While the main focus on water and steam chemistry in the new combined cycle plant is on the HRSG and steam turbine, chemistry limits are also important for water that is used to cool air feeding the gas turbine and water directly injected into the gas turbine. The limits suggested herein address all these water uses in a combined cycle plant. Small, low pressure HRSGs operating with soft water makeup are included in the document, although the suggested chemistry limits for these units are the same as for industrial watertube boilers. This document does not cover HRSGs used for enhanced oil recovery or thermal in-situ operations.
TYPES OF HRSGS
Many HRSG configurations create very complex flow patterns. During start-up, some tubes in the HRSG may remain stagnant or even flow in the opposite direction from operation. Lay-ups, particularly those in hot standby condition (daily cycling), are common and chemistry during this time is also important. These conditions dictate chemical operating limits that more closely approximate those for large, fossil-fuel boilers than the drum pressures would indicate.
Most recently built combined cycle power plants use demineralized water makeup. However, some existing units, primarily in industrial settings, operate at low pressures and use softened water makeup.
Units operating at higher pressures and using demineralized makeup are more sensitive to some types of corrosion than those on softened water. For example, softened water units rarely have issues with flow accelerated corrosion (FAC). Since the chemistry limits of HRSGs using softened water and demineralized water are significantly different, they are detailed in separate tables in this document.
Once-through heat recovery steam generators are HRSGs that do not include a conventional steam drum for the separation of water and steam. Water is pumped into the unit as a subcooled liquid and passes over the HRSG heat exchange surfaces where it is converted to steam. Since this HRSG design does not include provisions for concentration and mechanical removal of impurities in the system, feedwater quality in these units must match superheated steam purity requirements. Specific guidelines on water chemistry operating limits for these units should be obtained from the manufacturers.