6 gas turbine_fuel systems

Dr. Walid Abdelghaffar

Dr. Walid AbdelghaffarGas Turbine_Fuel Systems

This presentation provides introductory information on gas turbine fuel systems.

Fuel Systems


Gas Turbine Fuel Systems: PurposeThe purpose of a fuel system is to supply an exact amount of clean fuel to the engine under all operating conditions.

The amount of fuel is based on turbine speed and load requirements.

The fuel pressure required for a gas turbine is primarily a function of the compression ratio of the compressor section.

For example, the lower the compression ratio, the lower the fuel pressure requirement; the higher the compression ratio, the higher the fuel pressure requirement.

Fuel Systems


Gas Turbine Fuel Systems: Purpose

Almost any combustible fluid, either gaseous or liquid, can be used for turbine fuel.

Some gas turbines operate on both liquid fuel and fuel gas.

Both fuel gas and liquid fuel must be clean for efficient turbine operations. However, fuel requirement specifications differ among manufacturers.

The following information on fuel gas and liquid fuel requirements is provided to illustrate the differences in specifications.

Fuel Systems


Fuel Gas Requirements

The figure lists typical fuel gas requirements for gas turbine engines. These requirements are:

· lower heating value· supply pressure· gas temperature· fuel quality

Fuel Systems


Liquid Fuel Requirements

The figure lists typical liquid fuel requirements for gas turbine engines. These requirements are:

· fuel temperature· fuel viscosity· pour point· fuel quality

NOTE: Always check the turbine manufacturer's fuel specifications to ensure that the fuel meets the specifications for the gas turbine you are operating.

The main components of a fuel gas system and liquid fuel system are given next.

Fuel Systems


Fuel Gas System: ComponentsThe main components of a typical gas turbine fuel gas system are as follows:

· fuel shutoff valve (SOV)· vent valve· pressure control valve (PCV)· pressure indicator controller (PIC)· pressure safety valve(s) (PSV)· instruments and alarms· filter separators· control system· fuel gas heater (optional depending on gas dew point)

Fuel Systems


Liquid Fuel System: Components

The main components of a typical liquid fuel system are:· manifold· nozzles· pumps· filters· pressure switches· fuel control valves· solenoid-operating valves· control system

Fuel Systems


NOTE: Nozzles are not shown in the figure.

The main difference between the two systems is that the liquid fuel system has a storage tank and fuel pumps, whereas a fuel gas system does not.

Fuel Systems


Fuel Gas Supply System: PurposeThe purpose of the gas turbine fuel gas supply system is to provide the correct amount of clean, dry fuel gas to the engine under all operating conditions.

Each component of the fuel gas supply system is important to its overall operating efficiency.

Fuel Gas Supply System


Components & Operation

The main components of a typical fuel gas supply system are as follows:

· fuel shutoff valve (SOV)· vent valve· pressure control valve (PCV)· pressure indicator controller (PIC)· pressure safety valve(s) (PSV)· filter separator· control system

The purpose and operation of these components are discussed next.



Fuel Shutoff & Vent Valve

The purpose of the fuel shutoff valve is to allow, or prevent, the flow of fuel to the gas turbine fuel system.

The purpose of the vent valve is to release the pressure of trapped gas in the fuel lines.

The position of both valves is controlled by actuators, which are operated by instrument air or control oil.

Contd.



In the figure, a hand switch controls a four-way air valve to send instrument air pressure to either side of the shutoff valve actuator piston.

In some cases actuator piston movement is controlled by hydraulic control oil ported through a servo valve.

The actuator is operated by the hand switch or by the fire detection system by means of an interlock.



Fuel Shutoff & Vent Valve

The hand switch opens or closes the fuel shutoff valve through the control system. The switch may be located at the local control panel, the DCS, or both locations. In some systems, the shutoff valve is electrically operated.

The fire detection system sends a signal through the interlock when an engine fire is detected. The interlock closes the fuel shutoff valve and opens the vent valve.

Contd.



The interlock receives signals from the hand switch and the fire detection system to change the positions of the fuel shutoff valve and the vent valve.

When the fuel shutoff valve is closed, the vent valve must be open. The interlock is usually a part of the control system logic that closes the vent valve before the fuel shutoff valve opens.



PCV & PIC Valve

The figure shows fuel gas flow when the fuel shutoff valve is open. The first device located downstream of the shutoff valve is the pressure control valve (PCV).

In the figure, the pressure control valve regulates fuel gas pressure to the turbine according to instrument air signals from the pressure indicator controller (PIC).

The pressure indicator controller measures the fuel gas pressure, compares this pressure to the setpoint, and modulates the pressure control valve to maintain setpointpressure.



