Chemical Engineering December 2010 - 26

Cover Story
Feature Report
Heat Transfer
Fluid Leaks:
Break the
Fire Triangle
Extreme processing temperatures
present the greatest risk.
Know where leaks are most likely to
occur and how to prevent them
Conrad E. Gamble and Matthias Schopf
Solutia Inc.
M
anagement of process plants
requires the understanding
and application of the
fundamental tenets of safe
design, construction, and operation
of each facility. In that context, many
years of practical experience with systems
that use very high-temperature
organic heat-transfer media has led
to much knowledge regarding design
features to adopt and those that
should not be repeated.
For the media itself, the severe
stresses of extreme operating temperatures
involved can sometimes narrow
the number of candidate fluids to
one: the eutectic mixture of diphenyl
and diphenyl oxide (DP:DPO; Figure
1). This article shares best practices
that have been developed over the
years to ensure the safest possible use
of this heat transfer fluid (HTF). Effective
methods to identify and determine
the order of magnitude of leaks
are discussed. Meanwhile, a review of
the concepts necessary for effectively
maintaining a leak-tight system is
presented. And, for occasions in which
unexpected leaks do occur, a thorough
explanation of the fire risk is offered
and approaches that can minimize ignition
potential and property damage
are addressed.
IntRoduCtIon
Basics of system requirements
Process plants require heating and
usually cooling capabilities. A " typical "
HTF system is composed of an energy
source, such as fired heaters or waste
heat recovery systems; pumps to force
the fluid flow; an expansion tank to
absorb the volume expansion of the
fluid; and last but not least, a heat
consumer. High-temperature heattransfer
systems are usually closed
systems, and hence, a release of fluid
can typically only occur in case of accidents
or malfunctions. Heat transfer
media used in such systems are usually
water-and-steam, water based
fluids, mineral oils or synthetic heattransfer
fluids specially designed for
this purpose. Based on their physical
and chemical properties, such fluids
imply certain handling risks in addition
to the general risk of operating an
HTF system.
26 ChemiCal engineering www.Che.Com DeCember 2010
Besides classical process plants in
the chemical, petrochemical and plastic
industries, two other applications
have recently come into the focus by
utilizing large volumes of DP:DPO
HTFs (see box entitled Large volume
applications): electrical energy production
by concentrating solar power
(CSP) and converting natural gas into
synthetic oils in a process called gas to
liquids (GTL).
As with all HTF systems, the design
must accommodate the volume
expansion of the heating fluid, which
is typically provided by an expansion
or surge tank of sufficient volume and
headspace. For plants using DP:DPO
near its maximum temperature of
400°C (750°F), the volume expansion
from ambient temperature is greater
than 30% [1]. The expansion vessel is
typically pressurized to keep the HTF
in liquid phase and is equipped with
a vent system to permit the exit of
nitrogen, degradation products, and
a portion of DP:DPO into the ullage
vent-collection system. The extensive
network of HTF handling piping, instruments,
and vessels, combined with
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Chemical Engineering December 2010

Table of Contents for the Digital Edition of Chemical Engineering December 2010
