1. Field of the Invention
The present invention relates generally to a pressure relief method and apparatus, and more particularly, but not by way of limitation, to a method and rupture disk assembly for relieving predetermined positive pressure differential in one direction and/or predetermined negative pressure differential in the opposite direction.
2. Description of the Prior Art
A variety of pressure relief methods and devices of the rupture disk type have been developed and utilized heretofore. Generally, the devices include a rupture disk supported between a pair of complementary support members or flanges which are in turn connected to a relief connection in a vessel or system containing fluid pressure. When the fluid pressure within the vessel or system exceeds the predetermined design pressure of the rupture disk, rupture occurs allowing fluid pressure to be relieved unidirectionally from the vessel or system to the atmosphere or to another vessel or system.
Bidirectional pressure relief methods and rupture disk assemblies have also been developed and utilized heretofore, see, e.g., U.S. Pat. Nos. 3,091,359 to Wood, 4,079,854 to Shaw et al., 4,301,938 to Wood et al., 4,479,587 to Mundt et al. Such bidirectional rupture disk assemblies generally have included a flexible sealing member positioned adjacent a perforated rupture member so that when a predetermined positive pressure differential is exerted across the sealing member and rupture member, the rupture member ruptures causing the sealing member to also rupture and pressure to be relieved through the assembly. When a negative pressure differential is exerted on the sealing member by way of the perforations in the rupture member, the sealing member is caused to contact knife means positioned in the assembly adjacent thereto whereby the sealing member is severed and the negative pressure differential relieved. The term "positive pressure differential" is used hereinafter to mean a pressure differential exerted between first and second zones across a rupture disk assembly whereby the pressure in the first zone is above atmospheric pressure and is significantly higher than the pressure in the second zone. The term "negative pressure differential" is utilized hereinafter to mean a pressure differential exerted across the rupture disk assembly in the opposite direction, i.e., from the second zone to the first zone. While a negative pressure differential can occur as a result of the pressure in the second zone being above atmospheric pressure and being significantly higher than an above atmospheric pressure existing within the second zone, more typically such negative pressure differential is a result of a vacuum in the first zone and atmospheric or slightly higher pressure in the second zone.
While the prior bidirectional methods and apparatus have been utilized successfully, problems have been encountered in applications where negative pressure differential relief is required at very low specific levels. An example of such an application is a pressure vessel which normally operates under considerable internal pressure and requires positive pressure differential relief at a normal level, but which requires negative pressure differential relief at a very low level, e.g., when a vacuum occurs within the vessel whereby the negative pressure differential is between four inches and nine inches of water column with no opening occurring until the negative pressure differential exceeds four inches of water column. In such applications, prior methods and apparatus have often allowed the piercing and failure of the sealing member prior to when the negative pressure differential at which failure is supposed to occur is reached. This in turn, causes needless frequent shutdowns and replacements.
By the present invention, an improved method and rupture disk assembly are provided which obviate the problem described above as well as other problems associated with prior methods and devices.