OTSG and HRSG require better ...

OTSG and HRSG require better quality feed water than 1 ppm hardness.  Typically, mixed bed IX is required to reduce all salts to very low levels, in particular hardness and silica.  If your current IX system is sodium cycle softeners, I would suggest adding 2-pass RO downstream.  If your current IX system is strong acid cation followed by strong base anion, then mixed bed IX should be added downstream.  Better quality boiler feed water will significantly reduce boiler scale.