Upload
doanxuyen
View
222
Download
3
Embed Size (px)
Citation preview
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
3/3/2014 © 2013. CCI - Control Components Inc. All rights reserved. 2
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
3/3/2014 © 2013. CCI - Control Components Inc. All rights reserved. 3
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
3/3/2014 © 2013. CCI - Control Components Inc. All rights reserved. 5
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
3/3/2014 © 2013. CCI - Control Components Inc. All rights reserved. 6
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!!!
3/3/2014Proprietary and Confidential
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
3/3/2014 Proprietary and Confidential 9
(CCPP)Final Stage
Attemperation Challenges
Fluid & thermodynamic
Mechanical Challenges: Thermal Stress!
3/3/2014 Proprietary and Confidential 10
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
3/3/2014 © 2013. CCI - Control Components Inc. All rights reserved. 11
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
3/3/2014 12
• 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
© 2003. CCI –
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
3/3/2014 © 2013. CCI - Control Components Inc. All rights reserved. 14
Atomization/Evaporation of spraywater
• Desuperheating - 3 stages for successful
evaporation of water into steam
– Primary atomization
– Secondary atomization
– Tertiary evaporation (time related)
3/3/2014 15
• 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
3/3/201417
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
3/3/201418
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
3/3/2014 Proprietary and Confidential 19
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
3/3/2014 22
• 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
3/3/2014 © 2003. CCI – Control Components Inc. All rights reserved. P602. 23