The dry gas seal is the preferred method of shaft end sealing for nearly all new centrifugal compressors in hydrocarbon services. However, there are limits that warrant additional expert review, confirmation of user experience and perhaps special design and testing activities. These cautionary limits include:
• Surface speeds in excess of 400 feet per second
• Sealing pressures in excess of 2000 psig, and • Sealing temperatures in excess of 300°F (move pointer to yellow to see comments)
In addition, combinations of two or more values approaching the above warrant similar scrutiny. When checking user experience for similar services or conditions, it is important to go beyond the vendor installation list and actually contact at least some of the end users to confirm success and/or retrieve lessons learned. In most cases, the seal gas auxiliary system will require more scrutiny than the design of the seal assembly.
Retrofit applications can be more difficult to justify, since the cost of the auxiliaries for the original seal are already sunk. Typically, retrofits are justified primarily by reliability, and in some cases, seal performance. If credits associated with these improvements are insufficient, the following other factors should be included in the justification, if applicable:
• Elimination of venting gases from an oil seal trap system
• Elimination of driver power for seal oil pumps
• Reduction of main driver power draw due to lower seal parasitic loses (20-25 HP/seal for oil bushing seals)
• Elimination of make-up seal oil and disposal or reconditioning of contaminated seal oil
• Reduction of seal system auxiliaries maintenance and testing
• Elimination of motive gas consumption (steam and/or process gas) for labyrinth seal eductors
• A potential reduction in other utilities used by auxiliaries (air, N2, steam, cooling water, etc.)
The above must be offset by costs associated with dry gas seals, which are generally much less.
Retrofits on compressors with oil seals can have a significant impact on rotor dynamics. It is essential to have a rotor dynamic analysis conducted with expert review to identify potential problems and solutions. Retrofits on compressors with labyrinth seals will generally result in less rotor dynamic effect, but depending on geometries and existing rotor dynamic margins, there may be a potential issue. A pre-analysis expert review of these situations is suggested.
The selection table (Figure200-56, at the end of this section) is to assist in selecting an appropriate sealing arrangement and preliminary auxiliary scheme for most services. When using this table, also refer to the text of this section to better evaluate decisions and options.
Figure 200-57, Figure 200-58 and Figure 200-59 are schematics of typical dry gas seal auxiliaries used for the specific arrangements and schemes.