Chemical Engineering August 2022 - 23

Facts At Your Fingertips
Heat-Transfer Fluid System Venting
Department Editor: Scott Jenkins
H
eat transfer fluids (HTFs) provide
heating and cooling of
process equipment, including
reactors, autoclaves, distillation columns,
reboilers, mixers and dryers.
HTF system designs should provide
for effective system venting, both of
residual water at startup and of degradation
products during operation.
Water concerns
When commissioning new HTF systems
(Figure 1), a primary concern
should be the effects of water: new
systems can be vulnerable to excessive
pressures from residual water. Hydrostatic
pressure tests (leak checks)
conducted on the system during
manufacture or onsite after maintenance
can be a typical water source.
Complete water removal can be hindered
by traps and piping elevation
changes. The best system designs
provide piping installations with slopes
toward strategically placed low-point
drains. After water is drained, but prior
to filling, the system may be further
dried by purging warm, dry air (or N2)
through the system's circuits until the
exiting gas dewpoint reaches -34 to
-40°C, indicating moisture has been
adequately dried. Close attention to
the drying process will significantly
reduce the time needed to reach the
intended high operating temperatures
at start-up.
Removing moisture at start-up
Prior to circulation, ensure the cold
liquid level of the HTF in the system
is adequate. This is typically indicated
by the expansion-tank liquid level
instrument (Figure 1).
Next,
heat
the liquid slowly while circulating
throughout all piping circuits with the
assumption that water content may
be excessive. Valve A is closed, and
valves B and C are open. The HTF
is circulated through the expansion
tank and heats to just above 100°C.
This temperature forces the moisture
to flash into the vapor space of the
expansion tank. Valve E is open, and
the ingress of inert gas sweeps the
water vapors from the vapor space
downstream to a catch tank or flare
system. The process
continues until moisture
symptoms - including
pump cavitation,
erratic flowrate
at the discharge side
of the pump, and rattling,
knocking and
boiling sounds in the
expansion tank and
pipe - subside.
Once the HTF is
deemed adequately
dried, the fluid should
be capable of conEastman
FIGURE
1. Heat-transfer fluid systems need to be vented for residual
water at start-up and for fluid degradation products during operation
tinued heating to higher operating
temperatures. Typical valve alignment
during normal operation is for valves
A and B to be open, and valves C and
E to be closed. This valve placement
allows a lower temperature in the expansion
tank (commonly about 25%
of system volume), where its thermal
degradation rate is negligible.
Venting degradation products
In operation, the HTF deteriorates
at increasing rates as the operating
temperatures approach the bulk
operating maximum for the specific
HTF, altering the condition and composition
of the HTF. Thermal degradation
leads to the formation of both
high-boiling compounds, which increase
fluid viscosity and potential
solids formation that increase risks
of coke or fouling deposits, and lowboiling
compounds that decrease the
fluid's viscosity and that have boiling
points lower than the boiling range of
the HTF. Additionally, increasing lowboiling
content can lead to flashpoint
depression by 45°C or more.
The low boilers can be managed
by routine system venting. Systems
are best vented only when the concentration
of low boilers exceeds
recommended limits based on sample
analysis of the fluid.
The same procedure is followed
when commissioning a new system,
except higher temperatures are
needed. For many organic HTFs,
the venting procedure is conducted
at fluid temperatures near 180 to
200°C. This temperature range supCHEMICAL
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ports flashing into the vapor phase
and separation of the low boilers
from the heat transfer fluid for removal
without incurring a significant loss
of the HTF components. With valve
A closed and valves B and C open,
all the fluid flows through the expansion
tank. This process raises the
fluid temperature in the expansion
tank and increases the partial pressures
of the low-boiling degradation
products. This allows the low boilers
to flash into the vapor phase, where
their removal is supported by opening
valve E. Inert gas can be used to
efficiently sweep those vapors across
the surface of the liquid and out of
the vent line, where they can be condensed
and collected for disposal.
Circulation through the expansion
tank also ensures that all HTF benefits
from reducing levels of low-boilers.
After the venting process, return
to the typical valve alignment for normal
operation, where valves A and
B are open, and valves C and E are
closed. This arrangement provides for
the thermal expansion and contraction
of the HTF volume to and from
the expansion tank with temperature
changes. Continual venting and purging
is not recommended, as this can
deplete fractions of the HTF itself, creating
related changes in its properties,
performance and life expectancy. n
Editor's note: This content was drafted by Kapil Bathla, a product
development and customer technical support specialist at Eastman
Chemical Co. (Kingsport, Tenn.; www.eastman.com).
Sponsored by
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