Diffusion System Components Diffusion Burner Supply Line Emergency Stop Valve The fuel oil diffusion burner supply line emergency stop valve MBN14AA051 is used to enable or disenable
Trang 1DRAWING/DOCUMENT STATUS FOR REFERENCE
PROJECT :
CAMAU 1 750MW COMBINED CYCLE POWER PLANT
PETRO VIETNAM CPMB
s
DRAWING TITLE :
System Description Fuel Oil
Ursprung/Original Ursprung-Nr./Original-No Urspr.-PKZ-Nr Orig.-PC
Datum
Date
Name Maßstab
Scale N/A A4 UA/DCC Type XS00
gezeich
Drawn 06-04-10 STEENM Benennung/Title
Inhaltskennzeichen Contents Code bearb
Coord 06-04-10 LIEDTKE
geprüft
Abtlg
Dept P415 sgd
System Description Fuel Oil
Zähl.-Nr
Reg.-No 355056
Dienstst./Dept UNID Index/Rev Version
Blatt-Nr./Page-No
CM1-L1-M-MBN.08-355056
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Refer also to:
List of Measuring Instruments 3.1-0220
Settings, limits and measuring ranges of the items of
equipment listed here are given in the List of Measuring
Instruments, the List of Electrical Loads, the Equipment
List and the List of Control Settings (SREL) This
description only gives guideline values
Overview
The fuel oil system supplies the burners with fuel oil
and controls the volume of fuel injected into the
combustion chamber On shutdown, it ensures that the
flow of fuel oil is quickly and reliably shut off
Supply of fuel oil to the system must meet various
requirements, i.e the fuel oil must have a certain
pressure, temperature, and mass flow rate and be of a
precisely defined quality The requisite booster pump and
treatment system as well as the fuel oil tank are
described elsewhere
The fuel oil system branches downstream of the
injection pump to form two subsystems, one for each of
the two burner groups The diffusion burners and the
premix burners The diffusion system comprises a supply
line and return line since, for technical reasons, only a
portion of the fuel oil supplied to the diffusion burners is
actually injected into the combustion chambers The
premix system only has a supply line, i.e all of the fuel oil
entering the premix burners is also injected into the
combustion chambers
Startup and shutdown of the GT on fuel oil is always
performed in diffusion mode The premix system is
inactive during operation in diffusion mode Diffusion
mode is possible over the entire output range of the gas
turbine
In the upper output range, pollutant emissions and
thermal stresses on materials are significantly lower in
premix mode than in diffusion mode At low output in
premix mode, there is a marked increase in CO
emissions (unfavorable fuel-air ratio) and the flames
become unstable Premix mode is therefore only
permissible in the upper output range (above approx 50% of rated power) In premix mode, the diffusion burners are used to produce small pilot flames that are required to stabilize the premix flames This means that both subsystems are activated in premix mode
A large portion of this equipment, including the fuel oil filters, injection pump, as well as the emergency stop and control valves, are compactly arranged on the fuel oil package
The fuel oil system is designed as leak-tight as possible Whenever possible and appropriate, the main components of the fuel oil system are described in the following in the order in which the fuel oil passes through them
Components in the Supply Line
Venting
A venting line is connected at the highest point of the fuel oil line upstream of the duplex filters (MBN11AT001 and MBN11AT002) Any air which may have collected during standstill to form large bubbles is fed into the auxiliary return line via orifice MBN11BP001 An additional venting device is located in the upper portion of the filter Any air present as small bubbles in the flowing fuel oil, which may rise in the filters (through which the fuel oil slowly flows), is discharged by way of this vent and likewise fed into the auxiliary return line via orifice MBN11BP002 Shutoff valves MBN11AA501 and MBN11AA502 in the filter vent lines are always open during operation of the GT Sight glass MBN11CF501 is installed in the vent line downstream of the two orifices
Filter
The fuel oil filters (MBN11AT001 and MBN11AT002) remove all matter from the fuel oil that could damage downstream components such as the injection pump, fuel oil nozzles, and turbine blades Since these filters have a very fine mesh, it is necessary to pass the as-delivered fuel oil through a prefilter (which may comprise several stages) Doing so avoids the need for frequent changeover of the fuel oil filter for cleaning
This filter is a duplex filter If the pressure drop due to fouling exceeds a certain level, an alarm is annunciated
