l
o
主g
(Po -Pn)
1j2
m
l
二 0.9A
n + ln Pn Po
For staggered labyrinths， the equation can be written as
l
1j2
gm
l二 0.75A 主on (+ Poln -PPn n) Po
where:
m
l二leakage， lb/sec
A二leakage area of single throttling， sq ft
Po二absolute pressure before thelabyrinth， lb/sq ft
主o二specific volume before the labyrinth， cu ft/lbm
Pn二absolute pressure after the labyrinth， lbf/sq ft
n二 number of lands
The leakage of a labyrinth seal can be kept to a minimum by providing:
(1) minimum clearance between the seal lands and the seal sleeve， (2) sharpedges on the lands to reduce the flow discharge coefficient， and (3) grooves or steps in the flow path for reducing dynamic head carryover from stage to stage.
The labyrinth sleeve can be flexibly mounted to permit radial motion forself-aligning effects. In practice， a radial clearance of under 0.008 is difficult toachieve， except with very small high-precision machines. On largerturbines， clearances of 0.015.0.02 are generally used. .uring machine con-struction， it is important to measure and record these clearances because mechanical seizure or loss in aerodynamic efficiency can often be traced to incorrect labyrinth seal clearances.
The windback seal closely resembles the labyrinth but has an entirelydifferent operational principle. A screw-thread device winds theoil， whichis carried around the bore by the windage of theshaft， into an internal drain for return to the system as shown in Figure 13-23a.
Windback structures are extremely simple. .learances about the shaft areample， and the device has high reliability. When shaft speeds extend into thelow regions where windage effects are inadequate for effective operation， augmentation of windage can be achieved by special configurations of theshaft surface. Windbacks are also used as adjuncts to other types ofseals， asshown in Figure 13-23b. With circumferential seals， windbacks can be used to keep oil splash from reaching the seal carbons when coking problems exist. In oil-buffered seals for compressors they are used to direct the small internal leakage into a pressurized drain to effect practically complete recov-ery of the leakage.
Ring .Bushing. Seals
The restrictive ring seal is essentially a series of sleeves in which the boresform a small clearance around the shaft. Thus， the leakage is limited by the flow resistance in the restricted area and controlled by the laminar or turbulent friction. The API 617 codes characterize this type of seal. Most of the restric-tive-type seals are of the floating type rather than the fixed type. The floatingrings permit a much smaller leakage， and they can be of either the segmented type as shown in Figure 13-24a or the rigid type as shown in Figure 13-24b.
Because of the minimal contact between the stationary ring and the rotor，these seals， when properly designed， are ideal for high-speed rotating machinery.
When adequate lubrication and cooling fluid is available， the sealring，manufactured from babbitt-lined steel，bronze， orcarbon， will functionsatisfactorily. When the medium to be sealed is air orgas， carbon seal rings must be used. .arbon has self-lubricating properties. .ooling of the seal is provided by the leakage flow through the seal. .epending on the operating temperature and environment， aluminum alloys and silver are also used in the manufacture of the seal rings. Leakage limitation depends upon the type of flow and type of bushing. There are four types of flow: compressible andincompressible， each of which may be either laminar or turbulent. Ring seals are divided into two categories: fixed breakdown rings and floating break-downrings， according to whether or not they are fixed with respect to the stationary housing.
Fixed seal rings. The fixed seal ring consists of a long sleeve affixed to a housing in which the shaft rotates with small clearance. It is an inexpensiveassembly. However，since it isfixed， the seal behaves like a redundantbearing when rubbing occursand， like thelabyrinth， requires large clear-ances.Therefore， long assemblies must be used to keep leakage within reasonable limits. Since long seal assemblies aggravate alignment and rub-bing problems， sturdier shafts are required to keep operating speeds in a subcritical region. The fixed-bushing seal almost always operates withappreciable eccentricity. This， plus the combination of a large clearance and a large eccentricityratio， produces large leakages per unit length. Fixed-seal rings are therefore impractical where leakage is undesirable. 中國航空網 m.k6050.com 航空翻譯 www.aviation.cn 本文鏈接地址：燃氣渦輪工程手冊 Gas Turbine Engineering Handbook 2(89)
