FIELD OF INVENTION
The invention relates to fuel oil pressure control system of a gas turbine and
automatic change over of fuel injection from main fuel oil to stand by fuel oil and
vice versa during on load condition.
BACKGROUND OF THE INVENTION
Gas turbines play a major role as prime movers for many applications including
power generation.
Fuels play a critical role in power production. The cost and availability of fuel are
major planning considerations. Consequently the ability of any prime mover to
burn a wide range of fuels - or fuel flexibility continues to be of primary
importance.
A Gas Turbine has the capability to operate on more than one fuel. Accordingly,
many Gas Turbines are designed to operate on various gaseous as well as liquid
fuels to meet the objective of fuel flexibility.
In locations, where gas fuel is not readily available, the users opt for operating
the Gas Turbines on liquid fuels.
As shown in Figure 1, both the main as well as standby fuel oils are fed
according to prior art by separate fuel forwarding systems up to the three-way
valve (12) at the inlet of Gas Turbine. The three-way valve will admit either of
the fuel oils into the Gas Turbine at a time, based on the fuel oil selected in the
Gas Turbine control system.

Typical Fuel forwarding system of each of the fuel oil, basically consists of fuel oil
storage tank (1), Duplex strainers (2) fuel oil forwarding pumps (3), fuel oil
pressure control valve (4), fuel oil fine filters (5) fuel oil flow meters (6), and fuel
oil re-circulation lines (10) as indicated in the flow diagram.
storage tank ensures availability of the fuel oil for a given period of
time.
the fuel oil forwarding pumps will pump the fuel oil into the Gas
Turbine at the required pressure.
the fuel oil filtration system ensures the required cleanliness of oil by
the Gas Turbine.
the fuel oil flow meter measures the fuel oil being consumed by Gas
Turbine.
the fuel oil pressure control valve maintains the fuel oil pressure at the
inlet of Gas Turbine, within the required limits, at all conditions taking
into consideration of various factors such as :
(i) variations in pressure drops in the components of fuel oil forwarding
system at various fuel oil flows, based on the operating load of the Gas
Turbine.
(ii) density variations of fuel oil,
(iii) variations in oil level in the storage tank,

(iv) clogging conditions of fuel oil filters etc.
The fuel oil re-circulation line re-circulates a minimum amount of fuel oil back
to the storage tank to enable the pressure control valve to maintain the
required pressure, especially while the fuel stream is in standby mode.
The controls directly pertaining to the Gas Turbine proper are implemented
through the dedicated control system of the Gas Turbine [Gas Turbine
Control System] (15). The controls of the balance of the power plant
including controls of the fuel oil forwarding systems for the Gas Turbines are
normally operated through DCS system [Distributed Control System] (8).
The Gas Turbine requires liquid fuel at a specified minimum pressure at inlet
flange of the turbine. Fuel oil pressure, lower than the minimum specified at
the inlet of Gas Turbine, can lead to the damage of the Gas Turbine fuel oil
system components.
Gas Turbine control system, will trip the Gas Turbine, in case the fuel oil
supply pressure falls below the minimum specified, sensed by pressure switch
low-low, PSLL (16) at the inlet of the Gas Turbine.
Pressure Transmitter (7) is provided downstream of the pressure control
valve (4) of each of the fuel oil forwarding system. Based on the feed back
from the pressure signal from the respective pressure transmitters, the
pressure control valves maintain the required pressure at the inlet of Gas
Turbine, as per the control valve opening signal received from controller in
the DCS. Conventionally PID controllers are used as the pressure control
valve controllers in the DCS system (8).

Following are the sequence of events observed, during auto change over of
Gas Turbine on load from "main fuel oil" to standby fuel oil with conventional
PID control system.
1. Gas Turbine is operating on main fuel oil.
2. Pumping system of "standby fuel oil" is in running condition.
3. Pressure control valve in the "main fuel oil" forwarding system is in the
required high open condition based on the command from the PID
controller.
4. Pressure control valve in the standby fuel oil forwarding system is in the
partially open condition.
5. Malfunction of the "main fuel oil forwarding system is detected by low
pressure switch, PSL in the "main fuel oil" forwarding system.
6. Gas Turbine control system gives command to 3-way valve to divert its
position from "main fuel oil" to standby fuel oil, to enable auto change
over of Gas Turbine from "main fuel oil mode" to "standby fuel oil mode".
7. Pressure control valve in the standby fuel oil forwarding system is not able
to open quickly to the required level.
8. Gas Turbine is tripped on fuel oil pressure low-low at inlet of Gas Turbine.