on local display MBN11CP501 of differential pressure switch MBN11CP001; changeover to the other filter element must then be made manually It is absolutely imperative to ensure that this filter has already been filled with fuel oil using valve MBN11AA252 After draining a sufficient volume of fuel oil via the respective valve, MBN11AA401 or MBN11AA402, the fouled filter element
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can be removed from the filter housing and cleaned
Then the filter housing itself must be cleaned prior to
reinsertion of the clean filter element
Bladder Accumulator for Damping Pressure
Surges
Opening the emergency stop valves, starting up and
shutting down the injection pump, but in particular, rapid
closing of the emergency stop valves, induces sudden
flow velocity changes in the fuel oil lines that can cause
pressure surges of considerable amplitude Bladder
accumulators MBN11BB001 and MBN11BB002 dampen
these pressure peaks
Injection Pump
Injection pump MBN12AP001 boosts fuel oil pressure
to the level required for atomization in the burners This
pump is a centrifugal pump
If at least two of the pressure measuring transducers
MBN12CP101, MBN12CP102, MBN12CP103 signal that
pressure upstream of the pump is too low (for example
<1bar), pump startup is prevented If an additional limit
(for example <0.1bar) is violated during plant operation,
the pump is automatically shut down, causing the fuel oil
system to trip due to declining fuel oil pressure upstream
of the burners This protective shutdown is necessary due
to the risk of pump cavitation associated with insufficient
intake pressure
The pressure upstream of the pump can be read off
pressure gauge MBN12CP501; pressure gauge
MBN12CP502 indicates the pressure downstream of the
pump
Bearing temperatures are monitored at all four
bearings of the pump and electric motor using
dual-element temperature instruments (MBN12CT105 to
MBN12CT108) If both elements of any one of these
bearing temperature measuring points register excessive
temperature, the pump is shut down If one element of a
given measuring point is faulted and the other signals
excessive temperature, this also triggers pump shutdown
A pre-trip alarm is annunciated if both elements of a pair
are faulted
Seal Medium System for the Fuel Oil Injection
Pumps
The shaft of the fuel oil injection pump is equipped on
both the intake and discharge sides with a double
floating-ring shaft seal The narrow space between the
floating-ring seals is pressurized with seal medium The
seal medium pressure is greater than the fuel oil pressure
within the pump near the floating-ring seal, preventing the
escape of fuel oil from the injection pump via the shaft glands A mixture of water and glycol is used as the seal medium If the floating-ring seal is damaged, seal medium either escapes from or penetrates into the pump, from where it is fed to the burners Fuel oil cannot escape
to the surroundings, however
Each floating-ring seal is connected with a heat exchanger (MBN12AH001 and MBN12AH002) A helical rib on the pump shaft circulates seal medium within the respective floating-ring seal The seal medium temperature increases as it passes through the floating-ring seal This thermal energy is transferred to another loop of cooling water by the heat exchangers To ensure that cooling water does not ingress into the seal medium, the system pressure of the coolant loop must be lower than that of the seal medium
Temperature switches MBN12CT111 and MBN12CT112 upstream of the heat exchangers monitor the temperature of seal medium returning to the respective heat exchanger Supply and adjustment of cooling water shall be provided in the feedline or returnline of the coolant loop outside of the fuel oil package
Flowmeters MBN12CF111 and MBN12CF112 monitor the respective seal medium circle for leaks If the seal medium flow exceeds a certain limit, the fuel oil injection pump is shut down and the gas turbine trips
Seal medium is pumped by the seal medium pump from storage tank MBN12BB010 into the bladder accumulator MBN12BB012 which is charged with nitrogen and perform buffer storage functions Seal medium pump is started up when pressure switch MBN12CP014 detects that pressure has dropped below a previously-defined setting When the requisite pressure level has been restored, the pump is shut down Release for pump startup is given by level monitor MBN12CL011
in the storage tank MBN12BB010 Tank level can be checked locally using sight glass MBN12CL511
Pressure control valve MBN12AA161 ensures that pressure does not drop below a certain level (e g
26 bar) If pressure declines, seal medium is allowed to flow out of the bladder accumulator downstream of the pressure control valve The pressure that triggers supply
