Thermal Mass Flow Meter | Turbine Flow Meter | Open Channel Flow | Target Force Meters | Ultrasonic Flow Meters | Positive Displacement | Orifice | Venturi Tube | Vortex | Electromagnetic | Coriolis | Pitot Tube | Flow Nozzle
A flow meter is an instrument used to measure linear, nonlinear, mass or volumetric flow rate of a liquid or a gas. The basis of good flow meter selection is a clear understanding of the requirements of the particular application. Therefore, time should be invested in fully evaluating the nature of the process fluid and of the overall installation.
FLOW METER OPTIONS
Thermal Mass Flow Meter
Thermal mass flow meter operates independent of density, pressure, and viscosity. They 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.
Turbine Flow Meter
There are 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 turn-down 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%.
Open Channel Flow
Open channel flow monitoring is an older technique for measuring water flow rates in irrigation channels, streams, and storm water systems. In most applications for open channel flow, weirs and flumes are used. Traditional methods of measuring the level through weirs and flumes were by way of float sensors and measuring tapes, which are maintenance nightmares. Mechanical floats can be disrupted by debris and sediment buildup. The original method of monitoring flows was by way of a "weir stick" or ruler markings, this required personnel to check the readings at the location daily, weekly or monthly. Both traditional methods can have very high hidden costs in maintenance and labor charges, however, Niagara Meters & Niveclo offers a new solution to an outdated and expensive process.
Fixed Ultrasonic Flow Meters
2 types of Ultrasonic Meters are Doppler meters and time-of-travel (or transit) meters.
Doppler meters measure the frequency shifts caused by liquid flow. Two transducers are mounted in a case attached to one side of the pipe. A signal of known frequency is sent into the liquid to be measured. Solids, bubbles, or any discontinuity in the liquid, cause the pulse to be reflected to the receiver element. Because the liquid causing the reflection is moving, the frequency of the returned pulse is shifted. The frequency shift is proportional to the liquid's velocity.
Time-of-travel meters have transducers mounted on each side of the pipe. The configuration is such that the sound waves traveling between the devices are at a 45 deg. angle to the direction of liquid flow. The speed of the signal traveling between the transducers increases or decreases with the direction of transmission and the velocity of the liquid being measured. A time-differential relationship proportional to the flow can be obtained by transmitting the signal alternately in both directions. A limitation of time-of-travel meters is that the liquids being measured must be relatively free of entrained gas or solids to minimize signal scattering and absorption.
Target Force Meters
Sense and measure forces caused by liquid impacting on a target or drag-disk suspended in the liquid stream. A direct indication of the liquid flow rate is achieved by measuring the force exerted on the target. In its simplest form, the meter consists only of a hinged, swinging plate that moves outward, along with the liquid stream. In such cases, the device serves as a flow indicator.
A more sophisticated version uses a precision, low-level force transducer sensing element. The force of the target caused by the liquid flow is sensed by a strain gauge. The output signal from the gauge is indicative of the flow rate. Target meters are useful for measuring flows of dirty or corrosive liquids.
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.
Positive-displacement meters are good candidates for measuring the flows of viscous liquids or for use where a simple mechanical meter system is needed.
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. The plate obstructing the flow offers a precisely measured obstruction that narrows the pipe and forces the flowing fluid to constrict.
In the Venturi Tube the fluid flow rate is measured by reducing the cross sectional flow area in the flow path, generating a pressure difference. 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.
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 flow meter 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.
Direct mass measurement sets Coriolis flow meters 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 flow meters are universal meters. They use 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 turn down rate more than 100:1. The Coriolis meter can also be used to measure the fluids density.
Differential Pressure 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.
In-Line Meters have flow enter at end marked “IN” and forces the piston to move with it, against spring pressure, enough to pass given flow around piston
periphery. The knife edge of the
piston is visible through the
transparent housing; its position
under the printed scale gives the