Flow Measurement: Physical principle employed in various types of flow meters. PART 4 PREPARED BY: ESO OLUWATOBILOBA

Transcription

1 Flow Measurement: Physical principle employed in various types of flow meters. PART 4 PREPARED BY: ESO OLUWATOBILOBA

2 Pressure Differential Flow Meters In a differential pressure drop device the flow is calculated by measuring the pressure drop over an obstructions inserted in the flow. 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. Common types of pressure differential flow meters: Orifice Meter Venturi Meter Flow Nozzles

3 Electromagnetic Flow Meter An electromagnetic flowmeter operates on Faraday s law of electromagnetic induction, which states that a voltage will be induced when a conductor moves through a magnetic field. It is used in systems with electrically conductive liquids. The magnetic feld is created by energised coils outside the flow tube. It is based on a fundamental principle that the electromotive force of electric potential (E), is induced in a conductor of length (L), which moves with a velocity (U), through a magnetic field of magnetic flux (B). [ E = U*B*L ]

4 Vortex Flow Meter `flow measurement involves placing a bluff body (called a shedder bar) in the path of the fluid. As the fluid passes this bar, disturbances in the flow called vortices are created. The vortices trail behind the cylinder, alternatively from each side of the bluff body. The frequency at which these vortices alternate sides is essentially proportional to the flow rate of the fluid. The frequency is measured and the flow rate is calculated by the flowmeter electronics using the equation [ f = SV/L ] where f is the frequency of the vortices, L the characteristic length of the bluff body, V is the velocity of the flow over the bluff body, and S is the Strouhal number, which is essentially a constant for a given body shape within its operating limits.

5 Rotameter The meter consists of a float within vertical tube, tapered to an increasing cross-sectional area at its outlet. Flow entering through the bottom passes over the float (which is free to move). The equilibrium height of the float indicates the flow rate. The operating principle of a rotameter is based on the balance between the drag force and the weight (W), and buoyancy forces (FB), acting on the float in the moving fluid. [ W= FD + FB ].

6 Turbine Meter If a fluid moves through a pipe and acts on the vanes of a turbine, the turbine will start to spin and rotate. The rate of spin is measured to calculate the flow rate. In principle the exchange of momentum within the flow turns the rotor at a rational speed that is proportional to the flow rate. The rotor angular velocity (W), depends on the average flow velocity (u), the fund kinematic viscosity (v), through the meter bore of diameter (d1).

7 Ultrasonic Flow Meter Ultrasonic maters use sound waves to determine flow rate. A pair of transducers separated by some distance, fixed to the outside of a pipe wall and a reflector applied to the opposite outside wall of the pipe, to increase the signal to nose ratio. An ultrasonic wave emitted by one transducer passes through the fluid, reflects off the pipe was and received by the other transducer. The difference in transit time for a wave to travel from transducer 1 to 2 and from transducer 2 to 1 is directly related to the average velocity of flow in the pipe.

8 Thermal Flow Meter The rate at which energy(e), must be added to a flowing fluid to raise its temperature between two control surfaces is directly relates to the mass flow rate. [ E = mcpdt ], where cp is the fluid specific heat. Thermal meters use a heated sensing element isolated from the fluid flow path where the flow stream conducts heat from the sensing element. The conducted heat is directly proportional to the mass flow rate and the temperature difference is calculated to mass flow.