of medium from the accumulator is 2 bar higher than the pressure that triggers shutdown of the fuel oil injection pump If pressure drops below the setting of two of the three pressure switches (MBN12CP011/012/013), the fuel oil pump is shut down and the gas turbine trips
Temperature Monitoring
Due to the fact that the temperature level in the fuel oil system increases, particularly at low fuel oil flow rates the
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fuel oil temperature is also measured by a resistance
thermometer MBN11CT101 at the fuel oil package inlet If
the temperature in the fuel oil system is high (e g >
40°C), an alarm is annunciated
Measurement of Pump Discharge Pressure
Pump discharge pressure is determined using
pressure transducers MBN13CP101, MBN13CP102, and
MBN13CP103 This measurement is required for various
open- and closed-loop control functions
Minimum-flow Orifice and Shutoff Ball Valve in
the Bypass Line
When the injection pump MBN12AP001 is running
and the emergency stop valves MBN14AA051 and
MBN23AA051 have closed, pneumatically-actuated ball
valve MBN12AA053 in the bypass line is opened by
spring force to direct the flow into the auxiliary return line
Minimum-flow orifice MBN12BP004, which is installed
in the bypass line upstream of ball valve MBN12AA053,
is designed such that about 20% of the volumetric flow of
the injection pump operating point can be routed through
the bypass line when the ball valve is open
After ESV MBN14AA051 has opened, this ball valve
is closed pneumatically by actuator MBX98AS007
Spring force keeps the ball valve open when the gas
turbine is not running, because the supply of compressed
air is shut off
Safety valve
Safety valve MBN13AA181 prevents pressure
downstream of the injection pump from reaching
impermissible levels
Impermissible pressure increases can occur if
excessive, or if shutoff ball valve MBN12AA053 in the
minimum-flow line is spuriously closed when the
emergency stop valves MBN14AA051 and MBN23AA051
are closed The safety valve opens at a pressure of
approx 152bar
Diffusion System Components
Diffusion Burner Supply Line Emergency Stop
Valve
The fuel oil diffusion burner supply line emergency
stop valve MBN14AA051 is used to enable or disenable
the flow of fuel oil to the diffusion burners during startup
and shutdown, and during changeover from fuel oil to
natural gas operation and vice versa It is also closed in
the event of faults requiring immediate turbine shutdown (for example over speed trip)
The emergency stop valve is opened hydraulically (cf description “Hydraulic Oil System”) It is closed very rapidly and reliably by spring force
Diffusion Burner Supply Line Control Valve
The diffusion burner supply line control valve MBN14AA151 has two functions As a control valve, it regulates the amount of fuel oil supplied to the diffusion burners This valve also has an emergency stop function, i.e when trip is triggered, it is closed very rapidly and reliably by spring force, as is the diffusion burner supply line emergency stop valve
The control valve is actuated hydraulically (cf description “Hydraulic Oil System”)
Permanent-element Fuel Oil Filter (Diffusion)
Despite filtering of the fuel oil, isolated instances of contamination may occur, particularly after an extended outage of the fuel oil system To protect the fuel oil burners against impermissible soiling, permanent-element, fine-mesh filter MBN31AT001 is installed upstream of the fuel oil diffusion burner manifold Controlled shutdown of the GT is initiated if the differential pressure at filter MBN31AT001 exceeds the setting of pressure transducer MBN31CP101, for example >1.5bar
Fuel Oil Lines to the Combustion Chamber (Diffusion)
Fuel oil from the diffusion burner supply line is distributed to the 24 burners via a manifold mounted directly on the combustion chamber
The purge water line, which supplies water for purging the manifold and the burners following the shutdown of the diffusion system, is connected at the highest point of the diffusion burner supply line upstream of the manifold (cf description “Purge Water System”) That portion of the diffusion burner supply line is shaped like a siphon This ensures that residual purge water or condensate from the seal air does not flow into the diffusion burner supply line after shutdown of the diffusion system If this occurred, the fuel oil would rise, pass through the manifold, and enter the burners, where it would be coked
by the high material temperatures
In addition to the diffusion burner supply line, the seal air line fitted with pneumatically-actuated seal air ball valve MBN34AA051 is also connected to the manifold The seal air ball valve opens after the diffusion system is shut down and purging is completed During operation on
