{"id":105295,"date":"2020-12-21T09:44:46","date_gmt":"2020-12-21T15:44:46","guid":{"rendered":"https:\/\/www.power-eng.com\/?p=105295"},"modified":"2020-12-21T09:44:49","modified_gmt":"2020-12-21T15:44:49","slug":"hitting-the-layup-case-study-on-hrsg-corrosion-protection-techniques-at-nebraska-ccgt-power-plant","status":"publish","type":"post","link":"https:\/\/www.power-eng.com\/om\/hitting-the-layup-case-study-on-hrsg-corrosion-protection-techniques-at-nebraska-ccgt-power-plant\/","title":{"rendered":"Hitting the Layup: Case study on HRSG corrosion protection techniques at Nebraska CCGT power plant"},"content":{"rendered":"\n

By<\/strong> Brad Buecker, ChemTreat, and Dan Dixon, Lincoln Electric System<\/strong><\/p>\n\n\n\n

As coal-fired power plants are being retired as a result of environmental and economic factors, the primary bridge technology to fill power generation voids has been combined cycle power production. <\/p>\n\n\n\n

For those new to the industry, the combustion turbines of a combined cycle plant function on the Brayton thermodynamic cycle, while the heat recovery steam generators (with steam produced by exhaust heat from the gas turbine) operate on the classic Rankine cycle.\u00a0 Net efficiencies of modern combined cycle units now reach or exceed 60 percent, which is significantly higher than even the most advanced supercritical coal units.\u00a0 Logic suggests that plants with such high efficiencies would normally be base loaded, but that is generally not the case.\u00a0 Load swings associated with renewable technology operation require most combined cycle power plants to cycle regularly in load.<\/p>\n\n\n\n

See full PE coverage of all things combined cycle gas turbine<\/a><\/strong><\/p>\n\n\n\n

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Any time a unit comes offline, the potential for air ingress may exist at the small openings within the water\/steam network.  Oxygen can then attack in localized areas and cause serious damage to tubing, piping, and other components.  Oxygen corrosion may be particularly problematic if a unit cools significantly before the next operating phase.  Compounding this issue, corrosion products can dislodge upon startup and travel to the steam generator, where they precipitate on boiler tubes.  Porous iron oxide product deposition reduces heat transfer and can establish sites for under-deposit corrosion.  Unfortunately, plant management at both dedicated power plants and co-generation facilities often underestimate the severity of off-line corrosion and the need for well-designed layup procedures. <\/p>\n\n\n\n

From an early stage, the staff at Lincoln Electric System\u2019s (LES) Terry Bundy combined cycle plant in Lincoln, Nebraska recognized that proper layup was important to maintain the integrity of the twin combined cycle units.  This article outlines the layup methods the staff implemented, and the excellent protection the technologies have provided over the last decade-plus.  These lessons will also be applicable for hydrogen-fueled combined cycle plants, whose emergence offers interesting promise.<\/p>\n\n\n\n

Introductory Details<\/strong><\/p>\n\n\n\n

Power at the Terry Bundy plant is produced by two GE LM 6000 combustion turbines (47 MW maximum output per CT) and two Nooter-Eriksen dual-pressure HRSGs (no reheat) that feed a 26-MW steam turbine.  The plant also includes a stand-alone simple cycle turbine, also with a 47-MW output capacity.  These units regularly cycle on and off, frequently on a daily basis, during the summer.  At other times of the year, the units may be off for several weeks but need to be available for dispatch on short notice. <\/p>\n\n\n\n

Shortly after the units were commissioned in the mid-2000s, the plant staff converted the HRSG feedwater chemistry program from all-volatile treatment reducing [AVT(R)] to all-volatile treatment oxidizing [AVT(O)] to minimize the potential for flow-accelerated corrosion (FAC) in the condensate\/feedwater network.  Per AVT(O) guidelines, reducing agent\/oxygen scavenger feed is eliminated and ammonia (or possibly an ammonia\/amine blend) is utilized for feedwater treatment to maintain pH within a mid- to upper-9 range.  The small amount of dissolved oxygen that (normally) enters the feedwater via condenser air in-leakage is allowed to remain, as this chemistry in high-purity feedwater (\u00e2\u2030\u00a40.2 mS\/cm cation conductivity) induces formation of a tight a-hematite boundary layer on carbon steel.<\/p>\n\n\n\n

\"\"<\/figure><\/div>\n\n\n\n

Figure 1.\u00a0 The deep red a-hematite layer on the internals of the Terry Bundy HRSGs.<\/strong><\/p>\n\n\n\n

When plant personnel switched feedwater chemistry from AVT(R) to AVT(O), concerns were raised because the change eliminated the oxygen scavenger feed.  Generally speaking, unless the feedwater system contains copper alloys (virtually unknown in HRSGs), AVT(R) is not recommended, as the reducing agent may increase the risk of flow-accelerated corrosion. [1, 2]  Previously, when oxygen scavenger use was considered de rigueur<\/em> in all systems, a common short-term wet layup procedure was to maintain, or perhaps even increase, the reducing agent concentration at shutdown.  However, many treatment programs now follow the principle summarized in reference 3: \u201cMaintain [the] same oxidation\/reduction potential during wet layup as operation.\u201d  That principle suggests that if a unit is on AVT(O) during normal operation, do not switch to reducing conditions for wet layup.  Doing so can cause a shift back and forth between an environment that promotes the a-hematite oxide layer and then, at shutdown, a magnetite (Fe3<\/sub>O4<\/sub>) layer.  It can be difficult to establish a stable oxide layer in these conditions, and spalling of the protective layer and subsequent transport of corrosion products to the steam generators is possible.  However, in the early days of unit operation at Terry Bundy, off-line oxygen pitting was an issue.<\/p>\n\n\n\n

\"\"<\/figure><\/div>\n\n\n\n

Figure 2.\u00a0 Oxygen pitting in one of the high-pressure evaporator drums during early operation of the units.\u00a0<\/strong><\/p>\n\n\n\n

So, plant personnel needed solutions that continued to allow AVT(O) for normal operation with corrosion protection during the frequent downtimes.<\/p>\n\n\n\n

The LES Solution<\/strong><\/p>\n\n\n\n

To maintain constant conditions from normal operation to wet layup, LES personnel installed several systems and developed written procedures for shutdown conditions to help prevent simultaneous metal contact with water and air.  The primary modifications were:<\/p>\n\n\n\n