This article takes an in depth look at mass flow meters and their use.
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A mass flow meter measures the flow rate of a gas based on the convective transfer of heat on the surface of the flowing gas and uses temperature sensors with electric heaters in the flow or outside the pipe. Inertial flow meters are devices that measure mass flow rates of fluids passing a fixed point in a predetermined unit of time.
The names given to mass flow meters depend on the industry that uses them and includes flow gauge, flow indicator, liquid meter, or flow rate sensor. Mass flow meters have replaced other forms of flow rate measurement because of their accuracy and precision of flow measurement.
Flow technologies used to measure mass flow are Coriolis, or inertial, and thermal. Coriolis flow meters use the Coriolis effect, which states that a mass moving in a rotating system creates force perpendicular to the direction of the motion and rotational axis. A Coriolis-meter measures the inertia caused by a gas flowing through oscillating tubes and uses sensors to record the amplitude, frequency, and phase shift of the oscillations to determine mass flow.
Thermal mass flow meters use the principles of heat transfer using a heating element and temperature sensors. Gas, passing the sensors, creates thermal energy that increases its temperature, which can be used to determine the flow rate.
The image above is a generalized view of a mass flow meter inserted into a pipe to measure the flow rate.
Manufacturers include mounted temperature and pressure sensors with a flow sensor. They use flow meter electronics to calculate mass flow based on the equation of mass flow equals density multiplied by volume flow, multiplied by the area of the flow body where the area is constantly based on the flow body size. Density is calculated by measuring the pressure and temperature of the flow, with the velocity being measured by a rotating turbine or vortex sensor.
The measure of flow is an important part of controlling process conditions for a plant’s production, efficiency, and quality of products. In some instances, flow measurements are indicators of the performance of a process.
Though all mass flow meters measure flow rates, each type takes its measurements in different ways. There isn‘t any standardized method for checking flow rates. They vary according to the material being measured, the conditions, and the required accuracy.
Flow meters are necesary in production facilities to give precise and accurate readings regarding fluid flow to ensure maximum operational efficiency. Flow measurements provide indicators of the overall performance of the system.
The main function of mass flow meters is to measure variations in the flow caused by viscosity and density, which affect the accuracy of flow measurements. The effects of temperature on the density of fluids widely vary. Mass flow meters are used for fuel monitoring and balancing of fuels, which require an accuracy between ± 1%.
Below is a brief description of how a few flow meters work.
Direct mass flow measurement eliminates inaccuracies caused by the physical properties of fluids such as the difference between mass and volumetric flow. When direct mass flow measurements are taken, they are absolute measurements that are taken directly from the flow of the medium and are not affected by changes in pressure, temperature, viscosity, and density, which is a key reason for the use of mass flow meters.
Volumetric measuring instruments remain valid as long as the conditions and reference calibrations are strictly followed. Unfortunately, volumetric devices, such as variable area meters or turbine flow meters, are unable to notice temperature or pressure changes that affect the flow rate.
The Coriolis principle is the effect a moving rotating mass has on a body. The moving mass exerts force, called the Coriolis force, on the body, causing deformation that appears to deflect the body from its path. The force does not act directly on the body but on the body‘s motion, which is the principle used for Coriolis flow meters.
The Coriolis principle is very basic but extremely effective. It involves a tube that is energized by a fixed vibration. When a fluid passes through the tube, the mass flow momentum changes the vibration of the tube, which is a phase shift. From the phase shift, a linear output is obtained that is proportional to the flow. The Coriolis method for measuring of mass flow is independent of the type of material in the tube and can be applied to any gas or liquid.
Along with the phase shift frequency, it is possible to measure the natural frequency that changes in direct proportion to the fluid density. With the mass flow rate from the phase shift and the density, it is possible to also calculate the volume flow rate.
Coriolis meters are direct flow meters using the Coriolis effect. The flow direction is straight through the meter, allowing for higher flow rates and less pressure loss.
The simple diagram below offers a view of the Coriolis principle being applied.
