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Problems Associated with Cold Weather
Operation of Gas Turbines
J. DICKSON
Senior Engineer,
Compression Facilities,
TransCanada PipeLines,
Toronto, Ontario, Canada
The operation of gas turbines during conditions of extreme cold weather can present a
variety of problems. Foremost among these problems is the potential for snow and ice
buildup in the air intake system which can cause restrictions or interruptions in operation
and possible ingestion damage to the axial compressor section of the gas turbine. Other cold
weather operating problems include cold lubricating oil and oil system blockages following
periods of shutdown. Major maintenance on the equipment becomes very difficult, if not
impossible, during conditions of extremely cold weather unless the work can be carried out
in an adequately heated building. This paper expands on these problems and others and
outlines methods which have been developed to deal with them. Its content could be of
benefit to design engineers engaged in preparing specifications for gas turbine equipment to
be used in cold climates.
Contributed by the Cos Turbine Division of The American Society of Mechanical Engineers for
presentation at the Gas Turbine and Fluids Engineering Conference, Nen Orleans. La., March 21-25.1976.
Manuscript received at ASME Headquarters January 9,1976.
Copies ill be available until December I, 1976.
THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS) UNITED ENGINEERING CENTER, 345 EAST 47th STREET, NEW YORK, N.Y. 1C017
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Problems Associated with Cold Weather
Operation of Gas Turbines
.1. DICKSON
INTRODUCTION
IMPLICATIONS OF COLD WEATHER OPERATION
The operation of stationary gas turbines
during cold weather conditions presents a variety
of problems to the user. It is the objective of
this paper to provide the reader some degree of
insight into these problems and to examine the
various methods of solving them or reducing their
severity.
The information contained herein has, for
the most part, been based on the actual operation
of different types of gas turbines by TransCanada PipeLines over a 15-year period. The
winter climate which prevails along this system
can, at times, be as hostile as that found in the
far north with temperatures falling to -45 C accompanied by winds in excess 50 Km/hr producing
a wind chill factor in the neighborhood of -57 C.
In order to successfully operate turbomachinery in this type of environment, it is essential to develop a plant design and operating
philosophy which will meet the rather special
challenges presented by extremely cold weather.
This means that equipment specifications must be
prepared with a view to obtaining equipment which
operate reliably under severe weather conditions.
It also means that the design of buildings and
various secondary plant systems must be carried
out to ensure that the influence of cold weather
on them and on operating personnel is minimized.
Special procedures and instructions for station
attendants must be adopted to cope with the hazards presented by intake icing and its consequences: interruptions in service, restricted
operation, and possible damage to the unit.
Original equipment manufacturers (0.E.M.'s)
are very much aware of the implications of exposing gas turbines to very low ambient temperatures
and are continuously reviewing their own design
concepts to improve machine reliability. They
have generally been cooperative in providing information and advice to users who have sought to
incorporate new ideas into existing or future
equipment to circumvent problems which may have
developed during winter operating conditions.
Generally speaking, the operation of gas
turbines in extremes of cold weather leads to
consideration of the following:
I Direct operating problems
2 Potential maintenance difficulties
3 Machine performance.
The first two of these can be considered to be
problematic while the implications of machine perromance are potentially to the benefit of the
user with higher output power and lower specific
fuel consumption being realized at reduced ambient
temperatures.
Direct Operating problems
Experience has shown that the major operating problem areas involved with cold weather conditions are air intake system icing and cold lubricating oil,
There are a number of other miscellaneous
problems which can develop unless precautions are
taken to avoid them.
Most users would agree that the problem of
air intake icing is the most troublesome so it is
the author's intention to deal with it first and
in some detail.
Air Intake Icing. The Problem -- Ice formation in the air intake system represents a multiple problem to the operator. Firstly, it can
cause interruptions or restrictions in running
while severe inlet blockages are cleared and ice
accumulations are removed from the plenum chamber. If ice buildup is not cleared, it results
in a loss of performance due to the reduction of
inlet flow area and consequent increase in inlet
pressure loss. This results in a reduced mass
flow to the engine, reduced power output, and
thermal efficiency. In extreme cases of flow
blockage, it is possible to experience an axial
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compressor flow stall which would be fully developed and relatively violent.
