nooter eriksen presentation [2]
TRANSCRIPT
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Paul Gremaudand
Shaun Siegel
American Electric Power BRO Forum July 2010
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Nooter/Eriksen HRSGs at American Electric Power FacilitiesOologah - 2 Units (1999)
GE 7 FA turbines(3) pressure levels with supplemental firing
Waterford - 3 Units (2000)GE 7 FA turbines(3) pressure levels with supplemental firing
Stall - 2 Units (2007) – just began commercial operation (7/2010)Siemens 501 FD2 turbines
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Nooter/Eriksen HRSGs behind Siemens 501Fs and GE 7 FAs
Nooter/Eriksen has supplied HRSGs behind Siemens 501F machines on 41 projects for a total of 79 HRSGs.
Nooter/Eriksen HRSGs are behind 42% of the Siemens 501F machines
2010 AEP BRO Forum
Nooter/Eriksen has supplied HRSGs behind GE 7FA machines on 51 projects for a total of 115 HRSGs.
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Topics to be Addressed
1. General Overview of the Thermal Design of a HRSG
2. Issues/Concerns With HRSGs
2010 AEP BRO Forum
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Part 1: Thermal Design of HRSGs – An Overview
• HRSG materials• Tube finning• Heating surface configurations• Economizer design• Vapor lock• Cold end corrosion• Boiler pressure turndown
2010 AEP BRO Forum
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Tube Materials
• SA-213 T91 – HP Superheaters and Reheaters where tube wall temperature exceeds T23/T22/T11 limits
• SA-213 T23 – HP Superheaters with high MAWP’s and tube wall temperatures with T22/T11 limits
• SA-213 T22/T11 – HP Superheater and Reheater when tube wall temperatures above CS limits. LP evaporator for corrosion resistance.
2010 AEP BRO Forum
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Tube Materials Continued
• Carbon Steel – Evaporators and economizers where design temperatures are within carbon steel limits
• 2205 SS – LP economizer, preheaters where water is non-deareated.
2010 AEP BRO Forum
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Finning Configurations• Fin Density
2.5 – 7.0 fpi• Fin Height
0.25” – 1.0”• Fin Thickness
0.039” – 0.059”• Fin Segment
0.1772” for stainless and 0.15625” for CS
2010 AEP BRO Forum
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Drum Separator Design
2010 AEP BRO Forum
• Primary Separator- Sized for max sub-cooled flow
• Secondary Separator- Sized for max ρV2
• Benefits over cyclone type-Wider range of operation
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Water Side Characteristics• Pinch
– Temperature difference between fluid and exhaust gas. Recommend minimum of 15°F
• Approach– Temperature difference between saturation
temperature and temperature of incoming water. Design as small as 0°F
• Coil Distribution– Provide uniform heat transfer across row– Increases pressure of evaporator or pump
2010 AEP BRO Forum
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HRSG Heating Surface
2010 AEP BRO Forum
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Heating Surface Configurations
Half Circuit Single Circuit
2010 AEP BRO Forum
Double Circuit
Up Flow Down Flow
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HRSG Heating Surface
2010 AEP BRO Forum
Half Circuit
Double Circuit
Single Circuit
Up Flow
Stall Module #2 Waterford Module #4
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Heating Surface Configurations
• Half Circuit– Economizers and IP/LP Superheaters
• Single Circuit– Economizers and Superheaters
• Double Circuit– HP Superheaters and Reheaters
2010 AEP BRO Forum
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Competitor 1 Economizer Design
2010 AEP BRO Forum
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Competitor 1 Economizer Design• Design disadvantages
– Requires vent at every top header– Converge/diverge at every header– Possible vapor lock in down flow tubes– Potential for corrosion fatigue
2010 AEP BRO Forum
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Competitor 2 Economizer Design
2010 AEP BRO Forum
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Competitor 2 Economizer Design• Design disadvantages
– Requires vent at every top header– Converge/diverge at every lower header– Possible air entrapment at end of headers– Potential for corrosion fatigue
2010 AEP BRO Forum
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N/E Typical Economizer Design
2010 AEP BRO Forum
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N/E Economizer Design• Design advantages
– Does not require vents– Converge/diverge at inlet and outlet headers– Accommodates differential movement– Accommodates steaming– Operation, maintenance and repair.
