For more than 85 years, the rupture disk has served as an effective passive safety mechanism to protect against overpressure or potentially damaging vacuum conditions in tanks and other enclosed vessels. However, as these pressure relief devices become increasingly miniaturised to sizes as small as 1/8in to meet the demands of a new generation of smaller, lighter applications, the industry is running squarely into design and raw material challenges that often require re-engineering the product itself.
Fortunately, leading rupture disk manufacturers have embraced this challenge with novel structures and design elements that have led to a new category of miniaturised options from 1/8in to 1in at all ranges of pressure including low (15-1000 psi), medium (350-16,000 psi) and high (1,500-70,000 psi).
The beneficiaries are expected to be equipment manufacturers and design engineers currently developing the next generation of aircraft safety systems, fire suppression systems, lithium batteries and battery packs, cryogenic systems, bioreactors, refrigeration systems, chemical systems, and hydraulics.
Rupture Disks
When it comes to pressure relief devices, the two most common are safety valves (reclosing) and rupture disks, also known as bursting discs (non-reclosing). Rupture disks are designed to fail within milliseconds when a predetermined differential pressure, either positive or vacuum, is achieved. The device has a one-time-use membrane usually made out of various metals, but also exotic alloys and graphite.
In many applications the superior leak tightness and flow characteristics of a rupture disk are preferred over safety valves, which are known to leak. In fact, it is a common industry practice to install a rupture disk on the inlet side of a safety valve to economically protect against corrosion and provide the required leak tightness.
Transition to Miniature
In traditional applications, rupture disk sizes ranged from 1- 6in, with sizes up to 60in for the largest piping configurations. To install the product, the rupture disc is placed in a holding device, called a safety head, and installed between flanged pipe ends.
However, as equipment manufacturers strive to make their products smaller and lighter, the rupture disk industry has been challenged to deliver miniaturised options well under 1in diameter.
Due to a delicate balancing act between the shrinking diameters, a complicated choice between two rupture disk design philosophies, the limitations of the specific raw materials utilised for the membrane, and the variations required to meet the requirements for low, mid and high burst pressures have forced rupture disk manufacturers to the drawing board.
Forward-acting vs. reverse buckling
The first major decision when selecting a rupture disk of any size is the choice between forward-acting and reverse buckling technologies.
In the traditional forward-acting design, the loads are applied to the concave side of the disk. The thickness of the raw material employed and the diameter of the fitting in which it is mounted determine performance.
However, with this approach the rupture disk is prone to metal fatigue caused by aggressive cycling and operating conditions that can limit its operational life. Traditional rupture disk technology is additionally limited to applications having an operating to burst pressure ratio of 75% or less to avoid fatigue effects leading to potential unwanted activation.
Exacerbating the issue, forward-acting miniaturised disks with low set pressures require the use of tissue paper thin raw material that is fragile and prone to leakage when assembled.
This has caused a somewhat negative view of this type of disk even though it is still utilised in many static pressure applications and suffices for certain high pressure applications.
In a reverse buckling design, on the other hand, the dome is inverted toward the source of the load. 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 is able to resist operating pressures up to 100% of minimum burst pressure even under pressure cycling or pulsating conditions. The result is greater longevity, accuracy and reliability over time.
However, miniaturisation of reverse buckling technology presents its own unique challenges, according to Geof Brazier, Managing Director, BS&B Safety Systems, Custom Engineered Products Division.
BS&B Safety Systems invented the first rupture disk device in 1931. The company later pioneered reverse buckling technology and, more recently, the application of that approach to miniaturised disks as small as 1/8in at all ranges of pressure.
“As burst diameters decrease it becomes increasingly difficult to design a reverse buckling dome that will reliably collapse through such small orifice sizes,” said Brazier. “In many ways it can be like trying to fit a camel through the eye of a needle.”
To resolve this issue, BS&B has created novel structures that control the reversal of the rupture disk to always collapse in a predictable manner.
This includes, for example, a hybrid shape that combines reverse buckling and forward bulging characteristics that are pre-collapsed. In this type of design, a line of weakness is typically placed into the rupture disk structure to define a specific opening flow area when the reverse type disk activates.
Small nominal size rupture disks can also be very sensitive to the detailed characteristics of the orifice through which they burst, causing normal variations in holder support machined part dimensions to have an unwanted impact on burst pressure accuracy.
“With small size pressure relief devices, the influence of every feature of both the rupture disk and its holder is amplified,” explained Brazier.
For miniaturised products ¼in and up certified for high burst pressures up to 70,000 psi, BS&B manufactures the rupture disk and holder from a single piece of material, eliminating the usual connection between a rupture disk membrane and its support fitting. The rupture disk is intrinsically leak-tight by virtue of its unitary construction.
The devices are typically threaded, but are also available in configurations for welding, soldering, or crimping based on the application conditions and leak tightness requirements.
