An Indicating Rupture Disk for Gas Cylinders Thomas J. Bruno Thermophysics Division, National Bureau of Standards, Boulder, CO 80303
Almost all com~ressedeas cvlinders and containers (with the exception of lkcture bottles) are equipped with relief devices that provide for the rapid venting of the cylinder contents under conditions of excessive pressure or high temperature.' These situations may be caused by accidental overfilling of the cylinder or by the container being exposed to fire or excessive temperature. The pressure relief devices currently used are those involving a-fusible metal plug, a frangible rupture disk, or a combination of the two devices. Themost common type is the frangible rupture disk. Frangible rupture disks fail (i.e.. rupture under nonemergency conditions at ambient temperature and allowable internal pressure) for a variety of reasons. The most common cause of a s~ontaneousfailure is the develonment of corrosion stresses in the disk itself. The development of corrosion on the outside surface of the disk (the side of the disk not wetted by the gas in the container) is caused by the environment in which the cylinder is stored or used. Clearly, the acidic atmosphere of many laboratories is an example. A less obvious but more striking example is the hospital environment. Compressed gas cylinders that are used in hospitals must be equipped with chromium-plated valves. In order to maintain a pleasing appearance, hospital housekeeping personnel routinely clean and shine these valves, often with ammoniacal detergents. Theuse of strong cleaners can cause a corrosion failure of a rupture disk within 24 hours, especiallv of disks made from conoer. he consequences of a frangible disk failure were illustrated dramaticallv several months aeo in one of the laboratories in the ~ h e r m 6 ~ h ~ sDivision ics i f NBS. The frangible disk on a full nitrogen cylinder ruptured spontaneously, quickly releasing the contents of the cylinder. The effect of this pressure release on the laboratory was astounding. Expeusive equipment was blown about the room with cyclonic force. In one part of the lab, alarge cardboard barrel came to rest on top of a gas chromatograph. The entire room was strewn with papers and debris. Fortunately, the incident occurred in the evening, otherwise serious injury to personnel would probably have resulted. Currentlv. there is no reauired maintenance for these pressure relief de\.ires. Compreised gns suppliers only check the relief device housing for leaks after fillinr the c\4inder. It is not possible to examLe the surface of thehisk itself, since this would require removine the housine and therebv ruinine the sealing surface of the disk. We havetherefore fabricatei a device, referred to as an indicatina rupture disk, to allow a determination to be made as to the&p~,ximoterondirionof the sewice rupture disk itself. The device (see figure, consists of a collar (A) machined from a secrion of 3600 free-
Work performed at the National Bureau of Standards: paper not subject to copyright. Guide to Safe Handling of ~ompreisedGases: Matheson Gas Products. Inc.. 1983.
'
Photograp showmg the lndlcatmg npture ask on a housmg 5 mllar 10 the ho~sng shown an the eh Tne collar IAI and me md!calmg disk (81are shown by the arrows.
cutting brass. This collar contains a second rupture disk (B), identical to that in contact with the gas or fluid, that is held in the interior side of the relief device. This second disk. which is in the same general environment as the service disk inside, is easily accessible for visual examination with the unaided eye or with a hand-held mihoscope. The collar is machined to be press fit on the end to the relief device housing. Holes drilled in the brass collar (concentric with the holes in the disk housing itself) provide an unrestricted flow path. With this device in place, visual examination will reveal the possibility of a corrosion-damaged rupture disk, as shown bv the presence of nits and discoloration. In order to mimic the ~ ~ n d i t i oofuthe service disk properly, both the service disk and the indicatine disk should be installed simultaueously. This insures that-both disks have the same history. We believe that the indicating disk will be most valuable in work areas that have acidic environments. This would include inorganic synthesis labs and electroplating areas. The collar is easily machined on a small lathe, and the disks are obtainable (often at no cost) from commercial suppliers. Periodic inspection of the exposed surface of the indicating disk will help prevent the unexpected bursting of the disk and its attendant catastrophic presgure release. Volume 64 Number 6 June 1987
557
