Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to pressure release devices
for containers and especially to electrochemical cells which
are subject to undesirable increases of internal pressure.
Prior art pressure release devices include safety
valves, rupture membranes, shearing plugs, sharp points which
are forced through sheets and the like. A problem with all
these devices is that their inherent properties make it diffi-
cult to combine the characteristics of low cost, precision in
venting at a specific pressure, adequate sealing for the pres-
surized container and a degree of strength against accidental
triggering by mechanical abuse or malfunction. For example, a
safety valve, which is made to open when the force on the valve
exceeds that of a spring, is adequately precise with respect to
venting at a specific pressure. However, this is offset by the
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costly requirement for added parts, which are susceptible to
~; mechanical accidents, and precise fit for hermeticity which can
lead to increased cost. Additionally, such devices rely upon
the stability of a seal for hermetic closure and corrosive sùb-
r stances within a container could render this seal ineffective.
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~ RupturabLe membrane!J consist of a wea]cenec~ portion of
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a vessel or contailler wall which provides hermctic closure until
tl~e contained prcssurF is sufficient to tear the mem~rcine.
Though it is less compllcated in terms of material than the safety
valve, its curvature, thickness and material condition require
close control because the pressure at whic~ the membrane tears
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is deterMined Dot by the stable elastic properties of the
memorane `out by its ultimate plastic-deformation behavior which
is dependent upon composition and the mechanical and thermal
history of the material. Thus whi.le the seal is adequate the
cost of precision in the release pressure becomes high. Any
; cost saving realized by requiring less precision in venting
pressure is offset by the need for additional strength and
weight of the vessel or container to accommodate hiyher pressures.
Additionally, for small openings the membrane m~st be so thin
that protection Erom puncture and corrosion would also bécome
necessary, thus further increasing the cost. Simple grooves in
the wall of con:tainers such as have been used in capacitors have
the additional deficiencies of lengthy tear propayations and the
absence of vent closure after the excess pressure has been
released unless an expensive spring container is used. These
deficiencies can be especially detrimental if caustic materials
ai-e contained in the container.
Other methods for venting such as shearing the
periphery of a plug, or the forcing of a sharp tool through a
sheet, remain imprecise to the extent that the venting pressure
depends on the varying physicaL characteristics of the material
used in actuatincJ the venting process. Additionally, shearing
- plugs depend on adding materials of low shear strencJth to the
wall and these materials Inust be selected to be compatible with
the container contents.
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It is ~:herefore an ol~jec~ of the prc.sen~ lnvention to
provide a ~LesSul-e release device which inteyra~es the ~atures
of low cost, good precision in venting and an hermetic seal ~cr
. pressurized vessels or containers.
It is another object to provide a pressure release
device W21ic]l is an integral part of the containeL^ wall and
which requires no additional ioreigl~ material.
Another object is to provide a pressure release
device which can be formed to vent at a predeteLmined slow
or rapid rate.
It is still another object to provide a pressure
release device which will s~bstan~ially close af~ter venting
to inhibit ~urthe`r egress of container materials or the ingress
of foreign matter.
` - Another object is to provide a low cost pressure
; release device ~ihich c~an be formed by die pressiny.
Another cbject is to provide a pressure release device
which is based upon the predictable behavior of à bending strain
`rather thall a tensile strain.
It is another object to provide a method for pressure
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"- venting which is low cost, preclse within acceptable limits,
and ~equires no materials other than the walls of a container
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to be vented.
It-is yet another object to incorporate all of the
above features in a r~ressuriza~ container or vessel.
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Another object is to incorporate all of the above
features in an electrochemical cell which because of its con-
tents is either in a pressurized state or subject to a high
degree of pressure buildup.
According to a broad aspect of the present invention,
there is provided a pressure release device comprising channel
means having one or more interruptions therein whereby under
conditions of increasing pressure the channel means unfolds and ;
simultaneously therewith at least one of the interruptions is
stretched to rupture thereby ~orming a vent for release of the
pressure.
