1 April 2006
Rooftop Maker Solves Burning Problem
New clamp-on ultrasonic flow technology brings temperature control to hot process
By Ellen Fussell Policastro
Tallant Industries, Inc., a manufacturer of corrugated asphalt roofing panels, was searching for a solution to a burning problem that was causing delays in their process. Tallant's plant in Fredericksburg, Va., produces what they consider a revolutionary roofing product. The corrugated asphalt roofing materials consist of paper impregnated with bitumen for water resistance. A critical component of the company's production is bitumen storage. Bitumen presents a challenge in that high temperatures and its viscous, sticky nature require maintaining tight temperature control throughout the process.
During the process, manufacturers move bitumen from a heated vessel to a smelter where they heat it to 375°F (190°C) just prior to using it for impregnation and coating. They heat the bitumen vessels and smelter via 2-in pipes flowing heat transfer oil at 540°F (282°C). Maintaining the proper flow rate of the transfer oil is critical to getting the heat transfer efficiency they need to heat and maintain bitumen at the right process temperatures.
They typically measure heat transfer oil (HTO) on a 2-in pipe at a temperature higher than 540°F (282°C). The company said a diagnostic tool to measure this HTO circuit has not been available up to this point. The flowmeter they used in the past failed repeatedly because of the high temperature. "This application is extremely problematic for intrusive metering technologies because of the high temperatures, abrasive nature of the liquid, and high flow velocities" said Allan Cowden, Tallant's lead plant engineer. Because the company emphasizes safety, Cowden said they now must avoid potential leakage due to new metering technologies, such as flange connections. As a result, they needed a non-intrusive measurement method for such a highly flammable aggressive medium.
To overcome these limits, the company looked to high-temperature pipe mounting fixtures, which allow clamp-on flow measurement at temperatures of up to 752°F (400°C).
Cowden said the devices allowed the company to verify the problem wasn't in the HTO circuit, but was build-up of coke from the bitumen on the heat transfer coils.
By means of cooling fins, the transducers move away from the measurement point. Engineering of these plates allows areas reflecting heat to be 100 times larger compared to the areas conducting heat. This results in a significant temperature gradient between the pipe wall and the transducer surface area. The drop in temperature is large enough to permit standard temperature rated transducers to see use in high temperature applications exceeding 750°F without reliability or performance degradation.
"Flow measurement at high temperatures is problematic by itself," said John O'Brien business development and operations manager with Flexim Instruments LLC in Bohemia, N.Y. High media temperatures(>500°F) have in the past posed problems for non-intrusive ultrasonic manufacturers. Thermal stress in magnetic flowmeters causes ceramic liner cracks. So they're only suitable for measuring up to 356°F (180°C). Vortex shedding meters cover a higher temperature range than magnetic meters, but they are too expensive for large pipe diameters and wouldn't provide the lower flow rate performance this application needs. Even non-intrusive ultrasonic meters, "which are in principle ideal for corrosive and aggressive media," have seen limited use at high temperatures, he said. One reason is the sound coupling gels these meters require have a 482°F (250°C) maximum temperature tolerance. High temperatures accelerate the aging of the ultrasonic transducers' piezo elements and limit their useful operating life.
"The basic problem in measuring the flow of gases or liquids at high temperatures is survival of the transducers that generate and detect the ultrasonic signals," O'Brien said. "Their form is such that their heat radiating surfaces are several hundred times greater than their heat inducting surfaces. So the user can get a considerable temperature difference between the surface of the pipe and the transducer." Even when the temperature of the surface of the pipe is high, the temperature of the transducers will be less than the specified maximum temperature of the standard flow transducers. "So users can then do a measurement with standard general-purpose flow transducers."
Changing operating parameters (temperatures, pressure, flow velocities) and fluid parameters (change in fluid densities, particle, and gas content) will cause signal changes by altering the sound path angle (refraction angle) through the pipe wall and the fluid, causing a misalignment of the transducers, which install in a fixed position on the pipe. Signal evaluation algorithms compare the theoretical transducer position with the actual transducer position and compensate for such transducer misalignments, O'Brien said. Users calculate a theoretical transducer position via a feed-back loop involving temperatures and sound velocities. "This lets the meter achieve high accuracies even if the operating conditions are not equal to reference conditions," O'Brien said.
Because of the new fixtures, Tallant was able to conduct non-intrusive flow measurement at extreme temperatures. When fixed onto the pipe, the flow transducers sent the sound signals into the hot pipe wall via optimized metallic coupling plates, which decoupled the transducers thermally from the measuring point.
View the online version at www.isa.org/intech/20060402.
