BS&B Safety Systems
Today, cryogenics enables convenient storage of large quantities of industrial gases such as nitrogen, oxygen, carbon dioxide, argon, helium, and hydrogen that are vaporized from liquid to gas at time of use in support of many industrial processes ranging from steelmaking to medical systems and welding.
Additionally, cryogenic equipment provides stable, cold temperatures required to preserve biological samples, support superconducting magnets such as those used in medical imaging systems and particle physics experiments, as well as to support novel surgical procedures and materials research.
Within these systems, liquefied gases such as helium, hydrogen, nitrogen, carbon dioxide and oxygen are kept at very low temperatures since the boiling points for these gases are very low, ranging from –78.5 °C (–109.3 °F) for liquid carbon dioxide to –269 °C (–452.2 °F) for liquid helium. Due to its physical properties, as temperature rises, a very small amount of liquid can expand rapidly into a very large volume of gas.
For example, the expansion ratio of nitrogen is 694 — with 1 liter of liquid nitrogen becoming 694 liters of gaseous nitrogen at standard temperature and pressure (ambient conditions). If the insulation or other cooling methods used to maintain cryogenic temperature conditions for a liquid are lost, a rapid buildup of pressure will occur in any closed tank or vessel in which the liquid is contained.
For these reasons, cryogenic systems are equipped with pressure relief devices such as rupture disks to protect against rapid pressure rise caused by a sudden increase of heat into cryogenic systems, cryogenic shippers, cryostats, cannisters, and associated piping. For applications that utilize superfluid helium to cool superconducting magnets used in magnetic resonance imaging equipment, particle accelerators, and for semiconductor processing, rupture disks protect against the sudden catastrophic loss of insulating vacuum or insulating nitrogen in the storage vessel or experimental enclosure.
The rupture disk, which is a one-timeuse membrane made of various metals including exotic alloys, is designed to activate within milliseconds when a predetermined differential pressure is achieved. However, given the critical reliability of the equipment in operation and during storage/ transport, this demands high integrity pressure relief technology.
As a result, OEMs are increasingly turning to integrated rupture disk assemblies with all components combined by the manufacturer, as opposed to loose rupture disk and holder devices that leave much to chance. These assemblies are being tailored to the application, miniaturized, and utilize a wide range of standard and exotic materials, as required. This approach ensures the rupture disk device performs as expected, enhancing equipment safety, reliability, and longevity while simplifying installation and replacement.
Separate Components Versus Integrated Assemblies
Traditionally, rupture disks began as standalone components that are combined with the manufacturer’s separate holder device at the point of use. The installation actions of the user contribute significantly to the function of the rupture disk device. When installed improperly, the rupture disk may not burst at the expected set pressure. There is a delicate balance between the rupture disk membrane, its supporting holder, and the flanged, threaded, or other fastening arrangement used to locate the safety device on the protected equipment.
For this reason, an integrated rupture disk assembly is often a better choice than separable parts. Available ready-to-use and with no assembly required, integrated units are certified as a device to perform at the desired set pressure. The one-piece design allows for easier installation and quick removal if the rupture disk is activated.
The assembly includes the rupture disk and housing and is custom engineered to work with the user’s desired interface to the pressurized equipment. The devices are typically threaded or flanged, or even configured for industry specific connections such as CF/KF/biotech industry clamp connections/VCR couplings. The manufacturer combines the rupture disk and holder by welding, bolting, tube stub, or crimping based on the application conditions and leak tightness requirements.
“Cryogenic equipment OEMs are driven to deliver the safest operation as well as longest life and lowest cost of ownership to their customers,” says Geof Brazier, managing director of BS&B Safety Systems Custom Engineered Products Division. “The use of an integral assembly maximizes the longevity, proper function, and trouble-free service of the pressure relief technology.”
Integrated Assemblies — Rupture Disk Design
The most important considerations in rupture disk device design are having the right operating pressure and temperature information along with the expected service life, which is often expressed as a number of cycles the device is expected to endure during its lifetime. Since pressure and cycling varies depending on the application, each requires a specific engineering solution. Because user material selection can also determine the longevity of rupture disks, the devices can be manufactured from metals and alloys such as stainless steel, nickel, Monel, Inconel, and Hastelloy.
For a wide range of industries, it can be important for rupture disks to have a miniaturized reverse buckling capability in both standard and exotic materials. In a reverse buckling design, the dome of the rupture disk is inverted toward the pressure source. Burst pressure is accurately controlled by a combination of material properties and the shape of the domed structure. By loading the reverse buckling disk in compression, it can resist operating pressures up to 95 percent of minimum burst pressure even under pressure cycling or pulsating conditions. The result is greater longevity, accuracy, and reliability over time.
However, miniaturization of reverse buckling technology presents its own unique challenges. To resolve this issue, BS&B created novel structures that control the reversal of the rupture disk to always activate in a predictable manner. In this type of design, a line of weakness is also typically placed into the rupture disk structure to define a specific opening flow area when the reverse-type disk activates and also prevents fragmentation of the disk petal.
Small nominal size rupture disks are sensitive to the detailed characteristics of the orifice through which they burst. This requires strict control of normal variations in the disk holder. Due to cost, weight and other considerations, Brazier says that BS&B has increasingly received more requests for housings that are made out of plastics and composites.
Because customers are often accustomed to certain types of fittings to integrate into a piping scheme, different connections can be used on the housing. Threading is popular, but BS&B is increasingly utilizing several other connection types to attach the rupture disk assembly to the application. Once the integral assembly leaves the factory, the goal is that the set pressure cannot be altered.
While OEMs have relied on rupture disks in their cryogenic equipment, the availability of integrated, miniaturized rupture disk solutions tailored to the application in a variety of standard and exotic materials can significantly enhance equipment safety, compliance, and reliability even in extreme work conditions.