The Turn Down Ratio and accuracy can be compared with the orifice plate. The flow nozzle is relative simple and cheap, and available for many applications in many materials. Note that the manometry for a venturi tube or orifice should be installed below the hydraulic grade line or pipe.įlow nozzles are often used as measuring elements for air and gas flow in industrial applications. With proper instrumentation and flow calibrating, the Venturi Tube flowrate can be reduced to about 10% of its full scale range with proper accuracy. After the constricted area, the fluid is passes through a pressure recovery exit section, where up to 80% of the differential pressure generated at the constricted area, is recovered. In the Venturi Tube the fluid flowrate is measured by reducing the cross sectional flow area in the flow path, generating a pressure difference. Wear will reduce the accuracy.ĭue to simplicity and dependability, the Venturi tube flowmeter is often used in applications where it's necessary with higher Turn Down Ratios, or lower pressure drops, than the orifice plate can provide. A high accuracy depend on an orifice plate in good shape, with a sharp edge to the upstream side. Their accuracy are poor at low flow rates. The Turn Down Ratio for orifice plates are less than 5:1. The orifice plates are simple, cheap and can be delivered for almost any application and in any material. The plate obstructing the flow offers a precisely measured obstruction that narrows the pipe and forces the flowing fluid to constrict. With an orifice plate, the fluid flow is measured through the difference in pressure from the upstream side to the downstream side of a partially obstructed pipe. Note that it is common to use " head" instead of "pressure"Ĭommon types of differential pressure flow meters are: Ρ = density of fluid (kg/m 3, slugs/ft 3) The differential pressure flow meter is based on the Bernoulli Equation where the pressure drop and the further measured signal is a function of the square flow speed. In a differential pressure drop device the flow is calculated by measuring the pressure drop over an obstructions inserted in the flow.
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