A METHOD AND APPARATUS FOR THE ISOKINETIC SAMPLING OF A MULTIPHASE STREAM

Abstract

The present invention refers to a method for the isokinetic sampling of liquids and gases present in streams having many fluid phases, and to an apparatus suitable for achieving it. The method and apparatus have application in particular in the field of oil extraction, wherein, after the extraction of liquid and gaseous hydrocarbons possibly accompanied by water and suspended solids, it is necessary to know the composition of the mixture extracted and also the flow rate of the single phases.

Claims

1. An apparatus for the isokinetic sampling of the liquid and gaseous phases in a fluid multiphase stream, comprising: a sampling device for the separation of said stream in a sampled fraction having a flow-rate q.sub.C and a non-sampled fraction having a flow-rate q.sub.NC and for the sampling under substantially isokinetic conditions of a portion of fluid of said sampled fraction, said device comprising a tubular body inside of which said multiphase stream flows, and a sampling probe or sampling channel having an upper end open to the flow of said multiphase stream, and extending outside of said tubular body through an opening on it, two differential pressure measuring means in fluid communication with the interior of said tubular body for simultaneously measuring the pressure dropsΔ p.sub.C and Δp.sub.NC of said sampled and non-sampled fractions, caused by flow restrictions; means for the separation of the liquid and gaseous phases of said portion of sampled fraction; and means for the measurement of the flow-rates of the liquid and gaseous phases exiting from said separation means; wherein said device for the isokinetic sampling is provided with two flow restrictions in parallel between each other at said open end of said sampling probe or sampling channel, said restrictions having substantially annular form or, being such as to cause said pressure drops and to define two narrowed, concentric flow sections of circular perimeter that constitute respectively a sampling opening having a narrowed section of area A.sub.C for the flowing of said sampled fraction and a non-sampling opening having a narrowed section of area A.sub.NC for the flowing of said non-sampled fraction, said sections being such that
Δp.sub.c=α.sub.C.Math.q.sub.C.sup.2/A.sub.C.sup.2 and ΔP.sub.NC=α.sub.NC.Math.q.sub.NC.sup.2/A.sub.NC.sup.2 Wherein α.sub.C and α.sub.NC are a calibration coefficient of the section restriction through which the sampled fraction and, respectively, the non-sampled fraction flow, and they are such that and they are such that ΔP.sub.C=φ ΔP.sub.NC and α.sub.C=φα.sub.NC, wherein the coefficient φ is constant.

2. The apparatus according to claim 1, wherein said device for the isokinetic sampling is provided with two flow restrictions in parallel between each other, defining such respective narrowed flow sections so that the coefficient φ is between 0.9 and 1.1.

3. The apparatus according to claim 2, wherein said device for the isokinetic sampling is provided with two flow restrictions in parallel between each other, defining respective narrowed flow sections so that the coefficient φ is substantially equal to 1.

4. The apparatus according to claim 1, wherein said sampling probe consists of a tubular axial portion extending inside said tubular body for a part of it and coaxially to it and of a tubular radial portion, and said device is provided with two annular flow restrictions, a first restriction rigidly connected to said axial portion of the sampling probe and defining a circular, narrowed section for the flowing through of said sampled fraction through a sampling opening, and a second restriction rigidly connected to said tubular body and defining, together with said upper end of said probe, an annular narrowed section for the flowing through of said non-sampled fraction through a non-sampling opening.

5. The apparatus according to claim 4, wherein said second restriction consists of a nozzle-type fluid path restriction element.

6. The apparatus according to claim 1, further comprising a rim formed on the inner wall of the tubular body of the apparatus upstream of said restrictions.

7. A method for the isokinetic sampling of the liquid and gaseous phases in a fluid multiphase stream flowing inside a tubular body, comprising the sampling of a portion of said fluid stream entering through a sampling opening wherein there exist substantially isokinetic conditions, by means of a device for the isokinetic sampling as defined in claim 1.

8. The method according to claim 7, further comprising, before said sampling, the flowing of said multiphase fluid through a portion of said tubular body provided with a rim able to disrupt a possible film of liquids that forms along the wall of the tubular body itself

9. A method for the measurement of the liquid and gaseous flow-rates q.sub.L and q.sub.G of the liquid and gaseous phases in a fluid multiphase stream of a total flow-rate Q flowing inside a tubular body comprising the isokinetic sampling of a portion of said multiphase stream having a flow-rate q.sub.C according to the method defined in claim 7, followed by the separation of said portion of flow-rate q.sub.C in the single liquid and gaseous phases of flow-rates q.sub.L and q.sub.G, which are then measured.