Pressure Safety Valve

In the figure, a pressure safety valve (PSV) is located between the pressure control valve and the pressure indicator controller.

The purpose of the pressure safety valve is to vent excessive fuel gas pressure to the flare should the PCV malfunction.

Two other instruments receive fuel gas pressure from the same instrument line as the pressure indicator controller: the local pressure indicator (PI) and the pressure transmitter (PT). Contd.



The pressure transmitter signals gas pressure information to the DCS pressure indicator (PI), low pressure alarm (PAL), and high pressure alarm (PAH).

The fuel gas pressure can be read from the turbine control system CRT and from the DCS.



Filter SeparatorsThe purpose of the filter separators in the fuel gas supply system is to provide filtration and separation of the fuel gas before it enters the fuel control system.

The fuel gas should be relatively clean and dry by the time it reaches this part of the system.

Operators should monitor the filter separator sight glass (LG) during routine operating checks.

Any liquid accumulation must be drained off. If liquid levels in the filter separator become excessively high, the high level switch (LSH) signals the DCS high liquid level alarm (LAH) and an alarm is initiated.



Filter Separators

alarm is initiated.

Like the lube oil and some hydraulic oil filters, the filter separators are equipped with differential pressure indicators and alarms.

The system shown in the figure contains a high differential pressure indicator switch (PDISH) and a DCS differential pressure alarm (PDAH).

The differential pressure initiates the high differential pressure alarm when the high differential pressure setpoint is reached.

Operators should place the standby filter in operation and service the operating filter before the high differential pressure



Control SystemFuel gas flows from the filter to the gas turbine fuel system which is controlled by the turbine control system.

Several measuring devices are located between the filter separator and the gas turbine:

· pressure transmitter (PT)· flow transmitter (FT) - optional· temperature measuring elements (TE)

Contd.



These devices provide fuel gas supply information to the control system instruments.

This information may be used for pressure indicators, pressure recorders, pressure alarms, flow indicators, totalizers, recorders, temperature indicators, and temperature recorders.



Control System

The information is also analyzed and computed by the control system to schedule fuel flow to the engine as needed for speed and load requirements.



IntroductionThe figure shows a simplified gas turbine package fuel gas system.

It represents the system on the gas turbine being controlled.

Fuel Gas Control System


Fuel Gas Control System: Purpose

The purpose of a fuel gas control system is to provide required fuel flow and pressure to the engine. It uses specially designed components to accomplish this function.

A simplified view of the fuel gas control system is shown in the figure.



Fuel Gas Control System: Components

The main components of the fuel gas control system are:

· strainer· gas supply pressure switch· stop/speed ratio valve assembly (stop/ratio valve and gas control valve)· fuel gas pressure transducer· fuel vent solenoid valve· four linear variable differential transformers (LVDT) position sensors· two electrohydraulic servo valves· three gas pressure gauges· Speedtronic controls

The strainer is discussed first.



Strainer

The purpose of the strainer is to remove foreign particles from the fuel gas before it enters the stop/speed ratio valve assembly.

A blowdown connection on the bottom of the strainer body is used for periodic cleaning of the strainer screen.



Gas Supply Pressure Switch

The gas supply pressure switch is installed in the gas piping upstream of the stop/speed ratio valve.

This switch initiates an alarm when fuel gas pressure decreases below the setpoint.

Downstream of the gas supply pressure switch is the stop/speed ratio valve and gas control valve assembly.



Stop/Speed Ratio & Gas

· gas pressure regulation

Control ValveThe combination stop/speed ratio and gas control valve assembly contains the following independent valves:· stop/speed ratio valve· gas control valve

Both of these valves are actuated by hydraulic pressure through servos which receive the signal from the Speedtronic control system

The stop/speed ratio valve has two functions:

· a stop valve



Stop/Speed Ratio Valve

As a stop valve, the stop/speed ratio valve shuts off fuel gas flow during normal and emergency shutdowns.

The hydraulic dump valve, control oil, and servo valves control the action of the stop/speed ratio valve as a stop valve.

The hydraulic dump valve is located between the electrohydraulic servo valve and the actuating cylinder.

(Contd.)



When control oil pressure is low, a spring moves an internal spool to the "dump" position.

When hydraulic pressure is removed, a closing spring on the stop/speed ratio valve plug closes the valve.

Fuel gas flow to the gas control valve and gas turbine is stopped.



Stop/Speed Ratio Valve

As a pressure regulator, the stop/ speed ratio valve regulates the gas pressure to the gas control valve.

The stop/speed ratio valve is positioned by the actuating cylinder and servo valve according to FSR (Fuel Stroke Reference) signals from the stop/speed ratio valve control.

(Contd.)



