Instrumentation
Evaluating ultrasonic flowmetering
Oliver Foth describes how an ultrasonic flow metering solution has benefited a US chemical facility.
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Fig. 1. Rick McCamy of Eastman’s Flow Measurements Group measuring with a dual channel portable flowmeter FLUXUS F601 on an 18-in supply and return line simultaneously.
astman Chemical’s main facility in Kingsport, Tennessee, USA, is a huge challenge for anyone responsible for monitoring flow in their miles of pipe. Started in 1920, it now covers 4000 acres and contains 550 buildings. The plant site itself covers 900 acres. Eastman is divided into five business segments, each with extensive product lines. This results in every type of piping imaginable carrying gases, slurries, and an incredible number of industrial chemicals. And as flow metering becomes more and more critical, steps are being taken to enhance flow control. “Because the company has such a variety of piping and products, it periodically reassess its measurement capabilities
to take advantage of the latest technology, ‘ said Greg Harper, environmental and process analytics manager. “A short time ago the company thought it was time to do a thorough investigation of flow metering and see what recent advances had become available.” Much of Harper’s job as environmental and process
analytics chemist is to use portable flow meters to assure accuracy of installed flow meters, to troubleshoot process upsets, and to do flow checks on unmetered lines. Since accuracy is his main concern, he keeps up with the latest technology. “The need for accurate, representative flow data that we
could archive and access has been increasing exponentially, but there was nothing in place that could provide all the information we needed. Several internal development and engineering groups had looked into clamp-on ultrasonic flow meters with data logging capability, but nothing had been expanded to the company as a whole. Those that had the ultrasonics had varying degrees of success with older platforms of the meters, but for the most part these meters were collecting dust on a shelf. The problem was accuracy. I often heard such things as “We’ve had a clamp-on ultrasonic for years and I’ve never had much success with the thing” or “Clamp-on meters are very frustrating to use. You never know if they are giving you accurate readings, if you actually get any readings at all.” I believed that ultrasonic flow meters had vast potential, but had not lived up to it. So I decided to look into it deeper and see if there had been any advances.” Harper made calls to every clamp-on ultrasonic flow meter
manufacturer he could identify. “My intent was to get these meters in-house and test them
on identical process and utilities pipes. I identified 20 points throughout the plant. Several are in our coal gasification plant because it transports gasses and liquids and slurries of changing consistencies and temperatures. These and other sites around the plant would serve as my test bench.”
How ultrasonic works The technique most ultrasonic flow meters use is called transit-time difference. It exploits the fact that the transmission speed of an ultrasonic signal depends on the flow velocity of the carrier medium, kind of like a swimmer swimming against the current. The signal moves slower against the flow than with it.
When taking a measurement, the meter sends ultrasonic
pulses through the medium, one in the flow direction and one against it. The transducers alternate as emitters and receivers. The transit time of the signal going with the flow is shorter than the one going against. The meter measures transit-time
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