Rotor whirl stabilization of high performance turbomachinery which operates at supercritical speed is discussed. Basic whirl driving forces are reviewed. Stabilization and criteria are discussed. Damping seals are offered as a solution to whirl and high vibration problems. Concept, advantages, retrofitting, and limits of damping seals are explained. Dynamic and leakage properties are shown to require a rough stator surface for stability and efficiency. Typical seal characteristics are given for the case of the high pressure oxidizer turbopump of the Space Shuttle. Ways of implementation and bearing load effects are discussed

Vonpragenau, G. L.

English

NASA Technical Reports Server

DAMPING SEALS FOR TURBOMACHINERY 
George L. von Pragenau 
Systems Dynamics Laboratory 
NASA Marshall Space F l igh t  Center 
Huntsvil le ,  Alabam 35812 USA 
Abstract  
The paper addresses r o t o r  whir l  s t a b i l i z a t i o n  of 
high performance turbomachinery which o p w a t e s  a t  
s u p e r c r i t i c a l  speed. Basic whir l  drivzng fo rces  a r e  
reviewed. S t a b i l i z a t i o n  and c r i t e r i a  a r e  discussed. 
Damping s e a l s  a r e  offered a s  a so lu t ion  t o  whir l  and 
high v ib ra t ion  problems. Concept, advantages, r e t ro -  
f i t t i n g ,  and l i m i t s  of damping s e a l s  a r e  explained. 
IDynamic and'leakage p roper t i e s  a r e  shown t o  requ i re  a 
rough s t a t o r  surface  f o r  s t a b i l i t y  and e f f i c iency ,  
Typical s e a l  c h a r a c t e r i s t i c s  a r e  given f o r  the  case of 
t h e  high pressure oxidizer  turbopmp of t h e  Space 
Shutt le .  Ways of implementation and bearing load 
e f f e c t s  a r e  discussed. F e a s i b i l i t y  was f i r s t  proven 
a n a l y t i c a l l y  i n  a Center Director  Discret ionary Furad 
pro jec t  i n  f i s c a l  year 1981, The Rocketdyne Division 
o f  R o c b e l l  In ternat ional  has s ince  i n s t a l l e d  such 
s e a l s  i n  t h e  high pressure oxidizer  tu rbopup  f o r  
development t o  r a i s e  the  speed lhit and thus achieve 
higher t h r u s t  f o r  t h e  Space Shu t t l e  Main Engine. 
Introduct  ion 
High performance turbomchinery operates super- 
c r i t i c a l l y ,  i , e .  with speeds above r o t o r  resonances 
( c r i t i c a l  speeds). Thus more power is  being packed 
i n t o  lightweight u n i t s  unless speed must be l imi ted  t o  
prevent a r o t o r  from whirl ing and causing d 
speed l i m i t  challenged many a s  w e l l  a s  t h e  writer t o  
f ind  a solut ion,  not  without earnes t  prayer (whqh God 
answereq), and not without the  foundation layed by 
o the rs  such a s  D r .  Henry Black from Scotland, who i s  
w e l l  remembered . *I-4 
* Deceased January 17, 1980. 
https://ntrs.nasa.gov/search.jsp?R=19850018583 2020-03-20T19:06:49+00:00Z
Constant Whirl Forces 
mirl i s  an o r b i t a l  motion, superimposed upon t h e  
r o t a t i o n  of a ro tor .  Whirl fo rces  a r e  c o n c e p t a l l y  
known, but accurate data  a r e  not r ead i ly  ava i l ab le ,  
Whirl forces  (Fig. 1) a r e  constant  o r  increase with 
r o t o r  motion. 
