Chemical Engineering August 2013 - 25

Detection and
Prevention of
Department Editor: Scott Jenkins
he management of process plants requires
understanding and applying the fundamental
tenets of safe design, construction and
operation of facilities. This overview summarizes
best practices that have been developed
to ensure the safest possible use of DP:DPO
(diphenyl-diphenyl oxide) eutectic heat-transfer
fl uids. A review of the concepts necessary for
effectively maintaining a leak-tight heat-transfer-fl
uid (HTF) system is provided.
T
HTF system requirements
A " typical " HTF system is composed of an energy
source, such as fi red heaters or waste-heat
recovery systems, as well as pumps to force the
fl uid fl ow, an expansion tank to accomodate the
volume expansion of the fl uid and one or more
heat consumers.
High-temperature heat-transfer systems
are usually closed systems, so a release of
fl uid can only occur in the case of accidents
or malfunctions. As with all HTF systems,
the design must accommodate the volume
expansion of the heating fl uid. The extensive
network of piping, instruments, and vessels,
combined with elevated vapor pressures, can
increase the potential for leaks from systems if
adequate design and maintenance measures
are not incorporated.
Leak sources
Primary sources of leaks are fl anged connections,
fl exible connectors or rotary joints, and
pump seals. When insuffi cient fl exibility is provided
in piping networks, the resulting force
applied to the fl ange pair can reduce gasket
compression and lead to leakage. Stainlesssteel
fl exible hoses were extensively installed
on the early concentrating solar power (CSP)
systems. Experience in the mode of failure in
such hoses has found that the hoses develop
small cracks. A properly installed fl ex hose will
avoid torsion and misalignment, and maximize
its usable life. When fl exible hose is used in
place of rigid pipe, consult manufacturers'
recommendations of inspection and replacement
frequency.
Rotary joints have the advantage of greater
wall thickness, which give rise to a more
robust joint, yet maintaining the fl exibility to
support the demands of mobile piping. For
high-temperature service, the joints will typically
require periodic injection of graphite-based
packing to maintain leak-free performance in a
fi re-resistant design.
Standard centrifugal pumps will typically
use mechanical seals. While some seepage
is not uncommon, problems can arise
that can reduce seal life and lead to more
significant leakage (see " Pump and pump
seals " section).
When DP:DPO eutectic fluids freeze, the
material contracts in volume by over 6%. If
the product then melts between frozen plugs
of product or other mechanical boundaries,
tremendous pressure can result. The pressure
can lead to release of the product through the
weakest constraint. When thawing a frozen section
of piping or equipment, it is very important
to accommodate the expansion in volume into
unobstructed piping or equipment.
Component selection
Because the HTF is signifi cantly above its
normal boiling point at process temperatures,
adequate overpressure protection in the form of
pressure relief valves (PRVs) and rupture disks
must be provided with adequate capacity for
relief between selected points of isolation.
Piping, fl anges and gaskets. Seamless carbon
steel has been shown to be an appropriate
selection for piping with organic HTFs in plants
up to their maximum bulk-operating temperatures.
Fully welded construction in piping is
preferable to threaded fittings, which should
be avoided. Graphite-based, paste thread
sealants have demonstrated marginal success
with threaded connections, in cases where they
cannot be avoided, such as in instrument connections,
pump-casing drain plugs and others.
With any metallurgy employed, the linear
expansion and contraction of the piping must
be accounted for through the use of expansion
loops and fl exible connection members. Pipe
supports should be generously spaced to prevent
sagging. Where fl anges are necessary,
Class 300 and Class 600 ring-joint fl anges or
raised-face fl anges are commonly used. When
using raised-face fl anges, gaskets should have
a metal ring for blowout resistance, and graphite-fi
ller with 316 stainless-steel spiral windings
for fi re-resistance. A key necessity for fl anged
connections is to ensure that adequate and
uniform sealing compression (seating stress) is
provided on the gasket faces.
Pumps and pump seals. Pumps in high-temperature
service can have double mechanical
seals, or can be of a sealless design. Excessive
temperatures at mechanical seal faces can
vaporize the HTF, resulting in an absence of
lubrication and mechanical damage to sealface
materials. In order to avoid excessive
temperatures (which can create particles that
can erode the seal face and result in leakage),
cooling of the stuffi ng box and seal gland is
important to maintain lower temperatures and
also improve lubricity of the HTF.
Valves. Valves in DP:DPO service may include
forged or cast steel, or stainless steel bodies,
balls, plugs and disks. Bellows seal designs
can provide physical barriers for reduced
emissions and leaks. Soft-seat materials should
be avoided, since they can burn out in case
of fi re, potentially adding to the complexity
of the HTF release. Small valves with welded
end-connections should be considered to reduce
potential leak points, and larger valves should
be considered with fl anged end connections.
Relief valves can use engineered installations of
rupture disks beneath.
Leak detection
Human senses. When there is leakage, even
above its normal boiling point, the vapor
emitted can quickly condense to form a visible,
near-white mist cloud. For small leaks it may
be possible to observe liquid droplets present
at the source of the leak, such as valve stems,
pump seals, fl anges, and so on, as well as on
the ground.
Also, the odor threshold is 9 parts per
billion (ppb) for DP:DPO in air, making
detection by odor possible without exceeding
FACTS AT YOUR FINGERTIPS Sponsored by
DP:DPO Fluid Leaks
MAINTENANCE PRACTICES
Simply put, three keys for adequate
maintenance practice are:
* Respond to identified leaks promptly
with repairs
* Repairs should address the cause of the
failure, and not just the consequence
* Learn, document, and practice recommended
preventive maintenance of the
equipment
established airborne exposure limits for either
component. In indoor areas, the airborne HTF
may not dissipate readily, preventing easy
tracing back to the leak point. In outdoor environments,
the odor is more closely localized
around the leak point due to the more rapid
dissipation in air.
Process level indications. Today's installations
will have installed instrumentation for
the routine measurement of liquid HTF levels.
Changes in liquid level in vessels can be a
somewhat crude, but important component of
detecting material loss.
Specialized instrumentation. Instruments using
the principal of photoionization are well
suited for use in all kinds of process plants.
These instruments can readily detect concentrations
as low as the ppb range. For realtime
personnel exposure monitoring, the ppb sensitivity
is appropriate to quantify exposure. For
maintenance needs, parts-per-million (ppm)
sensitivity is suffi cient to determine orders of
magnitude of identifi ed leak sources so that
repairs can be scheduled on a prioritized
basis. The instruments are available as handheld
units or as fi xed-mount, continuous area
monitoring stations.
While not comprehensive, the above guidance
can help develop practices enabling a
safer system design, reduce leaks and lower
make-up fl uid costs.
References
1. Gamble, C.E., Schopf, M., " Heat Transfer Fluid
Leaks: Break the Fire Triangle, " Chem. Eng.,
Dec. 2010, pp. 26-33.
2. Solutia Inc., " Systems Design Data " , Pub. No.
7239193 version C, Therminol Heat Transfer
Fluids, Solutia Inc.
Notice. Although the information and recommendations
set forth herein are presented in good faith,
Eastman Chemical Company and its wholly owned
subsidiary Solutia Inc. make no representations
or warranties as to the completeness or accuracy
thereof. You must make your own determination of
its suitability and completeness for your own use,
the protection of the environment and the health
and safety of your employees and purchasers of
your products. Nothing contained herein is to be
construed as a recommendation to use any product,
process, equipment or formulation in conflict with
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will not infringe any patent. No representations or
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Chemical Engineering August 2013

Table of Contents for the Digital Edition of Chemical Engineering August 2013