The stop/speed ratio valve has two control loops:

· position control of the valve· gas pressure control to the inlet of the gas control valve referenced to the speed of the turbine

The stop/speed ratio valve position control loop functions by comparing an FSR signal with a valve position signal generated by the LVDTs (position sensors).



Pressure TransducerThe pressure control loop senses the

fuel gas pressure requirements.

fuel gas pressure exiting the stop/speed ratio valve.

The pressure transducer changes the gas pressure input signal to a DC voltage output signal. The output signal is relayed to the stop/speed ratio valve Speedtronic control.

The Speedtronic control compares the signals received from the intervalvepressure, position control loop, and turbine speed.

The control system then adjusts the stop/speed ratio valve to maintain



Gas Control Valve

The purpose of the gas control valve is to meter or supply fuel gas to the turbine based on turbine speed and requirements of the load or driven equipment. The valve is activated by an electronic signal from the GCV segment of Speedtronic control system.

The gas control valve position is detected by the LVDTs. The sensors signal the valve's position to the GCV Speedtronic control, which changes the signal to a DC signal.

The DC signal is compared with the FSR input signal.



Gas Control Valve

If the feedback signal is in error (differs) with the FSR, the control system signals the hydraulic servo valve to adjust the gas control valve in a direction to decrease the error.

This adjustment maintains a relationship between valve position and FSR.

The gas control valve then meters the correct fuel flow.

Located between the stop/speed ratio valve and gas control valve is the vent solenoid valve



Vent Solenoid Valve

vent valve is closed and remains closed until the turbine is shut down.

The purpose of the fuel vent solenoid valve is to vent fuel gas from the line between the stop/speed ratio and gas control valve when the gas turbine is shut down and prior to firing the machine during startup.

This venting prevents gas pressure buildup between the valves and fuel gas leakage through the gas control valve.

The solenoid valve vents the gas line when the solenoid is de-energized.

When the master control/protection circuit is energized, the solenoid is energized and the vent valve is closed.

When a turbine start signal is initiated, the



LVDTs (Position Sensors)Linear variable differential transformers (LVDTs) are position sensors attached to the valve stems. A fuel gas control system contains four LVDTs:

· two sense the position of the stop/speed ratio valve

· two sense the position of the gas control valve

( Contd.)



LVDTs sense the position of the SRV and the GCV and transmit signals to the appropriate Speedtronic control system segment.

Here the signals are changed into DC signals, which are compared to the FSR.

This signal is then transmitted to the servo valves, which regulate the flow of oil to the actuator cylinder, repositioning the SRV and the GCV.



Servo Valves

force that precisely positions the piston of the second-stage valve.

The electrohydraulic servo valves are two-stage, four-way flow control devices.

They provide directional and proportional hydraulic flow control in response to a low power DC input signal from the Speedtronic control system.

The servo valves control the direction and rate of movement of the pistons in the stop/speed ratio valve and gas control valve actuating cylinders.

The first-stage valve changes the small electric signal into a hydraulic



Servo Valves

The second-stage valve meters hydraulic pressure to and from the actuating cylinder.

Movement of the single-acting piston actuator is opposed by a spring in the gas control valve.

The last two components in the fuel gas control system are the pressure gauges and Speedtroniccontrol system.



Pressure GaugesThree gas pressure gauges are installed in the fuel gas supply line.

The upstream gauge measures fuel gas pressure entering the stop/speed ratio valve.

The middle gauge measures fuel gas pressure between the stop/speed ratio valve and the gas control valve.

The downstream gauge measures the pressure of the gas that has been metered to the fuel gas manifold and fuel nozzles.



Speedtronic Control SystemThe positions of the stop/speed ratio valve and gas control valve are controlled by an electrical signal from the Speedtronic control system.

The signal causes the electro-hydraulic servo valve to send oil to or release oil from the hydraulic cylinder that actuates the stop/speed ratio valve and gas control valve.

A signal from the position sensors (LVDTs) tells the Speedtroniccontrol system that the valve is in the correct position or that a position change is needed.



Speedtronic Control Components

The Speedtronic control system is a microcomputer control system that provides analog and digital signals to control and protect gas turbine operation.

The primary operating parameters of a gas turbine are start-up, temperature, and speed.

All are controlled by regulating fuel flow to the engine.

Contd.



Turbine operations are sensed and used as feedback signals to the Speedtronic control system.

The Speedtronic control system is also equipped with protective devices as a backup to the main system.



Control Loops

The Speedtronic control system consists of three major control loops:

· start-up and shutdown· speed· temperature

The output of these control loops is connected to a minimum value select logic circuit.

Contd.



