FIELD OF INVENTION
The invention relates to a method of uninterrupted on load change over of fuel
system from gas fuel mode to liquid fuel mode in a gas turbine operating with
fuel gas booster compressor. More particularly, the invention relates to a method
of uninterrupted on load change over of fuel system from gas to liquid by using a
new surge vessel system.
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 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 available at sufficient pressure, fuel gas
booster compressor system is provided to increase the fuel gas pressure as
required by Gas turbine.
In case of failure of running compressor, the fuel gas booster compressor
systems being complex, require considerable time for change over from running
compressor to standby compressor thus leading to interruption of fuel gas supply
to Gas turbine.
Most of the process plants depend on the power generated by the captive Gas
turbine power plants. Any interruption in power generation will upset the
operation of the process plant, leading to heavy commercial losses. Hence, there
is a need for standby fuel to ensure uninterrupted process plant operation.
Hence, many Gas turbine plants operate on "Gas Fuel" as main fuel and Liquid
fuel as standby fuel to ensure uninterrupted operation. Further, in order to have
uninterrupted power from gas turbine in case of failure of fuel gas booster
compressor, there is a need for on load change over of gas turbine from "fuel
gas mode" to "alternate liquid fuel mode".

OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method of uninterrupted
on load change over of fuel system from gas fuel mode to liquid fuel mode in a
Gas turbine which eliminates the disadvantages of prior art.
Another object of the invention is to propose a method of uninterrupted on load
change over of fuel system from gas fuel mode to liquid fuel mode in a Gas
turbine which ensures uninterrupted power generation in case of failure of fuel
gas booster compressor.
A still another object of the invention is to propose a method of uninterrupted on
load change over of fuel system from gas fuel mode to liquid fuel mode in a Gas
turbine without tripping of Gas turbine which is capable of saving huge time of
change over of fuel system from gas to liquid.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.1 - It shows a flow diagram of on base fuel oil system of a Gas turbine
Fig.2 - It shows a flow diagram of existing off-base fuel gas systems for Gas
turbines prior to invention

Fig.3 - It shows a flow diagram of proposed off-base system for Gas turbines as
per the invention
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
A typical on-base fuel oil system is shown in fig.l. The on-base fuel oil system
consists of liquid fuel oil stop valve (01), a main fuel oil pump (02) with
hydraulically controlled bypass valve (03), a high pressure fuel oil filter (04), flow
divider (05) equipped with speed sensors (06) and interconnected piping.
The main fuel oil pump (02) is driven by gas turbine shaft. The bypass valve (03)
adjusts the fuel flow into gas turbine by returning the excessive fuel oil to the
pump suction. The fuel oil, after final filtration in high pressure filter (04), will be
divided into each fuel nozzle assembly, mounted in each combustion chamber,
by the flow divider (05). The flow divider consists of a series of coupled gear
pumps on a common shaft with one gear pump for fuel nozzle. The speed of the
flow divider is proportional to fuel oil flow into the gas turbine, and is therefore
used to measure the actual fuel oil flow. Speed sensors (magnetic pickups) (06)
on the flow divider (05) provide flow indication, which can be used as feedback
signal for gas turbine fuel oil control system.

For the Gas turbines designed to operate on "fuel gas" as well as "liquid fuel",
the gas turbine combustion system will have separate fuel nozzles for gas fuel as
well as liquid fuel. Fuel from the nozzles is mixed with air in the combustion
chamber where combustion takes place.
Transfer sequence for Gas Turbines for change over from "gas fuel
mode" to "liquid fuel mode"
For a typical Gas turbine, when the Gas turbine is operating on "gas fuel mode",
and it is intended to change over to "liquid fuel mode" in a planned operation,
the following sequence of operations take place:
(a) On base liquid fuel stop valves (01) opens, 5 seconds after receiving the
signal for change-over to liquid fuel from gas fuel, by which time off-base
fuel forwarding pump (02) are already started. The electrical clutch for
on-base liquid fuel forwarding pumps engaged equally fast.
(b) The purge timer is started and the fuel oil command to gas turbine is
raised to a level that will allow filling of the system and outgoing fuel gas
command is lowered by an equivalent amount. The shaft-driven fuel
pump (02) starts slowly pumping liquid fuel into the fuel manifold. This
purging (filling of fuel oil lines) takes place in 10 seconds and the purge
timer times out.
(c) After the fuel has reached the fuel nozzles, the incoming liquid fuel is
ramped up to equal the total fuel demand, and the outgoing gas fuel is