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natural gas, cooled air from the gas turbine compressor
outlet is fed into the manifold so that air flows through the
fuel oil diffusion burners, preventing hot combustion
gasses from the combustion chamber from egression into
the fuel oil diffusion system To prevent coking due to the
compressor outlet temperature of about 400°C, seal air is
cooled between the compressor outlet and the seal air
ball valve Pressurized seal air also passes through
orifice MBN36BP001 into the return line manifold to help
ensure that check valves MBN36AA201 through
MBN36AA224, which close due to the weight of their
disks, remain closed during natural gas operation This
prevents the circulation of hot gas between the burners
which would normally occur due to slight differences in
pressure in the combustion chamber The diameter of the
orifice is so small that only insignificant cross flows of fuel
oil pass through during operation on fuel oil
To prevent fuel oil from flowing from the manifold into
the seal air line and from there to the compressor outlet
(risk of ignition!) during operation on fuel oil (postulated
defective seals in one of the seal air ball valves), a
leakage oil line is provided to drain the space between
the ball and valve body Solenoid valve MBN34AA002 is
open during fuel oil operation, allowing any leakage oil to
flow through orifice MBN34BP001 into leakage oil tank
MBN60BB001 If the amount of leakage oil is so great
that pressure upstream of this orifice exceeds a defined
limit, the fuel oil system trips when at least two of the
pressure switches MBN34CP001, MBN34CP002,
MBN34CP003 operate
The fuel oil returning from the diffusion burners flows
through the branch lines into the return line manifold In
the case of a fuel oil supply fault to one of the burners
during operation on fuel oil operation (for example swirl
chamber inlet slits clogged), the swing check valves
prevent the high pressure of the fuel oil in the return line
from causing fuel oil to pass from the return line manifold
through the branch line of the defective burner and into
the combustion chamber unswirled, where it could cause
damage
The return line to the fuel oil package has a
connection to the purge water system When changing
over from operation on NG to operation on FO, the return
line from the fuel oil diffusion burners is filled with purge
water prior to opening the return line ESV (see below)
When the fuel oil system is shut down, air pockets which
may form at the highest point of the return line are forced
into the supply line via orifice MBN36BP001 This
minimizes the amount of hot gas that could enter the
return line in the first few seconds after opening the return
line emergency stop valve Impermissible heating of the
burners is prevented in this way
Diffusion Burner Return Line Emergency Stop Valve
Return line emergency stop valve MBN52AA051 performs the same function in the fuel oil return line as the diffusion burner emergency stop valve MBN14AA051
in the supply line This valve is closed rapidly and reliably
by spring force and opened hydraulically (cf description
“Hydraulic Oil System”) If an “OPEN” command is issued
to the diffusion burner supply line ESV, the return line ESV must open within a short time, otherwise the entire flow through the diffusion burner supply line would be injected into the combustion chamber and this is not permissible Fuel oil system trip is triggered if the return line ESV does not open within a short time
Diffusion Burner Return Line Control Valve
The task of the diffusion burner return line control valve MBN53AA151 is to regulate the return line flow This, in turn, determines the volume of fuel injected, which is the difference between the supply line flow and the return line flow The control valve is actuated hydraulically (cf description “Hydraulic Oil System”) It also functions as a leak-tight second shutoff element for the return line Whenever the return line emergency stop valve is closed, the associated control valve also closes
Premix System Components
Premix Burner Emergency Stop Valve
Similar to the diffusion burner supply line emergency stop valve, the task of the premix burner emergency stop valve MBN23AA051 is to enable or disenable the supply
of fuel oil to the premix burners Furthermore, it is closed
in the event of faults requiring immediate shutdown of the gas turbine
The emergency stop valve is opened hydraulically (cf description “Hydraulic Oil System”) It is closed very rapidly and reliably by spring force
Premix Burner Supply Line Control Valve
