Materials Performance

DEC 2016

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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41 NACE INTERNATIONAL: VOL. 55, NO. 12 MATERIALS PERFORMANCE DECEMBER 2016 ria. Samples can be collected from specific points for testing planktonic bacteria. Bio- probes or coupons can be used for obtaining sessile bacterial counts. If a pipeline has a high bacteria count, the source of bacteria should be identified. If bacteria are gener- ated in the pipeline, then treatment with a biocide can solve the problem. If bacteria are not generated in the line, then the source of bacteria is upstream, and another strat- egy is required to mitigate the problem. Selection of Test Kits, Media for Culturing Bacteria Getting positive results when measur- ing bacteria counts can be very difficult (Figure 1). Because these bacteria require suitable cultures, media should be active to ensure the test bacteria survive. To deter- mine bacteria counts, standard methods like NACE TM0194 1 need to be followed; they provide complete details about the culture. Today, with many instant bacteria count checking products available in the marketplace, results may not be reliable if they are not developed on the standard test methods. Culture bottles should be main- tained and used before the expiration date. Tests should be carried out in duplicate or triplicate for more accurate results. Some standard organizations call for preparing the culture based on the produced water composition, mainly total dissolved solids. This type of culture gives readings on par with actual bacteria counts in the system. Sample Collection and Preparation Using sterilized bottles and needles for this test is important; using contaminated apparatus or sample bottles for bacterial tests can significantly change the results. Utmost care should be taken to avoid interference during these tests. For exam- ple, a water sample containing H 2 S can react with the iron nail inside the SRB cul- ture bottle, which could be mistaken as the presence of SRB. An analyst or chemist should observe the culture media for some time after injecting the sample. While bac- terial reactions take some time, H 2 S reacts immediately with the iron. To identify the bacteria count in water containing H 2 S, the sample can be heated to liberate H 2 S and avoid a reaction be- tween the H 2 S and the nail in SRB culture bottles. Heating the sample to tempera- tures higher than the system temperature can kill bacteria present in the water sam- ple, so care needs to be taken when heating samples in this type of test. Normally, a water sample will be col- lected and tested for bacteria count, but this will give a count of planktonic bacteria only. These bacteria move along with fluid. A sessile bacteria count is much more im- portant, because these bacteria adhere to the pipe and can cause more damage to the system than planktonic bacteria . Since f luid collected from the system will not provide an idea of the sessile bacteria count, bioprobes, studs, or coupons taken from the system should be used instead for sessile bacteria measurement. These are placed in the fluid flow and sessile bacteria adhere to them. The sessile bacteria sam- ple can be collected by swabbing the bio- prob es, stud s, or coupons with st erile buds. The bacteria sample can be placed in the sterile nonnutrient water immediately and th en inje ct ed into th e culture to incubate. MIC Testing There should not be any delay in inject- ing the bacteria sample into a culture. Ac- cording to NAC E TM0194, th e sample should be injected immediately after col- lection. Injecting the sample into a culture while in the field is preferred, in case the laboratory is located far from the field. The serial dilution method can be followed for testing. Checking the incubator temperature is also very important. Too many fluctuations in temperature can render inaccurate re- sults by killing test bacteria in the culture bottles. Maintaining the incubator temper- ature on par with the system temperature is required; only then can the test simulate the actual conditions of the system. Taking the culture vials out of the incubator daily or many times in a day to check the status can lead to inaccurate results because these bacteria are temperature-sensitive. Prevention of MIC by Chemical Treatment Biocides are used to eradicate microor- ganisms in pipelines. At least two biocides may be used alternately for better results and to reduce the immunity of bacteria. Biocides can be injected in a batch process FIGURE 1 The sensitivity of a MIC monitoring and prevention system is like a balloon with air. Just as any one nail can puncture a balloon, one of many possible factors can hamper the system.

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