Chemical Engineering October 2020 - 23

are outdoors, in non-air-conditioned
process bays, and facilities with
significant off-hours temperature
changes. Sterilization and cleaningin-place
are often overlooked as a
source of the same issue.
FIGURE 4. The softer copper ferrule shown in
the photo will never seal properly on the harder
stainless-steel tubing
FIGURE 2. Badly rounded threads like those shown
here are almost certain to leak
ened for longevity. Compression
fittings that are routinely
removed
and replaced will, over time and
dozens if not hundreds of cycles,
eventually start to leak and need to
be replaced. The more the fitting is
tightened, the shorter the time until
this can happen. Most manufacturers
recommend 1¼ turns as a good
compromise between longevity and
integrity. However, good-quality
compression fittings will often seal
with fewer turns. So some personnel
under-tighten compression fittings
routinely to extend their life. But this
results in a much greater chance of
leakage, as the safety factor inherent
in the manufacturer's recommendations
is no longer present.
Tubing inserted improperly. If tubing
is not inserted all the way into the
fitting and fails to bottom out, this will
result in a poor seal.
Debris and scratches. Fitting sealing
surfaces and tubing outside
surfaces are not generally treated
carefully enough. Dirt, grit or foreign
materials are often present, albeit in
small amounts, but enough to cause
gaps or scratches and promote leakage.
Poor work practices prevent
the fittings or the tubing from staying
clean enough. Tubing is often badly
scratched from poor handling practices,
being dragged along floors,
dirt, or other surfaces that produce
small scratches that make it more
difficult for the system to seal properly.
All could be prevented by proper
handling techniques.
Misalignment. Ferrules are left out,
damaged, or so badly misaligned
that sealing is compromised. Some
tubing is not aligned properly (Figure
3). Either it is placed in at an
angle or forced into the fitting in
such a way that the resulting forces
are working against the fitting itself,
promoting leakage.
Tubing hardness. For compression
fittings to work properly, the ferrules
must be harder than the tubing,
so they can bite into it (compress)
and make a good seal (Figure 4).
Thus,
every
quality
manufacturer
has a maximum recommended tubing
hardness. Sadly, most tubing is
bought from local suppliers with no
reference to this property. This often
results in tubing that is too hard for a
good seal and numerous unnecessary
leaks.
FIGURE 3. The tubing shown in this photo was
probably poorly aligned and felt like it was properly
inserted to bottom out in the fitting. It leaked
immediately upon startup and the entire piece had
to be replaced
Heating and cooling
Routine temperature changes always
create more leaks. Whether these
are heating or cooling is immaterial.
Large (>150°C) or rapid (>25°C/min)
temperature swings can cause leakage
in as little as one cycle. The author
has seen fittings that started out
leak-free and properly tightened become
less than hand tight at the end
of a heating or cooling cycle.
Ambient temperature swings,
often routinely ignored, are a significant
continuing problem. While
slower and of less magnitude, ambient
temperature swings happen with
a much higher frequency (every day).
The frequency more than compensates
for the smaller magnitude and
greatly promotes leakage. Common
problem areas include pipes that
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM OCTOBER 2020
Stress and torque
Pipe fittings that are subject to higher
stresses and torque tend to leak
more easily because these forces
work against the sealing surfaces
over time. Work on adjacent fittings
or components can cause two
leaks for each component touched
(one on each side). Hold backs, or
a wrench or pliers designed to prevent
the body of the fitting from moving,
are difficult to always coordinate
properly. Often, users respond to the
turning motion after the stationary fitting
starts to move, so it has already
been loosened. Other problem areas
include equipment, components
and pipe sections that are frequently
removed. These will leak not only at
the disconnect points, but also often
at nearby connections. Turning a
poorly supported valve increases the
potential for the torque to loosen the
fitting and cause a leak. Vibrations,
bumps and similar shocks from
equipment and operations all can
contribute to leakage.
Clearance issues
Pipe threads, unions and compression
fittings require clearance when
connecting and disconnecting to
allow components to be removed. It
is often difficult to provide sufficient
clearance, leading to problems including
improper
tightening,
bent
tubing
or sprung piping (piping that
has been forced together and has
significant residual stresses trying
FIGURE 5. Buying the right fitting would reduce
the potential leak points by two, by eliminating the
pipe bushing
23
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Chemical Engineering October 2020

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