Chemical Engineering December 2010 - 28

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ing surfaces during operation. Excessive
temperatures at the seal faces
can also vaporize the HTF resulting
in no lubrication and ultimately cause
mechanical damage to seal face materials.
This mechanism can create particulate
matter that erodes the seal
face and create a separation of seal
faces, thereby resulting in leakage. A
seal flush can be effective in minimizing
excessive seal wear by removing
harmful corrosive or fouling deposits
with filtered fluid [3].
It should be noted that while infrequent
insulation fires are experienced
in very-high-temperature organic
HTF systems, it is extremely rare
in DP:DPO systems. DP:DPO has a
lower relative-normal boiling point
than other high temperature HTFs,
permitting its more rapid evaporation
from soaked insulation before it can
be heated to its autoignition temperature
(AIT) by the oxidation exotherm
within porous insulation. Cellular
glass insulation has proven most effective
in interrupting the mechanism
of HTF insulation fires among most
organic HTF chemistries.
Consequences of leaks, fires
There are no positive consequences
of leaks and fires, except perhaps the
heightened awareness of design or
construction flaws, which leads to improvement.
The immediate negative
consequences include risks to human
life and health from released fluid
ignition and fire fighting response,
or inhalation of partially combusted
hydrocarbons. Personnel should be
trained and equipped to respond by
remotely isolating systems, and staying
out of the path of smoke, vapors
and liquid runoff. In CSP plants, the
amount of fluid contained in a 100-m
mirrored row with 70-mm O.D. heatcollection
elements will be limited
to nearly 0.33 m3 (87 gal), plus the
volume contained in piping up to the
isolation valve(s). Much of the HTF
released from piping containment
will flash into the vapor phase, and
then quickly condense into a mist
cloud. The balance will rapidly cool
in contact with equipment and earth,
remaining in liquid phase. Contaminated
soils will require disposal or
treatment per regulatory requireReduced
compression
of
gasket
Fixed restraint
Figure 2. Poor piping flexibility can cause leaks
Characteristic
Table 1. DP:DPO Key Fire-relaTeD PrOPerTies
°C
Method
Flash point (open cup)
Fire point
Flash point (closed cup)
ASTM D-92
ASTM D-92
ASTM D-93
Autoignition temperature ASTM E-659
Lower combustion limit
--Upper
combustion limit
--°F
Vol. % in air
124 255 --127
260 --110
230 --601
1,114 --149
260
149
260
ments,
possibly including incineration
or bioremediation. Hot fluid released
and collected should not be reused, as
it will have become oxidized and perhaps
contaminated. Equipment wetted
with HTF condensate should be
cleaned for proper housekeeping, and
to remove residue that could potentially
fuel a future fire.
In areas where leakage is of highest
risk (such as flex hose or rotary
joints, pump seals, valves or instrument
manifolds, and so on) efforts
should be made to limit the amount
of instrumentation and wiring potentially
exposed to fire. Doing so will
minimize the downtime required to
return the equipment into operation.
Any equipment, piping, instrumentation,
insulation or controls that have
been exposed to fire or DP:DPO spray
should be inspected by qualified engineers
and technicians for reliable operating
integrity, prior to returning it
into service.
28 ChemiCal engineering www.Che.Com DeCember 2010
300
500
300
500
0.8
0.5
3.3
6.2
Fluid ChemiStry and
inherent ProPertieS
Formulation
DP:DPO heat transfer fluids are a eutectic
mixture of diphenyl and diphenyl
oxide. These two components exhibit
the highest thermal stability available
among organic heat-transfer media.
Their eutectic ratio ensures the lowest
freeze point possible, thereby creating
the widest working range of temperature.
As diphenyl and diphenyl oxide
exhibit tremendous thermal stability,
the weakness of the specific fluid formulation
would then lie in its impurities.
Therefore, it is most desirable to have
very high assay of the two primary constituents,
and limit the percentage of
impurities possible. Doing so protects
thermal stability of the fluid and also
ensures that the physical properties of
the HTF will have minimal deviation
from its key components.
Low chlorides content (< 10 ppm)
ensures long life of stainless-steel
Deflection
Force
Force
Insulation
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Chemical Engineering December 2010

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