Process-Design Considerations for a Compressor Dry-Gas Seal-System Interface
The dry-gas seal (DGS) is a critical integrity component of the centrifugal or screw compressor, providing shaft sealing and pre- venting uncontrolled escape of process gas from the casing. Failure of this component in the compressor can result in plant outage and considerable revenue loss to the operating company.
The DGS relies on a very thin gas film that is formed between a stationary ring and a rotating ring. Pressurized and clean seal gas is introduced to work as the gas film, preventing leakage of the com- pressor casing gas. Minor seal-gas leakage from the gas seal is at low pressure, and is usually collected in an enclosed system for dis- posal (e.g., low-pressure or atmospheric flare).
Failure of the DGS seal is often not caused by its intrinsic design issues, but rather by aspects peripheral to the seal. The need for pres- surized seal gas necessitates the evaluation of possible sources of gas supply during normal operation and startup. Possible sources of supply evaluated in this study include high-pressure gas-export pipe- line, multitrain arrangement to supply gas from the operating train to the standby train, and the use of gas boosters. Seal-gas cleanliness de- mands fine gas filtration as mandatory before gas entry to the seals. Because the seal gas undergoes different levels of pressure reduction within the seal, potential liquid (or condensation) and, in some cases, solid (hydrate) formation in the gas seals must be studied together with its mitigating measures in the design to avoid seal failure. The possible presence of other contaminants because of sour-gas compo- nents is addressed, along with suggested treatment methods. Other design considerations, such as reverse rotation, depressurization lim- itations, and reverse pressurization, are also addressed.
Whether engineers are engaged in designing the gas-compres- sion system or in troubleshooting the facilities operation, a clear un- derstanding of these various aspects is important. This paper does not address the design of the DGS, which is proprietary to the man- ufacturer. On the basis of past experiences, this paper describes the various salient features and peripheral requirements of the DGS, and offers recommendations for interfacing with the compressor vendor from the process-system-design and -operation perspectives.
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