Gas Turbine Start System

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Dr. Walid Abdelghaffar Gas Turbine- Start Systems

System Function

¾

To accelerate the turbine engine up to

15%-20% where combustion light off occurs

¾

At 60% -66% gas producer speed ,

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Types of start systems

Pneumatic

Electrical

Electro hydraulic

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Introduction

Preceding lectures have discussed gas turbine engine components and

operation, oil systems, and fuel

systems. This lectures will describe gas turbine engine starting systems.

This lectures presents information

about a pneumatic starting system, its components, and operation. A Solar Centaur pneumatic starting system is used as an example.

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Pneumatic Starting

System: Purpose

The purpose of a gas turbine engine starting system is to provide power to:

· rotate the turbine shaft to starting

speed

· assist the turbine to self-sustaining speed after combustion occurs

Most gas turbine engines are started by starter power input to the main accessory gearbox. The gearbox is connected to the turbine rotor and compressor.

Any gas under pressure may be used as a power source for a pneumatic starting system.

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Starting System: Components

Natural gas must meet the manufacturer's specifications. A typical pneumatic starting system requires approximately 2600 scfm.

The main components of a Solar Centaur starting system are as follows:

· gas inlet strainer · pilot gas filter

· solenoid-operated pilot valve · starter motor shutoff valve · lubricator

· starter motors

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Starting System: Gas

Inlet Strainer

The gas inlet strainer is located at the gas turbine skid upstream of the shutoff valve. A shutoff valve is located downstream of the strainer to shut off the gas supply to the starter motor.

The strainer is a Y-shaped fitting that houses a removable cylindrical "strainer" screen.

A pilot supply line branches off from the main gas supply to the starting system. This line provides pneumatic pressure to the solenoid-activated pilot valve.

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Starting System: Pilot

Gas Filter

A pilot gas filter assembly, consisting of a 10-micron filter and a pressure-reducing orifice, is installed in the pilot supply line upstream of the pilot valve.

The filter prevents foreign matter from entering the solenoid-operated pilot valve. The pressure-reducing orifice creates a pressure drop in the pilot supply line. This pressure drop ensures that the pilot valve will operate properly in the event of

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Starting System: Pilot

Valve

The pilot valve is a three-way, solenoid-operated valve that is powered by 24-volt DC from the electrical system and actuated by the control system.

The pilot valve opens and closes the starter motor shutoff valve.

When the solenoid is de-energized, pilot gas pressure closes the starter motor shutoff valve. When the solenoid is energized, pilot gas pressure is vented, allowing the shutoff valve to open.

A pilot relief valve protects the filter and pilot valve from excessive pressure.

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Starting System: Starter

Shutoff Valve

The pilot-actuated starter motor shutoff valve controls gas flow from the supply line to the two starter motors.

The starter motor shutoff valve is installed upstream of the lubricator.

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Starting System: Lubricator

A lubricator, located downstream of the starter motor shutoff valve,

injects lubricating oil into the gas flow.

The purpose of the lubricator is to provide atomized lubricating oil to the starter motor vanes.

A sight dome is used to check oil flow. In addition, the lubricator bowl has an oil level sight glass.

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On Solar Centaur gas turbines, two vane-type, pneumatically operated starter motors are mounted on the starter adapter housing, located on the reduction gear.

The motors transmit starting power to the engine through a common overrunning clutch and shaft.

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Starting System: Starter

Motor

The figure shows a starter motor and the clutch, called a sprag or sprag clutch assembly. The clutch in this installation is in the housing that is mounted on the engine gearbox drive.

The pawls , driving or holding links of a ratchet that permit motion in one direction only, are forced inward by small springs to engage the sprag clutch ratchet. At a

preset engine speed, the pawls are thrown

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outward, disengaging the drive shaft assembly from the sprag clutch

ratchet. The sprag clutch ratchet and starter gear train coast to a stop and the drive shaft assembly containing the pawls continues to rotate at engine gearbox speed.