Secondly, there is the possibility of a
piece of ice which has formed within the air intake system breaking loose due to vibratory or
aerodynamic forces and being ingested by the engine and causing severe compressor damage.
Aircraft derivative engines are more vulnerable to suffering ice ingestion damage than
the heavier industrial gas turbines. Originally
designed with lightweight as a major criterion,
their axial compressors are inherently less robust in terms of airfoil thickness and resistance
to rotor blade deflection on ice impact. The engine manufacturers have recognized this fact and
have expended a great deal of effort in developing
stiffer, higher strength rotor blades for existing and new generation machines.
When a chunk of ice strikes a front row rotor blade, it tends to deflect the blade tip
about its principal neutral axis. If the axial
component of this tip deflection is great enough,
there can be mechanical interference with the
trailing edge of the adjacent inlet guide vane
(IGV). The action of the rotor blade striking
the IGV can result in some metal shedding and
possible secondary damage in later stages of the
compressor. This secondary damage is caused wholly
by the passage of metal fragments into the compressor. The first row of blading reduces ice
lumps to tiny crystals not damaging to later
stages.
There are basically two categories of icing
condition which can lead to ice buildup in the
inlet system. These are known as precipitate
icing and condensate icing. The precipitate icing
condition follows from the existence of free water
particles in the intake airstream at temperatures
below the freezing point. This free water can be
in the form of snow, sleet, freezing rain, suspended ice crystals, and/or supercooled water
droplets in certain fog conditions. Ice can accumulate when these free water particles strike
a cold surface.
Hoarfrost, a special case of precipitate
icing, occurs when quiescent, supersaturated ambient air is disturbed and comes in contact with
a cold surface. The rate of buildup of hoarfrost
can be very startling, and it has been known to
completely block off an inlet debris screen within
a period of 10 min. The conditions for hoarfrost
accumulation are generally associated with a waterfog consisting of super-cooled water droplets at
an ambient temperature range of -19 to 3 C.
Whereas precipitate icing can only take
place under certain meteorological conditions at
ambient temperatures below 0 C, condensate icing
can occur in the engine inlet annulus under conditions Of high humidity and at ambient temperatures
above the freezing point. This results from the
fact that there is a static temperature drop
caused by the acceleration of the airstream at the
compressor inlet which causes a portion of the
water vapor to condense. If the static temperature at the engine inlet is less than 0 C. the
condensed water can supercool and freeze into
ice crystals which adhere to the inlet surfaces.
"Condensate icing (as it is formed on gas turbine
inlet surfaces) does not exist as an atmospheric
condition per se, but is a situation induced by
the engine under a certain atmospheric condition"
[Reference (1), 1 page 81
The static temperature drop experienced at
the inlet throat of the engine is a function of
the ambient total temperature and the Mach number
at the compressor inlet annulus. For a typical
aircraft derivative engine at design operating
conditions and an ambient temperature around the
freezing point, the static temperature depression
experienced has been found to be about 6 C. Per
this same engine, the quantity of water vapor
which will actually condense or solidify by this
process at the compressor inlet surfaces has been
shown to be a maximum at an ambient temperature
of 6 C for a given relative humidity. This quantity decreases sharply with reductions in ambient
temperature. At -12 C, the quantity is only
about 30 percent of what it is at 6 C.
The intake system components of a typical
aircraft derivative installation are shown in Fig. 1. The airstream is drawn almost vertically
upward from the skirt area of the protective
weather hoods and then through a debris screen
and an external set of fixed louvres. It then
passes through the inertial cleaner where airborne contaminants are removed. The filtered
airstream carries on through the silencer baffles
and into the intake plenum which acts as a settling chamber to eliminate air disturbances prior
to entry into the engine bellmouth flare,
It has been found that the major icing
problem areas are as follows:
(a) Hoarfrost formation on the inlet debris
screens causing increased inlet pressure
drop.
(b) Ice accumulation around the area of the
bypass door which can break loose when
the door opens due to (a) above and fall
dangerously in the vicinity of the engine
intake bellmouth.
1 Numbers in parentheses designate References at end of paper.
2
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WEATHER
HOOD
AIRFLOW
• .......
