This article takes an in depth look at mass flow meters and their use.
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A mass flow meter is a way of measuring the volume or mass of a gas or liquid passing through a system at a specific point in the flow system. They are used to measure mass and volumetric flow rates. The names given to mass flow meters depend on the industry that uses them and include 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 resolution of flow measurement.
The two main categories of flow meters are volumetric and mass, which differ in how they measure flow and show their readings. Volumetric flow meters measure the volume of a liquid, while mass flow meters measure its mass.
Flow technologies used to measure mass flow are Coriolis, or inertial, and thermal. Coriolis flow meters use the Coriolis effect that 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 liquid or 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. Fluids, passing the sensors, create thermal energy that increases the fluid‘s 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 and 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 constant 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.
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 a necessity 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 density of fluids widely varies. 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.
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 be a deflection of 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.
Magnetic, ultrasonic, differential pressure, positive displacement, variable area, non-compensated vortex, and turbine meters are volumetric. For increased accuracy, these meters can be combined to provide pressure and temperature readings, with a flow computer, to produce mass flow readings, which is an indirect method for measuring mass flow. Indirect mass flow measurements are used when direct flow measurements are not sufficient.
Direct mass flow measurement eliminates inaccuracies caused by the physical properties of fluids. Mass measurement is not sensitive to changes in pressure, temperature, viscosity, and density. 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.
Pressure differential meters have four matched orifice plates in a Wheatstone bridge arrangement. 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 have two temperature sensors, which measure heat transfer as a fluid passes over a heated surface. Molecules from the material create heat transfer. The more molecules in contact with the heated surface, the greater the transfer.
The temperature sensor in a thermal mass flow meter is a reference and provides a measurement of temperature. The flow sensor is heated slightly above the temperature sensor. As material flows past the heated flow sensor, heat transfer occurs. The meter measures the amount of power required to maintain the temperature differential between the flow sensor and temperature sensor to supply the flow rate reading.
The diagram below shows the flow sensor on the top and the temperature sensor on the bottom.
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 blades of the rotor 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 rate of the flow, 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 open channel or closed conduit, where an open channel is open to the atmosphere and closed conduit is enclosed. With open channel flow, the force of gravity causes the flow. Closed conduit flow is caused by pressure differences in the conduit.
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 frequency of a reflected signal is modified by the velocity and direction of the fluid flow. If the fluid is moving towards the transducer, the frequency of the signal will increase. As it moves away, the frequency of the returning signal decreases. The frequency difference is equal to the reflected frequency minus the originating frequency and can be used to calculate the speed of fluid flow.
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.
There are different designs for turbine flow meters. In all versions, a fluid moves through a pipe and moves the vanes of a turbine. The rate of its spin measures the flow rate with an accuracy better than ± 0.1%.
Flow is calculated by measuring the pressure drop over caused by an obstruction in the flow. The process is based on the Bernoulli Equation where the pressure drop and the further measured signal is a function of the square flow speed.
A gyroscopic flow meter has a tube in a circular or square shape with oscillating vibration at a constant angular velocity on the A axis. As the fluid passes through the loop, precession occurs on the B axis, which is measured by the deviation of the sensor element. The torque on the rotating pipe is measured to determine the mass flow.
The design of a float flow meter includes suspending a float in a tapered pipe that slowly widens in line with the flow. When the fluid is pushed between the tapered pipe and the float, a pressure differential is produced. The created pressure causes the float to move upward and downward, pressure, caused by the weight of the float, generates equalization. The result of the contesting pressures creates the flow meter reading that is visible on the tapered pipe.
The tapered pipe is made of transparent material that is adjusted for the fluid flow. Readings are taken directly from the pipe. Other forms of float flow meters have magnets attached to the float that provide a magnetic sensor reading.
A laminar flow meter is a type of differential pressure flow measurement method. In the case of laminar flow, internal fluid friction forces are greater than inertial forces. Laminar flow does not have turbulence and has limited mixing of fluids. The pressure drop in a laminar flow stream is measurable and quantifiable, which is a measurement of the resistance to fluid motion that causes friction in the pipe walls.
Laminar flow meters consist of tubes that are placed in the pipe. The placement of tubes is wider than the diameter of the pipe and positioned to produce a slow moving flow velocity. The expanded size of the flow meter tubes slows the flow of the fluid that creates a pressure drop from when the fluid enters the flow and when it exits. The pressure drop is measured by a transmitter.
Magnetic flow meters measure flow without the use of moving parts. A magnetic field, following Faraday’s law regarding the voltage created by a conductor, in this case fluid, moving at right angles to a magnetic field, provides the rate of flow. The motion of the fluid through the magnetic field is the measurement of the rate of motion of the conductor or fluid. The faster the fluid flows through the magnetic field, the greater the voltage it generates.
Sensors in the walls of the pipe read the voltage signal and send it to a transmitter, which processes the signal to determine the flow rate.
A propeller flow meter measures the rate of flow using a propeller. The rotation of the propeller is proportional to the rate of the flow. As the blades of the propeller rotate, their motion is read by a totalizer that is mounted in the meter. The totalizer scales the received data and displays it. The type of totalizer is determined by the fluid being measured.
Pitot tube flow meters were first introduced over 275 years ago and are still used today. The measuring process for a pitot tube flow meter is based on the principle of differential pressure. The static pressure for a pipe is determined by the pressure upstream of the meter and is measured at right angles to the direction of the flow.
The impact pressure is the sum of the static and kinetic pressure, which is detected as the flow impacts the opening of the pilot. The calculation of the impact pressure is dependent on a small L shaped tube that faces the flow stream. As the velocity of the flow rises, the pitot tube changes from laminar to flat.
Hydraulic flow meters are used to determine the pressure in a hydraulic system and measure the volume and flow rate. They are essential for troubleshooting, testing, and maintenance.
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 a 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 a high accuracy liquid mass flow sensor 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 is 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 gases that are influenced by pressure and temperature.
When liquids or gases are pumped for energy use, the rate of flow 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.
Gases 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), LFM (linear feet per minute), 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 depends 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 increase in pressure is balanced by the drop in pressure.
Venturi effect measures the velocity of a fluid in a pipe using the 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 meters performance is measured by its amount of error and how precise its measurements are. 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 in the selection of the proper mass flow meter for an operation.
Flow of liquids and gases 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 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 that is released is dependent 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...
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...