Medical Design Briefs - November 2021 - 26

TECHNOLOGY LEADERS Gas/Fluid Handling/Pumps
Small nominal size
of
the
rupture
orifice
disks are sensitive to the detailed
characteristics
The correct torque value is critical for preventing leaks. (Credit: BS&B Safety Systems)
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, "
says 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 says.
26
Cov
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 percent 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
in. burst diameter from BS&B. Until
recently, obtaining disks of that size and
performance was impossible, " says 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, "
says Brazier.
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ToC
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 says the company
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, for
example, 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, "
says 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.
This article was written by Jeff Elliott, a
technical writer based in Torrance, CA. For
more information on BS&B Safety Systems,
visit http://info.hotims.com/79419-342.
Medical Design Briefs, November 2021
http://info.hotims.com/79419-342 http://www.medicaldesignbriefs.com
Medical Design Briefs - November 2021

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