SAFETY IN THE PLANT
FORWARD-ACTING VERSUS REVERSE BUCKLING The fi rst 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 fi tting 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. Traditional rupture disc technology is additionally limited to applications having an operating to burst pressure ratio of 75% or less to avoid fatigue eff ects leading to potential unwanted activation. Exacerbating the issue, forward-acting
miniaturised 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 utilised in many static pressure applications and suffi ces 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 disc 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.
THE MINIATURISATION CHALLENGE However, miniaturisation of reverse buckling technology presents its own unique challenges, says Geof Brazier, managing director of BS&B Safety Systems, Custom Engineered Products Division. BS&B Safety Systems invented the fi rst rupture disc
device in 1931. The company later pioneered reverse buckling technology and, more recently, the application of that approach to miniaturised discs as small as 1/8in at all ranges of pressure. “As burst diameters decrease it becomes increasingly
diffi cult to design a reverse buckling dome that will reliably collapse through such small orifi ce sizes,” says Brazier. “In many ways it can be like trying to fi t 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 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 defi ne a specifi c opening fl ow area when the reverse type disc activates. Small nominal size rupture discs can also be very
sensitive to the detailed characteristics of the orifi ce 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 infl uence of every feature of both the rupture disc and its holder is amplifi ed,” explains Brazier.
Forward-acting rupture discs
For miniaturised products 1/4in and up certifi ed
for high burst pressures up to 70,000 psi, BS&B manufactures the rupture disc and holder from a single piece of material, eliminating the usual connection between a rupture disc membrane and its support fi tting. The rupture disc is intrinsically leak-tight by virtue of its unitary construction. As for installation, Brazier says these miniature
rupture discs are available as a single, integrated assembly certifi ed to perform at the desired set pressure. The devices are typically threaded, but are also available in confi gurations for welding, soldering or crimping based on the application conditions and leak- tightness requirements. ■
For more information visit
www.bsbsystems.com
www.engineerlive.com 39
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