Indirect measurements are an approach used to measure things using alternate methods or properties. They are necessary when it is not possible to measure something directly such as the height of a building or the distance across a river. There are several mathematical formulas and calculations used to complete indirect measurement, which include use of the Pythagorean theorem, proportions, and geometric shapes.
Magnetic, ultrasonic, differential pressure, positive displacement, variable area, non-compensated vortex, and turbine meters are volumetric, but can be combined with pressure and temperature sensors using a flow computer to calculate mass flow. This type of measurement is indirect because it involves the use of several forms of sensors and computers to measure mass flow. The need for indirect measurement of mass flow is due to the inability of direct mass flow meters to meet the requirements of an application.
A DP meter is a very common form of flow meter, which has been around for over 100 years. It uses Bernauli’s law that says a difference in pressure causes flow velocity. Measuring the value of pressure changes can be used to calculate the velocity of flow using DP pressure flow gauges that cause a narrowing in the pipe and a decrease in pressure.
Differential Pressure meters have four matched orifice plates in a Wheatstone bridge arrangement, or resistance bridge, that calculates the unknown resistance by balancing the legs of the bridge circuit. A pump transfers fluid at a known rate from one branch of the bridge into another to create a reference flow. The differential pressure measured across the bridge is the mass flow rate.
Thermal mass flow meters are precision devices that are direct measuring instruments for gas mass flow measurement for various forms of gas. They measure gas flow using convective heat transfer by inserting a probe into the gas stream of a pipe, stack, or duct.
Two resistance temperature detector (RTD) sensors are placed at the tip of a thermal mass flow meter, measuring heat transfer as a fluid passes over a heated surface. One of the RTDs is heated by an integrated circuit, while the other, reference RTD, determines the temperature of the gas.
As the flow passes the heated sensor, molecules take heat away, causing the sensor to cool as energy escapes. The loss of heat creates a temperature difference between the heated sensor and reference sensor. The attached integrated circuit restores the lost energy in the heated sensor within a short time to adjust the overheat. The power required to sustain the overheat is the mass flow signal.
A turbine flow meter measures the energy of the flow using a rotor with blades that are angled so that they rapidly rotate, moving the rotor in either a clockwise or counterclockwise motion. The rotor blades are attached to a rod with bearings that allows for smooth rotation. The rapid movement of the flow increases the rotational rate of the blades, which spin the rod. To read the flow rate, magnets or temperature or pressure sensors are attached to the blades.
As the blades turn, they pass a small piece of metal located close to the meter. The time it takes for the blades to pass the metal piece provides an accurate reading of the rate of the flow. A turbine system works regardless of the direction of the flow.
Flow can be either an open channel or closed conduit, where an open channel is open to the atmosphere and a closed conduit is enclosed. With open channel flow, the force of gravity causes the flow. Pressure differences in the conduit cause closed conduit flow.
The list of the types and kinds of mass flow meters is very long and involved and changes with their industrial use. This discussion will examine Coriolis, ultrasonic, thermal, turbine, differential, positive displacement, vortex, and gyroscopic.
With Coriolis mass flowmeters, the fluid runs through U-shaped tubes vibrating in an angular harmonic oscillation. The tubes deform and an additional vibration component is added to the oscillation, which causes a measurable phase shift in the tubes. Coriolis flow meters are very accurate, better than ± 0.1%, with a turndown rate of more than a 100:1 and can be used to measure a fluid's density.
The popularity of thermal mass flow meters is due to their ability to take mass flow readings without any moving parts, which reduces maintenance and allows them to be used in demanding applications such as saturated gas. The operation of a thermal mass flow meter does not require any additional equipment such as temperature and pressure correction devices. Regardless of their design, thermal mass flow meters are exceptionally accurate and can be used over a wide range of media.
Thermal meters have two heated sensors in the fluid flow path. The flow stream generates heat from one of the sensors, which is proportional to the mass flow rate. The temperature difference between the sensors is the mass flow rate. The accuracy of a thermal mass flow meter depends on the reliability of its calibrations and variations in temperature, pressure, heat capacity, and the viscosity of the fluid.
