GB2126281A

GB2126281A - Pressure relieving devices

Info

Publication number: GB2126281A
Application number: GB08221094A
Authority: GB
Inventors: Robin Andras Benedek
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 1982-07-21
Filing date: 1982-07-21
Publication date: 1984-03-21

Abstract

This invention relates to pressure relief devices and particularly to such devices which are capable of operating at relatively low pressures. The device comprises a wall (1) made from a fluid-impermeable material, the wall including a weakened zone (3). The weakened zone shown is a part-sheared portion of the original wall (1). Several alternative ways of producing the weakened portions are described. In one example a disc or body is forced into an opening in the wall (1) and the junction treated with a sealant. In another example a body placed in an opening in the wall is held in place by forming indentations at the junction, the latter being subsequently coated with a sealant. <IMAGE>

Description

SPECIFICATION Pressure relief device This invention relatesto pressure relief devices and particularly to such devices which are capable of operating at relatively low pressures.

By a "pressure relief device" is meant a device which will permitthe escape offluid (liquid orgas) from a system, so asto relieve the pressure therein, when the difference between the fluid pressure in the system acting on one side ofthe device and the atmospheric or other pressure acting on the other side reaches a predetermined value or a value within a predetermined narrow range ofvalues.

The present invention is especially but not exclusively concerned with pressure relief devices which will operate at pressure within the rangefrom about 30to about 700 milibars (about 0.45 to about 10.15 psi).

Known pressure relief devices include pressure reliefvalves ofthe typethat are used on steam engines and also so-called "normal" and "reverse" bursting discs. These are generally made of metal, such as stainless steel or an aluminium alloy and consist of a domed central portion which is integral with a surroundingflatannularflange. In use, this flange is clamped in a fluid-tight manner between first and second metal collars normally by means of threaded bolts which pass through the first collar and screw into tapped holes in the second, with the heads ofthe bolts recessed below the surface ofthefirst collar and the ends ofthe bolts when screwed home not projecting beyond the surface of the second.

The resulting unitary combination of bursting disc and collars is generally, in turn, clamped in a fluid-tight manner between a pairofpipeflangesand, with 'normal' bursting discs,thearrangement is such that the higher pressure acts on the concave surface of the dome.

With a reverse bursting disc on the other hand,the higher pressure is arranged to act on the convex surface of the dome and, in this case, a cruciform assembly of cutting blades is disposed belowthe concave surface ofthe dome with their cutting edges facing the interior ofthe dome. Generallythesecutting edges are arranged roughly in the plane of that surface ofthe bursting discflangewhich is the more remotefrom the dome.

The nature and thickness ofthe material ofthe dome of a bursting disc and also the dimensions ofthe dome are carefully controlled so that at a temperature within a specified narrowtemperature range,the dome of a normal bursting disk will burst outwards when the 'pressure difference', that is the difference between the pressures acting on the concave and convex surfaces ofthe dome reaches a predetermined value.

In the case of a reverse bursting disc, the higher pressure acts on the convex surface ofthe dome ofthe disc so that at a predetermined pressure difference, the dome collapses inwards and is cut into two or more sections bythe cutting blades. It is forthis reason that such a disc is sometimes referred to as a "reverse buckling disc" or simply as a "buckling disc".

The lower limitof bursting pressure for a normal bursting disc, that is, the lower limit of the pressure difference at which the bursting ofthe dome will be effected, is typically, for a dome of 5cm diameter, about 1.75 bars (that is, about 25 psi). lfverythin aluminium foil is used forthe dome, however, bursting pressures as lowasabout300 milibars (about 4.5 psi) may be achieved.

A bursting disc having a bursting pressure in the region of 300 milibars is, however, a rather specialised, unconventional device which is difficultto manufacture, difficultto store and handle because of thefragilityofthe dome and pronetoirregularand unpredictable failure. The lower bursting pressure of a conventional normal bursting disc should be regarded, therefore, as about 1.75 bars.

An object ofthe present invention is to provide a pressure relief device which is relatively robust but which will operate reliably at pressure differences of from about30to about 700 milibars (about 0.45 to about 10.15 psi) and, in particular, at pressure differences of from about 50 to about 200 milibars (about 0.73 to about 2.9 psi).

A pressure relief device according to the present invention, comprises a wall madeoffluid-impermeable material and, forming part of the wall, a zone of reduced resistance to displacement with respectto the remainder ofthe wall, such that when the difference between the pressures acting on opposite sides of the said zone reaches or closely approximates a predetermined value, the said zone is subject to displacement so thatthe pressure on the higher pressure side ofthe zone is relieved. The said displacement may result in the expulsion of the zone from the wall orion the development of an opening between the zone and the wall through which fluid may escape.