The operation of the pneumatic starting system is explained next.

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Starting System: Operation

The operation of the pneumatic starting system is automatically actuated by the gas turbine control system panel.

The control system provides automatic starting and sequencing as the gas turbine is accelerated to operating speed.

During acceleration and operation, the control system monitors the gas turbine and driven equipment, such as a generator or compressor. If an operating malfunction occurs, the control system identifies the nature of the trouble and may initiate an emergency shutdown.

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System Operation: Start Sequence The following start sequence is not a start-up checklist. Before a gas turbine start is attempted:

· A thorough prestart inspection must have been completed.

· Prestart conditions must be satisfied. When the operator presses the START pushbutton:

· The RUN indicator lamp illuminates.

· Start/run control system relays are energized, and the start circuit is initiated.

· The pre/postlube pump starts, and prelubrication begins.

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System Operation: Start Plus 15 Seconds

The following events occur 15 seconds after the START button is pressed:

· The prelube timer times out, and the start relay remains energized up to 66% of engine speed.

· The solenoid-operated pilot valve is energized, and the starter motor shutoff valve opens.

· Starter motors begin to crank and accelerate the gas turbine.

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System Operation: Turbine At

15% Speed

When gas turbine speed reaches 15%:

· Purge timer relay inhibits fuel while purging the machine.

· Ignition begins when fuel is admitted 10 seconds after 15% speed. (Purge timer times out.)

· Lightoff occurs and combustion begins. · The engine continues to accelerate with assistance from the starter motors.

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System Operation:

Temperature @ 350°F

When engine turbine

temperature reaches 350°F:

· Ignition system is shut off.

· Pre/postlube pumps stop when engine-driven lube oil pump

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System Operation: Engine

Speed @ 66%

When engine speed reaches

approximately 66% or self-sustaining speed:

· The starter clutch overruns.

· The start-relay and start circuits are de-energized.

· The solenoid-operated pilot valve is de-energized.

· The starter motor shutoff valve is closed by pilot pressure.

· Acceleration continues.

The start cycle is complete at approximately 66% engine speed.

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Pneumatic Start System

G VALVE ACTUATORQUICK EXHAUST V G V STARTER MOTOR STARTER MOTOR G V G G O LUBRICATOR FROM LUBE OIL SYSTEM SHUT OFF VALVE PILOT AIR/GAS SUPPLY AIR/GAS SUPPLY STRAINER SHUT OFF VALVE LEGEND G GAS O OIL V VENT

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1. Clutch Lubrication

Fixed Orifice

2. Pneumatic Starter

Motor

3. Gas Shutoff Pilot

Solenoid Valve

4. Gas Shutoff Valve

PNEUMATIC start system –

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Hydraulic Starting System:

Purpose

The purpose of a hydraulic starting system is to supply high pressure hydraulic fluid to a hydraulic starter motor. This motor is mounted on the accessory gearbox.

The hydraulic starting system module and its associated piping, valving, and instrumentation is mounted on a self-contained skid.

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Hydraulic Starting System:

Components

The main components of a

hydraulic starting system are as follows:

· electric motor · hydraulic pump

· hydraulic fluid reservoir

· hydraulic starter motor (engine mounted)

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Electric Motor

The purpose of the electric motor is to drive the hydraulic pump.

The size of the electric motor depends on the capacity of the hydraulic start module. Larger engines require greater starting power.

A typical 55 gpm hydraulic starting system that operates at 5000 psig uses a 200 horsepower electric motor.

The motor normally operates on 480 volt, 3-phase AC and rotates at 1800 rpm on 60 hz or at 1500 rpm on 50 Hz.

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Hydraulic Pump

The hydraulic pump is an axial-piston, variable-displacement or variable-volume pump. This type of pump automatically increases or decreases the volume of fluid flow to limit output pressure.

