A steam generating plant requires very clean water and carefully controlled chemistry for proper operation of a boiler that runs at high temperatures and pressures. Careful monitoring and control of pH at strategic sampling points is required to prevent chemistry upsets, corrosion, equipment damage, system failure, and downtime. This monitoring of pH in pure water can be challenging due to signal drift, frequent recalibration, and/or poor agreement between on-line and lab/grab sample measurements. Typically, the more pure the water, the more challenging this becomes. When a tight pH range is required for proper operation of a system, a stable, accurate, and reliable pH on-line analyzer is essential.
The plant in this case study is a large lignite-fired power plant with three 750 once-through supercritical units. All three units operate at or near 3,950 psi and 1,000 ºF, utilizing All Volatile Treatment (AVT). Sampling points for pH in the steam cycle include the condensate pump discharge header, polisher effluent, boiler feed pump, and economizer inlet. Before installation of the Orion 2001SC ROSS pH electrodes and analyzers, the existing pH analyzers showed drift and required weekly calibration. In addition, pH electrodes had to be replaced every 9-12 months due to a shift in the reference potential (E0).
The plant started with a demo unit before purchasing the Thermo Orion pH analyzers. Installation of other pH electrodes typically showed pH drift of 0.2 pH in the first two days of operation. However, the Orion 2001SC ROSS pH electrodes held stable and did not have to be recalibrated. After running two months unattended, the analyzer still maintained good agreement with the benchtop grab sample results. This performance not only met, but exceeded, customer expectations. The plant purchased and installed 12 Thermo Orion pH analyzers in 2000. In 2009, three more sampling points were added with Orion pH analyzers installed for each.