Components of a Sprinkler: What Causes Corrosion?
Wet and dry sprinkler systems consist of metal pipe, water and trapped or compressed air. Any environment that places oxygen, metal and untreated water in prolonged contact with each other is subject to corrosion. The presence of air gaps in wet systems or undrained water in dry systems can be a contributing factor to promote corrosion.
Corrosion is a natural phenomenon that gradually destroys metal by a chemical or electrochemical reaction with its environment. The environment inside the pipe normally contains water, which is conductive. When the water comes in contact with the metal pipe, it promotes electrochemical reactions between itself and the metal. In steel pipe, these reactions result in the creation of iron oxide or rust.
Types of Corrosion
The most common forms of corrosion found in sprinkler systems are crevice corrosion, uniform corrosion and pitting. Crevice corrosion is a localized form of corrosion that occurs in crevices (beneath gaskets, in holes, beneath surface deposits, in thread and groove joints) exposed to a stagnant corrosive environment. Uniform corrosion is a general thinning involving the regular loss of a small quantity of metal evenly distributed over a large surface area of pipe. The increased use of thin wall pipe has made uniform corrosion more prevalent than in the past. Pitting is a localized form of corrosion that results in covered or open holes or cavities in the metal.
Another form or corrosion that can accelerate previously occurring corrosion is Microbiologically influenced corrosion (MIC). MIC is an electrochemical process, which involves bacteria that can accelerate previously occurring corrosion in both wet and dry pipe sprinkler systems. MIC is caused by several different types of microorganisms that live on nutrients in water and react with the products of other microorganism reactions and with piping material.
MIC generally results in the growth of biofilms and nodules within the piping. Biofilms, usually appearing as a black slime, can be flushed out. Nodules, however, are solid and will adhere to the pipe interior and will not flush. While MIC is not new, its association with fire protection systems is relatively new. Unlike many other industries, the water in a sprinkler system in generally dormant and there has not been established corrosion control practices in place.
What are the Impacts of Corrosion?
Corrosion can result in increased friction loss in sprinkler pipe, which can affect the output of a sprinkler system. In addition, if deposits are significant, they can completely block the orifice of a sprinkler head.
Beyond obvious external visual observations, leaks or obstructions can also be potential indicators of corrosion. Other indicators can be a noted reduced flow from Inspectors Test Connections or 2 in. drains during normal sprinkler system testing.
Takeaways from the NFPA
The “Standard for the Installation of Sprinkler Systems” indicates that sprinkler systems shall be maintained in accordance with NFPA 25, “Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems,” as follows:
26.1 General. A sprinkler system installed in accordance with this standard shall be properly inspected, tested, and maintained by the property owner or their authorized representative in accordance with NFPA 25, Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems, to provide at least the same level of performance and protection as designed.
States the following:
14.2 Obstruction Investigation and Prevention
14.2.1 An investigation of piping and branch line conditions shall be conducted every 5 years by opening a flushing connection at the end of one main and by removing a sprinkler toward the end of one branch line for the purpose of inspecting for the presence of foreign organic and inorganic material.
188.8.131.52 Alternative nondestructive examination methods shall be permitted.
184.108.40.206 Tubercules or slime, if found, shall be tested for indications of microbiologically influenced corrosion (MIC).
Addresses water supplies as follows:
23.1.5 Water Supply Treatment
220.127.116.11 Water supplies and environmental conditions shall be evaluated for the existence of microbes and conditions that contribute to microbiologically influenced corrosion (MIC). Where conditions are found that contribute to MIC, the owner(s) shall notify the sprinkler system installer and a plan shall be developed 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.
18.104.22.168 Water supplies and environmental conditions shall be evaluated for conditions that contribute to unusual corrosive properties. Where conditions are found that contribute to unusual corrosive properties, the owner(s) shall notify the sprinkler system installer and a plan shall be developed to treat the system using one of the following methods:
(1) Install a water pipe that is corrosion resistant.
(2) Treat all water that enters the system using an approved corrosion inhibitor.
(3) Implement an approved plan for monitoring the interior conditions of the pipe at established intervals and locations.
22.214.171.124 Where approved biocides and corrosion inhibitors are used together they shall be compatible with each other and system components.
The 2008 edition of FM Global Data Sheet 2-81, “Fire Protection System Inspection, Testing and Maintenance And Other Fire Loss Prevention Inspections,” indicates that when any of the following conditions exist, a flushing investigation should be performed:
- Discharge of obstructive material is noted during a yard main water test.
- Foreign material is noted in fire pumps, dry pipe valves or check valves.
- Plugging of pipe or foreign material noted coming from Inspector’s Test Connection.
- Failure to flush underground piping or surrounding public mains following new installations or repairs.
- Plugged sprinklers or piping found during building alterations or after a fire.
- Defective intake screens for fire pumps taking suction from open bodies of water.
If there is evidence of corrosion in your sprinkler systems during normal testing programs, Risk Logic can assist in helping determine what further action is needed.