Impact of Daily Cycling in
Combined Cycle Power Plant
Key Components in Steam Cycle
1
Agenda
• CCPP & overview of effects of frequently cycling on Key
Control Components of Steam cycle.
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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
3/3/2014
© 2013. CCI - Control Components Inc. All rights reserved.
3
Outlook for CCPP’s
New requirements
Security of
Supply
Security of
Supply
Focus on
erection,
operation and
maintenance of
power plants
Economic
Efficiency
Environmental
Compatibility
Environmental
Compatibility
Load
Flexibilty
Economic
Efficiency >60%
2010
Higher efficiency driving higher firing
temperatures & steam temperatures
Page 4
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
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
• 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
How much ∆P for single stage?
30 bar!
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High rangeability required for
– Start-up (HIGH ΔP)
– Full load (low ΔP)
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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:
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Solution = Single Combined Valve
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Multi Stage throughout stroke
Long Stroke
Characterised
Piston Actuator
One Valve, long stroke multi stage
characterization
Turbine Bypass Challenges
HRSG
Fuel
Gas turbine
Air
Steam for
NOx control
or STIG
HP
IP
LP water
IP water
HP water
LP
VS-BT
15
VLB-BTC
VST-SE
Requirements
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Critical for Quick Start-up
Potential for wet
steam/condensate
Higher ΔP’s
Higher Steam Flows
Growing use of ACC’s
Low Noise
Thermal Shock!!!
3/3/2014
Dual Boiler
Feedwater
pumps
4
VS-BT VS-BT
2
1
VDA-4
16
5
3
VS-BT
18
VDA-4
VS-BT
6
17
8
7
VLB-BTC
10
9
11
VDA-4
840
840
Solution
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840
13
LLP-stop
14
LLP
12
840
840
Condenser
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
8
Proprietary and Confidential
The target application “Attemperation”
• Interstage Attemperation normally
standard.
• Final Stage optional allows faster cold
start
Heat Recovery Steam Generator
(HRSG)
RH
SH
(CCPP)
Final Stage
3/3/2014
Proprietary and Confidential
9
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
• 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
• Wetted temperature sensors
• Damage to downstream
superheaters
• Damage to downstream pipes
• Forced shut-downs
3/3/2014
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Considerations for good temp control
• Distribution of spray water over cross
sectional flow.
• Good installation
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Upstream straight piping
Downstream straight piping
Thermal liner
Positioning temp sensor
© 2003. CCI –
Control
Components Inc.
All rights
reserved. P602.
3/3/2014
13
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)
• 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
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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.
3/3/2014
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Effect of nozzle direction
Nozzles spray in cross flow
Probe style spray with flow
Secondary atomizing velocity
=
Dynamic force
Surface tension
Primary = Penetration
Secondary = superior Atomization
Reliant on
Primary only
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
548°C
1018°F
3/3/2014
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
Solution
+
3/3/2014
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Attemperation giving PP Flexibility
1.
2.
3.
Most Plants have Interstage
Few utilize final stage only
More plant flexibility is using
Interstage and Final Stage
RH
SH
3/3/2014
Proprietary and Confidential
19
Attemperation giving PP Flexibility
1.
2.
3.
Most Plants have Interstage
Few utilize final stage only
More plant flexibility is using
Interstage and Final Stage
Final
Stage
3/3/2014
Proprietary and Confidential
20
Attemperation giving PP Flexibility
1.
2.
3.
Most Plants have Interstage
Few utilize final stage only
More plant flexibility is using
Interstage and Final Stage
RH
SH
Final
Stage
3/3/2014
Proprietary and Confidential
21
Considerations for thermal liner
Improved performance through steam flow profiling
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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
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.
3/3/2014
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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
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.
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