Like the diffusion burner supply line control valve, the premix burner control valve MBN23AA151 has two functions As a control valve, it regulates the amount of fuel oil supplied to the premix burners In addition, it also has a trip function, i.e spring force is used to very rapidly and reliably close this valve concurrent with the premix burner emergency stop valve when trip is triggered The control valve is actuated hydraulically (cf description “Hydraulic Oil System”)
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Permanent-element Fuel Oil Filter (Premix)
Despite filtering of the fuel oil, isolated instances of
contamination may occur, particularly after an extended
outage of the fuel oil system To protect the fuel oil
burners against impermissible soiling,
permanent-element, fine-mesh filter MBN41AT001 is installed
upstream of the fuel oil premix burner manifold If the
differential pressure at filter MBN41AT001 exceeds the
setting of pressure transducer MBN41CP101 (for
example >1.5bar), changeover is made from fuel oil
premix mode to fuel oil diffusion mode In addition, the
pretrip alarm “Differential pressure at fuel oil premix filter
too high” is annunciated
Fuel Oil Line to the Combustion Chamber
(Premix)
The premix burner supply line with purge water
connection and the seal air line (equipped with
pneumatically-actuated ball valve MBN44AA051) are
identical, in terms of purpose and configuration, to the
fuel oil diffusion burner supply line to the combustion
chamber
A drain line equipped with two drain valves
MBN45AA401 and MBN45AA402 is connected to the
lowest point of the premix burner manifold Their function
is described in greater detail below (cf “Trip during
Operation in Premix Mode”, and “Premix Burner Back
Purge”)
Burner Pressure Monitoring
Proper burner function requires sufficient pressure at
the burner inlet (diffusion, premix burner supply lines) to
atomize the fuel oil Otherwise, unburned fuel oil could
enter the combustion chamber (explosion hazard!) or hot
combustion chamber gases could flow in the reverse
direction into the burners and the fuel oil system
Consequently, the differential pressure between the fuel
oil supply line and the combustion chamber is monitored
For reasons of simplicity, the compressor outlet pressure,
which is only slightly higher, is used instead of the
combustion chamber pressure This parameter is
measured by pressure transducers MBA12CP101 and
MBA12CP102
Pressure transducers MBN14CP101 and
MBN14CP103 in the diffusion burner supply line and
pressure transducers MBN23CP101 and MBN23CP102
in the premix burner supply line are provided for
measuring fuel oil pressure at the burners
Differential pressure is calculated by the I&C system
from these instrument readings If pressure drops below a
certain differential pressure limit (appr 10bar in the
diffusion burner supply line and 2bar in the premix burner supply line, 2-of-3 logic), either fuel oil system trip or premix system partial trip is triggered
Pressure Relief of Diffusion and Premix Systems (Vented Oil Seal)
When the GT is at standstill, there is a danger that a slight amount of leakage oil may enter the combustion chamber via the ESVs due to pressure prevailing in the supply line or to the tank back pressure in the return line
An impermissibly large amount of oil could accumulate in the GT during a very lengthy outage To prevent this, the piping sections between the premix burner supply line valves (MBN23AA051, MBN23AA151), the diffusion burner supply line valves (MBN14AA051, MBN14AA151), and the diffusion burner return line valves (MBN52AA051 and MBN53AA151) is depressurized by opening solenoid valves MBN14AA501, MBN23AA501 and MBN52AA501 This diverts any oil leakages that occur into the leakage oil tank Very small diameter orifices installed in the relief lines (MBN23BP001, MBN14BP001, MBN52BP001) ensure that only an insignificant amount of oil can enter the leakage oil tank if a solenoid valve spuriously opens during fuel oil operation
Leakage Oil Tank
Leakage oil from the shutoff ball valves of the diffusion and premix burner seal air lines, the premix burner manifold drain line, the vented oil seal venting lines and from various drain lines is collected in the leakage oil tank MBN60BB001 This tank is provided with
a venting line that opens to the atmosphere
Leakage oil pump MBN60AP001 forwards oil leakages into the auxiliary return line It is automatically started up and shut down by level monitor MBN60CL001 Level switch MBN60CL002 annunciates “Leakage oil system faulted” if the leakage oil level exceeds a certain limit
Burner Assembly
