Chemical Engineering December 2010 - 27

Large voLume appLications
CSP plants
Concentrating solar power (CSP) plants require a reliable heat-transfer fluid
to efficiently absorb the sun's energy and transfer it to the steam generator.
The HTF is the single component of the system, which traverses the solarenergy
collector field (mirror arrays) and the power generation unit (power
block). The uninterrupted circulating flow of the fluid and thermal energy is
essential for system operation. Any resistance to flow reduces demonstrated
efficiency from theoretical levels, and interruptions in flow can take the entire
system offline.
The designs of existing and planned CSP plants that utilize DP:DPO heat
transfer fluid have similar features. Large circulation pumps capable of
delivering flowrates up to 10,000 gal/min or more provide the motive
force for circulating the fluid through field supply headers of decreasing
diameters extending to the farthest mirror rows. HTF flowrates through the
individual heat-collection element (HCE) rows are regulated to maintain
the desired HTF temperature in the return headers. The heated fluid is used
in large shell-and-tube heat exchangers to provide superheated steam,
which is then utilized in traditional power turbines. The HTF continually
circulates throughout the operational hours to support the movement of
solar energy from direct solar and thermal storage systems, if provided.
Figure 1. Extreme operating temperatures sometimes
narrow the number of candidate fluids to one:
the eutectic mixture of diphenyl and diphenyl oxide
(DP:DPO)
elevated vapor pressures, can increase
the potential for leaks from systems
if adequate design and maintenance
measures are not also incorporated.
Incident history of leaks, fires
Examination of the leak history from
DP:DPO handling systems indicates
that the primary sources are flanged
connections, flexible connectors or
rotary joints, and pump seals. Fireresistant
gaskets used in DP:DPO
service can require substantial compressive
force on the gasket face. In
cases where insufficient flexibility is
provided in piping networks (Figure
2), the resulting force applied to the
flange pair reduces compression on a
portion of the gasket, leading to leakage.
Such leaks can sometimes be addressed
by tightening sufficiently, but
only as a temporary repair. Permanent
repair will require gasket replacement
and improved piping flexibility.
Stainless-steel flexible hoses were
first installed on the early solar energy
generating systems (SEGS) in
the California Mojave Desert in the
1980s. Some of the original flex hose
GTL-processes
In the gas-to-liquids (GTL) process, natural gas is chemically converted
into fuel liquids. The base is the Fischer-Tropsch (F-T) process of converting
mixtures of hydrogen and carbon monoxide into liquid hydrocarbons. In
general, GTL technologies are three-step processes. The first step is conversion
of the natural gas into a hydrogen and carbon monoxide mixture usually
called synthesis gas or syngas. The second step is a catalytic reaction
of this syngas into a mixture of hydrocarbons (F-T). Finally, the last step
is a cracking and conversion of the hydrocarbons into the desired final
products [2]. Due to high temperature requirements of some of the process
steps, DP:DPO heat transfer fluid is utilized for process heat control. ❑
remain in service today, while most has
been replaced over time. Experience in
the mode of failure finds that the hose
develops small cracks through which
the DP:DPO fluid seeps slowly at first.
The leakage can be visually observed
as a darkened area on the outside of
the hose assembly. Prompt maintenance
can permit the piping section
to be shut down and isolated for hose
replacement prior to a rupture. When
proper experience and care is applied
during hose installation, the hose can
be kept free of the torsion and misalignment
that reduce its service life.
It is important to follow hose manufacturer
recommendations on inspection
and replacement frequency, as the
hose has smaller wall thickness than
rigid pipe.
Rotary joints have the advantage of
using greater wall thickness for a more
robust joint, yet have the flexibility to
support the daily cycling demands of
the mirrored rows of heat collection
elements (HCEs). Designs can incorporate
high-temperature, metal-to-metal
seals and can include provision for
injectable, graphite-based packing for
maintaining adequate sealing. Some
users, however, have reported that
some of the rotary joints failed more,
and without warning.
Whether from flexible hose or rotary
joints, the close proximity of these devices
to nearby ignition sources can
lead to the ignition, or even autoignition
of a released cloud of HTF mist if
surface temperatures exceed 1,100°F
(593°C). For safety of personnel, response
typically involves remote isolation
of the piping circuit to allow the
fire to burn itself out in remote or noncongested
areas. Alternate measures
may be appropriate to protect against
peripheral damage in more congested
areas. Extinguishing the fire prior to
stopping the HTF release may result
in later, repeated ignition. Operating
practices must incorporate the frequent
monitoring of the integrity of
field piping systems through regular
inspections and/or remote video surveillance,
or both.
Pumps in DP:DPO service can have
single or double mechanical seals.
Some of the pumped liquid may be
required to adequately lubricate sealChemiCal
engineering www.Che.Com DeCember 2010 27
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Chemical Engineering December 2010

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