Ultrasonic flowmeters move into the mainstream
According a new report, "Ultrasonic Flowmeter Worldwide Outlook," published by ARC Advisory Group, the latest generation of ultrasonic flowmeters has overcome the technology's tarnished past and stands poised to become the benchmark technology for monitoring and controlling flow. "Ultrasonic technology will increasingly become the technology of choice, and not merely an alternative to traditional flowmeter technologies," says author and ARC research director Wil Chin. Chin estimates the ultrasonic flowmeter market to grow from about $400 million in 2003 to over $600 million by 2008, rivaling the sale of magnetic flowmeters worldwide.
Innovation Fueling Growth
Technical innovation is fueling this explosive growth, as well as a proliferation of new suppliers. The Consumer Guide to Ultrasonic and Correlation Flowmeters, also recently published, identifies more than 70 ultrasonic flowmeter suppliers worldwide, of which 50 manufacture their own technology. Coauthor David W. Spitzer notes "many of these suppliers are not based in the traditional markets of North America and Western Europe. Significant suppliers are now to be found in Eastern Europe, Japan, and other Asian countries such as Korea, Singapore and China."
There are two basic ultrasonic flowmeter technologies: Doppler shift and transit time. Doppler-shift flowmeters operate by measuring the difference in frequency of the sound wave reflected off acoustic discontinuities, bubbles or particles in the flow stream. When there is flow, the frequency differential increases as flow increases. Transit-time flowmeters function by measuring the time differential between an ultrasonic signal sent upstream and a corresponding signal sent downstream. When there is flow, the time differential increases as flow increases (see Figure 1).Ultrasonic signals are used to measure the velocity and the major types
exploit them differently to compute a measurement.
Transit-time flowmeters can have single or multiple-signal paths. The more signal paths a transit-time flowmeter has, the higher its precision and the closer its accuracy is to that of a magnetic flowmeter. Multipath transit-time flowmeters have now been certified by AGA, API and OIML for custody transfer in oil and gas applications (see Figure 2).The more signal paths a transit-time flowmeter has, the more accurate it is.Portable and semi-portable clamp-on versions of transit-time and Doppler-shift flowmeters are offered as well. "Clamp-on meters," says Chin, "are looked at as test devices, primarily, because of the fact that the ultrasonic coupling grease degenerates over time." Chin points out that a new ultrasonic flowmeter from Endress+Hauser has a sensor saddle that does not require ultrasonic or acoustic coupling grease. According to Chin development in this area continues. "Caldon [a manufacturer of high precision transit-time ultrasonic instrumentation in Pittsburgh} has created an innovative coupling grease, which does not degrade."
Doppler-shift flowmeters have been relegated to a small niche application wise, because for many years, reliability and other issues tarnished their reputation among control engineers. Doppler-shift flowmeters, whether insertion (wetted sensor) or clamp-on, are used primarily in wastewater for sludge measurement, and in dredging and in mining for high concentration slurries.
The significant problem with clamp-on meters whether Doppler or transit time, according to Spitzer, is the inability to provide a traceable accuracy statement for volumetric flow rate. "Ultrasonic flowmeter performance claims often feature high reference accuracy. Note that the accuracy typically refers to the accuracy with which the velocity, not the volumetric flow is measured. An ultrasonic flowmeter that measures velocity with high accuracy may not measure volumetric flow with high accuracy."
Another significant issue with clamp-on flowmeters is the technology's differing ability to read through pipe linings. Bitumastic linings, for example, are more acoustically transparent than others, such as those made from Teflon.
Multipath transit-time flowmeters have for years been used for the measurement and custody transfer of high-value fluids and gases in large-diameter pipes. These devices are limited by the path geometry to large pipe sizes, and the number of sensors required has also increased their price. Typical multipath flowmeter are priced in the $30,000 to $40,000 range and higher. ARC's Chin points to Krohne as the category leader for liquids, and National Instruments for gas flows.
Single-path transit-time flowmeters were once quite common in spool-piece architectures, but over time the higher performance and increasingly lower cost of magnetic flowmeters and vortex flowmeters marginalized the more general application of these devices.
In recent years, several manufacturers have introduced a new group of spool-piece flowmeters, which are categorized as "general-purpose multipath" meters. These meters have fewer paths than the high precision multipath meters applied by the oil and gas industry, but have much higher accuracy. Because of their spool-piece architecture, spool-piece flowmeters are capable of being calibrated to a reference standard for volumetric flow, thus offering a significant improvement over clamp-on meters. Siemens' design deploys reflective targets in a specially shaped flow section. Krohne's configures its product with a three-beam design, and Controlotron produces a general purpose multipath meter in this category as well.
General-purpose multipath meters are being heavily marketed as replacements for all volumetric flowmeter types, including magnetic flowmeters and other devices such as turbine meters. An additional selling point of these devices is that, like all transit-time ultrasonic meters, they are able to measure nonconductive and low conductivity fluids that magnetic flowmeters cannot. These devices have significant new applications for transit-time ultrasonics. For example, Krohne claims its three-beam general purpose multipath meter can handle up to 5% solids, and up to 2% entrained air. Krohne claims it can offer its three-beam device as a cost competitive alternative to vortex types and with optional mass flow output, as an alternative to Coriolis flowmeters. Krohne claims, that permits the application of a single flowmeter technology across the entire process plant. Certainly, if this is the case then ARC's estimate of $600 million in worldwide sales for ultrasonic meters seems quite reasonable.
Closely allied to the transit-time ultrasonic flowmeter is the correlation flowmeter. Not all correlation flowmeters are ultrasonic flowmeters, but at least one transit-time flowmeter claims to be a correlation flowmeter. "Correlation flowmeters measure fluid velocity by measuring parameters associated with flowing stream at various locations in the piping," Spitzer says. In the ultrasonic example, a transducer pair is located downstream from another transducer pair, and cross-correlation computed to determine the velocity of the fluid. Other correlation flowmeters, such as the novel sonar array flowmeter produced by CiDRA Corporation, are not ultrasonic flow sensor based at all.
The article was first published in controlglobal