The present application is the national stage under 35 U.S.C. xc2xa7371 of international application PCT/IT99/00041, filed Feb. 19, 1999 which designated the United States, and which application was not published in the English language. 
1.  Field of the Invention
The present invention relates to a method for measuring the flow rate of each phase in a multiphase stream, for example a gas (or vapor) stream in the presence of a significant volume fraction of liquid.  The invention also relates to an apparatus for measuring the flow rate of the phases of a multiphase stream operating according to said method. 
2.  Description of the Prior Art
The measurement of the flow rate of a gas (or vapor stream in the presence of a significant volume fraction of liquid, or of a liquid stream in the presence of a gas is a common technical problem in the process industry.  The coexistance of two liquid phases is also a typical situation.  Examples of multiphase flows are encountered in the extraction of endogenic fluids, such as liquid hydrocarbons and water accompanied by gas; gaseous hydrocarbons flows (natural gas) accompanied by a liquid phase consisting of water, or geothermal fluids formed by a vapor phase consisting of steam, as a major part, and gas such as the H2S and a liquid phase essentially consisting of water. 
The measure of the flow rate of the prevailing phase in a multiphase fluid stream cannot, in general, be carried out directly, as the precision of the measure rapidly decreases as the volume fraction of the supplementary phase increases.  For this reason the separation of the single phases is first carried out to give raise to two monophase streams, the flow rate of which can then be measured in a easy and extremely precise way by means of known instruments. 
Since the solution involves the handling of the overall flow of fluid, the use of a very large separator is requested and not always this solution is logistically feasible and/or economically convenient, particularly when considerable flow rates are involved. 
UK patent application no.  2179156 discloses the solution of diverting a portion of the total flow of a triphase mixture flowing in a conduit and to separate the portion in two phases in order to measure the average density of the liquid phase.  Therefore, the withdrawal and separation procedure is not necessary when the liquid density is known.  In the above disclosure it is proposed in particular a meter operating on the basis of the detection, besides the liquid density, of the total volume flow rate of the mixture, and of the average density of the mixture in order to calculate the flow rate of each phase.  The apparatus according to the UK patent application involves using a volume meter of the total flow velocity, said meter having limits of use due to maintenance problems.  For example, in a sub-sea installation of the apparatus, repairing or substitution of it would be considerably expensive. 
Moreover, the measurement of the combined density of the mixture usually requires the use of a xcex3-rays absorption meter, which is highly expensive and difficult to install in a production plant.  In addition, if the vapor phase is the prevailing phase, the measure of the combined density of the mixture by means of a xcex3-ray densitometer is barely precise. 
An object of the invention is to provide a method for carrying out a sufficiently precise measure of the flow rate of each phase in a multiphase fluid stream, without the need of processing the whole stream, what would mean the division of the overall stream into a number of monophase streams, and without the above mentioned inconveniences of the metering devices based on the measure of the combined density of the liquid phase. 
Another object of the present invention is to provide an apparatus operating according to the above method. 
These objects are accomplished with the method and the relevant apparatus for the measure of the flow rate of each phase in a multiphase fluid stream according to the present invention, the main features of which are defined in the attached claims 1 and 9. 
The invention accordingly provides that in a measure section, where a fraction of the total flow is sampled by at least a probe, a condition of uniform phase velocity be established, i.e. that UL and UG (local velocities of the liquid phase and of the gas phase) be constant over that section even if, in general, UL and UG will have different values.  Under such condition, if A1 indicates the area of the flow section and A2 indicates the area of the sampling section, i.e. the effective section of the probe (or probes) through which the sampling is performed, the sampling can be defined isokinetic if the sampling flow rate is really equal to the ratio A2/A1 of the total flow rate of the fluid stream crossing section A1. 