STAATER
ME ON EMCINE
INLET SURFACES
_
Fig. 1 Typical aircraft derivative gas urbine
air intake system showing possible locat ons of
ice formation
(c) Ice buildup on any projection in the airflow path including the lip of the intake
bellmouth itself where it is not flush
with the plenum bulkhead wall.
(d) Icicle formation on the underside of the
silencer splitter baffles.
(e) Ice and snow which has accumulated on the
plenum chamber roof can melt and leak into
the intake system aided by the slight
vacuum which exists in the plenum.
(f) Condensate icing on the engine inlet
surfaces,
Ice Protection Techniques -- There are numerous approaches being used by 0.E.M.Is and users
to eliminate the problems associated with intake
icing. These methods are generally aimed at reducing to a minimum the amount of free water entry
into the air inlet system and to prevent ice formation by various means of that free water or
water vapor which does find its way into the system.
One method of reducing the access of free
water into the inlet is to use weather hoods.
They can be attached to the air inlet structure
and extend in an awning fashion out over the inlet 'louvres. The weather hood causes the intake
airflow to be drawn upward at low velocities of
the order of 10 fps. In so doing, it acts to
prevent the direct access of precipitation to
the filter intake louvres. Dry snow flakes, how-
, Fig. 2 Aircraft derivative engine intake flare
showing mounting location of ice accretion probe
ever, can still be arrested in their downward motion and be swept upward into the inlet louvres,
but experience has shown that they are effectively
separated from the airstream by the inertial
filter elements.
The use of debris screens during winter
operation is something of a controversial subject. Their usefulness in warm weather periods
to prevent the entry of miscellaneous debris to
the filter elements is obvious. However, during
cold weather operation, they do tend to present
an additional surface to the intake airstream
and can become blocked by the formation of ice
and/or hoarfrost. Experience has shown that it
is their propensity to act as a nucleation site
for ice formation that actually makes them a
rather useful component of the air intake system
during cold conditions. This apparent anomaly
is explained when one considers the fact that the
debris screens are normally mounted externally
and can be cleared relatively easily by brushing
with a stiff broom or by striking with a long
pole. The ice particles which are dislodged in
this way will either fall to the ground or be removed by the inertial filter elements. It is
reasonable to assume that if debris screens were
not used, there would be an increased probability
of ice and hoarfrost formations to occur within
the inertial filter cells which are not as accessible and more difficult to clear.
Where the unit is being operated unattended,
a rather different approach must be taken to the
problem of debris screen blockage and, of course,
to all icing problems in general. In this case,
some provision for automatic anti-icing of the
screen must be made. This consideration is dealt
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'
Projections
of
any
kind
into
the
airstream
with in subsequent paragraphs on the methods of
tend to present sites for ice accretion. Ice
air intake heating.
will
form on the upstream side of a protective
The selection of an air intake filter for
screen
located in front of the engine inlet under
the unit must be predicated on its ability to
certain
atmospheric conditions. The screen conadequately separate airborne contaminants from
figuration which has been found to provide adethe airstream on an all seasons basis. For sumquate protection along with an acceptable presmer operation, it may be necessary to use a media
sure drop when ice has formed on it is a 25-mm
type filter to obtain sufficient particulate reMesh of 3-Mm wires (3).
moval. However, experience has shown that the
A bypass door is normally provided downstream
media filter can become seriously blocked with
of
the
air filter and is designed to open autosnow and ice-fog crystals during winter conditions.
matically
if the inlet system should become
"By using an inertial air cleaner ahead of
plugged
and
increase the pressure depression.
the (media type) filter, most of the free water,
The
sealing
surface of the door is nearly always
either solid or liquid, can be removed from the
subject
to
some
air leakage and ice tends to form
airstream. In many cases, this may be sufficient
in
this
area.
It
is, therefore, desirable to loprotection for an air filter. Under some condicate
the
bypass
door
in such a position that ice
tions, the air cleaner may pass too great an
cannot
fall
immediately
in front of engine inlet
amount of ice-fog crystals or very fine snow to
when
the
door
opens.
avoid filter obstruction and either the filter
Aircraft derivative gas turbines are equipped
media will have to be removed or other measures
with
integral
anti-ice systems which protect
taken" (2).
against
ice
buildup
on compressor inlet surfaces.
