Attack of the Metal-Munching Microbes
Like creatures in a horror movie, tiny acid-producing microorganisms are eating away at some of our most important infrastructures, including power and petrochemical plants, pipelines, pulp and paper mills and transportation systems. Commonly known as microbiologically influenced corrosion (MIC), this little-understood phenomenon also is a cause of pipe failures (blockage and leakage) in automatic fire sprinkler systems throughout the world.
In a study of 155 cases of failed sprinkler components collected between 1994 and 2000, the FM Global metallurgical laboratory found evidence of MIC in about 40% of the cases. According to FM Global Property Loss Prevention Data Sheet 2-1, Prevention and Control of Internal Corrosion in Automatic Sprinkler Systems, corrosion, including MIC, was the fifth largest cause of sprinkler leakage losses, preceded by mechanical injury (e.g., forklift impact), freezing, defective equipment and accidental discharge.
Until recently, MIC has been implicated in sprinkler pipe failures primarily through empirical evidence such as pinhole leaks, particularly those that appear in relatively new sprinkler systems; sulfur odor; pitting; biofilm (black slime); flow restrictions or blockage; and the formation of characteristic nodules known as tubercles. In addition, basic laboratory tests (e.g., sulfur prints, stereomicroscopic and metallographic examination) have been used to identify the presence of certain byproducts of microbial activity, including the presence of sulfides in the corroded areas .
New diagnostic methodology
In recent years, advanced test methodologies have been used to make more conclusive diagnoses of MIC occurrences. Typically, suspect pipe sections alone, or in combination with water or sludge, are gathered and analyzed using several different types of tests. FM Global completed an MIC study earlier this year that employed a unique battery of tools to more accurately detect and diagnose MIC. The study was so successful, the testing protocol will now be used by the FM Global metallurgical laboratory to test failed sprinkler components.
FM Global scientists Geary Yee and Michael Whitbeck designed and conducted the MIC study as part of a strategic research initiative to address this little-understood cause of sprinkler system failure. The analytical and microbiological tools used in the study included:
Microbial community analysis (MCA)
Ion chromatography (IC)
Scanning electron microscopy (SEM)
According to Whitbeck, tests were chosen after a thorough review of prior MIC research. “We looked at the body of literature covering microbial influence in aqueous environments,” he said. “We also looked at the most promising models for how MIC develops and hypotheses for the metabolic characteristics of the organisms most commonly associated with MIC, such as sulfate-reducing bacteria (SRB).”
Catching MIC in the act
Proper test sample collection was the greatest challenge, said Yee. “Getting the right samples was tricky,” he explained. “We needed the samples, including pipe, supply water and water or sludge from inside the pipe, as quickly as possible and free from contamination. Once a site was identified as a possible investigation target, we would send out a sampling kit we devised with instructions on how to obtain a proper sample.”
Seven sites were selected in the United States, Australia and the United Kingdom. In some cases, multiple samples were collected from each site. Once collected, samples were sent by overnight delivery to the FM Global offices in Norwood, Mass. From there, certain sample components were distributed to outside laboratories for MCA and SEM with energy dispersive X-ray spectrometry and environmental SEM, which provided images of bacterial influence in sprinkler pipe corrosion. Immunoassay tests, IC and sulfide analysis were all conducted in the FM Global analytical laboratories in Norwood.
Sampled source water used to feed or test the sprinkler systems was compared with water collected inside the failed sprinkler pipes. MCA, a quantitative molecular technique used to genetically characterize and compare microbial community structures, was used to determine which bacterial species, present in the source water, survived and thrived inside the failed sprinkler pipes. Debris from inside the failed pipe also was analyzed with MCA. Bacteria found in the debris were likely responsible for MIC, noted the research report.
One advantage MCA offers over the traditional culturing approach is that it is quicker (few days vs. 1 to 2 weeks) and that all bacteria present in the sample are observed. With culturing, only about 1% of the bacteria present can be grown and analyzed. MCA creates a microbial community profile using bacterial DNA to identify a unique gene “fingerprint” for each bacteria species. Some of the bacteria and microbes implicated in MIC include:
Sulfate-reducing bacteria (SRB)
Sulfur-oxidizing bacteria (SOB)
Acid-producing bacteria (APB)
Iron-reducing bacteria (IRB)
Low-nutrient bacteria (LNB)
These and many other bacteria are naturally occurring and can be found in ground and surface waters, as well as in soils, particles, oils and other substances that can be present or introduced into sprinkler pipes before, during or after installation. According to the research report, SRB have been implicated in the corrosion of many types of metals, including cast iron, steel, stainless steel, copper and high nickel molybdenum alloys. SRB are anaerobic, requiring no oxygen to survive, and reduce sulfate to sulfide. This process can result in the production of hydrogen sulfide (H 2 S) and its characteristic rotten-egg odor.
MCA offers a new methodology for identifying the bacterial community structures responsible for MIC in failed fire sprinkler systems; however, further testing with this technique is still required. “I really wasn’t certain what to expect from this study,” Yee said. “We believe we picked a very good battery of tests in that each test complemented and confirmed the results of the others. The consistency of the results—and that they seemed to point toward MIC in almost every case—was somewhat unexpected.”
Conclusive diagnosis of MIC is an essential first step in the difficult process of MIC remediation. FM Global’s Chief Engineering Technical Specialist Paul Janusauskas, Montreal (Canada) operations, welcomes the new MIC test methodology. He has investigated an increasing number of MIC-related sprinkler system failures over the past few years. “It’s fair to say there are more MIC problems out there than we are aware of,” Janusauskas said. “We are seeing it more and more in dry-pipe systems, particularly in our saw mill and pulp-and-paper accounts. These operations use a high percentage of dry-pipe systems, such as in their lumber kiln areas and areas exposed to freezing temperatures. It seems to be a perfect breeding ground for MIC.”
