GE Power Systems

1 MS9001E STARTING SYSTEMA00205

MS9001E STARTING SYSTEM

General

Like other internal combustion engines, a gas tur-bine cannot produce torque at zero speed. The start-ing means system is used to start the gas turbinerolling, crank it to firing speed, and assist the firedturbine to self–sustaining speed. This is accom-plished by an induction motor starting device oper-ating through a torque converter coupled to theaccessory gear. This arrangement provides thecranking torque required by the turbine for startup.Also, at shutdown, this same equipment continues torotate the turbine rotor at slow speed for cooldownpurposes.

The starting means system components and controldevices are located and mounted in the accessorycompartment. The induction motor and torque con-verter are mounted on a separate base structure at-tached to the accessory base top deck. The startingmotor drives the torque converter through a flexiblecoupling. The torque converter is coupled to the ac-cessory gear and provides the required torque multi-plication for the starting motor to drive the turbine.The primary parts of the torque converter are the im-peller driven by the input shaft, the turbine wheelwhich drives the output shaft, and the stator whichdirects fluid from the impeller to the turbine at thecorrect angle to produce the required output torque.

The function of the accessory gear in this system isto drive a number of the control components as wellas to provide the connection between the startingmotor and the gas turbine compressor. The startingmeans is coupled to the accessory gear #1 outboardshaft and the accessory gear #1 inboard shaft is con-nected to the turbine compressor shaft flange by anoil–filled flexible coupling.

Starting Motor

The starting motor is a GE Co. Custom 8000 hori-zontal induction motor with anti–friction bearings.

This is a 1250 horsepower, 6000 volt, 3 phase, 3000rpm unit.

Torque Converter

The torque converter is a Voith Type EL 10 z FT.This unit is equipped with adjustable guide blades, afilling and draining valve, and a hydrodynamic turn-ing device. The torque converter receives its bearinglube oil from the turbine’s 1.7 bar (25 psi) bearingheader. The operating fluid for the torque convertercomes from the turbine’s auxiliary lube oil pumpdischarge. Minimum supply pressure for the operat-ing fluid is 6.9 bar (100 psi).

The torque converter is load responsive, meaningthe torque developed in the turbine section is great-est when the output shaft is stationary and decreaseswith increasing speed. In this way, the converteradapts itself automatically and steplessly to the re-sistance encountered.

An integral part of the torque converter is the motor–driven torque adjustor drive. This provides themeans for adjusting torque output within specifiedranges. Switches 33TM–1 through 33TM–4 providethe torque adjustment range limits and the upper andlower drive limits of the torque adjustor drive.Switches 33TM–1 and –2 are true torque switchesmounted in the torque–adjuster drive motor 88TM.If the mechanical resistance exceeds their setting,88TM is shut off. 33TM–3 and –4 are true limitswitches that shut off the torque motor at the maxi-mum travel in either direction. 33TM–1 through –4are set by Voith. 33TM–7 is the switch for the maxi-mum, or acceleration, torque setting and 33TM–8 isthe switch for the minimum, or firing, torque setting.Both of these switches are set in the field. Control ofthe torque converter is also achieved via integrallymounted solenoid valves 20TU–1 and 20TU–2. Re-fer to the Starting Means Schematic Piping Dia-gram.

GE Power Systems

2MS9001E STARTING SYSTEM A00205

Functional Description

General

Cranking and turning power are supplied by thestarting system during gas turbine startup and cool-down cycles. In the starting cycle there are three pri-mary functions provided by the starting equipment:start the gas turbine rolling (breakaway from stand-still); accelerate the gas turbine to a speed where itcan be fired; and after the turbine has fired, furtheraccelerate it to self–sustaining speed, a speed atwhich the gas turbine develops net positive poweroutput.

Startup Functions & Sequences

In the normal starting sequence, fluid is admittedinto the torque converter hydraulic circuit from theturbine lubrication system by energizing solenoidvalve 20TU–1 at the same time that starting motor88CR is energized. Breakaway is achieved and theturbine starts to rotate. The turbine begins to in-crease in speed and continues to accelerate untilminimum firing speed is attained and relay 14HMpicks up. When the turbine has reached this speed(14HM setpoint), the internal geometry of the torqueconverter is adjusted by torque–adjustor drive88TM to hold firing speed constant through thepurge cycle. The unit is held at crank speed longenough to exchange three to four times the exhaustsystem volume of air. At this point, 20TU–1 is de–energized, partially draining the torque converter,and turbine speed decreases below the 14HM drop–out point. Once 14HM drops out, 20TU–1 again en-ergizes and unit speed increases. As the unit onceagain passes by the 14HM setpoint, fuel is admittedand ignited. When flame is sensed, torque–adjustordrive 88TM gradually drives the vanes to the hightorque position, accelerating the unit while it is stillin the warm–up cycle. In this application, 14HMpicks–up at 10% rated speed.

Readjustment of the converter geometry (torque ad-justment) to the maximum torque setting allows thetorque converter to assist in accelerating the unit up

to self–sustaining speed. At this speed, a nominal60% TNH, solenoid valve 20TU–1 is de–energizedand cranking motor 88CR runs unloaded for 15 min-utes for cooldown. A crank and restart can be initi-ated at any time below 14HM speed.

When the torque converter is filled, heat generated ispassed to the lube oil by the constant flow of operat-ing oil through the converter to drain. An orifice inthe drain line maintains the operating pressure with-in the converter. When the torque converter isempty, the heat generated by the high–speed wind-age of the turbine wheels is dissipated by an oil sprayjet providing surface cooling.

Rotor Cooldown

To promote even cooling of the rotor and avoid rotor“bow” after unit shutdown, the gas turbine rotor isrotated by the “slow roll” function of the torque con-verter. When the the gas turbine is on cooldown, so-lenoid valve 20TU–2 permits a slow turning of therotor by injecting jets of oil under pressure onto thetorque converter turbine via nozzles arrangedaround the outer diameter of the converter. Duringslow roll, the converter guide blades are to be held inthe fully open position. 20TU–2 energizes to initiateslow roll when rotor speed decreases to approxi-mately 3.2% of rated speed (approximately 96 rpm)as determined by the SPEEDTRONIC control cir-cuits.

When the rotor is at rest and “COOLDOWN ON” isselected at the primary operator interface, the start-ing motor is used to break the rotor away from stand-still. At breakaway, 14HR drops out and the startingmotor is de–energized. The inertia of the startingmotor continues to accelerate the turbine rotor toslow roll speed.

GE Power Systems Training