sealing technology for aircraft gas turbine nbsp;· sealing technology for aircraft gas turbine...
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NASA TECHNICALM E MORANDUM
NASA TM X-71607
Note in table III that the seal leakage in-creases as the sliding speed increases (for anygiven pressure differential). This leakage increaseis due to a slight increase of the sealing gap be-cause the increased lift force produced by the liftpads (dynamic effect). As would be expected, theleakage increases as the pressure increases.
To further explore the operating limits of theself-acting seals, 150 hours of endurance operationat ambient temperature was conducted as follows:
Air temperature varied throughout the test butwas generally from 372 to 408 K (200 to 275 F).Inspection of the seals after the 150 hours revealedthat the wear to each seal was insignificant (lessthan 0.0012 mm (0.00005 in.)j.
A comparison of the gas leakage rates of thevarious seal configurations is shown in Fig. 5. Ingeneral, the plot shows that self-acting face sealhas the potential of significantly reducing leakageas compared to the conventional seals.
Of the conventional configurations, face sealsallowed the least air flow at high pressure differ-entials. Circumferential segmented seals are astight as face seals at moderate operating condi-tions; however, experience and the subject test pro-gram results have shown that at pressure differen-tials above 41.4 N/cm2 (60 psi) and speeds above107 m/sec (350 ft/sec), these (unbalanced) circum-ferential segmented seals rapidly wear out andfinally operate as labyrinths. In that case thereis little to choose between circumferential, rotat-ing ring, and labyrinth seals in terms of air flows.
To gain some perspective of the magnitude ofair flow under discussion, engine experience hasshown that excessive air flow into a bearing packageincorporating seals of the size used in the testprogram would be in the order of 0.012 kg/sec(0.029 Ib/sec). Taking midpoint values of the rangeof pressure differentials in Fig. 5, the face sealcould not meet this criterion at pressure differ-entials above approximately 85 N/cm2 (123 psi) ; andthe limiting pressure differential for circumferen-tial segmented seals (which wear rapidly), rotatingring seals, and simple labyrinths would be approxi-mately 40 N/cm2 (58 psi). The self-acting seal,however, did not reach the limiting leakage rate
and had a leakage of 0.0046 kg/sec (0.0102 Ib/sec)at a pressure differential of 107.6 N/cm^(156.0 psi). In general the self-acting seal hadabout one third the leakage of the conventionalface seal.
Gas Path Seals
Gas path sealing either in the primary or sec-ondary gas flow paths, is a complex engineeringproblem since these.seals operate in extreme andvarying temperature environments under large cen-trifugal stress conditions, high peripheral speeds,and must withstand occasional rubbing contact.These rotating and stationary seal elements can .move radially and axially with respect to one .another due to engine thermals and transient condi-.tions. However, in proposing improvements, itmust be kept in mind that the foremost requirementis reliability; that is, the seal system must notenter a self-destruct rub mode when hard rubs occurand adverse blade damage must not be produced.
Considering all the seals, the primary gaspath seals (Fig. 6) have the greatest impact onTSFC. Generally, the gas path sealing clearanceschange with each engine condition (idle, takeoff,climb, etc.); the support structure dimensionalchanges being large relative to the seal clear-ances. One portion of the design problem is thatof compensating for these relatively large dis-placements through design features and judicioususe of materials. The trend toward higher enginepressures and temperatures will tend to increasethese seal displacements.
Typically, a secondary gas path system con-tains flow restrictions in parallel and series.The functions of the secondary seals include cool-ing, maintaining proper thermal balance, pressuriz-ing, venting and thrust balancing. Because of thecavity interconnections across the seals, a prob-lem at one seal location can affect other parts ofthe system (such as changing the thrust bearingload). Changes in the turbine cooling flow are ofparticular concern since the trend is to use mate-rials at temperatures which result in the minimumacceptable low cycle thermal fatigue life. Thetotal seal clearance, of course, is a function ofdifferential thermal growth, wear, and erosion.The crux of the gas path seal (primary or secondaryflow) problem is to maintain control of clearancesthat need to be made as small as practical.
