Dry gas seals represent the latest technology for compressor shaft end sealing, and are currently the preferred sealing technology for most centrifugal compressor applications. Under dynamic (rotating) conditions, dry gas seals function as restrictive seals. Depending on the design and conditions, dry gas seals can behave either as restrictive or positive seals under static conditions. Similar to pump mechanical seals, dry gas seals use mating faces to create the sealing interface between the rotating and stationary parts. The seals depend on a fine balance between pressure forces, closure spring forces and aerodynamic forces that are created by very shallow grooves or depressions on one of the seal faces, as shown in Figure 200-49). This balance results in a face gap of about 0.0001″ to 0.0002″, through which the seal leaks at very low rates. Leak rates are approximately proportional to seal size, sealing pressure and rotational speed, and are influenced to a lesser extent on gas conditions. Depending on these parameters, leakage rates generally range from fractional SCFM to about 4 SCFM. Although the dry gas seal design concept first achieved significant commercial use in the early 1980’s, it can be traced back to the early 1950’s. Dry gas seal technology is presently also applied in both steam turbines and pumps, but this section will address only centrifugal compressor applications. Dry gas seals are an advancing technology in the petrochemical industry, so it is important to be aware of the age of information (including this Gray Manual section), as well as the duration of successful field experience for any given advance.
In general, dry gas seals offer the following primary advantages compared to other sealing technologies:
• lower leakage rates and improved pressure capability vs. other restrictive type seals, and
• simpler, more efficient and lower cost operation and auxiliaries vs. other positive type seals.
Dry gas seals can offer additional advantages as well, all of which should be considered in the economics if justification for gas seals is needed (see Application Considerations section). Justification is usually an issue for retrofits, but on new compressors, economics are favorable, especially if the alternative design requires expensive and/or inefficient auxiliaries (seal oil systems, eductor systems, etc.).
The primary advantages of gas seals are the result of an advanced and precise design that relies heavily on the proper operating environment. Reliable operation is extremely dependent on having seal gas (the gas seen by the seal faces) which is free of particulates and liquids. In addition, the reliability of designs can be compromised when approaching current experience envelopes in sealing pressure, sealing temperature and seal face surface speeds, either singularly or in combination. All of these issues focus on assuring the proper gas film and stress levels at the seal faces. Other vulnerabilities include seal face hang-up (caused by sticking of o-rings used for secondary sealing of the faces), reverse rotation, reverse pressurization and lube oil contamination of the seal faces. Many of these vulnerabilities are associated with earlier gas seal designs, and have been reduced or eliminated with design advances.