In today's world, facilities face the daunting challenge of minimizing the environmental impact of industrial processes. A primary motivation for industrial plants to control their environmental impact is government regulation. In the U.S., these regulations are issued at the national level by the Environmental Protection Agency (EPA) and similar agencies at the state and local levels. Before the U.S. government passed the first Clean Air Act in 1963, no federal regulations on gaseous emissions existed. The Clean Air Act initiated research to investigate techniques to monitor and control air pollution. During the 1970s and 1980s, the government passed legislation that established a limit on emissions of 10,000 parts per million (ppm) for gases defined as volatile hazardous air pollutants (VHAPs). Similarly in Europe, a series of accidents resulting in toxic chemical release lead to adoption of the European Commission directive 82/501/EEC targeting safety measures to prevent the release of hazardous chemicals. This threshold came down to 1,000 ppm in the 1990s. In some cases, these emissions restrictions were driven even lower by local authorities. The sealing industry has always worked to provide innovative solutions to meet the goals of the regulations and industry.
09/22/2015
Figure 1. Dual unpressurized liquid buffer seal (Courtesy of Flowserve)
Figure 2. Dual pressurized liquid barrier seal (Courtesy of Flowserve)
Table 1. Piping plans for dual unpressurized seals (Tables courtesy of FSA)
An Arrangement 2 seal can run on a liquid or gas buffer designs, also called containment seals, that have either dry contacting or non-contacting seal faces. These dual unpressurized seals are designed to contain and direct process leakage to a waste area for recovery or safe disposal. Dual unpressurized seals provide a margin of safety because the outer seal acts as a true backup seal, while the inner seal sees the rigorous operating conditions and high pressure drop across the seal faces. Testing has proven that dual unpressurized seals can achieve emissions control below 50 ppm. Although emissions into the buffer cavity are low, plant operators must manage vent and buffer systems properly to ensure emissions are properly recovered.
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.