Flow measurement
Flow Measurement is the process of measuring fluid in your plant or industry through a variety of different ways.
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count the number of times a fixed volume of fluid is filled
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indirectly calculate flow by measuring forces produced by the flowing stream to overcome a known constriction.
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measuring the velocity of fluid over a known area
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tracer methods
Types of flow meters
Mechanical flow meters
Measurement of flow is done by continuous displacement of fixd volume of fluid without letting the fluid out of the pipe. The filling and emptying of the fluid is timed for measurement. For transferring the fluid content methods like pistons and cylinders, gear teeth, rotating oval gears or a helical screw.
| Flowmeter | Description | Application |
|---|---|---|
| Piston meter/rotary piston | Semi – positive displacement using piston rotating within a chamber of known volume. Using gear mechanism for odometer type display. | domestic water measurement. |
| Oval gear meter | Positive displacement type using gears with inlet, outlet ports and a measurement chamber. The gears are fitted with permanent magnet connected to a reed system for flow rate measurement | High viscosity applications |
| Helical gear | Rotors made of semi spherical shape capable of holding fluids. The fluids move the rotor and measuring its rotation rate gives flow rate. | High viscosity applications |
| Nutating disk meter | A electrically mounted disk along the vertical axis wobbles on the fluid flow. The nutating action gives the flow rate | Residential applications, automatic Batching applications |
| Turbine flow meter | Not a displacement type device as the turbine does not hinder the flow of the fluid, the blades of the turbine placed in the path of fluid flow moves when the fluid hits the blades thus rotation per minute of the blades give the flow rate | high accuracy, excellent repeatability and low pressure drop, turbine flowmeter is widely used in industry to measure gas or liquid flow. |
| Woltman meter | A turbine flow type with helical vanes as in helical meter. | Potable water |
| Single jet meter | A impeller with vanes radially. Increases the pressure and flow of the fluid. | secondary meter in residential and small industrial applications and in beverage machines and similar applications. |
| Paddle wheel meter | Magnetic blades which on rotation is sensed by the magnetic sensor generating a voltage signal with frequency relative to the flow rate | The most common use is in a system where the fluid is like water. Not applicable for less viscos or turbulent appliations. |
| Multiple jet meter | Same as a single jet meter but with the fluid flow hitting the horizontally rotating vanes at multiple points n its circumference thus reducing wear and tear in the impeller | Accurate at low flow rate for domestic and light industrial applications |
| Pelton wheel | Similar to current generating vanes, but the rotation across the axis can be read as rate of flow | Low viscosity applications |
| Current meter | Propeller-type current meters with remote data acquisition | testing the efficiency of large turbines. |
Pressure based meters
These type of meters are based on Bernoulli’s principle measuring differential pressure or static or dynamic pressures
| Flowmeter | Description | Application |
|---|---|---|
| Venturi meter | The coefficient of discharge of Venturi meter ranges from 0.93 to 0.97 | flow rate in the transmission of gas through pipelines |
| Orifice plate | There are three type of orifice: concentric, eccentric, and segmental. | Single phase well mixed continuously flowing fluid that fills the pipeline. Used in turbulent |
| Dall tube | The pressure differential is typically measured using diaphragm pressure transducers with digital readout. Differential pressure produced is higher than Venturi tube | Larger pipelines |
| Pitot-tube | Measuring local fluid velocities. A volumetric rate of flow is determined by generating the velocity profile from measuring the velocity at different points in the flow. | Common applications are in air speed measurement for aircrafts and racing cars |
| Multi-hole pressure probe | Also called impact probes use Pitot tube theory for measurement in more than one dimension. | Aerodynamic testing of swirling flows, eg: flow of gas from a turbine blade. |
| Cone meters | Welded to the measurant. Pre calibrated. Uncertainty of ±0.5% | Measure gas,steam and liquids |
| Linear resistance meters | Reynolds numbers below 2000 | Viscous fluids with laminar flow. |
Variable area flow meter
Cross sectional area of the device varies with respect to the flow indicating a measurable flow rate.
| Flowmeter | Description | Application |
|---|---|---|
| Thorpe tube flowmeter | Rotameter type using weighted float to indicate flow rate. Accuracy is 1% | Used in medical gas applications |
| variable area orifice | flow through an orifice deflects the spring-loaded tapered plunger. |
Optical flow meters
Flow rate is determined by laser light that scatters when it hits the particles in the fluid, which is detected by a photodetector to produce a pulse signal.
