impact of daily cycling in combined cycle power · pdf fileimpact of daily cycling in combined...

Impact of Daily Cycling in

Combined Cycle Power PlantCombined Cycle Power Plant

Key Components in Steam Cycle

1

Agenda

• CCPP & overview of effects of frequently cycling on Key

Control Components of Steam cycle.

– Power Market Trends pertaining to CCPP

– Turbine Bypass (TBS)

– Feedwater Regulation (FREG)

– Attemperation (main focus of presentation)

• Attemperation issues, Thermal Shock & temperature

control critical for plant performance and reliability

• Understanding the attemperation fluid and

thermodynamic process & review of a proven solution.

• Conclusions

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Power Market Trends

• More Renewables drives the need for CCPP

flexibility

• CCPP’s have to change load faster

• Daily cycling and even double daily cycling.

• Faster Start-up times Required• Faster Start-up times Required

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Outlook for CCPP’s

Focus onerection,

operation andmaintenance of

Security of

Supply

New requirements

Security of

Supply

Environmental

Page 4

maintenance ofpower plants

Environmental

Compatibility

Economic

Efficiency Economic

Efficiency >60%

Load

Flexibilty

Environmental

Compatibility

2010Higher efficiency driving higher firing

temperatures & steam temperatures

Key Components in Steam Cycle

• Maintain and control flow of Steam Cycle:

Feedwater Regulator Valve (Drum Level Control

Valve).

• Critical for start-up, shut down and load rejection.

Turbine Bypass Valves are Critical for GT operation Turbine Bypass Valves are Critical for GT operation

and maintaining Steam Cycle pressure

• Control of steam temperature to Steam Turbine:

Interstage and Final Stage Attemperators

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How does this effect Steam Cycle

• Higher Efficiency in Gas Turbines lead to Higher Exhaust Gas Temperatures

• Steam Cycle is integral in operation, but follows GT!

• Leads to higher steam temperatures which increases Steam Cycle efficiency

Part Load Steam Temperatures increase• Part Load Steam Temperatures increase

• Water Control Valves frequently exposed to high ΔP

• Demands on HRSG’s:

– Frequent Starts Daily

– Ramping up load in Minutes (Thermal Stress)

– Once Thru/Smaller Drums etc

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Feedwater control Challenges

• High rangeability required for– Start-up (HIGH ΔP)

– Full load (low ΔP)• Most HRSG have fixed speed pump.• More frequent cycling• Increasing pressure drop• Instability problems when switching

between main & startup valves• Manual operation of startup valve• Solution:

– One Valve, long stroke multi stage

How much ∆P for single stage?

30 bar!

– One Valve, long stroke multi stage characterization

• Solution = Single Combined Valve– Multi Stage throughout stroke

– Long Stroke

– Characterised

– Piston Actuator

Turbine Bypass Challenges

Requirements

• Critical for Quick Start-up

• Potential for wet steam/condensate

HRSG

Air

FuelGas turbine

Steam forNOx controlor STIG

LP waterHP LPIP

15

16

18

4 2

3

1

5

IP waterHP water

Dual BoilerFeedwaterpumps

VST-SE

VS-BT

VDA-4

VLB-BTCVS-BT VS-BT

VDA-4

VS-BT

• Potential for wet steam/condensate

• Higher ΔP’s

• Higher Steam Flows

• Growing use of ACC’s

• Low Noise

• Thermal Shock!!!

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8

Condenser

617

9

12

13 14

87

10

VDA-4VS-BT

840

840

VDA-4

VLB-BTC

LLP

840840

11

LLP-stop

840

Solution

• Ensure supplier proven track record & CCPP experience

• Valve must modulate fast 3-5s

• Body and trim suited for High temp & thermal shock

• Angle Pattern and over the plug design

The target application “Attemperation”

Heat Recovery Steam Generator(HRSG)

• Interstage Attemperation normally

standard.

• Final Stage optional allows faster cold

start

SH

RH

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(CCPP)Final Stage

Attemperation Challenges

Fluid & thermodynamic

Mechanical Challenges: Thermal Stress!

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Function of Attemperator

• Control of Final Steam Temperature for Start up of

Turbine

• Control of final steam temperature during normal

operation

• Prevents overheating of downstream superheaters• Prevents overheating of downstream superheaters

• Used both Main and RH

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The Symptoms: Problems seen in the field

• Leakage!!

