January 2008

Chilling out with ultrasonic

University chilled water plant plays it cool with ultrasonic flowmeters

Fast Forward

  • Noninvasive flow measurement gaining ground in liquid and gas-flow measurement.
  • University chilled water plant sees improved efficiency, energy use with ultrasonic.
  • Transit-time is secret to ultrasonic magic.
By Ellen Fussell Policastro

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 (open-throat) 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 were-where 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.

Some manufacturers might prefer wetted models, in which the ultrasonic transducers are in direct contact with the flowing fluid, for certain permanent locations or for fluids difficult to measure due to their acoustic or physical properties, Scelzo said.

In either case, the ultrasonic transducers are located at an angle across the pipe so the ultrasonic pulses transmitted by one will be received by the other. The time it takes for a pulse to travel from the upstream transducer to the downstream transducer is shorter than the time it takes for the next pulse to travel in the opposite direction. "The reason is the flow of fluid in the pipeline speeds up the pulse traveling downstream and slows down the pulse traveling upstream. The difference of the two transit times is proportional to the velocity of flow of the fluid in the pipeline," Scelzo said. The meter calculates the volumetric flow rate of the fluid by multiplying its velocity by the cross sectional area of the inside pipe diameter.

Ultrasonic radar

The ultrasonic meter works somewhat like a policeman's radar, Menefee said. "A police radar sends out a signal that then hits an automobile. Then it sends out a second signal that hits it again. It's the time difference that helps calculate how fast the car is moving. It's the same with ultrasonic meters. It sends a signal through the pipe-through water moving in the pipe. It then sends out a second signal and measures the difference between the signals, and that way it calculates how fast the water is moving."

The water may be moving 7, 8, or 9 feet per second. "Measuring in feet per second helps us calculate volume because we know the size of the pipe," he said. "And we know the area of the pipe. And by knowing what the temperatures are, we can calculate energy and tons of cooling. So there's a calculation if you know the volume rate, cubic feet per minute, and temperatures. You can calculate the amount of cooling the building is using to air condition the building. If the water comes into the building at 40°F, and comes back out at 55°F, it has absorbed the heat out of the building.

"Before we did not know how much cooling each building was using. We knew the total for the whole system. The new ultrasonic meters will allow us to look at each building individually, and see which are performing well, which buildings are the most and least energy efficient. And they will help us improve our system."

Improve efficiency, energy use

The bottom line benefit to Indiana University is to improve efficiency in the chilled water distribution system and to better recognize where to address building problems and issues to improve efficiency. "This will allow us to better use the capacity we have," Menefee said. "By having this information, we'll look for ways to improve efficiency of the system and utilize capacity to save money in the long run."

With a big sustainability move on campus, Indiana University is looking at energy use and how to better utilize energy and reduce the carbon dioxide footprint, Menefee said. "The electricity we're using is being produced by coal fired plants. If we can reduce our electricity use, that's good for our bottom-line dollars and the environment-reducing the amount of carbon dioxide produced when the electricity is produced." Using the ultrasonic meters helps "because it will show us how the chilled water is being used and hopefully will point out how to improve efficiency and capacity utilization. Those are the two bottom lines."


Ellen Fussell Policastro is the associate editor of InTech. Her e-mail is efussellpolicastro@isa.org.




Ultrasonic primer

Ultrasonic flowmeters are so sensitive at and around zero flow, they operate and provide flow data during flow and no-flow conditions. They permit the system to detect and integrate very small leaks and extractions and allow for leak detection during no-flow conditions.

These meters are bi-directional in operation, so there is no need for additional instrument, valves, and expensive piping configuration. And they can detect different liquids because they measure the sonic velocity of liquids in the pipeline. They have the ability to infer density and viscosity and provide standard volume flow outputs.

With the transit-time technique in ultrasonic flow measurement, the transducer operates as both transmitter and receiver. Transit-time flowmeter benefits include no pressure drop or energy loss. There is little to no maintenance and no moving parts to wear or foul. It maintains calibration with no process shutdown during installation. There is no need to cut pipes and no potential for a leak point.

pg 42 Jan08

SOURCE: ISA EXPO 2007 presentation, "Pipeline Leak Detection Using Clamp-on Ultrasonic Technology," by Martin Dingman, Siemens ultrasonic flowmeter manager