There are a variety of HRSG configurations and operating pressures. For the purposes of this document two configurations have been considered: an industrial HRSG and a three-drum HRSG where the steam is used in a steam turbine to generate electrical power. These are shown in Figures 1–2. The chemical limits are always dictated by the steam purity requirements, the highest drum pressure on the HRSG, and, if applicable, by the highest pressure at the duct-fired condition.

Although the temperature of the turbine exhaust is much lower than the flue gas temperatures in an equivalent water-tube boiler, sudden heat flux changes and inefficient circulation in HRSGs, particularly during start-up, require a higher purity feedwater and a lower chemical treatment rate than would typically be associated with the respective operating pressures.

Depending on the configuration, the LP drum may be isolated from the higher-pressure drums and require its own chemistry or may be, in effect, a feedwater heater that feeds all the higher-pressure drums in the unit. In the latter case, only volatile chemicals can be used for treatment in the LP drum as water from the LP drum is often used as attemperation water for superheat and reheat. The intermediate pressure (IP) drum often receives blowdown from the high-pressure (HP) drum, therefore the IP drum chemistry is to a certain extent set by the HP drum chemistry. The HP drum chemistry is determined by its operating pressure when operating at its highest capacity.