The minimum value select logic circuit interfaces the speed, temperature, and start-up control output signals to FSR for fuel control.

Only the control segment (e.g., start-up, speed, or temperature) calling for the lowest voltage output is allowed to pass the gate to the fuel control system as controlling FSR voltage.

FSR control is the command signal for fuel



Start-Up & Shutdown Control Loops Proper speed sensing is an important part of the start-up and shutdown sequence control of the turbine.

The graphic shows the speed sensors used on G.E. gas turbines:· zero-speed detector· minimum-speed relay detector· accelerating relay speed detector· high-speed relay detector Contd.



The following is a basic description of how these sensors function.

If speed is zero, permissive logic allows clutch engagement and the cranking sequence for turbine start-up is initiated.

The zero-speed detector provides a signal when the turbine shaft starts rotating.



Start-Up & Shutdown Control Loops During the shutdown cycle, the zero-speed detector provides a signal to permit the ratchet gear, or turning device, to be placed in service in the cool-down sequence.

A minimum speed detector indicates that the turbine has reached the minimum firing speed before ignition.

The acceleration speed relay indicates that the turbine has reached approximately 40% to 50% speed in the acceleration cycle.

The high-speed sensor indicates that the turbine is at operating speed and that the accelerating sequence is complete.



Temperature Control LoopThe purpose of the temperature control loop is to limit the turbine firing temperature by regulating fuel flow.

The actual firing temperature is most difficult to measure and generally is not measured.

Contd.



Exhaust temperature is measurable and is proportional to the firing temperature.

Thermocouples mounted in the exhaust provide temperature feedback proportional to the firing temperature.

Air is more dense on cool days, causing the firing temperature to increase for a given speed.



Protective System

The increased firing temperature improves turbine efficiency, but the control system must prevent overfiring the machine.

This is accomplished by the control system lowering the temperature control point.

Protection systems are also provided to prevent abnormal conditions that can damage the turbine. Contd.



These control and protective systems are independent systems that back up the primary control systems.

The protective systems will trip the machine when overspeed or over-temperature trip conditions occur.

The over-temperature system protects the gas turbine against possible damage caused by overfiring.

It is a backup system that operates only after failure of the temperature control loops.



Supertonic Control: Basic Function

Under normal operating conditions, the temperature control system limits increases in fuel flow when the firing temperature limit is reached.

Contd.



If the system malfunctions, exhaust temperature can exceed control limits.

If a malfunction occurs, the over-temperature protection system provides an over-temperature alarm before it trips the gas turbine.

Speed control software changes the FSR based on the difference between the actual turbine generator speed/load and the speed/load reference set point.



Speedtronic Control: Basic Function

When the generator breaker is closed, on a power grid, speed is held relatively constant, or synchronous.

Fuel flow in excess of that necessary to maintain full speed/no load will increase power generator output capabilities instead of increasing turbine speed.

Isochronous controls hold turbine speed steady during load changes.

When there is a difference between turbine speed and setpoint, the electronic controls increase or decrease the FSR until there are no error signals.



Liquid Fuel System: Purpose

The purpose of a liquid fuel system is to deliver metered quantities of fuel, at the correct pressure, to the engine.

Gas turbine liquid fuels are liquid hydrocarbons similar to kerosene. Almost any combustible fluid can be used for turbine fuel, although high viscosity fuels present special problems.

The figure shows a typical liquid fuel system. Liquid fuel, or fuel oil, is stored in a storage vessel and routed to the gas turbine.

Liquid Fuel System


Liquid Fuel System ComponentsThe main components of a typical liquid fuel system are as follows:

· fuel storage tank· fuel oil pumps· pressure switches· fuel filters· manifold· nozzles· fuel control valve· solenoid-operated valves· control systemA fuel storage tank is discussed first.

Liquid Fuel System


Fuel Tank Storage & PSVThe purpose of a fuel storage tank is to store fuel oil received from an outside source. Storage tanks usually store enough fuel for 24 hours of gas turbine operation. They are sometimes called day tanks.

Fuel storage tanks are often equipped with the following devices:

· relief valve· pressure controls· pressure indicator· temperature indicator· level indicators· level controls and/or alarms

The relief valve (PSV) is a safety device designed to prevent excessive pressure in the tank.

Liquid Fuel System


Pressure, Temperature, & Level Controls

In the figure, the storage tank is protected against over-pressurization by pressure controls. These controls consist of a pressure transmitter (PT), controller (PIC), actuator (PY), and control valve (PV).

Operators can monitor the fuel oil storage tank pressure at local pressure indicators (PI).

A local temperature indicator (TI) and a level gauge (LG) are also provided.

Another level gauge, usually a sight glass, is installed on the boot of the tank.

