Chemical Engineering July 2015 - 58

ences typically need several years
to come to the attention of (and
be accepted by) code task-force
team(s) and then be incorporated
into the codes
* The teamwork that is involved
should have representatives from
different groups, vendors and others,
with various backgrounds
and goals, to prepare for compliance
of relevant codes. Note: the
main focus of the codes should
not necessarily be the best performance
and reliability (for the operators);
rather, the final (or agreedupon)
specification should be an
optimized set of requirements (or
sometimes compromised ones).
In other words, any user should
know that some items specified in
codes are not necessarily the best
possible technology for operators
and often it is necessary to specify
some requirements in addition tothe
codes
There is always a great concern
about the lubrication oil system (generally
called " the oil system " ). A design
that involves having the primary oil
pump driven by the main machinery
shaft is usually not acceptable for any
high-speed turbomachinery (including
the gas turbine). The high speed itself
is one reason; it is not usually matched
with lower speeds of oil pumps. The
use of a gear unit to connect the oil
pump to the turbomachinery shaft is
often a poor solution, because an oil
pump failure can mandate shutdown
of the turbomachinery unit.
Horizontal oil pumps are always
preferred. The oil pump selection
and packaging should be in a way
to minimize maintenance and operation
efforts. For instance, oil-suction
pipes should be arranged to provide
positive suction head on the
oil pumps, with a slope toward the
pump. Cast-iron casing is usually
not desirable for any equipment (or
machinery) in the machinery package
or auxiliary systems, because it
is a brittle material and can fail very
quickly in emergency situations, particularly
in the event of fire.
Generally, an auxiliary skid arrangement
should be optimized to balance
performance objectives and reliability
goals, while maintaining easy access
and maintenance requirements, and
compact design. Heat exchangers
are needed to cool the lubrication
oil, which is often heated during op58
eration.
TEMA C shell-andtube
heat exchangers (with
removable bundles) are wellknown
for auxiliaries, including
lubrication oil skids. The
oil pressure should be greater
than the cooling-water pressure,
to avoid the potential
for water leakage into the oil
in the case of an unexpected
problem (such as exchanger
tube crack or leakage). Water
is on the tube side and oil on
the shell side.
Plate-type heat exchangers are
FIGURE 4. A example of a very large gas turbine installation.
The air inlet filter and the exhaust gas stack are also shown.
not popular in turbomachinery assemblies
since they might clog or
experience some other operational
problems. An exception is for revamp
projects with limited available footprints;
but tubular heat exchangers
may be used for small packages.
Gas turbine heat recovery
For some gas-turbine packages,
heat recovery is the source of many
problems and issues. One reason
may be that in spite of the gas turbine,
which is offered in standard
models, the heat-recovery unit is
usually a custom-designed system.
The performance and reliability of
the gas turbine package depend
on the heat-recovery system. The
design, fabrication and operation
of such a heat-recovery system
as an integrated part of the overall
gas turbine package system. In the
most common form, the exhaust
gases from the gas turbine enter
the heat recovery steam-generating
(HRSG) system, where the energy
from the hot exhaust is used to heat
the water to produce steam.
Many HRSGs are designed in
different modules and sections. In
many cases, each HRSG has a preheater,
an economizer and a superheater.
The steam for modern steam
turbines is usually superheated.
Both vertical and horizontal HRSGs
are commonly employed.
By contrast, once-through steam
generators (OTSGs) are used in
some applications because they are
cheaper, simpler and more compact
compared to other HRSG designs.
OTSG systems do not have defined
economizer, evaporator or superheater
sections. In simple terms,
in an OTSG system, water enters
at one end and steam leaves at
the other end. There is no need for
drums, various sections and many
other auxiliaries (or accessories).
Typically, the hot gas exhaust
from a gas turbine has ample oxygen.
Therefore, the gas exhaust
can be used in another combustion
process to increase its temperature
for a better heat recovery
arrangement. The supplementary
firing at the waste-heat recovery
unit could be a feasible option to
achieve the maximum possible efficiency.
This design is also becoming
popular in modern CPI plants,
which require a better operationall
flexibility since, theoretically, supplementary
firing can offer some
operational flexibility.
However, this complex design
(supplementary firing and HRSG)
requires special care. For example,
the transfer duct (between the gas
turbine exhaust and the waste-heat
recovery unit) should have a sufficient
length to ensure complete
combustion and avoid direct flame
contact on the heat-transfer surfaces.
On the other hand, the duct
system length should be optimized,
in order to limit the heat loss, manufacturing
costs and overall system
footprint.
Control and monitoring
The temperature of the turbine blade
metal must be monitored to ensure
reliability of the first row of turbine
blades and other hot sections. The
use of pyrometers to sense the blade
metal's temperature is offered for
some gas-turbine packages for CPI
units. Very high pressures in air-axial
compressors of gas turbines have
caused these compressors to have
a very narrow operating range between
the surge and the choke. The
axial compressors of gas turbines
tend to be very sensitive to dirt and
fouling, and even slight modifications
to the blade (vane) angle.
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Chemical Engineering July 2015

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