The Causes and Cost of Sprinkler Corrosion

  • David Fuller Assistant vice president, senior engineering specialist, FM Global
  • Dr. Paul Su Senior lead research specialist, FM Global

Corrosion is found in all sprinkler systems and is one of the leading causes of maintenance and operation problems for fire protection systems. Corrosion damage and mineral deposits can cause pipe leakage, restrict water flow to sprinklers and impair mechanical operation of fire protection equipment, leaving otherwise protected facilities vulnerable to uncontrolled fire loss.

Several organizations, such as the National Fire Protection Association (NFPA), the Electric Power Research Institute (EPRI), FM Global and the European Fire Sprinkler Network (EFSN), have published documents and standards attempting to address the corrosion issue. Currently though, there is no widely accepted strategy either within the fire protection industry or the National Association of Corrosion Engineers (NACE International) to effectively mitigate these types of problems in fire protection systems.

In a recent research paper, Corrosion and Corrosion Mitigation in Fire Protection Systems, FM Global's Dr. Paul Su, senior lead research scientist, research, and David Fuller, assistant vice president, senior engineering technical specialist, engineering, provide a comprehensive explanation of the different types of corrosion, the contributing factors that can be found in fire protection systems and potential corrosion mitigation methods. Additionally, the paper provides insight into further studies that may yield cost-effective solutions for FM Global clients and the fire protection industry.

What is corrosion?
Corrosion involves the reaction between a metal or alloy and its environment. It is an irreversible interfacial process, which causes the gradual deterioration of metal surface by water, moisture or other corrosive chemicals.

While metal corrosion can have many different causes, it is generally characterized as having different forms or types based on various physical characteristics (see related story, "9 Types of Corrosion" on p. 41). This study specifically focused on corrosion issues in fire protection systems. Some fire protection systems are more susceptible to certain forms of corrosion and some types of corrosion, or a combination of types, are more likely to cause leaks or system malfunctions.

Corrosion in fire protection systems
Corrosion increases the lifecycle costs of fire protection systems and can become a significant issue for some owners of water-based systems. It can cause leaks that will reduce the amount of water available if the system is activated and can affect components like sprinklers, making them inoperable.

Corrosion can also cause tuberculation, the buildup of mineral deposits in the system's pipes. These knoblike mounds are frequently observed in steel and galvanized steel pipe, and are capable of obstructing water flow in pipes and/or plugging sprinklers.

The most common causes of these types of corrosion in fire protection systems are pipe weld corrosion, residual water in dry pipe systems and trapped air in wet pipe systems. Other frequent causes are corrosive water chemistry, oxygen injections into the system (fresh water recharged during regular maintenance), stagnant water, dead legs of pipe and microbiologically influenced corrosion (metabolic activities of microorganisms such as bacteria, fungi and algae).

Pipe weld corrosion
Corrosion of the weld seam is common for an electric-resistant welded steel pipe. This kind of damage may be attributed to the formation of unstable iron sulfides, along with high residual stresses and microstructure changes around weld seam areas. Such areas are created during the pipe manufacturing process and occur along the weld seam and other heat affected zones.

The use of stress-relieved steel pipes (Grade B in accordance with ASTM A795/A795M-13) can be beneficial in reducing weld seam corrosion. But the most effective way to mitigate weld seam corrosion is to orient the weld seams toward the building roof to prevent the weld from being located under deposits within the pipe.

Residual water in dry pipe systems
Residual water in dry pipe or preaction systems is the leading cause for corrosion and leakage of galvanized steel pipe in these systems. The presence of the air gap above the residual water results in high dissolved oxygen and carbon dioxide contents in the water and makes it corrosive to galvanized steel sprinkler pipe. New dry and preaction systems can develop through wall leakage within as little as two years after installation due to residual water.

Draining residual water from these dry systems after hydrotesting or maintenance inspection is the most effective way of mitigating corrosion. In practice, this can be difficult due to inadequate pitching of the pipe or use of pendent sprinkler drops that prevent all water from being drained from the system.

Another mitigation approach is to fill the dry pipe or preaction systems with nitrogen gas. Filling these systems with nitrogen gas removes oxygen and decreases oxygen-related electrochemical reactions.

Trapped air in wet pipe systems
Trapped air in wet pipe systems provides the oxygen source for steel pipe corrosion. Minimizing air pockets in wet pipe systems is recommended. An air release valve, which is capable of venting trapped air in the pipe, can mitigate this kind of corrosion. Limiting the introduction of oxygenated fresh water into the piping system can also be an effective mitigation strategy for wet pipe systems. Similar to the dry pipe or preaction systems, nitrogen gas can also be used as a supervisory gas to reduce oxygen concentrations in the sprinkler water of wet systems.

