New Damper Seals Increase Stability For High-Pressure Re-injection Compressor Along the coast of Angola on one of the world’s largest offshore compression platforms, Dresser-Rand centrifugal high-pressure re-injection compressors will be demonstrating technology which proves that the harder you work the machine, the more stable it becomes. The D-R compressors in Angola will be producing a discharge pressure of 6,625 psi (457 bar).  The dominant component that affects rotor stability in a back-to-back compressor is the damper seal at the division wall. And it is here that Dresser-Rand engineers have created the hole pattern damper seal to replace the standard honeycomb seal. The benefits of the new hole pattern damper seal include: Greater rotordynamics stability at higher pressures  Fewer compressor bodies per application  Up to 50 percent reduction of section-to-section leakage which reduces recycled flow and energy consumption compared to the older honeycomb design  Increased client confidence in compressor stability during long term operation.  In July, Dresser-Rand shipped the last of four trains to Okpo, South Korea, where Daewoo Shipbuilding and Marine Engineering is fabricating the platform topsides for ChevronTexaco and Cabinda Gulf Oil Company, an Angolan state oil company. The compression platforms are scheduled to be delivered to Angola by the end of December, followed by nearly a year of pre-commissioning and startup.  While the discharge pressure, flow, and power levels have been matched by earlier Dresser-Rand projects, this is the first time so much pressure rise has been done in so few compression stages. Furthermore, we believe the high pressure compressors of these trains set the benchmark in the industry for pressure rise across a single casing. These units develop over 4,300 psi (297 bar) differential pressure to reach the final discharge of 6,625 psi (457 bar).  “It’s a landmark project for us,” said Paul Dainora, Dresser-Rand project manager for the ChevronTexaco re-injection compressor in Olean, New York. The project consists of four trains, each with two DATUM® compressors driven by Rolls Royce gas turbines. “We were able to get the desired compression by using only two-bodytrains,” Dainora said “which helped the client save valuable space and weight on their platform.”  For the competition to achieve the desired discharge pressure of 6,625 psi (457 bar) would have meant a third compressor for each train, according to J. Jeffrey Moore, manager, development engineering at Dresser-Rand Olean Operations.  The key is the hole pattern damper seal. “The benefit is an increase in rotor stability with increasing discharge pressure,” Moore said. “The new damper seals permit higher speeds, pressures, and horsepower. It’s a paradigm shift in compressor design.  “We’re now designing a 12,000 psi compressor for a Gulf State project in the Middle East.” In the past, high-pressure centrifugal compressors suffered from rotordynamic instability. High pressure process gas produces considerable excitation and reaction forces on the rotor. As a result, D-R engineers developed swirl brakes and damper seals to reduce excitation and improve damping, greatly reducing the likelihood of rotordynamic instability.  “Traditionally, as the discharge pressure of a centrifugal compressor increases, the destabilizing forces increase while damping from the bearings remains constant,” Moore said. The resulting excitation and vibration problem can lead to mechanical damage to the compressor. When forward-driving forces exceed the resisting dissipation forces, self-excitation occurs in the first whirling mode of the rotor, according to Moore. “The result can be large subsynchronous vibration that is limited only by contact between the rotor and stator parts; this instability is not encountered until late in the project cycle,” he said. The consequences of which include costly downtime and hardware changes.  And if full-load testing hasn’t been performed at the factory, the instability problems may not be discovered until the unit is in the field. “Depending on the application, the loss of production and service expense can run upwards of millions of dollars,” Moore said. The goal of D-R engineers was to reverse the equation and provide a rotor that becomes more stable as pressure increases.