This article takes an in depth look at mass flow meters and their use.
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A mass flow meter gauges the flow rate of a gas by measuring the heat transfer on the gas's surface, utilizing temperature sensors and electric heaters either within the flow or outside the pipe. In contrast, inertial flow meters assess the mass flow rates of fluids as they pass a fixed point within a specified time frame.
Mass flow meters are known by various names depending on the industry, including flow gauge, flow indicator, liquid meter, or flow rate sensor. They have largely replaced other flow rate measurement methods due to their superior accuracy and precision.
There are several flow measurement technologies for mass flow, including Coriolis, inertial, and thermal methods. Coriolis flow meters operate on the Coriolis effect, where a mass moving in a rotating system generates a force perpendicular to both the motion and the rotational axis. These meters detect the inertia from gas flowing through oscillating tubes and use sensors to measure the amplitude, frequency, and phase shift of these oscillations to calculate mass flow.
Thermal mass flow meters rely on heat transfer principles, using a heating element and temperature sensors. As gas flows past the sensors, it generates thermal energy that raises its temperature, which can then be used to determine the flow rate.
The image above illustrates a general view of a mass flow meter installed in a pipe to measure the flow rate.
Manufacturers equip mass flow meters with integrated temperature and pressure sensors alongside the flow sensor. These meters use flow meter electronics to compute mass flow based on the formula: mass flow equals density multiplied by volumetric flow rate, which is then multiplied by the cross-sectional area of the flow path. The area remains constant, determined by the size of the flow body. Density is derived from pressure and temperature measurements, while velocity is assessed using a rotating turbine or vortex sensor.
Accurate flow measurement is crucial for controlling process conditions in a plant, impacting production efficiency and product quality. In some cases, flow measurements also serve as indicators of overall process performance.
Although all mass flow meters are designed to measure flow rates, they employ different methods for taking these measurements. There is no standardized technique for measuring flow rates; the method used depends on the material being measured, the operating conditions, and the required level of accuracy.
In production facilities, flow meters are essential for providing precise and accurate readings of fluid flow, which is crucial for achieving optimal operational efficiency. Accurate flow measurements also serve as indicators of the overall performance of the system.
The primary role of mass flow meters is to detect variations in flow resulting from changes in viscosity and density, which can impact measurement accuracy. Temperature variations can significantly affect fluid density. Mass flow meters are particularly important for applications such as fuel monitoring and balancing, where high accuracy, typically within ± 1%, is required.
Below is a brief overview of how several types of flow meters operate.
Direct mass flow measurement avoids inaccuracies linked to the physical properties of fluids, such as the distinction between mass and volumetric flow. These measurements are absolute and obtained directly from the flow of the medium, remaining unaffected by variations in pressure, temperature, viscosity, or density. This is a key advantage of using mass flow meters.
Volumetric measuring instruments can provide accurate readings if conditions and reference calibrations are strictly adhered to. However, devices such as variable area meters or turbine flow meters cannot detect changes in temperature or pressure that may influence 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 straightforward yet highly effective. It involves a tube that vibrates at a fixed frequency. As a fluid flows through this tube, the mass flow alters the tube's vibration, creating a phase shift. This phase shift produces a linear output that is directly proportional to the flow rate. The Coriolis method is versatile, applicable to any gas or liquid, regardless of its type.
In addition to measuring the phase shift frequency, the Coriolis method can also determine the natural frequency of the tube, which varies proportionally with the fluid's density. By combining the mass flow rate derived from the phase shift with the density measurement, the volume flow rate can also be calculated.
Coriolis meters are direct flow meters that utilize the Coriolis effect. They allow fluid to flow straight through the meter, which supports higher flow rates and reduces pressure loss.
The simple diagram below illustrates the application of the Coriolis principle.
Indirect measurements are methods used to determine values through alternate means or properties when direct measurement is not feasible, such as measuring the height of a building or the width of a river. These measurements often involve mathematical formulas and calculations, including the Pythagorean theorem, proportions, and geometric shapes.
Volumetric meters like magnetic, ultrasonic, differential pressure, positive displacement, variable area, non-compensated vortex, and turbine meters can be paired with pressure and temperature sensors through a flow computer to calculate mass flow. This process is considered indirect because it relies on various sensors and computational methods to derive mass flow measurements. Indirect measurement becomes necessary when direct mass flow meters cannot fulfill the specific requirements of an application.