10. The method according claim 9, further comprising a step wherein said flow-rates q.sub.L and q.sub.G of the liquid and gaseous phases, once measured and possibly combined, are re-introduced in said multiphase stream inside said tubular body.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0023] The characteristics and advantages of the apparatus of the invention and of the relative method of isokinetic sampling of the different phases in a multiphase flow will become clearer from the following description of an embodiment thereof given as an example and not for limiting purposes with reference to the attached drawings, in which:

[0024] FIG. 1 compares the schematic representation of a device for: (a) isokinetic sampling for monophase stream according to the prior art; (b) isokinetic sampling for multiphase stream according to the prior art; (c) Dual Orifice Sampling according to the invention;

[0025] FIG. 2 represents a perspective view, partially sectioned, of a first preferred embodiment of the Dual Orifice apparatus with annular section according to the invention;

[0026] FIG. 3 shows a perspective view, partially sectioned, of a second embodiment of the Dual Orifice apparatus with annular section according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0027] In the present invention, as described hereinafter in detail, it is presumed to conduct a continuous and simultaneous measurement of flow rate of a sampled multiphase fluid and of the same non-sampled multiphase fluid.

[0028] With reference to the figures, and in particular to FIG. 1c and to FIG. 2, an apparatus of the invention in a preferred embodiment thereof comprises an isokinetic sampling device 3 consisting of a tubular body 13, having a flow-through section A, inside which the multiphase stream of total flow rate Q flows.

[0029] On the inner wall of said tubular body 13 it is possible to form a rim 5 able to disrupt the possible film formed by the liquid that flows along the walls of the tubular body. Downstream of said rim 5, if present, a sampling probe 15 is positioned, essentially shaped like an L, consisting of a tubular axial portion 15a, extending inside the tubular body 13 for a part thereof, coaxially with respect to said body 13, and a tubular radial portion 15b.

[0030] Said axial portion 15a is oriented with its free end open in the direction opposite to the flow of the multiphase stream of total flow rate Q, at which an annular narrowing 17 is positioned, rigidly connected to the axial portion 15a of the probe 15 and such as to internally define a circular narrowed section, i.e. an orifice, of area A.sub.C, which constitutes a sampling opening 18 of the flow rate of sampled fluid q.sub.C, the underlying part of the axial portion 15a internally delimiting a vertical part 8a of the sampling channel of the probe.

[0031] The radial portion 15b of the sampling probe 15 extends from the axial region of the tubular body 3, communicating with an outlet duct 1 that extends radially and externally through an opening 2 obtained on the tubular body 13, internally delimiting a horizontal part 8b of the sampling channel.

[0032] At the same height as the sampling opening 18, inside the tubular body 13 and rigidly connected thereto, a second annular narrowing 14 is positioned having an outer diameter equal to the inner diameter of the tubular body 13 and an inner diameter greater than the outer diameter of the sampling probe 15 and such as to define, with the upper end of the sampling probe 15, an annular opening 16, i.e. an annular orifice, which constitutes the flow-through section of area A.sub.NC of the flow rate of non-sampled fluid q.sub.NC. An underlying non sampling channel with annular section 6a is limited laterally by the inner cylindrical wall of the tubular body 13 and by the outer cylindrical wall of the sampling probe 15. Beneath the sampling probe 15, the part of non-sampling channel 6a with annular section takes up a circular section in a part of non-sampling channel 6b coinciding with the inside of the cylindrical body 13.

[0033] Again with reference to FIG. 2, two differential pressure measurers 7C and 7NC are in fluid communication with the inside of the tubular body 13, at suitably selected parts, so as to simultaneously measure the pressure drops of the fluid of the multiphase stream, sampled and non-sampled respectively, caused by the respective flow restrictions, according to the method described in greater detail hereinafter.

[0034] FIG. 3 illustrates a second embodiment of the apparatus according to the present invention that, in the same way as the first embodiment of FIG. 1, also foresees flow restrictions with annular section, but the non-sampling opening with annular section is in this second embodiment defined between a nozzle-type fluid path restriction system 23, and the upper open free end of the sampling probe 24, inside which a second annular narrowing 26 is positioned, rigidly connected thereto and such as to internally define the circular sampling opening 27, arranged at the same height as the nozzle-type flow restriction 23.

[0035] In the same way as the first embodiment of FIG. 2, the second embodiment of FIG. 3 also foresees a sampling probe formed from an axial portion 24a and a radial portion 24b, the latter communicating with an outlet duct 28 that extends radially and externally through an opening 29 obtained on the tubular body 21. Inside the two portions of the probe, 24a and 24b, the horizontal part and the vertical part of the sampling channel are delimited, respectively indicated as 30a and 30b.

