Flow measurement is the process of quantifying the flow rate of a particular medium. The flow of liquids and gases, for example, is one of the most commonly measured variables in process industry. It is used in different industries for different purposes; from controlling secondary variables, such as heating by means of steam flow control, to consumption monitoring and billing. In short, flow can be defined as the volume or mass divided by the period of time.
Mathematically, flow can be represented as:
Mass Flow (Q) = Mass (kg) ÷ Time (s)
Volumetric Flow (Q) = Volume (m3) ÷ Time (s)
The SI units of measurement for mass and volumetric flow rate are kg/s and m3/s respectively, however, in practice different units such as kg/h, ton/h for mass flow, and l/min or m3/h for volumetric flow can be used.
It is also possible to determine the volumetric flow rate in a pipe or duct if you know the fluid velocity and the cross-sectional area of the pipe, whereby:
Volumetric Flow (Q) = Area (m2) ∙ velocity (m/s)
The output of this equation is the volumetric flow in the SI unit m3/s.
If the fluid density is known and constant, it is possible to obtain the mass flow by multiplying it with the volumetric flow rate.
Mass Flow (Q) = Volumetric flow (m3/s) ∙ Density (kg/m3)
The output of this equation is the mass flow in the SI unit kg/s.
In the case of the volumetric flow measurement of gases, gas particles have weaker intermolecular bonds than liquids, their density being subject to changes caused by pressure and temperature variations. For a direct comparison, the volumetric rates are often corrected to a reference condition of temperature and pressure. The two most used reference conditions are 0°C (32°F) and 1 bar (100 kPa) defined by IUPAC, and 20°C (68°F) and 1013.24 mbar (101.325 kPa) defined by NIST. In the mass flow measurement of gases, no compensation is required, as the mass is independent of temperature and pressure changes.
Most flow measurement principles are based either on fluid dynamics or fluid characteristics such as thermal, acoustic, and electromagnetic properties, with flow rates measured directly or calculated indirectly from other measured quantities. Due to different physical properties between liquids and gases, liquid flow measurement and gas flow measurement are often regarded separately. Depending on application requirements, a distinction is made between volume flow measurement and mass flow measurement. Depending on the measuring principles, flow meters can be installed in a variety of ways: inline, insertion or clamp installation.
Electromagnetic flow meters, also known as magnetic-inductive flow meters or simply as Magmeters, operate on the basis of Faraday's law of induction, being commonly used to measure the volumetric flow of conductive liquids, such as water, chemicals, or beverages, within various industries. The unobstructed measuring body causes virtually no pressure loss, making it a good option when measuring dense viscous media and, on account of its special wetted material and design, it can also be used for corrosive and abrasive liquids.
Electromagnetic flow meters are constructed with two field coils, installed on opposite sides of the measuring tube, producing a magnetic field. When the liquid passes through the field, it induces a voltage, which is measured by a pair of electrodes. The induced voltage is directly proportional to the flow velocity. Due to applied measurement principle, magmeters can only measure conductive liquids, and are not suitable for non-conductive liquids such as oils, hydrocarbons, or demineralised, ultra-pure and distilled water.
Video 1: Overview of the electromagnetic flow measuring principle
The Coriolis flow meters are one of the most versatile, advanced, and accurate flow meters available, operating based on the Coriolis effect, as first developed by French scientist, Gaspard-Gustave de Coriolis, in 1835. Coriolis flow meters can measure mass flow, density, and temperature, and can also calculate other variables such as volumetric flow, concentration, and viscosity. They are often used in applications that demand high accuracy and reliability, for example dosing applications within the food & beverage and pharmaceutical industries, or custody transfer in the chemical and oil & gas industries.
Video 2: Overview of the coriolis flow measuring principle
Coriolis flow meters can have different designs depending on the manufacturer and model, however, they all follow a same basic principle: a drive coil excites the sensing tube on a resonant frequency, while a pair of pickups installed in the inlet and in the outlet of the tube measures the frequency of the oscillating tube. When there is no flow, both extremities of the tube oscillate synchronously. As soon as there is flow in the pipe, on account of tube movement caused by the drive coil, the fluid movement imposes an alternating twist in the tube, and a phase-shift in the movement of both ends of the tube is detected by the pickups. The mass flow is then proportional to the time difference between the inlet and the outlet pickups responses.
The resonant frequency of the tube is dependent on the fluid density: the higher the density, the lower the resonant frequency will be. Considering this property, Coriolis flow meters will also accurately measure the density of liquids and, with the measured density, it is then also possible to calculate the volumetric flow. A built-in temperature sensor helps the Coriolis flow meter to calculate specific concentration values such as Brix, Plato, Baumé, and API.
