Steam Attemperation Valve and Desuperheater Driven Problems on HRSG's
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Transcript of Steam Attemperation Valve and Desuperheater Driven Problems on HRSG's
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28 March 2005
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Attemperation Frustrations
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Steam Attemperation Valve and Desuperheater Driven Problems on HRSGs
Main and Reheat Steam Attemperation
Turbine Bypass Operation
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Interstage Steam Attemperation Process is controlled by inlet
steam temperature to turbine Pro
Decreases potential for moisture content at turbine inlet
Decreases temperature in superheat/reheat tubing
Con System response is slow
and can cause cycling swings
Water carryover into the tubing possible
Thermal cycling of tubing
Control temp
Control temp
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Problems on Main and Reheat Steam Attemperation
Superheat and reheat tube failures Main/Reheat steam header failures Poor spray water control and thermal cycling Failure of spray probe components Plugging and blockage on spray valve and nozzle Leaking spray valves Steam leakage from flanged connections on main
steam line Broken liners
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Steam header and tube failures
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Increased superheat tube failures with years of operation
Downloaded from APTECH web site
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Unit age, years
Cum
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Fundamentals of steam attemperation The smaller the water droplets, the greater the
surface area of the liquid and the faster the droplets evaporate.
Spray nozzles generate a distribution of droplets with different sizes (Sauter number)
Both primary and secondary atomization break water droplets apart
Aerodynamic forces exceed surface tension forces break apart water droplets (Weber number)
Distribution of the water into the steam Time for steam to evaporate
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Fundamentals of steam attemperationSpray basic atomisation shape / Formen des Strahlzerfalls1. Rayleigh Regime
v water (Velocity)v amb
v water - v amb 0 Breakup way out of jet outlet Capillary + Viscosity Forces are
dominant Aerodynamic Resistance is small Droplets larger than jet diameter
2. First Wind-induced Regime
v water - v amb 2 m/s Breakup closer to the jet outlet due to higher relative motion, gives
additional aerodynamic Resistance Droplets are equal to jet diameter
Spray basic atomisation shape / Formen des Strahlzerfalls1. Rayleigh Regime
v water (Velocity)v amb
v water - v amb 0 Breakup way out of jet outlet Capillary + Viscosity Forces are
dominant Aerodynamic Resistance is small Droplets larger than jet diameter
1. Rayleigh Regime
v water (Velocity)v ambv water (Velocity)v amb
v water - v amb 0 Breakup way out of jet outlet Capillary + Viscosity Forces are
dominant Aerodynamic Resistance is small Droplets larger than jet diameter
2. First Wind-induced Regime
v water - v amb 2 m/s Breakup closer to the jet outlet due to higher relative motion, gives
additional aerodynamic Resistance Droplets are equal to jet diameter
2. First Wind-induced Regime
v water - v amb 2 m/s Breakup closer to the jet outlet due to higher relative motion, gives
additional aerodynamic Resistance Droplets are equal to jet diameter
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Fundamentals of steam attemperation3. Flapping Jet 4. Spray Jet
v water - v amb 10 m/s Breakup closer to the jet outlet Increasing of relative velocity Superposition of "O-mode"(dilatation)
and "1st-mode"(sinusoidal flapping)gives Instabilities
Droplets smaller than jet diameter
v water - v amb 50 m/s Breakup near to the jet outlet High relative velocity Increasing of shortwaving disturbance on
the surface due to the aerodynamic forces Droplets very much smaller than jet
diameter
3. Flapping Jet 4. Spray Jet
v water - v amb 10 m/s Breakup closer to the jet outlet Increasing of relative velocity Superposition of "O-mode"(dilatation)
and "1st-mode"(sinusoidal flapping)gives Instabilities
Droplets smaller than jet diameter
v water - v amb 50 m/s Breakup near to the jet outlet High relative velocity Increasing of shortwaving disturbance on
the surface due to the aerodynamic forces Droplets very much smaller than jet
diameter
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Water droplet size distribution by different nozzle designs
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19 35 63 114 205 370 668 1207Droplet diameter, microns
% V
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FC 526.07
Spray distribution into the steam
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Evaporation of water in steam
Finite amounts of time are required to heat the water and boil the water off
As the water evaporates, steam temperature decreases, reducing driving forces
The largest water droplets will be the last to evaporate, and an exponential increase in evaporation time can occur
Steam/Water Temperature Gradient vs Time
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0 2 4 6 8 10 12 14 16
Time
Deg
. F Steam TempWater Temp
Initial heating Boiling
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Traditional venturi nozzle
Fixed orifice nozzle
Venturi tube for mixing and thermal sleeve protection
Used on base load drum boiler and supercritical units
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Variable Area ProbeStyle Desuperheaters
Typically flanged to the main steam piping.
