One size doesn’t fit all for microbiologically-influenced corrosion
Many of us can see examples of corrosion in the form of rust on a daily basis: driving to and from work on highways or bridges, in buildings, parking structures and even in our cars.
Corrosion is the process which uniformly weakens or gradually destroys materials by electrochemical interactions. Microbiologically influenced corrosion, also known as MIC, is a form of corrosion defined in the Fire Protection Handbook as an electrochemical corrosion process that is concentrated and accelerated by the activity of specific bacteria within a fire-sprinkler system resulting in the premature failure of metallic system components.
MIC is not just a concern in the fire protection industry. Mechanical systems in other related fields, including cooling towers and gas pipelines, also are concerned with the presence of MIC in systems. However, these mechanical systems tend to be dynamic and have flow rates that are relatively constant. Fire-protection systems do not have constant flow rates and there are several differences when draining/refilling the fire-protection systems. MIC presents unique challenges to the fire-protection community, while also providing still-evolving solutions to prevent premature system failure.
Differentiating MIC from other forms of corrosion is difficult to substantiate. Although the electric power, gas and nuclear industries have well-documented data supporting MIC failures, fewer studies have been published in the fire-protection industry.
The National Fire Sprinkler Association (NFSA) conducted a survey in 1996 that included a questionnaire in its membership magazine requesting information on the experience of sprinkler system failures. NFSA received 40 responses from members in the U.S. and Canada, and concluded MIC was a “widespread problem.” However, it is important to note the qualifications of the individuals assessing the type of corrosion was not known, which is important due to the difficulties in distinguishing MIC from other forms of corrosion.
FM Global conducted a study based on a review of data from 1988 to 1997 that revealed corrosion as the fifth-leading cause of sprinkler system failures. In another study completed by the FM Metallurgical Laboratory, 155 cases of sprinkler system leaks between 1994 and 2000 were reviewed; MIC was identified to be present in about 40% of the cases.
The sources of MIC — not just a wet-pipe system problem
The presence of MIC in fire-protection systems has been around for a long time, but the source of corrosion has only recently been found. Although not conclusive, MIC predominately is thought to come from microorganisms with sulfate-reducing bacteria or acid-producing bacteria found in a system’s water source. NFPA 13, Standard for the Installation of Sprinkler Systems, requires water supplies and environmental conditions to be evaluated for the existence of microbes and conditions that contribute to MIC.
What many people do not realize is MIC is not limited to wet-pipe systems and can occur in dry systems as well. In accordance with NFPA 25, Standard for the Inspection, Testing and Maintenance of Water Based Fire Protection Systems, dry-pipe systems are required to undergo a “trip test” every three years or when the system is altered. In addition, the dry-pipe valve must be trip-tested annually. The trip test allows for a comparative assessment of the water delivery time that can reveal significant delays in system operation. Although critical to confirm system performance, this test can create the right atmospheric moisture content for some bacterial colonies to develop, causing MIC. To mitigate this problem, a complete draining and the subsequent use of truly dried air or nitrogen gas should be used.
A dry-pipe sprinkler system is a system that contains air or nitrogen under pressure and when a sprinkler head opens due to the heat of a fire, the release of pressure allows the water to flows into the piping system and out of the opened sprinklers.
MIC — Premature failure of fire-sprinkler systems
As stated above, MIC within a fire-sprinkler system can result in the premature failure of metallic system components. While steel piping typically is the first observed point of failure, MIC can be found in sprinkler orifice caps, control valves, fittings and supply tanks. The localized corrosion that develops can be in the form of pitting, crevices, cratering and filiform.
Two forms of premature failure have been identified. The first is failure of a system to hold water, which is a result of pinhole leaks from the localized and accelerated corrosion. This can cause significant damage to the system, resulting in water damage to the occupancy and the incurring detrimental financial implications.
The second form includes the failure of a system to operate as designed to achieve fire control from biological growth. Biological growth can be generated as a byproduct of microbial activity and can obstruct sprinkler system piping, changing the roughness of the interior of a pipe as well as the internal diameter of the pipe. The change in these two factors significantly impacts the hydraulics of a sprinkler system and could prevent water from reaching the hazard in the event of a fire.
The presence of MIC typically is first revealed with pinhole leaks in a system, but detecting MIC is difficult without conducting assessments. Differentiating MIC from other corrosion is difficult and even after these assessments, MIC may go undetected. If MIC is found in the system, test kits and services are available to analyze the test samples for the presence and extent of MIC within the system.
In accordance with NFPA 13, when conditions are found to contribute to MIC, the owner is required to notify the sprinkler system installer and develop a plan to treat the system using one of the following methods:
1) Install a water pipe that will not be affected by the MIC microbes;
2) Treat all water that enters the system using an approved biocide;
3) Implement an approved plan for monitoring the interior conditions of the pipe at established time intervals and locations; and
4) Install a corrosion monitoring station and monitor at established intervals.
NFPA 25 provides requirements for the inspection, testing and maintenance of water-based fire-protection systems, dedicating an entire chapter on how to conduct obstruction investigations. Internal obstruction assessments are required at least every five years for sprinkler systems unless another assessment frequency has been established by an approved risk analysis. The associated annex language provides several methods to conduct an internal obstruction assessment, including alternative examination methods such as video inspection equipment and ultrasonic technology. If organic or inorganic foreign material is found to obstruct pipe or sprinkler, or one of the 14 conditions specified in Section 14.3.1 of NFPA 25 exists, an internal obstruction investigation is required. An internal obstruction investigation requires an internal four-point inspection at a system’s control valve, sprinkler riser, cross main and branch line. If problems are detected in any of these four points, a complete flushing program is required to be conducted by qualified personnel.
What you can do
If MIC is found in a system, the fire-protection system should be assessed to determine the extent and severity of MIC. Unfortunately, there is no “one size fits all” solution. In some instances the affected portions may be able to be cleaned to remove obstructions, and in other circumstances portions of the system may need to be replaced.
It is vital to be proactive in the prevention of MIC within fire-protection systems by testing the water supply and determining the type of bacteria present for adequate treatment. If MIC is present, using corrosion-resistant piping or designing a system using thicker piping could buy time for the system’s life cycle or until a universally acceptable method is achieved.
In addition, analyzing the frequency of drain and alarm testing could reduce the biological and non-biological forms of corrosion that have the potential to grow when draining and refilling fire-protection systems.
Although there are no chemical-injection systems specifically listed or approved for fire-protection systems, this is another method to mitigate MIC only after several factors have been considered. For example, a complete toxicity review to confirm if the specific chemicals are nontoxic and will not impact firefighting properties or deteriorate other sprinkler system components made of rubber or polymer material (pipe couplings, O-rings, valves) is an option. In addition, the chemical(s) selected need to be analyzed to ensure the chemical(s) does not change the pipe surface roughness. Other considerations include adequate sensitive draining, which can be provided to guarantee the municipal water is not affected.
As the fire protection community continues to study MIC and develop requirements to mitigate premature system failures, refer to NFPA 13 and NFPA 25 for more information on this evolving issue.