The Such advanced technology comes at an elevated cost compared to other technologies, however, the investment pays for itself over time on account of the increase in accuracy and productivity that this type of device can bring.
Ultrasonic flow meters are volumetric flow meters, normally featuring a clamp-on design, which are installed externally in the pipe wall, with no contact with the measured media. The most common application for this type of flow meter is water measurement, especially on big pipes where other technologies become costly. They have also become popular within the water & wastewater industries, however, they also have other applications and some models are even designed for gas measurement. There are two different technologies available in the market: Transit Time and Doppler Effect, depending on the manufacturer and model.The Doppler Effect method requires particles or bubbles in the fluid. A transductor emits an ultrasonic signal is reflected in the moving particle or bubbles, returning at a frequency different to that originally emitted. This frequency shift is proportional to the flow velocity.
Figure 1 – Doppler effect method
Video 3: Overview of the ultrasonic flow measuring principle
Transit time method is the most common variation. It makes use of one or more pairs of sensors. Each sensor produces and will also receive the ultrasonic signal coming from the other sensor. Moving fluid in the pipe will change the time it takes for the signal to transit between the sensors, which is proportional to the flow velocity.
Ultrasonic flow meters can be good alternatives to electromagnetic flow meters, for example in case of non-conductive media and for larger pipe diameters. Because clamp-on ultrasonic flow meter sensors do not come into contact with the media, it can be a good option for corrosive fluids and high-pressure lines. Moreover, it is often used for retrofitting without interrupting a process or as portable meter for temporary measurement, for example, for auditing or flow validation.
Thermal mass flow meters work on the basis of the thermal dispersion principle, also known as King’s Law, whereby a moving fluid carries heat away from a heat source. This type of flow meter is commonly used as a compressed air meter and also for various types of gas, on account of it being a reliable yet inexpensive technology compared to other mass flow meters such as the Coriolis.
The thermal flow meter’s construction can vary depending on the manufacturer, but generally consists of two temperature sensors. One sensor acts as reference sensor, measuring the fluid temperature, while the second sensor is heated by an electrical current, creating a constant temperature difference between the reference sensor and the heated sensor. As soon as there is flow in the pipe, some of the heat is carried out by the fluid, cooling down the heated element. The electronics then will deliver more current to the heated sensor, ensuring that the temperature difference always remains constant. The mass flow can then be calculated based on the heated sensor’s current consumption.
Video 4: Overview of the thermal flow measuring principle
The thermal properties of the fluid must be known to the flow transmitter, as the thermal properties of each fluid are different, which affects the measurement. The most common gases used within the various industries are already pre-configured on most thermal flow meters.
There are two basic commercial thermal flow meter designs: insertion and inline. The insertion version is commonly used for larger pipes, whereby the probe is inserted directly into the pipe.
Video 5 - Overview of the turbine flow measurement principle
Turbine flow meters are volumetric flow meters and are one of the most well-known flow measurement technologies. Turbine flow meters can be used for the volumetric flow measurement of both liquid and gases, representing a good alternative when handling non-conductive liquids such as oils.Turbine flow meters have a relatively simple working principle, consisting of a rotor with blades mounted on a bearing, supported inside the meter by a central shaft. When there is flow in the pipe, the kinetic energy of the fluid will make the rotor spin. The movement of the blades is detected by a motion sensor, producing electronic pulses, which correspond to the volume quantity. The volumetric flow rate is then proportional to the pulse frequency.
Paddle wheel flow meters also known as impeller flow meters, are often regarded as a variant of the turbine flow meter, commonly used for simple applications, like water flow measurement. In place of turbines, the flow meters are equipped with a paddle wheel, which spins with the force from the fluid flow. The revolutions of the paddle wheel are proportional to the flow rate. The paddles’ movement is detected by a motion sensor, producing electronic pulses, which correspond to the volume quantity. On simpler models, the paddle wheel moves a set of gears that will act changing the counting digits. In this version, the instrument is entirely mechanical and requires no power supply.
Figure 2 - Paddle wheel flow meter
Video 6: Overview of the vortex flow measurement principle
Vortex flow meters are volumetric flow meters with a wide range of applications, from water to gas flow measurement. However, due to their resistance elevated temperatures and pressures, they are widely used in steam flow measurement applications; the most common application for this type of flow meter.
Vortex flow meters work on the basis of the vortex shedding principle, where a fluid flows past an obstacle, known as a bluff body, thus producing low-pressure zones behind the bluff body, and forming interspersed vortices on either side of the bluff body. A sensor installed after the bluff body measures the frequency at which the vortices are being formed and the flow velocity and volumetric flow rate are then proportional to the frequency at which the vortices occur.
