Medical devices and equipment depend on highly reliable, integrated, application-specific rupture disc solutions for critical life safety, diagnostic and analytical applications.
Rupture discs serve as an effective passive safety mechanism to protect against overpressure, such as in compressed specialty gases in laboratory and analytical instruments, magnetic resonance imaging, laser surgery, cryogenics and other other apps.
The disc, which is a single-use diaphragm composed of various metals, including exotic alloys, is designed to activate within milliseconds when a predetermined differential pressure is reached.
These discs also protect against overpressure conditions in the sanitary chamber of autoclaves, which rely on heat or pressurized steam to sterilize medical instruments and research laboratory equipment.
Often medical devices need to be very compact and low profile and integrated assemblies can help provide such capability with a simplified design.
Reliability in medical equipment is critical and this demands high integrity from the pressure relief technology used to protect low and high pressure OEM systems.
As a result, medical OEMs are increasingly turning to integrated rupture disc assemblies with all components combined by the manufacturer, as opposed to loose rupture disc and backing devices which leave much to chance.
These assemblies are application-specific, miniaturized, and use a wide range of standard and exotic materials, as required. This approach ensures that the rupture disc device performs as intended, improving equipment safety, reliability and longevity while simplifying installation and replacement.
Separate components versus integrated assemblies
Traditionally, rupture discs for medical devices started out as stand-alone components combined with the manufacturer’s separate support device at the point of use. The installation actions of the user contribute significantly to the operation of the bursting disc device.
When installed incorrectly, the rupture disc may not burst at the intended set pressure. There is a delicate balance between the rupture disc membrane, its supporting bracket, and the flanged, threaded, or other fastener used to attach the safety device to the protected equipment.
For this reason, for medical applications, an integrated rupture disc assembly is often a better choice than separable parts. Available out-of-the-box with no assembly required, built-in units are certified as operating at the desired set pressure.
The one-piece design allows for easier installation and quick removal if the rupture disc is activated.
The assembly includes the bursting disc and housing and is custom designed to operate with the user’s desired interface to the pressure equipment. Devices are typically threaded or flanged or even configured for industry specific connections such as CF/KF/Biotech/VCR couplings.
Rupture disc and carrier are combined by the manufacturer by welding, bolting, tube butting, gluing or crimping depending on application conditions and sealing requirements
There are additional benefits to this approach. Built-in assemblies prevent personnel from using unsafe or jury-rigged solutions to replace an activated rupture disc to save a few dollars or get equipment back online.
The physical characteristics of increasingly miniaturized rupture discs as small as 1/8″ can also make it difficult for personnel to grab the disc and place it in a separate holder.
“Medical device OEMs are committed to delivering the best performance while respecting the cost of ownership for their customers,” said Geof Brazier, general manager of BS&B Safety Systems’ custom engineered products division. “Using an integral assembly maximizes the longevity, smooth operation and trouble-free service of the decompression technology.”
The integrated assembly is ideal for many hydraulic, pneumatic and other low, medium and high pressure applications, including pumps, piston and bladder accumulators, motors, pressure vessels and piping.
Embedded Assemblies – Rupture Disc Design
According to Brazier, the most important considerations in designing bursting disc devices for medical applications are having the right operating pressure and temperature information as well as expected life.
This is often expressed as the number of cycles the device is expected to endure over its lifetime. Since the pressure and the cycle vary depending on the application, each requires a specific technical solution.
“Finding the right, highly reliable, cost-effective and application-specific solution for a medical OEM involves selecting the right disc technology, the right interface (welds, threads, compression fittings, single machined part) and the right options like dictated by codes and standards,” Brazier said.
Since the user’s material selection can also determine the longevity of rupture discs, devices can be fabricated from metals and alloys such as stainless steel, nickel, Monel, Inconel and Hastelloy.
According to Brazier, for medical applications, it can be important for rupture discs to have miniaturized reverse buckling capability in standard and exotic materials.
“When economy is the driving force, reverse buckling discs are typically made from materials such as nickel, aluminum and stainless steel. When aggressive conditions are required, more exotic materials like Monel, Inconel, Hastelloy, Titanium and even Tantalum can be used,” he said.
In almost all cases, “reverse buckling” rupture discs are used because they outperform the alternatives in terms of life.
In an inverted buckling design, the rupture disc dome is inverted toward the pressure source. The burst pressure is precisely controlled by a combination of material properties and the shape of the domed structure.
By loading the reverse buckling disc in compression, it can withstand operating pressures up to 95% of minimum burst pressure, even under pressure cycling or pulsating conditions. This results in greater longevity, precision and reliability over time.
“The process industry has relied on reverse buckling discs for decades. Now the technology is available to medical device OEMs in miniature form as small as 1/8″ burst diameter from BS&B. performance,” Brazier said.
However, the miniaturization of reverse buckling technology presents its own unique challenges. To solve this problem, BS&B has created new structures that control the reversal of the rupture disc to always activate in a predictable manner.
In this type of design, a line of weakness is also typically placed in the rupture disc structure to define a specific opening flow area when the inverted type disc activates and also prevents “petal” fragmentation. of the disk.
“The reverse buckling and therefore the compression of the material does certain things. First, the cyclability is much greater. Second, it allows you to get lower burst pressure from thicker materials, which helps improve accuracy as well as durability,” Brazier said.
Rupture discs of small nominal size are sensitive to the detailed characteristics of the orifice through which they burst. This requires strict control of the normal disc carrier variations.
“With small pressure relief devices, the influence of each characteristic of the rupture disc and its support is amplified,” explains Brazier. “With the correct media design and the correct rupture disc selection, customer expectations will be met and exceeded.”
Due to cost, weight and other considerations, Brazier said BS&B is getting more and more requests for plastic and composite enclosures.
Because customers are often accustomed to certain types of fittings to be incorporated into a piping diagram, different connections may be used on the enclosure. Threading is popular, but BS&B is increasingly using several other types of connection to attach the rupture disc assembly to the medical application.
Once the integral assembly leaves the factory, the intent is that the set pressure cannot be changed.
“If you’re relying on someone to put a loose disc in a system and then capture it by threading it, unless they follow the installation instructions and apply the correct torque value, there’s always a risk of leakage or the disc may not activate at the designed burst pressure,” says Brazier. “When welded into an assembly, the rupture disc is inherently sealed and the set burst pressure is fixed.”
While medical device OEMs have long relied on bursting discs in their gas, hydraulic and pneumatic equipment, compact design suitable for high cycle environments has been particularly challenging.
Fortunately, with the availability of application-specific, miniaturized, integrated rupture disc solutions in a variety of standard and exotic materials, OEMs can dramatically improve equipment safety, compliance, and reliability, even under harsh conditions. extreme work.
Jeff Elliott is a technical writer based in Torrance, California. He has researched and written on technology and industry issues for the past 20 years.