REVERSE DOMED BUCKLING DISCS, A PRESSURE RELIEF DEVICE, THEIR PERFORMANCE AND PROBLEMS. B. K. WATTON * G. W. BRODIE ** C. ENG. F.I. MECH.E.

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1 REVERSE DOMED BUCKLING DISCS, A PRESSURE RELIEF DEVICE, THEIR PERFORMANCE AND PROBLEMS. B. K. WATTON * G. W. BRODIE ** C. ENG. F.I. MECH.E. A widely used method employed for the relief of overpressure in reactors subject to Exothermic conditions is to fit reverse buckling discs to the vent system. The performance of such discs is usually superior to other types of safety discs. They are capable of withstanding heavy and pulsating working conditions even at elevated temperatures. Ihey will usually withstand vacuum conditions without the necessity of assistance from reverse pressure support. There are several designs of reverse buckling discs. The paper explains the operating features of the various designs and illustrates how mal-functioning can be experienced with these devices. 1) INTRODUCTION A Reverse Domed Bursting Disc is one which in operation has its dome against the direction of the bursting pressure. It is designed to fail, by buckling, bending or shearing forces. The performance of Reverse Domed Discs is generally superior to Conventional Simple Domed Discs, Conventional Slotted lined discs or Graphite Discs of flat form. In exothermic conditions however, a conventional disc is likely to begin to relieve pressure marginally faster than a reverse domed disc. 2) REVIEW OF REVERSE DOMED BURSTING DISCS There are currently available the following different types :- (.1) Discs which fail by being sheared radially by knife blades which are mounted on the vent side of the disc. (.2) Discs which fail by being sheared circumferentially by serrated cutters which are mounted on the vent side of the disc. (.3) Discs with preformed diametral grooves which on disc reversal tear open at the grooves. (.4) Discs which at reversal slip out of Holder. (.5) Discs which shear around the disc dome at reversal. These types are available in metal or pure Graphite. * MARSTON PALMER LIMITED, WOBASTON ROAD, FORDHOUSES, WOLVERHAMPTON. STAFFS ** 2, BARNFIELD CRESCENT, WELLINGTON, TELFORD, SHROPSHIRE. 51

2 3) GENERAL FEATURES OF REVERSE DOMED BURSTING DISCS (.1) COMPRESSIVE LOADING Conventional discs are caused to fail by tensile stresses induced by the bursting load. Reverse Domed discs are subject to Compressive loads (See Fig 1) (.2) MATERIAL THICKNESS Fig 1. As an example a 2" (50mm) nominal bore conventional simple domed disc when made in Stainless Steel to burst at 20 Bar 20 Deg.C. would require a material thickness of approximately " (0.04mm). For similar conditions using a reverse domed disc, the material thickness would be approximately 0.012" (0.30mm). This is 7.5 times the thickness for a conventional simple domed disc. Were Nickel to be used the thickness for a conventional simple domed disc would be about half that of a reverse domed disc. (.3) CREEP AND CORROSION Because material used to manufacture reverse domed discs is thicker and is in compression, creep problems can be generally ignored. Resistance to corrosion is also much improved. (.4) WORKING CONDITIONS Reverse domed discs subject to even violent pulsating conditions will not fail unless the working pressure/bursting pressure ratio recommended by the manufacturer is exceeded. This ratio is generally 0.9 but can be as high as 0.95 for some types. (.5) TEMPERATURE EFFECT All reverse domed discs which are to operate at temperatures other than ambient should be temperature tested. In conventional simple domed discs elevated or sub-zero temperatures can have a significant effect on the maximum working/bursting ratio. This is not the case with reverse domed discs. At high temperatures it should be possible to operate at a working/bursting ratio of 0.9. (.6) HYDRAULIC CONDITIONS Care must be excercised when reverse domed discs are to be used 52

