Gasket failure through crushing is a common problem that can lead to leaks and unplanned maintenance. While sophisticated torquing equipment is often used, achieving proper torque is difficult due to various factors. A better solution is improving gasket design. The article describes a new gasket design called a "fishbone gasket" that features rounded ribs and integral stop-steps to prevent over-compression of the gasket material and eliminate crushing. Laboratory tests found this design achieved better crush resistance and sealing performance compared to traditional spiral-wound and camprofile gasket designs. The stop-step concept could also be applied to other gasket types to improve reliability.

1. December 2014 | Pumps & Systems
78 PRACTICE & OPERATIONS
G
askets are crushed more
frequently than many end
users realize. Some main-
tenance professionals claim that
is all they see when they separate
a leaking flange. Many simply
replace the gasket rather than
investigating the cause of the
failure in depth.
Gasket crush is a significant
threat to plant reliability. This
problem causes leaks, which can
lead to a dangerous plant situation,
especially if a hazardous fluid is
involved. This issue is unpredict-
able and introduces variability into
plant operations.
All types of gaskets—metal,
nonmetal and composite—expe-
rience gasket crush. If a gasket
design fails on the first installa-
tion, plant personnel can replace
it with a new design during the
next plant shutdown. However,
if a gasket has worked nine times
without a problem, then leaks on
the tenth installation because of
gasket crush, an unplanned main-
tenance response is required.
Users have two options for
reducing gasket crush: better
torquing procedures, which can be
difficult, and better gasket design,
which is easier to implement.
Torquing Procedures
In a crushed gasket, different
compression patterns are vis-
ible—sometimes on one side of
the gasket, sometimes on opposite
sides and sometimes all over the
gasket. These patterns point to
over-compression. The most obvi-
ous conclusion is that something
was wrong with the torque, even
though sophisticated torque equip-
ment, such as torque wrenches
or bolt tensioners, is often used
during installation.
Torque equipment tension-
ing accuracy, which is generally
between 5 and 15 percent at
best, is based on a few important
assumptions, including:
• Bolts must be well-lubricated
with a lubricant of known
coefficient of friction, because
unlubricated bolts can
consume up to 50 percent of
the torque load.
• Torque typically is calculated
based on the bolt material, not
the required load of the gasket
material, which is usually too
high for the gasket need.
• Personnel only have access to
a recommended bolt load for
the cool flange when the opera-
tion has not yet started. The
crucial hot torque load value is
rarely available.
Design Improvement Reduces Gasket Crush
A compressible design and stop-step feature enhance strength and minimize leakage.
By Justin Zhao
AIGI Environmental Incorporated
Figure 1. Round-edged beveled ribs compress under
pressure. (Graphics courtesy of AIGI Environmental Inc.)

2. pumpsandsystems.com | December 2014
79
Maintenance workers torque
flanges before plant startup, but
when operational, flanges expand
because of process heat. Personnel
must also perform a hot torque
process to prevent leaks. Torquing
practice in the field is based on
skills and judgment.
Maintenance team members
sometimes hot-torque a flange
without a proper torque value for
that temperature, and leakage
persists. In this situation, main-
tenance personnel often continue
to tighten the bolts until the leak
stops. The gaskets are usually
crushed at this point.
Successful flange hot torquing is
a disappearing art because of fewer
senior maintenance staff who
have had many years of experience
performing the task.
Gasket Design
Designing better gaskets is a
much easier solution in practice
than attempting to deal with
torquing procedures and the art of
hot torquing.
Spiral-wound and camprofile
gaskets are the most commonly
used metal gasket types. The com-
pressibility and elasticity of spiral-
wound gaskets are useful sealing
characteristics. When paired with
the strength that a camprofile
gasket achieves through its unitary
metallic structure, gaskets start to
approach optimal design.
These gasket types also have
negative features—such as sealing
issues—that should be addressed
to improve gasket design. Both
types also provide variable per-
formance. Spiral wound gaskets
have a tendency to unwind, and
camprofile gaskets do not recover
well. An improved design would
also eliminate the flange damag-
ing characteristics of both types
by removing any sharp edges or
points. The optimal gasket would
be a solid, unitary metallic struc-
ture to prevent crushing. It would
incorporate a mechanism for
compressibility and elasticity for
improved sealing (see Figure 1).
To achieve a crush-proof gasket,
a simple feature called a stop-step
can prevent over-compression of
the flexible rib elements and elimi-
nate the possibility of destructive
compression of the overlaid sealing
element. These stop-steps can be
located outside the sealing area
but between the flexible ribs for
additional support (see Figure 2).
Successful flange hot torquing is a disappearing art because
of reductions in senior maintenance professionals who have
had many years of experience performing the task.
Figure 2. A stop-step feature prevents the flexible ribs from over-compressing.

3. December 2014 | Pumps & Systems
80 PRACTICE & OPERATIONS
An internal rounded stop-step,
which is integral to the design, pro-
vides a positive stop that prevents
over-stressing of the ribs and the
graphite-sealing element.
The stop-step ensures that the
ribs remain elastic and functioning
in their self-energizing capacity
because it prevents them from
being stressed beyond their elastic
limit. The same is true for the
graphite-sealing element. The ribs,
stop-step and graphite-sealing
element perform as an integrated
system to ensure maximum sealing
performance under bolt load.
A gasket with this design is more
difficult to make than construct-
ing the concept. Designers cannot
assume that any design element
on its own will work. They must
find a balance between the step
height, rib angle or curvature,
and thickness to achieve optimal
sealing while eliminating gasket
crushing and sealing element
over-compression.
The gasket prototype designs
shown in Figures 1 and 2 were
tested for crush resistance and
sealing. The results confirm the
effectiveness of the overall design
and the specific utility of the
stop-step (see Figures 3 and 6).
The stop-step, which prevents
gasket over-compression, can be
applied to other types of gaskets.
Both spiral wound and camprofile
gaskets can incorporate this design
feature (see Figures 4 and 5).
Gasket innovation is often a
difficult and slow process because
Figure 5. Sharp edges in a camprofile gasket can lead to damage.
Figure 4. Spiral-wound gaskets have a tendency to unwind.
Figure 3. Crush resistance test results, with a pressure of 205 MPa (29,732 psi)

4. pumpsandsystems.com | December 2014
81
of the limited number of gasket
features that can economically be
incorporated in a design that
must fit the space constraints
between two flanges. These
improvements are important as
gaskets move toward the
ultimate sealing reliability.
The stop-step design is one
small step on the path to a
new standard for metal gasket
reliability.
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— Describes (3) types and levels of dynamic analysis for
pumps and pump trains.
— Describes mathematical means to calculate the first
rotor lateral, rotor torsional, and structural read critical
frequencies within Level 1 analysis.
— Helps the end user avoid costly reliability problems
related to resonance of the rotor system and structure.
Justin Zhao is the patent inventor
of Fishbone Gaskets. He received a
Master of Science from University
of Chicago and an MBA from Rut-
gers University. For more informa-
tion, visit aigienvironmental.com.
Figure 6. A leakage test compared the three gasket types. The gasket stress
was 30 MPa (4,351 psi), and the nitrogen pressure was 4 MPa (560 psi).