7/29/2019 Clculo de sedes

1/3

DESIGN AND ST RESS ANALYSIS

CALCULATION OF POLYM ER SEALS FOR BALL VALVESE. G. Matyushin, L. A. Regush,A. Ya. Boiko, and I. V. Seme nov

UDC 621.646.6-72:678.5.001.24

The complexity of simultaneously taking into account a variety of factors affectingthe process of making ball valve seals hermetic is apparently the reason why there is anambiguous approach to the choice of the specific sealing pressure qh and the existence ofa large number of empirical formulas for the calculation [i, 2]. The present article dealswith a refined method of calculatin g polymer seals of ball valves.

Experimental investigations of the sealing properties of ball valve seals with liquidand gaseous media showed that for seals made of polymer materials, the dependence qh(p) iswell described by an equation of the form

q h = a + b P , ( i )where p is the pressure of the worki ng medium.

Let us examine a ball valve with floating plug ( Fig. i). As characteristic of thesealing of such a valve we adopt the following dependence:

q m ~ qo+ K P, ( 2 )where qme is the mean spec ific load on the sealing surface; qo is the mean specific loadindu ced by the pr ecom pres sion of the seal; K = (D + d)/ 4(D -- d) is a geometric c onstant; d,D are the inner and outer diameters, respectively, of the sealing surface.

On condition that in the closed valve position the worki ng medi um penetrates to thecenter of the sealing surface,

4Q oq o - ~ ( D ~ _ d ~) , ( 3 )

where Qo is the precomp ression of the seals.It can be seen that the chara cter istic s (2) depend on the ratio of the diamet ers d

and D. Tightness of the sealing joint of the ball valve in the pressure range of themedi um is ensured by the condition qme > q h , i.e., qo + Kp ~ a + bp.

Figure 2 shows the fields of permissible values of D of seals made of fluoroplastic -40with d = 20, 40, 60, and i00 mm depending on the pressure of the liquid medium. The minimalouter diameter Dmi of the sealing surface was determined by proceeding from the conditionqme < q~ with dependence (2) taken into account (here, qp is the maximally permissible loadfor the~seal material; in calculations for fluoroplastic-40 we ad opted qp = 20 MPa, q0 = 2

Fig. i. Diagra m of a ball valve wlthfloating plug.

Translated from Khimichesk oe i Neftyanoe Mashinostroenie, No. ii, pp. 17-18, November,1985.

0 0 0 9 - 2 3 5 5 / 8 5 / 1 1 1 2 - 0 5 2 9 5 0 9 . 5 0 9 1986 Plenum Publishing Corporation 529

7/29/2019 Clculo de sedes

2/3

8 0 - - - ~/ 1

,o F - -T ,T0 2 4 B 8 tO 12. 14p,l,, tPa

o..b;qme

o Pf P,~jFig. 2 Fig. 3

F ig . 2 . E v a l u a t i o n o f t h e a p p l i c a b i l i t y o f b a l l v a l v e s w i t hf l o a t i n g p l u g a n d f l u o r o p l a s t i c - 4 0 s e a l s i n o p e r a t i o n i n l i -q u i d m e d i a ( l - 4 ) c a l c u l a t i o n b y f or m u l a ( 4 ) ; 5 -8 ) Dm i =f(p); 9-12) Dmax; i, 5, 9) d = 20 mm; 2, 6, i0) d = 40 mm;3, 7, ii) d = 60 mm; 4, 8, 12) d = i0 0 mm.F ig . 3 . D e t e r m i n a t i o n o f t h e o p t i m a l c h a r a c t e r i s t i c o f b a l lvalves: i, 2) qme = f(P), ca lcul atio n by formula (4) and (2),respecti vely; 3, 4) qh = f(P), cal cula tion by formula (i)w i t h A q ( o p t i m a l c h a r a c t e r i s t i c ) t a k e n i n t o a c c o u n t a n d d i s -regarded, respectively.

MPa). The largest value of the outer dia meter is Dma x = 2R cos ~o (here R c is the radiuso f t h e s p h e r i c a l s u r f a c e o f t h e b a l l p l u g; i n t h e c a l c u l a t i o n s w e a d o p t e d R E i n a c c o r d a n c ew i t h t h e d a t a o f [ i] ). T h e m a x i m a l p e r m i s s i b l e p r e s s u r e w a s d e t e r m i n e d f r o m t h e c o n d i t i o nqp = qh: Pmax = (qp -- a)/b (the values o f a and b were calcu lated on the basis of expe rimen -t a l d a t a ) . I t c a n b e s e e n f r o m F i g. 2 t h a t d e p e n d i n g o n t h e p r e s s u r e o f t h e m e d i u m a n d t hei n n e r d i a m e t e r o f t h e s e a l i n g s u r f ac e , t h e r a n g e o f p e r m i s s i b l e v a l u e s o f t h e o u t e r d i a m e t e ris fairly wide (dashed areas).

