1.8 GT Fuel Gas System-9HA Gas Turbine Combine Cycle Power Plant - Operation

 1.0 SYSTEM DESCRIPTION

    1.1 General

• The main functions of the system are:
• To regulate the flow of gas fuel to the gas turbine
• To regulate the fuel fraction between the gas fuel line 
• To measure the gas fuel mass flow rate for performance perspective
• To isolate the turbine from the gas fie] supply in case of a safety event 
• To securely evacuate all the remaining gas fuel in the turbine and the Heat Recovery steam
• Generator to decrease the start-up time by skipping the purging phase

The gas fuel system is an assembly of valves, heat exchangers, piping and instrumentation where:

• A safety isolation valve, located outside of main turbine hall separates the gas turbine from the gas fuel supply in case of a safety event,
• A flow meter measures the mass flow rate of gas fuel to the turbine.
• Four gas control valves, operating in choked condition, regulate the flow of gas fuel and the split
• among the four premix circuits.

Page 2/9 of the P&ID 119T7367 shows the gas fuel flow meter with the Safety Shut-Off valve and the

safety vent valve. Page 3/9 shows the instrumentation at the inlet of the skid. Page 4/8 shows the

instrumentation and vent valve for the cavity upstream of the gas control valves. Pages 5/9, 6/9 and

7/9 show the gas control valves and manifold with the associated instrumentation. Page 8/9 shows the

temperature transmitters connected to the various thermocouples installed on the skid, as well as the

I DVP cabinet containing the digital positioners of gas control valves. Page 9/9 shows the purge credit

shut off air valves with the purge credit coriolis flow meter and the purge credit vent valve.

2.0 SYSTEN DEVICES FUNCTIONS

GE Code: Function

MG2-1: Coriolis mass flow meter measuring the gas fuel mass flow

96FM-1 Gas mass flow transmitter

VGI-1: Safely shuts off the gas fuel supply to the skid in case of safety trip

65VGI-1: Servo valve controlling the VGI-1 valve opening

VGV-1: Safely vents the cavity between the SSOV and the gas skid

VGS-1:Isolation valve cutting off the gas flow for shut down sequence

VGV-3:Vents the cavity between VGS and VGM

20VGM-10:Shuts off the gas fuel for shut down sequence

90VGM-10:Gas monitored valve (VGM-10) positioner

4VGM-10: TRIP relay for VGM-10 closing

VGV-4: Vents the cavity between VGM and gas control valves VGC

20VGC-1: Controls the gas fuel flow in PM1 circuit 

20VGC-2: Controls the gas fuel flow in PM2 circuit

20VGC-3: Controls the gas fuel flow in PM23circuit

20VGC-4: Controls the gas fuel flow in Quaternary circuit

3.0 SYSTEM COMPONENT DESCRIPTION

Before going into the gas fuel heating and conditioning system, the gas flows through the flow meter (MG2-1), used to validate the performance of the power plant, then it reaches the safety shut-off valve(VG1-I). This valve located outside of the main turbine hall is closed during a safety event to shut-off the gas supply to the turbine. In the meantime, the safety vent(VGV-I) opens to vent the gas between the safety shut-off valve and the gas fuel skid to prevent any potential hazard.

The gas stop valve(VGS-I) is the first isolation valve on the skid, installed within the gas turbine compartment. Upstream of it is installed pressure and temperature measurements. The gas modulation valve(VGM-I) is a pressure reduction valve used to decrease the pressure upstream of the gas control valves in order to increase their strokes within the allowable operating range of the hardware. The gas modulation valve controls its downstream pressure during acceleration of the turbine and remains fully opened in the loaded region.

Typically, VGC-I and 2 flow gas from light-off to 30% load while VGC-3 and 4 remain closed. The four gas control valves are opened above 3-% and  the gas fuel mass flow rate increase with load, while the repartition of gas between the lines is adapted based upon the load of the turbine and the control philosophy required by the combustion system.

4.0 SYSTEM OPERATION

4.1 General

The gas turbine software controls and monitors automatically all the devices of this system. 

4.2 Start-up

Initial state before start up is as follow:

· All the gas stop valves except VGI-1 and control valves are in closed position and vent valves in open position (except VGV-1).

· The vent valves are closed and VGS-1 is opened to pressurize the piping up to the upstream° VGM-1

During the startup sequence:

· The VGM-1 opens to bring gas fuel upstream of the gas control valves and controls i downstream pressure as an increasing function of gas turbine speed.

· At a typical 14% speed, the gas control valves (20VGC-1 and 20VGC-2) open and the gas turbine is ignited.

· After ignition, 20VGC-1 is closed while all the flow of gas fuel is transferred on 20VGC-2. flow of gas increases as the gas turbine accelerates.

· At a typical 60% speed, 20VGC-1 opens in addition to 20VGC-2.

· The gas turbine keeps accelerating up to Full Speed No Load and then reaches loaded operation.

4.3 NORMAL OPERATION

During normal operation:

· Starting from Full Speed No Load, the load increases with gas fuel through 20VGC-1 and 20VGC - 2..

· At a typical 20% load, 20VGC-3 and 20VGC-4 open to fuel the last two circuits, so the load can keep increasing.

· At a typical 30% load, the behavior of the gas turbine varies depending on the gas temperature.

· When the load decreases, th_{e gas} turbine follows the same load path in the reverse direction.

4.4 SHUTDOWN

During the shutdown sequence:

· When the gas turbine is at Full Speed No Load, the speed starts decreasing with gas flowing through 20VGC-1 and 20VGC-2. At a typical 60% speed, 20VGC-2 is fully closed so all the gas flows through 20VGC-1.

· The gas fuel flow and the speed keep decreasing until flameout, occurring at a typical 40% speed The gas fuel stop valves are closed and the vent valves are opened after flameout.

4.5  GAS LEAK TESTS

During the gas leak tests sequence when the gas turbine is shut down, the philosophy is to successively open and close the stop valves of the gas fuel line and monitor pressure decay or build-up in the cavities of known volume, allowing quantifying the leakage rate of the valves. For example, with gas pressurized at the beginning of the gas fuel line, the tightness class of VGS-1 is checked by closing VGS-1 and the two valves of the cavity downstream (20VGM-10 and VGV-3). Since the valve are closed, the pressure in the cavity increases due to the leakage through VGS-1. After a determined amount of time, the pressure in the cavity has to be below the maximum allowable limit so the tightness class of the valve is not compromised.

4.6 GAS TURBINE TRIP

In the event of a gas turbine trip:

· The two stop valves VGS-1 and VGM-1 are closed and the cavities of the module filled with gas fuel are vented to the atmosphere.

· If the trip is a safety trip, the VGI-1 is closed as well and the gas fuel from VGI-1 to the gas turbine is vented to the atmosphere.

4.7 LOAD REJECTION

When the circuit breakers open at load rejection:

· The gas turbine is brought back to its Full Speed No Load condition, with gas flowing through 20VGC-1 and 20VGC-2 and 20VGC-3 and 4 being closed.