A small charge pump is mounted on the head of the hydraulic pump.

The purpose of the charge pump is to prime the system and to ensure that air is purged from the system.

The charge pump is driven through the main hydraulic pump.

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Axial-Piston Pump

An axial-piston pump consists of several small reciprocating pumps in a common cylinder block and housing.

The head, which contains an inlet port and an outlet port, is attached to one end of the pump housing.

The cylinder block contains an odd number of cylinders (usually seven) equally spaced from the center.

Three cylinders are always connected to the inlet port, and three cylinders are always connected to the outlet port. One cylinder is located between the ports.

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Axial-Piston Pump

The cylinder block and pistons rotate inside the pump housing.

The pistons have connecting rods that are fastened to a swash plate by ball joints.

The drive shaft of the axial-piston pump rotates the swash plate and cylinder block so that the pistons move back and forth in the cylinder block, creating the pumping action.

Piston movement is almost overlapping, which results in a constant flow of

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Hydraulic Pump: Pumping

Action

The figure shows the pumping action of one piston as the cylinder block makes one revolution inside the pump housing.

When the piston is at point 1, it has just started its inward

movement and is only partly open to the inlet port.

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At points 2 and 3, the piston is moving inward and the space in the cylinder is being filled with hydraulic fluid.

At point 4, the piston is at the bottom of its stroke and it is not connected to either port at this time. This movement from point 1 to point 4 represents the first half of a revolution.

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Hydraulic Pump:

Pumping Action

During the last half of the revolution, the piston moves outward toward the head. The hydraulic fluid that filled the cylinder during the first half of the revolution is now forced out through the outlet port as the piston returns to point 1 to start another cycle.

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Variable Displacement Piston Pump

A variable-displacement piston-type pump provides a means for varying the length of the pumping stroke. The length of the pumping stroke determines the volume of hydraulic fluid that is discharged to the

system.

Pump stroke length is varied by changing the angle of the swash plate.

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The angle of the swash plate is controlled by two different means:

· A solenoid selects high or low flow operation.

· An internal compensator limits output pressure to 5000 psig by reducing the swash plate angle to reduce cylinder displacement.

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Hydraulic Starting System:

Operation

The typical hydraulic start system is a closed loop system with all return lines to the hydraulic fluid reservoir. The reservoir usually stores 40 to 50 gallons of hydraulic fluid.

Automatic temperature controls maintain hydraulic oil temperature between 50°F and 60°F.

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Alarms and shutdown switches are provided for high and low hydraulic oil temperatures and low liquid levels.

During starting system operation, the charge pump takes suction from the hydraulic fluid reservoir through a suction strainer.

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Hydraulic Starting System: Operation

Fluid from the charge pump flows

through a charge pump discharge filter and then to the main hydraulic pump. The charge pump provides over 300 psig to the main pump suction.

The main hydraulic pump provides up to 55 gpm at 5000 psig to the hydraulic starter motor.

High pressure fluid lines are installed between the starting system module and the hydraulic starter motor.

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Hydraulic Starting System:

Operation

The hydraulic starter motor is similar to the hydraulic pump.

The motor is variable displacement with a swash plate. The power

cylinders are arranged axially around a shaft. The angle of the swash plate on the shaft is

controlled by a speed-sensing mechanism.

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At speeds up to 2500 rpm, the swash plate is held at its maximum angle, and the pistons are working through their full stroke.

At speeds over 2500 rpm, the swash plate angle decreases. At about 4500 rpm, the swash plate is almost square to the shaft.

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Hydraulic Starting System:

Operation

The flow through the motor is equal to cylinder displacement times rpm. The flow gradually increases as the motor speed increases up to 2500 rpm. Flow then remains more or less constant up to cut-out speed (speed increasing, displacement decreasing). The starter converts high pressure fluid energy to shaft torque, which rotates the engine.

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The fluid discharges from the starter to the low pressure side of the system (about 150 psig) and returns through a filter to the supply pump inlet.