A Differential Pressure (DP) meter is a widely used flow meter with a history spanning over a century. It operates based on Bernoulli’s principle, which states that a pressure difference leads to changes in flow velocity. By measuring pressure changes, DP meters can determine flow velocity, utilizing pressure gauges that create a constriction in the pipe, resulting in a pressure drop.
Differential Pressure meters often feature four matched orifice plates arranged in a Wheatstone bridge configuration. This resistance bridge setup calculates the unknown resistance by balancing the bridge's legs. Fluid is pumped at a known rate from one branch of the bridge to another to establish a reference flow. The differential pressure across the bridge is then used to determine the mass flow rate.
Thermal mass flow meters are precision instruments designed for direct measurement of gas mass flow across various types of gases. They operate by measuring convective heat transfer, using a probe inserted into the gas stream within a pipe, stack, or duct.
These meters are equipped with two resistance temperature detector (RTD) sensors at the probe tip. One RTD is heated by an integrated circuit, while the other serves as a reference to measure the gas temperature. As gas flows past the heated sensor, it absorbs heat, causing the sensor to cool and resulting in a temperature difference between the heated sensor and the reference sensor.
The integrated circuit compensates for the heat loss by supplying additional energy to the heated sensor to maintain the desired temperature. The amount of power required to keep the sensor at this temperature is directly proportional to the mass flow rate of the gas, providing the mass flow measurement.
A turbine flow meter measures flow energy using a rotor equipped with angled blades that spin rapidly in response to the flow, either clockwise or counterclockwise. The rotor blades are mounted on a rod with bearings to ensure smooth rotation. As the flow rate increases, it drives the blades to spin faster, causing the rod to rotate more quickly. To measure the flow rate, the system uses magnets or sensors that detect changes in temperature or pressure attached to the blades.
As the blades rotate, they pass by a small metal target positioned near the meter. The time it takes for the blades to move past this metal piece is used to determine the flow rate accurately. This turbine system is capable of functioning effectively regardless of the flow direction.
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.
There is a wide range of mass flow meter types, each tailored to specific industrial applications. This discussion will focus on several key types: Coriolis, ultrasonic, thermal, turbine, differential pressure, positive displacement, vortex, and gyroscopic meters.
Coriolis mass flow meters operate by channeling fluid through U-shaped tubes that vibrate in an angular harmonic motion. The flow through these vibrating tubes causes deformation and adds an extra component to the oscillation, resulting in a measurable phase shift. These meters are known for their high accuracy, with a precision better than ±0.1%, a turndown ratio exceeding 100:1, and the capability to measure fluid density.
Thermal mass flow meters are favored for their ability to measure mass flow without moving parts, which minimizes maintenance and makes them suitable for challenging applications such as saturated gases. They function independently of additional equipment for temperature and pressure correction. Thermal mass flow meters are highly accurate and versatile, applicable across a broad range of media.
Thermal mass flow meters feature two heated sensors positioned in the fluid flow path. As the fluid moves, it carries heat away from one of the sensors, which correlates to the mass flow rate. The temperature difference between the two sensors indicates the mass flow rate. The accuracy of thermal mass flow meters relies on the precision of their calibration and can be affected by variations in temperature, pressure, heat capacity, and fluid viscosity.
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 - In an impeller mass flow meter, the fluid stream encounters two rotating elements: an impeller and a turbine, each with channels that allow fluid passage. The impeller, driven by a synchronous motor through a magnetic coupling, maintains a steady speed and imparts angular velocity to the fluid as it flows through the meter.
The turbine, positioned downstream of the impeller, absorbs the angular momentum of the flow and generates torque proportional to this momentum. A spring supports the turbine, causing it to deflect at an angle corresponding to the torque applied by the flow. This deflection provides the measurement of the mass flow rate.
Twin Turbine Mass Flow Meter - Twin turbine mass flow meters operate based on the principle of fluid inertia. The turbines are mounted on a single shaft and linked by a flexible calibration torsion element. A pickup is positioned over the turbines, and magnets are embedded in the turbine assemblies to facilitate accurate measurement.
The flexible coupling regulates the rotation of the turbines, causing them to turn together as a unit. This rotation generates an angular phase shift between the turbines, which correlates to the flow's angular momentum and mass. As the mass flow rate increases, the angular displacement between the turbines also increases.
