D E P E N D E N C E O F T H E R E L I A B I L I T Y O F AN

E X H A U S T V A C U U M S Y S T E M ON I T S

O P E R A T I O N R E G I M E

M. G . S h a k i r o v , F . D . P u t i l o v s k i i , a n d V. V. M e d v e d e v

UDC 621.52:62-19

T h e r e a r e va r ious exhaust vacuum s y s t e m s which p e r m i t one to obtain one degree or another of work- ing vacuum. We shal l examine the fundamental s c he m e s of some of them, whose e lements a r e common to mos t vacuum s y s t e m s . In Fig. 2 we give s c he m e s for vacuum s y s t e m s which p e r m i t one to obtain r e s idua l p r e s s u r e s f rom 1 �9 1 0 - 2 to 1 ' 10 -10 m m Hg.

F r o m this f igure i t is evident that the h igher the vacuum, the g r e a t e r the number of e l ements in the vacuum s y s t e m bywhose use it is obtained. While it is n e c e s s a r y to have only a mechan ica l vacuum pump and a vacuum valve, in the gene ra l case , to obtain a r e s idua l p r e s s u r e of 1 -10 -2 mm Hg, in o r d e r to ob- tain a r e s i d u a l p r e s s u r e down to 1 . 1 0 -20 mm Hg, it is n e c e s s a r y to have, be s i de s these, e i t he r a t ech- no log ica l ly complex and expensive m o l e c u l a r vacuum pump or a diffusion pump, a t rap , a magnet ic e l e c -

t r i c a l d i s c h a r g e pump, and two vacuum va lves .

In p r ac t i ce the following laws for d i s t r i bu t ion of a random valve a r e encountered : the exponential , no rmal , l o g a r i t h m i c a l l y no rma l , and Weibull law. Since only the qual i ta t ive s ide of the question is being cons ide r ed in the p r e s e n t a r t i c l e , for convenience in expos i t ion and g r a p h i c n e s s al l the d i s cus s ions a re c a r r i e d out for the case where the p r o b a b i l i t i e s of f a i l u r e - f r e e opera t ion of al l e l emen t s a r e d i s t r ibu ted by an exponent ia l law. In our case the random quantity i s the t ime of f a i l u r e - f r e e operat ion, t 0. In ac - co rdance with the exponent ia l law, we wr i t e the p robab i l i ty of f a i l u r e - f r e e opera t ion , P, in the fo rm

P(to) =exp (_~.t o) ' (I)

where A is the in tens i ty of f a i l u r e s .

Of course , in each spec i f i c case eve ry e l emen t wil l have i ts own d i s t r i bu t i on law; however, the qual i - ta t ive p ic tu re is not changed thereupon. It should be noted that the more complex a s y s t e m is, and the more e l emen t s it contains , the l e s s is i ts p robab i l i ty of f a i l u r e - f r e e opera t ion . We sha l l examine this in one of the e x a m p l e s .

We se t up the s t r u c t u r a l s chemes for r e l i ab i l i t y of vacuum s y s t e m s which a r e shown in Fig . 1. The s t r u c t u r a l s cheme of r e l i a b i l i t y is the g raph ica l ly r e p r e s e n t e d in te rdependence of e l emen t s in conformi ty with the effect of fa i lu re of individual e l emen t s on the r e l i a b i l i t y of the a r t i c l e . It wil l be cons i s ten t if the vacuum s y s t e m is so cons t ruc t ed that p r o p e r ac t ion of a l l i t s e l ements is r e q u i r e d for succes s fu l function- ing. S t ruc tu ra l s c h e m e s of r e l i a b i l i t y of vacuum s y s t e m s a re shown in F ig . 2.

