Mass Flow Meters

Chapter One – What is a Mass Flow Meter?

Mass flow meters measure 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 linear, nonlinear, 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, precision, and resolution of flow measurement.

The two main categories of mass 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.

Mass flow meters are further categorized as 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 fluid 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.

Chapter Two – How Does a Mass Flow Meter Work?

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.

Coriolis Principle

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.

The video below, from YouTube, offers a brief explanation of the Coriolis principle.

Explanation of the Coriolis Principle

Indirect Mass Flow Measurement

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

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 Methods

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 Meter

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.

Chapter Three – Types of Mass Flow Meters

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.

Types of Mass Flow Meters

Vortex

Obstructions in fluid flow create vortices in the downstream, which has a critical fluid flow speed, where vortex shedding occurs and the instance where alternating low pressure zones are generated.

The low pressure zones cause the obstruction to move towards the low pressure zone, where sensors gauge the vortices to measure the strength of the flow.

Coriolis

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.

Ultrasonic

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

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.

Turbine

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%.

Differential

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.

Positive Displacement

Positive displacement flow meters measure flow rate through the continuous filling and emptying of a chamber of known volume. They are driven by the flowing fluid, are the most accurate flowmeters available with measurement values within 0.1%, and directly measure volumetric flow rate. No power is required to run a positive displacement meter. They can handle conditions of high pressures, entrained gases, and suspended solids.

Positive displacement meters are used for nonabrasive fluids like heating oil, lubricants, additives for polymers, and vegetable oil.

The YouTube video below offers a brief explanation of positive displacement flow meters.

Positive Displacement Flow Meters

Gyroscopic Mass Flow Meter

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.

Chapter Four – Mass Flow Meter Types of Readings

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.

Units of 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.

Venturi Effect

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).

Accuracy

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:

• Flow Rate - %R
• Full Scale - %FS
• Calibrated Span - %CS
• Upper Range Limit - %URL

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.

Chapter Five – Flow Meter Accuracy Concerns

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:

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.

Air Bubbles:

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.

Deviations in the Flow:

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.

Pulsating Flow:

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.

Pipe Vibration:

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:

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:

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.

Conclusion

• Mass flow meters measure the volume or mass of a gas or liquid passing through a system at a fixed point.
• Mass flow meters measure linear, nonlinear, mass, and volumetric flow rates and have different names depending on the industry and their use.
• Liquid flow can be either open channel or closed conduit, where open channel is open to the atmosphere and closed conduit is enclosed.
• Flow of liquids and gases requires constant and vigilant monitoring with precise and accurate measurements and readings.
• 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.