Flow-Accelerated Corrosion in HRSGs
Jeff Anderson AEP Plant Engineering Programs August 10th, 2009
Helpful Articles Flow-Accelerated Corrosion in Fossil and Combined Cycle/HRSG Plants by Barry Dooley, Power Plant Chemistry, 2008 Assessments of HRSGs – Trends in Cycle Chemistry and Thermal Transient Performance By Barry Dooley and Bob Anderson in Power Plant Chemistry, 2009
For article copies:
[email protected]
Helpful Articles Flow-Accelerated Corrosion in Fossil and Combined Cycle/HRSG Plants by Barry Dooley, Power Plant Chemistry, 2008 Assessments of HRSGs – Trends in Cycle Chemistry and Thermal Transient Performance By Barry Dooley and Bob Anderson in Power Plant Chemistry, 2009
For article copies:
[email protected]
FAC The process whereby the normally protective oxide layer on carbon or low-alloy steel dissolves into a stream of flowing water or saturated steam. When the protective oxide layer on the carbon steel piping is dissolved into the fluid, a new protective oxide layer forms, and the cycle repeats until the pipe or vessel is thin enough to rupture. Influencing factors: fluid chemistry, fluid temperature, Influencing flow turbulence, and metal composition.
Corrosion Facts FAC is the leading cause of tube failure in HRSGs. The FAC corrosion products from the LP areas of the HRSG are deposited in the HP evaporator tubing and form the basis of the under-deposit corrosion damage mechanisms (hydrogen damage, acid phosphate corrosion, and caustic gouging). This LP-HP corrosion link forms the main focus of the cycle chemistry assessments in the plants, which if left unaddressed, will eventually lead to damage and failure by one or both mechanisms. Acting proactively will reduce the risk for both.
5
Simplified Mechanism of FAC Reaction between dissolved oxygen and metal surface forms a protective oxide layer (rust). FLOW Pipe ID
Pipe OD
protective oxide layer (rust)
6
Simplified Mechanism of FAC The oxide layer is dissolved into the flow stream.
FLOW ID
OD
7
Simplified Mechanism of FAC The oxide layer is replenished as the base metal is converted into more oxide. FLOW ID
OD
The oxide layer is again dissolved into the flow stream. The base metal again has to form a new protective oxide layer. The cycle repeats, thinning the pipe wall until a rupture occurs. FLOW ID
OD
8
Types of FAC Single-Phase FAC Occurs when the fluid is in the liquid water phase. Damage is characterized by a bumpy “orange peel” surface.
Two-Phase FAC Occurs when the fluid is in the saturated “wet” steam phase. Damage is characterized by a shiny black surface. Two-phase FAC is more aggressive than single-phase FAC due to the hyper-turbulent nature of wet steam versus water. Note: FAC does not occur in superheated “dry” steam environments.
Single-Phase Examples of FAC Examples of single-phase FAC. Left: FAC failure in an economizer inlet header. Right: FAC in a reducer which was attached to an HP FWH drain control valve. Two views of the surface appearance of single-phase FAC. Left: A close-up of the economizer inlet header tube above. Right: Microscopic view of an HRSG LP evaporator tube. In both cases the “horseshoes” point in the direction of flow.
Examples of FAC in HRSG LP evaporator tubing: A) single-phase FAC in a vertical tube. B) two-phase FAC in a vertical tube. C) two-phase FAC in a hairpin bend of a horizontal tube. D) microscopic view of the scalloped appearance that is always present with FAC.
Types of Oxide Layers Magnetite Black or gray surface color. Porous, easily dissolved, susceptible to FAC. Forms when cycle chemistry operates at low oxygen levels (reducing agent is used).
Hematite Red surface color. Dense, not easily dissolved, resists FAC. Forms from magnetite, on top of it. Forms when cycle chemistry operates at higher oxygen levels (reducing agent is not used).
Magnetite
Hematite with Magnetite
Two-phase FAC inside an LP feedwater heater.
Amos 3, #63 HP heater drain entry into the deaerator.
Welsh 1, 6” superheat attemperator supply piping. Nominal wall = 0.864”. Remaining wall, low point = 0.152”. FAC occurred when cycle chemistry used a reducing agent. Red hematite formed after agent was removed, then FAC
AEP’s FAC Program Philosophy We want to be proactive to protect against FAC rather than waiting for failures to occur. We want to optimize cycle chemistry to prevent damage while performing inspections to find existing damage. It must be recognized that chemistry alone cannot always eliminate FAC. An inspection program must be in place, combined with low-chrome alloy replacements.
