D. Turbines and Generators

8/10/2019 D. Turbines and Generators

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Turbines and Generators

Indonesia Customer Seminar

June 13 & 14 2012

Jakarta Indonesia


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Turbines & Generators

Purpose is to discus various potential problems withTurbine and Generator steam and water related systems.


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Typical power plant configuration


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Turbines & Generators

Steam TurbineFundamentals

TurbineGeneratorComponents

Critical SteamParameters


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Turbine-Generator Fundamentals

1. Steam at high temp & pressure containspotential

energy.

2. Potential energy converted to mechanical energyinturbinethrough steam expansion. Steam cools as itexpands to lower pressure.

3. Mechanical energyconverted to rotational energyinturbine.

Rotating blades and stationary blades alternate.

4. Rotational energy converted to electricityin generator.


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Typical Utility Turbine-Generator Set

Turbine: 3 Pressure Stages: LP IP HP

GeneratorExciter Crossover


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High Pressure/Intermediate Pressure Turbine

HP

Inlet

Valves

IP


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Low Pressure Turbine


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What the steam does as it passes through the turbine


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Nozzle block for the Impulse Stage


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Reaction Stage


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Turbine Blades are Airfoils


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Distribution of deposits on turbines


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Increased stress on larger blades in turbine


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GE STAG Non-Reheat Steam Turbine


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Overview of turbine generator system
http://upload.wikimedia.org/wikipedia/commons/6/6b/Turbine_generator_systems1.png


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Generators

Cooling is integral to generators

All have large lube-oil cooling

systems

Rotor / stator will have coolingSmall-Med ( 600 MW) Water cooled

stator, H2cooled rotor


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Generators

Stator

Water cooled

Water is on a separate systemwith condensate make-up, anddemin polisher on them.Separate closed loop.

G


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Generators

Generator rotor arrives as a single forging that ismachined mechanically.

Rotor winding of hard copper.

Rotor fans pump H2gas to cool rotor and stator

H2selected due to high thermal conductivity, lowwind resistance

H2ducted through hydrogen coolers for heat transferto cooling water.


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Copper Corrosion vs. pH and O2Oxygen at 200300 ppbmax corrosion for Cu.

8.0

7.0


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Two types of Stator Cooling

High Oxygen Stator Cooling System Saturated water with dissolved oxygen at +2 PPM.

Supposition that high oxygen on pure copper form Cupric Oxide(Cu0) film that is stable.

Low Oxygen Stator Cooling System Oxygen levels below 50 ppb.

Supposition that copper does not react with water with the absenceof oxygen


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Low Oxygen Stator Cooling cont.

The upper limit is set by the corrosion rate that the watercleansing system can handle.

The lower limit is set to the level where copper will notdeposit on any insulating surface in the water circuit such

as hose.

This is to avoid electrical tracking path to ground. It hasbetter heat transfer properties at copper/water interface anda lower copper ion release rate.


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High Oxygen Stator Cooling System


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Low Oxygen Stator Cooling System


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Stator Cooling General Information

The Stator Cooling Water (SCW) system is used

to provide a source of de-mineralize water togenerator winding for direct cooling of statorwinding and associated component.

Strainer are to remove debris in the 20 to 50

microns or large and filter for 3 micron.

The content of copper and iron in the SCW isnormally less than 20 ppb. High concentration ofeither could cause conductivity problem.


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Stator Cooling General Information

Many de-ionizing system use the mixed bed type,employing both a strongly acidic cation resin and a stronglybasic anion resin.

When no leaks are present in the system, hydrogen contentis minimum. High hydrogen content into SCW can causegas locking and conductivity problem (same as cationconductivity).


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Treatment Options

NOTE: These are set by the manufactureActually only 3 recommended programs


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Low Oxygen Neutral pH

Low-O2, neutral pH option.A thin layer of passivecuprous oxide forms in a lowdissolved oxygen and neutralpH regime. This treatment option is found in about 50% ofthe stator cooling systems in the power industry.

The water is fully oxygenated when the system is first filled.As the water circulates, it reacts with the copper in thesystem, the oxygen is consumed, and the dissolved oxygengradually approaches zero.

The dissolved oxygen is likely to remain at


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The dissolved oxygen is likely to remain at


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Air ingress can also cause significant increases in

the systems dissolved oxygen. Putting a nitrogencap on the stator cooling water head tank canminimize air in-leakage.

Carbon dioxide can enter the system via the

makeup water or along with the air.

As carbon dioxide is absorbed into the water, itdrops the pH to acidic levels, increasing thecorrosion rate of copper.


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Carbon dioxide can form bicarbonate and carbonate in thewater and exhaust the mixed bed polisher.

If the polisher is not changed when it is exhausted, thereleased carbonate can form insoluble copper carbonate in

the stator.


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Low O2high pH

Increasing the pH of the stator water to 89

significantly reduces the corrosive responseduring oxygen transitions.

The most direct method for increasing pH is to addcontrolled amounts of sodium hydroxide to the

water.

Initially, the sodium will be exchanged withhydrogen on the cation resin of the mixed bedpolisher, neutralizing the caustic.


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Low O2high pH

Raising the pH also makes it easier to measure

the pH of water in the system.

During shutdown, and particularly during a majorturbine outage, stator water can becomeoxygenated.

In a number of cases, deterioration of the statorcooling system occurred shortly after the unitcame back online from an extended outage.


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High O2Neutral pH

The other treatment alternative is to maintain a

highdissolved oxygen level in the cooling waterat all times.

It is estimated that 40% of water-based statorcooling systems operate with highdissolved

oxygen and neutral water chemistry.

In this treatment regime, CuO is formed on thecopper.

It will tightly adhere to the surface and create apassive layer on the metal.


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High O2Neutral pH

This layer tends to be thicker than the Cu2O

formed under low-oxygen conditions.

Because the dissolved oxygen will be depleted byits reaction with copper, at least initially, you mayneed to add air to the system to maintain sufficient

dissolved oxygen in the system.

Hi h O N t l H


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High O2Neutral pH

This chemical treatment is impervious to additionsof dissolved oxygen in the feedwater when it isoperating continuously under high (>2 ppm) levelsof dissolved oxygen. However, it may still besusceptible to low-pH corrosion from carbondioxide and carbonates if these are not removedby the mixed bed polisher.

If there is a hydrogen leak into the stator coolingwater system, the hydrogen can replace thedissolved oxygen and create lowdissolvedoxygen transients in the system, causing oxides tobe released.


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Monitoring SCW

Monitoring the health of stator water systems is

more about looking at a variety of relatedtemperatures and pressures than collecting grabsamples and running them for pH or dissolvedoxygen.

Water temperature is one example: An increase instator cooling water temperature puts the coolingwater system at higher risk for plugging.

Monitoring the makeup water usage in a statorcooling system is also important.


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Generator Exciter

The Exciter controls the magnetic field in the generator system

and controls the output voltage and/or current.

The Exciter is normally air cooled, with an air to water heat

exchanger. (exciter air cooler)


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Turbine / Generator Systems

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D. Turbines and Generators