Effect of Clearance
Reference 1 points out that gas leakage ef-fects become more acute as engine size decreasesbecause the ratio of engine circumference to engineflow area increases. For example, the leakageanalysis reported in Ref. 1 indicates that theengine flange and labyrinth seal leakage is 2 per-cent of the mass flow for a 5 pound per secondengine, and only 0.3 percent for a 120 pound persecond engine. Reference 1 presents a detailedanalysis of the clearance effects in a hypotheticalengine with a 5 pound per second air flow. Theengine was designed for zero leakage and perform-ance calculated; performance was then determinedwith typical leakage values (based on seal tests)assigned to each seal position. The results areshown in Fig. 7. The most significant losses arein the labyrinth seals, and in particular the lab-yrinth seals at the discharge of the centrifugal
compressor and at the turbine bearing locations.The calculated overall effect (which includes lossesthrough flanges and vane pivots) is a 21 percentloss in power and a 10 percent increase in SFC.
In large axial- compressors, overall efficiencylosses of 2 to 6 percent have been indicated^) .andtypical data are shown in Fig. 8 where compressorefficiency is plotted as a function of clearance toblade span ratio.
In turbines the blade tip clearances tend to belarge because of significant thermal growths andthermal transients. Tip clearance to blade height'ratios of 1 to 4 percent are common. The efficiencyloss, however, depends on the degree of turbine re-action because the pressure difference across thestage has a direct effect on.the amount of leakagethrough the tip clearance. Experimental efficiencyeffects for reaction and impulse turbines have beenpublished;(16,17) these data are shown in Fig. 9.In general high reaction turbines show about a3 percent loss in efficiency for an, increase in tipclearance of 1 percent of blade height; for impulseturbines the loss is about 1.6 percent for an in-crease in clearance of 1 percent of blade height.
Effect of Shroud Geometry
Stall margin is affected by both clearance andseal shroud geometry (casing treatment). The term"casing treatment" has been used when holes, slots,grooves, etc., are added to the shroud surface inthe region over the blade tips. Many geometrieshave been investigated.(3,18) jn general, it hasbeen found that stall margin can be improved byadding casing treatment if stall inception is atthe outer wall. Reference 19 attributes the im-provement to reduced downwash from the tip leakageflow. . .
Effect of Rub Interaction
As previously stated, clearance control is thecrux of the seal problem and generally, the sealingclearances change with each engine" condition. Rel-ative displacements of the seal components can begrouped into four categories:'^)
(1) Symmetric differential growth - This effectis principally due to thermal growth differences,but pressure and body forces are usually also sig-nificant. It is vital to consider not only thetotal growth but also the thermal response rate.Thus, the shroud cooling and mounting of the sealto the mating parts represents an integrated designproblem.
(2) Ovalization - An example is displacementdue to localized introduction of hot gas thatcauses asymmetric displacements. Also mountingpractice may allow case sag.
(3) Beam bending displacements - These occurfor various reasons; some are: rotor thermal bow,mounting, aeroelastic vibration of seal parts, andmaneuver loads. (Maneuver displacements can belarge.) .
(4) Machining tolerance buildup
In attempting to maintain close clearances,accumulation of adverse displacements causes rubs(usually localized). If no wear occurs to the
blade (or knife edge) and all wear occurs in theshroud, then only the local clearance increase isgenerated. If wear occurs to the blades (knife),then the clearance has increased around its fullannulus. Thus, an ideal situation would be zerowear to blades with all wear occurring in theshroud; a practical goal is 10 percent of the wearto the blade (knife) and 90 percent in the shroud;this can be termed a 10:1 rub tolerance.
Intimately associated with rub tolerance isthe thermal response of the seal system when a ruboccurs. This thermal response can be divided intotwo parts - an overall response and a local re-sponse .
The overall response is dependent on the rubseverity and the overall thermal expansion of thestatic and rotating parts. The differential expan-sions can lead to a self-destruct wear mode. Thatis, the rub itself generates enough heat to causethermal growth to increase the severity of the rub.This type of rub interaction has been responsiblefor a series of engine failures. Thus, the firstseal design criteria is avoidance of self-destructrub interaction.
It has been found that the wear, in stable rubinteractions, is dependent on the initial sealclearance. In some engines if the labyrinth sealis assembled with a small initial clearance, a rubinteraction is severe and produces significant wearthat results