Chemical Engineering December 2010 - 30

Cover Story
With any metallurgy employed, the
linear expansion and contraction of
the piping must be accommodated by
use of expansion loops or flexible connection
members, and pipe supports
should be generously spaced to prevent
sagging. When thin-walled expansion
joints are selected, manufacturers'
recommendations for inspection
and replacement frequency should be
strictly followed. Piping should be provided
with " shoes " , which can slide unrestrained
axially on the pipe supports
with thermal cycles. Where flanges are
necessary, Class 300 and Class 600
ring-joint flanges or raised-face flanges
are commonly used. While the number
of flanged pairs should be minimized
to reduce the number of potential
leak points and fugitive emissions,
designers should consider maintainability
and install such connections to
all major pieces of equipment and in
other key locations for ease of servicing
equipment (Figure 3).
When used with raised-face flanges,
gaskets should have a metal ring for
blow-out resistance and graphite-filler
within 316 stainless-steel, spiral windings
for fire resistance. A number of
manufacturers offer such gaskets, and
some designs provide more-exotic-alloy
spiral windings for improved memory
and longer use-life for cycling temperature
services. A key necessity for
flanged connections is to ensure that
adequate and uniform sealing compression
(seating stress) is provided on the
gasket faces. Data on compression force
of gasket surfaces are available from
gasket manufacturers, which will then
determine proper bolt/stud/nut selection
and torque requirements.
For non-circular gaskets, such as
in some pump casings and valve bonnets,
foil-inserted graphite has been
successfully used. Where larger gaskets
are required for heat exchanger
heads with partitions, and so on, foilinserted
graphite, tang or corrugated
metal-inserted graphite can be used.
With any gasket type selected, proper
installation techniques prescribed by
the manufacturer should be followed.
Pumps and pump seals
Pumps in HTF service are generally
of cast steel for withstanding the
stresses of thermal shock. Pumps in
high temperature service can have
double mechanical seals, or can be of
a sealless design. The pumped fluid is
used to lubricate and cool bearings in
either type. Excessive temperatures
at mechanical seal faces (typically
tungsten carbide, stellite, or silicon
carbide) can vaporize the HTF resulting
in no lubrication and mechanical
damage to seal face materials. This
mechanism can create particles that
can erode the seal face and create a
separation of seal faces resulting in
leakage. Therefore, stuffing box and
seal gland cooling is important to
maintain lower temperatures and also
improved lubricity of the HTF. A seal
flush can be effective in minimizing
excessive seal face wear and particle
accumulation on flex metal bellows by
removing harmful deposits with filtered
fluid [15].
Valves
Valves in DP:DPO service may include
forged- or cast-steel, or stainless-steel
bodies, balls, plugs and
disks. 13-Chrome (Cr) stems and
13-Cr or stellited seats should be
considered. Fire-resistant graphite
packing should be specified in select
configurations available. Bellows seal
designs can provide physical barriers
for reduced emissions and leaks.
With frequent updates of valve feature
availability, the designer should
always consult with the valve manufacturer
to consider the latest innovations
available. Soft-seat materials
should be avoided since they can burn
out in case of fire, 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
flanged end-connections. Connecting
piping should have adequate flexibility
to minimize torque induced from
thermal expansion and contraction of
piping applied to flanges, which may
develop leaks.
Maintenance practices
Simply put, three keys for adequate
maintenance practice are as follows:
1. Respond to identified leaks promptly
with repairs
2. Repairs should address the cause of
30 ChemiCal engineering www.Che.Com DeCember 2010
the failure, and not just the consequences
of the failure
3. Learn, document, and practice the
recommended preventive maintenance
of the equipment
Leak DeteCtion
Human senses
DP:DPO fluid can be operated in liquid
or vapor phases. When there is
leakage, even above its normal boiling
point, the vapor emitted can quickly
condense to form a visible, near-white
mist cloud. The cloud quickly increases
its transparency as it dissipates. For
small leaks it may be possible to observe
liquid droplets present at the
source of the leak, such as valve stems,
pump seals, flanges, and so on, as well
as on the ground. As mentioned earlier,
a small leak from a flex hose can
appear as a darkened, wetted area.
When cool ambient conditions permit,
frozen DP:DPO may be observed as
white crystalline solid. Visual detection
capability for leaks can be greatly
enhanced by use of remote video-surveillance
monitoring.
The DP:DPO eutectic mixture
has been measured to have an odor
threshold of 9 parts per billion (ppb) in
air, making detection by odor possible
without exceeding 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 of
the components in air. While odor may
be a sensitive detection technique, the
use of instrumentation is a preferred
detection-and-measurement method
with superior quantifying capability
for leak tracing.
Specialized instrumentation
Instruments using the principal of
photoionization are well suited for use
in CSP and process plants. Typically,
these instruments can readily detect
DP:DPO concentrations in air as low
as the ppb range. For realtime personnel
exposure monitoring, the ppb
sensitivity is appropriate to quantify
exposure at levels below established
occupational exposure limits for the
fluid's components. For maintenance
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Chemical Engineering December 2010

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