Two common types of turbine mass flow meters are impeller and twin turbine. The two different designs have similar processing methods, including fluid moving through a pipe turning the vanes of a turbine. The rate of its spin measures the flow rate with an accuracy of 2% for gasses or steam and 1.5% for liquids. Since turbine meters measure velocity with the help of temperature sensors and pressure sensors, they do not have the precise accuracy of other flow meters.
Impeller Turbine Mass Flow Meters - An impeller mass flow meter has two rotating elements directly in the path of the fluid stream, an impeller and turbine, which have channels through which the fluid can pass. The impeller moves at a consistent speed powered by a synchronous motor using a magnetic coupling and gives an angular velocity to the flow through the meter.
The turbine is downstream from the impeller and removes the angular speed of the flow and gets torque proportional to the angular momentum. A spring holds the turbine that deflects at an angle that is proportional to the torque of the flow, which produces the measure of mass flow.
Twin Turbine Mass Flow Meter - Twin turbine mass flow meters work on the principle of fluid inertia. The turbines are mounted on a single shaft and are connected with a calibration torsion piece that is flexible. A pick up is mounted over the turbines, and magnets are placed in the assemblies of the turbines.
The movement of the turbines is controlled by the flexible coupling, which causes the assembly to rotate in unison. As they turn, an angular phase shift develops between the turbines that are the angular momentum of the flow and the mass of the flow. As the mass flow rate rises, the angular displacement between the turbines increases.
A gyroscopic mass flow meter works on the same principle as a Coriolis mass flow meter. It has a C shaped pipe and a T shaped leaf spring that functions like a tuning fork. When the tuning fork is excited by an electromagnetic field, it subjects the pipe to a Coriolis type of acceleration. The produced forces deflect the C shape pipe in an amount that is inversely proportional to the stiffness of the pipe but proportional to the mass flow rate.
The deflection of the pipe is measured during the tuning fork oscillation. The output from the deflection is a pulse width modulation that is proportional to the mass flow rate.
Calorimetric flow meters are a form of thermal flow meter that measures the asymmetrical temperature of a fluid flow. Two heat sensors are placed around a heating element and use the physical laws of heat to measure the flow rate.
One of the sensors is continuously heated and measures the temperature of the heating element. The second sensor measures the heat of the fluid in the pipe. The higher the flow rate, the smaller is the difference in temperature between the two sensors. The concept is based on the idea of the cooling effect of flowing media. When the cooling media passes the heat element, it collects heat. The more the media that passes, the greater the cooling effect.
A flow switch controls the flow through the use of a reed switch, paddle, or relay that sends a message to the machine that controls the system. The relayed information tells the control system to either turn on or off. It is a method for damage control and protection of the system. The information provided by a flow switch assists in controlling the flow rate. As the flow passes, a paddle connected to the flow switch will get displaced and activate the switch.
Unlike other forms of flow meters that use mechanical methods to obtain a flow rate, a digital flow meter displays the flow rate accurately and precisely without the use of any form of mechanism. Common methods used to collect data by a digital flow meter are magnetic flow meters and ultrasonic flow meters. The sophistication of digital flow meters allows them to interpret data in multiple ways and allows them to be interfaced with other electronic devices.
Air flow meters provide multiple readings that include the air flow rate, its volume, and mass, depending on the design of the flow meter. Though they are referred to as air flow meters, they are also capable of measuring different gases such as nitrogen, helium, and hydrogen. There are four types of air flow meters, which are hot wire, vane, cup anemometer, and pitot. Each of the various types use a different method for reading air flow.
A significant cost of operating a shipping and transport system is fuel, which represents 50% to 70% of the overall cost of the operation. For this reason, fuel flow meters, and their accurate readings, have become a necessity in the transfer of fuel.