Water chemistry
The corrosiveness of sprinkler water in fire protection systems is significantly influenced by water quality. The chemistry of the water—concentration of dissolved gases (O2, CO2) and dissolved anions (chloride, sulfate), pH, alkalinity and the presence of microorganisms—will impact the corrosiveness of the water. Higher dissolved oxygen concentrations, for example, greatly increase the corrosion rate. For this reason, only treated water, such as municipal water, should be used as the source of sprinkler water and untreated raw water should be avoided.

Corrosion monitoring
There are many recommended methods for monitoring corrosion in sprinkler systems, such as coupon measurements, internal visual inspections and ultrasonic thickness measurements. The effectiveness of some of these techniques may be questionable since the most severe corrosion is often localized. Internal visual examination using a borescope or video-borescope is the most practical and effective method for evaluating corrosion in fire protection systems.

Plastic materials
Plastic materials (CPVC pipes and fittings) have successfully been used in fire protection systems for more than 30 years owing to their corrosion resistance properties, long-term durability, and ease of production and installation. However, similar to metals, plastic products do suffer degradation and failure in service.

Failure of CPVC piping can occur when they are exposed to organic chemicals and put under stress. The plastic can absorb organic chemicals, reducing its strength. This absorption process is accelerated when the plastic is under applied stress, which can lead to environmental stress cracking (ESC), the most common failure of CPVC piping in fire protection systems. Paints, adhesives, cleaning agents, lubricants and other organic fluids with modest hydrogen bonding can weaken the plastic.

Some compatibility issues between CPVC material and its service environment are known and have been addressed by Lubrizol Corporation in its "FBCTM System Compatible Program." This program is designed to test chemical compatibility of Lubrizol's CPVC products with a broad range of chemicals, which may come in contact with CPVC in the field. In addition, UL (Underwriters Laboratories), Lubrizol, NSF International (National Sanitation Foundation) and the fire protection industry have formed a committee to write the standard for CPVC in fire protection systems.

FM Global recommends applying only those chemicals approved by Lubrizol, FM Approvals and UL for hybrid CPVC fire protection systems to avoid potential ESC failures.

Other factors
FM Global's loss data (1982 – 2001) show that dry pipe and preaction systems are involved in 59 percent of obstructed sprinkler system fire losses. This loss data analysis predominantly involves black steel pipe. The hard and dense corrosion tubercles in black steel pipe were found to be the most common obstructing material. Galvanized steel pipe is therefore recommended for dry or preaction systems, to minimize the likelihood that sprinklers become clogged when activated.

Plastic material (CPVC pipes and fittings) has successfully been used in fire protection systems for more than 30 years due to corrosion resistance properties, long-term durability, and ease of production and installation. However, similar to metal, plastic products suffer degradation and failure in service.

Corrosive environments
Many industrial plants remove smoke, fumes and dust generated during the industrial process through exhaust ductwork systems, some of which are made of plastics or fiber- reinforced plastics (FRP). These systems include flue gas desulphurization systems, metal acid pickling ducts, chemical industry exhaust systems, power generation industry exhaust systems, and pulp and paper industry exhaust systems.

These duct systems create highly corrosive atmospheres where short circuits, hot work, friction and spontaneous ignition of deposits can cause fires. Fire damage to these systems can lead to interruption of plant operations for an extended period of time with substantial financial losses. Some sprinkler systems have proven ineffective in protecting against fires originating within these ducts, partly because of the highly corrosive atmosphere.

FM Global has developed a corrosion- resistant fire sprinkler system for industrial exhaust ducts. The system was designed based on results from laboratory and field tests, along with data from manufacturers. This system is comprised of corrosion-resistant sprinkler nozzles, linear heat detector (LHD) wires and fire control panels, flexible sprinkler connections and other accessories. It is designed to be suitable to protect combustible exhaust ducts from fires in extremely corrosive environments.

Further study
FM Global research has identified two areas that require further study: the use of corrosion inhibitors and the value of water filtration.

Corrosion inhibitors are regularly used in many industries to mitigate corrosion, such as boiler water treatment, cooling water systems and wastewater treatment. Each service environment requires special combinations of corrosion inhibitors to effectively and efficiently mitigate corrosion.

Corrosion inhibitors could also be used for fire protection systems, but systematic study and field testing needs to be done. Compatibility tests of these chemicals with other materials used in fire protection systems (CPVC and other types of plastic piping, sprinkler heads, O-rings, rubber gaskets) should be carried out before any field applications.

Water filtration or treatment systems should also be explored. Treating source water (ion exchange or chemical precipitation) could remove aggressive anions from the sprinkler water, greatly reducing water corrosivity.

To view the research paper, Corrosion and Corrosion Mitigation in Fire Protection Systems, go to fmglobal.com/research and click on Research Technical Reports.

 


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