Flow-Accelerated Corrosion

Flow-Accelerated Corrosion

Many utilities and cogeneration plants are finding that flow-accelerated corrosion (FAC) is causing waste age of materials, downtime, and additional maintenance concerns. FAC is defined as the localized rapid metal loss resulting in tube wall thinning of carbon and low

196 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

Sh. Evap. SCR Evap. Eco.

10 1—Unfired 110,000 Ib/h at 725 F, 600 psig 2 2—Fired 150,000 Ib/h at 782 F, 600 psig 3—Fired 200,000 Ib/h at 800 F, 600 psig

900 4—Fired 250,000 Ib/h at 800 F, 600 psig erature, °F

HRSG gas temperature profiles at various gas inlet conditions. 100

netite dissolution, ppb 30 9.05

Temperature, °C

FIGURE 4.28

FAC as a function of temperature and pH of water. alloy piping. FAC is known to occur in the region of 125°C–220°C as shown in Figure 4.28. In

HRSGs, the evaporators and economizers in the LP (low pressure) and IP (Intermediate pres- sure) sections are affected. FAC is the absence of protective metal oxide layer that when pres- ent limits corrosion in boiler systems. Without this protection, the surface is free to react with the passing water (or two-phase flow), and metal loss is rapid. Factors affecting FAC are flow velocity, geometry, metallurgy, dissolved oxygen concentration, and temperature. Feeder connections with sharp bends are common in HRSGs. These multiple feeders connect the downcomer pipes to the evaporator circuits and take the steam–water mixture to the drum.

Past industry practices believed that all of dissolved oxygen must be eliminated from feed water to control corrosion. To deoxygenate the feed water, oxygen was mechanically

Waste Heat Boilers 197

removed in the condenser–deaerator system with an addition of an oxygen scavenger such as hydrazine to maintain a residual of 40–100 ppb hydrazine. This caused the feed water to become more and more reducing and has produced the opposite effect of producing a protective layer. The normally protective magnetite layer in carbon and low alloy steel dis- solves into a stream of flowing water or two-phase flow. Both the pH and temperature and level of dissolved oxygen influence the stability and solubility of the magnetite oxide layer. The difference in wall loss due to FAC is 100 times greater at 1 ppb dissolved oxygen in feed water than at 20 ppb. Hence, maintaining some residual oxygen will reduce the FAC.

In Germany, the maximum oxygen in feed water has been increased from 0.02 to 0.1 (20–100 ppb) mg/kg. Flow velocity is also lowered to reduce the erosion of the magnetite layer. HRSGs have sharp bends in economizers, evaporators that are prone to FAC. FAC also develops at flow disturbances such as elbows, bends, reducers, tees, and steam attem- perating lines. Sometimes the bends are made of low chromium alloys to reduce FAC. In oxygenated treatment, oxygen is deliberately introduced in the condensate and feed water system. PH of feed water is raised to 9.2–9.6 for FAC effect to be minimal by an addition of ammonia. It has been found that two-phase FAC is difficult to control chemically and hence 1.25 chromium steels are suggested for such areas. All volatile treatment used in some boiler including once-through units to avoid deposition of solids leads to the dissolu- tion of the protective magnetite.