The operators at Buckeye petroleum terminal in Tampa, Florida, faced a challenge. As one of the largest independent petroleum pipeline systems in the U.S., Buckeye was experienced in solving complex problems involving petroleum product transportation, but this one was unusual.
Buckeye was installing a new connection from the Port of Tampa Bay Central Manifold to its Tampa Bay South storage and distribution system. The problem was that three 16-inch pipes would need to be installed in a combination of underground and aboveground areas that were not accessible at all times. The new lines would be subject to operational regulations established under the authority of the U.S. Coast Guard.
“Thirty-five percent of the lines run aboveground through a neighboring terminal where we do not have 24-hour access,” said David Martin, senior project manager for Buckeye who was in charge of finding a solution that would meet the Coast Guard’s requirements. “The remaining 65 percent is underground through the Port of Tampa Industrial Park. The Coast Guard regulation for ship discharge is that the lines need to be visually inspected for integrity on an hourly basis during off-load operations. The combination of buried lines and limited access created a compliance issue for us.”
Paul Hutz, Buckeye’s terminal operator at Tampa Bay said, “The Coast Guard rules gave us some difficult choices to make. One obvious solution was to put a double wall around each pipe with interstitial monitoring to assess hourly line integrity, which would eliminate room for one of our three 16-inch pipes. Since we handle regular gas, premium gas and diesel fuel at this terminal, operating on two lines was not an option. On top of that, it would have been a very expensive solution. The other choice was to come up with a system that would accurately monitor the petroleum products as they left the ship and as they arrived at the terminal to assure there were no losses.”
With roots dating back to 1886, Buckeye has developed and refined methods and processes to manage its 6,000-mile pipeline system in an ever-changing regulatory climate.
Martin’s first job was to find a monitoring technology that could validate that the same amount of petroleum product discharged from a ship arrived at the terminal.
“After doing the needed research, we identified mass balance flow monitoring as the most economical technique to quickly and accurately provide the constant oversight of the line integrity during product movements,” he said.
Mass balance flow monitoring
In essence, mass balance uses flowmeters that have the hardware and software to compensate for flow variables such as pressure and temperature changes that might erroneously signal a nonexistent line integrity issue. They would need flowmeters that could be configured to remotely report via satellite communications directly to Buckeye’s 24-hour SCADA Center.
Identifying the right flowmeter
“I knew I wanted a flowmeter that was nonintrusive and highly accurate with a solid operational history to communicate to our control system,” said Martin.
Those requirements instantly eliminated almost all typical metering and leak detecting systems, including turbine, vortex and magnetic flowmeters.
Martin discovered an application being used at a Buckeye facility in West Chicago that used ultrasonic flow measurement. Although the application was totally different than his, he could see that it might provide a monitoring solution for his needs.
How ultrasonic flowmetering works
“One of the major benefits of ultrasonic flowmeters is that, unlike traditional meters, they contain no moving parts and do not need frequent calibration and maintenance,” said Joe Tierney, FLEXIM America’s Gulf regional manager. “Measurements are made using the transit-time difference method. It exploits the fact that the transmission speed of an ultrasonic signal is affected by the flow velocity of the fluid. An ultrasonic signal moves slower against the flow direction of the medium and faster with the flow direction.
“The meter sends ultrasonic pulses through the medium, one with the flow direction and the second against it. The meter’s transducers work alternately as transmitter and receiver. The transit time of the signal sent in the flow direction is shorter than that of the signal sent against the flow. The meter measures the transit time difference and calculates the average flow velocity. Since the ultrasound signals propagate in solids, the meter can be mounted directly onto the exterior of the pipe noninvasively.”
Ultrasonic metering background
Ultrasonic metering technology has significantly improved since it was introduced in the 1970s. Today ultrasonic spool meters are a popular choice for natural gas custody transfer measurement. The accuracy achieved by these meters is 0.05 to 0.2 percent. Clamp-on meters are not as accurate because they are not calibrated with a spool section. Therefore, they contain an uncertainty of about 1 percent after installation. When taken to a calibration laboratory, some clamp-on meters can show accuracy and repeatability similar to ultrasonic spool meters in the 0.1 to 0.25 percent range. This makes the clamp-on meter a good selection for leak detection because the meters can be calibrated on the pipeline to one another, which then allows for low leak detection thresholds.