The p robab i l i ty , P, of f a i l u r e - f r e e opera t ion of a sequent ia l s y s t e m for a t ime t o is defined by the

fo rmula

P(to) =PI'P2"P3...P,, (2)

where P1, P2, P3, �9 �9 �9 Pn a re the p r o b a b i l i t i e s of f a i l u r e - f r e e opera t ion of the e l emen t s which en te r into the vacuum s y s t e m . If we take the t ime of opera t ion of one of i ts e l emen t s for the t ime of opera t ion of the whole s y s t e m and wr i te down P1, P2 . . . . , Pn f rom fo rmula (1), then the p robab i l i t y of f a i l u r e - f r e e o p e r a -

t ion of the s y s t e m wil l have the following fo rm:

T r a n s l a t e d f rom Khimicheskoe i Neftyanoe Mash inos t roen ie , No. 10, pp. 33-35, O c t o b e r , 1974. "

�9 19 75 Plenum Publishing Corporation, 22 7 West 17th Street, New York, N. Y. 10011. No part o f this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission o f the publisher. A copy o f this article is available from the publisher for $15.00.

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a

. 7

c

E

d

( - 3 z " - - 4 J 2 3' 1 . / / /

( -J , 5 _ _

F i g . 1

-ZI"

e

F i g . 1.

a)

vacuum ~vzIve

2

b)

c)

d)

F ig . 2

S c h e m e s of v a c u u m s y s t e m s which p e r m i t one to ob t a in v a r i o u s r e s i d u a l p r e s s u r e s : a) down to 1 "10 -2 m m Hg; b) down to 1 �9 10 -3 m m Hg; c) down to 1 �9 10 m m Hg; d) down to 1 �9 10 -1~ m m Hg; e) down to 1 �9 10 -~~ m m Hg; 1) m e c h a n i c a l v a c u u m pump; 2) v a c u u m v a l v e ; 3) v o l u m e be ing p u m p e d out; 4) m a n o m e t e r ; 5) d o u b l e - r o t o r v a c u u m pump; 6) d i f fu s ion pump; 7) v a c u u m v a l v e ; 8) m o l e c u l a r v a c u u m p u m p ; 9) t r a p ; 10) m a g n e t i c e l e c t r i c a l d i s c h a r g e p u m p ; 11) p o w e r s u p p l y b l o c k f o r m a g n e t i c e l e c t r i c a l d i s c h a r g e p u m p .

F i g . 2. S t r u c t u r a l s c h e m e s fo r r e l i a b i l i t y of v a c u u m s y s t e m s which p e r m i t one to ob ta in v a r i o u s r e s i d u a l p r e s s u r e s : a) down to 1 . 1 0 -2 r a m ; b) down to 1 "10 -6 m m Hg; c) down to 1 . 1 0 -1~ m m Hg; d) down to 1 �9 10 - l ~ m m Hg; Xi) i n t e n s i t y of f a i l u r e s of i - t h o r i : t h p a r t which e n t e r s in to a g iven e l e m e n t of the v a c u u m s y s t e m ; ni) n u m b e r of uni t p a r t s ; t 0) o p e r a t i o n t i m e of e l e m e n t ; 1) m e c h - a n i c a l v a l v e ; 2) v a c u u m v a l v e ; 3) d i f f u s i o n p u m p ; 4) v a c u u m s e a l ; 5) m o l e c u l a r pump; 6) t r a p ; 7) m a g n e t i c - d i s c h a r g e p u m p .

P(to~-exp(~k~to)exp(--),eto)...exp(--knto)=exP[-- to(kl+L,,+ka+...+ )-n)] exp --t:~ ' (3)

F r o m E q s . (2) and (3) i t i s ev iden t tha t the m o r e e l e m e n t s t h e r e a r e in a v a c u u m s y s t e m , the l e s s i s the p r o b a b i l i t y of i t s f a i l u r e - f r e e o p e r a t i o n . It was shown above that the h i g h e r the va c uum, the m o r e e l e - m e n t s e n t e r in to the s y s t e m by which th i s v a c u u m i s a t t a i n e d . Consequen t ly , the c o n c l u s i o n m i g h t be d r a w n tha t the h i g h e r the v a c u u m which i s g iven by a s y s t e m , the l e s s i s the p r o b a b i l i t y of f a i l u r e - f r e e o p e r a t i o n of th i s s y s t e m . H o w e v e r , t h i s i s not c o m p l e t e l y so, b e c a u s e the r e l i a b i l i t y of o p e r a t i o n of v a c u u m s y s t e m e l e m e n t s such as v a c u u m p u m p s d e p e n d s on the o p e r a t i o n r e g i m e , and the o p e r a t i o n r e g i m e v a r i e s wi th

c h a n g e in p r e s s u r e .

