Chemical Engineering December 2010 - 29

Figure 3. Pipe flanges are potential sources of leaks. Design considerations are
important here
system components without excessive
risk of stress-corrosion cracking.
There are SEGS systems, for example,
that have been utilizing low-chloridecontent
heat-transfer fluid since the
1980s with no reported cracking issues.
Low chlorides content should
therefore be a requirement of DP:DPO
fluids used in plants to help maintain
longterm mechanical integrity.
Tendency toward leakage
Operators of high temperature, organic
heat-transfer fluids should be
aware of the potential for leakage of
DP:DPO from components that may
be leak-tight when pressure tested
with water. It has been noted that,
" the physical and solvent properties
of organic fluids allow the fluids to
penetrate ordinary valve and pump
packing and bleed through porous materials
including some cast irons " [4].
For piping flanges, fire-resistant gaskets
are preferred, which can require
higher compression to adequately seal
as compared to softer gasket materials.
To help minimize leak potential
and keep the DP:DPO confined within
the system, engineers and safety professionals
have developed key practices
as outlined later in the section on
preventing leakage.
When DP:DPO eutectics 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, possibly leading
to release of the product through the
weakest constraint, such as flanged
connections, valve stems, pump seals,
and so on. When thawing a frozen section
of piping or equipment, it is very
important to accommodate the expansion
in volume into unobstructed piping
or equipment.
Key fire-related properties
Key properties of interest in gauging
the potential for fire with organic heat
transfer fluid are provided for DP:DPO
in Table 1.
Minimum ignition energy (MIE)
is a measure of an amount of energy
below which an explosive mixture in
air will not be affected, and above
which the mixture can be affected.
Measured values for heat transfer
media are not readily found in published
literature. As T. H. Pratt explains,
the MIE concept applies only
to capacitive spark discharge and
that, based upon numerous factors,
" one must recognize that MIEs vary
with most everything " [5]. Taking this
into account, NFPA 77-2007 states
that the MIE for most saturated hydrocarbon
vapors is near 0.25 mJ, and
that mists can have MIE values one
or two orders of magnitude higher [6].
This value can have considerable deviation
from the MIE experienced in
actual field environments.
Preventing leakage,
comPonent selection
Reliable system design
Complete guidance on system reliability
cannot be adequately covered
within the constraints of this article;
however, many important concepts
are provided herein in support of designers
and operators. Economics of
projects often require use of the lowest
cost materials that are fully compatible
for the service to be utilized.
Since the mid-1970s and the development
of the first low-chloride (<
10 ppm) formulation of heat transfer
fluids, stainless-steel metallurgy for
instrument tubing and specialized
equipment could be used without fear
of chlorides-related stress cracking
potential. It is important to note that
any significant extension of operating
temperatures above those currently
used in DP:DPO HTF systems would
require more exotic and costly alloys
for piping and equipment, as well as
the development of satisfactory, new
heat-transfer fluids. Useful reference
standards and codes are included in
the References section [7-14].
With the HTF being significantly
above its normal boiling point during
operation, 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. With proper design considerations,
the PRVs can be installed in
series with rupture disks to minimize
fugitive emissions from the system.
General good practices for HTF system
design are typically available from the
HTF manufacturer, including information
on system components, filtration
and expansion tank design.
Piping, flanges and gaskets
Seamless carbon steel has been demonstrated
as an appropriate piping material
for use with organic HTFs in plants
up to their maximum bulk operating
temperatures. Threaded fittings in piping
are to be avoided in preference to allwelded
construction. Graphite-based,
paste thread sealants have demonstrated
marginal success with threaded
connections that cannot be avoided,
such as instrument connections, pumpcasing
drain plugs, and so on.
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Chemical Engineering December 2010

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