Solenoid control valves are sequenced and controlled by the main unit control panel.

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Hydraulic Starting System:

Operation

When a start is initiated, the system accelerates the gas

generator from static conditions to self-sustaining idle speed.

The hydraulic starter motor has an overrunning clutch, which

disconnects the starter shaft from the gas turbine during engine operation.

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Hydraulic Clutch

Coupling between

torque converter and

asses gear arranged for

engage by hydraulic

piston

Hydraulic piston

Hydraulic oil inlet pipe

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Introduction

This is the last in the series about gas turbine starting systems. Previous parts provided important information about pneumatic and hydraulic starting systems.

This part focuses on the diesel starting system, its components, and operation. The figure represents a typical diesel starting system.

The part begins with the purpose of a diesel starting system.

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Diesel Starting System:

Purpose

The purpose of a diesel starting system is to provide power to start the turbine axial flow compressor.

A diesel starting system performs three primary functions during the start cycle:

· start the turbine roll (breakaway from standstill)

· accelerate the gas turbine to firing speed · assist the gas turbine to self-sustaining speed A diesel starting system has three main

components. Each is described in the information that follows.

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Diesel Starting System:

Components

The main components of a diesel starting system are:

· diesel engine · torque converter

· hydraulic ratchet system

The three components are graphically represented in the figure. The diesel engine is discussed first.

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Diesel Engine

A diesel engine is the primary component of a gas turbine diesel starting system. It is used to rotate (crank) the gas turbine for start-up.

A diesel engine is an internal combustion engine that converts the heat of fuel into work in the cylinders of the engine.

Diesel engine operation is based on the reciprocating (upward and downward) movement (stroke) of a piston in a cylinder.

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In a two-cycle engine, the combustion cycle is completed in each cylinder during one revolution of the crankshaft.

The diesel starting engine for G.E. Frame 5 gas turbines is a 12 cylinder, two-cycle engine rated at 2300 rpm.

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Two-Cycle Diesel Engine

In a two-cycle engine, four events occur during one piston stroke. The events are:

· scavenging (air intake) · compression (fuel injected) · power (fuel ignition)

· exhaust

The action taking place during each of these events is shown in the figure.

The next component discussed is the torque converter.

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Torque Converter

The torque converter is a hydraulic device that transmits the torque

(turning force) of the diesel engine to the gas turbine through the ratchet jaw clutch.

The hydraulic ratchet's jaw clutch couples and uncouples the torque converter and diesel engine from the gas turbine.

Rotation of the hydraulic turbine causes the output shaft of the torque converter to rotate. This turning force (torque) is transmitted to the gas turbine through a starting clutch.

The third component of the diesel starting system is the hydraulic ratchet assembly. The purpose of this component is discussed next.

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Hydraulic Ratchet

Assembly

The purpose of the hydraulic ratchet assembly is to assist the starting device to begin the

rotation of a gas turbine rotor at breakaway.

The ratchet system also rotates the gas turbine rotor during the cooldown cycle, after gas turbine shutdown.

The ratchet is controlled partially by the starting control system.

The diesel starting system has several auxiliary systems that are essential to its operation. These auxiliary systems are discussed next.

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Diesel Starting System:

Auxiliary Systems

A diesel engine has the

following auxiliary systems:

· cooling · lube oil · air

· fuel

· starter control

The first system discussed is the cooling system.

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Cooling Systems

A diesel engine uses one of two

types of cooling systems:

· radiator and fan · heat exchanger

Both systems use a centrifugal water pump to circulate coolant and a

thermostat to maintain operating temperature.

The radiator and fan cooling system is discussed first.

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Cooling Systems: Radiator

&

Fan

In a radiator and fan cooling system, the coolant is drawn from the lower section of the radiator by the water pump.

The coolant is then forced through the lube oil cooler, into the cylinder block, and up through the cylinder heads.