L e t us a n a l y z e the d e p e n d e n c e of the r e l i a b i l i t y of a m e c h a n i c a l pump, a d i f fu s ion pump, a m o l e c u l a r v a c u u m pump, and a m a g n e t i c e l e c t r i c a l d i s c h a r g e pump which e n t e r in to a v a c u u m s y s t e m on t h e i r o p e r a - t ion . Wi th th i s o b j e c t i v e we e x a m i n e fo r e a c h of t h e m the d e p e n d e n c e of the o p e r a t i o n of the e l e m e n t s which p r e d e t e r m i n e r e l i a b i l i t y on the d e g r e e of v a c u u m .

tn a m e c h a n i c a l v a c u u m pump, the e l e m e n t s which p r e d e t e r m i n e i t s r e l i a b i l i t y a r e the d r i v e , the r o t o r , the pump cups , and the v a l v e s . H o w e v e r , i t is to be noted tha t a change in d e g r e e of v a c u u m does not e x e r t an e f f ec t on the o p e r a t i o n of the cup, t h e r e f o r e the cup, s r e l i a b i l i t y d o e s not depend on the d e - g r e e of v a c u u m . As the l i m i t i n g r e s i d u a l p r e s s u r e i s a t t a ined , a v e r y s l i g h t amoun t of gas e n t e r s into the

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pump. Under these conditions, the space under the d ischarge valve turns out to be essent ial ly under vac- uum until it is filled with oil. Thereupon the gas which might play the role of a damper between the oil and valve is lacking in this space. But since the oil is pract ica l ly incompress ib le and the valve has a cer ta in moment of inertia, the p r e s s u r e under the valve can be ra ther large, and in some cases it can reach s e v e r - al tens of k i lograms per square cen t imer [1]. : There fore a sudden hydraulic shock takes place in these ele- ments and, as a result , a reduction in the reliabil i ty of valve operation.

An increase in pump reliabil i ty is achieved by feeding a smal l amount of controlled air into the com- p ress ion side; this plays the role of damper between the oil and valve [2]. The impairment in limiting residual p r e s s u r e caused thereby is slight and essent ial ly does not affect the pump cha rac te r i s t i c s .

For a mechanical pump, the work expanded in the p rocess of polytropic compress ion can be written in the following form:

x - - 1 [ ] x : p ~ - 1

where Pl and Pa are, respect ively, the p r e s s u r e s in the pump inlet and outlet; V s is the volume of the pump chamber ; and x is the polytropie compress ion coefficient. The dependence of cycle operation on pump in- take p re s su re , Pl, has a maximum, which is reached when Pl = P a / x x / x - l .

It is known [3] that the curve for dependence of the power required by the pump motor on pressure , W = f(p), has a maximum in the p re s su re region 200-300 ram, and is reduced when the vacuum becomes higher . The minimum power required by the motor is in the region of the (limiting) residual p r e s su re of the pump. Consequently, with increase in vacuum the pump operation regime is lightened, and the units and par t s which make up the drive and ro tor begin to operate with s t r e s s e s which are considerably less than nominal. There fo re the probabil i ty of their failure is reduced, and consequently their reliabil i ty r i ses . F rom this it follows that, in the general case, the reliabil i ty of mechanical vacuum pumps r i ses with increase in vacuum.

The elements of a diffusion pump which predetermine its reliability are the working liquid (the oil), the heater , and the cooling jacket. The magnitude of the vacuum does not affect the operation of the hea ter or cooling jacket, therefore their reliabil i ty does not depend on the degree of vacuum. The element which determines the dependence of reliabil i ty of diffusion pump operation on degree of vacuum is the working liquid. If the pump is pumping out large gas s t r e ams (especially air), then scorching (oxidation) of the working liquid will take place; it changes its proper t ies , it cokes, and carbon scale appears in the vapor line. All this impai rs the pump operation, and may cause a fai lure.

