Miniaturized Rupture Disks Deliver Protection Against Over-Pressurization at Low, Medium, + High Set Burst Pressures
For more than 85 years, the rupture disc has served as an effective passive safety mechanism to protect against overpressure or potentially damaging vacuum conditions in oil and gas storage tanks and other enclosed vessels. However, as these pressure relief devices become increasingly miniaturized to sizes as small as 3.2 millimetres (1/8 inch) 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 reengineering the product itself.
Fortunately, leading rupture disc manufacturers have embraced this challenge with novel structures and design elements that have led to a new category of miniaturized options from 3.2 mm to 3.54 centimetres (one inch) at all ranges of pressure, including low 103 kilo-Pascal — 6,895 kPa (15 — 1,000 psi), medium 2,413 — 110,316 kPa (350 – 16,000 psi), and high 10,342 — 482,633 kPa (1,500 – 70,000 psi).
The beneficiaries are expected to be equipment manufacturers and design engineers currently developing the next generation of oil, gas, and petrochemical processing systems.
Evolution of Rupture Discs
To under the challenge requires an understanding of the origins of rupture disc technology. When it comes to pressure relief devices, the two most common are safety valves (reclosing) and rupture discs, also known as bursting discs (non-reclosing). Rupture discs 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 from various metals, but all exotic alloys and graphite.
In many applications, the superior leak tightness and flow characteristics of a rupture disc are preferred over safety valves, which are known to leak. In fact, it is a common industry practice to install a rupture disc on the inlet side of a safety valve to economically protect against corrosion and provide the required leak tightness.
Transition to Miniature
For decades, rupture discs have been used in petrochemical, chemical, and oilfield applications. In these, rupture disc sizes range from 2.54 cm (one inch) to 15 cm (six inches), with larger sizes up to 152 cm (60 inches) 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 disc industry has been challenged to deliver miniaturized options well under 2.54 cm (one inch) in diameter.
Due to a delicate balancing act between the shrinking diameters, a complicated choice between two rupture disc design philosophies, the limitations of the specific raw materials used for the membrane, and the variations required to meet the requirements for low, mid, and high burst pressures have forced rupture disc manufacturers to the drawing board.
Competing Designs: Forward-Acting versus Reverse Buckling
The first major decision when selecting any rupture disc 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 disc. 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 disc is prone to metal fatigue caused by aggressive cycling and operating conditions that can limit its operational life. This technology is also limited to applications having an operating to burst pressure ratio of 75 per cent or less, to avoid fatigue effects leading to potential unwanted activation.
Exacerbating the issue, forward-acting miniaturized discs 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 disc, even though it is still used in many static pressure applications, and suffices for certain high-pressure applications.
In 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 dome structure. By loading the reverse buckling disc in compression, it can resist operating pressures up to 100 per cent of minimum burst pressure, even under pressure cycling or pulsating conditions. The result is longer longevity, accuracy, and reliability over time.
Miniaturization Challenge
However, miniaturization of reverse buckling technology presents its own unique challenges, says Geof Brazier, managing director of custom engineered products, BS&B Safety Systems, the company which invented the first rupture disc in 1931. It later pioneered reverse buckling technology and, more recently, the application of that approach to miniaturized discs as small as 3.2 mm (1/8 inch) 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,” says Brazier. “In many ways, it can be like trying to fit a camel through the eye of the needle.”
To resolve this issue, BS&B created novel structures that control the reversal of the rupture disc 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 disc structure to define a specific opening flow area when the reverse type disc activates.
“With small size pressure relief devices, the influence of every feature of both the rupture disc and its holder is amplified,” explains Brazier.
For more information, please visit www.bsbsystems.com.
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