CA1081083A - Rupture disc pressure relief device - Google Patents

Abstract

ABSTRACT
A safe pressure relief assembly of the rupture disc type guards against pressure differentials beyond a predetermined range extending both above and below zero and includes a positive relief disc which is perforated to permit negative flow without rupturing, a sealing disc normally blocking the perforations, a cutting member positioned in spaced relation to the sealing disc and extending a substanial transverse distance thereacross and a sealing disc support member having a stay arrangement projecting into negative supporting engagement with the sealing disc, the stay arrangement having a resistance to collapse sufficient to retain the sealing disc out of contact with the cutting member only up to a predetermined negative pressure.
The subcombination of sealing disc, stay arrangement and cutting member is separately useful, especially as a very low pressure relief device.

Description

~ 3 This application is a division of application nu~ber 273,217 filed March 4, 1977.
This invention relates to safety pressure relief devices and more particularly to improvements in rupture disc arrangements for protection against both excessive pressure and excessive vacuum.
In recent years rupture discshave found increasing use as highly reliable pressure relief devices primarily to guard against the explosive conditions created ~hen a vessel, for any reason, is subject to excessive internal pre~sure. ~n certain applications, the same vessel for which over- -pressure protection is indicated, also requires protection against excessive negative pres.sure or vacuum therein. By way of example, in the food process-ing and pharmaceutical manufacturing industries, relatively thin-walled stainless steel tanks are commonly utilized which normally contain process reactions involving controlled positive internal pressure, but subject to drops in pressure to a point substantially below atmospheric pressure. In absence of vacuum relief, such vessels may buckle inwardly under atmospheric pressure, causing considerable damage and down time. Typically, the positive pressure limit ~ill be great compared to the negati~e pressure limit, for example, 5 to 50 lb./in. . Yet, accurate control o both is important, the former primarily or the safety of personnel and equipment, the latter not only for equipment preservation, but for process preservation, since the opening of a vessel to ambient air can produce severe product contamination.
Therefore, such manuacturin~ arrangements ha~e often used ~arious multiple de~ices Qr protectingagaIns~tboth overpressures and under pressure, includ-ing complex and expensive mechanical vent valves and vacuum breakers.
Positive relief rupture discs, such as the type shown in United States ~atent Na. 3,881,629, heretoore have been modi~ed by placing knife bladqs, such as that s:hQw~n ~n United States Patent No. 3,834,581, beneath the - :- . . ~ - , . . . ,, . . :

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flexible seal member and placing additional openings through the rupture disc. The arrangement then operates as a normal forward-rupturing disc under positive pressure but will cause cutting of the seal upon negative pressure, allowing reverse relief flow past the disc. In many instances this proved unsatis~actory because virtually any degree of vacuum caused the flexible seal to be cut or "pinholed", thereby often requiring produc-tion shut-down even though the vacuum experienced was well within the range easily tolerated by the vessel and contents.
According to the present invention there is provided, in a safety pressure relief assembly of the rupture disc type having a relief disc and a knife blade si~ed and positioned to sever said disc along a substantial transverse extent thereof in response to a pressure applied to said disc urging said disc onto said knife blade, the improvement of: ~a) stay structure supporting said disc out of contact with said blade up to a pre-determined pressure, said stay structure comprising support members bendable with said disc to positions providing a substantially open pathway for flow past said disc after said severing.
The relief disc is preferably a flexible sealing disc. The support members may be of finger-like configuration.
In the accompanying drawings, wherein are set forth by way of illustration and example certain embodiments of this invention:
Figure 1 is a somewhat schematic, vertical cross-sectional view of a safety pressure relief assembly embodying this invention;
Figure 2 is an exploded perspective view, on a reduced scale, showing various elements, and an optional element, of the assembly;
Flgure 3 is a vertical cross-sectional view, on a reduced scale, , showing one type of sealing disc support member in association with a seal-ing dlsc and cutting member;