These and other objects, features and advantages will
become more evident from the detailed description below as well
as from the drawings, in which~
Figure 1 is a plan view of the end wall of a container
embodying the pressure release device of the invention.
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Figure 2 is a perspective view of Figure 1 with a
portion, taken along line 2-2 of Figure 1, shown in section.
This figure shows a specific embodiment of the container of the
invention wherein it is used for an electrochemical cell.
;~ Figure 3a is an enlarged sectional view taken along
line 3a-3a of Figure 1.
Figure 3b is an enlarged sectional view similar to
3a of another embodiment of the invention in which there is
provided an additional reinforcing sectionO
Figure 4 shows the portion of the container where
venting takes place. In this figure another embodiment of the
invention is shown, namely one in which the shape of the vent-
ing area differs from that shown in Fig. 1.
- 30 Figure 5 is a perspective view of another embodiment
of the invention wherein the pressure release device is incor-
porated into a side wall of the container.
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Figure 6 is a perspective view of another embodiment
of a container made in accordance with the invention.
Figure 7 is a graph showing the general relationship
;~ between displacement of the disc in the embodiment :hown in Fig. 1
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,,arid,venting pressui-e.
~ The pl-esent invention envisions a pressure release .
: device integratecl into a container or vessel wall in which a
, stress caused by pressure is concentrated and direc~ed at ~
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: preselected portion of the contai.ner wall causing this
preselected portion to tear under an undesirable increase of
pressure thereby forming a vent for release of the pressure.
- The.concentration of stress is obtained because the normal
contour of a container wall llas been a].tered to provide two
,~ 10 deformed and more extensible.sections between WhlCIl is a small
undeformed or less deformed and relat-ively inextensible section
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of wall. This undeformed section is adapted to rupture when
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.: , the pre,ssure level within the container reaches ,a predetermined
level. As the.internal pressure increases either the end or
side walls of the container are forced outwardly and the
deformed sections of the container wall will tend to unfold
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~: while undergoing relatively slight bending strain. However,
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thls unfoLding can be achieved only with stretching of the
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~ ; unde.form~ section of wall that is between tlle two deformed
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portions thereof. This section, referred to herein as a
~,: "bridge", "bridging" or "bridging area", cannot extend in the
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~ same way as the deformed portions and therefore is subjected to
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~ , . high tens.ile stress which leads ineluctably to the tearing or,
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rupturing of the bridging area thereby forming a vent. The
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~ :. deformations will advantageously be elongated and in the form
.:of channels arranged so that an end of one is aligned with an
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.~ - end of another with a bridge therebetween. The container
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.~ . material can be of metal such as steeL or nickel-plated steel
or of plastlcs or rubber which can contain pressurizcd contents
: 30 and have:predictable movement under stress.
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. Th.e cPntainer materials most suitAb.. le'~ox th.e us~e : .. .. ...
~' of the structuxes are those'that are capa,bl'e Qf under~o~n~
substantial inelastic eLon~ation befoxe b,rea~In~
: ~eferrin~ no~.to th~ drawin~s~ F,i~uxe 1 sh.ows the .
end ~f a container 10 w,hich'has been di.e pxessed and~Qx dee~ ' '
drawn to include annular cha'nnels llr12 as. the de~rmed aXe~s.
Th.e ends 61 a-d, edges. 62 a-d and bases: 63 a-~b ~f the channels
are preferably of rounded confi~uxation7 thus th.e ch~nneI~s, can be
for~ed by draw,in~ and stxetchin~ pxocesses .with~ut the de~elo~ent
. 10 of weak spots that c~uld be susceptible to c~rXQsion ~r ~echanical ,~.
.,~ breaka~e, However Qthex s,h.~pes. cQuld alsQ be u~s:ed. These ,,,
:~'' chann~els 11, 12 axe capable~ of un,~old;na undex pre$'sure by
.,, under~oin~ bendin~ ,stxain. It m,atters little whether~ the
, ch.annels extend ~nwardly into the'containex ~X outw.ardly ~xo~ :.
it, so lon~ as extensibilt,ty undex pressure. I$~ possible, ;. . .
