Chemical Engineering December 2010 - 31

Water fog mist suppression system
Fog nozzles
Photocell
detector
Vapor mist
Heat
transfer fluid
piping
Water
header
Deluge
valve
Figure 4. Vapor or mist detection systems automatically detect the presence
of clouds of DP:DPO and can automatically trigger water-fog deluge systems and
alarms, initiate forced ventilation, and de-energize area equipment
needs, parts-per-million (ppm) sensitivity
is sufficient to determine orders
of magnitude of identified leak sources
so that repairs can be scheduled on
a prioritized basis. The instruments
can be purchased as handheld units
for lightweight mobility, or as fixedmount,
continuous area monitoring
stations. The devices pull air samples
through the instrument for measurements.
Drawbacks include inability to
distinguish DP:DPO vapor from other
measureable organics and combustibles
(although, this is not a problem in
most CSP plant areas), and tendency
to condense vapors within the sample
inlet when sampling high concentration
leak points. The internal condensation
can require the instrument to
be offline until disassembled for thorough
cleaning. Experience in handling
the unit can quickly overcome
this issue. Cost of these instruments
currently varies from approximately
$3,000 for small, ppm handheld models,
to $5,000 and higher for the ppb
and fixed-mounted systems. They can
be purchased with intrinsically safe
certifications for use in hazardous
areas.
To prevent the potential ignition of
vapor- and mist-cloud leaks within indoor
areas with reduced ventilation,
custom-developed instrumentation
has been installed to automatically detect
the presence of clouds of DP:DPO
with the capability of automatically
triggering water-fog deluge systems
and alarms, initiating forced ventilation,
and de-energizing area equipment
with a continuous monitoring capability.
This design has incorporated
the use of multiple-sensor actuation
to avoid accidental activation from
only one monitor (Figure 4). The use
of water-fog systems has been found
in testing to rapidly reduce airborne
DP:DPO concentrations to below combustible
concentrations in air [16].
It is important to follow the manufacturer
recommendations with respect
to the use and maintenance of
these instruments.
Process level indications
Today's facilities will typically have
installed instrumentation for the
routine measurement of liquid level
in process vessels, including bulk
storage tanks, surge or expansion
tanks and ullage vessels, condensate
tanks, and others. Modern instrumentation
often permits excellent
level-trend capability so that
response time to unexpected level
changes is greatly enhanced by use
of configured deviation alarms with
a distributed control system (DCS).
Common suitable instrumentation
types can include displacer, differential
pressure and radar within their
respective design limitations of temperature.
Externally mounted floats
within stainless-steel chambers can
provide 0 to 100% level indication
and remote, continuous monitoring
on DCS screens. Changes in liquid
level in vessels can be a somewhat
crude, but important component of
detection material loss.
I/P
Potential fire scenarios
The prerequisite to a fire is a leak from
point sources, such as described in the
earlier section on incident history.
Insulation fires. Insulation fires are
not uncommon with high-temperature,
organic heat-transfer fluids. However,
in one extensive study of such fires by
Britton, he concludes, " Least prone
should be DP:DPO types, which have
very high AITs (above 500°C) and
relatively low flash-points of around
130°C " [17]. DP:DPO-soaked insulation
apparently avoids the insulation
fire mechanism by its more rapid rate
of evaporation out of the insulation
than other higher-boiling heat-transfer-fluid
chemistries, while avoiding
the close approach of its autoignition
temperature.
Spark- or flame-produced ignition.
With DP:DPO fluid, a more expected
cause of fire would be from spark- or
flame-produced ignition. Releases of
liquid DP:DPO above its fire point of
127°C (260°F) are susceptible to ignition
from area sources, such as exposed
electrical contacts, spark-producing
tools, open flames and so on. However,
hot HTF can rapidly cool once released
into its environment, both by evaporative
cooling and by conducting thermal
energy into the heat sinks provided by
the ground, equipment and so on. A
good design practice is to provide sloping
of underlying surfaces such that
any liquid release can drain away from
process equipment, thereby minimizing
potential ignition. Also, once any
release of HTF is observed, all sparkproducing
work should be halted until
the situation has stabilized.
Autoignition. Autoignition of a
DP:DPO cloud has been reported in
one CSP plant where the HTF leaked
in close proximity to the heat collection
element (HCE). When a vapor or mist
cloud of DP:DPO within its combustible
limits of concentration in air contacts
a surface at temperatures above
the autoignition point of the HTF, it
can ignite. The cloud is consumed rapidly
and can continue to burn at the
source of ongoing leakage until its flow
is stopped. Where possible, designers
are encouraged to consider incorporating
remote isolation capability for piping
circuits, so that the fuel to the fire
can be safely interrupted.
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Chemical Engineering December 2010

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