• Design disadvantage– Vapor lock
2010 AEP BRO Forum
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Vapor Lock
• Vapor Lock – When tube flow is not able to clear the return bend.
• Variables– Heating surface arrangements– Fluid temperature and pressure– Flow through coil
• Most likely to occur in low pressure economizers
2010 AEP BRO Forum
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Vapor Lock Cont.
2010 AEP BRO Forum
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Cold End Corrosion
• When exhaust gas temperature goes below the sulfur dew point or contacts a surface colder than the dew point
• Control the inlet feedwater temperature above dew point
• Three methods of prevention– Preheat evaporator (patent pending) – Recirculation– External heat exchanger
2010 AEP BRO Forum
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Cold End Corrosion Prevention
2010 AEP BRO Forum
Recirculation
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Cold End Corrosion Prevention
2010 AEP BRO Forum
External heat exchanger
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Boiler Pressure Turndown
• Boiler performance envelope is based on the original design
• Minimum of 50% of HP base load pressure • Potential problems with turndown
– Velocity in pipes– Velocity in evaporators– Stability in evaporators
2010 AEP BRO Forum
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Part 2: Issues/Concerns With HRSGs
• HRSG Inlet Duct Liner Issues
• Distribution Grid Issues
• Condensate Management – Desuperheater Drain Pots & Coil Drains
• Improvements to Address Cycling of HRSGs
• Flow Accelerated Corrosion
• Backpressure Issues/Economizer Surface Reduction
2010 AEP BRO Forum
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Inlet Duct Floor and Sidewall Details Floor is typically sloped 10-18 degrees
Floor and sidewall liners have additional stiffening
2010 AEP BRO Forum
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Inlet Duct Floor and Sidewall Details Inlet Duct Floor and sidewalls are stiffened up to the distribution grid or the first coil.
2010 AEP BRO Forum
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Inlet Duct Liner Damage
Deformed Liner Plate
Catastrophic damage to sidewall liner
Catastrophic damage to floor liner
2010 AEP BRO Forum
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Inlet Duct Liner SolutionsWhat can be done when you have inlet duct liner damage?
Add stiffeners on the casing sidewall or floor
Add intermediate liner pins
Increase the thickness of the liner plate
Install additional batten channels
Additional liner stiffening – backup angles
2010 AEP BRO Forum
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Inlet Duct Distribution GridWhat is it?
Stiffened, Flat Plate with 40% to 60% open area
Usually made from 300 series stainless steel
Located in the inlet duct
What is the purpose of the distribution grid?
Used to get proper flow distribution in the HRSG
Typically required for units with supplemental firing
2010 AEP BRO Forum
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Typical Distribution Grid
2010 AEP BRO Forum
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Typical Distribution Grid Components
Gas Flow
2010 AEP BRO Forum
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Distribution Grid Damage
Pin Weld Failure at Sidewall Support pedestal failure at weld to casing
Local deformation at support plate Shear block weld failure2010 AEP BRO Forum
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Some Potential Grid Modifications
(Incorporated into New Designs)Modifications can and have been made to address many grid failure issues.
Addition/modification of the floor restraintsAddition/modification of the sidewall restraintsAddition of a gusset at the sidewall restraint plateMove location of sidewall restraint – remove the offsetAddition of stiffener bars on grid fabric
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Reheater and HP Superheater
Condensate ManagementModifications have been made to address removal of condensate during startup, operation, and shutdown.