; The brid~es. 13, 14 between the ends of channeIs 11~ 12 are '. .::~
the points at which ventin~ t,a,kesi place because the b~idge ~$ ;~
,' subjected to tensile str~in beyond its rupture point while the '''
'. channels still ha~e substantial extensibility, Gxo~yes. 15~ 16 ,
. . 20 are either cut Qr pressed as axcuate ~xooVes which'extend
~, across the bxidges and d~wn the ends of th.e adjacent channels
,,. since th.ese are re~i4ns of txansition from the hi~h'stxain
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': at the undeformed portions o~ the'brid~es to thé low stXain
,:`, at the fully de~oxmed portiQns o~ the channels, The ~r~oyes
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15, 16 of the brid~e aXeas 13~ 14 are relatively small yet
~ pxoYide su~icient yent~n~ as Qpposed to the si.~ple ~roQyes
`:~. of the prior art whIch m,ust be lar~er to be'effectiye ~nd
"'.~ therefoxe would have the dis:advanta~es of bein~ more susceptible
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tQ mechanical or corr~siQn da~age~and to ~or~ti.on o~ wide openings.
High'tensile:stres,s. is concentrated in the ~ro~ved
~ortiQns 15~ 16 o~ the brid~e areas 13, 14 and not in the
adi~inin~ areas such as:the~ Quter' annulus 6S adja,ce'nt the brid~e
~xeas. The depth o~ the ~ro~Ve is substantiall~ detexminative
of the de~xee 4~ thi~ concen~tration and a ~roo~e hav~n~ ~ depth
at le,a,st ~bQut hal~ o~. th:e'ori~inal w:~ll th.~ckne,ss. is~ usually
~de~uate.,
: The width'at the~ba'se o~ each ~r~QVe ,i.s m.,a,de ~uch
tha't the ru~ture which..occurs.in th.e ~rQo~e t~kes pl~ce durin~
th~t ~t ~ the ~ut,w,a,rd.dis~lacement Q~ the disc 19 ~or ~h~ch
the excess~ye internal~pressure yaXies le~s,t, I,n oxde~ to
ensu~e ~Qdex~tely unifor~;s:tr~in across the ~r~oYe b~se and
tQ ~pid pxe~ature'ru~ture it i,s pxe~exable that the
junctuxes p~ the ~r~ave'bas:e with'the side w~lls of the ~X~Ye
~xe rounded~,~nd the ~ateria,l thickness, Q~ the ~Qoye ba,s,e
between thes:e juncture.s: is:.appxoxi~ately cQnstante The desixed
ba,se width depends upon ~and i:s a function of the contalnex
materials characteri.s,tic;elon~ation to rupture. undex tens~le
~ 20 stxess~ and 4f the struc*ure-detexmined pullin~ ~p~xt
o~ the ~r~ove w~lls-as~excessive'internal pressuXe ~ncrea,$es~
The m~texi~l o,~ the ~ro~.ve hase is st~ai.ned in direct prop~Xtion
to the. di,~s,placement thatl ~idens th.e ~rOQ~e~ ~nd in inYex~s~e pXQ-
~xtign tQ its initial ~idth~ Thus~ f~X ex~ple.f the pxe~e~red
~ x~oye b~se width f~r th.e m~:~e'extensible annealed ~ld ~-teel W~ll
,. be .~ade $~Allex than th~t ~sr the sa~e steeI which'h~s been
- ~ w~xk-hardened. ~ matexial allowin~ severa~ old extenS~on
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before rupture, such as annealed pure copper or fine silver,
lead, rubber, or some plastics, would require a much narrower
groove, for the same structure, than that suited to mild
steel.