Multiple nozzles are exposed as the valve plug opens
Spray water nozzles and valve are immersed in the steam line
Nozzles face downstream and inject water with the steam flow
Most common desuperheater in CCPP steam attemperation
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Spray Ring Desuperheaters Normally applied in
large mass flow systems
Individual streams break into small droplets
Liners are often used when steam is attemperated close to saturation.
Fixed orifice, rangeabiltiy is limited to between 3:1 and 5:1
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Spray Nozzle Desuperheaters Applied in large and small
mass flow systems Liners are often used because
of directed water jets. Nozzles can be designed to be
serviceable. Nozzle can take higher
pressure drops than spray ring.
Fixed orifice, rangeabiltiy is limited to between 5:1 and 15:1
DS STEAM
H20
H20
SHSTEAM
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Spring Loaded Spray Nozzle
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Spring Loaded Spray Nozzles
Spring loading increases the minimum discharge pressure before injecting water into the steam
Provides a higher water velocity discharge into the steam flow
Provide a very thin sheet injection at low flow rates Can be provided in multiple point entry injection or
single point middle injection Places the critical control valve outside the steam
flow environment.
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Nozzle Spray Patterns, High Speed Photography
Spray nozzle Whirl nozzle Spring loaded nozzle
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Problems seen on Spray Control Probes
Lack of throttling control, stair step flow response
Cracked extension pipes Missing and damaged nozzles Fracture of tack weld and loosening of
lower housing Wear on the valve trim Sticking/galling with trash sensitivity Leakage in the closed position Damaged upstream Spray water
Block Valves
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Problems seen in the field on Spray Probes
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HP and HRH Attemperator Steam Solutions
Separate spray water control and spray water injection functions Location is the Key to low thermal stresses Simplify components in high temperature transition
zones. Use high temperature components in high thermal
shock zones. Provide thermal isolation for spray water headers to
steam headers.
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Spray Nozzle Design
Thermally isolated spray water housing and header
Spring loaded spray nozzles for fine atomization
High temperature spring and nozzle materials
No welding to retain threaded components.
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Cure for poor atomization, desuperheater design
Variable orifice nozzle for best primary atomization
Increase steam velocity with liner to increase velocity at water injection and protect steam piping
Spray nozzles create back pressure to spray water valve
Over-travel nozzles for passing contamination
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Multiple Spring Loaded Nozzle Assembly
Multi-nozzle design Variable geometry spring
loaded nozzles High capacity Nozzle rangeability 50:1
or more. System rangeability
limited water control valve and steam flows
Used for superheater & reheater applications, turbine bypass, aux. steam applications
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Direct Probe Replacement
Remove the water control valve from the high temp. steam
Multiple spring loaded nozzles
No piping modifications required
Multistage characterized water control valve
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Superheat water spray valve operating pressure differentials
2 x 500 MW units Fixed speed
motor driven feedpump
Operation in 1x1 and 2x1 results in variable steam turbine pressure with fixed water pressures 0
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10 Days of Data (10 min intervals)
Pres
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[psi
g]
HP FeedwaterPressure
HP SteamPressure
~1500 psid
~700 psid
~2000 psid
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Valve Solutions to solve the spray water control problem
High Rangeability Multistage design
Highly characterized for spray applications, generally modified equal percentage
Provide a tight shutoff design, FCI-70 Class V.
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Velocity control customized characterization
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% Flow
% Stroke
Modified Linear
Linear
Modified Equal %
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Clogging and Filtration Recommendations
Temporary filters may be needed during plant commissioning
100 micron filtration recommended on many designs
Separate strainers for at least several months of commissioning
Use spray water valve trims as back-up protection devices
Partial clogging of nozzles can cause directed water flow at steam pipe walls.