The gas turbines 24 burner assemblies (MBM12AV001 through MBM12AV024) each comprise several individual burners In addition to the two fuel oil burners (diffusion burners and premix burners), the burner assemblies also include means for operation on natural gas (natural gas diffusion burner, natural gas premix burner, natural gas pilot burner) All burner assemblies are completely identical They are uniformly distributed around the circumference to achieve the most uniform possible temperature field in the annular combustion chamber
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The fuel oil diffusion burners atomize the fuel oil so
that it can burn completely in the combustion chamber
The fuel oil fed through the diffusion burner supply line
enters the swirl chamber of the diffusion burner nozzles
where it is split into a partial flow that is injected into the
combustion chamber and a partial flow that flows to the
fuel oil return line The split is a function of the
backpressure in the fuel oil return line, which in turn is
determined by the lift of diffusion burner control valve
MBN53AA151 With a widely opened control valve, return
line pressure is low; consequently, a large volume of oil
flows into the return line and only a small volume is
injected A nearly closed control valve results in a high
return line pressure Only a small volume of oil is returned
and a correspondingly large volume of fuel oil is injected
The amount of fuel oil entering the swirl chambers of
the burner nozzles varies only slightly with the amount of
fuel injected The injected jet of oil thus has a nearly
constant turbulence, ensuring good atomization over the
entire operating range A correspondingly large amount of
oil is supplied to the burners even during startup
Whereas the diffusion oil is injected directly into the
combustion chamber via a central nozzle in the burner
assembly, the premix oil is atomized using numerous
small nozzles arranged in the primary air duct (diagonal
swirler) of the burner assembly This thoroughly mixes
the air and fuel prior to entry into the combustion
chamber, hence the name premix combustion
Startup, Fuel Oil Operation, Shutdown
The sequence in which items of fuel oil system
equipment are operated or actuated during startup,
operation and shutdown is described below All of these
processes are fully automatic
Startup
The booster pump system is started up with the gas
turbine at standstill (or in turning gear mode)
The diffusion burner supply line and return line control
valves are actuated to their startup lift settings The
startup equipment (generator and startup frequency
converter) begins to accelerate the gas turbine If at least
two of the pressure measuring transducers
MBN12CP101, MBN12CP102, and MBN12CP103
upstream of the injection pump signal sufficient pressure,
the injection pump is started when the gas turbine
reaches a certain speed Since the emergency stop
valves are still closed, the fuel oil delivered by the
injection pump flows through the minimum flow line into
the return line Fuel oil pressure downstream of the pump
increases to its maximum
When the gas turbine has reached a speed of S.TURB.31 (for example 11% of rated speed), ignition gas is fed into the natural gas diffusion burners and ignited Thereafter the diffusion burner supply and return line emergency stop valves are simultaneously opened
so that fuel oil flows through the diffusion burners and is ignited by the ignition flames Once a certain speed is exceeded, the ignition gas flow is shut off; the fuel oil diffusion flames burn stably on their own At a turbine speed of S.TURB.55 (for example 35% of rated speed), the volume of fuel oil injected is increased with a linear time function At turbine speeds above S.TURB.56 (for example 60% of rated speed), the slope of this time function becomes greater and the volume of fuel oil injected increases more rapidly When rated speed is reached, the speed controller assumes control of the diffusion burner control valve actuators The turbine-generator is then synchronized with the electric power grid
Diffusion Mode
After synchronizing, the lift of the diffusion burner control valves and thereby the volume of fuel oil injected are set by the load controller Initially the gas turbine is operated in diffusion mode
The diffusion burner supply line control valve is used
to regulate the amount of fuel oil supplied in accordance with a stored characteristic as a function of the injection set point By so doing, the mass flow in the diffusion burner supply line is kept within a range favorable for the pumps and burners The diffusion burner return line control valve regulates the amount of fuel oil actually injected
The changeover to premix mode can be made once output exceeds a certain level, for example 50% of base load
Filling of the Fuel Oil Premix System
