Chemistry in support of flexible operation

Flexible plant operation impacts HRSG chemistry. New Zealand-based David Addison, principal, Thermal Chemistry Ltd, began Day Two with optimized cycle chemistry, stating: “If baseload HRSG (cycle) chemistry is not optimized, it will never be optimized for flexible operation.” The theme of operational change would continue from Day One of AHUG’s Fifth Annual Meeting, so Addison set clear definitions:

Base load

      • Almost always online, steady load, high load factor.

      • Infrequent starts, mainly cold starts.

Two-shifting/cycling

      • Frequent stops and starts.

      • Large load changes (load following) during operation.

Flexible/fast start

      • Plants able to have accelerated hot, warm, and cold starts.

      • Used with both once-through and drum-type HRSGs.

      • Appropriate for two-shifting/cycling operation.

As discussed the previous day, flexibility is critical in today’s power generation market. This includes the new world of fossil generation impacted by solar and wind generation, and in Australasia geothermal to some degree.

Addison further set the stage for cycle chemistry’s challenges:

Startup chemistry

      • Sampling and analysis systems must produce a quality sample within the flows and pressures required.

      • Dosing systems must establish control and set points.

      • Blowdown and water demand will strain the water treatment plant.

      • Risk of condenser tube leaks will rise with cooling-water pump starts and bypass operations.

      • Availability of plant personnel will be stretched; everyone will be busy.

Large load swings

      • There will be pressure and flow issues, phosphate hideout, and carryover.

A good starting point, stated Addison, is the IAPWS optimized program and readily available (and free) Technical Guidance Documents (TGD) published by the International Association for the Properties of Steam and Water.

Standard base parameters were then listed:

      • Feedwater, AVT(O) or OT (oxygenated treatment).

      • LP evaporator drum (feedforward), AVT(O) or OT.

      • LP/IP/HP evaporator (once-through), OT.

      • LP/IP evaporator drum (standalone), phosphate treatment (PT) or caustic treatment (CT).

      • HP evaporator drum, AVT(O) or PT/CT.

      • Instrumentation (refer to IAPWS TGD, available at www.iapws.org).

      • Carryover testing (refer to IAPWS TGD).

      • Corrosion-product sampling and analysis (refer to IAPWS TGD).

For dosing, the standard is AVT(O) or OT feedwater chemistry with no oxygen scavenger used at any time. AVT(O) should provide 5 to 10 ppb dissolved oxygen.

Feedwater pH control is critical, and the fully automatic control loop should maintain a 9.8 control point under all startup and operating conditions.

Evaporator pH control also is critical. Automatic phosphate or caustic control ensures stable evaporator pH and minimizes over- or under-dosing issues.

pH control is critical, too, for mitigating single- and two-phase flow-accelerated corrosion (FAC) and to minimize the risk of contaminates in the system.   

With phosphate treatment, hideout can be a challenge, particularly above 1450 to 1520 psig. Fast starts and load changes can aggravate the situation in some HRSG designs. If using trisodium phosphate only, no corrosion issues are caused by hideout. But with mono- or di-sodium phosphate there is major acid phosphate corrosion risk. Therefore, the standard recommendation is TSP only.

Phosphate should be maintained above 0.3 ppm for minimum protection, along with proper pH for FAC protection. If there is a need to go lower (phosphate) then NaOH can be added. Sometimes, a conversion to caustic treatment (with no hideout issues) is needed. The IAPWS TGD provides clear guidance on this in Section 6.4.

Once-through HRSGs are suited to flexible operation with AVT/OT chemistry. They do, however, require proper condensate polishing for correct feedwater quality.

Sampling and analysis. Addison then turned to sampling and analysis. Minimum instrumentation should be in line with the IAPWS TGD (2015 revision) which includes fast- start/flexible HRSG advice in Section 4.7:

      • Water flushing of automated analyzers.

      • Short sample lines/local sample conditions.

      • Degassed conductivity (CACE) on condensate and superheated steam.

Corrosion products. It is critical to know total iron levels, a key chemistry parameter. IAPWS is currently working on more guidance for flexible operating regimes.

Carryover. Perhaps the most robust discussion centered on carryover (photo). For HRSGs with drums, flexible operation leads to high carryover risk, which is conducive to superheater and steam-turbine damage. This risk increases during startups and rapid changes in load. Thus drum level control is critical and should be fully validated against saturated-steam analysis and carryover testing (in line with IAPWS TGD). Continuous online saturated-steam sampling, and CACE and sodium analysis, are strongly recommended.

AHUG fig 4

In general, carryover increases with HRSG operating pressure. Simple steam-drum designs have higher carryover limits, but offer poorer steam purity. Lower drum pressures reduce the carryover risk, but risks escalate with fast starts and flexible operation.

Consistent with others, Addison stressed the importance of proper layup and storage procedures and equipment. In summary, short-term wet layup of an HRSG requires nitrogen capping; long-term wet layup requires nitrogen capping, HRSG circulation, mechanical dissolved oxygen control, and pH control. Dry storage for both the HRSG and steam turbine requires automatic dehumidification systems.

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