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Table 2. API piping plans for dual pressurized seals
Table 3. Lowest emissions capability by seal type
Dual pressurized gas seals accomplish the same zero emissions objective but use a gas barrier in the seal cavity. This technology, adapted from centrifugal gas compressor seals, uses topographical features in the seal face to generate hydrodynamic lubrication of the seal faces with pressurized gas which keeps the seal faces from contacting. Non-contacting containment seal faces also employ features for hydrodynamic support.
Figure 3. Dual unpressurized liquid buffer seal (Courtesy of Flowserve)
This revolutionary achievement led to the implementation of gas seals in a variety of applications. Compared with dual wet seals, dual gas seals generate little heat or wear because of face separation, use simpler support systems, and, in many cases, use an inert gas such as nitrogen as the barrier medium. In addition, the support system does not require fluid circulation. Dual gas seals also effectively use less energy consumption than their liquid counterparts because of the absence of friction between the seal faces. Users must implement proper instrumentation to ensure the barrier gas is clean and properly pressurized higher than the process at all times.
Long-term reliability of any dual mechanical seal and the achievement of zero process emissions are strongly dependent on the lasting reliability of the support system. Dual gas seals also effectively use less energy consumption than their liquid counterparts because of the absence of friction between the seal faces.
Over time, the seal industry has developed a variety of innovative solutions—single, dual, wet and dry—to continuously reduce fugitive emissions. Emissions control has come a long way, from high-balanced single seals that keep emissions below 500 ppm to dual pressurized liquid seals allowing true zero emissions. Each type of seal—Arrangements 1, 2 and 3—are viable options for low-emissions sealing, but the sealing option selected will depend on the application. When selecting a seal for a zero emissions application, users must define the zero emissions criteria.
Acceptable emissions can take on a different definition depending on location and process fluid, as local government defines the acceptable VHAPs emission limit. Sealing solutions may become more elaborate as the total allowable emissions limit trends toward zero, but they are fully able to achieve the goal.