To ensure safety and reliability, aerospace original equipment manufacturers (OEMs) depend on integrated, specific, rupture disk solutions for applications ranging from compressed gas cylinders to propulsion systems, aircraft wheels, environmental and fire protection equipment, and fuel storage systems. Rupture disks serve as an effective passive safety mechanism to protect against overpressure in many such aerospace applications. The disk, which is 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.
Aerospace equipment reliability is essential and demands high integrity from the pressure relief technology used to protect low- and high-pressure OEM systems. Instead of loose rupture disk and holder devices, OEMs are increasingly turning to integrated rupture disk assemblies with all components combined by the manufacturer. These assemblies are tailored to the application, miniaturized, and use a wide range of standard and exotic materials. This approach ensures the rupture disk device performs as expected, enhancing equipment safety, reliability, and longevity while simplifying installation and replacement.
The integrated assembly is also ideal for numerous hydraulic, pneumatic, and other low-, medium- and high-pressure applications including pumps, piston & bladder accumulators, propulsion systems, pressure vessels, and piping.
From satellites to aircraft to drones, tailoring integrated rupture disk applications for use with lightweight, compact materials such as titanium and aluminum are also important since it takes more energy to get heavier vehicles off the ground.
When tremendous corrosion resistance is required for aggressive fluid conditions, titanium is often the material of choice. Where light weight and economy are required, an aluminum welded assembly may be the right solution.
Separate components versus integrated assemblies
Traditionally in aerospace, rupture disks begin as standalone components that are combined with the manufacturer’s separate holder 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/KFVCR couplings. The rupture disk and holder are combined by the manufacturer by welding, bolting, tube stub, adhesive bonding, or crimping based on the application conditions and leak tightness requirements.
This approach has additional advantages. Integrated assemblies can be mistake-proofed by design to ensure correct direction of installation such as by use of a different screw thread configuration at the inlet and outlet of the device. The physical characteristics of increasingly miniaturized rupture disks as small as 1/8″ can also make it challenging for personnel to pick up the disk and place it into a separate holder.
“Aerospace OEMs are driven to deliver the best performance while respecting the budget of their customers, says Geof Brazier, Managing Director of BS&B Safety Systems Custom Engineered Products Division. “The use of an integral assembly maximizes the quality assurance for the pressure relief technology by providing a ready to use component.”
Integrated assemblies – rupture disk design
According to Brazier, the most important considerations in rupture disk device design for aerospace are having the right operating pressure and temperature information along with the dimensional constraints of the application. Service performance is sometimes expressed as the number of cycles the device is expected to endure during its lifetime. Since pressure and cycling varies depending on the application along with the space available and weight that is acceptable, each requires a custom engineered solution.
“Coming up with a good, high reliability, cost-effective, and application specific solution for an aerospace OEM involves selecting the right disk technology, the correct interface (weld, screw threads, compression fittings, single machined part), and the right options as dictated by the codes and standards or end-user validation requirements,” Brazier says.
Because user material selection can also be very specific to the application conditions, rupture disk device can be manufactured from metals and alloys such as stainless steel, nickel, aluminum, Monel, Inconel, titanium, columbium [niobium], and Hastelloy.
For aerospace applications, it can be important for rupture disks to have a miniaturized reverse buckling capability in both standard and exotic materials, Brazier notes.
In almost all cases, reverse-buckling rupture disks are used because they outperform the alternatives in accuracy and resistance to normal operating conditions.
In a reverse-buckling design, the rupture disk’s dome 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% of minimum burst pressure even under pressure-cycling or pulsating conditions. The result is greater longevity, accuracy, and reliability.
“The process industry has relied on reverse-buckling disks for decades. Now the technology is available to aerospace OEMs in miniature form as small as 1/8″ burst diameter from BS&B. Until recently, obtaining disks of that size and performance was impossible,” Brazier says.
He adds that the benefits of such miniaturized, reverse-buckling disks include the lowest possible burst pressure ratings in small diameters, enabling low profile, light-weight design, superior performance in cycling service conditions, minimal or no fragmentation upon activation, and the ability to withstand full vacuum or back pressure without extra support components.
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 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 retains the disk petal within the assembly housing.
“Reverse buckling – and therefore having the material in compression – does a few things,” Brazier says. “Number one, repeatable structural integrity is achieved. Second, it allows you to obtain a lower burst pressure from thicker materials, which contributes to enhanced accuracy as well as durability.”
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.
“With small size pressure relief devices, the influence of every feature of both the rupture disk and its holder is amplified,” Brazier explains. “With the correct design of the holder and the correct rupture disk selection, the customer’s expectations will be achieved and 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 BS&B is increasingly using several other connection types to attach the rupture disk assembly to the application. Once the integral assembly leaves the factory, the set pressure is fixed, and the device is ready for use.
“If you rely on someone to put a loose disk in a system and then capture it by threading over the top of it, unless they follow the installation instructions and apply the correct torque value, there is still potential for a leak or the disk may not activate at the designed burst pressure,” Brazier warns. “When welded into an assembly, the rupture disk device is intrinsically leak tight and the set-burst pressure fixed.”
While aerospace OEMs have long relied on rupture disks in their compressed gas, hydraulic and pneumatic equipment, high and low pressure, high-cycling environments have been particularly challenging. Fortunately, with the availability of integrated, miniaturized rupture disk solutions tailored to the application in a variety of standard and exotic materials, aerospace OEMs can significantly enhance equipment safety, compliance, and reliability even in extreme work conditions.
About the author: Jeff Elliott is a Torrance, California-based technical writer who has researched and written about industrial technologies and issues for the past 15 years.