Synchronous constant  forces  a r e  caused by mass 
unbalance, runout rubbing, and s e a l  runout. Subsyn- 
chronous constant fo rces  a r e  generated by r o t a t h g  
Coulomb f r i c t i o n  fo rces  i n  r o t o r  jo in ts .5  The con- 
s t a n t  fo rces  produce a forward whirl  l i m i t  cycle  
response a t  synchronous o r  subsynchronous frequencies,  
Well balanced r o t o r s  with minute runout y i e l d  low 
vibra t ions ,  however, Coulorab f r i c t i o n  i n  r o t o r  j o i n t s  
can cause damging r o t o r  v ibra t ions .  Coulomb f r i c t i o n  
whirl  can be reduced with lubr ica t ing  t h e  r o t o r  j o i n t s  
and spl ines .  The whirl  can be avoided wi th  t i g h t  but 
nonlubricated in ter ference  f i t s ,  J o i n t  locklng with 
a high r o t o r  mbalance  i s  not  recomended. 
Proport ional  Whirl Forces 
Increasing whir l  forces  can be most des t ruct ive .  
The simplest representa t ion of t h e  forces  is  a propor- 
t i o n a l  r e la t ionsh ip  which couples r o t o r  de f lec t ions  by 
skewed-symmetric matrices t o  t ransverse  whir l  forces ,  
Rotor h y s t e r e s i s  ,6 s e a l  gaps, 1 journal  bearings, 7 tur-  
bine blade bypass ,8 and impeller in te rac t ions  d r i v e  
usually subsynchronous forward whirls .  External r o t o r  
rubbing d r ives  a backward whir l  t h a t  f o r  hard rubbing 
can become supersynchronous (Fig, 1). 
Rotor S t a b i l i z a t i o n  
The purpose of r o t o r  s t a b i l i z a t i o n  is  t o  negate 
t h e  whirl  regenerat ion of proport ional  forces  while 
reducing t h e  whir l  l i m i t  cycle caused by constant  
forces.  Rotor suspension damping, s t i f f n e s s ,  and 
asymmetry a r e  whirl  s t a b i l i z e r s  ordered a f t e r  t h e i r  
ef f ic iency.  Natural damping is  usually too low t o  
prevent whirl  i n  high pressure  applicat ions.  There- 
fo re ,  squeeze f i l m  dampers a r e  used i n  tmbomchinery  

not only f o r  v ib ra t ion  a t tenuat ion but a l s o  f o r  whir l  
prevention.4 However, such dampers a r e  unsuitable f o r  
cryogenic appl ica t ions .  The d i f f i c u l t i e s  prompted the  
development of damping seals .  9 
Damping s e a l s  a r e  l i k e  annular sea l s ,  except f o r  
rough inner surfaces on s t a t o r s  (Fig. 2). Annular 
s e a l s  a r e  a f i r s t  s t e p  and were implemented t o  elimin- 
a t e  whirl  from the  high pressure turbopump (NPFTP) of 
the  Space Shu t t l e  Main Engine (SSME).2,3 However, 
such s e a l s  can d e s t a b i l i z e  a ro to r  resonance below 1/2 
t h e  speed frequency. Damping s e a l s  improve t h i s  r a t i o  
t o  < 114, reduce leakage, and increase damping. Damp- 
ing s e a l s  were recent ly  i n s t a l l e d  i n  a high pressure 
oxidizer  turbopump (HPOTP) f o r  SSME development a s  
shown i n  Figure 3. 
S t a b i l i t y  Cr i t e r ion  
The s t a b i l i t y  condit ion of a s ing le  mass r o t o r  
(Fig. 4) s u f f i c e s  t o  examine the  s e a l  e f fec t .  Also, 
more complex models a r e  represented, considering one 
resonant mode t h a t  i s  only negl ig ib ly  detuned by the  
sea l .  Equation (1) descr ibes  the  i n t e r n a l  s e a l  whir l  
e f f e c t  with the  whir l  f a c t o r  Q proport ional  t o  the  
s e a l  damping C. The Couette f a c t o r  c of equation (2) 
i s  the  r a t i o  of c i rcumferent ia l  bulk flow veloci ty  v 
t o  r o t o r  surface  ve loc i ty  w. il is  the  r o t o r  speed i n  
rad / s ,  Equation (3) is  the  c h a r a c t e r i s t i c  equation of 
t h e  model i n  Figure 4. Conformal mapping, 10 subsititut- 
ing with equation ( I ) ,  and reordering produce the  
inequal i ty  (4) which s e t s  t h e  speed l i m i t .  Ky and KZ 
a r e  the  l a t e r a l  suspension s t i f f n e s s e s  i n  y and z ,  ms 
is  the  r o t o r  mass, and s is t h e  Laplace operator .  R 1  
i n  equation (5) def ines  the  f i r s t  c r i t i c a l  speed o r  
resonance. 
