Steam purity requirements and the amount of carryover, mechanical and vaporous, of the evaporator water into the steam will dictate the chemistry of the IP and HP drums. The vaporous carryover is a function of the total dissolved solids and other contaminant concentrations in the drum, operating pressure of the drum and the pH of the evaporator water. The mechanical carryover is a function of the drum design and condition of the steam/water separators, the operating pressure, the drum water level control, the steam header pressure stability, the steaming rate and the magnitude of sudden surges in the steaming rate.

Conventional water-tube boilers are conservatively designed to produce steam that contains less than 0.1 to 0.2% mechanical carryover for a drum operating between 1001-2400 psig (7.0-16.6 MPa), according to American Boiler Manufacturers Association (ABMA) standards. Measured carryover is typically lower as long as the drum is operating at the design level and the separation equipment is working properly. The percent mechanical carryover from any drum that supplies steam to the turbine should be checked every six months by comparing the sodium in the drum water (grab sample) versus the sodium in the saturated steam (on-line analyzer).

Attemperation or desuperheating water provides a direct route for possible additional steam contamination. Depending on the purity of the feedwater and the amount of attemperation needed during start-up, duct burner use, or other operating conditions, the amount contributed from this source can be significant.

Species such as chlorides, sulfur compounds, and hydroxides in steam can cause corrosion of steam turbine materials. Other species such as silica and copper compounds can cause deposits and reduce turbine efficiency. There are various chemical regimes for the feedwater and drums that can be used to achieve the steam chemistry requirements listed in Tables 10 and 11. The selection of a chemical regime is a site-specific decision based on a number of factors including:

  • • Configuration (number of drums)
  • • Materials of water- and/or steam-touched equipment
  • • Operating pressure of HP and IP drum
  • • Operating Conditions (base-load, cycling, peaking)
  • • Balance of plant equipment

The advantages and disadvantages of common chemical treatment regimes are discussed below.

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