When the engine is at normal operating temperature, the coolant passes through the thermostat housings into the top part of the radiator.

In the radiator, coolant temperature is reduced by heat exchange with the airflow through the radiator created by the fan.

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Cooling Systems: Radiator

& Fan

When starting a cold engine or when the coolant is below operating

temperature, coolant flow is restricted by the thermostat.

A bypass tube permits water

recirculation in the engine during warm-up.

The heat exchanger cooling system is discussed next.

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Cooling Systems: Heat

Exchanger

In a heat exchanger cooling system, the coolant is pumped through the heat exchanger by the water pump. The coolant then passes through the engine oil cooler, cylinder block, cylinder heads, exhaust manifolds, and thermostat

housings.

A bypass tube permits coolant recirculation when the thermostat is closed. When the thermostat is open, coolant returns to the heat exchanger for cooling and recirculation. As coolant passes through the core of the heat exchanger, the coolant temperature is lowered by heat exchange with raw water. Raw water is supplied by a raw water pump from an outside source

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Lube Oil System

The diesel engine lube oil system ensures positive lubrication at all times.

This lube oil system has the same purpose and components as other lube oil systems discussed in previous lessons.

Diesel engine lube oil provides full pressure lubrication for all main, connecting rod, and camshaft

bearings; piston pins; gear train; and drive gear. Lube oil also cools the piston.

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Air System

The purpose of a diesel engine air system is to provide air for

scavenging (removing) exhaust

gases from the engine cylinders and for combustion.

In a two-cycle diesel, a charge of air is forced into the cylinders by the blower(s). Each cylinder is filled with fresh, clean air.

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The air charge thoroughly sweeps all of the burned gases out through the exhaust valve ports.

The air helps to cool internal engine parts, particularly the exhaust valves. The diesel starting fuel system is discussed next.

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Fuel System

The main components of the diesel engine fuel system are:

· diesel fuel tank · fuel charge pump · canister tank

· main fuel pump · filters

· fuel header · fuel injectors

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A diesel fuel tank is built into the base of the gas turbine.

The fuel charge pump is driven by the diesel engine camshaft. This pump supplies fuel from the base tank to a small canister tank mounted on the side of the engine.

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Fuel System

The main fuel pump takes suction from the canister tank. It pumps the fuel under pressure through the fuel filter to the fuel header.

The main fuel pump is a positive

displacement gear pump that is attached to the blower.

The fuel header delivers fuel to the fuel injectors. The fuel is filtered through filter elements in the injectors and then

atomized through small spray tip orifices into the combustion chamber of each cylinder.

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Control System

This section discusses the starter control system.

The primary diesel controls are:

· starter

· speed control system · stop mechanism

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Control System

The starter is a DC electric starter motor. The starter motor is attached to the diesel engine block.

Lead-acid storage batteries provide electric power for starting.

Diesel engine speed is controlled by the

variable speed governor. A hydraulic actuator positions the governor speed control lever. A small, engine-driven pump supplies engine oil for actuator operation.

When the accelerating solenoid is energized, the governor lever is driven to the high

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Control System

When the solenoid is de-energized, the lever is returned to the idle position. The diesel engine stop mechanism is a solenoid connected to the governor. When the stop solenoid is energized, it activates the shutdown mechanism, shutting off fuel and stopping the engine.

Electronic logic in the gas turbine control panel provides automatic sequencing of the starter and control solenoids for normal diesel engine start-up, for normal and emergency engine shutdown, and for exercising and testing the engine.

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Control System

To protect the diesel engine, electronic logic also monitors the starting clutch position, the engine lube oil pressure and the engine speed.

Alarms and emergency shutdowns are

initiated for out-of-limit operating conditions.

If the diesel engine is tripped when the throttle is in the full open position, the throttle does not reset.

The next diesel engine restart will be made with the throttle set at full power. The throttle lever should be manually reset before the next start attempt.