Figure 4 is a plan ~iew, on the scale of Figure 1, showing various cooperating elements in assembled relation with portions broken away to reveal details af construction and relative position;
~ igure 5 is an elevational view of the assem~l~, on a scale, with a portion broken away revealing the relief disc and sealing disc ruptured in the positi~e direction and folded outwardly of the relief passageway and away from the passageway axis, and is found on the same sheet as ~igure 1;
Figure 6 is a view similar to Figure 5 but showing the cut sealing disc and support stays folded inwardl~ of the relief passageway and away from the passageway axis;
Figure 7 is a plan vie~, on a reduced scale, shQwing a modified form af sealing disc support member;
Figures 8 and 9 are plan views similar to Figure 7 and showing further modified forms of sealing disc support members;
Figure 10 is a fragmentary cross-sectional ele~ation, on an enlarged scale o~er Figure 1, showing a rupture disc, sealing disc, sealing disc support member and optional support member support, in assembled relation; and Figure 11 is a fragmentary crass-sectional elevation, on a further enlarged scale, illustrating an alternative construction wherein ~he sealing disc support member is connected to the sealing disc, and is found on the same sheet as Figure 1.
Material thicknesses in the drawings are often shown greatly exaggerated for illustration.
Referring to the drawings in more detail:
The reference .numeral 1 generally indicates one example of safety pressure relief assembly embodying this invention. The assembly 1 is shown mounted bet~een inlet and outlet supporting rings 2 and 3 ~hich are, in turn, clamped bet~een a pair of con~entional annular pipe flanges ~ and 5 . : . , . . : . . . ..

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having a plurality of bolts 6 disposed through openings 7 positioned spaced relationship about the peripheries thereof. ~t is to be understood, however, that other types of mounting arrangements may be used withou~
departing from the scope of this invention, for example, that shown in United States Patent No. 3,834,581, wherein a disc cutting member is supported within a centering ring having a flange secured between the same clamping surfaces as the disc flange. With this type of arrangement the inlet and outlet rings 2 and 3 may not be needed, their function being essentially assumed by the pipe flanges.
The assembly 1 normally blocks flow out of and into a pressure relief passage~ay 8 which communicates with a pressure ~essel or ~he like for the purpose o~ protecti~n against bQth overpreSsure and excessive vacuum conditions therein. Upon actuation of the assembly, as described below, p~sitive ~orward~ flow 9 ~Figure 5) and negative ~reverse) flow 10 ~Figure 6~ are permitted through the outlet passageway 11 which then c~nstitutes a continuation of the relief passageway 8.
The assemhl~ 1 comprises a relie disc 1~, of the forward ruptur-ing typeJ for example, similar to that shown in United States Patent No.
3,881,629, having a concave-convex portion 13 connected to a surrounding annular flange portion 14. In this example the flange portion 14 is flat, however, other flange configurations such as the known 30 degree seat or angular seat types may be used without departing from the scope of this invention.
The concave-convex portion 13 includes slits lS therein radiating outwardly, from a central area 16 thereof, to locations near the annular flange portion 14, thereby defining a plurality of sector shaped leaves 17 such that, upon forward rupture as shown in Figure 5, tearing takes place within ~he central area 16, allowing the relief disc to open as is further discussed below.

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'' .'' . ' " ' ' ', '' ' '' '', ' ' ' '.' ' ''.' ' ' ', ' ,`,".'"'' , '"' '' ' " . " ' `' The relief disc 12, in this example, further has a plurality of openings or perforations 18 through the sector shaped leaves 17, permitting rapid fluid flow past the concave-convex portion 13 in either direction without rupture, in absence of the sealing disc~ now described.
A sealing disc 22 may be constructed of various gas impervious, flexible, easily deformable and relatively flimsy materials, including synthetic resin poly~ers, such as .001-.OOS inch thick ~eflon*, which have little resistance against deflection under pressure differentials in the range desired and which are not subject to substantial ~`atig~e characteris-tics upon being slightly deformed in response to a large number of pressurereversing cycles. Under certain conditions, very thin metal stock may meet the necessary requirements. The sealing disc 22, in this example, has an annular flange portion 23, which, when assembled with the relief disc 12 is at least partially coextensive with the disc flange portion 1~, although, if desirable, suitable gaskets or sealing materials ~not shown) may be loca~ed therebetween. The sealing disc 22 here includes a central concave-convex portion 24 ~hich, upon assembly, is nested into the relief disc concave-convex portion 13, thereby blocking the perforations 18 to normally prevent -flow therethrough in the forward direction 9. The relief disc 12 and sealing disc 22, together, resist positive differential pressure in the passageway 8 up to a predetermined point when applied to the concave side 25 of the disc 22, Figure 1.
A sealing disc support member 30 may be constructed of various materials, including selected metals and synthetic resin polymers, and, in this example, has an annular flange 31, preferably at least partially co-exten-sive with the sealing disc flange portion 23 and three fingers or finger-like stays 32 integral therewith. The stays 32 form a saddle-like structure, pro-jecting inwar~ly and axially of the flange 31 into supporting engagement with, *Trade Mark ... . . ~ , . . ..

