1 FAC General
Recognition, control, and alleviation of FAC should be one of the most important management-supported aspects of operating combined-cycle and cogeneration plants powered by gas turbines, it caused by internal wall thinning of low-chromium-content carbon steel, which occurs because the magnetite layer dissolves when exposed to the temperature, velocity, chemical, and geometric factors associated with FAC. When corrodents enter the cycle, the porous oxide layer allows under-deposit corrosion mechanisms to proceed rapidly. Pressure-part failures are a likely result , Figure 1, FAC cause LP evaporator tube leaking.
1.1 single phase FAC
the reducing environment produced by oxygen scavenger feed results in single-phase flow-accelerated corrosion (FAC).
Single-phase FAC. The attack occurs at flow disturbances such as elbows in feedwater piping and economizers, feedwater heater drains, locations downstream of valves and reducing fittings, attemperator piping, and, most notably for the combined-cycle industry, in low-pressure (LP) economizers and evaporators, and to a lesser extent in intermediate-pressure (IP) circuits. The effect of single-phase FAC is further illustrated below. Fig 1
Fig 1 FAC cause the LP economizer tube leaking
Metal loss occurs gradually until the remaining material at the affected location can no longer withstand the process pressure, whereupon catastrophic failure occurs. The thinning is due to the combination of a reducing environment and localized fluid flow disturbances, which cause dissolution of ferrous ions (Fe+2) from the metal and metal oxide matrix.
EPRI research also showed that iron dissolution is greatly influenced by solution pH and temperature.
As the chart illustrates, corrosion reaches a maximum at 150 o C. Thus, feedwater systems and HRSG low-pressure evaporators are particularly susceptible locations. Also note the influence of pH, as reflected by ammonia concentration, on the corrosion characteristics. As we shall see, this factor is quite important about control of FAC.
oxygenated treatment (OT) to minimize carbon steel corrosion and iron dissolution in supercritical steam generators. The key component of the program was deliberate injection of pure oxygen into the condensate/feedwater network to establish oxygen residuals of up to 300 ppb.A small, continuous feed of pure oxygen may be required to boost the concentration.
For new HRSGs, single-phase FAC control can also be addressed by materials selection. The addition of a small amount of chromium in the material at FAC-susceptible locations virtually eliminates the corrosion.
1.2 Two phase FAC
This corrosion mechanism occurs where water flashes to steam, resulting in a mixed-phase fluid. It is usually happen at HP evaporation the temperature range is higher than single phase FAC
1.3 Another FAC impact factor
The primary damage from FAC was due to high local flow velocities, such as local flow maldistribution due to blockage, high local heat input or transient conditions, or high bulk flow rates resulting in local areas of secondary flow FAC damage. High bulk rates can be the result of high duct firing, resulting in high steam production rates or nonoptimized flow paths
Duct firing to allow peak power production, In some cases, high steam export made it difficult to achieve reliable water chemistry control, increasing FAC wear. Also, at full output, water flows through the areas of the HRSG most susceptible to FAC had high velocities. This led to significant local FAC wear in some areas of high-flow/nonuniform flow patterns in the range of 0.75 to 1 mills/khr, while other areas of the HRSG showed little or no FAC wear (<0.5 mills/khr). The local flow conditions produce a large increase in wear independent of overall water chemistry and temperature. Management of this FAC wear is largely a matter of modification of local flow paths and materials.
FAC wear rates in other areas of the same component away from the local high flows are quite low (<0.15 mills/khr), resulting in pressure part lifetime greater than 400,000 operating hours.
The chromium-rich oxide layer that forms on these materials in boiler service has a very low solubility compared to magnetite in the FAC temperature range.
2 ESEC Operation Challenge
ESEC pay enough attention to monitory FAC , during the outage time, completed the fully inspection for the steam side use borescope and visual inspection.
Iron removed primarily by dissolution during the FAC process is transported into HP, IP drum make up valve to cause the valve slowing open to 100%, then need to open bypass valve to keep plant at based load
For example HRSG$1 HP drum level control valve 9A at 2016-01-20 251MW 296t/h feed water valve open 41.8% open, but at May 22nd 2019 the drum water level 9A control open to 96%, the bypass valve 9B also open to 96% to keep 100% based load.
May 28, 2019 outage time open the valve and found the seriously oxide iron build on the trim as figure 2,
New trim replaced at this outage as figure 3 :
Entire inside of the HP drum, the dissolved fine magnetite particle still pass through the 9A valve trim attached to surface of inside drum surface (grey color) as figure 4:
June 2nd 2019 new valve open 47% at based load, no need bypass valve 9B open, but due to magnetite layer dissolves process (FAC) ongoing and plus running longer time at based load , up to 2021-01-24 CT1 261 MW, 9A valve open up to 75% ( 309t/h), obvious the control valve open faster than last time.