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Diesel Starting System:

Operation

The following describes the operation of a diesel starting system. The procedure

assumes the gas turbine is off cooldown, in a ready to start condition with all pre-start-up checks complete.

When START is selected:

· starting clutch is engaged

· starting motor is energized to start the diesel engine

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The diesel engine idles through a 2-minute warm-up cycle.

The acceleration solenoid is energized to accelerate the diesel engine to max speed for gas turbine breakaway with hydraulic ratchet assistance.

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Operation

Following gas turbine breakaway:

· acceleration solenoid is de-energized and the diesel engine slows to approximately 1900 rpm

· acceleration stop solenoid controls diesel engine speed until the gas turbine is sequenced through the warm-up cycle

· acceleration stop solenoid is de-energized · acceleration solenoid is energized to accelerate the diesel engine to maximum rpm for acceleration of the gas turbine

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Operation

At diesel engine speed of 3000 to 3400 rpm:

· clutch automatically disengages · acceleration solenoid is

de-energized

At this time, the diesel engine: · returns to idle speed

· idles through a cooldown period · stops when the stop solenoid is energized

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Trouble Shooting Guide

FAIL TO CRANK

1. Check For Engine Rotation.

2. Check Magnetic Pickup And Circuit.

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Fail To Start

1. Check Oil Temperature

2. Check Operation Of Guide Vanes

3. Check Operation Of Bleed Valve

4. Check Operation Of Fuel Actuator

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Ignition Failure

1. Check T5 On Restart Attempt.

2. Check Torch

And Igniter Assemblies

3. Check T5 Thermocouples

4. Check Pilot Fuel

Pressure Regulator

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Scheduled Performance Tasks Table

1. Clean strainers in supply lines to start system.

2. Check start system and associated wiring and

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Trouble Shooting Guide

FAIL TO CRANK

1. Check For Engine Rotation.

2. Check Magnetic Pickup And Circuit.

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Fail To Start

1. Check Oil Temperature

2. Check Operation Of Guide Vanes

3. Check Operation Of Bleed Valve

4. Check Operation Of Fuel Actuator

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Ignition Failure

1. Check T5 On Restart Attempt.

2. Check Torch

And Igniter Assemblies

3. Check T5 Thermocouples

4. Check Pilot Fuel

Pressure Regulator

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Gas Turbine Start System

System Function

¾

To accelerate the turbine engine up to

15%-20% where combustion light off occurs

¾

At 60% -66% gas producer speed ,

Types of start systems



Pneumatic



Electrical



Electro hydraulic

Introduction

Pneumatic Starting

System: Purpose

Starting System: Components

Starting System: Gas

Inlet Strainer

Starting System: Pilot

Gas Filter

Starting System: Pilot

Valve

Starting System: Starter

Shutoff Valve

Starting System: Lubricator

Starting System: Starter

Motor

System Operation: Turbine At

15% Speed

System Operation:

Temperature @ 350°F

System Operation: Engine

Speed @ 66%

Pneumatic Start System

1. Clutch Lubrication

Fixed Orifice

2. Pneumatic Starter

Motor

3. Gas Shutoff Pilot

Solenoid Valve

4. Gas Shutoff Valve

PNEUMATIC start system –

Hydraulic Starting System:

Purpose

Hydraulic Starting System:

Components

Electric Motor

Hydraulic Pump

Axial-Piston Pump

Axial-Piston Pump

Hydraulic Pump: Pumping

Action

Hydraulic Pump:

Pumping Action

Hydraulic Starting System:

Operation

Hydraulic Starting System:

Operation

Hydraulic Starting System:

Operation

Hydraulic Starting System:

Operation

Hydraulic Clutch



Coupling between

torque converter and

asses gear arranged for

engage by hydraulic

piston



Hydraulic piston



Hydraulic oil inlet pipe

Introduction

Diesel Starting System:

Purpose

Diesel Starting System:

Components