The calorimetric flowmeter can achieve relatively high accuracy at low flow rates.
There is many different manufacturing design of turbine flow meters, but in general they are all based on the same simple principle:
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.
The turndown ratios may be more than 100:1 if the turbine meter is calibrated for a single fluid and used at constant conditions. Accuracy may be better than +/-0,1%.
An obstruction in a fluid flow creates vortices in a downstream flow. Every obstruction has a critical fluid flow speed at which vortex shedding occurs. Vortex shedding is the instance where alternating low pressure zones are generated in the downstream.
These alternating low pressure zones cause the obstruction to move towards the low pressure zone. With sensors gauging the vortices the strength of the flow can be measured.
An electromagnetic flowmeter operate on Faraday's law of electromagnetic induction that states that a voltage will be induced when a conductor moves through a magnetic field. The liquid serves as the conductor and the magnetic field is created by energized coils outside the flow tube.
The voltage produced is directly proportional to the flow rate. Two electrodes mounted in the pipe wall detect the voltage which is measured by a secondary element.
Electromagnetic flow meters can measure difficult and corrosive liquids and slurries, and they can measure flow in both directions with equal accuracy.
Electromagnetic flow meters have a relatively high power consumption and can only be used for electrical conductive fluids as water.
The effect of motion of a sound source and its effect on the frequency of the sound was observed and described by Christian Johann Doppler.
The frequency of the reflected signal is modified by the velocity and direction of the fluid flow
If a fluid is moving towards a transducer, the frequency of the returning signal will increase. As fluid moves away from a transducer, the frequency of the returning signal decrease.
The frequency difference is equal to the reflected frequency minus the originating frequency and can be use to calculate the fluid flow speed.
The positive displacement flow meter measures process fluid flow by precision-fitted rotors as flow measuring elements. Known and fixed volumes are displaced between the rotors. The rotation of the rotors are proportional to the volume of the fluid being displaced.
The number of rotations of the rotor is counted by an integral electronic pulse transmitter and converted to volume and flow rate.
The positive displacement rotor construction can be done in several ways:
The positive displacement flowmeter may be used for all relatively nonabrasive fluids such as heating oils, lubrication oils, polymer additives, animal and vegetable fat, printing ink, Dichlorodifluoromethane R-12, and many more.
Accuracy may be up to 0.1% of full rate with a TurnDown of 70:1 or more.
Mass meters measure the mass flow rate directly.
The thermal mass flowmeter operates independent of density, pressure, and viscosity. 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.
The accuracy of the thermal mass flow device depends on the calibrations reliability of the actual process and variations in the temperature, pressure, flow rate, heat capacity and viscosity of the fluid.
Direct mass measurement sets Coriolis flowmeters apart from other technologies. Mass measurement is not sensitive to changes in pressure, temperature, viscosity and density. With the ability to measure liquids, slurries and gases, Coriolis flowmeters are universal meters.
Coriolis Mass Flowmeter uses the Coriolis effect to measure the amount of mass moving through the element. The fluid to be measured runs through a U-shaped tube that is caused to vibrate in an angular harmonic oscillation. Due to the Coriolis forces, the tubes will deform and an additional vibration component will be added to the oscillation. This additional component causes a phase shift on some places of the tubes which can be measured with sensors.
The Coriolis flow meters are in general very accurate, better than +/-0,1% with an turndown rate more than 100:1. The Coriolis meter can also be used to measure the fluids density.
A common method of measuring flow through an open channel is to measure the height of the liquid as it passes over an obstruction as a flume or weir in the channel.
Common used is the Sharp-Crested Weir, the V-Notch Weir, the Cipolletti weir, the Rectangular-Notch Weir, the Parshall Flume or Venturi Flume.
Important factors when selecting flow metering devices are
These factors are more or less related to each other. Example - cost of flow meters increases with accuracy and life time quality.
Flow metering principles - Orifice, Venturi, Flow Nozzles, Pitot Tubes, Target, Variable Area, Positive Displacement, Turbine, Vortex, Electromagnetic, Ultrasonic Doppler, Ultrasonic Time-of-travel, Mass Coriolis, Mass Thermal, Weir V-notch, Flume Parshall and Sluice Gate flow meters and more.
Instrumentation and process control systems - design and documentation.
Conservation of energy in a non-viscous, incompressible fluid at steady flow.
Calculate the discharge length from the open end of a partially filled horizontal pipe.
A limited comparison of flowmeter principles - regarding service, rangeability, pressure loss, typical accuracy, upstream pipe diameters, viscosity and relative costs.
Introduction to accuracy in flow measurement devices.
Turndown ratio (Rangeability) can be used to compare flow measurement devices like orifices, venturi meters etc.
Weirs can be used to measure flow rates in open channels and rivers - common for water supply and sewage plants.
Nozzles discharging air volume.
The orifice, nozzle and venturi flow rate meters makes the use of the Bernoulli Equation to calculate fluid flow rate using pressure difference through obstructions in the flow.
Sluice gates can be used to control and measure volume flow rates in open channels and rivers, mainly in connection to hydro power plants.
The doppler effect is the change in sound frequency due to the relative motion between a source and a listener.
Steam flow through orifices - for steam pressures ranging 2 - 300 psi
Steam leaks through orifices for pressures ranging 250 - 600 psi
Inclined and vertical u-tube manometers used to measure differential pressure in flow meters like pitot tubes, orifices and nozzles.
An basic introduction to the ultrasonic Doppler and Time of Flight Flow Meters.
Flow rate or discharge in an open conduit, channel or river can be calculated with the velocity-area principle.
The vortex shedding frequency created around a bluff body and flow rate.
Standards for weir flow measurements.
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