Liquid Fuel System


Level Control & Shutoff Valve

A level transmitter (LT) signals fuel level information to the DCS level indicator (LI). The level indicator contains alarms for high level, low level, and low-low level. The low-low level alarm is usually accompanied by a unit shutdown.

In the figure, the storage tank shutoff valve (MOV) is motor operated. The valve is opened or closed by a hand switch (HS).

Liquid Fuel System


Fuel Oil Pumps & Hand SwitchesThe purpose of fuel oil pumps is to deliver fuel oil, under pressure, to the gas turbine fuel control system. Fuel oil pumps are started and stopped by a local HOA hand switch (HS).

HOA means hand/off/automatic.

When the hand switch is in the H position, the fuel oil pump is under manual control. When in the O position, the pump is off. (Contd.)

Liquid Fuel System


When both pump switches are placed in the A position, the running pump becomes the lead pump and the other pump is in auto standby.

If the lead pump fails when operating with both pumps in auto, the standby pump will start automatically with no interruption in fuel supply.

Liquid Fuel System


Fuel Boost Inlet Pump

There are two types of liquid fuel system pumps:

· fuel boost inlet· high pressure fuel

The purpose of a fuel boost inlet pump is to raise the inlet fuel pressure to the pressure that is required for proper fuel system operation.

(Contd.)

Liquid Fuel System


The boost pump is installed upstream from the low pressure duplex fuel filters.

An electric boost pump is a rotary, positive-displacement, gear, motor-driven pump. The pump takes fuel from the low pressure liquid fuel supply and delivers it to the fuel system inlet at the required pressure.

Liquid Fuel System


Hp Fuel Pump, Pressure Switch, & Gauge

The high pressure fuel pump is a gear-type, positive displacement pump and may either be engine-driven or electric motor-driven.

The low fuel pressure switch senses fuel pressure to the high pressure fuel pump. This switch initiates engine shutdown if fuel pressure decreases below the setpoint.

A pressure gauge, with the switch, indicates the inlet fuel pressure.

Liquid Fuel System


Fuel Filters

Fuel oil flows from the fuel oil pump to the gas turbine via the pump discharge header. In the figure, a filter removes solid particles from the fuel oil.

In a liquid fuel system, filters are of two types:

· low pressure duplex filters· high pressure fuel filter

The duplex filter assembly incorporates two parallel-mounted filters equipped with a selector valve, filter check valves, and a differential pressure switch.

Liquid Fuel System


Fuel Filters

Each filter contains two replaceable filter elements with a 10-micron nominal rating, connected to the fuel system through the control valve so that fuel flow may be directed through either filter. This arrangement serves two purposes:

· servicing of the inactive filter during engine operation· manual transfer to the clean filter without engine shutdown (Contd.)

Liquid Fuel System


The high pressure fuel filter is installed in the fuel line between the high pressure fuel pump and the fuel control valve. It contains a replaceable filter element rated at 40 microns nominal.

Liquid Fuel System


Fuel ManifoldThe purpose of a fuel manifold is to divide a single fuel supply into several outlet streams.

A liquid fuel manifold usually has an inlet and outlet orifice called a boss.

There is an inlet boss and an outlet boss for each fuel nozzle.

Injector tube assemblies are connected to the fuel outlet bosses and carry the fuel to the fuel nozzle.

A liquid fuel manifold is sometimes called a fuel flow divider manifold. It incorporates inlet and outlet fuel connections for manifold-to-injector tube assemblies that carry the liquid fuel to the fuel nozzles.

Liquid Fuel System


Fuel ManifoldSolar gas turbines may also have an air assist manifold. Air assist manifold-to-injector tube assemblies carry fuel-atomizing air to the fuel nozzles.

The fuel and air mixture ratio is the weight of combustor primary air in relation to the weight of the fuel. A specific proportion of air is needed for efficient operation.

Liquid Fuel System


Fuel NozzlesFuel nozzles are located in the inlet of the combustor.

The purpose of fuel nozzles is to deliver highly atomized fuel in a controlled spray pattern in the combustors.

Fuel nozzles are of three types:

· simplex· duplex· air blast

Liquid Fuel System


Simplex Fuel NozzleThe simplex fuel nozzle has a small orifice that provides one spray pattern.

The simplex nozzle has a set of vanes, called flutes, that give a swirling motion to the fuel. This motion reduces the axial velocity of the fuel and provides better mixing of fuel and air.

Duplex fuel nozzles are:· single line· dual line

Liquid Fuel System


Single Line Duplex Fuel Nozzle

The single line duplex fuel nozzle receives fuel at one inlet port.

A flow divider in the nozzle distributes fuel through two spray orifices. The inner orifice sprays at a wide angle. The outer orifice opens at a preset pressure and sprays the primary fuel.

