AMMONIA AND AMINE USAGE
Many combined cycle power plant designs contain no copper alloys in the steam/water cycle. In these cases, and where superheated steam is attemperated with feedwater, ammonia or high-basicity amines are recommended to maintain the feedwater pH sufficiently alkaline to maximize formation of protective iron oxide and minimize iron transport.
For plants that contain copper alloys in either the steam condenser or in process components (in cases of a steam host), there is a variety of amines that may be considered as an alternate to ammonia, as excessive ammonia causes grooving and stress corrosion cracking in copper alloys.
In systems with mixed materials, the feedwater must also be maintained in a lower range of 8.8-9.3 pH. Air inleakage and potential organic compound degradation, including the degradation of amines and oxygen scavengers may increase the amount of amine required to maintain this pH range and increase the likelihood for corrosion of copper components.
As superheated steam temperatures approach or exceed 1000°F (540°C), owners/operators must be aware that any organic compound, naturally occurring or intentionally added for chemistry control - including amines, can degrade to form acetates, formates and other organic acids. These breakdown products may increase cation conductivity to the point where the value is higher than the recommended normal operating limits.