In one case, a large lumber company near Montreal experienced MIC-related leakage in its dry-pipe systems after only five years of service. Most sprinkler systems are designed to last 40 years or more. A review of the company’s dry-pipe system revealed it had not been properly drained after trip testing, actuation due to leakage and/or after any false trips. The investigation report concluded future MIC damage could be avoided if replacement piping is properly pitched for improved drainage, and care is taken by the company to properly drain the system after testing or accidental discharges.
According to Janusauskas, when MIC is suspected or confirmed in the field, the client is given D. S. 2-1, which contains the latest recommendations for the identification, prevention and mitigation of MIC. It provides step-by-step guidance in the prevention of MIC in new sprinkler systems and MIC control in existing systems, including current mitigation tools and treatments. Generally, any leaking pipes must be replaced. Where MIC is suspected, heavier gauge pipe, such as Schedule 40, is suggested in place of the more common Schedule 10 or 5.
FM Global is continuing its search for new and creative ways to identify and treat MIC. Recently, FM Global’s Toronto (Canada) operations conducted a study of videoscope technology for the assessment of dry-pipe sprinkler systems. The purpose of the study was to determine if videoscope inspections could be used as a substitute for traditional flushing investigations during the deep winter months in Canada when extreme cold prevents flushing, or when the system is installed over highly sensitive electronic equipment.
Bennie Vincent, an FM Global senior research engineer in the protection research group, based in Norwood, selected the hardware for the videoscope pilot conducted by Toronto operations. He went to Canada to witness one of the investigations and brought back samples for analysis. Based on the laboratory analyses that showed clear bacterial contamination in several of the samples, along with the clear video images of pipe interiors provided by the videoscope, he concluded the system offers promise, not only as a tool for assessing internal pipe corrosion and potential sprinkler system impairment, but also as an MIC detection tool as well.
“I think there is a combination of factors that has led to an increased awareness and reports of MIC,” Vincent said. “Twenty years ago, you didn’t hear about MIC or this type of problem, certainly not to the extent we do today. I think the thinner gauge pipe that’s commonly used today, along with an increase in sprinkler system testing that may leave residual water in these dry-pipe systems—particularly those that may not have proper drainage—may be one explanation for the increase in MIC incidents. It’s difficult to know for sure. We’ve come a long way toward understanding the problem, but we still have a long way to go as well.”
It’s all about MIC
Microbiologically influenced corrosion (MIC) almost always occurs in conjunction with other types of corrosion, including rust or other oxidation, scaling and galvanic corrosion. MIC not only forms in wet sprinkler systems, but also in dry-pipe systems where water is left standing, either from poor drainage, condensation, too frequent testing or valve leakage. In fact, dry-pipe systems with remnant water offer a highly favorable environment for MIC and have experienced very aggressive MIC attacks, with system failure occurring in a year or less in some cases.
Unlike uniform corrosion such as rust, MIC is typically localized and appears to develop in a three-step process. In the first step, bacteria and other microbes settle and attach themselves to favorable areas inside the pipe, such as nutrient-rich areas, crevices, joints and other imperfections. These aerobic bacteria (which thrive in oxygenated water) begin to thrive and create large colonies. These colonies produce byproducts that can include biofilm (black slime, typically) at the attachment site.
In step two, anaerobic bacteria (requiring little or no oxygen to survive), such as sulfate-reducing bacteria (SRB), penetrate and become established below the biofilm. The biofilm, and the aerobic bacteria that inhabit it, effectively filters oxygen out of the water entering the biofilm, creating an environment favorable for SRBs and other anaerobic bacteria. In this phase, a corrosion cell begins to form, covered by the biofilm or a nodule, known as a tubercle. SRB obtain energy by reducing or oxidizing inorganic sulfur compounds and producing hydrogen sulfide as a byproduct. This leads to the development of an acidic environment inside the tubercle, which begins to corrode the pipe wall forming a corrosion pit.
In step three, as the bacterial colonies continue to grow, increasingly larger tubercles are produced. Like tiny chemical factories, the tubercles develop increasingly acidic interiors, while at the same time protecting the bacterial colonies from outside attack. The corrosion pit continues to expand and eventually breaks through the pipe wall in the form of pinhole leaks. Long before leaks occur, however, tubercles and other MIC deposits can reduce the effective flow capacity of the sprinkler pipe and lead to blocked sprinkler heads.
In a case of suspected MIC reported by the Iowa State Fire Marshall and the American Fire Sprinkler Association, a small fire in a nursing home laundry room grew into a serious situation when a sprinkler head failed to operate. The fire was controlled by the local fire department. It was later discovered that the fusible link had functioned, but the sprinkler head was completely clogged with hardened, rust-colored debris. Every sprinkler head in the wing where the fire occurred was inspected and found to be clogged. Laboratory testing of pipe and debris samples subsequently confirmed high levels of bacteria. This, in combination with thick deposits and pinhole leaks in other pipes in the facility, led investigators to conclude MIC was to blame.
FM Global Property Loss Prevention Data Sheet 2-1, Prevention and Control of Internal Corrosion in Automatic Sprinkler Systems, recommends taking the following actions to avoid MIC in dry-pipe systems:
Avoid the use of roll-grooved joints—these can promote water accumulation and provide corrosion sites;
Install pipe with the proper pitch to promote drainage of all test water and/or condensate;
Pressurize the system using dry nitrogen (from cylinders or plant supply) and an air supply as a backup;
Alternatively, install a dehumidification system for supply air;
Keep low-point drains clean and drain condensate as often as needed to prevent water buildup; and
Fix air leaks to keep system as tight as possible.