Follow IAPWS recommendations to mitigate operational issues

Seventy percent of combined-cycle plant damage can be traced to poor chemistry, which contributes to tube failures via FAC, under-deposit corrosion, corrosion fatigue, stress corrosion cracking, and pitting. Corrosion-product transport leads to deposits in HP evaporators and steam turbines, and in air-cooled condensers. Steam-turbine damage in the phase-transition zone results from pitting, stress corrosion, and corrosion-fatigue cracking. Damage also results from deposition in the HP, IP, and LP sections.

Traditional searches for cause looked at HRSG design, turbine design, and feedwater system design. But it is now apparent that other causal elements were missing. Plants can be at fault by:

      • Not addressing cycle-chemistry basics.

      • Not installing a fundamental level of instrumentation.

      • Not using the international standards of cycle chemistry.

      • Not using total iron as an indicator.

      • Not having an integrated management program to prevent repeat cycle-chemistry situations.

Identification of repeat cycle-chemistry situations is a powerful (yet still overlooked) tool. AHUG Chairman Dooley then reviewed plant assessments using repeat cycle-chemistry analysis, leading to categories within these recurring situations. Tied for first position are corrosion products and instrumentation. Next was drum carryover, followed by failure to challenge the system chemistry status quo, then HP evaporator deposits.

An alarming fact: 37% of HRSGs worldwide are using reducing agents, at very high risk.

Dooley, the executive secretary of IAPWS, and Gary Joy, chairman of the Australian national committee for IAPWS, ended Day Two with an update of that organization’s activities and by reminding participants of the value of its Technical Guidance Documents. Updates of greatest interest to readers were the following:

      • A TGD on film-forming amines will be released in 2016 and will include specifics on their safe and effective use.

      • The TGDs on instrumentation and treatment for fast-start and frequently started HRSGs, published June 2015, are being updated.

      • The TGD “HRSG HP Evaporator Sampling for Internal Deposit Determination” will be published in 2016.                                          

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