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ROTATIONAL DIRECTION
from the mating ring. .uring normal operation， the running gap is approxi-mately 3 microns. Under pressurization， the forces exerted on the seal are hydrostatic and are present whether the mating ring is stationary or rotating. Hydrodynamic forces are generated only upon rotation. The mating ring consisting of the logarithmic spiral grooves is the key to generating these hydrodynamic forces.
.uring operation， the grooves in the mating ring generate a hydrody-namic force that causes the primary ring to separate from the mating ring creating a ""running gap'' between thetwo rings， which effectively seals against the process gas. .uring normaloperation， the running gap isapproximately 3 microns. A sealing gas is injected into theseal， providingthe working fluid， which establishes the running gap.
.perating Range of .ry Gas Seals
Gases ranging from inert gases such as nitrogen to highly toxic gaseous mixtures of natural gas and hydrogen sulfide can be sealed utilizing the optimum seal arrangements. The operating range of the spiral grooved dry gas seals is as follows:
Sealed Pressure:2，400 psi (165 Bar)
Temperature: 500 oF (260 o.)
Surface Speed: 500 ft./sec. (152 m/sec)
M.W.: 2.60
.ry Gas Seal .aterials
The gas composition， contaminants in the gasstream， operating tempera-tures and process conditions dictate the choice of materials. The most common materials of construction are as follows:
Mating Ring: Tungsten .arbide， Silicon .arbide
Primary Ring: .arbon， Silicon .arbide
O-Rings: Elastomers(.iton， ""Kalrez'')
Hardware: 300 or 400 series ss (Sleeves， discs， retainer rings)
.oil Springs: 316ss， Hastelloy
.ry Gas Seal Systems
The use of dry gas seals requires a system designed to supply sealing gas to the seal as a working fluid for the running gap. These gas seal systems are normally supplied by the compressor OEM mounted on the compressor baseplate. There are two basic types of gas seal systems， differential pressure ( .P) control and flow control. .ifferential control systems control the supply of seal gas to the seal by regulating the seal gas pressure to a pre-determined value typically 15 psi (1 Bar) above the sealing pressure. This is accomplished through the use of a differential pressure control valve. Flow control systems control the supply of seal gas to the seal by regulating the seal gas flow through an orifice upstream of the seal. This is accomplished through the use of a differential pressure control valve monitoring pressures on either side of the orifice.
.ry Gas Seal .egradation
.ontamination of the seal by foreign objects leads to seal failures. The running gap between the primary and mating gas seal rings is typically around 3 microns. Injection of any type of solids or liquids into this very narrow seal running gap can cause degradation of seal performance. This would create excessive gas leakage to the vent and eventual failure of the seal.
Since the typical operating gaps between the two sealing surfaces rangefrom 0.0001in to 0.0003in， the resultant leakage is very small in magnitude.Under conditions of static pressurization beyond 50.75 psi (3.4.5.17bar)， the seal leaks a very small amount. This leakage increases with increasing pressure and reduces with increasing temperature. Increased viscosity of gases at higher temperatures reduces the amount of seal leakage. For exam-ple， 4-in. (101.6 mm) shaft seal on a natural gas compressor statically pres-surized to1，000 psi (69 bar) will leak about 1 scfm (0.03 scmm). Underdynamic condition， due to the pumping effect of the spiralgrooves， the leakage increases as well.
The power loss can also be increased with seal contamination. The seal surfaces being noncontacting under dynamic conditions the power loss asso-ciated with dry gas seals is very small. The power loss for a 10-in. (254 mm) seal operating at 1000 psi (69 Bar)and10，000 rpm is about 12.14 kW.With damage seal surfaces， these losses can be increased by 20.30%.
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燃气涡轮工程手册 Gas Turbine Engineering Handbook 2(95)