3 in hydraulic applications. The reversal speed of a reverse domed disc is extremely rapid. The smaller the disc, the faster the speed, low pressure discs slower than higher pressures. e.g. A disc of 18" (450mm) nominal bore designed to reverse at 0.5 psi (0.035 Bar) would reverse in a time of 40 milli seconds. A disc of 4" (100mm) nominal bore designed to reverse at 10 Bar would reverse in a time of 5 micro seconds. The reason for caution lies in the fact that the speed of build up of Hydraulic pressure is often too slow for most of the reverse domed designs. Users intending to employ reverse domed discs in Hydraulic circuits should make certain from suppliers that the design offered will perform safely in ALL circumstances. (.7) PROOF TESTING The claims of manufacturers with regard to the high working/ bursting ratio coupled with relatively low tolerances appears to be treated with some sceptiscism. Users therefore sometimes consider it necessary to "proof test" discs before use, particularly where exothermic conditions are possible. Often in such cases, the user "proof tests" hydraulically, testing in excess of the maximum permitted working pressure. Should these conditions take place the pressure will deform the dome. This condition is not usually detected or appreciated by users who see something which appears to be perfect. The disc, now destroyed of its original strength is assembled to plant and slowly reverses at a pressure well below the specified burst pressure. This loss of speed at a much lower pressure results in most designs of reverse domed discs failing to open and vent as intended causing a potentially highly dangerous situation, particularly where exothermic conditions apply. Testing should wherever possible be carried out using gas as the pressure medium. (.8) PRECISE TOLERANCES In conventional simple domed discs the material thickness is directly proportional to the burst pressure. This is not the case in reverse domed designs. For a given thickness of material a range of burst pressures can be produced. This is achieved by variation of dome heights. This feature produces a further advantage. Because a manufacturer can vary the burst performance in this manner precise and close tolerances can be achieved without the necessity of using a manufacturing range. (.9) DISC MOUNTING Most designs require to be installed following the manufacturers instructions carefully. Reverse domed discs must be securely mounted and the arrangements should ensure that under load a disc is prevented from radial spread. If such a disc is allowed this freedom the performance is likely to be impaired. One feature is that radial spread reduces the height of dome causing a low burst pressure. 53

4 4) DESIGN PARTICULARS OF REVERSE DOMED DISCS WHICH OPEN BY SHEARING THE MEMBRANE (.1) KNIFE BLADED TYPES The form of disc is offered by several suppliers. It was originally patented in The arrangement comprises a pair of flanges between which the disc is gripped. This clamping pressure is often provided by the plant flanges when the unit is installed. In some cases the holder flanges can be secured by a specified torque applied to the disc clamping screws. Where this feature is provided it is still necessary to tighten the plant flanges to a given value. In the vent flange of the holder, 3, 4 or more radially disposed cutting blades are positioned. Sometimes these are welded to the flange, in other cases, they are mounted in a separate ring which is recessed into the flange. (see Fig. 2). As pressure is applied to the discs the dome initially collapses at the edge fanning out across the diameter of the disc, at high speed. This causes the dome to be impaled on the knife blades which shear the material into segments, thus allowing the system to vent. (.2) SERRATED EDGE CUTTER TYPE In most features this type is similar to the knife bladed. On the vent side is mounted a ring which is provided with serrations for approximately 65% of the circumference. The remaining space is used to fashion a radius edged segment. (See Fig 3) As the disc reverses, the dome is forced onto the serrations which rip the material for about 65% of the circumference. The free material opens flap fashion turning about the segment remaining attached to the flange of the disc at. this point. 54

5 Fig. 3 5) REVERSE DOMED DISCS WHICH OPEN BY TEARING THE MEMBRANE (.1) GROOVED TYPE The dome is produced with a flange, after doming grooves are produced in the vent side of the disc in cruciform pattern. To obtain the correct burst pressures a second doming operation may be necessary. The disc is clamped under specified loading in a pair of flanges. When pressurised to destruction the disc reverses in the normal manner into the vent bore flange. As this takes place the weakened section at the grooves is torn open along the groove lengths, petalling out to produce a clean vent bore, (see Fig 4). Fig 4. 55

