POWER June 2011 - 60

COMBUSTION TURBINES
paid for every hour that the plant operates.
The filter should remove 100% of particles
in the inlet air that are 3 microns and larger
and, at minimum, 99% of particles 0.5
to 3 microns. Be sure to include an entry
screen to prevent debris from entering the
filter house; orient the air inlet, louver,
or cowling to minimize entry of driving
rain, snow, or sand; also ensure that there
is good access to all parts of the air filter
module for maintenance and filter element
replacement.
The optimum duct design will:
■ Minimize the number of direction changes
required before the air enters the compressor,
including required turning vanes
(to ensure uniform flow distribution and
avoid resonance).
■ Limit the inlet air velocity to between 20
meters (m)/s and 30 m/s.
■ Design ducts to be sufficiently rigid to
avoid vibration (plate 5 to 10 millimeters
thick is generally used).
■ Include plenty of man-ways for cleaning
and inspection.
■ Include a differential-pressure alarm for
each stage of filtration.
At the end of construction, thoroughly
clean the air side of the inlet filter and ductwork
to eliminate any objects, no matter how
small, that could come loose during operation
and cause catastrophic damage to the CT.
If the CT is located close to the seacoast,
sea salt ingestion will rapidly cause sulfidation
or hot section corrosion, so make sure
the filter system materials and level of filtration
are proper for the location.
Here are a few other tips for selecting
equipment:
■ Don't skimp on the materials inside the filter
house and ductwork. The filter house,
inlet silencer, and perforated plate elements
should be fabricated from suitable
grades of stainless steel.
■ Make sure the silencers have a rigid structure
to prevent damage due to acoustical
or mechanical resonances or differential
thermal expansion.
■ The ducting and casing design should permit
field balancing in the end planes of the
rotors without requiring the removal of
major casing components.
When determining the arrangement of
the plant, make sure the air inlet is upstream
of the exhaust stack during prevailing
wind conditions to avoid recirculation
of exhaust gases under any projected wind
conditions. A good recommendation is to
keep a minimum horizontal separation of
60
idea is to include a full-size, redundant
shell and tube lube oil cooler configured
with a removable tube bundle and redundant
oil filters with removable elements.
Also be sure to use stainless steel for all
lube oil piping and valves-accept nothing
less.
Each oil supply line to critical components
should be individually monitored,
mainly for oil pressure. We recommend
that the oil reservoir retention time should
be at least 8 minutes. For aero-derivative
CTs (that have antifriction bearings and
use synthetic lubrication oil), the turbine
lubricating oil system usually should be
separate from the driven equipment (such
as the generator) lubricating oil system.
We also strongly recommend that CTs
equipped with antifriction bearings should
be instrumented with metal chip detection,
an online metallic debris monitoring
system. Industrial CTs, which normally
use hydrodynamic bearings and mineral
oil-based lubricating oil, typically have a
single integrated oil system for the entire
driveline.
Clean Fuel. Designing a combustor
is a complex task, often likened to lighting
a match in a hurricane. For most CTs,
there are two distinct configurations: the
can-annular (a number of combustor cans
arranged around the circumference of the
CT) and the annular design, which includes
the single-can option.
The design of the fuel supply system
is critical and requires special attention.
Always include a fuel strainer (a Y-type
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alarm is required in any robust fuel system
design. Also consider including a fuel gas
superheater designed to deliver 40C fuel
gas to prevent condensate mist carryover
or hydrate formation. If fuel gas compression
is required, screw compressors always
seem to be the optimal selection.
The fuel control system should also include
a shutoff valve (separate from the
fuel control valve) that stops all fuel flow
to the turbine on any shutdown condition
(local and remote tripping) and that cannot
open until all permissive firing conditions
are satisfied. Fuel shutoff valves should
have a remote shutdown actuator and a
partial stroke feature to permit field checking
of the operability of the shutoff valve
during normal operation of the CT.
Driveline Design
The rule of thumb for power generation
packages is that the generator shaft diameter
should be equal to or greater than the
CT shaft diameter. For mechanical drive,
shafts should have approximately the same
diameter. When torsional vibration problems
appear, the primary cause is the lack
of a comprehensive torsional analysis, improper
coupling selection (mainly flexible
couplings), and lack of proper operation
and maintenance.
Most original equipment manufacturer
(OEM) designs make certain that the blade
natural frequencies do not coincide with
any source of excitation from 10% below
minimum governed speed to 10% above
maximum continuous speed. For the enPOWER
| June 2011
It's often said that the key to achieving
maximum CT life is keeping the combustion
air, fuel, and lubricating oil clean.
7.5 m. The air inlet (elevated a minimum
of 5 m) and CT exhaust should also be
located outside a three-dimensional fire
hazardous zone and outside any classified
electrical area.
Clean Oil. A good gauge of the oil cooling
system design quality is to confirm that
the inlet oil temperature and oil temperature
rise through the bearing are less than
50C and 33C, respectively. Another good
strainer with stainless steel internals) and
a blowdown system (manual valve) for
purging and warming up the fuel system
for approximately 20 minutes prior to
starting. A manual valve closed about 2
minutes after starting should be included
as well as a safety shutdown valve. The
plot limit valve (fail safe) for trip on gas
knockout drum high liquid level (manual
and automatic) preceded by a high level
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POWER June 2011

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