Accurate flow measurement is a critical component of many commercial and industrial processes. Flow meters are instruments designed to quantify the rate or volume of a moving
fluid—either liquid or gas—in an open or closed conduit. Whether determining the proper concentrations of ingredients in manufacturing, measuring fuel usage, ensuring proper
flow for cooling equipment, or monitoring municipal water and sewer services; flow meters serve in a wide variety of applications. Because of this, a number of flow measurement
technologies have been developed. Each of these technologies comes with particular advantages and disadvantages. Understanding the needs of an application is always the first step
towards selecting the proper flow meter.
Flow Measurement
Flow measurement can be described in either of two ways:
Volumetric flow in which Q = AV, meaning that the volume of fluid passing through a flow meter (Q) is equal to the cross-sectional area of the pipe (A) times the average
velocity of the fluid (V). The only flow meter technology that measures volume directly is the positive displacement flow meter, however, other types of flow meters measure the velocity
of the flowing stream to determine the volumetric flow. Examples of flow meter technologies that measure velocity include electromagnetic, turbine, ultrasonic, and vortex flow meters.
Mass flow in which W = RQ, meaning that the mass flow of fluid passing through a flow meter (W) is equal to the fluid density (R) times the volume of the fluid (Q). Examples
of flow meter technologies that measure mass flow include Coriolis mass and thermal flow meters.
Other types of flow meters, notably differential pressure and variable area flow meters, do not measure volume, velocity or mass, but rather measure flow by inferring its value from other
measured parameters.
Flow Meter Technology
Coriolis Mass Flow Meters
Coriolis meters make direct mass flow measurements based upon the Coriolis effect: the deflection of moving objects when they are viewed in a rotating reference frame. Coriolis flow meters
artificially introduce a Coriolis acceleration into the flowing stream. As the fluid is "deflected", the forces generated cause an extremely slight distortion or 'twisting action' of the
measuring tube that is directly proportional to the mass flow rate. This distortion is picked up by special sensors and converted to an output signal.
Coriolis mass flow meters can provide flow (mass or volume), density, and temperature measurements of liquids and gases all within a single meter. Since the measurement principle is independent
of the physical fluid properties, these meters typically have a very high accuracy. The lack of straight pipe requirements and moving parts makes them a very attractive alternative to other flow
meters.
Differential Pressure Flow Meters
Differential Pressure flow meters measure the velocity of fluids by reading the pressure loss across a pipe constriction. These meters can contain laminar plates, an orifice, nozzle, or Venturi
tube to create an artificial constriction. Highly sensitive pressure sensors measure the pressure before and after the constriction. According to Bernoulli's principle, the pressure drop across the
constriction is proportional to the square of the flow rate. The higher the pressure drop, the higher the flow rate.
Differential pressure flow meters utilize a robust, time proven measuring technique for a wide range of clean liquids and gases. The meters are available in a wide range of line sizes with wide
temperature and pressure ranges. Installation is relatively easy and the meters often offer temperature and pressure measurements as well, measurements of mass flow compensation . Care should be
taken with highly viscous liquids, though, as accuracy can be adversely affected or not achieved.
Magmeters / Electromagnetic Flow Meters
Electromagnetic flow meters are volumetric flow meters that measure the voltage created when conductive liquids move through a magnetic field. According to Faraday's Law, the voltage induced across
any conductor as it moves at right angles through a magnetic field is proportional to the velocity of that conductor. With magmeters, the liquid serves as the conductor and the magnetic field is
created by energized coils outside the flow tube. Electrodes detect the voltage which is directly proportional to the flow rate.
Electromagnetic flow meters can measure corrosive liquids and slurries, and have the ability to measure flow in both directions with equal accuracy. A conducting fluid and a non-conducting pipe
liner are required. Magmeters will generally not work with hydrocarbons, distilled water and many non-aqueous solutions. They are also ideal for applications where low pressure drop and low
maintenance are required.
