Corrosion in Low-Temperature Fluids with Low Dissolved Gases: the Case of Condensate Pipelines in Malitbog-South Sambaloran Sector, Tongonan, Philippines

Abstract

The steam turbines of the Malitbog-South Sambaloran power plant, (i.e. consisting of 3 units x 77 MWe) in Tongonan geothermal field generate a total of 90 kg/s condensate coming from the condenser hotwell pump. These condensates with temperature of 50-60?C are disposed through injection process in five wells using a five-kilometer long alvenius pipeline. The alvenius pipes are made up of segments of 15.2-cm diameter by three-meter long pipes, with metal clamps and rubber seals. The main component of the alvenius pipe is galvanized iron with internal and external zinc galvanic coatings.

The condensate fluids from the power plant have substantial amount of dissolved CO2 and H2S (50 and 6 ppm, respectively). These dissolved gases produce weak acid solutions with pH ranging from 5.2 to 6.4, depending on the amount of gases. Simple stoichiometry has identified H2S to be the major determinant of acidity, with contributions from CO2. The corrosive potential of the fluid is aggravated by the presence of dissolved O2 (1 to 3 ppm) picked up from the cooling towers and the open pressure-reducing basin. The weak acid and dissolved oxygen caused uniform, pitting and possibly galvanic corrosion along the pipeline. Leaks and cracking became regular operational problems since the reaction of the condensate with the pipeline favored formation of unstable film of hematite and pyrite.