Evaluating ultrasonic flowmetering

Paul Boughton
Eastman 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 difference and determines the average flow velocity of the medium. Since ultrasonic signals propagate in solids, the meter can be mounted directly on the pipe and measure flow non-invasively, eliminating any need to cut the pipe.

The tests

Harper said: "One of my first tests for all the meters was in the parking lot where we have a 30-inch water line. Aside from accuracy, I was looking for ease of installation. After all, I have to go all over the plant measuring flow and I don't want to waste time setting up. I was also looking for reliability, data logging capability, diagnostic tools, and a good battery life. The thing with the water line was that if it took too long to set up the meter or the accuracy wasn't there on such an easy task, there was no point going into the other test sites."

And the winner is ...

Harper continued: "I'm not going to name those meters or the others that didn't make the cut because some were good products with good accuracy and I don't want to make them look bad. But at the end of six months, one meter met all of my standards, but especially for ease of set up and accuracy. It was from Flexim.

"They had two distinct advantages over the others. First, they were a combination gas/fluid meter so I only had to buy one meter, not two. Second, their installation is a snap. The main problem with the others was installation. Flexim has a fixed frequency on their transducers. It doesn't matter what they are measuring, they keep that frequency. The others use a sweep frequency and it takes quite a bit of time to set those things up and put them on the pipe. Then you have to find a 'sweet spot' and even after you've found the spot, you still have to adjust the transducers to get an accurate measurement."

John O'Brien, general manager of Flexim Americas Corporation, said: "There are many ways of getting a signal to a pipe. Many use multiple transducers which operate at different frequencies to overcome the various types of pipes and wall thicknesses and types of liquids. Flexim uses the same transducer for everything, but they filter the transmission pattern and either increase the power to the transducer or break up the pattern of transmissions to adapt to the real world conditions. Power to the transducer may go from 15 volts transmit to 90 volts transmit. The meter sends approximately 1000 pulses back a second in 500 pairs, and the system automatically realises a change in the consistency of the liquid. Not even coal slurries are a big challenge. The system automatically realises it's losing signal and increases power to the transducers. It's a combination of software, clever transducers, and signal processing. It can monitor anything from quarter-inch tubing to a 30-foot penstock.

"I think we can safely say that ultrasonic flow meters have finally met their potential. I have taken more than 600 readings with this portable meter and my success rate is 99.8per cent, and that includes pipes containing acids, water, gasses, and slurries at several hundred degrees. At the end of the day, I collect all the data dumps from the built-in data logger and export it to my spread sheet program.

"And our internal customers have gained faith in ultrasonics and have bought ultrasonics to replace their older meters. And the concerns about accuracy are a thing of the past. One of our clients was doing an addition process and having some problems with the feed rates. They had a plate flow meter on the line, but weren't getting consistent measurements. I took the portable over there and spent three days with them. They liked the results, but weren't sure about the accuracy. So I took the portable meter to our metrology group. They had a coriolis flowmeter set up and we bench tested the Flexim versus it and got a correlation of.9999."

Next steps

Harper continues: "We currently have five of the ultrasonic meters in our group. Four of the meters are the Fluxus F601 Liquid and one is the Fluxus G601 Gas unit. The G601 Gas meter is actually a multi-meter in that it has the capability of measuring liquids as well. Most of the applications we have are for liquid measurements but with a plant this size the ability to measure gas flows from outside the pipe is definitely a plus. The specifications of this meter states a minimum requirement of 100 psi to measure gas flows but the lowest pressure line that I have personally attempted to measure is 140 psi. That particular application was for a nitrogen header and the meter worked well.

"At the present time our Flow Measurements Group consists of three people (me and two Utility Operators). Our future plans are to possibly add addition people to the group but for now our model is working. As news of our having this ability spreads throughout the plant we are finding a need to do some 'creative scheduling' given that we are a small group, but that is a good problem to have. We are continually finding new applications for this technology. When we first started this service we were doing straight forward flow measurements. They were basically chemical addition type measurements expressed as GPM or lbs/min. We also did a fair amount of check metering/troubleshooting existing flow meters. We have started to expand our repertoire by now adding heat flow (BTU) type measurements. Our meters are configured for temperature inputs and given the fact that these 601 units are dual channel allows us to measure supplies and returns simultaneously. This energy meter functionality is starting to pay off. We have never had this ability before and are just now starting to apply it. This is definitely something we will keep out eyes on."

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Oliver Foth is with FLEXIM GmbH is based in Berlin, Germany. www.flexim.de

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