Positive Displacement Flow Meters
Positive displacement flow meters measure the volumetric flow rate of a moving fluid or gas by way of precision-fitted gears or rotors containing cavities through which precisely known volumes of
fluid pass. A basic analogy would be holding a bucket below a tap, filling it to a set level, then quickly replacing it with another bucket and timing the rate at which the buckets are filled (or
the total number of buckets for the "totalized" flow).
Positive displacement flow meters are very accurate and have high turndown. They work best with clean, non-corrosive, and non-erosive liquids and gases, although some models will tolerate some
impurities. They require no straight runs of pipe for fluid flow stream conditioning though pressure drop can be an issue. They are widely used in custody transfer and are applied on residential
home natural gas and water metering.
There are two common types of positive displacement flow meters. Nutating disk meters feature a circular disk mounted on a ball inside a precision fitted measuring chamber. As
the liquid flows through the chamber, the disk rotates and wobbles upon the ball. Each rotation causes a predictable wobble which creates a cavity of a known size through which the liquid passes.
By using an indicator or totalizer, the number of rotations can be counted and the flow rate determined.
Oval gear meters use oval shaped gear-toothed rotors that rotate within a chamber. As these rotors turn, they sweep out and trap a very precise volume of fluid between the outer
oval shape of the gears and the inner chamber walls. The flow rate is then calculated based on the number of times these compartments are filled and emptied.
Rotameters / Variable Area Flow Meters
Variable area flow meters / rotameters are among the oldest and most mature principles in flow measurement. Based upon Bernoulli's theorem, these meters consist of a uniformly tapered flow tube,
a float, and a measurement scale. As a gas or liquid is introduced into the tube the float rises, its weight supported by the fluid flowing underneath, until the entire volume of fluid can flow
past the float. The position of the float corresponds to a point on the tube's measurement scale and provides an indication of the fluid's flow rate.
The operating principle of variable area meters is as simple as it is reliable. They are generally inexpensive, easy to install and feature low, nearly constant, pressure drop. However, concern
for orientation of rotameters (floats) must be observed, as they must be mounted vertically and have moderate accuracy. Variable area flow meters are generally not suitable for low-flow applications.
Thermal Flow Meters
Thermal flow meters measure mass flow rate by means of measuring the heat conducted from a heated surface to the flowing fluid. Relying on the principle that a fluid flowing past a heated
temperature sensor removes a known quantity of heat as it passes, thermal flow meters measure either how much electrical power is required to maintain the temperature of the heated sensor or the
temperature difference between the heated sensor and the flow stream. Either of those values is directly proportional to the mass flow rate.
Thermal flow meters are used almost entirely for gas flow applications. Their design and construction make them popular for a number of reasons. They feature no moving parts, have nearly
unobstructed flow path, require no temperature or pressure corrections, and retain accuracy over a wide range of flow rates. Straight pipe runs can be reduced by using dual-plate flow conditioning
elements and installation is very simple with minimal pipe intrusions.
Turbine / Paddlewheel Flow Meters
Turbine or paddlewheel flow meters are mechanical meters that have a freely rotating turbine set in the path of a fluid stream. The flowing liquid or gas causes the turbine to spin upon its axis.
The rate of spin will be proportional to the velocity of the flow. The simple and reliable design of turbine meters makes them popular choices for large commercial and industrial users such as gas
companies and municipal water districts.
Turbine meters are less accurate than some other types of flow meters but since the measuring element does not severely restrict the path of flow, they are able to measure high flow rates with low
pressure loss. Though versatile, turbine meters do best in applications with constant conditions in liquids such as water or lower viscosity fluids. Strainers are generally required to be installed
in front of the meter to protect the measuring element from gravel or other debris that could enter the flow system.
Ultrasonic Flow Meters
Ultrasonic flow meters utilize sound waves to measure the velocity of a fluid from which the volumetric flow rate can be calculated. Unlike most flow meters, ultrasonic meters do not include any
moving parts and thus are more reliable, accurate and provide maintenance free operation. Since ultrasonic signals can also penetrate solid materials, the transducers can be mounted onto the outside
of the pipe offering completely non-invasive measurement eliminating chemical compatibility issues, pressure restrictions, and pressure loss.