LEAKAGE A 
NOTE: 
SEAL STATOR PART 
TRIANGULAR 
POCKETS FOR 
ROUGH STATOR 
CLEARANCES 
EXAGGERATED 
SEAL ROTOR PART 
- LOW LEVEL CIRCULATION 
(COUETTE FLOW) 
WINDERS ROTOR WHIRL 
ROTOR 
Figure  2 .  Damping Sea l  Concept. 
Figure 3. HPOTP Damping Seal  R e t r o f i t .  
ROTOR WlTH SPEED 52 
AND MASS MS 
STATOR 
GAP WlTH COUETTE FLOW 
VELOCITY V 
Figure 4. Simple Rotor Model. 
The s t a b i l i t y  l i m i t  of inequal i ty  (4) is  high f o r  
low c values, high r o t o r  resonances, and asymmetric 
r o t o r  suspensions. Asymmetry can be coincidenta l  
which makes symmetry a s a f e r  assumption f o r  predic t ing 
speed l i m i t s .  External whir l  e f f e c t s ,  such as turbine  
blade bypass, can be lumped i n t o  Q by r a i s i n g  c. Con- 
versely,  ex te rna l  damping, such a s  from bearings, can 
be lumped i n t o  C by reducing c t o  maintain the  same Q. 
m u s  external  e f f e c t s  a r e  equivalently represented 
i n t e r n a l l y  by t h e  Couette f a c t o r  c which becomes a 
measure of s t a b i l i t y  f o r  comparing seals and other  
r o t o r  dynamic components. 
Flow F r i c t i o n  
The flow v e l o c i t i e s  r e l a t i v e  t o  the  r o t o r  and the  
s t a t o r  d i f f e r  because of ro ta t ion .  The v e l o c i t i e s  
describe bulk flow so  turbulent  flow can be t r ea ted  
s imi la r  t o  p ipe  ,flow. Equations ( 6 )  and (7) describe 
shear stresses T, and T, respect ive  t o  the  r o t o r  and 
s t a t o r .  f r  and f s  a r e  respect ively  t h e  r o t o r  and 
s t a t o r  surface  f r i c t i o n s  which general ly d i f f e r .  u is 
the  a x i a l  bulk flow ve loc i ty  and p is the  f l u i d  mass 
density.  The s e a l  gap pressure gradients  a r e  given i n  
equations (8) and (9) f o r  the  a x i a l  and c i rcumferent ia l  
d i rec t ions  x and q respectively.  h is  t h e  gap thick- 
ness, p the  pressure,  and t t h e  t i m e .  Note the-pro-  
j ec t ions  of t h e  shear s t r e s s e s  from equations ( 6 )  and 
(7). The Reynolds numbers Rr and Rs i n  equations (10) 
and (11) a r e  taken i n  the  respect ive  bulk flow direc-  
t i o n s  of t h e  r o t o r  and the  s t a t o r .  p is t h e  f l u i d  
v iscos i ty .  The respect ive  r o t o r  and s t a t o r  f r i c t i o n  
f a c t o r s  f r  and f of equations (12) and (13) a r e  
Hoody" approximation with 2h a s  hydraul ica l ly  equiva- 
Lent pipe diameter D . l l  The e f f e c t i v e  roughnesses kr 
and ks reach pocket depth values f o r  wel l  c i r c u l a t i n g  
pockets. The t r i angu la r  pockets of Figure 2 should be 
bes t  f o r  obtaining high ks values. 
