Chemical Engineering July 2018 - 36

Cover Story
KEYS TO MAINTAINING HEAT-TRANSFER-FLUID QUALITY
1. Match heat transfer fluid to the process temperature demands.
2. Avoid problem-causing contaminants.
3. Protect against heat-transfer-fluid oxidation and excessive purging and venting.
4. Identify and correct causes of high-temperature excursions.
5. Establish a working relationship with heat-transfer-fluid supplier technical resources.
6.
Establish and maintain appropriate administrative and engineering controls.
Some foreign contaminants can have
such low thermal stability that they quickly
form coke deposits within heater coils,
leading to coil overheating, blistering and
possible rupture. Monitoring of heater performance
to detect the onset of such consequences
can include heat-transfer-fluid
temperature rise, pressure drop, and flowrates
through each coil pass of the heater.
With properly balanced flowrates, these
measured parameters should be common
to each pass. Monitoring of heater stackgas
temperatures, fuel usage rates, and
coil skin temperatures using infrared imaging
can also be helpful.
Some HTF chemistries can exhibit increased
thermal degradation rates when
affected by elevated concentrations of impurities
or contaminants that can act as
free radicals to promote molecular changes
[4]. As a result, the usable life of the heat
transfer fluid is diminished. When production
units cannot afford to shut down for
a proper identification and repair of leaking
equipment and replacement of the
fluid, some have chosen to perform partial
fluid replacements online. This is a costly
approach to extend the operation until a
planned downtime, and the subsequent
total fluid replacement remains necessary.
Contaminated fluid may have additional
disposal issues to manage as well, such
as one user whose process inadvertently
partially halogenated the heat transfer fluid
due to in-leakage. The named impacts to
operations all act toward increased frequency
of unplanned downtimes and increased
maintenance costs.
Corrective actions
What if the heat transfer fluid has already
fallen victim to one or more of the problems
mentioned? The priority is to correct the
root cause(s) that led to the deteriorated
fluid quality, whether it be equipment failure,
human error or other issues. The fluid
quality should be assessed. This typically
involves the fluid experts consulting with
the heating system owner's experts to de36
termine
if the fluid quality requires improvement.
Analysis of a representative sample
of the suspect fluid should be conducted.
Fluid quality adjustments can involve a partial
to full fluid replacement. This will dilute
any contaminants, degradation products,
insoluble solids, moisture, and acidity proportional
to the fraction of fluid replaced.
Added attention to fluid filtration can be an
appropriate measure for further insoluble
solids removal.
Severe situations where fouling and deposits
of solids have occurred can require
emptying the system for a more thorough
cleaning of the equipment and piping. Options
can include solvent cleaning, organic
fluid flushing, or water-based chemical
cleaning [5]. Consultation with the heattransfer-fluid
supplier can be helpful in
determining the approach best suited for
restoring the system cleanliness, while
minimizing costs of cleaning and fluid disposal
and keeping the turnaround time to
a minimum.
If the heater coils may have been compromised,
one useful technique is thermal
scanning. While limited to designs permitting
the view of an imaging camera through
an inspection port for visibility of coils near
the combustion zone, this approach can be
used to trend coil skin temperatures over
time to assess the potential ongoing accumulation
of internal coke fouling. The coke
can act as an insulation layer, resisting heat
transfer into the fluid. As a result, the coil wall
or skin temperature increases compared to
normal. With diligence in periodically measuring
coil skin temperatures, it is possible
to have early warning that can prompt corrective
measures prior to coil failure. It is important
to realize that without baseline temperature
measurements when coils are free
of coke, measurements taken later with coke
deposits present cannot be judged as " normal "
or " elevated. "
If the heat transfer fluid is being excessively
thermally stressed due to process
temperature requirements being beyond
the capability of the fluid to withstand, alternate
fluid chemistries should be considered.
Consultation with the heat-transferfluid
supplier can usually identify at least
one or two viable alternative
fluids
greater thermal stability to consider. With
any upgrade of fluid chemistries, a variety
of engineering evaluations and actions
should be undertaken, including potential
changes in fluid volume expansion (expansion
tank sizing), materials of compatCHEMICAL
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