Method and apparatus for the isokinetic sampling of a multiphase stream

An apparatus for isokinetic sampling of the liquid and gaseous phases in a fluid multiphase stream, comprising:

• a sampling device for separation of said stream in a sampled fraction having a flow-rate qC and a non-sampled fraction having a flow-rate qNC and for 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,
• two differential pressure gauges in fluid communication with the interior of said tubular body for simultaneously measuring pressure drops ΔpC and ΔpNC of said sampled and non-sampled fractions, caused by flow restrictions;
• a liquid-gas separator for separating the liquid and gaseous phases of said portion of sampled fraction;
• and
• a flow meter for measuring 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 located at the same flow cross section, both normal to the flow direction, at said open end of said sampling probe or sampling channel, said restrictions having substantially annular form, 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 AC for the flowing of said sampled fraction and a non-sampling opening having a narrowed section of area ANC for the flowing of said non-sampled fraction, said sections being such that
• ΔpC=αC·qC2AC2 and ΔpNC=αNC·qNC2/ANC2
• wherein αC and αNC are a calibration coefficient of the section restriction through which the sampled fraction and, respectively, the non-sampled fraction flow, and are such that ΔpC=ϕΔpNC and αC=ϕαNC, wherein the coefficient ϕ is constant.