Couette Flow 
The r o t o r  generates a Couette flow which is  
es tabl ished downstream of the  s e a l  i n l e t  according t o  
equation (14) which follows from equation (9) f o r  t h e  
case  of neg l ig ib le  c i rcumferent ia l  pressure gradients  
and flow accelera t ions .  Equations (15) and (16) a r e  
t h e  asymptodes f o r  u = 0 and u = which enclose t h e  
Couette f a c t o r  c f o r  a l l  surface f r i c t i o n  r a t i o s .  
Figure 5 shows c a s  a function of surface  f r i c t i o n  
r a t i o  and t h e  a x i a l  ve loc i ty  r a t i o  ulw. c is low f o r  
high s t a t o r  f r i c t i o n .  The f r i c t i o n  r a t i o  is approxi- 
mately t h e  cubic roo t  of t h e  roughness r a t i o ,  a s  the  
equations (12) and (13) indicate .  Theref ore ,  l a r g e  
roughness r a t i o s  a r e  required. 
Figure  5. Couette fac to r  c = v / w  versus the  
f r i c t i o n  and axial veloci ty  r a t i o s .  
namic Seal Parahileters 
The i n t eg ra l  dynamic e f f e c t  of a s ea l  i s  described 
by s t i f f n e s s  K, damping 6 ,  m a s  m, trhir1 coupling 
en@\ and gyro coupling cQm8 as shorn i n  vector equa- 
t ion  (17), 
The deta i led  der ivat ions  a r e  given i n  reference 9, 
except f o r  minor d i f ferences  i n  rearranging frequency 
square terms herein,  Some p a r t s  of the der ivat ion a r e  
repeated f o r  cont inui ty ,  Equations (18) through (22) 
define the dynamic parameters and t h e i r  response t o  
the whirl frequency a, The time constants a r e  sm81, 
The basic s t i f f n e s s  KO, damping Go, and induced mass 
are. given i n  equations ( 2 3 )  through (25) f o r  i n l e t  
l o s s  f l ;  r e s i s t i v i t i e s  F, G, and H; dimensions PI, L, 
and r of s ea l  gap, s ea l  length,  and s e a l  radius ;  a x i a l  
veloci ty  u; and mass density p. The r e s i s t i v i t i e s  a r e  
given by the eqmtions (26) through (32). The time 
constants are defined by the equations (33) through 
(36) * 
2  KO = (nr /h) u (l+f TL (Fm) /2 (23) 
c = (nr /h) ( T / ~ ~ ) L ~  (F*) / l 2  (24 )  
The flow induced masses i n  equation (17) a r e  
dip01 masses t h a t  a r e  neg l ig ib le  agains t  the  r o t o r  
mass. The whirl  coupling is indeed proport ional  t o  
the  Couette f a c t o r  c a s  shown e a r l i e r  i n  equation (1) .  
Table 1 gives the  dimensions of d iverse  i n l e t  and out- 
l e t  s e a l s  of t h e  HPOTP preburner pump impeller (Fig. 
3). Table 2 l ists  the  a n a l y t i c a l  r e s u l t s  f o r  laby- 
r i n t h ,  damping, and smooth annular sea ls .  Table 2 
shows t h a t  the  leakage r a t e s  of labyr in th  and damping 
s e a l s  a r e  comparable, while t h e  smooth annular s e a l s  
leak 1.6 t i m e s  more. Tests  by Rocketdyne produced 
the  same r a t i o .  The whirl  r a t i o  of damping s e a l s  is 
< 1/4 but 0 . 5  f o r  the  o ther  sea l s .  Damping s e a l s  have 
the  highest  damping while the  smooth annular s e a l s  
show the  highest  s t i f f n e s s .  
