MAKEUP WATER AND CONDENSATE RETURN
Makeup water and condensate return for cogeneration units should be sufficiently pure to meet the feedwater requirements when these streams are introduced into the HRSG, Tables 6-9. Water treatment equipment such as filters and ion exchange polishers may be required to treat condensate prior to returning it to the HRSG. As a minimum, each source of process return should be monitored continuously for specific conductivity, pH, turbidity, or other parameters specific to the anticipated impurities.
LP DRUM CHEMISTRY
In some configurations, each of the HRSG drums in a multi-drum configuration is fed in parallel from the hotwell. In others, the LP drum is the feedwater source for higher pressure sections and the drums are in series to one another. The chemical treatment of the LP drum varies according to the specific configuration. Where water from the LP drum is used for attemperation, it must meet the same chemical purity standards as the steam limits. Attemperation rates during startup may be very high (>10%) and may significantly affect the purity of the steam.
In cases where demineralized makeup is used and the feedwater to and blowdown from the LP drum are in parallel with those of the other drums, that is to say that water from the LP drum does not feed any other higher pressure location, the LP drum can be treated with phosphate. The use of solid alkali will mitigate two-phase FAC in the low-pressure evaporator.
In systems containing copper alloys, the LP drum should not be the source of feedwater to the higher pressure drums. Ammonia cannot be used to maintain sufficiently high pH to prevent corrosion (FAC) in the LP drum if copper is used in the steam turbine condenser or in steam host systems.