Overview of Flow Measurement
Flowmeters are devices that measure the amount of liquid, gas or vapor that passes through them. Some flowmeters measure flow as the amount of fluid passing through the flowmeter during a time period (such as 100 liters per minute). Other flowmeters measure the totalized amount of fluid that has passed through the flowmeter (such as 100 liters).
Flowmeters consist of a primary device, transducer and transmitter. The transducer senses the fluid that passes through the primary device. The transmitter produces a usable flow signal from the raw transducer signal. These components are often combined, so the actual flowmeter may be one or more physical devices.
Flow measurement can be described by
Q = A · v, which means that the volume of fluid passing through a flowmeter is equal to the cross-sectional area of the pipe (A) times the average velocity of the fluid (v); and
W = r · Q, which means that the mass flow of fluid passing through a flowmeter (A) is equal to the fluid density (r) times the volume of the fluid (Q).
Volumetric flowmeters directly measure the volume of fluid (Q) passing through the flowmeter. The only flowmeter technology that measures volume directly is the positive displacement flowmeter.
Velocity flowmeters utilize techniques that measure the velocity (v) of the flowing stream to determine the volumetric flow. Examples of flowmeter technologies that measure velocity include magnetic, turbine, ultrasonic, and vortex shedding and fluidic flowmeters.
Mass flowmeters utilize techniques that measure the mass flow (W) of the flowing stream. Examples of flowmeter technologies that measure mass flow include Coriolis mass and thermal flowmeters.
Inferential flowmeters do not measure volume, velocity or mass, but rather measure flow by inferring its value from other measured parameters. Examples of flowmeter technologies that measure inferentially include differential pressure, target and variable area flowmeters.
Flow computers are often used to compensate flow measurements for actual process conditions, such as pressure, temperature, viscosity, and composition.
Additional flowmeter technologies include flowmeters that measure liquid flowing in an open channel, and insertion flowmeters that measure flow at one location in a pipe and use this measurement to infer the flow in the entire pipe. Insertion flow measurement systems often use a flow computer to compensate for hydraulic effects.
Installation Cautions for Flowmeters
In liquid service, be sure that the flowmeter is installed such that it remains full of liquid, because gas/vapor in the flowmeter can alter its geometry and adversely affect accuracy.
In gas/vapor service, be sure that the flowmeter is installed such that the flowmeter remains full of gas/vapor, because liquid in the flowmeter can alter its geometry and adversely affect accuracy.
Disturbances located upstream (and sometimes downstream) of the flowmeter, such as pipe elbows and control valves, can adversely affect measurement accuracy, because the flowmeter may not be able to accurately measure disturbed flow streams. Be sure to locate control valves downstream of the flowmeter so their flow disturbances are not introduced directly into the flowmeter (as they would be if located upstream). Also, be sure to properly design the upstream and downstream piping with sufficient straight run to remove disturbances that can affect measurement accuracy.
Be especially careful when flow is two-phase, such as liquid/gas flow and liquid/solid flow, because these flows can adversely affect the accuracy of many flowmeters. Be careful because some flowmeters can become plugged and stop working in liquid/solid flow streams.
Application Cautions for Flowmeters
Each type of flowmeter has its own specific applications and installation constraints. There is no “one size fits all” flowmeter. The way to select the right flowmeter is to use the application as your guide, not the technology. Many of these technologies will all work well on many applications. If you start with the application, you can select the technology you wish to use based on accuracy, cost, durability and reliability, rather than trying to make the technology you chose fit the application you have.