6 (.2) WELDED TYPE The dome of the disc is removed from its flanges by machining and mounted in a recesses metal support ring. The joint between the two is then sealed by welding. (see Fig. 5) Fig 5. In operation the dome collapses in the normal manner. As it passes through the support ring the edge of the flange is ripped from the support ring at the weld. Arrangements down stream are usually provided to catch the spent disc. 6) GRAPHITE REVERSE DOMED DISC The most recent and unique examples of reverse domed discs are those made from pure Graphite. They peform in a similar manner to those made from metal and have similar advantageous operating characteristics. The dome is surrounded by a flange used to secure the disc in its holder mounting. (see Fig 6). On reversal the dome collapses and shears around the edge, freeing the disc to provide venting. Fig 6. 7) REVERSE DOMED DISCS WITHOUT CUTTING OR SHEARING CHARACTERISTICS (.1) POSITIVELY CLAMPED TYPE In this design, the disc is not provided with an identity tag. 56

7 The dome is surrounded by a very narrow flange which on mounting in the holder is precisely positioned in a suitable recess. A clamp ring is provided with screws which clamp the disc to the holder to a specified low torque loading. The system is sealed by a gasket positioned between the pressure side of the disc and the holder. The clamping pressure from the plant flanges has no influence on disc performance. (see Fig 7.) Fig 7. (.2) TYPE IN PRECISE HEIGHT OF RECESS IN ASSOCIATION WITH '0' RINGS Similar in every way to (7.1) but in this case the clamping ring is secured metal to metal with the holder providing a precise recess height slightly longer than the thickness of the disc material. This arrangement avoids necessity for torque specifying either in the holder or plant flanges. The system is sealed by an '0' ring positioned in the holder body. '0' Rings are specified to suit processing media and/or temperature of operation. The usual materials are Viton A, P.T.F.E. or Metal. (see Fig 8). 57

8 (.3) POSITIVELY CLAMPED PROTECTED BY GRAPHITE By a combination of (7.1) and (6) a disc of composite form is mounted as in (7.1) and the graphite in this case also forms the sealing gasket. In operation types (7.1), (7.2) & (7.3) the disc inverts at the edge, rapidly fanning out to the opposite disc edges. This action results in a force which results in a radial pull of the edges towards the disc centre. This combined with the process pressure propels the disc towards the vent where usually suitable arresting devices are employed to catch the spent disc. (.4) TYPE WHICH COMBINES VENTING WITH VACUUM RELIEF This design is similar in every feature excepting that the metal disc is perforated through the dome with a number of circular holes. The graphite membrane seals the system from positive pressure. In the reverse situation where the vessel is subject to vacuum or where a reverse pressure must be relieved, the graphite is pressurised in an analagous manner to that of a simple domed disc, the holes in the disc allowing the pressure to reach the graphite. This bursts the graphite at the pressure required. (.5) POSITIVELY CLAMPED TAGGED DISCS A further design which operates similarly to (7.1) is provided with a tag. This serves to arrest the disc on bursting. (see Fig 9). Fig 9. "Applications" for patents have been filed for two arrangements. In both cases the disc is surrounded by a flange of similar thickness to the disc material. System sealing in one case is accomplished by a plastic gasket straddling the disc and flange. The other utilises a welding process to join the two parts together. In operation the disc is parted from the joint and hinges on a bore preparation to be retained by the disc tag. 8) PROBLEMS RELATED TO REVERSE DOMED DISCS IN OPERATION (.1) (GENERAL) Ref. (3.6) It is imperative that the speed of reversal is not 58

9 hindered in any way if satisfactory venting is to be achieved. In most of the examples described above difficulties may be experienced by not having sufficient compressed gas on the pressure side of the disc. The exception from this requirement is (6) the graphite reverse domed disc. (see Figs 10 and 11). Fig 10. Fig 11. Secondly, if for any reason the dome of reverse domed disc be deformed, e.g. by mechanical damage in either the form of very shallow local depressions or indentations of the true spherical dome, this will result in, a) the disc reversing under pressure below its specified bursting pressure. b) the speed of reversal will take place at a fraction of that necessary to produce normal venting conditions with metal discs. A further point to note is that reverse domed discs do not give venting relief as quickly as do conventional simple domed discs. 59