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or in close proximity to, the concave side 25 of the sealing disc 22 and com-prise relatively thin, elongated, spherically and planarly curved, bridging members which respectively extend from a peripheral flange anchor point 33 to a similar anchor point 34 located a substantial angular distance away on the support member annulus or passageway 35. The individual stay arrangement is here repeated three times, once for each stay 32, so as to produce a cir- -cumferentially symmetrical pattern about the passageway 35, with the stay legs of adjacent stays being generally parallel and spaced-apart, as best shown in Figure 4, so as to form a three-slotted, star-like aperture 36.
A cutting member 40, in this example, is suitably mounted as by welding, on the inlet ring 2 and radiates outwardly from an axially project-ing central point 41, in three legs 42, although, as noted above, other mounting structures may be used. Also, other shapes having a different number of legs are feasible. Each of the legs ~2 have an elongated, sharp-ened ~preferably surgically ground) blade edge 43 positioned in axially spaced relation from the sealing disc concave side 25 and extending a substantial -t-ransverse distance across the concave-convex portion 24. The blade edges 43 are here generally horizontal at the inner surface of the passageway 8 and, as they approach the center of the passageway, angle sharply, e.g., 30 degrees, axially ~upwardly in Figure 1), to the central point 41, thereby producing a generally pyramidal shape roughly approximating the concave curva-ture on the sealing disc side 25.
The stays 32 are positioned in predetermined offset relation with respect to the cutting member 40 so that the three-slotted aperture 36 is aligned both axially and circumferentially with the cutting member blades 42, whereby the sealing disc 22 is exposed to the respective blade edges along a substantial radial extent thereof for cutting therethrough upon deflec-tion of the sealing disc thereagainst, Figures 1 and 4.

The stays 3~ are designed to exhibit a resistance to bending or deformation sufficient to retain the flexible sealing disc out of contact with the cutting member blade edges 43 only up to a p~edetermined positive differential pressure applied to the convex side 45 of the sealing disc.
Such pressure will be experienced, in this example, through the relief disc perforations 18 and will essentially have no effect upon the relief disc 12.
In operation, positive differential pressure in the relief passageway 8 i5 resisted by the sealing disc 22, through support by the relief disc 12, up to the point where the stresses in the relief disc exceed the material strength thereof, causing the disc 12 to tear within the central area 16, whereupon both the relief disc 12 and sealing disc 22 ~ which has little appreciable strength when unsupported) will open as shown in Figure 5.
The leaves 17 and portions of the disc 22 will bend outwardly of the relief passageway and away from the passageway axis, providing a free and substan-tially open path for fluid flow 9 and pressure relief within the vessel ~not shown) into which the relief passageway communicates. Unless the relief flow 9 is sufficient to cause bending or deforming forces on the stays are not affected thereby and due to the open configuration of the support member 30, ofer little resistance to the flow. With reference to Figure 4, n~ethat there is considerably more relief space in the annulus or passage-way 35 than stay material, allowing ample relief area without displacement of the stays 32, unless flow velocity is relatively high and the stay material relatively thin and weak.
~ hen a vacuum condition exists in the passageway 8, the stays 32 support the sealing disc concave-convex portion 2~ against de1ection onto the cutting member blade edges 43 up to a predetermined pressure diferentia-l applied to the convex side 45. When this is exceeded, the stays 32 collapse, whereupon the sealin~ disc concave-convex portion 24 engages the blade edges .. ~ . . .. .
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- - -43, causing cutting therethrough along multiple substantial transverse extents thereof. The sealing disc concave side 25 will normally first con-tact the blade edges 43 at the point 41, producing an initial puncture which, under the force of the pressure differential, will rapidly spread in this example, into three radially exp~nding and expansive slits corresponding to the extent of the cutting member blade edges 43. The three sealing disc leaves 46 thus produced will easily and rapidly bend, with the stays 32, past the blade edges 43, inwardly toward the passageway 8 and away from the passageway axis, producing a large free flow path for rapidly relieving the excessive vacuum detected in the vessel (not shown). The vacuum relief flow 10 passes through the relief disc perforations 18 and, therefore, normally does not involve the relief disc 12 and it remains essentially unaffected. Ilo~ever, under certain combinations of conditions when the vacuum pressure differential is relatively high and the relief disc 12 is relatively weak in buckling resistance, the disc 12 may collapse downwardly onto the blade edges 43 where it will be supported against further deflect-ion, or under extreme conditions, cut and bent further past the edges in segments so as to more completely open the relief passageway.
A support ring 50 may be optionally utilized to provide additional