B y u s i n g t h e f o r m u l a s s u g g e s t e d i n [2 ] fo r d e t e r m i n i n g t h e w i d t h o f t h e s e a l i n g s u r f a c e

~n k V 2d n P n (4 )(here n is the chord of the seali ng surface; d is the nomin al free diameter; Pn is then o m i n a l w o r k i n g p r e s s u r e ; k i s a c o e f f i c i e n t o f t h e m a t e r i a l ( f or f l u o r o p l a s t i c k = 0 . 35 ) ) o nt h e c o n d i t i o n t h a t d = d w e p l o t t e d t h e d e p e n d e n c e s D ( p ) . T h e o b t a i n e d f i e l d s o f p e r m i s -sible outer diamet ers D are bounded by the line ABCEFK; the limit values of the funct ionD ( p) s a t i s f y i n g t h e i n t r o d u c e d c o n s t r a i n t s a r e b o u n d e d b y t h e l l n e N P Q S . I t m a y b e n o t e dt h a t w h e n f o r m u l a ( 4) i s us e d , t h e r a n g e o f p e r m i s s i b l e p r e s s u r e s o f t h e m e d i u m i s c o n s i d e r -a b l y r e d u c e d . F o r i n s t a n c e , v a l v e s w i t h d = 20 m m a n d f l u o r o p l a s t i c - 4 0 s e a l s ma y o p e r a t ew i t h l i q u i d m e d i a a t p r e s s u r e s u p t o 1 1 . 3 M P a w h e r e a s w h e n t h e w i d t h o f t h e s e a l i n g b e l t i sc a l c u l a t e d b y f o r m u l a ( 4 ) , t h e m a x i m u m p r e s s u r e i s 3 M P a.

T h e c h a r a c t e r i s t i c q m e = f (P ) o f a b a l l v a l v e w i t h s e a l s , c a l c u l a t e d b y f o r m u l a ( 4 ) ,is shown in Fig. 3. It can be seen that wit h incre asing press ure the mean speci fic loadconsi derab ly exceeds the value qh; this leads to intens e wear of the seali ng surface. Toe n s u r e t h a t a v a l v e w i t h a b r o a d s e a l i n g b e l t i s t i g h t i n t h e p r e s s u r e r a n g e O - -p ma , i t i sn e c e s s a r y t o p r o v i d e a s p e c i f i c l o a d q o t h at c o n s i d e r a b l y e x c e e d s t h e r e q u i r e d l o a d c o r r e s -pondi ng to the value of a. This, most of all, causes an increase in the torque neces saryf o r r o t a t i n g t h e s p h e r i c a l p l ug . T h e o p t i m a l c h a r a c t e r i s t i c o f t h e b a l l v a l v e i n t h e w o r k -ing pres sure range O-p~, where the specific loads sl ightly ex ceed the values of qh, can beobtai ned on the conditio n that the coeffic ients b and K in Eqs. (I) and (2) are equal toe a c h ot h e r. W i t h t h i s c o n d i t i o n t a k e n in t o a c c o u n t , w e o b t a i n a n e x p r e s s i o n f o r c a l c u l a t i n gt h e o p t i m a l o u t e r d i a m e t e r o f t h e s e a l i n g s u r f a c e

~(4 b + 1)O = - 4 b - - 1

530

7/29/2019 Clculo de sedes

3/3

The optimal precompr ession of the sealing can be determined from Eq. (3) by adoptingqo = a + Aq (her e, Aq is the margin of r eliability):

(D 2 - d 2) ( a ~ A _ q )4

We determined experimen tally the service life of seals made of fluoroplastic-40, calcu-lated by the method of [2] and by the method su ggeste d by us (working med ium is air wi th atempera ture of 20~ 5~ qo = 3.5 MPa). The results of the experim ents are prese ntedbelow:

Diameter, mm:of nomin al free flow d 50 80innerof the sealin g surface d 55 86outer 59.3 92.8of the sealing surface D* 68.4 107

Number of operating cycles up to 7.36"103 4.82'103deterioration of sealing 9.17.103 5.78"i0 s

*The data in the numera tor and in the denom inator were calcu lated for sealsby the method r ecommen ded in [2] and suggested in the present article, re-spectively.

It can be seen that when seals of ball valves with floating plugs are calculated by thenewly devised method, their life can be increased by 20-25%.

LITERATURE CITEDi. A. F. Bykov, Fittings wit h Ball Valve for Hydra ulic Systems [in Russian], Mashino stroe-

nie, Moscow (1971).2. P. A. Zhunev, Yu. M. Kotilevski i, N. I. F!erov et al., Pipeline Valves [in Russian],

Mashinostroenie, Mosc ow (1967).

UNLOADING MECHANISMS OF AUTOMATIC SYSTEMS IN STORING AND TRANSPORTO. P. Mulyukin, D, E. Chegodaev, UDC 621.6 42.2: 66.02 5.621 :65.0 11.56Yu. I. Kondrasho v, and F. M. Sha kirov

Relieving the pressure of spring elements on shutters of automatic valves in storageand transport ou tside of or as a part of pneumatic--hydraulic systems (PH S) is a topicaltask. Such relief or unloading is particularly important for shutters with considerablespecifi c pressur e (sealing force) in the zone of contact betw een the seat and the valve.Obviously, in the seal of a shutter lo aded by the force of a spring, relaxational, adhesive,and other processes occur acceleratedly, and the reversed loads on the shutter arising inthe transport of an automatic valve cause periodic impacts of the valve against the seat.These circumstances cause premature aging of the seal and reduce its life.

Depending on the purpose of the stop valve fittings, the requirements concerning tight-ness and life, and special features of storage, transport, and operation, we distinguishtwo groups of mechanism for relieving shutters as part of an automatic system: before plac-ing in the syste m (see Fig. la) and before a nd after placi ng in the syste m (see Fig. i,b-f). The devices of the second group are distinguished by being universal while those ofthe first group are simple in design and more reliable.

According to their interactio n with the shutters, the relieving mechanisms are sub-divided into contact and contactless ones. In the contact mechanisms the shutter elementsare relieved of the spring pressure for the perio d of storage or transport as a result ofmechanical contact between the power element of the unloading mechan ism and the shutter.Mechanisms in which there is no such contact in storage and transport of the systems areclassed as contactless mechanisms.

Translated from Khimicheskoe i Neftyanoe Mashinostroenie, No. ii, pp. 18-19, November,1985.

0009-2355/85/1112-0531509.50 9 1986 Plenum Publishing Corporation 531