[0036] Beneath the non-sampling opening a non-sampling channel 22a is formed having an annular section that, beneath the sampling probe 24, coincides with the inside of the tubular body 21 in a channel having a circular section 22b.

[0037] The embodiments of the apparatus of the invention illustrated above are therefore characterised by the presence of flow restrictions inside the tubular body of the apparatus, which define narrow sections, i.e. orifices, for the multiphase fluid to flow through, one for the sampled fluid (18, 27) to flow through and at least one for the non-sampled fluid (16, 25) to pass through, which can differ in shape, but in any case are arranged parallel to one another. Such flow restrictions make it possible to carry out an effective mixing of the multiphase stream of total flow rate Q at the sampling point, and also allow the simultaneous measurement of the pressure drops of the sampled fluid and of the non-sampled fluid through suitable differential pressure measurers (7C, 7NC). In all of the configurations proposed, a non-sampling differential pressure measurer (7NC) is able to measure the pressure difference between the total fluid entering the sampling section (6, 22) and the non-sampled fluid flowing in the non-sampling section (6a, 22a) respectively upstream and downstream of the dual orifice. Similarly, a sampling differential pressure measurer (7C) is able to measure the pressure difference between the total fluid entering the sampling section (6, 22) and the sampled fluid flowing in the sampling section (8a, 30a), respectively upstream and downstream of the dual orifice.

[0038] According to a preferred embodiment of the present apparatus, a rim 5 is formed on the inner wall of the tubular body (13, 21) of the apparatus upstream of the aforementioned flow restrictions so as to create a discontinuity element on the inner wall of the tubular body and disrupt the possible film formed in the multiphase stream flowing along the walls, in this way promoting greater uniformity in the composition of the multiphase stream itself.

[0039] The rim 5 can for example consist of a ring of low thickness with respect to the diameter of the tubular body (for example of thickness corresponding to about 5% of the diameter of the tubular body) having a triangular or trapezoidal section.

[0040] The method for measuring the liquid and gaseous flow-rates q.sub.L and q.sub.G of single liquid and gaseous phases present in a stream of multiphase fluid of total flow rate Q flowing inside a tubular body according to the present invention comprises the following steps:

[0041] (i) sampling a portion of fluid q.sub.C entering through a sampling opening of section A.sub.C in which substantially isokinetic conditions occur through an isokinetic sampling device like the one of the present apparatus described above;

[0042] (ii) separation of said portion of sampled fluid q.sub.C in the single liquid and gaseous phases of flow rate respectively q.sub.L and q.sub.G;

[0043] (iii) measurement of said flow-rates q.sub.L and q.sub.G of the separated liquid and gaseous phases of said sampled portion of fluid.

[0044] In the present isokinetic device a portion of fluid of flow rate q.sub.C that enters through the sampling opening (18, 27) with narrowed section of area A.sub.C flows through the duct of the sampling probe (8a,8b, 30a,30b) and is sampled. The remaining portion of fluid of flow rate q.sub.NC, on the other hand, enters through the non-sampling opening (16, 25) with narrowed section of area A.sub.NC and flows inside the non-sampling duct (6a, 6b, 22a, 22b).

[0045] An innovative aspect of the apparatus and method of the present invention is represented by the way in which isokinetic sampling conditions are ensured. In the two flow restrictions the sampled flow rate q.sub.C, flowing through a flow restriction, causes a load loss Δp.sub.C that in the case of turbulent motion of the phases can be expressed as

Δp.sub.c=α.sub.C.Math.q.sub.C.sup.2/A.sub.C.sup.2   (3)

where α.sub.C is the calibration coefficient of the flow restriction, a coefficient that, at least theoretically, at high flow speeds, depends exclusively on the geometry of the system and does not depend on the flow rate of the phases, i.e. for the case of flow of a single phase, on the Reynolds number of the flow.

[0046] An analogous relationship applies to the flow rate of non-sampled fluid:

Δp.sub.NC=α.sub.NC.Math.q.sub.NC.sup.2/A.sub.NC.sup.2,   (4)

where α.sub.NC is the calibration coefficient of the flow restriction flowed through by the non-sampled fraction of the overall flow.

[0047] The geometry of the present apparatus, characterised by the presence of two flow restrictions parallel to each other, is such that the ratio α.sub.C/α.sub.NC does not change even for significant variations of the physical properties and of the flow-rates of the phases, i.e. α.sub.C=φα.sub.NC, where the coefficient φ is constant.