It is important that the air bleed fans of
Hot
compressor
discharge
air passes through a
the scavenged inertial air cleaner are continusolenoid
valve
into
the
hollow
components in the
ously operational. The efficiency of the air
cleaner drops drastically, and there is a tendency inlet annulus and bleeds into the main airstream.
This raises the temperature of the critical inlet
for it to become plugged when the fans are not
surfaces to a point where ice cannot form on them.
operating.
The on-engine system must always be actiA logical approach to plenum chamber design
vated before any visible signs of ice are present.
in general is necessary. The plenum enclosure
The general practice has been to provide an anticshould be as large as is feasible to reduce airstream approach velocities to the inlet bellmouth. ipatory type of detector probe inside the plenum
In combination with this, a long horizontal run is chamber which monitors both ambient temperature
desirable in order to avoid the possibility of ice and relative humidity. This ice condition detector
falling from the elevated components of the system activates the system when it detects an atmospheric
condition conducive to the formation of condensate
into close proximity of the engine inlet.
icing. Unfortunately, the humidity sensing porThe plenum chamber must be leakproof. This
tion of the most commonly used detector has been
is particularly true for the ceiling area immefound to be unreliable and user companies have
diately in front of and above the intake bellbeen forced to find alternative means of detecting
mouth. In order to avoid the accumulation of
the imminent formation of condensate icing.
free water on the outside of the plenum roof, this
An ice accretion detector is one such alportion of the inlet chamber can be located withternative. The ice accretion detector can be
in the main building enclosure. Where this is
mounted on the inlet bellmouth flare as shown in
done, it is important that the plenum walls be
Fig, 2 so that the sensor head projects into the
insulated against heat transfer from the building
accelerating airstream just ahead of the compressor
to avoid the melting of dry snow which may have
inlet surfaces. This device and its electronic
accumulated adjacent to the walls. If this norcontroller
will produce an icing signal when ice
mally harmless dry snow should refreeze as ice,
accretion
on
the sensing element reaches a predeit can be dangerous from an ingestion standpoint.
termined
thickness
(typically, 0.5 mm or less).
Where the installation is attended, it is
The
probe
has
a
built-in
heating element to de-ice
desirable to provide a facility for adequate
itself after an ice condition has been detected.
visual monitoring. A sufficient number of winThe use of compressor discharge air for
dows and high intensity lamps should be installed
anti-icing represents a parasitic bleed on the
to allow easy viewing of the engine inIet and
axial compressor. It reduces thermal efficiency
other areas of the plenum chamber. Interior
and available horsepower. Furthermore, when it
painting of the walls with a non-peeling high
is applied only to the compressor inlet surfaces,
reflectivity paint has been found effective in
it does nothing to prevent the formation of ice
aiding viewing.
4
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•
on the air intake components upstream of the engine (e.g., the inlet debris screens, etc.).
In recent years, a singular approach has
been taken to solve the problems of both inlet
restriction due to precipitation icing and condensate ice formation on engine inlet surfaces.
This involves the heating of the bulk inlet airflow upstream of the air filters by using one of
the following schemes:
(a) Exhaust gas recycling; direct reinjection
or by passing the exhaust gases through
a heat exchanger
(b) Compressor discharge air mixing
(c) Engine external vent air mixing (where
available).
Anti-icing may be accomplished by using
either a "total" or a "partial" heating approach.
Total heating involves raising the temperature
of the intake air to a specified level above
freezing to prevent the formation of condensate
icing on the engine inlet surfaces. This level
will depend on the static temperature drop experienced at the compressor inlet annulus. Total
heating necessarily precludes the formation of
precipitate icing. Output power will be reduced
in proportion to the actual inlet temperature
versus the ambient temperature.