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Water injection directly downstream of elbows or valves Non-uniform
steam velocity profile generated
Water distribution is not uniform
Water is carried into pipe wall and drops out
Pipe cracking can occur
Overall steam velocity an issue
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Improper Piping Design
Increase distance from the elbow to the water injection point
Add flow straighteners to the piping to obtain a more uniform flow regime
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Installation recommendations
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Drain issues
Improper draining causes: Water hammer Thermal shocks and gradients
Check location and size of drains Use the drains more aggressively during start-up Sizing drains for not only condensate formation but
also for leaking spray valves Drains cant be sized for open spray valves
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Turbine Bypass Valve ApplicationsCombined Cycle Power Plant
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HP to CRH bypass system and adverse affects on HRSG life HP steam valve leakage and overheating of CRH
headers Operation of the water valve without steam flow Isolation of the bypass steam valve Continual operation of the bypass system
HP spray water valve leakage and water collection in the downstream pipe. Water carryover into the HRH tubing Water-hammer
Poor HP steam bypass control results in pressure fluctuations causing drum level changes and alarms
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Poor HRH steam bypass control results in pressure fluctuations causing drum level changes and alarms
Poor HRH bypass operation can cause unit trips Operation/leakage of the water valve without steam
flow can cause water-hammer Operation near saturation during desuperheating can
cause loss of spray water control High condenser enthalpy can cause unit trips system
HRH to condenser bypass system and affect on HRSG life
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Turbine Bypass System issues
Use high integrity turbine bypass systems Use same high integrity desuperheating systems as
used on HP and HRH steam attemperation Use proper sequencing of steam and water valves
to prevent water hammer and thermal shock issues Use feedforward control logic for both unit trips and
for spray water desuperheating control Proper design of piping systems for distances for
desuperheating Proper draining of the turbine bypass system
downstream of the bypass valve
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High Integrity Bypass Systems
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Desuperheater with flow conditioning Desuperheater with flow conditioning and pressure reductionand pressure reduction
Variable area Nozzle Desuperheating after
Pressure Reduction Nozzles configured to
give even cross section distribution
Water injection at high velocity and turbulence points in steam flow
Steam flow acts as thermal Shield
Quick Change Nozzle Assy
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Sequencing of Steam and Spray valves Water valves need to be interlocked to prevent
opening if the steam valve is closed. Opening
Open steam valve Once steam valve closed limit switch has cleared,
signal water valve to open Review systems for minimum recommended flow
Closing Bring steam valve to recommended minimum lift and
hold Close water valve Close steam valve
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Water Injection Control on Bypass Systems
Temperature Control Temperature measurement and feedback controls
amount of water injected. Feedforward Control
P1/T1 and Valve Position Control for estimating steam flow
P1/T1/P2 measurement for estimation of steam flow using the dump tube as a flow meter
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Temperature Control
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Enthalpy Control Using Backpressure for Flow Measurement
SprayWaterValve
Trh
Fw =f(p)Pdump
SteamControlValve
HRH Trh = Steam Inlet Temp.Tsw = Spray Water Inlet Temp.Pdump = Dump Tube Press.Fw = Water Flow
Tsw
Fw = f(Pdump, Tw, Trh)
Prh
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Feed Forward Enthalpy Control Using Valve Position for Flow Estimation
hrh=f(p,T)
Fw=Fst
SprayWaterValve
Trh
hdes-hw
hrh-hdes
Fw =f(p)Yst
Fst=f(Yst,p,T)
pdump
Prh
SteamControlValve
HRH T = TemperatureP = PressureF = Flowh = EnthalpyYst = Valve Stroke
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Temperature vs Enthalpy Control
Longer straight distances and distances to condenser on temperature control
More potential for oscillation with temperature control
Lower steam discharge enthalpy to condenser with enthalpy control
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Desuperheating Distances, Recommendations (rules of thumb)
Distance to first elbow 0.1 seconds at maximum steam velocity for most
desuperheaters 0.05 seconds for steam assist
Distance to Pipe Spec Change 16 ft (5 m)
Distance to Temperature Sensor 0.2 seconds at maximum steam velocity for < 15% water
injection 0.3 seconds at maximum steam velocity for > 15% water
injection. Temperature set point 18 F (10 C) above saturation
The above values can change based on steam SH, water steam ratios, required accuracy, and desuperheater type.
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HRSG and Plant Upgrades Most HRSG and CCPP plants were designed for
base load operation Most CCPP are in high cycle operation due to the
high cost of fuel and cycling load demands Replacement and upgrades of attemperation
hardware can reduce thermal cycling and other problems seen at plants
Separation of the water control function from high temperature steam will improve service life
Steam attemperation and turbine bypass systems are integrated units that need to be designed to fit specific piping systems
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Publications/Helpful References
Attemperators, Combined Cycle Journal, 1Q 2005 Advances in Desuperheating Technology for
Reliable Performance of Combined Cycle Power Plants, Proceedings of PWR2005, PWR-2005-50108
Polished Performance, Refined HRSG Designs and O&M Practices Boost Plant Performance, Power Engineering, February 2005.
Avoid Desuperheater Problems with Quality Equipment, Proper Installation, Tight Process Control, Combined Cycle Journal, Fall 2004