EXPERIMENTAL SEALS

FOR A. G.

D. E.

INVESTIGATION

GASEOUS

OF

MECHANICAL

MEDIA

Domashnev, L. I. Tkach, Lazarev, and A. D. Gopius

UDC 62-762 (2 06.3). 001.5

Much work has been devoted to the investigation of m e c h a n i c a l s e a l s [1-4, ere.]. However, this published data applies only to m e c h a n i c a l s e a l s for iiquids and cannot be extrapolated to m e c h a n i c a l s e a l s for g a s e s . T h i s a r t i c l e p r e s e n t s the r e s u l t s of an investigation of m e c h a n i c a l s e a l s , operating in g a s e s , The a i m of the investigation was to c l a r i f y the r e l a t i o n s h i p of the p e r m e a b i l i t y K of the sealing p a i r s to a c o m plex of i n t e r r e l a t e d f a c t o r s , which d e t e r m i n e the efficiency of the seal a s s e m b l y . A s c h e m a t i c of the e x p e r i m e n t a l unit for this study is shown in Fig. 1. The unit is mounted on a verti. cal plate. A 0.7 kW de m o t o r , through a V - b e l t t r a n s m i s s i o n , d r i v e s the upper shaft 1 with yoke 5 and rotating ring i2 mounted on its lower end. Torque is t r a n s m i t t e d to the rotating ring by guide 4. Ring 12 is self-aligning with r e s p e c t to the s t a t i o n a r y ring due to the p r e s e n c e of the elastic gasket 15. Rubber ring 11 s e r v e s as the secondary seal. The lower ring 13 is mounted on the enlarged section of the lower shaft 19 which is located in bronze bushing 18 which, in turn, is mounted on ball b e a r i n g s 17. The g a s e o u s m e d i u m inside the seal is fed through nozzle 16. P r e s s u r e between the sealing rings is exerted through l e v e r 8 and weight 9. Hydraulic seal 2 p e r m i t s m e a s u r e m e n t of the leaking g a s and c o n t r o l of t e m p e r a t u r e of the contact p a i r . Leakage is m e a s u r e d by connecting a tube to nozzle 10 of the hydraulic seal. T h i s tube t e r m i n a t e s at the bottom of a m e a s u r i n g cylinder. The liquid column in the m e a suring cylinder is balanced by a t m o s p h e r i c p r e s s u r e and is displaced by g a s flowing through the hydraulic s e a l . Cooling w a t e r supply and outlet a r e through nozzles 3 and 14. Coefficient of f r i c t i o n was m e a s u r e d by a t e n s o m e t r i c circuit consisting of a single point PS1-02 p o t e n t i o m e t e r and a type T P - U t e n s o m e t r t c attachment. Wire t e n s o t r a n s m i t t e r s w e r e glued to a r m 7~ the s t r e s s was t r a n s m i t t e d by guide 6. !

9 tl r _13

1--6

t5 ~6

~2 ..... t

-

---t

-

0 42 ~0 g6 Fig. 1 F~.2 Fig. 1. Schematic of the e x p e r i m e n t a l unit.

~8

Fig. 2. C u r v e s of b e a r i n g s u r f a c e s : i) AG-1500-S05 and 2) KhlSN9T. T r a n s l a t e d f r o m Khimicheskoe i Neftyanoe M a s h i n o s t r o e n i e , No. 11, pp. 8-10, November, 1970. 1971 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever without permission of the publisher. A copy o[ this article is available from the publisher for $15.00.

899

Qcalc, cm3/h

dK~rcy Q' cm~/h

000

q, kg/cm2

"'

2so /~ 12,ol i ~

e44"

