The Ins and Outs of Industrial Flow Measurement

May 20, 2010
ABB Talks on Principles and Practice of Flow Measurement

Ron DiGiacomo, business development manager for flow technologies in ABB's instrumentation business unit, gave a one-hour dash through the principles and practice of flow measurement this week at ABB Automation & Power World in Houston. He started by saying that even though open-channel techniques are used to measure flow, he was only going to talk about liquid and gas measurements in closed pipes. Good thing, because he used his entire hour for an extremely dense, fact-filled seminar.

"In a $14-billion market for industrial controls, flow takes up approximately 28%," DiGiacomo said. "You can see why it is a critical technology, especially for ABB. By virtue of our acquisitions we've managed to assemble a portfolio of flow technologies that is among the broadest in the industry."

"Why measure flow?" he asked. "The common reasons are custody transfer, measuring fluid passing from a supplier to a customer, product integrity, especially in blending applications, efficiency indication for flows of both liquids and gases, as a process variable for control during energy transfer application, and for safety."

He continued, "First, let's get some common terminology straight. We are going to talk about accuracy, repeatability and turndown. We will see how Reynolds numbers work, and we'll talk about pressure and temperature effects on flow. We will also see how flow profile—laminar, transitional and turbulent—affects flow measurement accuracy, and why you need to consider upstream and downstream piping when you're designing a flow application."

"Repeatability does not mean accuracy, but poor repeatability means poor accuracy." ABB's Ron DiGiacomo discussed the many subtleties of industrial flow measurement during his presentation at ABB Automation & Power World.DiGiacomo then sped through the categories of flow measurement and the types of devices in each. "Flow meter types," he said, "can be divided into inferential measurement, like differential pressure or variable area, velocity measurement devices such as magnetic, ultrasonic, vortex and swirl meters, volume, that is, positive displacement, and direct measurement of mass flow using Coriolis technology."

"Magmeters used to be the most used flow device other than differential pressure transmitters," DiGiacomo said. "But recently sales of Coriolis mass flow meters have overcome the magmeter lead. There are still more magmeters sold, but the dollar volume of Coriolis meter sales is higher. This is because the Coriolis meter is the most accurate meter on the market, and people are placing a premium on accuracy."

"Accuracy," DiGiacomo went on, "is the extent to which a given measurement agrees with the value of the measured quantity. It is one of the most important considerations in selecting a flow meter, and it tremendously impacts the customer's quality, billing and process efficiency. Rangeability, or turndown, is the ratio of maximum and minimum flow rates under which a flow meter can maintain measurement accuracy. Repeatability is the ability of a flow meter to produce the same measurement each time it measures a flow. And sometimes, repeatability is more important than accuracy."

"It is important to remember," DiGiacomo went on, as he showed several slides of William Tell and the apple to drive his point home: "Repeatability does not mean accuracy, but poor repeatability means poor accuracy. If your accuracy is good, your repeatability will also be good."

There are two ways accuracy is stated in flow measurement. One is percent of rate or percent of measured value. That's highly accurate, DiGiacomo noted, across the entire turndown of the meter. The other is percent of maximum flow. This, while accurate at the very top end of a meter's range, becomes progressively less accurate the closer the flow gets to zero.

"Something else to remember," he said, "is that meters are calibrated in clean shiny labs to something called ‘reference accuracy,' which means very little when you get out into the real world." For example, differential pressure (DP) flow meter accuracy is typically 3% to 5% over 3:1 turndown, while Coriolis accuracy is 0.10% over 80:1 turndown. Vortex is 0.65% over 20:1 and magnetic flow meters are 0.5% over 30:1. 

"What you have to do is capture the application data and consider the following: What is the performance requirement over the flow range? How much permanent pressure loss is acceptable? How much upstream and downstream piping is required, and what is the correct measurement for the application—mass or volume?"

DiGiacomo talked about rangeability and turndown and their effects on meter sizing and accuracy. "This is really important," he said. "For example, consider a magmeter that is carrying the entire water flow of a county into the wastewater plant. During the day, flow is just charging through there, but at night when people are sleeping, there's not much flow. Unfortunately, you still have to measure accurately."

Then he talked about Reynolds Number. In 1883, the British mechanical engineer Osborne Reynolds proposed a single, dimensionless ratio to describe the velocity profile of flowing fluids:  Re = VDρ/μ, where D is the pipe diameter, V is the fluid velocity, ρ is the fluid density, and μ is the fluid viscosity. Reynolds' findings stated that at low Reynolds numbers (below 2000), flow is dominated by viscous forces, and the velocity profile is elongated, and at high Reynolds numbers (above 20,000), the flow is dominated by inertial forces, resulting in a more uniform, flat velocity profile. There is a transition zone between a Re number of 2000 and one of 4000 (which flow meter designers try to avoid).

As important as Reynolds number, DiGiacomo stated, is viscosity. "Viscosity can be thought of as fluid friction," he said. He talked about viscosity effects and the physics of pipeline construction. "When you go around an elbow," he said, "some of the flow actually eddies backward. That's why you need straight run upstream and downstream of your meter."

DiGiacomo talked about density, pressure and temperature effects on volumetric flow, and then he waltzed his audience through a discussion of specific gravity, or relative density.

After he discussed the characteristics of the various types of flow measurement devices, (differential pressure, turbine, variable area, magnetic, vortex, swirlmeter, wedge meter, and, especially, Coriolis flow meters) he provided some guidance on what to do, and especially what not to do. "Keep your velocity in liquids between 1 and 12 ft/seconds, and between 50-200 ft/seconds in gas flows," he said. "Always make sure your Reynolds Number is higher than 4,000."

Figure 2
DiGiacomo showed a really useful chart (see Figure 2) that is part of a wall chart ABB has been providing to flow engineers for more than thirty years. The chart is color-coded (green means "go", red means "stop," and yellow means "proceed with caution"), and provides a quick reference for what you can use—and what you should not use—in a wide variety of liquid and gas flow applications.

He ended his presentation with a teaser: If a penny is tossed into the ocean, will it displace more or less water than the same penny placed on the deck of a boat? Come to DiGiacomo's presentation at next year's ABB Automation & Power World—April 19-21 in Orlando for the solution!