Reverse Rotation Capability of Spiral-Groove Gas Face Seals

Abstract

The dry-running spiral groove face seals, used in centrifugal compressors, are known for trouble-free operation and reduced operating and maintenance costs. This type of seal creates a gas film between the faces which prevents contact while running. Generally, the film stiffness of a unidirectional spiral groove seal is much higher than that of a corresponding bidirectional seal. In reverse rotation, however, a spiral-groove seal loses its film stiffness and may even become negative depending on operating pressure and speed as well as the groove design parameters. Some compressors during their operating cycle may run in the reverse direction for short periods of time. This paper describes the analysis, design and testing of a spiral groove seal wit reverse rotation capability. A seal designed for a 220 mm shaft was analyzed and the spiral groove geometries optimized for good forward rotational stiffness at design pressure and speed, and acceptable reverse rotation capability at lower speed levels. The parameters for optimization include number of grooves, groove angle, groove depth and land-to-groove width ratio. The optimized spiral groove design was machined into smaller prototype hardware and tested in a test cell. Test conditions were modified to simulate an actual groove operating environment. A computerized real-time data acqusition system was utilized to record speed, pressure, temperature, leakage, and vibration readings. The seals were operated in both directions, forward and reverse, and cycled to insure reliability. These seals showed good operating characteristics in forward rotation even with high vibration levels. At significant pressure levels, the optimized spiral groove configuration is a robust design in both directions of rotation. At reduced pressure levels the reverse rotation testing evidenced slight face contact but the temperature levels and amount of wear were acceptable. The seal performance is directly related to the pressure, speed, and time of reverse rotation.