A gyroscopic mass flow meter operates on principles similar to those of a Coriolis mass flow meter. It features a C-shaped pipe and a T-shaped leaf spring that functions akin to a tuning fork. When the tuning fork is energized by an electromagnetic field, it imparts Coriolis-like acceleration to the pipe. This results in forces that cause the C-shaped pipe to deflect. The extent of this deflection is inversely related to the pipe’s stiffness and directly proportional to the mass flow rate.
The deflection of the pipe is measured while the tuning fork oscillates. The resulting measurement is converted into a pulse width modulation signal, which is proportional to the mass flow rate.
Calorimetric flow meters are a type of thermal flow meter that measures the temperature difference in a fluid flow. These meters utilize two heat sensors placed around a heating element to gauge the flow rate based on thermal principles.
One sensor continuously heats the element and monitors its temperature, while the other sensor measures the temperature of the fluid in the pipe. As the flow rate increases, the temperature difference between the two sensors decreases. This principle relies on the cooling effect of the flowing fluid: as more fluid passes over the heating element, it absorbs more heat, resulting in a greater cooling effect on the element.
A flow switch manages the flow using a reed switch, paddle, or relay that communicates with the control system. The system then uses this information to turn the flow on or off. This method helps prevent damage and ensures system protection. By displacing a paddle connected to the flow switch, the switch is activated to control the flow rate.
Unlike traditional flow meters that rely on mechanical methods, a digital flow meter provides accurate and precise flow rate readings without mechanical components. Digital flow meters often use magnetic or ultrasonic technologies to gather data. Their advanced design allows them to process data in various formats and integrate with other electronic systems.
Air flow meters can measure several parameters including air flow rate, volume, and mass based on their design. While primarily known for measuring air, these meters can also assess other gases like nitrogen, helium, and hydrogen. They come in four types: hot wire, vane, cup anemometer, and pitot, each utilizing a unique method to measure air flow.
Fuel represents a major portion of the cost in shipping and transport, accounting for 50% to 70% of operational expenses. Therefore, accurate readings from fuel flow meters are essential for effective fuel management.
Fuel flow meters often employ the Coriolis effect to measure mass flow rate, simplifying the process by eliminating the need for complex calculations. They operate based on the principles of thermodynamic heat conduction and are unaffected by fuel density, pressure, or viscosity.
An orifice plate flow meter is a type of differential pressure meter known for its durability and affordability. It measures flow by using an orifice plate that restricts the flow, creating a differential pressure. Positioned between pipe flanges, this meter operates on the principle that a fluid's pressure and velocity are interconnected: an increase in velocity results in a decrease in pressure, and vice versa.
Water flow meters come in various types, including paddle wheel, positive displacement, magnetic, and ultrasonic. The choice of meter depends on whether the water flow is in an open or closed channel. These meters are positioned throughout the water flow system, from the source to the point of distribution.
The flow rate is typically recorded in cubic meters or liters and is displayed on either an electronic or mechanical device, depending on the system's complexity and design.
Peak flow meters are medical devices used to assess the efficiency of lung air expulsion. A patient exhales forcefully through the device's mouthpiece, which gauges the airflow in liters per minute and displays the result on a numerical scale.
By using a peak flow meter, one can assess the degree of airway constriction and determine if it may indicate a significant health issue. These meters are useful for monitoring daily breathing patterns and serve as a basis for additional diagnostic evaluation.
Microfluidic thermal flow sensors offer precise measurement for ultra-low flow rates in liquid mass flow monitoring. Operating in digital mode, these sensors must be connected to a specific reader. Microfluidics involve handling extremely small fluid volumes, down to femtoliters (fL), which is one quadrillionth of a liter. At such small scales, fluid behavior differs from that in larger volumes.
Microfluidic sensors are advantageous because they can analyze smaller sample volumes and perform multiple operations simultaneously due to their compact design, providing high-quality data.
Mass flow measurement can be categorized into mass or volumetric types. Mass flow gauges the number of molecules within a gas, while volumetric flow assesses the space between these molecules. Both measurements are affected by changes in pressure and temperature.
Volumetric flow rate quantifies the three-dimensional volume a gas occupies as it moves through the device under specific pressure and temperature conditions, reflecting the true flow rate.
