To illustrate the general principle of operation, consider a flow stream that abruptly changes color from red to green. The color of the fluid could be sensed at two points that are one meter apart in the piping. If the second sensor detects the green fluid one second after the first sensor detects the green fluid, the velocity in the pipe could be calculated to be one meter per second. There are various flowmeter designs that utilize different measurements to determine the flow rate. Note that correlation flowmeters do not disturb the fluid flow, but rather passively or actively sense its characteristics.

Correlation flowmeters are typically applied to fluids in the turbulent flow regime and other fluid flows with coherent disturbances, such as slurries.

A multiple pressure sensor array attached to the outside of the pipe is used to measure pressure gradients caused by fluid vortices that travel down the pipe as a natural consequence of the fluid flow. These vortices are passively sensed at each pressure sensor. Computational techniques derived from sonar technology are used to determine the velocity of the fluid. This technology can also be used to determine the speed of sound of the fluid in the pipe.

An ultrasonic transmitter/receiver pair is located at a fixed distance downstream of another ultrasonic transmitter/receiver pair. Their respective ultrasonic beams are used to actively sense turbulent eddies in the pipe. Cross-correlation computational techniques are used to determine the velocity of the fluid.

Excerpted from The Consumer Guide to Ultrasonic and Correlation Flowmeters.

Repeatability is the ability of a flowmeter to reproduce a measurement each time a set of conditions is repeated. Flow measurements taken using a flowmeter exhibiting poor repeatability would be chaotic. For example, if measurements were taken of a known flow rate of 100 units per minute, a flowmeter with poor repeatability might measure 85, 101 and 93 units per minute on three consecutive days. If these measurements were used to feed material to a process at a given flow rate, different amounts of material would be fed to the process at each of these times. The operator would be at a loss to determine what the flow setting should be to obtain a flow of 100 units per minute. This amount of variation could be detrimental to the operation of the plant.

In a similar test, a flowmeter with better repeatability might measure 96, 94 and 95 units per minute. Note that the difference between the measurements is smaller, that is, the measurement is more repeatable. From experience, the operator will find that setting the flow rate at 95 units per minute results in the desired plant operation of 100 units per minute. As such, one could make an argument that to operate the plant in a steady manner it is desirable to use flowmeters that are repeatable. Note that the flowmeter setting does not correspond to the desired flow.

Accuracy is the ability of the ability of the flowmeter to produce an output that corresponds to the characteristic curve of the flowmeter. Note that a flowmeter that is not repeatable cannot be accurate. Stated differently, if the output of the flowmeter is chaotic, it cannot correspond closely with the characteristic curve. Therefore, in order for a flowmeter to be accurate, it must be repeatable.

In a test similar to that described above, an accurate flowmeter might measure 101, 99, and 100 units per minute, and the operator will find that setting the flow rate at 100 units per minute results in the desired plant operation of 100 units per minute. Note that the flowmeter setting does correspond to the desired flow.

Many applications will be able to function with a flowmeter that is repeatable. If this is all that is necessary, maybe a repeatable flowmeter should be installed. For example, if a tank level increases, its level controller will increase its effluent flow setting to maintain the level setting. In this example, it is seemingly not important to accurately measure the flow rate, but it would be beneficial if the flowmeter were repeatable so that equal flow setting changes result in equal flow changes. Repeatable flowmeters are often less expensive than accurate flowmeters, providing another incentive for their use.

However, often overlooked is the opportunity and sometime necessity of performing process calculations in order to ensure proper economic process operation or improve the process. For example, if the effluent flowmeter cited above is the overhead take-off of a distillation column, the flow measurement may be used to calculate the column reflux flow rate. The original plant design may not have contemplated this, but operating experience may have determined that this control strategy will provide better control of the column. Many such applications exist, and many of these are “discovered” after the plant is operating. Sometimes, repeatable flowmeters must be replaced with accurate flowmeters to obtain operating data and/or accommodate these improvements.

Accurate flowmeters are desirable because they are repeatable and yield measurements that closely reflect the true flow rate. Repeatable flowmeters may not yield accurate measurements, but they will perform in the same manner under the same conditions. Which is appropriate depends in part on the application and budget.

This article originally appeared in Flow Control magazine.

When first exposed to flowmeters, I was told that flowmeters require straight run consisting of 10 diameters upstream of the flowmeter and 5 diameters downstream of the flowmeter. Myths such as this, and other similar rules of thumb, abound in a world where details are things to avoid because they just seem to get in the way.

Flowmeters are typically designed to operate properly with homogeneous single-phase fluids that have a fully developed non-swirling velocity profile entering the flowmeter. This can be accomplished by installing sufficient straight run. In Answer A, a fully developed velocity profile can be generated using sufficient straight run. However, this is not the only way that a fully developed velocity profile can be developed.

Depending upon the design, flow conditioners remove swirl, velocity profile distortion or both, from the fluid stream. Installing a flow conditioner upstream of the flowmeter to remove swirl and/or distortion can generate a fully developed velocity profile without straight run per se. In other words, development of an adequate velocity profile that may require a long straight run of pipe might be accomplished in but a few diameters by applying a flow conditioner. In a way, straight run could be considered to be a type of flow conditioner, albeit a rather long one.

When velocity profile distortion can affect flowmeter operation, manufacturers should provide a recommendation for straight run (Answer B). Note that this information is often not found in the flowmeter specification, but rather in the installation manual. Again, the object of the recommendation is to provide a fully developed velocity profile at the flowmeter inlet. This can also be accomplished in a shorter piping section by using a flow conditioner.

As previously stated, most flowmeters that require straight run do so to generate a fully developed velocity profile at the flowmeter inlet. However, sometimes the purpose is mechanical, such as the case of a Coriolis mass flowmeter that requires rigid supports located a prescribed number of diameters upstream and downstream of the flowmeter. This installation effectively becomes a straight run (Answer C), but the straight run has more to do with the proper functioning of the flowmeter, and not much to do about generating an adequate velocity profile.

One of the “tricks of the trade” when sufficient straight run is not available is to consider installing a flowmeter that does not require straight run (Answer D) and avoid the velocity profile issue altogether.

Additional Complicating Factors

The question implies that a fully developed velocity profile is required for the flowmeter to operate properly. However, the operating principles of some flowmeters are such that they are not affected by velocity profile and have no straight run requirements. Other flowmeters are designed to be a combination of a flow conditioner and a flowmeter that has minimal or no straight run requirements. When one gets into the details, there can be literally thousands of choices.

This article originally appeared in Flow Control magazine.