Therefore
(i) The response time of the pressure control valves have no limitations
for quick opening application.
(ii) The PID controllers are capable of maintaining the required fuel oil
pressure during normal operation of the Gas Turbine. However, PID
controllers alone are not capable for enabling auto change over of Gas
Turbine from one fuel oil to other fuel oil.
Hence, there is a need for a fuel oil pressure control system which also
ensures auto change over of Gas Turbine from "main fuel oil" to
"standby fuel oil" and vice versa.
Most of the process plants depend on the power generated by the captive Gas
Turbine Power Plants. Any interruption in the power generation upsets the
operation of the process plant, leading to heavy economic losses. Hence, there is
a need for a standby fuel oil injection system which is enabled to operate on
multi-fuel basis corresponding to the fuel used by the gas turbine, and further
enabled to automatically switch over from main-fuel oil operating mode to stand
by oil operating mode, and vice-versa.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an automated device to
simultaneously control fuel oil pressure of the main fuel injection system and a
standby fuel injection system in a gas turbine plant on load condition.

Another object of the invention is to propose an automated device to
simultaneously control fuel oil pressure of the main fuel injection system and a
standby fuel injection system in a gas turbine plant on load condition, which
allows an automatic change over of the gas turbine plant on load condition from
main fuel oil to standby fuel oil supply system and vice versa.
A still another object of the invention is to propose an automated device to
simultaneously control fuel oil pressure of the main fuel injection system and a
standby fuel injection system in a gas turbine plant on load condition, which
enables an uninterrupted operation of a gas turbine plant.
A further object of the invention is to propose an automated device to
simultaneously control fuel oil pressure of the main fuel injection system and a
standby fuel injection system in a gas turbine plant on load condition, which
provides fuel flexibility to a gas turbine to operate under two different fuel oils.
SUMMARY OF THE INVENTION
Accordingly, there is provided In a dual-fuel gas turbine plant having at least
one dedicated control system, and a fuel supply system comprising one each fuel
forwarding system for main fuel oil and stand by fuel oil enabled to supply fuel-
oil to a three-way valve disposed at an inlet of the gas turbine provided with a
low pressure switch (PSLL), the three-way valve is configured to allow ingress of
one of the dual fuel at one point of time based on selection by the dedicated
control system, the fuel forwarding systems, each having a pressure control
valve with respective transmitter controllable by a distributed control system
(DCS), an improved fuel oil pressure control system for auto change over of the
gas turbine between main fuel oil to standby fuel oil mode, the improvement is

characterized in that the system is enabled to incorporated with a control logic
and receive load reference signals of the gas turbine continuously in the
distributed control system from the dedicated control system; acquire via the low
pressure switch signals relating to a pressure below a threshold value at the inlet
of the gas turbine operating at any of said dual-fuel mode transmit a
corresponding signal to the 3-way valve to change position from the existing to
the second fuel-mode, a simultaneous signal being sent by the dedicated control
system to the distributed control system for change-over; automatically attain a
percentage of opening of the pressure control valve of the respective forwarding
system, corresponding to the DCS signal, based on load reference signal; and
transfer pressure control after successful change over to a PID-Controller in-built
in the distributed control system, the percentage of opening of the pressure
control valve during auto changeover being determined by the control logic and
implemented within fraction of seconds.
In order to ensure uninterrupted operation of Gas Turbine, due to any problem
in the operating fuel oil supply system, the invented device makes possible for
the Gas Turbine to change over to the standby fuel oil system, without
interruption in the power generation.
According to the invention, a control logic has been developed and programmed
in the DCS system for the Fuel oil pressure control system for auto change over
from "main fuel oil" to "standby fuel oil" and vice versa of Gas Turbine on load.
According to the invention, whereinever the Gas Turbine is operating on "main
fuel oil", the "standby fuel oil" pumping system is normally kept in operation.
Similarly, whenever the Gas Turbine is operating on "standby fuel oil", the "main
fuel oil" pumping system is normally kept in operation. Thus facilitates automatic