Mass flow meters determine the number of molecules passing through the instrument, expressed as a volumetric flow rate based on the space these molecules occupy at standard temperature and pressure.
Mass flow meters deliver data through various measurements and rely on the force exerted by the flowing stream when it impacts an obstruction, which can also offer a velocity measurement.
Gas and liquid flow are measured in units such as liters or kilograms per second, reflecting density. For liquids, density remains constant regardless of external conditions, whereas gases are affected by changes in pressure and temperature.
When pumping liquids or gases for energy applications, the flow rate is often measured in gigajoules per hour or BTUs per day. A flow computer calculates the energy flow rate using both mass and volumetric flow rates.
Measuring gases can be challenging because their volume fluctuates with changes in temperature, pressure, or heating. Gas flow rates on mass flow meters might be indicated as actual or standard, including units such 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).
Thermal, Coriolis, and controller meters are among the most effective for measuring gas flow rates.
The units for measuring liquids vary by application and industry, and can include gallons per minute, liters per second, bushels per minute, or cubic meters per second.
The Venturi effect describes the reduction in fluid pressure that occurs when the fluid flows through a constricted section of a pipe. As the fluid velocity increases in the narrowed area, its pressure decreases. This pressure drop is balanced by the increase in velocity.
To measure fluid velocity in a pipe using the Venturi effect, Bernoulli's equation is applied, which indicates that as the velocity of a liquid increases, the pressure decreases proportionally. The flow rate is often expressed in gallons per minute, liters per second, or cubic meters per second. It is calculated using the formula Q = A × v, where Q represents the liquid flow rate, A is the pipe's cross-sectional area in square meters, and v is the velocity of the liquid in meters per second.
A flow meter's performance is evaluated based on its error rate and measurement precision. The accuracy of a flow meter is typically expressed as a percentage of:
In evaluating flow rate accuracy, it's important to present calculations as percentages relative to the actual flow rate, categorized as minimum, normal, or maximum. These metrics are crucial for choosing the appropriate mass flow meter for specific applications.
Monitoring the flow of liquids and gases demands continuous attention to ensure precise and accurate measurements. Any errors in readings, calculations, or adjustments can lead to reduced efficiency and possible damage to equipment. Identifying the root causes of issues with meter readings can help avert repairs and production delays. The following are some conditions that might cause challenges with mass flow meter readings or lead to meter damage.
Slurry consists of tiny particles ranging from less than 60 to 100 microns and can be categorized as either settling or non-settling. These particles can be abrasive, causing wear on flow meters, or they may coagulate and block the flow path.
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.
In a straight pipe, the flow velocity of a fluid is typically consistent and steady. However, bends or angles in the pipe can disrupt this uniformity, leading to changes in flow velocity and causing it to become irregular, either deviating from the center or creating swirling patterns. The extent of measurement error will vary based on the degree of these irregularities.
Pulsations arise from the acceleration and deceleration of fluid flow, potentially surpassing the mass flow meter’s measurement range. This results in meter readings that underestimate the actual flow rate. Reciprocating pumps are a common source of this issue. To mitigate pulsations, a damper like an accumulator can be used, or the response time of the flow meter can be extended.
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 happens when tiny metal particles from groundwater crystallize and adhere to pipe walls. Over time, this buildup narrows the flow path, restricting liquid movement. Scaling can also affect flow meters, particularly those with paddle wheels or floating components, leading to inaccuracies in their measurements.
Slime consists of living organisms like algae, bacteria, and microorganisms, and can be either sticky or muddy. Similar to scaling, rust, sludge, and slurry, slime can obstruct a mass flow meter by clogging it or impeding fluid flow. Additionally, the electrical conductivity of slime may lead to inaccurate measurements.
The calorimetric principle enables the measurement of flow velocity and media pressure. It uses two sensors to track heat transfer and determine the flow rate, independent of the media's electrical conductivity, viscosity, or density.
The measurement process relies on the cooling effect of the media on a heated sensor. The mass of the media affects the extent of cooling experienced by the sensor. The media must release heat from its highest temperature, and this heat transfer is calculated based on the temperature difference and mass flow rate. This method measures changes in the state variables of the media to compute heat transfer accurately.
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 flow switch is a device that measures the flow rate and liquid pressure within a duct, loop, or system. Flow sensor and flow indication are other names for this switch. The primary purpose of a flow switch is to...
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...