Therefore, in the case of isokinetic sampling of the phases in a section in which uniform flow conditions are established (UL and UG constant through the section), if qL and qG are the liquid and gas flow rates measured in the sampled fraction and QL and QG are the overall mass flow rates of the liquid and gas flowing through section A1, the following relations apply:                               Q          L                =                                            A              1                                      A              2                                ·                      q            L                                              (1a)                                          Q          G                =                                            A              1                                      A              2                                ·                      q            G                                              (1b)            
If q=qG+qL is the total sampled flow rate and Q=QG+QL the total flow rate flowing throughthe measure section, summing the respective left-hand sides and right-hand sides of equations 1a and 1b we obtain:                               q          Q                =                              A            2                                A            1                                              (        2        )            
Dividing the equation 1a by equation 1b we also obtain:                                           Q            L                                Q            G                          =                              q            L                                q            G                                              (        3        )            
Therefore, in the case of isokinetic sampling of the phases in a section in which uniform flow conditions are established, the total mass flow rates of liquid and gas QL and QG can theoretically be obtained directly from qL and qG measured after sampling and separation on the basis of the relations (1a) and (1b). 
According to the invention, the overall flow rate Q or one of its fractions QG or QL is obtained by measuring the pressure drop xcex94p of the flow in a constricted section of area A1, typically defined by a nozzle, near the sampling point on the basis of the relation, generally valid for a multiphase system:
xcex94p=xcex1.Q2xe2x80x83xe2x80x83(4) 
where xcex1 (which has the dimension of a specific volume) is the calibration coefficient of the nozzle, depending on the geometry of the constricted section, the physical properties of the phases and the value of the ratio QL/QG between the flow rates of the phases.  For a given geometry and small variations of physical properties and phase flow rates, xcex1 is constant with good approximation. 
A suitable use of equation (1a), (1b), (2), (3) and (4) allows the flow rates of the single phases QL and QG to be determined according to some possible procedures that will be described below.  It is worth noting that the results of these procedures are affected by substantially negligible errors even if the condition (2) of isokinetic sampling is only approximately met.  As a matter of fact, in a condition in which the gas phase is the prevailing one, even if the sampling is only approximately isokinetic equation (1a), which allows to obtain QL from the measure of the flow rate of the sampled liquid qL and the known value of A1/A2, is anyway valid.  In the cases in which the liquid phase if prevailing, even if the sampling is only approximately isokinetic, equation (3) is anyway valid.  In both cases of prevailing gas or of prevailing liquid, the measure of QL or of the ratio QL/QG in combination with equation (4) allow to obtain Q and the phase flow rates, QL and QG. 
An important feature of the invention lies in the way by which uniform velocity conditions are established in the sampling section.  In the case of prevailing flow of a liquid phase, uniform velocity conditions of the phases can be easily established by carrying out the sampling downstream of a constricted section of the conduit, for example the outlet section of a nozzle, so as to have conditions of high turbolence of the phases.  Before the nozzle a mixer or a static mixer can be provided for. 
If the prevailing phase is a gas (or a vapor) and dispersed annular flow conditions are established in the conduit, the average velocity of the phases UG and UL are such that UG can be considerably greater than UL and also the liquid can flow in the conduit as a film at the conduit walls and as dispersed droplets in the gas phase.  In this case the liquid velocity in the film, UL is considerably lower than the velocity of the droplets entrained by the gas and it is difficult to establish conditions of uniform velocity of the liquid in the sampling section.  In order to achieve this result, it is not sufficient to carry out the sampling downstream of the constricted section of a nozzle, but it is necessary that the sampling section be preceded by one or more sections of the conduit and the nozzle in which the liquid film is removed from the wall and, as a consequence, atomised and accelerated by the gas phase. 
The presence of mixers, nozzles and sections of suitable geometry may be not sufficient to guarantee homogeneous flow conditions at the sampling section, in particular when pressure drops through the sampling system must be kept as low as possible.  In order to overcome this possibile drawback and, in general, to reduce the errors to a minimum, it is advisable to carry out the sampling by using more probes symmetrically arranged and in suitable positions. 
The sampled flow rates of the liquid and the gas qL and qG are determined by using respective meters of the known type for monophase stream, after the phase separation is carried out in a gas-liquid separator of the known type, which has to process a small portion only of the overall flow rate (approximately 5% to 15% of the total flow rate, said figure being equal to A2/A1) and, therefore, can have a simple structure and a very small volume.  After the measure the sampled streams are taken into the main stream again.