i

9 O. Io<:,-

0

0

0~5 1,0 /,Sv,m/sec

f,0

~5

2,0

~5 P2, kg~ m2

0

100

200

]00

oggQexp,

cma/h

Fig. 4

Fig. 3

Fig. 5 Fig. 3. Leakage Q and permeability K as a function of relative velocity v of sealing rings (Kh18N9TAG-1500-S05 pair): ) Q = f(v); . . . . ) K = f(v); 1) q = 0.91 k g / e m 2, P2 = 1.4 kg/cm2; 2) q = 1.5 kg/cm2, P2 = 2.2 kg/cm2; 3) q = 0.83 k g / c m 2, P2 = 1.0 k g / c m ~. Fig. 4. Lines of constant leakage for the pair Kh18N9T-AG-1500-505 at v = 0.65 m / s e c . Fig. 5. C o r r e l a t i o n curve of calculated and experimental values of leakage. The t e m p e r a t u r e gradient of the test pieces was m e a s u r e d by a six-point potentiometer, using C h r o r e e l - Copel t h e r m o c o u p l e s . The t e s t s were c a r r i e d out using the following pairs: metal (Khl8N9T) - graphite (AG-1500- S05 antifriction impregnated with lead and tin, UG-20F impregnated with furyl resin, and 4 K h 1 3 - 2 P - 1 0 0 0 F S impregnated with p h e n o l - f o r m a l d e h y d e resin) at sealed medium (nitrogen) p r e s s u r e of 0.5-5 k g / c m 2. Unit p r e s s u r e exerted on the sealing rings was v a r i e d f r o m 0.3 to 6 k g / c m 2. The outer and inner d i a m e t e r s of the contact rings were 50 and 35 ram, respectively. During the e x p e r i m e n t s the t e m p e r a t u r e of the pair was maintained constant (25-30~ to eliminate the t e m p e r a t u r e effect on leakage. T e m p e r a t u r e was maintained by cooling water which also s e r v e d as the hydraulic seal. The formula in [5] was used to c o r r e l a t e and explain the experimental results. This formula is derived f r o m D a r c y ' s law for i s o t h e r m a l gas flow in a porous medium. When leakage is m e a s u r e d in v o l u m e t r i c units at an external p r e s s u r e Pl this formula is

2~ hK (p~--p,)2 Q -

,

(1)

In ~ Pi where Q is the v o l u m e t r i c flow in cm3/sec, h is the nominal gap in the mechanical seal joint in cm, K is the permeability coefficient i n d a r c y s , Pl is the absolute external p r e s s u r e in k g / c m 2, P2 is the absolute p r e s s u r e of the fluid being seaied in k g / c m 2, ~ is the v i s c o s i t y of the fluid in cP, and r 1 and r 2 a r e the outer and inner radii of the rings in cm. Gap h in the mechanical seal joint was calculated, using the m i c r o r o u g h n e s s of the surface, the unit p r e s s u r e on the contact pair, and the mechanical p r o p e r t i e s of the p a i r m a t e r i a l s , by the formula [5] h= (R=~+R~2)(1-~8) The heights of m i c r o r o u g h n e s s of the sealing rings Rzl and Rz2 and relative approach e were d e t e r mined f r o m p r o f i l o g r a m s taken of the test pieces along and a c r o s s the friction path. The p r o f i l o g r a m s were taken using a p r o f i l o g r a p h - p r o f i l o m e t e r of the V ~ I - k a l i b r s y s t e m . A special device, mounted on the table of the instrument which rotated the test piece at 0.2 rpm about its own axis, was constructed to obtain p r o f i l o g r a m s along the friction path. A p r o f i l o g r a m with a 30 x horizontal enlargement could be taken using this device. Analysis of the p r o f i l o g r a m s showed that, a f t e r a period of running in, a roughness equal to a c l a s s 8-9 finish was established. This is c h a r a c t e r i s t i c for the selected pair m a t e r i a l s and friction conditions. Relative approach e was d e t e r m i n e d by the f o r m u l a [6] for elastic contact using a spherical model of roughness given in [5]. The coefficient u = ~l + u2, which depends on the shape and height distribution of m i c r o r o u g h n e s s , was determined using the c u r v e s of bearing s u r f a c e s shown in Fig. 2. The dashed lines in this figure r e p r e s e n t c u r v e s drawn f r o m the equation 900

TABLE 1

~,--bC,

l o,o I

kg/o

~. . . . . . . ......

I o,.

i

l

I ,,o o

I 0,,3410,,38[0.~4210,,4sl 0,,4,1 0,m] 0,,4910,,~0]o,152 04,1

where ~ is the relative a r e a of actual contact, b~ = 3.35, and ,2 = 3 for KhlSNgT steel, b I = 9.5 and v 1 = 3 for AG-1500-S05 graphite.

Some values of relative approach and gap as a function of unit p r e s s u r e for the contact pair Kh18. N 9 T - A G - 1 5 0 0 - 5 0 5 are given in Table 1. The r e s u l t s obtained f r o m the investigation of the effect of fluid p r e s s u r e , width of the sealing collar, and viscosity on leakage a r e adequately d e s c r i b e d by equation (1). E x p e r i m e n t s conducted with contact widths of 2.5, 5, and 7.5 m m showed that gas leakage is s o m e what lower with i n c r e a s e d contact width. On the basis of the proposed m e c h a n i s m of sealing [5] this can be explained by increased labyrinth length and resultant increased hydraulic resistance, in addition, wider contact increases power consumption and deviations from flatness. All the above factors must be considered in designing sealing assemblies and an optimum solution must be found. According to the proposed sealing mechanism one of the most important characteristics of the efficiency of the seal assembly is permeability K. Permeability is determined under laboratory conditions by direct measurement of the flow of liquid or gas of a certain viscosity across a unit area with pressure differential equal to unity. From_ e x p r e s s i o n (i) we get Q[tln r~ rl