The partial heating method of anti-icing
involves raising the intake air temperature to a
point where its relative humidity is reduced to
60 percent or less. "Under this condition, ice
crystals will not form from supercooled water
droplets (hoarfrost) and ice crystals already
present (ice-fog) will tend to evaporate. The
maximum inlet temperature rise required is about
11 C and typically 5 C would be sufficient" (2).
The success of the partial heating method depends
very heavily on the ability of the air intake system to remove free moisture. If moisture is not
being effectively removed, it can freeze within
the intake system in the temperatures experienced
with partial heating. There is again a loss in
output power capability commensurate with the increase in ambient air temperature.
The direct recycling of exhaust gas presents
the most convenient form of available heat for
most installations. It contains enough oxygen to
be used in the air supply for combustion. It
does, however, contain about 3 percent moisture,
and, therefore, an understanding of its operating
characteristics is required to employ this technique successfully. Additional heat must always
be added to compensate for the increase in moisture content of the air/exhaust mixture. The direct injection of exhaust gases can also lead to
intake system and compressor fouling. The amount
of fouling is dependent on the fuel being used and
the combustion characteristics of the turbine.
Gaseous fuels result in fewer fouling problems
'
than liquid fuels.
Where there is concern over equipment fouling, it is advisable to pass the hot exhaust gases
through a heat exchanger to keep them separate
from the intake airstream. After passing through
the heat exchanger, the exhaust gases are sent to
atmosphere through a secondary exhaust duct. In
order to force enough hot gas into the heat exchanger, it is necessary to install an adjustable
damper in the main exhaust stack. This damper is
never closed completely, but can be adjusted to
force additional hot gas flow to the heat exchanger when higher inlet temperature rises are
required. The heat exchanger system tends to be
somewhat more complex than the direct reinjection
method, but it does avoid the problems of added
moisture and fouling.
The use of compressor discharge air for
heating is desirable from the standpoint that it
is normally clean and dry. However, the performance penalty associated with its use is high.
For a typical installation at an ambient temperature of -4 C and 100 percent R.H., there is a
loss in available output power of 7.6 percent
and an increase in specific fuel consumption of
3.7 percent in order to achieve a partial heating
effect (i.e., an increase in ambient temperature
of 6 C and an R.H. of about 60 percent).
The direct mixing of exhaust gases into the
intake air to achieve the same reduction in R.N.
presents a slightly better alternative from a
performance standpoint. For the same initial
conditions, there would be a loss in output power
of 6.3 percent and an increase in SFC of 3.1 percent. The large loss in power is attributable
to the fact that the intake air temperature must
be raised by about 10 C in this case to achieve
the necessary reduction in R.H.
As a comparison, if exhaust gases were
passed through a heat exchanger on the same machine to produce this reduction in relative humidity, it would result in a loss of output power
of only about 3.6 percent and an increase in specific fuel consumption of 1.9 percent. This calculation does not account for the slight duct
losses which are introduced by a heat exchanger
in the inlet and a damper in the exhaust duct.
In the event that there is a supply of engine external vent air available, such as exists
on the Rolls-Royce Avon, it is a natural choice
for use as an air intake heating source. Fig. 3
is a sectional view of an intake system which
shows the piping arrangement used to direct the
5
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TO ATMOSPHERE
120
CONSTANT NI
T, BIASED NI
PERCENT OF
DESIGN POWER
AT BUTTERFLY VALVE
AND ACTUATOR
11
VENT AIR FROM
ENGINE
100
\PERCENT OF
RATED SPECIFIC
RMLCONSUNWTION
0
Fig. 3 Typical air intake system showing location
of the external vent air piping and control valve
vent air to a location upstream of the intake
filters.
The hot air mixture originates from two connections on the gas generator known as the low
pressure cooling air manifold and 15th stage seal
vent. This air is used within the engine for
bearing cooling and sealing; turbine casing cooling and axial compressor thrust balancing requirements before being vented overboard. It is waste
heat, free for the using, which normally is continuously vented to atmosphere.
The system delivers approximately 1 kg/sec
of low pressure 260 C air. This air can be directed through a three-way butterfly valve to the
"piccolo" piping surrounding the perimeter of the
intake louvres or alternatively to atmosphere.
The hot air passes through a series of 1.3-cm-dia
holes at 5.1-cm spacings in the "piccolo" pipe to
mix directly.with the inlet airstream.
