Jul. 15, 2024
Flow sensors can be found across various industrial, medical and aerospace applications. Flow is defined as the mass or volume of a fluid that passes per unit of time. In practice, flow sensors (or flow meters) are essential in every operation that requires measuring the mass or volume of fluids or gases that are dispensed, distributed, or consumed per unit time.
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A flow meter (alternatively called flow sensor) is an instrument that has been manufactured to measure with precision the rate of flow in a pipe. The accurate gas or liquid flow measurement ensures a safe, efficient, and environmentally compliant operation in many applications.
Measuring flow can vary from a very simple principle to a very complicated process.
The way of measurement really depends on the technology used. In this article, we are going to look at the eight most commonly used technologies in gas or liquid flow measurement. These flow meter types are:
For each application that demands gas flow measurement, different challenges might arise that require careful attention and consideration. Some of them include:
Of course, there is no single, all-in-one technology that can be implemented for all operational requirements, performance, and conditions.
There are at least 8 common gas flow measurement technologies being used today, all with their strengths and limitations. By understanding the advantages and disadvantages of each, costly mistakes can be prevented.
Electromagnetic flow meters detect flow by using Faraday&#;s Law of induction. Inside an electromagnetic flow meter, there is an electromagnetic coil that generates a magnetic field, and electrodes that capture electromotive force (voltage). As the fluid flows through the pipe, the electromagnetic field changes due to the forces generated by induction. These changes are then translated to flow rate.
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Best applied to: Electromagnetic flow meters are primarily used in food industries, chemical applications, natural gas supplies, and power utilities as they are largely unaffected by changes in pressure, density, and temperature.
Vortex flowmeters make use of a principle called the von Kármán effect. According to this principle, flow will alternately generate vortices when passing by a bluff body. A bluff body has a broad, flat front. In a vortex meter, the bluff body is a piece of material with a broad, flat front that extends vertically into the flowstream.
Flow velocity is proportional to the frequency of the vortices. Flowrate is calculated by multiplying the area of the pipe times the velocity of the flow. In some cases, vortex meters require the use of straightening vanes or straight upstream piping to eliminate distorted flow patterns and swirl. Low flowrates present a problem for vortex meters because they generate vortices irregularly under low flow conditions.
The accuracy of vortex meters is from medium to high, depending on the model and manufacturer. In addition to liquid and gas flow measurement, vortex flowmeters are widely used to measure steam flow.
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Best applied to: Vortex flow-meters are more commonly used in power generation and heat-supply systems such as compressed air, saturated steam, superheated steam etc.
This is classified as a turbine flow meter. Paddle wheel flow meters are generally divided into two mechanical classes
The flow and the revolutions of the paddle wheel are proportional to each other. Thus, by spinning the paddle wheel with the force from the flowing fluid, it becomes possible to measure the rate of this flow from the number of revolutions. By embedding a magnet in the rotation axis and on the edge of the paddle, pulses can be extracted as signals, converting the number of revolutions into the flow rate.
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Best applied to: Paddle wheel flow meters can be used in fume scrubbers, reverse osmosis and in various other fields.
Thermal dispersion flow meters use heat to measure the flow rate of a fluid. The usual structure is that there is a heating element in the middle and two temperature sensors on either side of the heating element. As gas flows, the heat is transferred towards the direction of the flow and the temperature sensor upstream is &#;getting colder&#; while the downstream temperature sensor is &#;getting hotter&#;. The flow rate can be calculated by measuring the difference between the temperature sensors.
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Best applied to: Some of the typical applications can be found in the medical and industrial fields such as respiratory devices, anesthesia equipment, CPAP devices and central gas monitoring systems.
ES Systems has developed two distinct product series of MEMS thermal sensors, ESRF-ESF and ESRF-HF.
This is one of the simplest flow measurement technologies. The method usually involves float in a tapered pipe. When the fluid is forced in between the tapered pipe and the float, a differential pressure is generated which causes the float to be moved accordingly. You can measure the flow rate by reading the visual scale of the meter.
Further reading: What is differential pressure?
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Best applied to: They are widely used for numerous applications including chemicals, compressed air, and other gases.
Ultrasonic flow sensors measure the volumetric flow rates of a wide variety of fluids relying on ultrasound and the Doppler Effect.
This technology is very accurate and is independent of the pressure, temperature, and viscosity of the medium. In idle operation, the transmitter sends ultrasonic waves that are bounced in the pipe and perceived from the ultrasonic sensor. Since there is no fluid movement, the frequency of the received signal is the same as the transmitter. Once the flow starts, the frequency of the received waves is either higher or lower (depending on the direction of the flow) than the one transmitted.
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This frequency difference can be translated to flow rate.
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Best applied to: Ultrasonic flow sensors have many applications, spanning from process flow to custody flow.
Differential pressure sensors measure flow through capacitive pressure sensors using Bernoulli&#;s equation. Differential pressure flow meters use laminar plates, an orifice, nozzle, or Venturi tube to create an artificial constriction then measure the pressure loss of fluids as they pass that constriction. The higher the pressure drop, the higher the flow rate. These rugged, accurate meters are ideal for a wide range of clean liquids and gases.
