Chilling out with ultrasonic

InTech, Jan 2008 by Policastro, Ellen Fussell

University chilled water plant plays it cool with ultrasonic flowmeters

Manufacturers have been using noninvasive flowmeters for years because of their reliability, ease, low maintenance, and accuracy. Smurfit Stone Corp., a world-wide paperboard and paper-based packaging manufacturer, likes the straight-through (op en-thro at) design of the magnetic flow tubes, which prevent plugging from pulp stock, chemicals, slurries, and other liquids with suspended solids. Over the past 12 years, John Osborne, a mechanical engineer at the plant's West Point, Va., location has installed Coriolis meters on starch slurry to measure consistency. "Before, we had to take samples manually and test them in the lab," he said. "These meters have allowed continuous measurement and given us a degree of consistency control we have not had before."

Control and accurate measurement are behind the ultrasonic meters Mark Menefee uses at the chilled water plant in Indiana University in Bloomington, Ind. With 65 buildings over 6.9 million square feet on campus, the university had no way of metering chilled water use in the buildings, only at the production plant. The university is now in the process of converting to a system for nearly 25,000 tons of cooling. Menefee, the university's assistant director of utilities, oversees eight chillers (with a total capacity of 15,000 tons), which pump water around the 65 campus buildings attached to the system.

Before deciding on ultrasonic meters for Indiana University, Menefee and his team were trying to better control the university's chilled water system "so we could understand where the most and least efficient buildings werewhere this chilled water was going," Menefee said. "We didn't know before what the true cooling load for these buildings really was. We had estimates but didn't know. We're trying to get more information so we can control the system better. Because even with 15,000 tons we're short some cooling on the hottest days in summer," he said. At each building, Menefee's team will install a meter to measure the flow of chilled water into that building, which will help the utilities team "understand our system better and be more efficient.

"We chose to go with ultrasonic because it was less invasive. We didn't have to have an outage of the system in order to install," he said. "It installs on the outside of the pipe and provides the accuracy we need."

Another advantage in installing the meters is the university will not have to shut down the cooling system to each building during installation. Noninvasive is so important, otherwise you have to shut valves and drill a hole in the pipe, and that would require an outage to install a meter. "We would have to shut cooling off to stop water from flowing and take pressure off the pipe. This was so much faster and provided better accuracy." Plus, Menefee's team has experience with these kinds of meters at the production facility. "So we'll be able to get quite a few of these done pretty quickly, in three months, before the next cooling season," Menefee said.

Clamp-on technology

Clamp-on ultrasonic flow metering technology was a good choice for a retrofit application like the one at Indiana University because clamp-on transit-time ultrasonic meters use a pair of ultrasonic transducers clamped onto the outside of existing pipes to measure the flow rate inside the pipe without cutting the pipe or otherwise penetrating the pipe wall. This keeps installation costs low because there is no need to cut open the piping system. "Opening the system in itself can be costly and may lead to unexpected expenses if the system is old and the pipe work is fragile," said Michael J. Scelzo, a chemical engineer and flowmeter technical manager at GE Sensing in Billerica, Mass.

Other areas where clamp-on meters make sense include when measuring sanitary or toxic flows, or in highly corrosive or erosive applications. Anywhere there is a reason not to break the pipe wall or penetrate the pressure boundary is a good place to use clamp-on ultrasonic meters. In addition to permanent installations, clamp-on meters also lend themselves well to portable or temporary use. Armed with a battery powered portable meter, a trained operator can clamp the transducers onto an exposed pipe and be measuring flow rate in less than five minutes.

Using the ultrasonic transit-time principle, clamp-on ultrasonic flow meters can see use for liquid and gas flow measurement. With a growing demand for clean-burning natural gas in the Western U.S., one gas transmission company with a design capacity of more than 1.7 billion cubic feet per day uses clamp-on flowmeters on fixed and changeable locations where technicians perform check metering on valves and other metering equipment mounted along the pipeline.

The fixed-location flowmeters provide flow rates back to the main control systems to implement in the overall control scheme, while the transportable flowmeters provide comparison readings to validate the readings from the fixed meter locations. In both cases, the meters mount to the outside of the pipes, eliminating the need to cut into the pipe or interrupt the flow This also increases the speed of installation. Technicians travel to various locations and clamp on the portable meters and document the flow rates to compare with data collected by the data acquisition systems.