In the h igh-vacuum region, at a p r e s s u r e close to the limiting res idual p ressure , gas s t r eams are small , scorching or coking of the oil does not occur, and the re l iabi l i ty of operation is increased .

The elements which prede te rmine rel iabil i ty in molecular vacuum pumps are the drive, the o i i s y s - tem, and the ro tor . The magnitude of the vacuum does not affect the operat ion of the oil sys tem; there - fore its reliabil i ty does not depend on the vacuum. The pump drive power is consumed in overcoming f r i c - tion in the ro tor bear ings and in per forming work in pumping gas. The lower the p res su re , the less are the forces which act on the ro to r blades. Consequently, the power required by the pump motor dec reases as the p re s su re becomes less .

In the ext remely high vacuum region, where the p r e s s u r e is about 1 - 10 -l~ mm Hg, the gas density is so smal l that power is essent ia l ly consumed 0nly in overcoming fr ict ional forces in the ro tor bear ings. The reduction in power is a consequence of the decrease in the loads which act on the drive elements, and this, in turn, leads to an increase in rel iabil i ty and a r ise in the serv ice life of the drive e lements .

The electrode block is the main element which prede te rmines the reliabil i ty of operation of a mag- netic e lec t r i ca l d ischarge pump. A change in p r e s s u r e exer ts a considerable effect on the rel iabil i ty of operat ion of each cell of the electrode block.

The magnitude of the discharge cur rent can be determined f rom the following equation [4]:

I = ~ r~ lz , ~craB~

where e is the charge on an e lect ron; m is the mass of an electron; l is the anode length; e0 is the dielec- t r ic constant; v i is the ionization frequency; ~0 is the coll ision frequency; ra is the drift radius; and B is the magnitude of the magnetic field: It should be noted that the ionization frequency, like the coll ision frequency, is proport ional to p r e s su re ; therefore the discharge cur ren t is also proport ional to p re s su re .

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Ia the pressure region from I �9 10 -3 to 1 �9 10 -4 mm Hg, where the discharge current is high (~ I000 #A), three basic phenomena are observed which reduce pump operation reliability: the intensive bombardment of the cathode plates with ions causes an intense atomization of the cathode material, which, in turn, leads to a rapid metallization of the insulators and, as a result, to a loss of their resistance; a large discharge cur-

rent leads to a local heating of the block and a strong gas evolution of previously pumped gas from its cells; and large gas streams (especially of hydrogen) lead to delamination of the film from the walls of the anode cells, dusting off of this film, and short circuiting inside the electrode block. These phenomena become weaker at pressures of 1 �9 I0 -G to 1 �9 10 -9 mm Hg, where the discharge current is I0 to 0.01 ~A.

In [I], data are given to show that at a pressure of 1 "10 -4 mm Hg the service life of a pump is 400 h; at a pressure of 1 �9 10 -5 ram, 4000 h; and at lower pressures it increases proportionally. Thus, the re- liability of a magnetic electric discharge pump is greater, the higher the degree of vacuum.

It is to be noted that the magnetic electrical discharge pump has one very important advantage, which distinguishes it from the other high-vacuum pumping media, namely it does not require fore-vacuum pump- ing after coming into operational regime. In the high vacuum region the fore-vacuum part of the system is switched off, and the reliability of operation is determined only by the reliability of the magnetic electrical discharge pump.

I.

2.

3.

4.

LITERATURE CITED

]3. D. pau6r, High-Vacuum Pumping Equipment [in Russian], Energiya, Moscow (1969), p. 527. British patent No. 925009 (1963). V. I. Kuznetsov, Mechanical Vacuum Pumps [in Russian], Gosenergoizdat, Moscow-Leningrad (1959), p. 279.

I. G. A. Vasil'ev, Magnetic Discharge Pumps [in Russian], Energiya, Moscow (1970), p. 113.

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