The present invention will now be described, where appropriate, with reference to the accompanying drawings which illustrate a number of embodiments thereof.

In the embodiment shown in Figure 1, reference numeral 1, is aflat plate orwall from which a rectangular portion is removed to leave a rectangular frame 2. A rectangular, relatively rigid plate 3 is located withintheframe2 by means of a sheet of aluminium foil 4which is secured to the plate 3 and to partofthewall 1 surroundingtheframe2bysome suitable adhesive, such asARALDITE (Registered Trade Mark) so asto leave a small clearance 5, about 0.0625 of an inch, say, between theframe 2 and the plate 3.

A pressure Pacts on the plate 3 as shown in Figure 2 (a section taken on X-X of Figure 1) and, since 3 is relativelyrigid,thisactsasa piston and the total force acting on 3 (pressure x area) is transferred to the relatively thin strip of aluminium foil bridging the gap between the plate and the frame. Th is thin strip of foil yields at a predetermined value of the pressure difference (P-P') and the pressure P acting on the higher pressure side ofthe wall and plate is relieved.

In the embodiment illustrated diagrammatically in Figure3, a zone 3, conveniently a disc, is part sheared from the wall 1 as shown. The high pressure acts in the direction ofthe arrow P and the pressure difference, namely, the value of (P-P') at which the zone 3 will be displaced depends upon the extent of the unsheared portion 6.

In an alternative embodiment, a disc or body of some other shape is forced into an interference fit in a corresponding opening in a wall and any tendency for fluidto escapefrom the higher pressuresidethrough the wall is inhibited by treating the junction between the body and the wall with a sealant. Suitable sealants are silicone rubber preparations, EVO-STIK (Reg isteredTrade Mark) etc.... but we preferto use heat resistant sealants.

In yet another embodiment, a body is initially a clearance fit in an aperture formed in a wall. In this embodiment a series of indentations is formed atthe junction between the respective peripheries so as to set-up a series of interference fits. In this way body is held within the aperture and an area including at least the said peripheries is coated with a sealant thereby rendering fluid tight that which was originally a clearance fit.

If desired, the series of interference fits may be extended so as to merge with each otherto form a continuous interference fit between the peripheries of the body and the aperture. This continuous interference fit may be formed by a roller mounted to follow a path which conforms to the peripheries ofthe body and the aperture. The roller may be constrained to operate on and thereby deform either the peripheral region of the aperture orthe peripheral region of the body. Alternatively, the roller may bridgethejunction between the peripheries ofthe body and the aperture and thereby deform the peripheral regions of both the aperture and the body to create the interference fit. To assist in the deformation, the circumferential surface ofthe roller is convex in diametral cross-section.

The single roller may be replaced bytwo coaxial and side-by-side rollers disposed to operate on opposite sides of the junction between the aperture and the body. The effect of two coaxial rollers may be otained using a single roller having a circumferential groove of width greater than the clearance fit between the aperture and the body.

When a single roller is used, we preferto operate in the region of the periphery of the body so as to minimise distortion of the wall. Yet again, the deformation to produce the interference of one or other or both ofthe peripheral regions ofthe aperture and the body in the neighbourhood ofthejunction between the body and the aperture may be achieved by coining or pening. We prefer, again, to deform the peripheral region ofthe body alone.

The body and the aperture may be circular, rectangular or other symmetrical or assymetrical shape and may be of uniform or non-uniform cross-section.

Yet again the body may be formed from that part of the wall removed to create the aperture.

In each ofthe embodiments ofthe invention involving one or more interference fits in an aperture in a wall of a body which constitutes a displaceable zone the extent and tightness of the interference fit, as well as the dimensions ofthe body and the material or materials ofthe body and the wall govern the pressure difference at which the body will be displaced and the pressure relieved. A number oftests were carried out using a single roilerto produce a continuous interference fit between the peripheries of the body and aperture as previously described. In these tests the wall and a disc-shaped body were made from 16 swg halfhard rolledaluminiumsheetandtheaperturewas 6" in diameter.

The continuous interference fits were created by forming depressions.

1) in the wall close to the periphery of the aperture; 2) in the body close to the periphery of the body; 3) in both the body and the wall bridging the junction between the peripheries and the body.

The roller used in these tests had a diameter of 1/2", a thickness of " and the circumferential edge was rounded at a radius 118". The depressions formed by the rollers in successive tests had depths of 3,5,10,15 and 25 thou. inch.

Further tests were carried out where the material of the wall and the body was 18 swg mild steel and the aperture was 51/2" in diameter.

