HRSG User's Group: Improving steam-plant reliability, durability and profitability
Replacement Projects for HRSG Tube-Modules
Max Begeman, NextEra Energy Resources, Inc.
Instead of patching up one individual tube leak after another, for a temporary repair, some users of aging HRSGs have opted to remove and replace their entire tube-modules, for a long-term repair (Fig 1).
The tube-module replacements completed by Begeman and his team originated in a formalized HRSG cleaning program— which revealed that significant corrosion was occurring in the low-pressure Evaporator (lpev), and the low-pressure Economizer (lpec) sections of their HRSGs, and was causing an efficiency-robbing increase in gas-turbine back-pressure.
A total of 26 HRSGs were cleaned in this program, using the relatively new percussion-cleaning method, in which a special lance is introduced into the area between the tube-modules. A bag at the end of the lance is inflated with a combustible mixture and then ignited, to cause a shock-wave that scrubs iron-oxides from the tubes.
The amount of iron-oxide that Begeman and his team removed from each HRSG weighed as much as 27 tons, and was as deep as 25 inches (Fig 2).
Figure1: More module retrofits are being done, as many HRSGs are reaching their “mid-life crisis”.
Figure 2: If you’re shoveling out two-foot-deep piles of iron-oxide from your HRSG, it may be time for a tube-module retrofit.
Analysis of the iron-oxides, and of tube samples cut out and sent to a lab, confirmed that flow-accelerated corrosion (FAC) was the damage-mechanism and that these lpev and lpec modules were nearing the end of their useful life. Therefore, Begeman and his team analyzed the financial feasibility of a complete replacement of the tube-modules, and they concluded that the projects were economically justified for 14 individual HRSGs.
The 14 HRSGs were commissioned in the early 2000s, although the accumulations of iron-oxide debris did not correspond directly to their age. The accumulations corresponded most directly to the differences in HRSG configuration—such as the presence of duct burners, or a selective catalytic reduction (SCR) system.
Begeman and his team specified that the replacement modules will be fabricated of an upgraded material—using T11 steel instead of the original plain-carbon steel, for improved resistance to FAC. The next step was removing the old modules and installing the new ones. This required the construction of a sled-and support-system, which accessed the HRSG from the side, rather than the top (Fig 3).
Figure 3: These extensive projects need room for cranes to maneuver.
With the tube-module retrofits now completed in all 14 HRSGs, Begeman and his team have taken the next pro-active step in their maintenance of HRSGs. Now, he explained, they are initiating a formalized tube-failure prevention program, across their entire fleet of HRSGs. Such a program—which has been effectively used in conventional boilers for many years—requires (1) the collection, trending, and analysis of key data; (2) the determination of the root-cause of failure; and (3) implementation of a long-term solution. In the Q & A session that followed this presentation, Begeman’s fellow HRSG users, along with the designers and consultants in the audience, applauded him for taking this next step.