(Contd.)

Liquid Fuel System


The higher volume and higher pressure fuel flow from the outer orifice narrows the spray pattern so that fuel does not touch the combustion liner.

Single line nozzles also use a spin chamber for each orifice. This chamber provides efficient fuel mixing and fuel-air residence time over different fuel pressures.

The head of the fuel nozzle usually has air holes that provide some primary air for combustion.

This air also cools and cleans the nozzle head and spray orifices.

Liquid Fuel System


Dual Line Duplex Fuel Nozzle

The dual line duplex fuel nozzle is similar to the single line except it does not have a flow divider to separate primary and secondary fuel.

The duplex fuel nozzle has a fuel inlet port for each spray orifice.

The dual line duplex nozzle contains flutes and cooling air orifices.

Liquid Fuel System


Air Blast Fuel Nozzle

The air blast fuel nozzle enhances the atomization process and produces finer fuel droplets.

This nozzle is more effective during start-up when low fuel pressure causes atomization problems.

By using a high velocity airflow, air blast nozzles atomize the fuel more completely than can be accomplished by only pressurized fuel.

(Contd.)

Liquid Fuel System


A cone-shaped, atomized spray pattern provides a large fuel surface of very fine fuel droplets.

This pattern optimizes mixing of fuel and air and ensures the highest heat release from the fuel for more complete combustion.

The most desirable flame pattern occurs at higher compressor discharge pressures. During start-up and other off-rated speeds, flame length increases because of low compression.

Liquid Fuel System


Compression Ratio

Recall from a previous lesson that a gas turbine engine must maintain its rated compression ratio for efficient operation.

Compression ratio is the amount of discharge pressure in pounds per square inch absolute (psia) over suction pressure in psia.

As compressor discharge pressure increases, fuel pressure to the engine also increases. As the engine nears rated speed, fuel flow is regulated according to load requirements.

Liquid Fuel System


Fuel Control Valve: Purpose & ComponentsTurbine speed is controlled by the fuel control valve.

The purpose of a fuel control valve is to:

provide the correct air/fuel ratio to the combustion section· regulate fuel flow to control engine speed and exhaust temperature

The main components of a liquid fuel control valve are:

· fuel metering valve· fuel topping actuator solenoid· Pcd bleed valve

Liquid Fuel System


Solenoid Operation ValvesThe fuel valve assembly is mounted on the fuel control assembly. The assembly is an explosion-proof junction box, on which are mounted four solenoid-operated valves.

These valves and their functions are described next.

The two-way, normally closed bypass valve connects fuel flow from the fuel control valve to a return line leading to the fuel filter inlet.

The valve remains closed during start-up and operation. Upon engine shutdown, the bypass valve opens.

Liquid Fuel System


Two-Way Fuel ValveThe two-way, normally closed fuel valve operates in conjunction with the bypass valve during the start-up cycle and normal engine operation.

The fuel valve opens during the start-up sequence when the bypass valve closes. This directs metered fuel flow to the fuel nozzles.

During engine shutdown, the valve closes.

Liquid Fuel System


Torch & Purge ValvesThe two-way, normally closed torch valve functions for only a short time during engine start-up.

Ten seconds after the engine reaches 15% speed, the valve opens, fuel is directed to the igniter torch assembly, and ignition occurs.

At a predetermined engine temperature, the valve closes and combustion is self-sustained.

The normally closed purge valve is a two-way valve connecting the fuel supply line to the fuel nozzles and to a return line.

During start-up, the valve opens. The purge valve closes when the engine reaches 15% speed.

Liquid Fuel System


Fuel Valve Linkage AssemblyOn engine shutdown, the purge valve again opens to permit drainage (purging) of the liquid fuel lines. The valve closes as engine speed drops below 15% speed.

The linkage assembly is the interconnecting device between the fuel control valve and the electrohydraulicservo actuator.

The assembly consists of a linkage rod and rod ends. One end of the rod is attached to the servo actuator output shaft, and the opposite end is connected to the fuel control valve fuel metering lever.

Movement of the servo actuator results in repositioning the fuel metering valve.

Liquid Fuel System


Fuel Topping Actuator Solenoid

The fuel topping actuator solenoid operates on input signals from the electrical control system to decrease fuel flow during the start-up cycle if turbine temperature exceeds a preset limit.

When energized, the fuel topping actuator solenoid reduces the metering valve open position.

Liquid Fuel System


Pcd Bleed Valve

The Pcd bleed valve is a solenoid-actuated, normally open, three-way valve mounted on the liquid fuel control valve housing.

The Pcd bleed valve operates on input signals from the electrical control system.