Pl

K= The e x p e r i m e n t s showed that the mechanical p r o p e r t i e s of the contact pair, the p r e s s u r e on the contact pair, the s u r f a c e roughness, the p r e s s u r e of the fluid being sealed, and the relative velocity of the r i n g s have a g r e a t effect on the permeability. The effect of p r e s s u r e of the fluid being sealed on the permeability can be explained as follows: at low p r e s s u r e s the gas has insufficient energy to o v e r c o m e the hydraulic r e s i s t a n c e of the labyrinths and p o r e s which obstruct its passage so that no leakage o c c u r s , rt should be noted that no leakage here r e f e r s to the absence of a continuous s t r e a m . The gas m e t e r used was ng~ sensitive enough to m e a s u r e diffusion leakage. A c c u r a c y of leakage d e t e r m i n a t i o n was 5 cm3/h. At some higher p r e s s u r e ( p r e s s u r e on the contact pair kept constant;) the gas finds one or m o r e p a s s a g e s for leakage. Higher p r e s s u r e i n c r e a s e s the number of such p a s s a g e s resulting in i n c r e a s e d p e r m e a b i l i t y of the pair. Gas leakage as a function of relative velocity of the rings was experimentally investigated over a range of 0.1-2 m / s e e . Higher relative velocities resulted in some reduction of gas leakage due to the change in hydrodynamic conditions and the i n c r e a s e d hydraulic r e s i s t a n c e of the labyrinths. Experimental r e s u l t s can be approximated by a straight line at an angle to the a b s c i s s a (velocity). The angle is l a r g e r at g r e a t e r leakage (Fig. 3). Correspondingly at higher velocities the permeability of the pair d e c r e a s e s . However, this relationship is d e s c r i b e d by a family of parallel lines (Fig. 3). I n c r e a s e d p r e s s u r e on the contact pair at eonstant fluid p r e s s u r e sharply reduces the permeability of the seal joint. This i n c r e a s e c a u s e s a c l o s e r approach which, in turn, reduces the labyrinth a r e a inc r e a s i n g hydraulic r e s i s t a n c e . Lines of constant leakage, drawn f r o m the relationships of gas leakage vs p r e s s u r e of the fluid being sealed and permeability vs p r e s s u r e on the contact pair, a r e shown in Fig. 4. These lines can be used to determine the unit p r e s s u r e on the sealing pair r e q u i r e d to achieve a given leakage at different values of sealed fluid p r e s s u r e . The selection of this unit p r e s s u r e depends not only on the p r e s s u r e of the fluid being sealed but also on the m a t e r i a l s of the sealing rings. Such c u r v e s must t h e r e f o r e be derived for each combination of sealing m a t e r i a l s . The m i e r o g e o m e t r y of the sealing rings, their devtatior~ from flatness, as well as the method of their f a b r i c a t i o n and finishing, must meet the technical r e q u i r e m e n t s for finishing and fabricating sealing r i n g s f o r mechanical seals. The ratio of q/p~ at constant leakage d e c r e a s e s with i n c r e a s i n g fluid p r e s s u r e and unit p r e s s u r e on the sealing pair.

901

Experimental data confirm, with adequate accuracy, the calculated relationships obtained ia [5] based on the assumption of gas flow in a porous medium. The deviation was • 15~c (Fig. 5). Permeability is a constant parameter for a given contact pair with known microgeometry under fixed operating conditions and can be used to classify sealing pairs. L ITERATURE 1o

2. 3. 4. 5. 6.

902

CITED

A. I. Golubev, Modern Seals f o r Rotating Shafts [in Russian], Mashgiz, Moscow (1963). L. I. Mamon, M. A . Lokshin, and G. L Shkurupii, Koks i khimiya, No. 9 (1964). D. F. Denny, Wear, 4, No. 1 (1961). E. Mayer, Machine Design, 32, No. 5 (1960). L. I. Tkach and A. D. D o m a s h n e r , Khim. i Neft. M a s h i n o s t r . , No. 11 (1968). N. B. Demkin, in the s y m p o s i u m : New D e v e l o p m e n t s in the T h e o r y of F r i c t i o n [in Russian], Nauka, Moscow (1966).