Apparatus and method for measuring the flow-rate of a multiphase fluid stream

An apparatus for measuring the liquid and gas flow-rates QL and QG in a multiphase stream which flows in a pipe, comprising:

• a measuring unit which coaxially intercepts two portions of the pipe, including:
• a) an isokinetic sampling device for the sampling of a portion of said multiphase stream, suitable for separating said multiphase stream into a sampled fraction and a non-sampled fraction, said device comprising a tubular body coaxial to the portions of said pipe and a distributing body positioned inside said tubular body suitable for generating a uniform radial distribution of the flow of said multiphase stream entering into n channels, of which m are sampling channels, annularly arranged on said distributing body which guarantee that the sampled fraction will have properties, in particular fractions by volume of the present phases and rates, almost identical to those of the non-sampled fraction;
• b) differential pressure measuring means between said sampled fraction and said non-sampled fraction situated downstream of the sampling;
• c) a flow restriction, having a reduced passage section with respect to the section of the pipe, situated downstream of said isokinetic sampling device equipped with a differential pressure measuring means associated with the flow restriction;
• separation means for separating the liquid and gaseous phase of said sampled fraction in said isokinetic sampling apparatus;
• measuring means at the outlet of said separation means for producing measuring signals of the liquid and gas flow-rates in the sampled fraction;
• regulation means, installed after the remixing of the liquid and gaseous fractions leaving the separator for controlling the sampled flow-rate by means of said isokinetic sampling device;
• data processing means suitable for receiving and processing the signals coming from the pressure indicators and flow-rate measurers and sending operative signals to said regulation means for varying the flow-rate of the sampled fraction in said sampling device.