Integrated rupture disk assemblies for cryogenic equipment and storage systems

Cryogenics areused to provide convenient storage of large quantities of industrial gases, such as nitrogen, oxygen, carbon dioxide (CO₂), argon, helium and hydrogen (H₂), that are vaporized from liquid to gas when used in support of many industrial processes ranging from steelmaking to medical systems and welding. Additionally, cryogenic equipment provides the consistently cold temperatures required to preserve biological samples, support superconducting magnets—such as those used in medical imaging systems and particle physics experiments—and support novel surgical procedures and materials research. 

Within these systems, liquefied gaseslikehelium, H₂, nitrogen, CO₂ and oxygen are kept at very low temperatures, since the boiling points for these gases are very low, ranging from –78.5°C (–109°F) for liquid CO2to–269°C (–452°F) for liquid helium. Due to its physical properties, as temperatures rise,a small amount of liquid can expand rapidly into a large volume of gas.  

For example, the expansion ratio of nitrogen is 694—with 1 liter (l) of liquid nitrogen becoming 694 l 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 systemsare equipped with pressure relief devices (e.g., 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 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—a one-time-use membrane made of various metals,including exotic alloys—is designed to activate within milliseconds when a pre-determined differential pressure is achieved. However, given the critical reliability of the equipment in operation and during storage/transport, high-integrity pressure relief technology is required. 

As a result, original equipment manufacturers (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 tailored to the application 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. Rupture disksbegan as standalone components 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.The rupture disk may not burst at the expected set pressure when installed improperly.A delicate balance exists among the rupture diskmembrane, its supportingholder and the flanged, threaded or other fastening arrangement used to locate the safety device on the protected equipment. 

Therefore, 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 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,flangedor configured for industry-specific connections, such as CF/KF/biotech industry clamp connections/vacuum coupling radiation 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. 

This approach offers additional advantages. Integrated assemblies prevent personnel from utilizing unsafe or jury-rigged solutions to replace an activated rupture disk to save a few dollars or rush equipment back online. The physical characteristics of increasingly miniaturized rupture disks as small as 1/8 in.can also make it challenging for personnel to pick up the disk and place it into a separate holder. 

Cryogenic equipment OEMs are driven to deliver the safest operation, longest life and lowest cost of ownership. The use of an integral assembly maximizes the pressure relief technology's longevity, proper function and trouble-free service. 

Integrated assemblies—rupture disk design. The most important considerations in rupture disk design are having the correct operating pressure and temperature information, along with the expected service life—the number of cycles the device is expected to endure during its lifetime. Since pressure and cycling vary depending on the application, each requires a specific engineering solution. 

Producing a good, high-reliability, cost-effective and application-specific solution for cryogenic equipment involves selecting the right disk technology, interface and options as dictated by the codes and standards. 

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. 

When economics is the driver, reverse buckling disks are typically made from nickel, aluminum and stainless steel. More exotic materials like monel, inconel, hastelloy, titanium and tantalum can be used where aggressive conditions are required. In most cases, reverse buckling rupture disks are utilized because they outperform the alternatives with respect to service life. 

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. Loading the reverse buckling disk in compression can resist operating pressures up to 95% of minimum burst pressure, even under pressure cycling or pulsating conditions. The result is greater longevity, accuracy and reliability over time. 

The process industry has relied on reverse buckling disks for decades. The technology is now available to OEMs in miniature form as small as 1/8-in. burst diameter. Until recently, obtaining disks of that size and performance was impossible.However, the miniaturization of reverse buckling technology presents unique challenges. Novel structures have been created that control the reversal of the rupture disk to always activate predictably. 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 prevents fragmentation of the disk petal. 

Reverse buckling and, therefore, having the material in compression, does a few things. First, the cyclability is much greater. Second, it allows you to obtain a lower burst pressure from thicker materials, which contributes to enhanced accuracy and durability. Smallnominalsize rupture disks are sensitive to the detailed characteristics of the orifice through which they burst, and this requires strict control of normal variations in the disk holder. 

With smallsize pressure relief devices, the influence of every feature of both the rupture disk and its holder is amplified. With the correct design of the holder and the correct rupture disk selection, the customer's expectations will be exceeded. 

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 several other connection types are available to attach therupture disk assembly to the application.Once the integral assembly leaves the factory, the goal is that the set pressure cannot be altered. 

If a loose disk is placed in a system and then captured by threading over the top,unless the installation instructions are followed and the correct torque value applied, there is potential for a leak, or the disk may not activate at the designed burst pressure.When welded into an assembly, the rupture disk is intrinsically leak-tight, and the set-burst pressure fixed. 

Takeaway. 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. GP

GEOF BRAZIER is the Managing Director of BS&B Safety Systems’ Custom Engineered Products Division. The author can be reached at sales@bsbsystems.com.