The purpose of the Pcd bleed valve is to receive or vent Pcd from or to the acceleration limiter.

Liquid Fuel System


Pcd & Turbine Speed MeasurementDuring engine operation, the valve closes and Pcd is admitted to the acceleration limiter through an orifice. Upon engine shutdown, the valve opens and vents the acceleration limiter.

An increase or decrease in the turbine load causes a corresponding change in the turbine speed.

Turbine speed can be measured by the following means:

· flyweight governor· magnetic sensors

The flyweight governor is discussed next.

Liquid Fuel System


Flyweight GovernorA flyweight governor consists of a pair of weights, called flyweights, a tension spring, and a governor rod.

The governor rod rotates, and centrifugal force moves the flyweights apart.

Movement in the flyweight position also repositions the governor rod. The rod connects the action of the flyweights to the throttle valve, which controls fuel flow.

Liquid Fuel System


Magnetic Sensors & Main Fuel Actuator

Magnetic or pickup sensors consist of a permanent magnet, wrapped with a coil in a sealed case. The sensors are mounted around a gear wheel on the gas turbine rotor shaft.

Each sensor generates an electrical signal proportional to engine speed. A signal is sent each time a gear tooth passes under the sensor.

( Contd.)

Liquid Fuel System


Modern gas turbines use an electric or electronically controlled main fuel actuator to control turbine speed.

The main fuel actuator controller can be programmed to maintain a constant gas producer turbine speed, a constant power turbine speed, or a constant compressor discharge pressure, depending on the requirements of the system.

Liquid Fuel System


Probes & Sensors

The controller receives signals from the:

· magnetic proximity probes or sensors

· exhaust gas temperature sensors

Shaft speed may be monitored by magnetic proximity probes positioned near the shaft. These are called "key phasors." These probes emit an electromagnetic field that fluctuates each time the key phasor slot passes the probe. One fluctuation in the circuit equals one revolution of the shaft.(Contd.)

Liquid Fuel System


Shaft rpm is compared with speed setpoints by the main fuel actuator controller.

The controller then increases or decreases fuel flow until the desired speed is reached.

Liquid Fuel System


Temperature Sensors

Exhaust gas temperature is the most critical of all gas turbine engine operating parameters.

Exhaust gas temperature sensors send signals to the main fuel actuator controller. The exhaust gas temperature is monitored by several thermocouples, which signal temperature information to the engine temperature controller.

(Contd.)

Liquid Fuel System


Several different terms are used to describe exhaust gas temperatures:

· Turbine Inlet Temperature (TIT): temperature is monitored upstream of the turbine wheel(s)

· Interstage Turbine Temperature (ITT): temperature is taken at an intermediate position between multiple turbine wheels

· Exhaust Gas Temperature (EGT), or Turbine Outlet Temperature (TOT) is taken downstream of the turbine wheel(s)

Liquid Fuel System


Liquid Fuel System Operation

Introduction

The preceding lesson explained liquid fuel system components and their function.


Liquid Fuel System: Purpose

The purpose of a liquid fuel system is to deliver clean liquid fuel to the engine in correct volumes at the correct pressure.

The delivery of the fuel is controlled by a microcomputer control system.

This system monitors all phases of engine operation, from start-up through shutdown.

(Contd.)

Liquid Fuel System


System Operation:

acceleration schedule.

Start-up Sequence1) The start-up sequence of the fuel system is initiated when the start switch is pressed.

2) The load/speed sensing control unit (governor) is energized.

3) A signal is transmitted to the electrohydraulic servo actuator. The actuator retracts and moves the fuel control linkage toward the maximum fuel position when servo oil pressure builds up.

This action moves the metering valve lever from the minimum fuel stop position. This also allows the acceleration limiter to progressively enrich the fuel and air mixture in accordance with the

Liquid Fuel System


System Operation: Start-Up Sequence

Engine temperature control is offset during start-up. The impending high temperature alarm and high turbine temperature shutdown setpoints are temporarily increased approximately 50°F.

During the start-up sequence when the engine is operating between 0% and 15% speed, the liquid fuel purge solenoid valve is energized and remains open until 10 seconds after 15% engine speed is reached.

The electric liquid fuel boost pumps are energized, if used.

Liquid Fuel System


System Operation: Start-Up Sequence

The air assist solenoid-operated shutoff valve is energized (opened). As the engine accelerates, the fuel pressure, Pcd, and engine oil pressure increases.

If 15% engine speed is not reached in 30 seconds after starters begin to crank, engine shutdown is initiated and FAIL TO CRANK malfunction is indicated.