Table 1. WOTP Preburner Pump Seal  Dimensions f o r  
I n l e t  and Outlet a t  F u l l  Power 30,367 RPM 
PBP Seals  Gap Diam. Leng. Pocket Press 
Dimension m i l s 1  inch inch m i l s  2 p s i  3 
I n l e t  Laby. 6 2.85 0.63 230 2006 
I n l e t  Damp. 6 2.898 0.66 lO(20) 2006 
I n l e t  Smooth 6 2.898 0.66 0.032 2006 
Outlet  Laby. 4 3.345 0.87 80 6505 
OutletDamp. 4 3.3371 0.88 lO(20) 6505 
Outlet  Smooth 4 3.3371 0.88 0.032 6505 
NOTE: 1 = r a d i a l  gap, 2 - e f f e c t i v e  pocket depth 
(ac tual  depth), 3 = pressure d i f ference .  
Table 2. HPOTP Preburner Pump Sea l s '  Dynamic f o r  
I n l e t  and Ou t l e t  a t  F u l l  Power 30,367 RPM 
PBP Sea l s  S t i f .  Whirl Damp. Rat io  Leak 
Dimension Klb l in  Klb l in  Lbs/ in  c % Lb/s 
I n l e t  Laby. 2 2 1 50 4.94 
I n l e t  Damp. 106 46 5 7 23 5.18 
I n l e t  Smooth 151 105 53 50 7.90 
Ou t l e t  Laby. 20 36 23 50 5.13 
Ou t l e t  Damp. 447 281 388 2 2 5.09 
Out le t  Smooth 935 544 302 5 0 8.51 
NOTE: % r a t i o  t imes maximum speed must be below 1st 
c r i t i c a l  speed. Analysis  i s  f o r  0.25 i n l e t  
l o s s ,  2.593-8 ps i - s  l o x  v i s c o s i t y ,  and 
1.05E-4 p s i ( s ~ i n . ) ~  l o x  dens i ty .  
Sea l  I n s t a l l a t i o n  
Figure 3 i l l u s t r a t e s  a r e t r o f i t t i n g  c a s e  where 
i n l e t  and o u t l e t  l a b y r i n t h  s e a l s  w e r e  rep laced  by 
damping s e a l s  t o  i n h i b i t  a 450 Hz w h i r l  a t  30,000 
RPM. Damping seals f i t  always i n t o  t h e  space of 
l a b y r i n t h  seals. Leakages a r e  comparable, bu t  s t i f f -  
n e s s  and damping are much h igher  as seen  i n  Table 2. 
Both changes i n h i b i t  wh i r l ,  lower v i b r a t i o n s ,  and 
inc rease  c r i t i c a l  speed. Damping seals seem t o  be 
more e f f e c t i v e  than squeeze f i l m  dampers, s impler ,  and 
smaller. Damping s e a l s  are suggested in s t ead  of 
smooth annular  s e a l s  t o  reduce l e a k s  and t o  suppress  
whir 1. 
The h igh  s t i f f n e s s  of damping seals must be, 
however, considered when r ep lac ing  l a b y r i n t h  s e a l s .  
E c c e n t r i c i t i e s  between bea r ings  and s e a l s  can cause 
unexpected s i d e  loads.  
So f t  Sea l s  
The s i d e  loads  can be  reduced wi th  small eccen- 
t r i c i t i e s ,  i f  t i g h t  t o l e r ance  c o s t s  are n o t  prohib i -  
t i v e ,  So f t  s e a l s  a r e  another  concept worth 
considering. Damping s e a l s  become s o f t e r  as rough- 
ness is increased. Axial flow is thus reduced and t h e  
f l u i d  compressibi l i ty much enhanced. Another way is  
the inse r t ion  of a labyr in th  s e a l  downstream t o  shorten 
a damping s e a l  without changing leakage. 