ASME Section I – 2004 edition 2006 addenda1. Includes new section PHRSG2. Contains mandatory requirements for DSHR drain pots3. Contains mandatory requirements for RH & SH
drains
If condensate is not removed…… bad things happen!
2010 AEP BRO Forum
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Condensate Management - DrainingBuckled tubes caused by uneven condensate clearing in coils or piping
2010 AEP BRO Forum
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Condensate Management
Water
No steam flow, hot tubes
Steam cooled tubes
2010 AEP BRO Forum
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Condensate ManagementNow what actually happens to cause this?
• Steam flow is low or prior to initiation of steam flow• Water filling lower header and tubes blocks steam• With low steam flow, only a few tubes clear• Cleared tubes are steam cooled • High expansion differences between hot and cool tubes• Cooler tubes go into tension and potentially yield• At cool down, tubes go into compression and buckle• If there are bends, very high bending stresses created• Normally no failure if the tubes are straight
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Condensate ManagementAvoid condensate moving between pressure levels or from other components
• Multiple pressure levels connected via blowdown or flash tank or other collection device
• It is possible to pressurize the collection device forcing water back into lower pressure components
• It is also possible to move condensate due to pressure differences between coils and external piping
• Recommend incorporating details to ensure adequate condensate removal.
2010 AEP BRO Forum
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Condensate Management
Critical Issue - Draining of all piping low points
1. Piping normally at bottom of unit
2. Cross over piping between HP Superheaters or Reheaters
3. Inlet piping from interface on Cold Reheat lines
2010 AEP BRO Forum
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Condensate ManagementTest condensate detection and removal system in Reheater and HP Superheater drains
2010 AEP BRO Forum
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Reheater and HP Superheater Desuperheaters
Why are desuperheaters used?
Protect components of the steam turbine
Nooter/Eriksen typically locates a desuperheater in the piping between the HP Superheater and Reheater coils
Potential Issues1. Un-evaporated water - overspray2. Leakage – valve failure3. Inconsistent operation – spraying at low steam flow
2010 AEP BRO Forum
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Desuperheaters
Damage from desuperheater quenching
2010 AEP BRO Forum
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Reheater and HP Superheater DesuperheatersHow have failures been addressed?
•New desuperheaters are not integral type•Control valve is separate from the spray nozzle•Experience with this type of desuperheater has been very positive
2010 AEP BRO Forum
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Reheater and HP Superheater Desuperheater Modifications
Drain pots before the DSHR, after the DSHR or both
Drain pot
Line is Sloped
Desuperheater
2010 AEP BRO Forum
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Reheater Bypass SystemEliminates the need for a desuperheater
Improvement in reliability
Improves efficiency since you are not spraying water into the steam.
Does not affect steam purity
CRHHRH
Reheater Bypass
2010 AEP BRO Forum
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Improvements to Address Cycling
1. Tube stubs at Tube/Header Connection
2. Spring Supported Coils
3. Internal coil flexibility
2010 AEP BRO Forum
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Optional Stub Tube to Header Connection
Better for cycling
Better weld NDE
2010 AEP BRO Forum
Minimizes header thickness, therefore lowers thermal stress
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Spring Support of Header
Fixed Header
Floating Header of Same Coil
2010 AEP BRO Forum
Outlet Header is Supported by Spring Supported Manifold Pipe
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Reheater and HP Superheater Coil Support Modifications
Spring cans being installed
2010 AEP BRO Forum
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Importance of Coil Flexibilities
• Two important aspects of coil flexibilities:
1. Row to row temperature differences (start up and normal operation)
2. Piping layouts
2010 AEP BRO Forum
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Row to Row Temperature Differences•SH/RH each row different temperature
•Different ways to absorb movements
1. Internal coil flexibilities
2. Allow parts to move freely
•Support SH & RH from spring supports
•Orders of magnitude lower stress
General Rule: Allowing parts to move freely is always better than relying on internal coil flexibilities
2010 AEP BRO Forum
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Flow Accelerated Corrosion (FAC)
2010 AEP BRO Forum
Primarily a water chemistry problem but Heat Recovery Steam Generator (HRSG) design and operation will influence the rate of metal loss.
