas . 9 36 and 40 Water. 11 39 and 37 Homogenised 12 38 and 37 Sample displacing and Single gas . Phase Fluid. Ditto. 52 35 and 40 Sample displacing higher density fluid (usually water) .

Varying 52 35 and 40 Elbow meter Mixture of density, sample Gas, Oil and density and phase Water or any fractions are two. correlated. Liquid Slugs. 52 35 and 37 Samples of slugs initially displace gas. and 35 only Correction to be as chamber applied if fills. entrained gas escapes from sample. Elbow meter density, sample density and phase fractions are correlated.

Sand Flushing 11 39 and 40 Any sand or the with water. like swept out across floppy 33.

The sample fluid displaces existing fluid in space 34. In general, low density samples enter from top and displace higher density fluid at the bottom, while high density samples enter from the bottom and displace the lower density fluid at the top. Slug flow sampling is the exception with the sampled liquid slugs entering from the top and displacing the low density fluid (gas) at the top. When long periods of homogenised or single phase flow occur there may be no gas or water available and any sampling would be as above table with the sample allowed to flush through the chamber space. All valves 35, 36, 37, 38, 39 and 40 are closed during the heating and subsequent cooling cycle.

It may be advantageous to excite one or more of the differential pressure sensing means 43, 44 and 45 to encourage mixing of the sample to give a good temperature reading.

It may be advantageous to have floppies 32 and 33 with stepped edges such that the clamping is distanced from the inlets and exits from the chamber space 34 to assist venting of gas and draining of liquids.

The separator can be connected to fluid flow meters, other than those described, to take advantage of the clear signals produced by the successive passing of slugs of each component phase of the multi-phase fluid.

In Fig. 8 the horizontal section 3 of the duct of the separator may divide into branches which connect with the inclined sections 5 of the duct, prior to entry into the metering means . This embodiment is useful where the length of the apparatus requires to be economised.

In Fig. 9 there is shown a further alternative embodiment, wherein the horizontal section 3 of the duct is connected via bends or the like to the inclined section 5. A yet further embodiment is shown in Fig. 10 which again enables a reduction in the product length, without compromising the distance of the flow path. In the embodiment shown in Fig. 10, the feed pipe 2 is divided into two branches which enter the enlarged duct 3 at respective inlets, the horizontal sections 3 communicating with the inclined section 5. In Figs. 11-13 yet further compact arrangements or embodiments are shown which incorporate folded and branched sections.

Tests have been run over a wide range of gas to liquid ratios which show the formation of single phase slugs at low frequencies which produce clear signals on conductive probes, impact probes and elbow meter differential pressure transducer.

Further modifications and improvements may be incorporated without departing from the scope of the invention herein intended.

Claims

1 A separator for separating homogenous slugs or bubbles from a multi-phase fluid, comprises a duct having a fluid inlet and a fluid outlet, wherein the fluid outlet communicates with a metering means .

2 A separator as claimed in Claim 1, wherein at least part of the length of the duct is inclined upwardly relative to the direction of fluid flow.

3 A separator as claimed in Claim 1 or Claim 2, wherein the duct also includes a horizontal section upstream of the inclined section.

4 A separator as claimed in any one of the preceding Claims, wherein the duct inlet communicates with a feed pipeline, and wherein the inlet has a substantially larger flow area than the pipeline, such that the formation and growth of slugs or large bubbles is encouraged as the multi-phase fluid flows into the inlet.

5 A separator as claimed in any one of the preceding Claims, wherein the length of the duct is sufficient to allow for slug growth throughout passage of the fluid through the duct.

6 A separator as claimed in any one of the preceding Claims, further comprising one or more tapping outlets for taking samples of gas or vapour, liquid mixtures, high density fluid and single phase or homogenized fluid.

7 A separator as claimed in Claim 6, wherein one or more of said tapping outlets comprise or comprises a collecting chamber for flushing purposes.

8 A separator as claimed in any one of the preceding Claims, comprising a means for trapping sand particles or other debris.

9 A separator as claimed in Claim 8, wherein said means is integral with a tapping point for taking samples of liquid mixtures, said tapping point being shaped to provide a sand trap.

10 A separator as claimed in any one of the preceding Claims, wherein at least one end of the duct is domed.

11 A metering means comprising a metering passage and measuring devices for measuring the pressure and temperature of fluid within the metering passage.

12 A metering means as claimed in Claim 11, further comprising gamma ray densiometers for measuring density and for sensing slug fronts and ends, enabling measurement of slug velocity and length.

13 A metering means as claimed in Claim 11 or 12, wherein the metering passage includes a venturi.

14 A metering means as claimed in any one of Claims 11 to 13, wherein the metering passage comprises a duct and two or more elbow meters.

15 A metering means as claimed in Claim 14, wherein at least one elbow meter indicates the mean density of fluid passing therethrough. 16 A sampler comprising a body assembly, upper and lower diaphragms, the body assembly and diaphragms together forming a chamber space for holding a sample, the sampler further comprising heating means and temperature measuring means .

17 A sampler as claimed in Claim 16, further comprising pressure sensing means for measuring the pressure of the sample in the chamber space.

18 A separator as claimed in Claim 16 or 17, wherein the chamber space is associated with an inlet and outlet, wherein the inlet and outlets are inclined to assist in clearing of previous samples and/or flushing.

19 A sampler as claimed in any one of Claims 16 to 18, wherein the heating means may be used for dewaxing purposes .

20 A sampler as claimed in any one of Claims 16 to 19, wherein the internal surfaces of the body assembly and two diaphragms which define the chamber space are coated with a non-stick surface, such as polytetrafluoroethylene (PTFE) .

21 A sampler as claimed in any one of Claims 16 to 20, wherein the diaphragms are sloped or inclined at a slight angle to assist the venting of gas and draining of liquid respectively from the chamber space.

22 Apparatus for analysing components of a multi- phase fluid comprising a separator, metering means and a sampler. 23 Apparatus as claimed in Claim 22, wherein the separator is as claimed in any one of Claims 1 to 10, the metering means is as claimed in any one of Claims 11 to 15, and the sampler is as claimed in any one of Claims 16 to 22.

24 A method of analysing components of a multi-phase fluid, comprising the steps of forming homogenized slugs or large bubbles of one or more of the components within the fluid and successively passing the slugs or large bubbles into a metering means for the purpose of measuring mass or volume fluid flow.

25 A method as claimed in Claim 24, making use of apparatus as claimed in Claims 22 or 23.

26 A method of metering with an associated separator which forms slugs or large bubbles wherein the metering is by sensing each slug or large bubble front and end passing two fixed points to give slug velocity and length.

27 A method as claimed in Claim 26, wherein the density is sensed by one or both of the two fixed sensors.

28 A method of metering where samples are allowed to enter a chamber with isolation means, wherein the sample average density is measured by differential pressure sensor means, followed by heating, thus raising the temperature and the pressure of the sample, thereby giving data such as to enable the calculation of the component fractions (oil, gas and water) and the density of one fraction (usually oil) with the other two densities by other means, including sampling of concentrated samples like gas and water.