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Theory: Venturi meter and orifice meter are the commonly used flow meters for measuring mass/volumetric flow rate or velocity of the flowing fluid. These flow meters are also known as variable head meters. They are categorized as full-bore meter as measurement of the fluid takes place when it flows through a conduit or channel. Venturi meter: The venturi meter has a converging conical inlet, a cylindrical throat and a diverging recovery cone. It has no projections into the fluid, no sharp corners and no sudden changes in contour. The following figure shows the venturi meter with uniform cylindrical section before converging entrance, a throat and divergent outlet. D (^) d Pressure taps The converging inlet section decreases the area of the fluid stream, causing the velocity to increase and the pressure to decrease. The low pressure is measured in the center of the cylindrical throat as the pressure will be at its lowest value, where neither the pressure nor the velocity will be changing. As the fluid enters the diverging section the pressure is largely
The small effect of the kinetic energy factors α 1 and α 2 are also taken into account in the definition of Cv. Volumetric flow rate Qa can be calculated as: Qa = V 2 S 2 = ……………………… (5) where, S 2 is the cross sectional area of throat in m^2. Substituting (P 1 – P 2 ) = ρgH in above equation (5) we get, Qa = V 2 S 2 = …………………………. (6) where H is the manometric height difference * (specific gravity of manometric fluid – specific gravity of manometric fluid of water).
Measurement Of Blood In Vessels It is used for the measurement of volume flow of blood through vessels. It can be done by two ways .One way is to insert an accurately calibrated Venturi meter made of glass into the circulation. This method requires an anticoagulant, but is accurate method and is sensitive to even slight changes in flow. Other way is by producing a constriction (making it narrower ) in a vessel by means of a ligature near a branch, which can be used as a side tube, thus transforming the vessel itself into a Venturi meter. Second method is subject to greater error in calibration for absolute flows but its advantage is that it does not require any type of anticoagulant. The Flow of Chemicals in Pipelines One of the advantage of using Venturimeter is that temperatures and pressures of chemicals in a pipeline do not affect the accuracy of this device .Due to this reason we useVenturiflowmeter in crude oil pipelines. Crude oil pipelines, such as the ones in Alaska, are exposed to extreme temperatures during winter. Another great advantage of using thisflowmeter in volatile and cold environments is that it does not contain any moving part so there is no chance of freezing and breaking due to thermal expansion.