Further reading: Capacitive vs Piezoresistive Pressure Sensors &#; Differences & applications
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Best applied to: Due to their compatibility, there are used in many industries, such as power supply, food and beverage, medical, aerospace, and HVAC.
ES Systems have designed the ESCP-BMS1 differential pressure sensor which has:
The main working principle of coriolis flow meters is the use of a vibrating tube where the flow of gas can cause changes in frequency, or phase shift proportional to the mass flow rate. At an idle state, the tube vibrates at a predefined frequency. As the fluid flow begins, the vibration of the tube alters proportional to the flow rate of the medium. This change in vibration is measured by sensors across the tube and then translated to flow rate.
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Best applied to: They are often applied in many different industrial sectors that need a measurement of sanitary and corrosive but relative clean gases.
Before making your choice about a gas flow meter type, you should consider the location of the manufacturer&#;s installation requirements. It is possible that a stable gas-flow profile upstream and downstream from the point of meter installation might be required. Moreover, the degree of maintenance needed should be considered as some technologies differ from each other.
Further Reading: Converting velocity to volumetric flow rate
With long experience in designing innovative flow sensors, ES Systems provide high &#; end products with cutting edge technology, high-performance capabilities, and other unique features. Choose the one that covers the needs of your business and industry.
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There are four natural gas meter types often used for flow measurement. They are mass flow meters, velocity flow meters, differential pressure, and PD meters.
A flow meter is a precision instrument that measures a pipe&#;s gas flow rate (or liquid flow). While there are four main meter styles for flow measurement, here are three characteristics to determine flow:
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Since gas is compressible accurate gas flow measurement is difficult. As the temperature increases, the gas molecules move further apart. Conversely, as the pressure increases, the gas molecules move closer together.
Most gas flow meters (differential pressure, turbine, positive displacement, vortex shedding) measure the gas flow at the actual operating conditions. This flow rate is ACFM (actual cubic feet per minute). However, it is more important to adjust or correct the flow rate for a particular pressure and temperature. This adjusted flow rate is often called STP (standard pressure and temperature) and is usually in units of SCFM (standard cubic feet per minute).
For this reason, most gas flow meters require pressure and temperature correction to convert the flow rate from operating conditions (ACFM) to standard conditions (SCFM).
Mass flow meters determine mass flow passing through the meter. Two types deserve mention here:
Coriolis flowmeters provide a direct mass flow measurement based upon the fluid&#;s deflection force moving through a vibrating tube. These meters are highly accurate with high turndown capabilities and are independent of fluid properties. They are also costly to purchase and install and unsuitable for larger pipe sizes.
Thermal mass flow meters measure the mass flow based on heat transfer from a heated element. The gas molecules create the heat transfer; the greater the number of gas molecules in contact with the heated surface, the greater the heat transfer. This flow measurement method depends only on the number of gas molecules and is independent of the gas pressure and temperature; therefore, additional pressure and temperature equipment are unnecessary. They also provide excellent accuracy and repeatability and are easy to install.
In a velocity meter type, the rate of the medium passing through the meter determines the measurement.
Turbine flow meters measure volumetric flow based on fluid flowing past a free-spinning rotor, each revolution corresponding to a specific volume of gas or liquid. The meters have high turndown and accuracy. Unfortunately, because of the meter&#;s moving parts, its use is limited to clean dry gases only in gas applications, and pressure and temperature compensation are required.
Ultrasonic flow meters measure the difference in pulses transit time that travels from a downstream transducer to the upstream sensor, compared to the upstream transducer back to the downstream transducer. This meter style is highly accurate but very expensive, and pressure and temperature measurements are required.
The vortex gas flow meter has a shedder bar (an obstruction) in the flow path, causing the fluid to flow around the shedder bar and creating vortices on the backside of the bar. The frequency of vortex generation is a function of the gas velocity. Fluid velocity is determined based on the principle known as the Kármán effect. The frequency of vortex shedding is independent of the fluid composition. The meter requires temperature and pressure compensation and needs a minimum flow rate to produce vortices.
Differential pressure flow meters calculate flow by measuring pressure drop over an obstruction inserted in the flow path. Common types of flow elements are orifice plates, flow nozzles, venturi tubes, and averaging pitot tubes.
The orifice plate is a differential pressure meter frequently used for natural gas measurement. It measures volumetric flow, not mass flow. This meter&#;s limitations include reduced low flow sensitivity, limited turndown, and a pressure drop, impacting operating costs. Additionally, it requires temperature and pressure correction to achieve mass flow since it&#;s a volumetric flow meter.
An averaging pitot tube is a differential-pressure flow measurement device commonly used for combustion air measurement. The device has limitations with measuring gas flow, especially low-flow sensitivity and turndown. The measure is contingent upon achieving velocity pressure, and if the current is too low, the user may not obtain adequate signals.
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Positive displacement meters require fluid to displace components mechanically and measure volumetric flow at the operating temperature and pressure. While they have sufficient accuracy, pressure and temperature compensation are needed to achieve mass flow, and since they have moving parts, the user must consider gas cleanliness. A PD meter may be called a PD flow meter or a volumetric flow meter. An example of a PD meter is the diaphragm meter.
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eBook&#;An Industry Guide for the Use of Thermal Mass Flow Meters: Oil & Gas Production, Waste Management, Steel. Download the eBook for FREE.
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