Clamp-on Flow Meter Gains Firm Hold

Site finds ultrasonic device provides easy installation and accuracy.

By Greg Harper, Eastman Chemical Co.

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Eastman Chemical's main facility in Kingsport, Tenn., is a huge challenge for anyone responsible for monitoring flow in its miles of pipe. Started in 1920, the complex now stretches over 4,000 acres and contains 550 buildings. The main plant site alone covers 900 acres. The facility makes a wide range of products to support Eastman's five business segments. So, not surprisingly, it boasts every type of piping imaginable, carrying gases, slurries and an incredible number of industrial chemicals.

Because we have such a variety of piping and products, we periodically reassess our measurement capabilities. In 2009, we decided to thoroughly investigate flow metering to see if we could benefit from recent advances.

Much of my job as an environmental and process analytics chemist is to use portable devices to ensure accuracy of installed flow meters, to troubleshoot process upsets and to do flow checks on unmetered lines. Accuracy is my main concern. So, I try my best to keep up with the latest technology.

The need for accurate, representative flow data that we could archive and access has been increasing exponentially, but nothing in place could provide all the information we required. Several internal development and engineering groups had looked independently into earlier-generation clamp-on ultrasonic flow meters with data logging capability and had varying degrees of success with them. For the most part these meters were collecting dust on a shelf. The problem was accuracy. I often heard things such as: "We've had a clamp-on ultrasonic for years and I've never had much luck 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 such meters had vast potential but hadn't lived up to it. So, I resolved to see if there had been any advances.

I called every clamp-on ultrasonic flow meter manufacturer I could identify. My intent was to get these meters in-house and test each of them on process and utilities pipes. I selected 20 points throughout the site; several are in our coal gasification plant, which transports gases and liquids as well as slurries of changing consistency and temperature.
 
HOW ULTRASONIC WORKS
The technique most ultrasonic flow meters use is called transit-time difference. The meter sends ultrasonic pulses through the medium, one in the flow direction and one against it. 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. Because ultrasonic signals propagate in solids, you can mount the meter directly on a pipe and measure flow non-invasively, eliminating any need to cut the pipe.

The tests. One of my first tests for all the meters was in a parking lot where we have a 30-in water line. Aside from accuracy, I focused on 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 also was looking for reliability, data logging capability, diagnostic tools and good battery life.

The water line provided telling results. If a meter took too long to set up or didn't give adequate accuracy on such an easy task, there was no point trying it at the other test sites. Some meters didn't make it out of the parking lot.

And the winner is… I'm not going to name those meters or, for that matter, the others I didn't ultimately select -- 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, especially for ease of set up and accuracy. It was from Flexim.

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