Gas Compressor Dry gas seals (Spiral groove seals)ByEr. Laxman Singh Sankhla B.E.Mech., Chartered EngineerJodhpur, IndiaMail ID: laxman9992001@yahoo.co.inDry gas seals (Spiral groove seals)Traditionally, the sealing of rotating shafts on gas compressors has been achieved by the use of Carbon ring seals, labyrinth seals and double contacting end face seals with pressurized liquid barrier to provide lubrication and cooling.These installations as mentioned above need complicated, space-consuming seal oil systems are expensive, inefficient and require continual maintenance. Ever increasing shaft sizes, shaft speeds and pressures generated considerable friction losses in sealing oil films which represents lost energy. In addition, there is the problem of contamination of the gas by the seal oil, and of the seal oil by the gas. This requires very expensive separation systems, and on off-shore gas rigs is virtually impossible to achieve. As a result the oil has to be off-loaded and shipped to land based installations for processing. The dry running, non-contacting gas seal, however, operates by establishing a very thin gas film between the rotating faces, thus requiring no auxiliary seal oil system, absorbing very little power and reducing wear rates to near zero values.A dry running gas seal resembles an ordinary stationary mechanical face seal, having the same basic components: face ring, seat ring, retainer, coil springs, secondary seal, sleeve, etc.

The main differences are: wider seal faces, specially shaped face ring and a spiral groove, Rayleigh pad or other pattern on one of the sealing faces. A typical example is shown below.

The rotating seat ring, usually a very hard, stiff, wear resistant material is axially fixed, whereas the stationary face ring, made from a relatively softer, more flexible self-lubricating material like carbon, is free to move in the axial direction to establish a dynamic equilibrium sealing gap between the two rings. SLEEVESMATING RINGRETAINERCLAMPING NUTPRIMARY RINGSECONDARY O-ringSHAFTSPRINGPROCESS SIDEDRY GAS SEAL ASSEMBLY

SPIRAL GROOVES ON ROTATING FACE (Mating ring)SEAL CARTRIDGEProcess SideBearing SidePrimary RingFirst StageSecond StageSpring(Not visible)Splines(For positive drive)TYPICAL DRY-RUNNING, NON-CONTACTING, TANDEM GAS SEAL(John Crane Type 28))

Deep feed grooveShallow pocketsSealing damSpiral grooveRidgeSealing dam

SPIRAL GROOVE AND RAYLEIGHT PAD SEAL ROTATING FACES

BIDIRECTIONAL GROOVE DESIGN

Centrifugal compressors require seals around the rotating shaft to prevent air or gases from escaping where the shaft exits the compressor casing. Over-hung compressors have a seal on only the inboard (motor) side.Dry seals operate mechanically under the opposing force created by hydrodynamic grooves and static pressure. As shown in exhibit, hydrodynamic grooves are etched into the surface of the rotating ring affixed to the compressor shaft. When the compressor is not rotating, the stationary ring in the seal housing is pressed against the rotating ring by springs. When the compressor shaft rotates at high speed, compressed gas has only one pathway to leak down the shaft, and that is between the rotating and stationary rings. This gas is pumped between the rings by grooves in the rotating ring.

Process Gas Tries to Leak Between Rings Rotating Ring

Compressor End

Stationary Ring

Spinning Shaft

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