This further series of tests was divided into two parts. In the first part of the tests the continuous interference fit was created by simultaneously forming a depression in both the body and the wall bridging the junction between the aperture and the body.

These simultaneous depressions were formed using a roller of diameter 1" and width 1/2" with a generally "V"-shaped groove formed centrally around the periphery of the roller, the rollerthus resembling a pulley. The two simultaneouslyformed depressions had a depth falling underthe range of 3-5 thou. inch and the continuous interference fitformed thereby was found to fail at a pressure difference of approximately 1 psi. In the second partofthetestsasingle rollerwas used similarto that detailed above and used for the tests carried out on the 16 swg sheet aluminium wall and body. In the second part ofthe tests the continuous interference fit was obtained by forming a depressionofdepth5thou. inch inthebody close to the peripherythereof. Again this was found to fail at a pressure difference of approximately 1 psi.

Figure 4 shows yet a further embodiment using two bodies 7 and 8 of which 7 is disposed on one side of the wall 1 to overlap and close the aperture and is disposed on the other side of the wall 1 to overlap and close the aperture. The two bodies 7,8 sealingly engagethewall and the spacetherebetween is partiallyevacuatedtodrawthe bodiesagainstthe appropriate wall surfaces. A sealant or sealing rings 9, 10 may be disposed between the bodies 7,8 and the respective wall surfaces as shown.

If it is required thatthe device should fail at, for example, a pressure of 1 psi above ambient-pressure and the plate 7 is designed to fail when a pressure difference of 10 psi exists across it, it may be made to fail at psi if the partial vacuum is -9 psi. Failure ofthe device or displacement of the body 8 may be assisted by biasing means not shown disposed to urge the body8awayfromthewall 1.

The embodimentofthe invention ofthe invention involving one or more interference fits between a body and an aperture in a wall as previously described are particularly well adapted to provide explosion relief panels operating at a pressure difference of approximately 1 psi. Such explosion relief panels may be used in the walls of chambers in which combustible gases are burnt as in certain types of domestic gas fires. In the case of domestic gas fires we preferto make the wall and the body from stainless steel.

CLAIMS: Filed on 17.8.83 1. A pressure relief device comprising a wall made from a fluid-impereable material, the wall including a zone which has a reduced resistance to displacement with respectto the remainder ofthe wall, such that when the difference between the pressure acting on opposite sides ofthe said zone reaches or closely approximates a predetermined value, the said zone is subjectto displacement so that the pressure on the higher pressure side ofthe zone is relieved.

2. A pressure relief device according to Claim 1 wherein the said displacement results in expulsion of the zone from the wall thereby relieving the pressure on the higher pressure side ofthe zone.

3. A pressure relief device according to Claim 1 wherein the said displacement results in the develop mentofan opening between the zone and the wall thereby relieving the pressure on the higher pressure side ofthe zone.

4. A pressure relief device according to any one of Claims 1 to 3 wherein the zone and the wall are separated buy a clearance, a bridge member sealingly interconnecting the wall and the zone and bridging the clearance, the bridge member being made from a material which yields at a predetermined value of the pressure difference acting on opposite sides ofthe zone thereby relieving the pressure on the higher side ofthe zone.

5. A pressure relief device according to Claim 1 wherein the zone is partially sheared from the wall therebyforming a line of weakness between the zone and the wall, the line of weakness yielding at a predetermined value ofthe pressure difference acting on oppositesidesofthezonethereby relieving the pressure on the higher pressure side ofthe zone.

6. A pressure relief device according to Claim 1 or Claim 2 wherein the zone is an interferencefitwithin the wall.

7. A pressure relief device according to Claim 1 or Claim 2wherein the peripheries ofthe wall and the zone initially present a clearance fit to each other and wherein the zone is retained in the wall by means of a seriesofindentationsformed atthejunctionbetween the said respective peripheries.

8. A pressure relief device according to Claim 6 or Claim 7 wherein the junction between the zone and the wall is sealed.

9. A pressure relief device according to Claim 7 wherein adjacent indentations of the said series of interference fits merge to form a continuous interfer encefit between the peripheries of the zone and the wall.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. be used in the walls of chambers in which combustible gases are burnt as in certain types of domestic gas fires. In the case of domestic gas fires we preferto make the wall and the body from stainless steel. CLAIMS: Filed on 17.8.83

1. A pressure relief device comprising a wall made from a fluid-impereable material, the wall including a zone which has a reduced resistance to displacement with respectto the remainder ofthe wall, such that when the difference between the pressure acting on opposite sides ofthe said zone reaches or closely approximates a predetermined value, the said zone is subjectto displacement so that the pressure on the higher pressure side ofthe zone is relieved.