If the gr~oves in a metal have been formed by
, cold pressing~the base material is hard and relatively ln-
extensible, but a simple anneal restores its ductibility and
stabilizes i~s mechanical and chemical properties. The
grooves, with the adjoining~areas including the bridges and the
- ends of the channels, can be annealed by application of heat
sufficient to make the entire areas 15,16 red hot for about
a second. It has been found that by annealing the metal in
the area where rupture is desired it is possible to provide
more eAYact control over the mom~nt of venting by counteracting
the effects of cold working the metal container. The
annealing also provides the groove area,` which is the weakest
part of the container structure, and the surrounding area,
which has been subjected to forming-stresses, with a degree of
resistance against corrosion and attack from materials contained
S 20 -within the container of which the groove is a part~and also
reduces the susceptibility to mechanical injury.
- ~ In the embodiments shown in Figures 1-4 the channels
11,12 have a central disc 19 therebetween. When the pressure
in the conkainer e,xceeds normal operating pressure the central
disc 19 will be forced outward. Thls will, ln turn, place
botll the c]-allnels 11,12 under ~endinc3 stress and the bridge
~ area 13,14 under tensile stress. Figure 2 shows the movement
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of the channels 11, 12 under this stress, by dotted line 20.
The pressure force existing in the container is shown by
arrows. The dotted line 20 shows that the channels have been
unfolded partially but not completely at the time of venting.
When the venting pressure has been reached, the groove is
stretched to the shape as in the embodiment where the container
is formed of mild steel, shown by dotted line 21, roughly
35-50% wider than originally, and is ruptured or torn. It
is desirable to have the groove tear while the deformed
portions are still extensible because this allows further
opening of the vent if the excess pressure is not relieved.
However, these tears are limited to the bridge area because
as the channels unfold, the ends of the channels do
not undergo appreciable tensile stress. The tensile load
released by the ruptured bridge does not propagate the tear
into the channel regions since the area adjacent to the tear
has retained its strength even when the channels have fully
unfolded. Simple grooves do not have this characteristic
since the areas adjacent to the ends of the groove have been
subjected to full tensile stress causing weakening and when
the groove ruptures the adjacent area does not have sufficient
strength to contain the tearO Limiting of the tears to the
chosen vent regions is generally aided by the greater thickness
of the channel material. Thus, the groove 16 shown in Figure
3a provides a limited vent in any metal that is sufficiently
ductile to allow deep drawing, even when internal pressure
after venting remains high enough to unfold the channels fully.
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The channel material will, of course, advantageouisily be o~
such thickness as to permit unfolding without bending fracture.
For homogeneous rigid materials, includiny those for which
reinforcement is impractical, means are needed to reduce tensile
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stress, across the ends of a vent crack, enough to prevent
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further rupture. Such means are shown in the embodiment of
Figure 3b where the end of the groove 35 dips below the lowest
level of the rest of the channel. The thinned portions 38, 39
uncler the groove are compressed as the channel unfolds, off-
10 ~ setting the tension that otherwise could extend the groove
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- crack. Thus, there generally should be a differ-ential in
strain between the vent area and the desired end of the vent
accomplished by these or other similar means.
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~ith tlle elastic-recovery movement of the channels
~; after venting, the vent that has opened w~ close
sufficiently to inhibit container material from escaping and
` ~ ~ foreiyn material from entering the container . This is especially
important with electrochemical cells containing substances
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; ~ which are incompatible with ambient atmospheric materials.
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This latter feature tends not to be present in vents that
consLst only of a thinned groove in the wall of a container.
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~ ~ The disc 19, by its mOveMent shown by the dot,ted
7j~ line 20, exerts a pulling force on both the cbannels and the
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iS bridge area. This force produces small, predominately bending
strains in the channels with concomltant large outward
displacemeht of the disc, and on the groove 15 ln the bridge
area 13 a large tensile strain which increases until the
groove is eventually ruptured.