The supply line downstream of the premix burner control valve could be empty after extended outages, major inspections and on initial startup of the fuel oil premix system Automatic filling of the premix burner supply line is required, if the gas turbine has not operated
in fuel oil premix mode for the last 24 hours When filling
is required, before the changeover from diffusion mode to premix mode the premix burner emergency stop valve opens, and the premix burner control valve is set to its minimum lift setting The fuel oil premix burner supply lines fill with fuel oil in the direction of the combustion chamber The gas turbine is monitored during this time
“Fuel oil premix system trip during initial filling” is triggered if a power output increase of 2 % and thus an
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increased input of fuel into the combustion chamber is
detected The gas turbine changes back into fuel oil
diffusion mode (see below), the premix burner supply line
is now completely filled with fuel oil and the gas turbine
can now be changed over to fuel oil premix mode
Changeover from Diffusion Mode to Premix
Mode
Once the conditions for changeover to premix mode
operation have been satisfied, the premix burner seal air
ball valve MBN44AA051 is closed and the premix burner
control valve is actuated to its startup lift setting
The premix burners must first be cooled with water to
prevent the inflowing fuel oil from coking (the premix
burner nozzles are surrounded by hot air supplied by the
compressor at a temperature of approx 400°C) To do
this, the manifold is filled with water via the purge water
line In the process, a small amount of water enters the
combustion chamber via the lower burners As soon as
the manifold is completely filled with water, the supply of
purge water is shut off and the ESV opened The flow of
oil initially forces the water through the premix burner
nozzles, which cools these items After a brief transition
period, all nozzles are supplied with oil and stable premix
flames form
The control valves for fuel supply to the premix and
diffusion burners are then actuated such that although the
total volume of oil injected remains constant, the premix
fraction increases and the diffusion fraction decreases
Changeover is completed when the diffusion burners only
receive the minimum flow of fuel that is necessary for
formation of the pilot flames
Premix Mode
In premix mode, the amount of fuel required for the
desired output is controlled using the premix burner
control valve, whereas the diffusion system supplies the
minimum amount of fuel required for the pilot flames
Changeover from Premix Mode to Diffusion
Mode
To change back to diffusion mode, the diffusion
fraction is increased and the premix fraction decreased,
initially maintaining a constant output A premix system
trip is triggered shortly before the point at which stable
premix flames are no longer possible The supply of
purge water to the premix system is activated shortly
before closing the premix burner ESV MBN23AA051 and
the premix CV MBN23AA151 Pressure at the premix
nozzles (produced by the purge water pump) still exists
after trip This ensures that any oil remaining is displaced
from the burners during the first seconds of the purge
operation, fully atomizes, and does not form an oil film on the burner parts which could cause coking
After the purging operation has been completed, the manifold remains filled with purge water to the height of the outlet of the lowest burner The premix burner manifold drain valves MBN45AA401 and MBN45AA402, are opened briefly during the purging operation This also purges the premix burner manifold drain line with water, ensuring that during subsequent diffusion mode operation, no oil can reenter the premix burners from this line, which would cause clogging of the premix burner nozzles in the lowest burners
Upon completion of purging, the premix burner seal air ball valve opens, i.e cooled air from the compressor outlet is supplied to the premix burners The gas turbine
is now running in diffusion mode
Shutdown
First the output of the GT (which is operating in diffusion mode) is reduced The generator is disconnected from the grid at a very low positive output Fuel oil system trip is then triggered, i.e the diffusion burner emergency stop and control valves in the supply and return lines close and the injection pump is shut down
The diffusion burners and their manifold are then purged with water With the return line ESV, and return line CV, the water forces the remaining fuel oil out of the supply lines into the return line
The gas turbine coasts down without combustion until
it reaches turning speed The fuel oil system is ready for the next startup
