Download - Nooter Eriksen Presentation [2]

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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

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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

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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

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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.

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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.

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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

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Drum Separator Design

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• 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

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HRSG Heating Surface

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Heating Surface Configurations

Half Circuit Single Circuit

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Double Circuit

Up Flow Down Flow

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HRSG Heating Surface

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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

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Competitor 1 Economizer Design

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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

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Competitor 2 Economizer Design

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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

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N/E Typical Economizer Design

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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

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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

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Vapor Lock Cont.

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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

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Cold End Corrosion Prevention

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Recirculation

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Cold End Corrosion Prevention

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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

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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

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Inlet Duct Floor and Sidewall Details Floor is typically sloped 10-18 degrees

Floor and sidewall liners have additional stiffening

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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.

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Inlet Duct Liner Damage

Deformed Liner Plate

Catastrophic damage to sidewall liner

Catastrophic damage to floor liner

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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

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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

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Typical Distribution Grid

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Typical Distribution Grid Components

Gas Flow

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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!

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Condensate Management - DrainingBuckled tubes caused by uneven condensate clearing in coils or piping

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Condensate Management

Water

No steam flow, hot tubes

Steam cooled tubes

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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.

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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

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Condensate ManagementTest condensate detection and removal system in Reheater and HP Superheater drains

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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

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Desuperheaters

Damage from desuperheater quenching

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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

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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

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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

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Improvements to Address Cycling

1. Tube stubs at Tube/Header Connection

2. Spring Supported Coils

3. Internal coil flexibility

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Optional Stub Tube to Header Connection

Better for cycling

Better weld NDE

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Minimizes header thickness, therefore lowers thermal stress

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Spring Support of Header

Fixed Header

Floating Header of Same Coil

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Outlet Header is Supported by Spring Supported Manifold Pipe

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Reheater and HP Superheater Coil Support Modifications

Spring cans being installed

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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

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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

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Flow Accelerated Corrosion (FAC)

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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:

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•pH of water in LP system

•Presence of reducing agents

•Velocities in piping/tubing

•Temperature

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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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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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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:

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•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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•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

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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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Rust build-up Ammonia salt build-up

HP/IP Economizer

LP Evaporator

LP Superheater

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Limit Ammonia salt formation• Improve AIG design and tuning• Improve NH3 control• Add catalyst for lower slip design• Delay ammonia feed at startup

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Reduce rust build-up• Close damper for heat retention when offline

Minimize causes of back pressure increase

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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

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Wash Coils To Remove Salts

Adding caustic to wash water may neutralize pH

Ammonia may be liberated

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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.

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