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Figure 2 shows container 5 having a pressure release
device of this invention as it is used in a typical electro-
chemical cell that in some instances can develop excessive
~nternal pressure. The cell shown is one in which the anode
~2 is lithium and the cathode depolarizer is S02. Separator
mats 23 separate the anode 22 from the cathode current collector
24, After the cell has been made hermetic the So~ is
introduced into it. An insulating disc 25 separates the cell
components from the outer cell wall and the pressure release
mechanism however this disc is not a hermetic separator and
- excess fluid and its concomitant pressure can circumvent this
disc and operate on the pressure release device integrated into
the cell wall.
Figure 4 relates to an embodiment similar to that
of Figure 1 and shows the ends 32 of adjacent channels 71,72
and the bridging area 73 therebetween. Groove 30 is of
lenticular shape with its width approaching zero as it
approaches the end of the most deformed portion of the channels
71,72. Differing shapes of groove alter the venting process.
A groove of uniform width such as that shown in Figure 1
ruptures first at the center of the bridge area 33 forming a ~;
lenticular slit whose extremities progress down the channel ends,
with continued outward motion of disc 19, until they reach the
ends of the thinned (or extensible) material at the base of the
- groove. The groove embodiment as shown in Figure 4 is
strained roughly the same amount throughout its length and
ruptures in a roughly full-length but initially very narrow
slit extending along the full length of the thinned base of the
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groove. The resulting partial decrease of restraint on the
~isc 19 allo~s more abrupt vent opening than that of the
uniform width groove at the time of rupture, with a more
rapid decrease of pressure. Thus the venting action can be
varied or controlled from a slow, pressure-regulating release
to an abrupt opening of a substantial vent.
Figure 5 shows another embodiment of this invention
in which a cylindrical container 2 is provided with a pressure
release device in the form of circumferential deformations
40,41 in the side wall thereof. Thoug}i the deformations 40,41
are shown as facing outward, th~ direction has little bearing
on the pressure release device's operation. In this embodiment
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the direction of the operative tension is axial as shown by
the arrows, and the defor~ations can unfold to increase the
axial length of the container. In this way the tenslle
stress is concentrated on the b}idge area 42 and particularly
in the groove 43 which tears under this strain while the
deformations 40,41 still are capable of further unbending as
previously explained.
Figure 6 shows another variation in structure in
which the channels 51,~52 are formed in the cylindrical side
- wall of a container but are not circumferential in form.
Instead the channels 51,52 are in a direction parallel to the
axis of tlle cylinder. Excessive pressure within the container
distends its circumference and the channels 51,52 unfold with
only a bendiny strain, while the bridgin~J area 53 is subjected
to high tellslle strain causing tile wall ~o rup~ure a~ g~oove
54.
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It should be noted that the structures of Figs. 1-4 havc
two characteristics not found in the embodiments of ~igs. 5
and 6. First, in the ormer the vent is formed in a bridge bet-
ween two ~uite difEerent members--a central disc l9 and an
outer annular rlng 6~5-- rather than between two like members.
The outer annulus, with the adjoining part of tlle container
walls, is subjec~ed to a contractile force alony tlle diameter
passing through the bridyes, when the disc is forced outward
by excesslve internal pressure. The width oE this annulus will
d~sira~ly be sufficient to support the contractile force with-
out undercJoing plastic deformation so that the force can
.! , ' continue to increase and to strain the groove base material
until it ruptures. Second, as the disc is displaced outwardly
Erom the plane of the outer annulus under e~cessive internal
pressure, the bridges ar~ rotated slightly from their original
plane but undergo little lnitial elongation, thereby providing
a degree of yrotection against acciden-tal release by pressure
vent formation in the bridge area.
