Water treatment programs for boilers and heat-recovery steam generator (HRSG) units appropriately focus on chemistry controls during normal operation. However, rates of corrosion product transport and deposition can be much greater in boilers and HRSG units during start-up than during routine operation. For that reason, in addition to standard programs for monitoring and control of corrosion product transport, deposition, and underdeposit corrosion, consider adding contingency plans for boiler water chemistry holds in the start-up process.
A number of years ago a member of our organization visited a power plant to conduct a water treatment practice review. At the time, iron deposits on the turbine and significant iron in the blowdown samples indicated that the inventory of iron oxides in the steam cycle and iron oxide transport rates to the boiler and turbine were excessive. A procedure of blowing down the boiler during start-up to reduce the inventory of iron oxides was recommended as a remedial measure. This type of procedure has been commonly recommended at other facilities following chemical cleanings and occasionally in response to upsets resulting in excessive suspended solids in the boiler water.
Station personnel observed a number of start-ups and developed a start-up curve of iron versus pressure. The original curve extended from 500 psig to 2,700 psig (3.4 to 18.6 MPa) with iron concentrations of 2,650 ppb to 25 ppb (as Fe3O4) in the boiler water. It is our understanding that these iron concentrations were found to be essentially the pseudo-equilibrium levels that were achieved in a reasonable amount of time as a result of normal start-up practices.
In 2007 we were contacted again by this client to perform a steam/water cycle chemistry review. The station continued to utilize the start-up curve for boiler water iron levels. Station staff believed that their iron transport was well controlled during normal operation and start-ups, but they wanted an independent evaluation of the chemistry program.
Although station personnel monitored iron content in the boiler water during start-ups, they were not performing routine iron testing of the feedwater samples. Ordinarily, waterside deposits can be removed by chemical cleaning before failures occur. Deposits can lead to high temperatures and blistering (Figure 1), high and cyclic temperatures that lead to creep and thermal fatigue cracking and fireside corrosion (Figure 2), underdeposit concentration mechanisms and underdeposit corrosion (Figure 3), and boiler tube failures. In lieu of the feedwater corrosion product data, we assessed iron transport based on tube deposit accumulation rates.
1. Excessive boiler tube deposits and blistering. These preboiler corrosion products were deposited on the inner diameter surface near welds on the smooth tube portion. Deposits were up to seven times heavier on smooth tube than on rifled tube surfaces (~375 g/ft2) . The tube experienced short-term overheating (blistering) as well as other damage. Courtesy: Sheppard T. Powell Associates LLC
2. Heavy boiler tube deposits, overheating, fatigue, and fireside corrosion. The end view and outer diameter of the tube are shown in these images. The tube had heavy (146 g/ft2) internal deposits and had experienced long-term overheating, creep, thermal fatigue cracking, and fireside corrosion. Courtesy: Sheppard T. Powell Associates LLC
3. Boiler tube failure. The left side of the photograph shows the crack on the tube’s outer diameter. The right side shows an end view of the blistering and cracking from hydrogen damage. Heavy deposits formed on the tube surface, causing high temperatures, swelling, underdeposit concentration of trace acidic salts in the boiler water, underdeposit corrosion, hydrogen damage, and tube failures. Courtesy: Sheppard T. Powell Associates LLC
Since the most recent cleaning, the deposit accumulation rates for the two units had been 7.8 g/ft2/year and 8.9 g/ft2/year, and the total deposit loading during our visit was well above the level at which a chemical cleaning is recommended. When plants have accumulation rates over about 5 g/ft2/year, it is a sign of poorer-than-average control of iron transport and/or an unusually high localized deposition/corrosion mechanism. Plants with <1 to 2 g/ft2/year of deposit accumulation rate have good control.
For an earlier period with a higher deposit accumulation rate (10.7 g/ft2/year), iron transport rates were estimated to be about 26 ppb as iron in the feedwater (based on material removed and estimated total feedwater flow). For the more recent period, the iron transport rate was estimated to be 18 to 22 ppb as iron in the feedwater. The data also indicated that the rate of iron oxide transport during normal operation remains elevated.
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