Actual field testing of the system has demonstrated that an increase in ambient temperature
of 3.6 C and reduction in relative humidity of 30
percent can be achieved. The system has also been
found very effective in keeping the inlet debris
screens free from hoarfrost accumulation. It is
a partial heating concept, designed to keep the
relative humidity of the intake air at a level
where condensate icing is unlikely to form. In
order to achieve this R.H. reduction, there is a
loss of 1.8 percent in available output power and
an increase in specific fuel consumption of 0.8
percent for an ambient temperature of about -4 C.
Lubricating Oil. It is very important that
the O.E.M. is consulted as regards the best lubricating oil to be used in the machine for the
0
"
80
-40 -30 -20 -10
0
W
213 30 40
50
AIR INTAKE TEMPERATURE ( °C2)
Fig. 4 Effect of varying compressor air inlet
temperature on maximum power and specific fuel
consumption for a typical aircraft derivative
base load rated gas turbine
climatic conditions which will prevail.
The problems wit lube oil in cold weather
operation are related to two areas: cold oil
(high viscosity) and oil cooler operation.
The problem of cold oil can be the consequence of a prolonged period of shutdown. With
the gas turbine out of service, the temperature
of the building enclosure and the equipment contained therein will be reduced. The temperature
which will be maintained will be function of the
outside ambient conditions and the design of the
building and heating system. In order to keep
the lubricating oil at a satisfactory temperature
where it will be immediately usable when the unit
is called for, tank immersion heaters must be
employed. In addition to this, provision can be
made to continuously circulate the warm lube oil
through the machine. The oil pumps can be turned
on automatically as a function of decreasing
building temperature. Circulation of the oil has
the effect of maintaining all oil passages and
filters in a warm condition thereby preventing
blockages. A period of pre-lube prior to startup is always necessary to ensure that there is
an adequate supply of correct viscosity oil to
the bearings.
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Makeup oil must be stored inside a heated
building to maintain it in a pourable condition.
In order to reduce the problems of adding and
handling oil during severely cold conditions, it
is advisable to install an adequately sized makeup tank, complete with pump, inside the building
housing the gas turbine.
The design of the lube oil cooler and the
method by which it is operated are important considerations. Because an oil cooler must be sized
to handle the heat-rejection load of summer conditions, it will naturally be somewhat oversized
for cold weather operation. There are a number
of approaches Which have been used by turbine
manufacturers to eliminate the problem of overcooling the oil during cold conditions. The solutions basically involve a thermostatic control of
the amount of airflow and/or oil flow through the
cooler. Automatic or manual inside/outside louvre
control can be used to add further flexibility
and to exclude direct cold wind access to the
cooler during shutdown conditions.
During shutdown conditions, oil trapped in
the cooler can become congealed. This can lead
to complete blockage of the heat exchanger core
and create a problem of oil starvation to the
bearings when the machine is restarted.
There are several ways of avoiding oil
cooler blockage. If the oil cooler has been designed to thermostatically modulate airflow only
(no oil bypass), then warm oil can be continuously
circulated through the core during shutdown. If
the design is predicated on an oil bypass principle, the access of oil to the cooler may be blocked
by a thermostatic mixing valve during a shutdown
condition. In this event, it is imperative that
the oil cooler core be drained manually or automatically after every shutdown.
Miscellaneous Problems. The location of
equipment within the building enclosure must be
arranged so that cold ventilation air from wall
louvres cannot impinge directly on it. In the
case of the gas turbine itself, this can lead to
casing or support distortion. For ancillary
equipment, such as a foam fire protection system,
it can result in freezing of the water and foam
concentrate making the system inoperable.
Regardless of where it may be located
within the building, a foam fire protection system should be equipped with insulated water and
concentrate tanks and immersion heaters. The
piping which runs to the foam generator(s) must
always be flushed and completely drained after
testing the system.
Where instrument air is being used for control systems, it must be effectively dried and
should be carried in insulated and/or heat traced
pipes to prevent freeze off.
Experience has shown that it is advisable
to install buried insulated piping conduit between
individual equipment buildings. This conduit can
be designed to carry the heating supply/return
lines along with fire water and compressed air
piping. The slight heat transfer from the heating system lines serves to keep the water and air
piping operational at all times. The somewhat
higher cost of this type of multi-pipe conduit
over conventional installations is easily offset
by the operating benefits.
