Chemical Engineering November 2014 - 47

Flanges become distorted when
flexing occurs at the flange, usually
due to uncontrolled bolt loads,
which leads to leakage at points
where compression is too low for
sealing. A flange can be checked for
distortion by taking a face run-out
reading of the flange as it is rotated
slowly on a portable lathe. Lasers
can also be used for more exact readings.
Furthermore, by employing a
non-drying blue pigment and an accurate
straightedge, the amount of
warpage or waviness in the flange
face can be detected by observing
the contact points. Machining of the
faces may be required to restore the
flange face to the required flatness
and desired sealing surface. Even
with relatively rigid flanges, bolt
loads can cause a certain amount
of bending in the flange between
bolt centers.
Internal pressure also adds to
the loading on flanges, gaskets and
bolts. Internal pressure can cause
bolt elongation, add to flange bowing
and cause gasket blowouts. The
internal pressure creates forces
that act to separate the flanges. The
bolt cross-section in Figure 1 shows
the directional path of applied internal
hydraulic pressure through
the element. The bolt loads must
be uniform and accurately applied
to keep the stress in the gasket, resisting
gasket blowout. Needless to
say, given the variety and intricate
details that go into the makeup of
a bolted connection - namely, gasket
type, sealing-face finish and the
flatness and stiffness of the flange
- simply adding more bolt load
to a leaking joint may not always
be the most suitable solution. It is
important to investigate the situation
and confirm that additional
tightening is the best path moving
forward. Other options may include
flange machining or upgrading the
bolts and gaskets themselves.
Bolt behavior
Any tightening method applied will
result in stretching of the bolt. The
bolt behaves like a spring, in that
it has a certain elastic limit that
it can reach and still return to its
original shape and size when the
Figure 2. A hydraulic nut on a heat-exchanger
flange helps to maintain proper
tightness, since the nut stays in place
even after a breakout
force is removed without incurring
any distortion. If a bolt is in tension,
the stress it experiences is
a tensile stress, representing the
amount of force applied. Tensile
strength is the maximum tensionapplied
stress a fastener can support
before it fractures. The amount
of distortion occurring in a part, as
long as it is kept within the material's
elastic limits, will be directly
proportional to the applied force. If
the force exceeds these limits, the
result is permanent set. Permanent
set of a bolt refers to the elongation
that remains in a fastener after it
is unloaded, and is also commonly
known as necking or yielding.
Simply put, necking occurs when a
bolt is stretched past the spring-like
state and the bolt becomes longer
than it originally was. In this situation,
the bolt is overstretched and
the material to create this length
has been displaced from other areas
of the bolt. Typically, bolt material is
transferred from the diameter when
this elongation occurs. It is not visibly
apparent when the stud bolt is
engaged, but this material migration
can cause major problems if ignored
or unnoticed. Necking decreases a
part's strength, which therefore reduces
the integrity of the bolted joint
as a whole. When the bolt needs to
be tightened again, there will be
a new stress-strain curve and the
thread pitch (how closely the threads
in a screw or bolt are spaced) will
change, even on the threads that
were not engaged during tightening.
Hence, when comparing a new
or properly used bolt to a bolt of the
same makeup that has previously
exceeded its capacity, the threads
will not match.
Yield strength is another important
parameter in bolt tensioning.
It is defined as the tension-applied
stress at which the fastener experiences
a specified amount of
permanent deformation. When
tightening a bolt, it is crucial to remain
below the yield strength up
to the proof load. The proof load is
typically defined as the tensionapplied
load that the fastener
must support without evidence of
any deformation.
Unfortunately, it is a common
misconception that simply tightening
a joint is something of a cure-all
for leaking connections. Of course,
there is the possibility that tightening
the joint will correct the issue.
However, the likelihood also exists
that the joint was previously overtightened
and therefore has compounded
problem areas, including
necking issues or distorted flange
faces. This brings up a key point
that illustrates how essential it
is to maintain accurate records of
the work performed on each application.
Detailed documentation
saves time and resources regarding
the continued maintenance and
performance of any asset.
Bolt-tightening methods
Several methods of bolt tightening
are in practice currently. Bolting
practices and technologies have
evolved in many ways over the
years. In the past, the tightness of
a bolted joint relied almost entirely
on the physical strength of the individual
operating the manual torque
wrench. As technological efforts
progressed, the pneumatic impact
wrench was introduced to simplify
control and provide improved accuracy.
Hydraulic torqueing followed,
further allowing individuals to
rely more heavily on the tool itself
rather than personal strength. Yet
another method, hydraulic tensioning,
is a more recent development
that can drastically reduce inaccuracies
and elevate the level of safety
in tightening applications.
Hydraulic nuts are a very versatile
type of tool used in tensioning
procedures. Installation applications
for hydraulic nuts include
heat exchangers (Figure 2) and reChemiCal
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Chemical Engineering November 2014

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