Chemical Engineering May 2022 - 52

APPLICATION EXAMPLE
I
nstalled in a drainage sump pit near a
tank or pump yard, an oil-leak detector
float sensor provides leak detection of
petrochemicals, vegetable oils and more.
Such a sensor combines vibronic and
conductive technologies with dual-level
logic to distinguish the presence of specific
fluids for effective leak monitoring.
The conductive probe detects waterEndress+Hauser
just
1/8 in. in a 30-ft diameter storage
tank - a common size at petrochemical
facilities - represents over
60 gal of fluid leaked. As a result, accuracy
must be measured in fractions
of millimeters. Many radar level gages
can provide accuracy of 0.5 mm
(Figure 3).
For many hydrocarbons and chemicals,
tank instrumentation must also
incorporate temperature compensation,
because chemical volume
expands and contracts with temperature
changes. These volumetric
fluctuations cause levels to change,
even when no fluid escapes or enters
a tank. Temperature sensors with multiple
measurement points and highprecision
accuracy are required for
such applications.
Combined vibronic and conductive level-sensing
technologies can identify the presence and
types of liquids in sumps and dikes
based liquids, while the vibronic tuning fork
confirms the presence of oil or air. Equipped
with process diagnostics, these instruments
transmit status data if a cable fails
or liquid freezes, assuring failsafe operation.
Advanced monitoring software works together
with advanced instrumentation by
executing algorithms that generate notifications
when detecting anomalies or certain
conditions. For example, software can
notify operators of a high rate-of-change in
a tank level, signaling a process issue that
requires attention.
❑
Leak detection and prevention
Chemical leaks can wreak catastrophic
consequences
Monitoring
level measurement with
a software system is the best way
to determine when level changes
should and should not be occurring.
When the level drops at a time
it should be stable or rising, the system
must alert facility personnel of a
possible leak or spill.
Monitor the dike
extremely
quickly, and the best method to
mitigate damage is to prevent their
occurrence or, at the very least, expediently
detect them. Doing so requires
proactive measurement. Level
instrumentation can safely and reliably
monitor the contents of a tank,
annunciating an alarm in the event of
an overfill event or leak.
As the first line of defense in tank
leak detection, a continuous level
instrument must always identify
tank-level fluctuations. If a tank level
ever decreases when its associated
control system is not actively lowering
it, this signals the presence of a
leak, and the monitoring system must
notify plant staff. Demonstrating the
criticality of detail in level measurement,
an unexpected drop in level of
52
The second line of leak-detection
defense is equipping level switches
inside retention dikes. All accumulation,
including rainwater after a
storm, must be removed to maintain
dike availability for catching spills or
leaks. Level switches to reliably indicate
any liquid, such as tuning forks,
are best suited for this application.
There are many specialty instruments
that indicate the presence
of liquid and its conductive properties.
This helps identify the liquid to
distinguish whether it is rainwater
(conductive) or a hydrocarbon (nonconductive).
By combining a vibrating
tuning fork, which detects liquid
regardless of its conductivity, and
a conductivity switch, facility personnel
and software systems can
identify the type of accumulation in
the dike.
When the dike is empty of all liquid,
neither the tuning fork nor the
conductivity switch report any liquid.
When the dike is full of water, both
the tuning fork and the conductivity
switch report liquid presence, assuming
the water is conductive, as is typically
the case with rainwater. When
the dike contains a nonconductive liquid,
the tuning fork reports presence
Endress+Hauser
FIGURE 4. Computer-based tank monitoring tools
can promptly alert operators of diagnostic and
process issues to minimize plant disruption
of a liquid, but the conductivity switch
does not, indicating a spill or leak of a
nonconductive chemical.
Maintenance is key
Accurate instrumentation is the cornerstone
for any safety system's reliability.
One of the greatest challenges
in maintaining instrumentation reliability
is a lack of awareness when
issues arise.
Many modern instruments, particularly
smart instruments, transmit
process and device diagnostics,
in addition to primary process variables.
By monitoring these diagnostic
data at central processing points
and implementing notifications, control
systems can alert operators of
automatically detected instrumentation
issues, identified directly by the
instruments (Figure 4).
Diagnostic information is useful
for implementing predictive maintenance
programs because instrumentation
usually begins to register
issues prior to complete failure. This
can help maintenance staff focus
their
efforts where most needed,
reducing unplanned downtime,
along with unnecessary and costly
instrument replacement.
Incident response
The more quickly a SIS can identify
chemical spills and leaks, the better
a facility can control incidents and
respond appropriately. The right level
instrumentation can provide early
indication of issues, and then dispatch
alerts to mitigate risk. When
an incident occurs, a well defined
response can make the difference
between acceptable recovery and
catastrophic harm to equipment, the
environment or humans.
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