Ultrasonics: A Primer
Accurately and non-intrusively measuring flow rates of liquids from outside the pipe has long been the goal of process engineers and the promise of ultrasonic technology. Advances in clamp-on ultrasonic flow metering have made this a practical and affordable reality. Fluids and pipe materials in industrial applications are mostly conductive to sound, so ultrasound technology is an ideal method in principle.
Advances in clamp-on transit-time flow metering have allowed applications never imagined a few short years ago. These multifunctional meters can now not only provide a user with precise and reliable flow information, but they provide a window into the process with available advanced diagnostics and output parameters. Yet, it's important to understand, when reviewing the suitability of a meter for any application, not all transit-time designs and systems are created equal.
In today's flowmeter marketplace, different technologies and products compete for customers. The user is interested in increased reliability, accuracy, reproducibility, and cost benefit. At the same time, the number of possible applications is growing fast, from liquids to gas to steam, and sometimes into areas where conventional metering technologies don't offer effective solutions.
Ultrasonic technology suffered from an initial poor reputation because products were not yet ready to meet industrial challenges. Misapplications and subsequent failures added to the negative image. Beginning in the 1980s, the emergence of micro processor technology facilitated fast development in ultrasonic measurement technology. Today digital signal processing and micro processor architecture enable reliable and exact flow measurement.
The technology has overcome its negative reputation and is the fastest growing flow measurement technology, offering no pressure loss; hardly any installation costs; and at least for clamp-on style, no contact with the medium you are measuring.
Leaning on Transit Time
"Using transit time is like crossing a river and flowing downstream with the current faster than if you're moving upstream against the current. So the difference in time is what tells you the average velocity," said Zaki Husain, staff engineer, Energy Technology Co. of Chevron Corp. in Houston. "Even the cleanest fluid will have some particles in it," Husain said. So for good measurement, you should use transit time "because if you have only one path going to diameter, you just look at the average velocity. But if you use multiple paths, it gives you a better average."
For measuring liquid at Chevron, Husain said they use ultrasonics for processes such as upstream production, midstream transportation, and downstream refinery and marketing. All areas can use ultrasonic flowmeters. What's important to Chevron in ultrasonic technology is it has "a lot of bells and whistles," Husain said. It's important to have a warning to tell you when different things are going wrong, he said, and ultrasonic technology can measure the speed of sound (automatic information based on the time difference and time of travel), depending on the density of the fluid. "So the speed of sound can serve as a cross check," he said. "If I have multiple paths, and the speed of sound from each path doesn't match, that's an indication something is wrong. Suppose I have more than two. If all the other paths have the same speed of sound and one is off, I know that one is having problems. Ultrasonic meters can reveal the average flow in the path of the sound and the speed of sound through the fluid, he said.
There are some size limitations in ultrasonic flowmeters. Six inches and above is optimum. There are meters measuring below those line sizes, but other meters could be economically more feasible, Husain said. "We're going more with ultrasonic in large sizes especially. The advantage is improving the meter in the field," Husain said. This can be difficult because you must process data and then send a signal, so there's a delay. "Some meters give you instantaneous velocity as it's happening," Husain said. "If you do field proving, you need a large prover or multiple proving to get reliable data, so it poses some difficulty."
"Of all ultrasound meter styles, the clamp-on version is the most flexible," said John O'Brien business development and operations manager with Flexim Instruments LLC in Bohemia, N.Y. These meters work with transducers mounted on the outside of a pipe. "They are just not exposed to chemical, pressure, or mechanical effects. You can install this type during flow operation with little effort."
With clamp-on, there are always pairs of transmitters, Husain said. "One is a transmitter the other is a receiver, thus the term transceiver. Both can receive or transmit. One's located downstream, the other upstream. The upstream one transmits the sound. The downstream one sends the signal to the upstream transceiver."
Eliminating zero drift and zero offset error on small pipe applications (less than 6 in OD) has always been a great challenge for all clamp-on transit-time manufacturers, as even the smallest amount of drift or zero offset can result in large calibration errors.
Dopler shift ultrasonic technology can have errors somewhere around 5% uncertainty, Husain said. So transit time is the preferred method for good measurement. "With dopler shift, there have to be particles in the flow," Husain said. The particles reflect an acoustic wave. The feedback and shift in frequency determine the flow rate. "Obviously it depends on location and size of particles and concentration put together," Husain said. "That's why it's not as precise."
Ultrasonic Flow Metering Devices and Their Applications to Royalty and Custody Transfer Measurements – ET CON 2004, by Youssef F. Basrawi
"Cutting Out the Noise," By Murry Magness
Industrial Flow Measurement Engineering (EI10)