Hard Seals  
The hard s e a l  approach takes  advantage of high 
s e a l  s t i f f n e s s  and uses the  s e a l s  f o r  supporting t h e  
ro to r .  C r i t i c a l  speeds a r e  so  increased and loads of 
pump and turbine  discharges a r e  shared by t h e  sea l s .  
Tight to lerances  ( e c c e n t r i c i t i e s )  can be circumvented 
by loose  f i t t i n g s  f o r  the  bearings '  outer  races. The 
bearings support the  r o t o r  a t  s t a r t u p  and cutoff  when 
the s e a l s  a r e  unpressurized o r  s o f t .  A t  operat ion 
speed t h e  s e a l s  a r e  s t i f f  and can take  over. Good 
bearing l i f e  i s  expected because the  bearings f l o a t  
mostly during t h e i r  operat ion i n  a loadless  condition. 
Outer race  spinning due t o  f l o a t i n g  can be prevented 
with a n t i r o t a t i o n  devices. 
Conclusion 
Damping s e a l s  a r e  new technology f o r  improving 
turbomachinery operations. The shown proper t i e s  
encourage many appl ica t ions  from r e t r o f i t t i n g  machines 
t h a t  a r e  beset  by whir l  problems t o  new and more 
e f f i c i e n t  designs. We a r e  g r a t e f u l  and do not  take 
t h i s  progress f o r  granted - be blessed t h e  same way. 
References 
1, Black, H. F.: Ef fec t s  of Hydraulic Forces i n  
Annular Pressure Seals  on t h e  Vibrations of Cen- 
t r i f u g a l  Pump Rotors. Journal M.E.Sc. vol .  11, 
no. 2, p. 206, 1969. 
2, Childs, D. W. : The Space Shu t t l e  Main Engine 
High-pressure Fuel Turbopwnp Rotor-Dynamic 
I n s t a b i l i t y  Problem. Trans. ASME, Journal of 
Engineering f o r  Power, vol.  100, p. 48, January 
1978. 
3. Ek, M. C.: Solving Subsynchronous Whirl i n  t h e  
High-Pressure Hydrogen Turbomachinery of t h e  
SSME. AIAA Journa l  of Spacecraf t  and Rockets, 
vo l .  17, no. 3 ,  p. 208, May-June 1980. 
4. Ehrich, F. F. and Childs ,  D. W.: Self-Excited 
Vibra t ion  i n  High-Performance Turbomachinery. 
Mechanical Engineering vol .  106, no. 5, p. 66, 
May 1984. 
5. Kimball, A. L.: I n t e r n a l  F r i c t i o n  Theory of 
Shaf t  Whirling. General E l e c t r i c  Review, vol .  
27, no. 4, p. 244, A p r i l  1924. 
6. B u c c i a r e l l i ,  L. L.: On t h e  I n s t a b i l i t y  of Rotat- 
ing  Sha f t s  due t o  I n t e r n a l  Damping. Journa l  of 
Applied Mechanics, vo l .  49, p. 425, June 1982. 
7. Newkirk, B. L., and Taylor,  H. D.: Shaf t  Whipping 
due t o  O i l  Action i n  Journa l  Bearing. General 
E l e c t r i c  Review, vol .  28, p. 559, 1925. 
8 .  Alford,  J.: P r o t e c t i n g  Turbomachinery from Self-  
Exci ted Rotor Whirl. ASME Transac t ions ,  Jou rna l  
of Engineering f o r  Po.wer, p. 333, October 1965. 
9. von Pragenau, G. L.: Damping Sea l s  f o r  Turbo- 
machinery. NASA Technical  Paper no. 1987, March 
1982. 
10. Nyquist ,  H.: Regeneration Theory. B e l l  Systems 
Technical  Journa l ,  vo l .  11, p. 126, January 1932. 
11. Moody, L. F.: F r i c t i o n  Fac to r s  f o r  Pipe Flow. 
Trans. ASME, New York, p .  671, November 1944. 


Damping seals for turbomachinery

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