The present invention relates generally to the field of chemical treatment of water used for hydrostatic tests, and, more particularly, to a method and chemistry for the reduction of oxygen and microbiologically influenced corrosion in systems requiring a hydrostatic or leak test.
Water used for hydrostatic or leak testing is typically left in pipelines from a couple of days to years. Thus, a pipeline undergoing a hydrostatic test may become quite vulnerable to oxygen corrosion and microbiologically influenced corrosion (MIC). MIC is often established during preparational phases of plant construction or during hydrostatic testing phases, when water is first introduced into the system. Therefore, this period is critical for prevention of oxygen corrosion and MIC and therefore, the quality of the water used for hydrostatic test is of vital importance.
In relation to a hydrostatic test, the main factors that cause corrosion of steel, leading to pipeline damage, are dissolved oxygen and bacteria present in the water used for the hydrostatic test. To prevent oxygen corrosion, oxygen scavengers such as ammonium or sodium bisulfites are commonly used. Preventive measures for control of MIC include biocide treatment, pH adjustment, or sulfate ion removal.
The use of biocides has proven to be effective in controlling MIC. However, most biocides add toxicity to the treated water. The toxicity of the water creates a problem because after the completion of a hydrostatic test the water must be discharged from the tested system, often with severe environmental constraints. For example, if the water is discharged into the ocean under U.S. jurisdiction, hydrostatic test water must pass certain aquatic toxicity tests. Due to such environmental constraints, the chemical selected to treat hydrotest water must not only inhibit oxygen corrosion and MIC, but must also comply with environmental requirements when discharged.
The adjustment of pH inhibits the bacterial growth to a certain extent. Further, the mere adjustment of the pH of water does not protect pipe surfaces from oxygen corrosion, while potentially causing mineral scale problems. Thus, there remains a need for a chemical treatment regimen for water used in hydrostatic test procedures that reduces or prohibits oxygen and microbiologically influenced corrosion of ferrous metals while also meeting environmental constraints placed on the discharge of such water. The present invention is directed to solving this problem in the art.
The present invention combines a chemical treatment strategy, including biocide, oxygen scavengers, and a scale inhibitor, with an adjustment of pH. This combination chemical stratagem provides an effective option for treating hydrotest water for inhibition of oxygen corrosion, inhibition of MIC, and the safe discharge of water into the environment following the performance of a hydrostatic test.
The primary benefit of the combination treatment plan is reduced biocide usage in a high ( greater than 9.0) pH brine. The reduced overall chemical usage facilitates meeting environmental guidelines while also minimizing chemical cost. The method of the invention therefore comprises adding an oxygen scavenger to remove oxygen and prevent or minimize oxygen corrosion; raising the hydrotest water pH, typically in excess of pH 9.5 (this may be accomplished with many bases, although sodium hydroxide is typical); adding a biocide in reduced amount (the ability of the biocide to function in reduced quantity is allowed by the increase pH of hydrotest brine); and adding a scale inhibitor to inhibit scale (an increase in pH of the hydrotest brine can increase the tendency to form mineral scales, such as calcium or magnesium carbonate).
The order of the addition of the chemicals to the hydrotest water may be important, particularly if using oxygen scavengers such as bisulfite that can react with and impair the performance of many biocides. If the selected oxygen scanvenger adversely reacts with the biocide, the it is important to add oxygen scavenger prior to adding biocide and to allow adequate time for reaction.
Based on the previous scenario, a typical application would be, to first inject a package consisting of sodium hydroxide solution, oxygen scavenger, and scale inhibitor into hydrotest water. This would then be followed by injection of biocide and used for the hydrotest application. If there are no negative interactions between the selected chemicals, then all of the chemicals may be injected simultaneously or in a single package.
These and other benefits, features, and advantages of the invention will be apparent to those skilled in the art from a review of the following detailed description along with the accompanying drawings.