change over of Gas Turbine to the other fuel, in case of problem in the operating
fuel system, thus ensuring uninterrupted operation of the Gas Turbine plant. The
problem in the operating fuel oil system is sensed by the pressure switch low,
PSL mounted in the respective fuel oil forwarding system.
When a Gas Turbine is operating at load on "main fuel oil", under these
conditions, the pressure control valve in the "main fuel oil" forwarding system
will be at corresponding higher opening condition.
At the same time, pressure control valve in the "standby fuel oil" forwarding
system will be in the partially open condition, as there is no flow of this fuel oil
into the Gas Turbine, any only recirculation flow is taking place.
Any malfunction in the "main fuel oil" forwarding system as detected by a Low
Pressure Switch, PSL in the "main fuel oil" forwarding system, is sensed by the
Gas Turbine control system. Gas Turbine control system gives command to 3-
way valve (12) to divert its position from "main fuel oil" to "standby fuel oil", to
enable auto change over of Gas Turbine from "main fuel oil mode" to "standby
fuel oil mode".
At this instance, the opening of the pressure control valve in the "standby fuel
oil" system, shall increase to the required higher opening condition automatically
from the partially open condition, quickly. Otherwise, the pressure of the fuel oil
at the inlet of Gas Turbine will fall drastically leading to the trip of the Gas
Turbine.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 Illustrates : Flow diagram of fuel oil supply system of a typical Gas
Turbines Plant, operating on "main fuel oil" as well as "standby fuel oil" [with
Conventional PID Pressure Control System]
Figure 2 - Shows a schematic view of a fuel oil supply system of a dual fuel gas
turbine plant with an automated device for simultaneously controlling fuel oil
pressure in the fuel oil supply system enabling auto change-over from main-fuel
operating mode to stand by fuel operating mode and vice-versa in accordance
with the present invention.
DETAIL DESCRIPTION OF THE INVENTION
In order to ensure auto change over of Gas Turbine, from "main fuel oil" to
"standby fuel oil" and vice versa, without tripping the Gas Turbine, the present
invention provides an automated device for simultaneously controlling fuel oil
pressure in the fuel oil supply system of the gas turbine to allow auto-
changeover of fuel-operating mode of the gas turbine.
As shown in Figure 2, during auto change over of a Gas Turbine from "main fuel
oil" to "standby fuel oil" the operating parameters remain as under:
1. Gas Turbine is operating on "main fuel oil" (1)
2. Pumping system of "standby fuel oil" is in running condition (1)

3. Pressure control valve (4) in the "main fuel oil" forwarding system is in
a high open condition based on the command from the PID controller
(8).
4. Pressure control valve (4) in the "standby fuel oil" forwarding system is
in a partially open condition.
5. "Gas Turbine load reference signal" is continuously fed to the DCS
system (8-8) from the Gas Turbine control system (15).
6. Malfunctioning in the "main fuel oil" forwarding system if detected by
the low pressure switch PSL (11), is sensed by the gas Turbine control
system (15). The Gas Turbine control system (15) transmits a signal to
3-way valve (12) to divert its position from "main fuel oil" to "standby
fuel oil", to enable auto change over of the gas turbine from "main fuel
oil mode" to "standby fuel oil mode", with a parallel "main fuel oil to
standby fuel oil change over signal to the DCS (8-8)".
7. The load reference signal from the Gas Turbine is continuously
available to the DCS "scaling" means (18). The load reference signal
indicates the present load on the Gas Turbine at that moment. The
'scaling' function contains a Nook-up' table having data for converting
load reference signal into the required pressure control valve opening,
corresponding to present load on the Gas Turbine at that moment.
On receiving "Main fuel oil" to "standby fuel oil" change over signal from the Gas
Turbine control system (15), the "SR" flip flop (19) changes the position of a
selector switch from position "AC" to position "BC" bypassing the output from the
"PID" controller (8). In position "BC" the output of the scaling function (18) is fed
to the pressure control valve (4) for a duration of 't' seconds. During the time