4 ESEC solution
Mechanism of the FAC now is very clear, for this big challenge, as top power plant, the management and engineer act very fast and pay highly attention for this, several measures come out to fight this challenge as:
- Outage inspection the drum, and borescope the downcomer from drum and use borescope to check the inside of the economizer tube bending area.
- Test iron per weekly to monitory, to ensure iron levels of less than 2 ppb in the feedwater and less than 5 ppb in the evaporators
- Stop oxygen scavenger
The designed to add the oxygen scavenger to the water side to stop the corrosion, but consider the FAC, stop to use the oxygen scavenger, the skid is converted to the cooling anti-foam, oxygen scavenger increase the area affected by FAC, as well as the corrosion rate, by increasing the solubility of the protective magnetite layer in the HRSG.
- Prepare the inject air to condenser to increase oxygen level and stop FAC
Incorporate ability to control air ingress to condenser or suction side of condensate pumps to allow us to achieve desired target DO levels of 20 ppb. Also incorporate air pump for BFW suction DO. Something like picture below would probably work well
reduce excessive magnetite being formed in HP and IP economizers which result in plugging of 9A and IP Drum level control valves. More importantly, FAC concerns with economizers prevail. Minimize iron moving into solution in HP and IP (&LP) economizers. Increased DO will allow more hematite and promote different magnetite composition that will be less prone to liberating and ending up in the 9A & IP Drum LCVs. More importantly, FAC at economizer can lead to very expensive repair and represents safety concern as well. Incorporate ability to ingress air at condensate pump suction to target 20ppb on condensate system. Incorporate oxygen cylinder (less preferred) or instrument air or air pump to be able to inject at LP coil or BFW suction to target 7 ppb.
The most commonly used oxidant is oxygen gas. Air and liquid oxygen are also utilized in some plants. Air, however, contains undesirable contaminants such as carbon dioxide and, therefore, its use is generally discouraged.
So absolute micron particle filter plus organics and oil filter (coalescing filter) and CO2 scrubbing cartridge filter IF you are going to use instrument air or an air pump. Chemtreat leans away from pure oxygen due to corrosivity when it enters the piping.
- New Dissolved O2 meters
EPRI recommendations, the dissolved oxygen target for feedwater under AVT(O) is 5-10 ppb to mitigate Flow Accelerated Corrosion (FAC) Refer to the following EPRI material:
The manufacturer has suggested upgrading to the Waltron 9065 at a cost of $8,905 net each. This is not a direct replacement, but the sensors would fit in the existing flow-cell and the new transmitters would have to be remounted.
I&C Department recommends the replacement of the existing Waltron setup with an M4 Knick dual channel digital transmitter and associated digital dissolved oxygen sensor that can read as low as 6 ppb.
Also we have plans to upgrade the pH sensors in this panel to M4 Knick, and this would give us the ability to use the same diagnostic tools for the DO sensors.
The existing Waltron Aqualyzer 9061C Compact Dissolved Oxygen Analyzers have become obsolete and require replacement, as replacement sensors and parts can no longer be ordered. M4knick 706 with Memosens along with the dual channel digital transmitter
Installation to follow EPRI recommendations
Note that this MOC covers the scope of the tie-in to the BFW pump suction piping only. At this time, the system will remain in feedwater service with design conditions of 1380 kPa and 198C. Assemblies are to be fabricated and tested in the shop, prior to field installation at the proposed 6″ 150# flange. A duplicate assembly will also be fabricated for future use on Unit 1.
- Increase the PH level on HRSG water side
Ammonia is adding into condensate system, to increase the set point form PH from 9.6 to 10 which use the specific conductive as control process to increase the condensate PH hence to increase the HP and IP drum PH.
- Using high tech Inspection Technology.
Past efforts at improving technology for HRSG nondestructive examination (NDE) have focused on inspection in the finned tube areas not currently accessible to many NDE techniques. A bigger problem for FAC is poor accessibility of bare tube segments at headers of horizontal gas path HRSGs. Only the outermost tube rows are typically accessible, and often only part of the tube circumference for these. Borescope is used and plus UT thickness test, but access is usually limited or requires cutting into the header or connecting piping. Advances in digital radiography and other techniques offer prospects for improved assessment of FAC damage. New technologies for under fin thickness measurement, digital radiography, and advanced borescope to help characterize FAC in lower temperature area, such as economizer tube
- Add economizer outlet sample point, more accuracy test iron and compare with economizer inlet point to understand FAC progress situation
5 Results
Management of FAC risks is a life-of-plant issue and not a one-time problem that can be fixed and then forgotten about, above measurement results will be update in the future.