Liquid Fuel System


System Operation: Run Sequence

At 15% engine speed plus 10 seconds:

1) The Pcd bleed valve opens and begins to act on the acceleration limiter.2) The purge valve closes.3) The torch valve and the fuel valve open.4) The electric motor-driven main fuel pump starts, if used.5) The ignition relay and ignition exciter are energized. Spark plug starts firing.6) Fuel flows through the torch valve to the torch. (Contd.)

Liquid Fuel System



7) Torch fuel is atomized by air assist pressure and is ignited in combustion air.

8) Metered fuel from the fuel control valve flows through the fuel valve and the torch bias relief valve to the fuel nozzles.

This fuel flow is then atomized by air assist pressure.

The torch flame ignites the fuel and air mixture from the fuel nozzles, beginning combustion.

The engine continues to accelerate. When engine temperature reaches setpoint, approximately 350°F, the torch valve and ignition are de-energized.

Liquid Fuel System



If turbine temperature has not reached the setpoint in 25 seconds after attaining 15% engine speed, engine shutdown is initiated and IGNITION FAIL malfunction is indicated.

If high pressure fuel pump suction pressure is lower than the setpointof the low fuel pressure switch, approximately 7 psig, 25 seconds after attaining 15% engine speed, engine shutdown is initiated and LOW FUEL PRESS malfunction is indicated. (Contd.)

Liquid Fuel System


After light-off occurs, turbine temperature increases rapidly.

If temperature exceeds setpoint while accelerating to 90% engine speed, engine temperature control warning IMPENDING HIGH ENGINE TEMPERATURE is initiated.

Liquid Fuel System



Fuel topping solenoid valve is energized.

Fuel flow is reduced to topping flow until turbine temperature decreases to normal. The fuel topping solenoid is then de-energized.

The fuel topping solenoid operates with on-off action if the over-temperature condition persists until 90% engine speed is attained.

If the temperature topping circuit malfunctions, a further temperature increase will activate a backup shutdown circuit.

Liquid Fuel System



When engine speed reaches 66%, the engine start system and the atomizing air assist shutoff valve are both de-energized.

Fuel atomizing air is then supplied by Pcd.

Liquid Fuel System


System Operation: Run SequenceWhen engine speed reaches 90%:

1) The electronic load/speed controller (governor) takes control of the electrohydraulic servo actuator and positions the fuel control linkage to accelerate to operating speed.

2) Offset setpoints are transferred to normal operating values for engine temperature control. (Contd.)

Liquid Fuel System



3) The topping control circuit is de-energized.

4) The temperature shutdown timer is armed.

5) Fuel is metered to the engine according to the demand of the electronic control system, based on load, speed, or temperature.

(Contd.)

Liquid Fuel System


6) Engine speed increases to operating speed.

7) The following events take place if a turbine over-temperature condition occurs when the engine is operating above 90% speed:

8) At approximately 1,155°F, the engine shutdown timer is de-energized.

9) After a 5-second delay, allowing for transient over-temperatures, HIGH ENG TEMP alarm is indicated.

10) At approximately 1200°F, engine shutdown is immediate and HIGH ENG TEMP is indicated and engine shutdown is initiated.

Liquid Fuel System


System Operation: Shutdown Sequence

Shutdown of the fuel system operation occurs in sequence when the stop switch or emergency stop switch (Local Panel) is pressed.

When start/run relays are de-energized:

1) The postlube timer relay begins to time out.

(Contd.)

Liquid Fuel System


2) The pre/postlube pump motor is energized.

3) The purge valve opens to purge the fuel system until engine speed decreases below 15%.

4) The fuel bypass valve opens to the filter outlet line.

5) The Pcd bleed valve opens to cutoff Pcd air and vent the fuel control valve.

6) Fuel valve closes to fuel injectors.

7) Control power to the electronic load/speed sensing control isstopped, and the governor is deactivated.

(Contd.)

Liquid Fuel System


8) Main electric fuel pump, if used, and fuel boost pump are both de-energized.

9) When the fuel supply to the engine is cut off, combustion stops and the engine begins to decelerate.

10) When engine speed decreases below 15%, the purge valve and the fuel bypass valve close.

11) The engine coasts to a stop.

Liquid Fuel System


System Operation: Shutdown Sequence

55 minutes after the stop switch is pressed:

1) The postlube timer relay times out.

2) The pre/postlube oil pump is de-energized.

3) The postlube cycle is complete.

4) The master control switch can be turned off and the engine is ready for restart.

Liquid Fuel System


Fuel Gas System


Fuel Gas Metering Valve (General Electric )


Monitoring Screen for the Fuel Gas Control System


Fuel Oil System


Dual Fuel Systems

Dr. Walid Abdelghaffar

6 gas turbine_fuel systems

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