period of Y seconds, the pressure control valve (4) will be ramped up to the
corresponding percentage of opening, thus enabling a successful change over of
the Gas Turbine.
After 't' seconds the position of the selector switch is changed from "BC" to "AC"
by the "SR" flip flop (19), transferring the control of the pressure control valve
(4) back to the PID controller within the DCS (8-8).
Duration of 't' seconds is set in an "On delay (20) timer" to reset the 'SR' flip flop
(19).

WE CLAIM :
1. An improved fuel oil pressure control system for auto change over of a
dual-fuel gas turbine between main fuel oil to standby fuel oil mode,
the dual-fuel gas turbine plant having at least one dedicated control
system, and a fuel supply system comprising one each main and
stand-by fuel forwarding system to supply one of the dual-fuel to a
three-way valve disposed at an inlet of the gas turbine, the valve is
provided with a low pressure switch (PSLL), and is configured to allow
ingress of only one of the dual fuel at one point of time based on a
selection by the dedicated control system, the fuel forwarding systems,
each having a pressure control valve with respective transmitter
controllable by a distributed control system (DCS),
the system is incorporated with a control logic and enabled to :
receive load reference signals from the gas turbine continuously in the
distributed control system from the dedicated control system;
acquire via the low pressure switch signals relating to a pressure below
a threshold value at the inlet of the gas turbine operating at any of
said dual-fuel mode.
transmit a corresponding signal to the 3-way valve to change position
from the existing fuel mode to the other fuel-mode, a simultaneous
signal being sent by the dedicated control system to the distributed
control system for change-over;

automatically attain a percentage of opening of the pressure control
valve of the respective forwarding system, corresponding to the DCS
signal, based on load reference signal; and
transfer the pressure control function after successful change over to a
PID-Controller in-built in the distributed control system, the percentage
of opening of the pressure control valve during auto changeover being
determined by the control logic and implemented within fraction of
seconds.
2. An improved fuel oil pressure control system for auto change over of a
dual-fuel gas turbine between main fuel oil to standby fuel oil mode, as
substantially described and illustrated herein with reference to the
accompanying drawings.

ABSTRACT

TITLE "AN IMPROVED FUEL-OIL PRESSURE CONTROL SYSTEM
FOR AUTO CHANGEOVER OF A GAS TURBINE BETWEEN
MAIN FUEL TO STANDBY FUEL OIL MODE"
The invention relates to an improved fuel oil pressure control system for auto
change over of a dual-fuel gas turbine between main fuel oil to standby fuel oil
mode, the dual-fuel gas turbine plant having at least one dedicated control
system, and a fuel supply system comprising one each main and stand-by fuel
forwarding system to supply one of the dual-fuel to a three-way valve disposed
at an inlet of the gas turbine, the valve is provided with a low pressure switch
(PSLL), and is configured to allow ingress of only one of the dual fuel at one
point of time based on a selection by the dedicated control system, the fuel
forwarding systems, each having a pressure control valve with respective
transmitter controllable by a distributed control system (DCS), the system is
incorporated with a control logic and enabled to : receive load reference signals
from the gas turbine continuously in the distributed control system from the
dedicated control system; acquire via the low pressure switch signals relating to
a pressure below a threshold value at the inlet of the gas turbine operating at
any of said dual-fuel mode; transmit a corresponding signal to the 3-way valve to
change position from the existing fuel mode to the other fuel-mode, a
simultaneous signal being sent by the dedicated control system to the distributed
control system for change-over; automatically attain a percentage of opening of
the pressure control valve of the respective forwarding system, corresponding to
the DCS signal, based on load reference signal; and transfer the pressure control
function after successful change over to a PID-Controller in-built in the
distributed control system, the percentage of opening of the pressure control
valve during auto changeover being determined by the control logic and
implemented within fraction of seconds.