[0048] This corresponds to the following condition:

ΔP.sub.C=φΔP.sub.NC,   (5)

which ensures that the speed of the phases in the two orifices is the same. If the aforementioned condition (5) occurs, the preceding equations give:

q.sub.C/A.sub.C=q.sub.NC/A.sub.NC,   (6)

Indeed, since q.sub.C+q.sub.NC=Q and A.sub.C+A.sub.NC=A, also

q.sub.C/A.sub.C=Q/A,   (7)

as required for an isokinetic sampling. The verification of the actual isokinetic nature of the sampling is carried out by checking that the relationship (7) is satisfied.

[0049] For the purposes of optimal operation of the method of the invention, it is possible to define geometries of the present apparatus such as to make the aforementioned coefficient φ substantially equal to 1. Indeed, in the at least two flow restrictions the flow rate of the respective fluid that flows through them is linked to the load loss determined by the passage through the restriction and to the section of the restriction itself by the following general relationship:

[00001]$\begin{array}{cc}q={\mathrm{cA}}_0.Math.\sqrt{\frac{2.Math.\Delta .Math..Math.P.Math.\text{/}.Math.\rho }{1-{\left(A_0.Math.\text{/}.Math.A\right)}^2}}& \left(8\right)\end{array}$

in which q is the flow rate of a generic fluid flowing through a restriction of section A.sub.0 created in a tube of total section A, ΔP is the load loss of the fluid determined by its flowing in the restriction and p is the density of the fluid, whereas the coefficient c in the aforementioned relationship (8) gives a measurement of the load loss in the system that is caused by the narrowing of section. In a commonly used apparatus for the measurement of the flow rate of a mono-phase fluid (orifice, nozzle, Venturi tube), the coefficient c depends on the geometry of the system and on the value of the Reynolds number. For high values of the Reynolds number, the coefficient c depends exclusively on the geometry of the system and in practice is constant as the speed and the viscosity of the fluid change. When the parameter φ is equal to 1, the two coefficients c for the portions of sampled and non-sampled fluid are equal to each other, the load losses through the two flow restrictions are totally analogous. For values of φ close to 1, for example comprised between 0.9 and 1.1, the fluid-dynamic behaviour of the sampled and non-sampled fluids that flows through the at least two restrictions therefore remains totally analogous also in terms of load losses.

[0050] Experimentally, it has been found that, in the case of monophase flows, by suitably selecting the geometric parameters of the at least two flow restrictions, for example like in the embodiments of the present apparatus illustrated in FIGS. 2-3, the values of the coefficient φ remain not only constant as the Reynolds number of the flow varies, but also comprised in a narrow range of values comprised between 0.9 and 1.1 also being able to take on values substantially equal to 1.

[0051] The separation of the liquid and gaseous phases in step ii) of the present method can be carried out using any conventional liquid-gas separator, positioned in fluid communication with the sampling device. Such a separator, having to treat only a small part of the overall flow rate, approximately comprised between 5% and 15% thereof, can be of simple structure and of very low volume.

[0052] The sampling flow-rates after the measurement in step iii), which can be carried out with monophase stream measurers also of the conventional type, possibly joined back together, are re-inserted in the main stream.

[0053] According to a preferred embodiment of the present method, the multiphase fluid is made to flow through the tubular body provided with a rim 5 upstream of the sampling, which disrupts the possible film of liquids formed along the wall of the tubular body itself, thus promoting the mixing of the total multiphase stream before sampling.

[0054] An important advantage of the present apparatus with respect to those mentioned above of the prior art is represented by the fact that the particular internal structure of the isokinetic sampling device is not a complex or bulky structure, provided with relative constructive simplicity and of low cost with respect to known devices.

[0055] A further important advantage of the apparatus of the invention is represented by the fact that both the mixing between the phases present in the multiphase stream and the measurement of the pressure drop are carried out exclusively at the two flow restrictions arranged parallel to each other, where the sampling of a portion of flow rate q.sub.C of the total flow of flow rate Q is also carried out. Therefore, in the apparatus of the invention the pressure drops caused by the mixing section of the phases that is necessary in the devices of the prior art before the sampling section do not occur, and nor do the pressure drops caused by the calibrated flange that is located downstream of the sampling section in known devices, where it is used for the continuous control of the process. The overall pressure losses in the apparatus according to the present invention are therefore significantly reduced if compared with those that can be detected in apparatuses of the prior art.

[0056] The present invention has been described up to here with reference to a preferred embodiment thereof. It should be understood that there can be other embodiments that derive from the same inventive core, all of which are covered by the scope of protection of the claims given hereafter.