The complex seal systems incorporate many different types of components to provide the most efficient sealing. Figure 13-32 shows a system that includes three different types of seals. The labyrinth seal initially provides the restriction that prevents the polymers contained in the process gas from clogging the seal rings. The labyrinth seal is followed by the two segmentedcircumferential contact seals and the four segmented restrictive-ring seals， which are primary seals in this combination. The primary restrictive-ring seals are followed by four circumferential segmented seal rings. A buffer gas is also introduced at the first set of circumferential contactseals， and aneductor is situated in the middle of the rear circumferential seals.Thus， this sealing system is very efficient in preventing any leakage and also for utiliz-ing educted gas in the process.
Gas compressors operating on highly toxic or flammable gases may require redundant systems to assure no leakages.In many applications， such asrefrigerationgas， buffer seals are required with the liquid-buffered face seal. A popular technique is to use a buffered labyrinth seal with a liquid seal.
Associated .il System
One of the advantages of mechanical contact seals is that the associated seal oil supply system may be relatively simple compared to the systemrequired with other types of seals， as seen in Figure 13-33. The relatively high oil-to-gas differential and wide allowable range allow simple differential regulators to be used to control the oil supply system rather than a complex overhead tank arrangement. The dark lines in Figure 13-34 represent the seal oil system used for this type of seal. Seal oil is taken from a controlled header ""A'' and dropped to the required .P via a relatively inexpensive regulator control. The sensing point for this .P control is off the contaminated drain cavity on the high-pressure end of the compressor. By sensing off the high-pressureend， a minimum of .P of oil to gas is always held on both ends of the compressor. Any pressurizing of the contaminated drain cavity due to buffer gas being used is also automatically followed by using a sensing point located in thecontaminated drain oil cavity. In the system shown， the ""uncontaminated oil'' combines immediately with lube oil and returns to the reservoir where the ""contaminated oil'' can be trapped by a drainer and automatically drained to be optionally discarded or returned to the reservoir via a degasing tank.

.ry Gas Seals
The use of dry gas seals in process gas centrifugal compressors hasincreased over the last 30years， replacing traditional oil film seals in most applications. Over 85% of centrifugal gas compressors manufactured today are equipped with dry gas seals.
.ry gas seals are basically mechanical faceseals， consisting of a matingring， which rotates and a primaryring， which is stationary. A cross-sectional view of a dry gas seal is shown in Figure 13-35. The rotating assembly consists of the mating ring (with spiral grooves) mounted on a shaft sleeve held in place axially with a clamp sleeve and a locknut. It is typically pin driven. The mating ring with spiral grooves and the primary ring are held within the retainer assembly. The stationary assembly consists of the primary ring mounted in a retainer assembly held stationary within thecompressor housing. Under static conditions， the primary and mating rings are held in contact due to the spring load on the primary ring.
The spiral groove pattern， for a clockwise rotation， on the mating ring is shown in Figure 13-36. The operating principle of the spiral grooved gas seal is that of a hydrostatic and hydrodynamic force balance. As gas enters thegrooves， it is sheared towards the center. The sealing dam acts as a restric-tion to the gasoutflow， thereby raising the pressure upstream of the dam.This increased pressure causes the flexibly mounted， primary ring to separate
SEAL GAS

SPIRAL
　
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本文鏈接地址：燃氣渦輪工程手冊 Gas Turbine Engineering Handbook 2(94)