2~ and more uniform support to the stays 32 in the area of the anchor points 33 and 34, particularly under conditions where the sealing disc support member 30 is produced from very thin stock and/or the mounting ring supports or flanges are pitted or corroded.
Alternate examples of sealing disc support members are shown in Figures 7, 8, and 9. Figure 7 utilizes inwardly directed, cantilever mounted, fingers or stays 51 secured to the inner periphery 52 of an annular flange 53. The stays 51 may be riveted at 54 or otherwise integrally with the flange 53 by suitable punching and deforming operations. In this structure, ... ... ..... . , -, , . . ~ . -, , .: : . , :

the cutting member blade edges will radiate rom the center into the spaces between the respective stays 51, but not necessarily eq~i~istant therebetween.
In Figure 8, the sealing disc support member 55 covers much more of the space within the annulus 56 within the flange 57. ~ere, the stays ~re relatively large area, circular segments 58 ~hich are supported on the flange 57 at bridging points 59, the segments being separated in a triangular pattern to form a three-legged aperture 60, which, in the assembly, will align with the cutting member legs as described above in connection with Figure 4. This structure is intended for relatively firmer support of the sealing disc, or example, where the sealing disc material is particularly flimsy or higher pressure operating characteristics are soùght. In this form, operation of the assembly in the positive direction will usually in-volve the bending of the segments 58 outwardly with the sealing disc and relief disc.
Figure 9 shows a further type of support member 63 which is similar in shape and function to that described in connection with Figure 8, but having a larger number of stays or segments 64. The member 63 is adapted to be used with a six-legged cutting member rather than the three-legged member described above.
Lf desired, a suitable seal ~not shown) of thin, weak material maybe placed within the passageway 8 to protect the cutting member 40 and sealing disc support member from corrosive or other adverse effects of moist-ure, chemicals, etc., which may be present within the pressure vessel during normal operation. Such seals are known in the industry and do not appreciably affect the operation of the relief assembly except in the very lowest opera-tional ranges, where other precautions to so protect the assembly may be indicated.
Modified support characteristics may be produced by connecting the _ g _ : ::, ' . . - : ~ . `
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support member stays to the sealing disc. Referring to Figure ll, a sealing disc support member stay 66 is secured to a Teflon sealing disc 67 by means of a suitable adhesive 68~ the Teflon being etched on its concave side 69 prior to the application of the adhesive in order to produce the desired bond. In utilizing structures such as that shown in Fi~ure 11, the sealing disc will coact with the support member stays 66 to produce altered support-ing and collapsing characteristics over non-connected arrangements.
It has been found that the above described structure permits a great range of both positive and negative protection, with the two limits varying considerably in magnitude, such as 50 lb./in.2 positive vessel pressure limit and ~ lb./in.2 negative pressure limit, although it will be apparent to those skilled in this art that great variations in both pressure limits are feasible in utilizing this invention.
Under conditions where positive pressure limits need not be con-trolled by the assembly, a suitable screen or like structure ~not shown) may be substituted for the relief disc 12 to support the weak sealing disc against positive pressure which would otherwise deflect same away from the support stays. Reverse or vacuum limit operation would not be affected thereby, so long as sufficient relief openings were provided in the sub-stitute structure.
Further, the subcombination of sealin~g disc 22, support members30, 53, 55, etc., and cutting member 40, as described, form the basis of an independently useful rupture disc assembly suitable for protection against overpressures in the relatively low ranges, e.g., one to 10 inches of water column, where heretofore rupture discs were not considered reliable.
~ t is to be understood that while certain forms of this inven-tion have been illustrated and described, it is not to be limited thereto except insofar such limitations are included in the following claims.

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Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a safety pressure relief assembly of the rupture disc type having a relief disc and a knife blade sized and positioned to sever said disc along a substantial transverse extent thereof in response to a pressure applied to said disc urging said disc onto said knife blade, the improvement of:
(a) stay structure supporting said disc out of contact with said blade up to a predetermined pressure, said stay structure comprising support members bend-able with said disc to positions providing a substantially open pathway for flow past said disc after said severing.

2. In a safety pressure relief assembly of the rupture disc type having a relief disc and a knife blade sized and positioned to sever said disc along a substantial transverse extent thereof in response to a pressure applied to said disc urging said disc onto said knife blade, the improvement of:
(a) stay structure supporting said disc out of contact with said blade up to a predetermined pressure, said stay structure comprising finger-like support members bendable with said disc to positions providing a substantially open pathway for flow past said disc after said severing.

3. An assembly according to claim 1 or 2 wherein the relief disc is a flexible sealing disc.