Ultrasonic flow meters are affected by the acoustic properties of the fluid and can be impacted by temperature, density, viscosity and suspended particulates depending on the exact flow meter.
Homogenous fluids, as well as, advanced digital signaling can eliminate many of the problems associated with noise and variations in liquid chemistry.
There are two types of ultrasonic flow meters:
Transit time flow meters measure the travel time of two sound waves. One wave travels the same direction as the flow while the other travels against the flow. At zero flow, sensors
receive both waves at the same time, i.e., without transit time delay. As the fluid moves, it takes an increasingly longer time for the downstream wave to reach the upstream sensor. This measured
"transit time difference" is directly proportional to the flow velocity and therefore to flow volume. Transit time flow meters require the fluid to be free from suspended solids or gas bubbles and
in a closed and full piping system.
Doppler-shift flow meters operate on the principle that the wavelength of an approaching sound source is shorter than the wavelength of that same source as it is moving away. A
transducer emits a sound wave which reflects off entrained particles or bubbles back to the transducer. The measured difference in the wavelengths of the transmitted signal versus the reflected
signal is proportional to the process' velocity. Doppler flow meters are used for slurries, liquids with bubbles, or gases with sound-reflecting particles. They can also be adapted for use in open
channels by integrating with level transmitters.
Vortex Flow Meters
Vortex flow meters use an obstruction, known as a bluff body, in the flow stream to create downstream vortices which are alternately formed on either side of the bluff body. As these vortices are
shed from the bluff body, they create alternating low and high pressure zones that oscillate at particular frequencies directly proportional to the velocity of the fluid. The flow rate can be
calculated from the fluid velocity.
Vortex flow meters are universally suitable for measuring liquids, gases and steam while remaining largely unaffected by changes in pressure, temperature and viscosity. Without moving parts, vortex
meters are easy to install and require little maintenance. The measuring signal is not subject to drift. Consequently, vortex meters can operate an entire life long without recalibration. Due to the
nature of a minimum required velocity for each bluff body, vortex meters will tend to need higher velocities and may have some difficulty reading low flow rates.
Additional Flow Accessories
Flow Indicators
Flow Meter Indicators are simple devices that provide visual indication, often through the use of a float or paddle, that there is movement of fluid in the process line.
Flow Meter Monitors
Flow meter monitors are accessories that, generally speaking, convert the signal sent by a flow meter into a viewable flow rate. Though sometimes flow monitors are simple indicators, they often
include sophisticated programming that allow control functions as well as other high-level operations.
Flow Switches
Flow switches are devices designed to trigger an action – such as on/off—based upon a preset flow setpoint. Flow switches may or may not read the flow rate.
Flow Transmitters
Flow transmitters are versatile instruments that may serve a number of functions. Basic transmitters may serve simply to relay the signal from the flow meter to a display. More sophisticated models
may include control functions and/or advanced communications as part of an integrated flow system.
Flow Regulators
Flow regulators are simple valves that keep flow constant by decreasing the cross section of an orifice proportionally as the pressure increases. They are particularly suitable for networks supplying
several users, as they can maintain the flow rate over a wide range of pressures.
Selecting a Flow Meter
The basis of good flow meter selection is a clear understanding of the requirements of the particular application. Therefore, time should be invested in fully evaluating the nature of the process
fluid and of the overall installation.
- What is the fluid being measured by the flow meter(s) (air, water, etc…)?
- Do you require rate measurement and/or totalization from the flow meter?
- If the liquid is not water, what viscosity is the liquid?
- Is the fluid clean?
- Do you require a local display on the flow meter or do you need an electronic signal output?
- What is the minimum and maximum flow rate for the flow meter?
- What is the minimum and maximum process pressure?
- What is the minimum and maximum process temperature?
- Is the fluid chemically compatible with the flow meter wetted parts?
- If this is a process application, what is the size of the pipe?
If you have any questions or need any help selecting a flow meter, please contact us at sales@instrumart.com or 1-800-884-4967 to speak with an applications engineer.