The concept of fuel flow meters follows the Coriolis effect for measuring mass flow rate, which eliminates the need for mathematical calculations. It operates on the thermodynamic heat conduction principle and is not dependent on density, pressure, or the viscosity of the fuel.
An orifice plate flow meter is a differential pressure type flow meter and is used for heavy duty applications due its resilience and low cost. The flow is measured by an orifice plate that reduces or restricts and creates differential pressure. It is fitted between pipe flanges and operates on the principle that pressure and velocity of a fluid are related. If the velocity increases, the pressure decreases. If the velocity decreases, the pressure increases.
There are several forms of water flow meters, which include paddle wheel, positive displacement, magnetic, and ultrasonic. The type of water flow meter is dependent on the type of water flow that is being measured, open or closed channel. Water flow meters are strategically placed throughout a water flow system from the source to where the water is dispensed.
The flow rate is measured in cubic meters or liters and is registered on an electronic or mechanical device depending on the sophistication and design of the system.
Peak flow meters are a medical tool used to measure how well the lungs expel air. The patient blows a blast of air through a mouthpiece of the peak flow meter, which measures the force of air in liters per minute and provides a reading on a numerical scale.
The use of a peak flow meter determines how narrow the airway is and if it is narrow enough to be a serious symptom. Peak flow meters can be used to measure daily breathing habits and offer a reference for further diagnosis.
Microfluidic thermal flow sensors are highly accurate liquid mass flow sensors for ultra-flow rate monitoring. They work in a digital mode and have to be connected to a special sensor reader. Microfluidics deal with small volumes of fluids down to femtoliters (fL), which is a quadrillionth of a liter. Fluids at that level behave differently than they do in normal conditions.
The benefits of MFS devices include their ability to analyze less volume of samples. Several operations can be performed at the same time thanks to their compact construction and provide excellent data.
Mass flow measurement is either mass or volumetric, where mass flow measures the number of molecules in a gas, while volumetric measures the space between molecules. Measurements are influenced by pressure and temperature.
Volumetric flow rate measures the three dimensional space a gas occupies as it flows through the instrument under measured pressure and temperature, which is the actual flow rate.
Mass flow meters measure the number of molecules that flow through the instrument as expressed as a volumetric flow rate, which is the space molecules occupy when measured under standard temperature and pressure.
Mass flow meters provide data using a variety of measurements and depend on the force produced by the flowing stream as it strikes an obstruction in the stream, which can also provide a velocity measurement.
Gas and liquid flow are measured in units as liters or kilograms per second, which is a measurement of density. In the case of liquids, density is unrelated to the surrounding conditions, which is not the case with gasses that are influenced by pressure and temperature.
When liquids or gasses are pumped for energy use, the flow rate is measured in gigajoules per hour or BTUs per day. A flow computer uses the mass and volumetric flow rate to determine the energy flow rate.
Gasses are difficult to measure since their volume changes when heated, cooled, or placed under pressure. When reading the gas flow rate on a mass flow meter, it may be expressed as actual or standard as acm/h (actual cubic meters per hour), sm3/sec (standard cubic meters per second), kscm/h (thousand standard cubic meters per hour), or MMSCFD (million standard cubic feet per day).
The best meters for measuring gas flow rate are thermal, Coriolis, or controllers.
The units used to measure liquids depend on the application and industry but can be in gallons per minute, liters per second, bushels per minute, or cubic meters per second.
The venturi effect is the reduction of fluid pressure when it flows through a constricted space. The velocity of the fluid increases, while its pressure decreases. The drop in pressure balances the increase in pressure.
Venturi effect measures the velocity of a fluid in a pipe using Bernoulli's equation that states that the velocity of a liquid increases in proportion to a decrease in pressure. The flow rate is in gallons per minute, liters per second, or cubic meters per second using the flow rate formula of Q (liquid flow rate) = A (pipe area in square meters) multiplied by v (velocity of the liquid in meters per second).
