Flexible operation, cycle chemistry dominate discussions at European HRSG Forum

The two themes dominating presentations and discussions at the fourth annual meeting of the European HRSG Forum were the need for greater operational flexibility and the importance of cycle chemistry for achieving high reliability of Rankine cycle components. EHF2016, held May 9-11 in Prague, Czech Republic, chaired by Dr R Barry Dooley of Structural Integrity Associates Inc, was arranged by Ladi Bursik of BHT GmbH. Open discussion sessions were led by Bob Anderson of Competitive Power Resources Inc (see companion article on new USA HRSG Forum).

The large amount of renewables generation installed in Europe, which has priority on the grid when the wind blows and sun shines, demands that combined-cycle plants shut down more often and start up much faster than they were originally designed to do. Several presentations addressed the need for increasingly flexible operation, how to accomplish it, and ways to avoid damaging the plant while pushing it harder.

Flexibility. Pascal Decoussemaeker and Anand Nagasayanam, GE Power, Switzerland, presented their company’s approach for getting more power on the grid earlier in the startup. Key elements of GE’s approach include the following:

      • Configure the generating unit’s fuel systems to use the purge credit approved by NFPA in 2011.

      • Provide auxiliary steam to seal the steam turbine and use vacuum ring pumps to establish condenser vacuum prior to startup.

      • Automate key “plant ready to start” pre-checks.

      • Optimize among gas-turbine (GT) exhaust characteristics, HRSG heating surfaces, and attemperators to quickly provide the required steam conditions to roll, accelerate, and load the steam turbine with the GT at high load.

      • Optimize water and steam chemistry monitoring to quickly meet steam purity requirements.

      • Use passive (stack damper and improved stack insulation) and active (steam sparging) HRSG heat conservation during layup to achieve faster startup.

      • Use active (hot air circulation) steam-turbine heat conservation during layup to achieve earlier roll off and faster acceleration.

Important to remember is that all of the above must be accomplished while paying attention to GT, HRSG, and steamer stresses and life consumption.

Cycle chemistry of HRSG-equipped combined-cycle and cogeneration plants continues to be at the forefront of concerns and problems for operators in Europe and worldwide with the major failure/damage mechanisms being flow-accelerated corrosion (FAC) and under-deposit corrosion (UDC).

The primary technical aspect associated with both mechanisms is the sampling and monitoring of total iron corrosion products. To assist operators in addressing the UDC mechanisms, the International Assn for the Properties of Water and Steam (IAPWS) has developed a new Technical Guidance Document on sampling HP evaporator tubing and a deposits map to determine when an HRSG should be cleaned. IAPWS supports user activities directly, and indirectly with proactive participation in EHF, the Canadian HRSG Forum (CHF), the Russian HRSG Forum (RHF), and the Australasian HRSG Users Group (AHUG).

The more frequent shutdowns and startups associated with flexible operation force generating units to spend more time in layup than would have been expected at the design stage. Because of the need to startup quickly when called, wet layup is required. Michael Rziha presented Siemens’ approach to preventing corrosion damage during layup, and equally important, minimizing transport of iron oxide during the restart.

Rziha noted that layup plans should include both HRSG internal surfaces in contact with water/steam and external heat-transfer surfaces in contact with hot gas, as well as the steam turbine, condenser steam-side, and the waterside of some heat exchangers. Common layup errors he identified included the following:

      • Elevation of pH as the only action during layup.

      • Dosing of reducing agents during shutdown periods (in all-ferrous systems).

      • No preservation actions during short layup periods.

      • No detailed plan for implementation of layup measures.

      • No plan for controlling the layup.

      • Waiting too long to apply layup measures.

Rziha offered this advice to operators:

      • Proper layup is very important.

      • Layup measures should be executed for shutdowns longer than 24 hours.

      • Layup procedures should be as flexible as possible.

      • Prepare detailed plans for different scenarios in advance.

      • Always control the layup conditions frequently and routinely.

      • Update and refine plans based on experience.

      • Do not underestimate stand-still corrosion.

      • Do not forget the gas side of the HRSG.

Filming amines. In response to the considerable interest in film forming products (FFP) expressed by many of the 80 attendees at the meeting, Dooley offered that IAPWS has developed a guidance document to help owner/operators by assuring these products are applied, monitored, and analyzed properly. The IAPWS executive secretary noted that FFPs are being applied in a wider series of HRSGs—even including a triple-pressure HRSG with a Benson once-through HP section, which was reported on at the meeting. An international conference on FFP will be held in early 2017; write Dooley @ bdooley@structint.com to receive information as it becomes available.

Trends in cycle chemistry and thermal transient performance were the subject of a joint presentation by Dooley and Anderson based on assessments of more than 50 combined-cycle plants over about the last 10 years. One of the first public reports on this work, based on material gathered during their first nine plant reviews, was published in CCJ 1Q/2009. The duo updated this work, indicating which trends identified continue today, which do not. The most frequent cycle-chemistry shortfalls, presented by Dooley, are the following:

      • Minimum level of chemistry monitoring, as recommended by IAPWS, not installed or maintained.

      • Iron oxide levels in boiler water not know or too high.

      • Drum carryover is not measured.

      • Deposit loading and deposition rate in HP evaporator tubes not known (samples not taken).

      • Inadequate protection during layup.

      • Inadequate monitoring of contaminant ingress and/or failure to set and follow action levels.

      • Dosing of proprietary chemicals without knowledge of constituents.

      • Dosing of incorrect chemicals (typically a reducing agent).

Anderson followed Dooley with a commentary on the most frequent causes of avoidable and damaging thermal transients—including these:

      • Attemperator spray water leaking into steam pipe.

      • Inappropriate attemperator-spray block/control valve logic.

      • Failure to routinely inspect attemperator hardware.

      • Poor draining of HP superheater during startup.

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