The amount of straight run usually required to obtain the best accuracy is 10 to 5 upstream/downstream diameters,” said Tierney. “So a 10-inch pipe needs 150 inches or 12.5 feet of straight pipe. We find that we can squeeze this down to 5 diameters upstream and still maintain the meter accuracy with multibeam meters.”
Martin chose an ultrasonic meter that was a clamp-on, nonintrusive meter with the ability to monitor and compensate for changes in temperature and pressure.
“While we have been supplying ultrasonic meters to several Buckeye locations over the years, this was a new application for us,” said Jim Pletcher, president of one of FLEXIM’s major suppliers of monitoring systems, Technical Devices Inc. “It was similar to custody transfer, which is a familiar internal application for ultrasonic meters within Buckeye’s facilities, but its main goal was leak identification.”
Tierney said, “Typically, we would have recommended a two-channel meter, which would meet all of Buckeye’s operational criteria — nonintrusive, heat and pressure compensation, minimal maintenance. But line integrity identification created more demands for accuracy because higher accuracy meant earlier detection of line balance issues.”
A four-channel solution
“Fortunately, [we found a] recently introduced four-channel system. While the two-channel meters offered accuracy of one-half of 1 percent, the new four-channel improved that to a range from a tenth to a quarter of 1 percent,” Tierney said.
With more restrictions in ecologically sensitive areas, the introduction of the system was timely. It offered greater accuracy because it doubles the number of ultrasonic paths. A two-channel generates four reflective paths, while a four-channel generates eight reflective paths. The eight beams illuminate more fluid, producing readings that are immune to turbulence-induced variations in the flow rate that are not reflective of real flow rate changes.
The meters use algorithms based on standards TP-25, ASTM1250 and D4311, which convert actual volumetric flow to standard volume flow. Standard volume is temperature- and pressure-compensated flow, equivalent to the mass flow needed for mass balance leak detection. Imagine this scenario: If the liquid going into the pipe is hotter than when it exits the pipe, the volume of liquid entering at the source site would not be the same as that arriving at the destination site because volume expands with temperature. That is why standard volume or mass balance is necessary. The supplied meters can provide mass and standard volume outputs, relieving the customer’s SCADA unit of this task.
The Buckeye mass balance leak detection algorithms take care of desensitizing the system during line start and stops where the pipeline packs and unpacks, which might lead to a false leak alarm. Additionally, several analytical integration intervals are maintained by the system with evermore-sensitive leak detection thresholds to ensure high performance from the metering gear by smoothing out short-term flow noise.
Martin and Pletcher created a proposal for the Coast Guard based on the accuracy and repeatability of the four-channel ultrasonic meters, the minimum maintenance and calibration, and Buckeye’s proprietary rapid leak repair protocols.
“The Coast Guard reviewed our proposal and accepted it as presented,” said Martin. “Once two meters were installed on each end of the three discharge lines, we were able to verify that the barrel of product that was being discharged from the ship was the same barrel received and stored at the terminal.
“The accuracy has met Buckeye’s needs and satisfied the Coast Guard’s oversight requirement during marine discharge activities. We have been up and operating for more than a year and there have been no issues. One nice thing about the ultrasonics is that if there is an issue, they are easy to access to make any changes. But, since there are no moving parts, I don’t see any issues in the future.”
At other locations, including its operation at Corpus Christi, Texas, Buckeye uses ultrasonic meters for internal custody transfer between operating entities at the same location. As far as the new four-channel meters are concerned, Buckeye engineers are working with a major oil producer on an application similar to that at Tampa Bay. They expect the four-channel meter to gain acceptance in the oil and gas industry as a more economical solution.
With the advantage of not having to shut down operations and undergo costly pipe works for installation, clamp-on meters capable of high accuracy and repeatability will be used more and more for these high-end applications.
Izzy Rivera is service manager for FLEXIM Americas. He can be reached at email@example.com or 631-492-2300. Jack Sine is a freelance writer specializing in the flow measurement, power, chemical and HVACR industries. He can be reached at firstname.lastname@example.org or 845-831-6578.