• Excessive Condensate Drainage

• Water hammer events

• Cracking Probes

• Poor control during Startup or

Shutdown

• Operating below Set Point

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• Operating below Set Point

• Wetted temperature sensors

• Damage to downstream

superheaters

• Damage to downstream pipes

• Forced shut-downs

Considerations for good temp control

• Distribution of spray water over cross sectional flow.

• Good installation – Upstream straight piping

– Downstream straight piping

– Thermal liner

– Positioning temp sensor

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Control

Components Inc.

All rights

reserved. P602.

3/3/2014 13

– Positioning temp sensor

Importance of Atomization/Evaporation

• Spray Water must be evaporated

rapidly (measurement/damage limitation)

• Spray water needs to be controlled

over wide range of conditions (high

ΔP and high turndown)ΔP and high turndown)

• Piping distances between

superheater headers minimal

• Typical distances 10-18m (relates to

residence time 0.2-0.3s)

• Prevent forced shut-down

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Atomization/Evaporation of spraywater

• Desuperheating - 3 stages for successful

evaporation of water into steam

– Primary atomization

– Secondary atomization

– Tertiary evaporation (time related)

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• Primary –Mechanical Atomization

• Provides good atomization/spray pattern regardless of flow

• Incorporates swirl to maximize coverage

• Provides protection to spraywater valve

• Self cleaning with regard to debris

Relative velocity drives the breakup: the higher the better.

Effect of nozzle direction

Nozzles spray in cross flow Probe style spray with flow

Reliant on

Primary onlyPrimary = Penetration

Secondary = superior Atomization

tensionSurface

force Dynamic =

Secondary atomizing velocity

Traditional Probe Style in Cycling Duty

• Hot steam + cold water = thermal shock cracks for probes

• 400°C of temperature difference on a heavy wall and

mechanical components inside hot steam flow

• Additional stress components: Vibration (vortices)

Mechanical (full ∆P) & Bending Moment

• Control components in hot steam path

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548°C

1018°F 153°C

307°F

Traditional Probe Style in Cycling Duty

• Hot steam + cold water = thermal shock cracks for probes

• 400°C of temperature difference on a heavy wall and

mechanical components inside hot steam flow

• Additional stress components: Vibration (vortices)

Mechanical (full ∆P) & Bending Moment

• Control components in hot steam path

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Solution

+

Attemperation giving PP Flexibility

1. Most Plants have Interstage

2. Few utilize final stage only

3. More plant flexibility is using

Interstage and Final Stage

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SH

RH

Attemperation giving PP Flexibility

1. Most Plants have Interstage

2. Few utilize final stage only

3. More plant flexibility is using

Interstage and Final Stage

3/3/2014 Proprietary and Confidential 20

Final

Stage

Attemperation giving PP Flexibility

1. Most Plants have Interstage

2. Few utilize final stage only

3. More plant flexibility is using

Interstage and Final Stage

3/3/2014 Proprietary and Confidential 21

SH

RH

Final

Stage

• Protects steam pipe under stress from impingement of relatively cool water

droplets

• Reduce diameter to increase velocity (secondary atomization effected by v2)

• Reduces cross sectional area making coverage of 100% of steam easier

• Thermal shield assists with heat transfer & evaporation of water

• Assists with atomization of un-atomized water in the vortexes

Considerations for thermal liner

Improved performance through steam flow profiling

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• Assists with atomization of un-atomized water in the vortexes

• Steam is profiled at point of water injection, minimising risk of water impingement.

• 100% of the steam is used in atomisation/evaporation process (Traditional liners

are 85-90% Coverage) which becomes more important as you approach sat temp.

Conclusions

• Trend of Flexible/Cyclic CCPP’s is putting high demands on key steam cycle control equipment.

• Additional care should be taken in specification and selection of components. Reliability + Performance

• Consider careful Attemperation design concerning resistance to thermal shock. Reliability

• Need to attemperation design suited to providing small • Need to attemperation design suited to providing small average droplet size <125micron over all operating conditions. Performance and Reduced installed cost

• Attemperation Design should be such to ensure even coverage of atomized spraywater over cross section of steam flow. Reliability (HRSG Components) & Performance

• If Final Stage (Terminal) Attemperators are requirement then Fastest evaporation time required. Turbine water ingress! Performance

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