ramped down to zero. Since, total energy to the gas turbine is held
reasonably constant, load variations for a properly matched and tuned
system are minimal. This period takes about 30 seconds.
As per event (c) above, while the liquid fuel is ramped up to equal the
fuel demand, the outgoing gas fuel is ramped down to zero. Accordingly,
the effective time period for liquid fuel (or) gas fuel consumption is half of
30 seconds, i.e. 15 seconds (=30/2), during event (c).
Even though, the transfer sequence takes place in a period of 45 seconds
[=(5+10+30)], as explained above, the effective period of fuel gas
consumption during transfer period is 30 seconds [=(5+10+(30/2))].
From the above, fuel gas is consumed during the transfer sequence for
an effective period of 30 seconds [5+10+(30/2)].
Hence, Gas turbines with existing system, trip, in case of failure of "fuel
gas booster compressor", as the fuel gas supply to Gas turbine (A) is
immediately stopped and the Gas turbine requires continuity of "fuel gas"
supply at least for a period of 30 seconds for on load change over to
"liquid fuel" mode.

Hence, in order to have uninterrupted power from Gas turbine (D) in case
of failure of fuel gas booster compressor, there is a need for continuity of
fuel gas supply for a time period for 30 seconds, by the time change over
to liquid fuel takes place.
Accordingly, in the present invention, the uninterrupted on load change over of
fuel system from "gas fuel mode" to "liquid fuel mode" in case of failure of gas
booster compressor supplying pressurized gas fuel to Gas turbine is done by
using a new fuel gas surge vessel system. For this period of 30 seconds the fuel
gas is supplied from fuel gas surge vessel (v).
As shown in fig.2, off-base fuel gas systems for Gas turbine prior to invention
consists of
a) Fuel gas treatment system before booster compressor (A)
b) Fuel gas booster compressor system (B)
c) Fuel gas treatment system after booster compressor (C)
d) Gas turbine (D)
As shown in fig.3, off-base fuel gas system for Gas turbine as per invention
consists of
a) Fuel gas treatment system before booster gas compressor (A)
b) Fuel gas booster compressor system (B)
c) Fuel gas surge vessel (V)

d) Fuel gas treatment system after booster compressor (C)
e) Gas turbine (D)
The capacity of the surge vessel shall be sized in such a way that it can cater to
the maximum effective fuel gas requirement of gas turbine for 30 seconds by
expanding from "normal-fuel gas operating pressure of fuel gas system" to
"minimum allowable fuel gas pressure by Gas turbine".
In case of failure of fuel gas booster compressor, the compressor trip signal shall
be provided as an initiation signal to Gas turbine control system. The Gas turbine
control system immediately initiates the transfer sequence from 'fuel gas' mode
to "liquid fuel mode" and the Gas turbine is changed over from 'fuel gas' mode to
"liquid fuel mode" in a time period of 45 seconds. Even though, fuel gas supply is
stopped from fuel gas booster compressor, the surge vessel provides the
required fuel gas supply to the Gas turbine during the fuel transfer.

WE CLAIM
1. A method of uninterrupted on load change over of fuel system from
gas fuel mode to liquid fuel mode in a gas turbine (D) operating
with fuel gas booster compressor comprising the steps of:
receiving the signal for change over to liquid fuel from gas fuel;
opening of liquid fuel stop valves (01) after 5 seconds of receiving
the signal for change-over;
starting of on-base fuel forwarding pumps (02) with an
engagement of electrical clutch of the said pump;
starting of the purge timer raising the level of fuel oil command to
gas turbine;
filling the system with fuel oil by pumping liquid fuel into the fuel
manifold in 10 seconds;
ramping up the incoming liquid fuel to total fuel demand;
ramping down the outgoing gas fuel to zero;
characterised in that supply of fuel gas is effected for 30 seconds
from fuel gas surge vessel (V) for making an uninterrupted onload
change over.

A method of uninterrupted on load change over of fuel system from gas fuel
mode to liquid fuel mode in a gas turbine consists of receiving the signal for
change over to liquid fuel from gas fuel and opening of liquid fuel stop valves
(01) after 5 seconds of receiving the signal for change over when fuel forwarding
pump (02) starts along with an engagement of electrical clutch of the pump
(02). The purge timer then starts to raise the level of fuel oil command to gas
turbine (D) and the system is filled with fuel oil by pumping liquid fuel into the
fuel manifold in 10 seconds. The incoming liquid fuel is then ramped up to equal
the total fuel demand and the outgoing gas fuel is ramped down to zero. Supply
of fuel gas is delivered during change over period from fuel gas surge vessel (V)
for an uninterrupted on load change over.