A solution and a mystery

Overhead lines carry vapors away from vacuum flashers. Researchers believe that the flow in these overhead lines cause structural vibrations seen in the units. These vibrations, if unmitigated, can cause fatigue failures in the vacuum flashers' nozzles and associated piping.

Much of this vibration can be overcome with what is aptly called a vibration reducer. "It's just a pipe with holes in it that provides resistance to the flow in the flasher and dampens the resonance between the fluid and the flasher at the right frequency," says Danesh Tafti. Tafti was a research scientist at NCSA for 10 years before taking a faculty position in the mechanical engineering department at Virginia Tech in 2002. He continues research for the Alliance, developing GenIDLEST for the academic and industrial community at large.

The team from NCSA and Shell used their model to isolate the most energetic modes in the flow. Then the team members could see the vibration reducer in action, suppressing these modes and thus allaying vibrations. They also found that the vibration reducer significantly affects the variation in pressure at points in the flow. The difference in pressure between two such points is known as a pressure drop.
		Vertical velocity vibrations in modeled vacuum flasher with and without vibration reducer. Enlarge image.

"The same phenomenon responsible for the vibrations also increases the pressure drop through the vacuum flasher. This increased pressure drop is detrimental to the process. It limits the capacity and requires additional energy input, which increases the company's energy costs. The vibration reducer, in addition to suppressing vibrations, decreases the pressure drop and significantly improves the process economics," Menon says.

The vibration reducer also introduces a mystery. Researchers aren't sure how the simple little gadget works. "At this point, we don't know the exact mechanism, we only know the effects," Tafti says. "It's isolating the exact mechanism that we'd like to continue to pursue."