Same old cycle chemistry issues persist

Barry Dooley, a senior associate at Structural Integrity Associates Inc and executive secretary of IAPWS, provided participants in the HRSG Forum with Bob Anderson a global view of optimum cycle chemistry for HRSGs and combined-cycle plants. He consolidated information from aligned HRSG sessions in Australasia, Europe, Russia, and Canada. Dooley and Anderson share the chairmanship duties for these conferences. 

A well-known and highly-respected presenter and cycle-chemistry expert, Dooley repeated that flow accelerated corrosion (FAC) is a paramount concern, and proper chemistry must either be designed into or incorporated into each system installation. He also noted, with a sense of frustration, that under-deposit corrosion and FAC are still occurring at the same rate as 20 years ago. In the US alone, three incidences of FAC-induced failure were reported in the past year.

But he stressed that this can be prevented with proper cycle chemistry.

Failure repetition was the underlying theme, and Dooley referenced a summary table of data from 185 plants showing repeat problem situations. He then presented the repeat cycle-chemistry events specific to combined-cycle plants and HRSGs.

Repeat causes of corrosion and corrosion products in HRSGs include, among others:

      • Inadequate online alarmed instrumentation.

      • Drum carryover.

      • Not challenging the (site) status quo.

      • Lack of shutdown protection.

      • Boiler waterwall/evaporator deposition.

      • No action plans for repeat situations.

Carryover also must be addressed because it leads to steam-turbine failures in the phase transition zone. “Steam-turbine issues in combined-cycle plants are increasing,” he noted.

Dooley also stated emphatically that “air-cooled condenser chemistry absolutely controls the plant chemistry.”

For FAC control, he noted the need to identify both single- and two-phase activity, and to distinguish between the two. To emphasize the seriousness and what we do know, Dooley stated that single- and two-phase FAC are still occurring worldwide, and locations have remained constant for more than 15 years. He then gave details and background, along with relevant IAPWS rules for avoidance.

Dooley then pointed out typical FAC locations in horizontal- and vertical-gas-path systems. His discussion featured some common ways to identify both location and type.

Then another rule of thumb: Two-phase is controlled by pH; single-phase is controlled by the oxidizing power of the liquid. Proper levels of each were discussed. He also noted that amines and film-forming products must be used with extreme care.

Under-deposit corrosion (UDC) was also explained and reviewed, with specific histories, causes, examples, and treatments.

Relevant IAPWS Technical Guidance Documents (TGDs) also were listed and reviewed.

EPRI’s Mike Caravaggio, senior program manager for major component reliability, followed Dooley with “Achieving Cycle Chemistry Excellence in HRSGs.” This included current industry statistics, examples of chemistry-related damage to both HRSGs and turbines (for example, FAC and iron transport, hydrogen), and a preferred instrumentation suite. Caravaggio concentrated on what he labeled “practical steps” to achieving proper and obtainable chemistry control.

Topics included the most common tube failure areas (LP economizer and HP evaporator), turbulence and FAC risk, UDC and hydrogen damage, and corrosion fatigue (now increasing with aging of the fleet).

Iron transport and oxygen levels also received high attention.

Caravaggio then presented the case study of a 700-MW plant that has shown dramatic improvement reducing iron transport by using an ammonia and neutralizing amine blend.

Concluding thoughts included understanding the damage mechanisms and chemistry control options, performance monitoring, and the critical importance of acting on the measured results. All too often, it became clear, such action requirements are overlooked (or simply ignored).

Water/steam sampling. Manual Sigrist, Swan Systeme AG, discussed specific tools to monitor cycle chemistry, emphasizing refurbishments of existing plant sampling equipment. Systems in legacy powerplants typically have high O&M costs, low monitoring reliability, and can increase risk of collateral damage to major components. Space for new instrumentation can be limited.

For refurbishments, he emphasized the “project concept phase” to define options and obtain commercial justification. Personnel safety should always be included as a valid economic factor.

He then reviewed examples of both refurbishments and new installations, in both cases putting strong emphasis on operator training.

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