Selection of an effective chemical treatment program for heat recovery steam generators requires an analysis of the equipment provided, the operating requirements of the system, and the personnel and equipment available to control the water chemistry. This paper discusses the advantages and limitations of the various treatment approaches commonly employed, and provides a flow diagram to assist in selecting a suitable program for various HRSG system operating requirements.
Selecting a chemical treatment program that will be effective in a gas turbine heat recovery steam generator (HRSG) system is not a simple task. Efficient, reliable, safe operation of HRSGs requires a water treatment program that is engineered to meet the needs of the individual system. An engineered approach to treatment selection and application is required because HRSGs and their operating requirements vary significantly from facility to facility. The type of equipment, the operating requirements of that equipment and the personnel available for management of the water treatment program should all be considered when selecting a chemical treatment program.
HRSG EQUIPMENT AND OPERATION
HRSGs can operate at pressures ranging from 5 to 2500 psig (0.13 to 17.3 MPa), and it is common to have high pressure (HP), intermediate pressure (IP) and low pressure (LP) units in the same facility. HP unit operating pressures vary from 900 to 2500 psig (6.30 to 17.3 MPa), and sometimes this unit operates with sliding pressure. IP unit operating pressures are usually in the 200 to 500 psig (1.48 to 3.54 MPa) range, while LP unit operating pressures are commonly in the 5 to 100 psig (0.13 to 0.79 MPa) range. While most units are "drum boilers," some are "once through" steam generators.
Some HRSG units are almost base loaded while others cycle on and off frequently, with 50 to 100 cycles per year, or even daily cycling, not uncommon. Many operate strictly to produce electric power and consequently have nearly 100 % steam turbine condensate return. However, other units send steam to a host and may not return condensate, or may return condensate from a processing plant that is subject to significant contamination. Some units cascade blowdown from higher pressure drums to lower pressure drums, limiting treatment flexibility and complicating control. Often the low pressure section of the HRSG provides feedwater for HP and IP sections as well as for steam attemperation.
Preoperational treatment
While a detailed discussion of preoperational cleaning and storage is beyond the scope of this paper it must be noted that this is an extremely important part of the process. Placing units in service that have deposition and corrosion from improper cleaning and storage procedures will inevitably lead to operating problems.
Feedwater and condensate treatment
Feedwater and condensate systems must be protected from oxygen corrosion and flow accelerated corrosion (FAC). In many new systems designed strictly for power generation, only ferrous alloys are present. When feedwater of high purity is maintained, the control of dissolved oxygen is not critical and levels greater than 5 ppb (microequivalents per liter) may prove to be beneficial in reducing single phase FAC. 1 However, if significant oxygen is present, contamination of the feedwater due to condenser leakage can lead to rapid corrosion and equipment failure. In addition, if copper alloys are present, oxygen levels greater than 5 ppb (microequivalents per liter) can lead to rapid corrosion of copper alloys, especially in systems treated with ammonia for pH control.
Consequently, oxygen scavengers are recommended for all systems