Most FAC through wall failures have occurred in high velocity areas of low pressure evaporators.
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Flow Accelerated CorrosionMajor factors:
2010 AEP BRO Forum
•pH of water in LP system
•Presence of reducing agents
•Velocities in piping/tubing
•Temperature
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Flow Accelerated Corrosion (FAC)
2010 AEP BRO Forum
Solubility of magnetite is at a maximum in the normal operating conditions for the LP pressure level.
Turbulence increases the local velocities and therefore the mass transfer and dissolution of the magnetite layer.
Where Does it Occur and Why?
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Flow Accelerated Corrosion (FAC)
2010 AEP BRO Forum
Gas bypass causes higher temperatures and more steam production
Tubes that produce the most steam are most susceptible to FAC
Tube BundlePlan View
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Flow Accelerated Corrosion (FAC)
2010 AEP BRO Forum
Two-phase FAC in LP Evaporator
Single phase FAC in LP Evaporator
Potential location of FAC due to orifice
Bottom of LP Evaporator Top of LP Evaporator
Potential location of FAC
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Flow Accelerated CorrosionNooter/Eriksen Addresses this Potential Problem the following ways:
2010 AEP BRO Forum
•LP Evaporator is designed using 1 ¼ Cr steel
•Velocities in tubes are evaluated for all operational cases
•Minimize bends in risers and piping
•Orifice plates have been re-located
•Better guidance regarding water chemistry
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Flow Accelerated Corrosion (FAC)
2010 AEP BRO Forum
•Generally use basic water chemistry guidelines proposed by EPRI
•Oxygen scavengers are not recommended
•Keep LP drum water pH of 9.4 or greater
•Ammonia is the recommended alkalizing agent for LP evaporators
Nooter/Eriksen’s Current Philosophy on Water Chemistry
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HRSG Elevated Gas Side Pressure
Affect -Decreases efficiency of the combustion turbine and the HRSG
2010 AEP BRO Forum
Typical Causes -•Rust build up on coil fins of cycling units•Ammonia salt build up on coil fins in units with SCR system
Prevalent in the last (2) modules of HRSG due to temperature of the coils
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HRSG Elevated Gas Side Pressure
2010 AEP BRO Forum
Rust build-up Ammonia salt build-up
HP/IP Economizer
LP Evaporator
LP Superheater
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HRSG Elevated Gas Side Pressure
Limit Ammonia salt formation• Improve AIG design and tuning• Improve NH3 control• Add catalyst for lower slip design• Delay ammonia feed at startup
2010 AEP BRO Forum
Reduce rust build-up• Close damper for heat retention when offline
Minimize causes of back pressure increase
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HRSG Elevated Gas Side Pressure
2010 AEP BRO Forum
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HRSG Elevated Gas Side Pressure
2010 AEP BRO Forum
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HRSG Elevated Gas Side Pressure
2010 AEP BRO Forum
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HRSG Elevated Gas Side Pressure
What can be done to remove salts and rust?
•Traditional methods – dry ice blasting, high volume water flushing, air blowing (dry ice blasting has worked very well at several plants – but it is configuration and company dependent)
•Non traditional – coil vibration, sonic vibration, steam cleaning (similar system to power washing), sootblowers
2010 AEP BRO Forum
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HRSG Elevated Gas Side Pressure
Wash Coils To Remove Salts
Adding caustic to wash water may neutralize pH
Ammonia may be liberated
2010 AEP BRO Forum
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Removal of Heating Surface in an Economizer to Improve Gas Side Cleaning
(4) LP Economizer tube rows were removed to make an access lane. The LP Evaporator and LP Economizer can now be cleaned from both sides.
Original Layout After Modifications2010 AEP BRO Forum
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