Steam Attemperation Valve and Desuperheater Driven Problems on HRSG's

28 March 2005

1

Attemperation Frustrations

3/28/2005 2003. CCI Control Components Inc. CCI is the Valve Doctor . All rights reserved. P102 2

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


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


Increased superheat tube failures with years of operation

Downloaded from APTECH web site

0

20

40

60

80

0 2 4 6 8 10

Unit age, years

Cum

ulat

ive

failu

res

of S

H tu

bes

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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


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



1. Rayleigh Regime

v water (Velocity)v ambv water (Velocity)v amb









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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


Water droplet size distribution by different nozzle designs

0

5

10

15

19 35 63 114 205 370 668 1207Droplet diameter, microns

% V

olum

e

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FC 526.07

Spray distribution into the steam


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

300

400

500

600

700

800

900

1000

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


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


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


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


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


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.


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


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

500

1000

1500

2000

2500

3000

3500

10 Days of Data (10 min intervals)

Pres

sure

[psi

g]

HP FeedwaterPressure

HP SteamPressure

~1500 psid

~700 psid

~2000 psid


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

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

% Flow

% Stroke

Modified Linear

Linear

Modified Equal %


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


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


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


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


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


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


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


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


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.


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

Steam Attemperation Valve and Desuperheater Driven Problems on HRSG's

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