Figure 7 illustrates the motion o~ disc l9 in~
Figure l as a function of internal pressure. The inclusion of
the channeLs in the container end wall allows substantial
outward motion of the disc with minor strain of channel
material. As the disc moves outward, the internal pressure
chanyes only gradually so that a pressure plateau is reached
as shown between points A and B ~ollowed by more rapid
pressure increase whell the channels arc unfolded to the point
.. . .
where ~urther unfoldillg becomes diEEicult. Though su~stantial
motion of clisc l9 continues within the plateau, the pressure
within tlle cell varies little. 'l'hus the bridcJe area is
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10644~5
sevexly strained during this ~ove~ent as prev~ously explained
and ruptures durin~ th;s predictable pressure plateau makin~ ,
,it possible to predeter~ine wi'th xeasonable pxecision the
internal pxessuxe at which:the vent will form, In order to .'
enSuxe rupture of the groove'w.ith;n a predetermined desired
pxeSSure ran~e such as the'p~essure plateau the width.o~ the :
~ro~ye ca,n be varied~ ~ith ,a, ~reater width caus~ng rupture
t~wArds, the upper end o~.. th.e cUXve and a lessex ~idth.causin~ '
xupture t~o,w~rds the lowe'r end of the sa~e cUxve. Because
of the ~elatiye sizes of the ~rooves and the channeIs the
~reatest p~rt of the force exerted by excessiYe intexnal .
pxes.suxe ~s directed to unfoldin~ the de~oxm~tions or channels ''.
with only a minor part of the force exerted to place the ' ,.
grooves under tensile strain, Once the ~rooYe has been
extended beyond ~ts proportional li~it~ both the min~r
~orce that widens this section to ruptuxe and the ma~or foxce
un~oldin~ the deformation axe li.ttle chan~ed, Thus;r while
the intexnal contai.ner pressure xem.ains s,ubs,tantI.a~
con$tant durin~ the unfolding o~ the de~oxmati,ons~ the
bxi.d~e section is sevexeIy strained to rupture. It 1,s thexe~oxe
the repxoducible mQVement o~ the de~ormations. ~hich detex.mines '
the pressure at which the yent is ~ormed. The position : :
of the plateau as it relates to ventin~ pressure is dependent~ . :
to a lar~e extent~ upon th.e thickness o~ the w.all o~ the
container at the ch.anneIs, Thus the cont~inex can be:~de
to yent ~t a hi~her pressuXe if the thicknes.'s of the cha,nnel
wall ~.s increased and a,t a lower pressure if decreased,
Sim~larl~Y~ i~ the channel ~re annealed the pressure pl~te~u
is ~ttained more quickly ~nd at a lowex intern~l pxessure leyel,
Ventin~ will thus ~ccuX ~t a lower pres'suxet The follo~w,in~
examples set ~orth the characteristics of a specific type of
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containex.
Standard D-size'cell containers as shown in Fi~uxe
2 wexe~rmed by deep-drawin~ mild steel to 1.3 inch:outside
d~.~eter ,Q2Q inch conta,iner ihicknes's, channels o~ .9 inch
rid~e dia~etex and ,08 inch:'depth with the channel sides ~orming
an an~le ~ 80, with each~'oth.ex. The ~rooVes, trayexsin~ the
,,, bx~d~es between adjoinin~ ch'annel ridge5, WeXe pressed about
,01 ,i,nch.deep and had a ~axi~um width o~ ~bout~j,a25 inch ~t the
centeX Q~ th.e brid~es. ~ter local annealing c~ the bxid~es
,a,nd adjoining cha,nnel ends.~ the cpnt~inexs vented a,t 450 + 25 psi.
~elea,s.e pxessures beyond these'li~its were found when substitution
.021 inch stock sti:fened the channels to incXease the vent
pres$,ure to 485 + 25 ps~i., Such'yarlables~ o~ cQurSer are
controlled in m~nufacturin~ to ~aintaln speci~ied dim,en$ions
~nd ~atexi~l conditiQn, '.These pres'sure ranaes ~oX yenting axe
su~icientl~ pxecise and~in the use~ul ra,n~e o,~ opeX~tin~
~es:sures ~X use fox he'x~eti,cally sealedr ~re5$uxized~ electr~-
che~ical cells.
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