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Impact of Daily Cycling in
Combined Cycle Power PlantCombined Cycle Power Plant
Key Components in Steam Cycle
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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
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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
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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
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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
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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
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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.
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– Positioning temp sensor
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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.
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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
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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
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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
+
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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
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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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Final
Stage
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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
Final
Stage
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• 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.
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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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