Protective devices, such as temperature
and pressure switches and their associated sensing lines, should be located inside a heated
building. Exposure to cold temperature can result in sluggish response or changes in calibration. It may be necessary to provide localized
heating on these and other devices, such as batteries, battery chargers, and control valves to
ensure their correct operation when the unit is
restarted following a shutdown.
Maintenance Considerations
Where the gas turbine is to be operated in
cold climates, a design philosophy geared toward
ease of maintenance must be adopted by the user
company.
The main building must be adequately insulated and supported by avheating system with a
capacity predicated on the worst expected climatic
conditions of wind and ambient temperature. Unit
heaters must be located in positions within the
building enclosure to provide sufficient warmth
for personnel engaged in maintenance on all major
components of the gas turbine and associated
equipment.
There should be adequate space within the
building to enable all maintenance to be done
inside. Personnel efficiency has been found to
be reduced drastically where workers are exposed
to extremes of cold temperature. Overhead cranes
must be adequately sized and designed to lift and
move the heaviest piece of equipment to a convenient location on the operating floor for maintenance purposes.
Ventilation openings in the building must
be capable of being well sealed against cold air
ingress and heat loss during periods of shutdown
for maintenance. It is not advisable to install
windows in the building walls because of the high
heat loss and cold air infiltration associated
with them. Electrical lighting must be relied on
exclusively for interior illumination.
Buildings should be designed with bay doors
and floor loadings capable of supporting the
weight of a truck carrying maintenance components.
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This will ensure that most material handling is
carried out with a minimum exposure to the outside
elements.
Adequate snow removal equipment is necessary
to maintain clear access from major transportation
routes to the location of the gas turbine building.
Performance Considerations
The performance of a gas turbine is directly
effected by the temperature of the air entering
the compressor. As ambient temperature falls,
the density of the air increases, and, for a
constant compressor speed, this results in an
increased pressure ratio, mass flow and, therefore, output power from the gas turbine.
From a typical machine, the output power
will increase by about 0.8 percent for each celsius degree decrease in ambient temperature.
Reference to Fig. 4 illustrates this increasing
power capability for a typical aircraft derivative gas turbine.
There are, however, limits on obtaining
increasing output power. As ambient temperature
is reduced and mass flow and pressure ratio are
increased, there is a point reached where the
axial compressor will not handle any additional
inlet airflow. It is in a choked condition and
becomes susceptible to compressor stall.
This choking condition will never actually
be encountered within the operating envelope defined by the control system being used. Axial
compressor speed will be reduced in accordance
with an ambient temperature bias signal to limit
maximum inlet airflow in advance of the onset of
this type of aerodynamic instability.
There will also be output power limitations
imposed by the mechanical design characteristics
of the driven equipment. In order to restrict
the output power from the gas producer, the control system may limit axial compressor speed or
discharge pressure as a function of air inlet
temperature. This is illustrated in Fig. 4 in
the Ti biased N1 section of the curve.
Thermal efficiency is increased as air inlet temperature falls. The amount of improvement
is a function of the individual axial compressor
design and the maximum power envelope defined by
the control system. For the typical aircraft
derivative gas turbine represented in Fig. 4, a
reduction in specific fuel consumption of about
15 percent is achieved where the machine is operated at -40 C as compared to operation at + 40 C.
REFERENCES
1 Chappell, M. S., and Grabe, W., "Icing
Problems on Stationary Gas Turbine PoWer Plants,"
National Research Council of Canada, April 1974.
2 Patton, R. E., "Gas Turbine Operation in
Extreme Cold Climate," Solar Division International
Harvester Company, Fourth Turbomachinery Symposium, Oct. 1975.
3 Chappell, M. S., and Grabe, W., "Some
Icing Tests on Selected Screen samples," Laboratory Technical Report-Eng-17, National Research
Council of Canada, Dec. 1972.
4 Albone, T., "Icing Problems; Stationary
Engine Operating Experience," TransCanada PipeLines Limited; National Research Council of Canada
Gas Turbine Operations and Maintenance Symposium,
Oct. 19i4.
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