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 for more than 85 years. As these pressure-relief devices become increasingly miniaturized — to sizes as small as 0.125" — design and raw material challenges often require reengineering the product itself. In response, rupture disk manufacturers have responded with novel structures and design elements that have yielded miniaturized options from 0.125 to 1" at all ranges of pressure.
When it comes to pressure-relief devices, the two most common are safety valves (reclosing) and rupture disks (non-reclosing). Rupture disks are designed to fail within milliseconds when a predetermined differential pressure is achieved. The first major decision when selecting a rupture disk 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. With this approach, however, the rupture disk may be prone to metal fatigue caused by aggressive cycling and operating conditions that can limit its operational life. Traditional rupture-disk technology also is typically limited to applications having an operating to burst pressure ratio of 75 percent or less to avoid fatigue effects leading to potential unwanted activation.
In a reverse-buckling design, the dome is inverted toward the source of the load. Burst pressure is 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 percent of minimum burst pressure even under pressure cycling or pulsating conditions. The result is longevity, accuracy and reliability over time.
Miniaturization of reverse buckling technology, however, presents its own unique challenges, says Geof Brazier, managing director of Tulsa-based BS&B Safety Systems, Custom Engineered Products Division.
“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 a needle.”
To resolve this issue, BS&B has created 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 be 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,” explains Brazier.