Vortex flow meters can measure the flow rate of both liquids and gases. However, it requires a minimum flow velocity in order that the vortices can be formed. Vortex flow meters are volumetric flow meters, however, with the help of temperature and pressure sensors, they can calculate the mass flow of gases and steam. Some models even include integrated temperature and pressure sensors.
Differential pressure flow meters, commonly referred to as DP meters, are one of the most commonly used and versatile flow measurement techniques, suitable for the volumetric flow measurement of liquids, gases and steam. They utilise the pressure difference induced by primary elements such as orifice plates, nozzles, venturi tubes, Pitot tubes etc. One primary area of application is measuring steam and condensates at high temperatures. Pitot tubes are more suitable for situations where the loss of pressure is undesirable, or for large pipe diameters.
Restrictive primary elements, such as the orifice plates and venturi tubes, cause a drop in pressure in the line. By measuring the pressure before and after the restriction. It is then possible to determine the volumetric flow. Different shapes and types of restrictive primary element can be used for various applications. The most common type is the orifice plate, which can also feature a variety of shapes and designs.
Figure 3 – Differential pressure flow meter with orifice plate
Figure 4 – Averaging Pitot tube
The insertion type primary elements, such as the Pitot tubes, measures the dynamic pressure. It is the sum of the line static pressure plus the pressure exerted by the fluid movement against the sensor element, while performing a separate measurement of just the static pressure alone. The flow velocity and the volumetric flow can be calculated based on the pressure difference between the two measurement points.
Because of its design, the insertion type primary elements will also cause pressure loss. However it is lower than the pressure loss cause by an orifice plate, for example.
Variable area flow meters, also known as floating element flow meters or rotameters, are volumetric flow meters with a relatively simple construction and reduced cost, for the measurement of the flow of gases and liquids. Variable area flow meters are frequently used for simple flow monitoring due to their low cost, especially versions with tapered and scaled glass tubes, where only a local indication is necessary. Variable area flow meters are particularly suited for measuring flows at the low end of the volume scale.
Variable area flow meters consist of a vertical tube, made of a transparent material such as glass, which progressively widens in diameter, and a float, often made of glass or metal. In short, the fluid flow exerts a force against the float, pushing it upwards and, on account of the gravitational force, the weight of the float and the tapered tube design, an equilibrium is achieved within the system and the float remains still in a fixed position, indicating the current flow rate. The float position can be viewed on a scale indicating the flow rate, while more advanced models feature an integrated sensor to detect the float position and transmit a proportional electrical signal, e.g. 4-20 mA.
Due to varying fluid, there exist different tube and float designs, with different materials, and these should be selected according to the intended application, otherwise the performance and function of the device may be compromised.
Figure 5 – Variable area flow meter
The number of designs and technologies and the variety of applications can make the selection of flow meter complicated, with selection influenced by a variety of factors.
First, it is important to understand the measurement problem or measuring task, in other words, why a flow meter is required. Typical tasks include: monitoring, controlling, dosing or filling, batching and switching. Several of the requirements placed on a flow meter can be derived from the specific task, for example, batching applications demand high accuracy and special batch functions are required. For controlling tasks, compromised accuracy is tolerated, but high measurement repeatability is a requirement.
After defining the measuring task, the following basic considerations or requirements will affect the selection of your flow meter:
1) What is the fluid to be measured?
The selection of the flow meter is based primarily on the fluid that needs to be measured. Certain technologies will perform better than others depending on the fluid type, while some will not work at all for certain fluids, for example, electromagnetic flow meters will not work for measuring gas flow. With this in mind, it is important that you check if the flow meter technology is compatible with the application.
2) What are the process conditions?
It is important to understand the process conditions where the flow meter will be installed, such as: expected flow rate, process temperature, and process pressure. The flow meter must be sized in accordance with these conditions to ensure good performance and durability.
3) Where is the flow meter going to be installed?
The installation conditions can also be a factor when selecting a flow meter. It is important that you are aware of the pipe diameter, process connection, inlet and outlet run, and the presence of sources of flow disturbance, such as control valves, fittings, pumps, etc.
4) What are the ambient conditions?
It is worth knowing if the flow meter is to be installed in an accessible location where workers have easy access to it. Ambient temperature, humidity, dust, and weather exposure should also be considered.
5) What is the required flow meter performance level?
Varying levels of performance may be needed for different applications. By understanding the process requirements, a flow meter technology with compatible performance levels can be selected.
6) Which investments are planned?
Initial investment costs, flow meter features, maintenance costs, and calibration costs should be considered when selecting a flow meter. A cheaper flow meter made of inexpensive parts, may become expensive in the long run due to the required maintenance and re-calibration.
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