Medical devices and equipment depend on highly reliable integrated, application specific, rupture disk solutions for critical life safety, diagnostic and analytical applications.
Rupture disks serve as an effective passive safety mechanism to protect against overpressure, such as in compressed specialty gases in laboratory and analytical instrumentation, magnetic resonance imaging, laser surgery, cryogenic and other 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.
Such disks also protect against over-pressure conditions in the sanitary chamber of autoclaves, which rely on pressurized heat or steam to sterilize medical instruments and research labware.
Often, medical devices must be very compact and low profile and integral assemblies can help to provide such capability with streamlined design.
Medical equipment reliability is essential 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 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, medical device 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, for medical applications 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/VCR 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 requirement
There are additional advantages to this approach. 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” can also make it challenging for personnel to pick up the disk and place it into a separate holder.
“Medical device OEMs are driven to deliver the best performance while respecting cost of ownership for their customers, said 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.”
The integrated assembly is ideal for numerous hydraulic, pneumatic and other low, medium and high-pressure applications including pumps, piston & bladder accumulators, engines, pressure vessels and piping.
Integrated Assemblies - Rupture disk design
According to Brazier, the most important considerations in rupture disk device design for medical applications are having the right operating pressure and temperature information along with the expected service life.
This 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.
“Coming up with a good, high reliability, cost-effective and application specific solution for a medical device 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,” said Brazier.
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.
According to Brazier, for medical applications it can be important for rupture disks to have a miniaturized reverse buckling capability in both standard and exotic materials.
“Where economics is the driver, reverse buckling disks are typically made from materials such as nickel, aluminum and stainless steel. Where 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 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.
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 over time.
“The process industry has relied on reverse buckling disks for decades. Now the technology is available to medical device 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,” said Brazier.
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."
“Reverse buckling and therefore having the material in compression does a few things. Number one, the cyclability is much greater. Second, it allows you to obtain a lower burst pressure from thicker materials, which contributes to enhanced accuracy as well as durability,” said Brazier.
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,” explains Brazier. “With the correct design of the holder and the correct rupture disk selection, the customer’s expectations will be achieved and exceeded.”
Due to cost, weight and other considerations, Brazier said 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 medical application.
Once the integral assembly leaves the factory, the goal is that the set pressure cannot be altered.
“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,” explains Brazier. “When welded into an assembly, the rupture disk is intrinsically leak tight and the set-burst pressure fixed.”
While medical device OEMs have long relied on rupture disks in their gas-utilizing, hydraulic and pneumatic equipment, compact design suited to 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, OEMs can significantly enhance equipment safety, compliance and reliability even in extreme work conditions.
Jeff Elliott is a Torrance, California based technical writer. He has researched and written about industrial technologies and issues for the past 20 years.