The paper will discuss the effects of impurities in HRSG boiling waters on steam purity and carryover into
the steam turbine. The equation used for predicting vaporous silica carryover and the equation used for
predicting mechanical carryover of other impurities will be applied to determine for a given operating
pressure and pH, the allowable impurity in the HRSG evaporator circuits. The determination of the final
Total Steam Purity, which includes attemperation water, will be provided in a fashion that allows the user
to assess such without interpolations of multiple graphs. This then allows for the setting of upper impurity
control limit values allowable within the boiling waters of multiple drum steam generators and ease of
assessment of off specification steam purity. Conditions leading to off specification steam purity will also
be itemized to provide site operating personnel directions to investigate for steam purity control
restoration.

Key words: heat recovery steam generators (HRSG), total steam purity, mechanical carryover, vaporous
carryover, attemperation water quality, feedwater quality, internal water treatment regimes, CPT
(Congruent Phosphate Treatment), PT (Phosphate Treatment, Phosphate Continuum), AVT (All Volatile
Treatment; O, Oxygenated or R, Reducing), OT (Oxygenated Treatment), CT (Caustic Treatment), boiler
water carryover, steam separation, steam quality, sodium, silica, chloride, sulphate, conductivity, cation
conductivity, degassed cation conductivity, velocity, specific volume, disengagement space, normal
operating level (NOL), steam limits, action levels, saturated steam, superheated steam, reheated steam,
steam sampling, isokinetic flow.