The speed of the particle when it moves from one point to another is detected from which flow rate is calculated.
Since it does not have any moving parts, nor dependent on the temperature , with light from the laser being constant, they are stable requiring only one installation point with less maintenance and error occurrence is less.
Application : Measuring flow from 0.1 m/s to faster than 100 m/s and used in challenging environments with high temperature, low flow rates, high pressure, high humidity, pipe vibration and acoustic noise. Eg: flare gases from oil wells and refineries
Open-channel flow measurement
When the fluid flow (liquids) in a channel with bounded only in three sides with one side open to air, measurement of flow rate take place by the following methods
| Method | Description | Device used |
|---|---|---|
| Level to flow | Using a small dam like structure as the reference point, the depth of water is measured at a point. From this flow rate can be calculated by a formula Q = (KH)^X , H is the depth, X is depended on the secondary instrument used | bubblers, ultrasonic, float, and differential pressure |
| Area/velocity | From the depth measurement cross sectional area A is confirmed and the average velocity is measured. The product gives the flow rate | Doppler and propeller methods |
| Dye testing | A known amount of solute is mixed and concentration after complete mixing is measured. Dilution rate is proportional to flow rate | Dye or salt |
| Acoustic Doppler velocimetry | velocity at a single point is measured instantaneous at regular short intervals . | Using doppler shift effect |
Thermal mass flow meters
Measured using heated components in conjugation with temperature sensor. Flow of fluids, transfers heat from the heating element to the fluid and the change in temperature is measured. Knowing the specific heat and density of the fluid helps calculate the flow rate. Sometimes compensation for the temperature and pressure will be needed if the fluid does not have constant specific heat and density.
Microscale structures of these flow meters are developed in MEMS sensor applications.
Application: flow rate of compressed gas, as long as it is not corrosive or reactive
Eg: Mass air flow (MAF) used in internal combustion engines.
Vortex Flow meter
When a bluff object is placed in the flow of fluid, it creates vortexes which is determined using piezoelectric crystals that give small voltage pulses. The vortices formed are proportional to the flow rate. The frequency of the voltage pulses give an idea of the flow rate.
Sonar Flow Meter
Non-intrusive flow meters that depend on the speed of sound for measurement.
Application: slurries, corrosive fluids, multiphase fluids where flow meter cannot come in contact with the measurand.
Electrical flow meters
| Method | Description | Application |
|---|---|---|
| Magnetic flow meter | Working on Faradays laws of electromagnetic induction. Applying a magnetic field around a non conducting tube will create a potential difference proportional to the rate of flow of the conducting fluid | Conducting fluids |
| Lorenz force velocimetry | Measuring the Lorenz force due to the interaction between the conducting fluid and the applied magnetic field. | Velocity measurement in conducting fluids. |
| Doppler effectUltrasonic flow meters | Using ultrasounds and detecting the reflected ultrasound waves. The change in frequencies gives the flow rate | Used in large number of application for any fluids as long as the speed of the fluid is known. Eg: Blood flow |
| Transit time Ultrasonic flow meter | Detecting the time taken for the ultrasound waves to propagate up and down the stream gives the flowrate | Used in large number of application for any fluids as long as the speed of the fluid is known. |
| Coriolis flow meters | Due to Coriolis effect, the distortion in a vibrating tube gives the mass flow and the density of the fluid measured | Natural gas flow |
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