The Control of FAC 1) Operate with an oxidizing chemistry (remove reducing agent) to control single-phase FAC. 2) Operate with an elevated pH (around 9.8) to control twophase FAC. 3) Monitor the color of the LP and IP drums. 4) Monitor the total iron concentration in the condensate, feedwater, and in each drum to evaluate how the first two approaches are working (The “Rule of 2 and 5” is the goal: less than 2 ppb total iron in the condensate/feedwater and less than 5 ppb total iron in each drum).
HRSG Areas of Concern (single and two phase)
1. Economizer/preheater tubes at inlet headers. 2. Economizer/preheater tube bends where steaming takes place. 3. LP evaporator inlet headers which have a contortuous fluid entry path and where orifices are installed. 4. Verticle LP evaporator tubes, especially in bends near outlet headers. 5. LP evaporator transition headers. 6. LP riser tubes/pipes to the LP drum. 7. LP drum internals. 8. IP economizer inlet headers. 9. IP economizer outlet headers, especially in nearby bends which have steaming. 10. IP riser tubes/pipes to the IP drum. 11. IP evaporator tubes on triple-pressure units operated at reduced pressure.
HRSG Areas of Concern Waterford - 33 inspections, no damage found. Lawrenceburg - ? Comanche & Northeastern 1 - Extensive damage found: -- Elbows around the LP economizer headers (at NE1 the inlet and outlet headers were replaced with P11 material because of the damage in the hard-to-reach bends). -- The entire feedwater, attemperator supply, by-pass, and recirculation lines (the smaller diameter lines appear to be corroding faster than the FW piping due to the tighter radius bends). --Since they have found so much FAC in these areas, they are systematically replacing the lines as money permits.
AEP’s FAC Program Database Scorecard Data Storage Support
FAC Database FAC Database Location Legend: Required feedwater and heater drain inspection locations. Third phase locations (additional pts. chosen by the plant). Y/N
Plant AM AM AM AM AM AM AM AM AM AM AM AM AM AM AM AM AM AM
Unit 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Location Feedwater, 18", D/S of flow nozzle Feedwater, 18", D/S of flow nozzle Feedwater, 18", D/S of check valve RH Attemp., 6", D/S of flow nozzle RH Atemp., 4", 90 deg elbow Heater #8, Drain, U/S of control valve, 10" Heater #8, Drain, U/S of control valve, 8" Heater #8, Drain, U/S of control valve, 4" Heater #8, Drain, D/S of control valve, 4" Heater #8, Drain, D/S of control valve, 8" Heater #8, Drain, D/S of control valve, 12" Heater #8, Alt Drain to Deaerator Elbows Heater #7, Drain, U/S of control valve, 12" Heater #7, Drain, U/S of control valve, 8" Heater #7, Drain, D/S of control valve, 8" Heater #7, Drain, D/S of control valve, 12" Heater #7, Alt Drain to Deaerator Elbows Heater #4, Drain, U/S of control valve, 6"
Req'd Loc Y Y Y Y Y
Y Y Y
Y Y
Comments Lege nd: Wall loss found; piping was repaired/replaced. Wall loss found; reinspection needed at a later date. --> Submitted data does not conform to Circ. Letter.
Date Inspected Comments 05-04-99 Some apparent wall loss likely due to boring. Reinspect in 05-04-99 05-04-99 05-04-99 Some apparent wall loss likely due to boring. Reinspect in 05-04-99
5/1/05 5/1/05 5/1/05
Data not submitted. Data not submitted. Replaced deaerator drain elbows in 2 Data not submitted.
FAC Scorecard Phase 3 2008-2012
Attemp Supply
Misc. BFP
FWH Alt Drains
4 Req’d
4 Req’d
4 Req’d
Conden FWH -sate Shells 4 Req’d
2 Req’d
FWH Vents
Plant's Choice
Phase 3 Total
4 Req’d
10 Req’d
32 Req’d
AEP’s FAC Program
Data Storage Support
Questions?
Examples of two-phase FAC in deaerators. Example A is located adjacent to an HP cascading drain entry. Example B is directly in the path of flashing steam from another drain entry. In both cases the two-phase FAC areas are easily seen by a black, shiny, extremely thin layer of magnetite. There may also be pitting on the surface. The red areas indicate where single-phase water has provided a protective hematite oxide film.
Potential-pH Diagram for Iron 1.6 Passive Oxidizing (+)
Potential, V (ORP)
0
Corrosive
Iron-Chromium H em a ti t e Oxide
M a g ne ti t e Reducing (-)
Immune
-1.6 0
7 pH
14