10 (.2) KNIFE BLADED TYPES (see 4.1) Subject to conditions explained above (8.1) a disc may reverse at lower than specified burst pressure, and simply lay on the cutting blades. This causes a major obstruction to venting. In exothermic conditions it could cause the system to experience pressure conditions above the safe working pressure, (see Figs. 12, 13, 14 and 15). Figs. 12 and 13. Fig 14. In the smaller discs sizes 6" (150mm) nominal bore and below discs at low or sometimes medium pressures will not have sufficient energy at venting to fully open the disc. This may result in the necessity to use a larger nominal bore simply to obtain the desired venting capacity. Corroded, damaged, or blunt knife blades could result in inefficient shearing of the disc at bursting. Even when blades 60

11 are sharp, fragmentation of discs can be experienced. Discs are rarely cut cleanly. Fig 15. One type is provided with an anti-fragmentation ring. This is a device which is positioned between the disc and clamping device to avoid the tearing of discs at the edge of the dome. Patterns of normal disc bursting show fragmentation in some cases which clearly questions the effectiveness of this feature. Disc holding arrangements should positively prevent disc spread when subject to pressure. Should a disc by some mischance be mounted dome to vent, then from (3.2) the system is clearly unsafe. (.3) SERRATED EDGE CUTTER TYPE (see 4.2) This example of reverse domed disc is subject almost to all the problems in (8.2). The serrated edges are not sharp. In cases of malfunctioning due to slow reversal it is possible for the disc to lie on the serrations causing a complete obstruction to the vent system. (see Figs. 16, 17, 18). Fig

12 Fig 17. (.4) GROOVED TYPE (see 5.1) Fig 18. Because of the difficulty of manufacture the tolerances are usually not as small as with discs not using grooves. In normal operation the disc usually gives good clear venting but where the dome has been damaged there can be insufficient tearing of the grooves, or alternatively the petals are not forced outwards to give a clear vent bore, (see Figs 19, 20 & 21). In cases of incorrect mounting i.e. dome to vent, the pressure would open the disc, but at a pressure well above the safe pressure for the system. (.5) WELDED TYPE (see 5.2.) Since this disc is welded to a support ring it is necessary to have sufficient energy to reverse the disc and tear the weld so as to release the disc to the vent. Slow reversals result in the disc inverting but usually remaining firmly attached to the 62

13 Fig 19. Fig. 20. Fig

14 Support Ring (see Fig. 22). Fig 22. A disc incorrectly mounted, dome to vent, would usually require much more than specified burst pressure to remove the disc from the support ring. (.6) GRAPHITE REVERSE BUCKLING DISCS (see 6) The problems with this disc are minimal. It will respond to both Hydraulic and Gas pressure. If incorrectly mounted, dome to vent, the disc would operate in an analagous manner to a conventional simple domed disc. In some cases, the burst pressure in this mode would be slightly higher than the specified burst pressure, in other cases lower. (.7) SLIP OUT TYPES (see 7.1 and 7.2) These discs having a very narrow flange and no tag, could be predamaged as described in (8.1) In operation such a disc would slowly reverse. Whether or not the disc would part from the holder is dependant on the nominal bore and the pressure at which the disc is reversed. (see Fig 23). There are at least 3 possibilities :- a) the disc could vent normally. b) the disc could reverse, be pulled radially inward causing a pressure leak of the system. c) the disc could reverse but remain in the holder and continue to seal the system. Fig

15 In case b) the loss of system pressure would indicate that an examination of the disc assembly is required. In case c) there would be no indication that the disc was in an inverted condition. However the disc after reversal would be in a relatively weak condition gripped and supported by a small flange. As the pressure increased the disc would be removed from the holder. This pressure depends principally on the nominal bore. The larger the disc, the lower the pressure of disc release. In cases of incorrect mounting, ie. dome to vent the same principal applies. This design vents by removing the disc from the holders, ie. no cutting features. Therefore discs of very high pressure can be made. In these cases the problems of arresting a disc after burst must receive special attention. (.8) POSITIVELY CLAMPED TAGGED DISCS (see 7.5) This disc is new and therefore one can only analyse the design features generally and comment on probabilities. Much of the content in (8.7) should apply. However the patent application describes two different disc onstructions. It could be expected that the type sealed by welding would produce problems similar to those described in (8.5) above. In both cases the disc is intended to be retained after bursting by the tag. Due to the enormous speeds involved, the disc may be ripped free from the tag. 65