Trip During Operation in Premix Mode
If a fault makes it necessary to immediately shut down the GT during operation in premix mode, the premix and diffusion burner emergency stop and control valves are closed simultaneously and the injection pump is shut down During the subsequent purging operation, substantial amounts of oil still in the lines must be prevented from entering the combustion chamber This oil could undergo uncontrolled ignition on the hot walls or enter the exhaust system as unburned fuel oil mist Consequently, first the diffusion system alone is purged
as described under “Shutdown” In doing so, most of the oil is forced into the return line At the same time, premix burner manifold drain valves MBN45AA401 and MBN45AA402 are opened, and the remaining (low) combustion chamber pressure forces the remaining oil from the premix burner manifold and the premix burners into the leakage oil tank The solenoid valves close after
a defined delay
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Once purging of the diffusion system has been
completed, the premix system is purged by opening the
premix purge water solenoid valve for a defined period
Premix Burner Back Purge
The premix burner back purging is to remove coke
residuals from the fuel oil premix burner should they
occur Obstructed premix burner nozzles cause
unbalanced temperature loads upstream of the turbine
blades The premix burner back purge sequence takes
place prior to a switch over from fuel oil diffusion mode to
fuel oil premix mode, therefore the gas turbine has to run
for a certain time in fuel oil diffusion mode, so that the fuel
oil premix burners are sufficiently hot
Potential residues on the hot inner premix burner
surface may first be detached by quenching with water
For this purpose, the premix burners are filled with water
via the premix purge water solenoid valves, as described
above The premix seal air ball valve MBN44AA051
remains closed during the purging process After the
quenching, the purge water valves are closed and the
premix manifold drain valves MBN45AA401 and
MBN45AA402 are opened Compressor discharge air
now forces the detached coke residuals to leave the
premix burners and the premix manifold into the fuel oil
leakage tank MBN60BB001
The process is directly repeated one more time After
this the premix burner system is filled with water for the
third time, the premix emergency stop valve
MBN23AA051 is opened, and the normal transfer to
premix operation begins
Correct operation of the premix burner back purge
sequence is monitored by the temperature measuring
point MBN45CT101A/B (duplex PT100 resistance
temperature detector) A warning message is issued at
high temperatures (> 100°C), excessive temperatures
(> 150°C) leads to shutdown of the gas turbine in 2-of-2
evaluation
The solenoid valve MBN45AA501, which is installed
between the premix manifold drain valves MBN45AA401
and MBN45AA402, remains open when the premix burner back purge sequence is not active Upstream of the orifice MBN45BP001, the pressure is monitored via the pressure switch MBN45CP001 to detect a possible leak of the drain valves
Fuel Changeover
The following paragraph only gives a brief overview A detailed description of the fuel changeover from operation
on natural gas to fuel oil is given in description “Fuel Change Over”
To make the changeover from natural gas operation
to fuel oil operation, the gas turbine must first be operated
in natural gas premix mode Automatic changeover can
be made to fuel oil diffusion mode when the gas turbine is operating in the requisite output window (approx 60% to 80%) Assuming that fuel oil premix mode and a specific output have been selected, automatic changeover to this mode and output can be initiated once changeover to operation on fuel oil in diffusion mode has been completed
To make the changeover from fuel oil operation to natural gas operation, the gas turbine must first be operated in fuel oil diffusion mode Once the gas turbine
is running in fuel oil diffusion mode in the requisite output window (approx 60% to 80%), the machine can be automatically changed over to natural gas premix mode
Miscellaneous
In addition to several optional measuring points, the following local pressure indicators are provided for maintenance and checking purposes: differential pressure across the fuel oil filter (MBN11CP501), pressure upstream of the injection pump (MBN12CP501), pressure downstream of the injection pump (MBN12CP502), and pressure in the diffusion return line (MBN52CP101) Valves are provided at various locations for draining piping and certain components of the fuel oil system These drain valves must remain closed whenever the gas turbine is running