A flow meter's performance is measured by its amount of error and how precise its measurements are. The Accuracy of a flow meter is expressed in percentages of:
When discussing flow rate accuracy, calculations should be expressed in percentages of the actual rate, which can be minimum, normal, or maximum. These determinations can help select the proper mass flow meter for an operation.
The flow of liquids and gasses requires constant and vigilant monitoring with precise and accurate measurements and readings. Errors in readings, calculations, and adjustments cause a decrease in efficiency and potential damage to equipment. Understanding the causes of the problems with meter readings can prevent potential repairs and stoppage of production. Below are some examples of conditions that can cause difficulties with mass flow meter readings or damage to the meter.
Slurry contains minute particles of less than 60 to 100 microns and can be settling or non-settling. The particles in the slurry can be abrasive and wear down a flow meter or coagulate and clog the line.
In open systems, exposed to the air, impurities, and air can be blended with a fluid to form bubbles. In vortex flow meters, air bubbles prevent the creation of vortices. In ultrasonic flow meters, they prevent ultrasonic waves resulting in malfunctions and inaccurate readings.
When a fluid is flowing through a straight pipe, flow velocity is uniform and stable. Bends or angles in a pipe cause the flow velocity to change and become irregular, drifting away from the center of the pipe or swirling. The amount of measurement error will depend on the amount of irregularity.
Pulsations are caused by the acceleration and deceleration of the fluid flow, which may exceed the range of the mass flow meter. The meter reading will be smaller than the actual flow rate. Reciprocating pumps are known to cause this problem. Pulsations can be reduced by a damper, such as an accumulator. Increasing the flow meter‘s time of response is another measure.
There are many varieties of ways that pipes can be caused to vibrate, which include the operation of machinery near the pipe or the opening and closing of valves. In some instances, when a fluid is introduced into a pipe, it can cause a vibration. Coriolis and vortex meters will not provide proper measurements in those conditions. This is not true of ultrasonic flow meters, which are not influenced by vibrations.
Scaling occurs when small pieces of metal from groundwater crystallize and become attached to the walls of pipes. As scaling builds up, the flow path narrows, obstructing liquid flow. Scaling can also attach to the flow meter. Flow meters with paddle wheels or floating elements will have errors in their readings caused by scaling.
Slime is living organisms such as algae, bacteria, and microorganisms, which can be sticky or muddy. Much like scaling, rust, sludge, and slurry, slime can blog a mass flow meter by clogging it or obstructing the flow of fluids. Slime has electrical conductivity, which may also cause inaccurate readings.
The calorimetric principle allows the measuring of flow velocity and media pressure. Two sensors are used to monitor the transformation of heat to determine the flow rate, which is possible regardless of electrical conductivity, viscosity, and density of the media.
The measuring process depends on the cooling of a heated sensor by the media. The mass of the media is responsible for the amount to which the sensor is cooled. The body of the media with the highest temperature must release energy in the form of heat. The amount of heat released depends on the temperature difference and the mass flow rate, which is the measurement of the changes in the state variables of a body to calculate heat transfer.
A Coriolis flow meter measures mass by the inertia of a liquid or gas flowing through a vibrating tube that is equipped with a set of sensors at the inlets and outlets of the meter. The increased movement of the flow produces measurable oscillation that is proportional to the mass...
A magnetic flow meter is a volumetric flow meter that uses electrodes connected to the liquid flow to measure the velocity of fluids in a tube or pipe. The unique design of magnetic flow meters allows them to take their readings without the need for any moving parts...
A rotameter is a device that measures the flow of fluid volume per unit time in a closed tube. There are diverse rotameters applications, including chemical injection/dosing and tank blanketing. A rotameter can be said...
An ultrasonic flow meter measures the flow of a liquid or gas by sending ultrasonic waves across a pipe, containing the flow in the direction of the flow and in the opposite direction of the flow. The ultrasonic waves and the velocity of...
A flow meter is a flow rate measuring device used to determine the linear or nonlinear mass and volumetric flow of a liquid or a gas. The many names of flow meters include flow gauge, flow indicator, liquid meter, and flow rate sensor...