Centrifugal Compressors
Summary
A centrifugal compressor is a mechanical device used to increase the pressure of a natural gas stream. Increased pressure is needed for the treatment or processing of natural gas and for facilitating the movement of natural gas from production, gathering and boosting, and processing sites via transmission to customer distribution systems. Centrifugal compressors contain rotating shafts that require seals to prevent the high-pressure natural gas from escaping the compressor casing. Traditionally, these seals used high pressure oil as a barrier against escaping gas; these seals are referred to as “wet seals”. Alternatively, centrifugal compressors can be equipped with mechanical seals, called “dry seals,” which have substantially lower emissions.
Description
A centrifugal compressor uses a rotating shaft to increase the velocity of the natural gas and direct the gas to a divergent duct section that converts the velocity energy to pressure energy. Centrifugal compressors have a series of rings to prevent gas from escaping where the shaft exits the compressor casing. Seals are used to prevent these releases – either wet seals that use a specialty oil or dry seals that use a mechanical barrier. Wet seals use a viscous oil under high pressure between a center ring (attached to the rotating shaft) and a ring on each side (stationary in the seal housing) pressed against the rotating ring with springs. A thin film of oil flows between the rings both to lubricate and to act as a leak barrier. The perimeter of the stationary rings is sealed in the case by rubber “O-rings” to prevent leakage of seal oil around the stationary rings. Very little process gas escapes through the oil barrier created between the stationary and rotating rings or through the outboard labyrinth seal, but gas is entrained and absorbed by the oil at the compressor side (inboard) seal oil/gas interface, thus contaminating the seal oil. The seal oil is purged of the entrained gas (using heaters, degassing tanks, and degassing techniques) to maintain its viscosity and lubricity before mixing with the uncontaminated outboard oil in a sump for recirculation back to the seals. This process results in methane emissions where the separated gas is vented to the atmosphere. Figure 1 depicts a centrifugal compressor with a wet seal configuration. On the other hand, compressors with a dry seal configuration use the opposing force created by hydrodynamic grooves and springs to provide a seal. The opposing forces create a thin gap of high pressure gas between the rings through which little gas can leak. The rings do not wear or need lubrication because they are not in contact with each other.
Note that natural gas leaks may occur at other components connected to the compressor, such as flanges, valves, and fittings, during compressor operation. For more information about these emissions, see the Equipment Leaks page.
References
U.S. Environmental Protection Agency. (2014, April). Oil and natural gas sector compressors – Report for Oil and Natural Gas Sector Compressors Review Panel. https://www.ourenergypolicy.org/wp-content/uploads/2014/04/epa-compressors.pdf
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