The problem of corrosion in metal fire sprinkler systems is drawing more attention, as evidenced by the popularity of the subject at industry events.
Corrosion was the subject of three presentations at Fire Sprinkler International 2016 (FSI) in Munich in April and two seminars at the American Fire Sprinkler Association Convention & Exhibition (AFSA) in Nashville in September.
What I heard at those shows ought to be disturbing to everyone in the industry. Corrosion of metal sprinkler pipe is hardly new, but we are still trying to gauge the full extent of the problem and how best to communicate it to our customers, many of whom are largely unaware of the severity of the issue. The presentations were accompanied by photos of pipes that had been corroded or clogged by sludge.
Nearly all of the presentations began with a sobering chemistry lesson on the inevitability of corrosion. The reaction is simple: oxygenated air + water + most metals = corrosion. And corrosion leads to leaks, partially obstructed or blocked pipes and sprinkler heads, system failure and worse.
In a presentation at the FSI conference in Munich, Jean-Philippe Roisin of FM Approvals described the typical corrosion process in a metal sprinkler pipe. Once the interior of the pipe begins to corrode, whether from oxidation, erosion or other causes, deposits form at the bottom of the pipe. Corrosion cells then form under the deposits while tubercles grow above the cells. The tubercles can impede the flow of water in the pipe while corrosion cells can cause pinhole leaks in the pipe exterior.
Corrosion was listed by FM Approvals as the fourth-most common cause of sprinkler leakage losses from 2001-15, behind freezing, mechanical injury and defective equipment.
In our everyday lives, we’re used to associating corrosion with water, so customers can be forgiven for assuming that wet systems are more prone to corrosion than dry ones. But actually, the opposite is true.
Corrosion typically advances less quickly in wet systems because there is less air (oxygen) present, though, of course, oxygen is dissolved in the water. Each flush and fill of the system introduces additional oxygen, which can accelerate corrosion. Air pockets in wet systems also provide oxygen.
In a presentation at the National Fire Protection Association conference in Nashville, Scott Futrell of Futrell Fire Consult & Design in Minnesota cited research showing that when oxygen in trapped air reacts with steel pipe in a typical 1,000-gallon wet pipe sprinkler system, it produces 5.8 pounds of iron oxide while dissolved oxygen in the water produces only 0.002 pounds of iron oxide.
Though all metal pipe will corrode, dry pipe systems are more susceptible than wet pipe systems. That’s because there is no such thing as a truly dry system. Hydrostatic pressure testing of systems during commissioning introduces water and condensation can also be a problem.
Residual water in dry pipe or pre-action systems is the leading cause for corrosion and leakage of galvanized steel pipe, damage which can occur as soon as two years after installation.
In another presentation in Munich, Josh Tihen of Potter Electric Signal Co., sounded the alarm about the assumed life expectancy of wet sprinkler systems. Fifty years or “the life of the building” have been typical responses to the question about how long a fire sprinkler system will last. Turns out those estimates could be greatly exaggerated.
Tihen’s presentation cited research by VdS, the German fire safety firm. This data found that 73 percent of dry systems have significant corrosion issues at 12.5 years old and 35 percent of wet systems have significant corrosion issues at 25 years after installation. For that reason, VdS guidelines specify that wet systems be thoroughly inspected after 25 years and dry systems after 12.5 years, adding that the inspections include an internal examination with an endoscope and a check of the residual wall thickness with ultrasonic measurement. That’s more relaxed than the NFPA recommendation of inspections every five years.
And while the reports spoke about averages, the seminars were illustrated with photos of extreme cases, badly corroded and obstructed pipes five years old or newer.
While the majority of corrosion is caused by metal oxidation, another significant cause is microbiologically influenced corrosion (MIC), or corrosion caused in the pipe interior by the growth of microorganisms, including bacteria, microalgae and fungi. That’s a different cause than galvanic or oxygen corrosion, but the results can be just as disastrous: pinhole leaks, obstructions, tubercles and exterior rusting and condensation.
In her presentation at IFS in Munich, Mascha van Hofweegen of the Dutch consulting firm KWA Advisors, spoke about MIC in the Netherlands. She said building owners and others in the Netherlands generally don’t think they have a problem with corrosion in their fire sprinkler systems because certified systems have their ITC valves inspected annually, though inspections are relatively uncommon. “This does not mean the system will stop a starting fire,” she said. “As long as no leakages occur, it’s assumed corrosion is no problem at all.”
Noting that almost no metal is resistant to MIC, she said that MIC occurs in many types of systems and across a wide range of pH and temperatures. The ideal systems for MIC damage are those in which the water is seldom supplemented and does not circulate; ones with lots of dead end pipes; and those that are at ambient temperatures. She showed the audience pictures of MIC corrosion in stainless, carbon and galvanized steel.
In addition to corrosion, bacteria can cause sludge, she said, which obstructs pipes, particularly pipes at lower levels of a system, and pendent sprinklers.
Common treatments for MIC include chemical and disinfectant dosing (biocides), UV and thermal disinfection, changing environmental conditions, cathodic protection, microbiological resistant coatings and the use of corrosion-resistant materials.
An alternative to the above is using CPVC pipe rather than metal. Unlike metal, CPVC has a natural immunity to MIC and is not susceptible to oxygen corrosion. Experience demonstrates that CPVC will remain corrosion free for 50 years of service life.
In a 2009 report on corrosion, the European Fire Sprinkler Network noted that: “CPVC plastic pipe does not corrode, nor does it support the development of biological films as readily as steel. In this latter respect, CPVC is as resistant as 316-grade stainless steel.”
However, CPVC is approved only for light hazard commercial and residential applications. And, while it does not corrode, it can suffer from stress fractures and deterioration caused by chemical incompatibility with solvents and other construction materials.
FM Global recommends following programs such as Lubrizol’s FBC System Compatible Program, which works with manufacturers and others in the industry to test the compatibility of its BlazeMaster CPVC pipe with various chemicals with which it might come into contact.
The seminars I attended were also full of advice for preventing corrosion in wet and dry systems.
To mitigate the potential for corrosion in dry systems, FM Approval’s Roisin recommended the following steps:
- Ensure the system is properly pitched
- Install low point drains and keep them clean
- Avoid using rolled groove joints because they can cause water accumulation
- Install an air drying system
- Fix air leaks to keep the system as tight as possible
Another approach for dry systems is to replace the oxygenated air with nitrogen, an inert gas that doesn’t interact with metal or cause corrosion.
Advice on how to prevent corrosion in wet systems largely concerned eliminating the air pockets through automatic or manual air release valves at system high points.
Another problem can be eliminated even before the metal pipe is installed. Pipe weld corrosion can occur along weld seams and other heat-affected zones. FM Approvals’ Roisin said stress-relieved steel pipes should be used and the weld seams should be oriented upward.
The fire sprinkler industry is addressing the corrosion issue through research and development of ways to avoid and treat the problem. Among the most promising steps are the use of nitrogen gas and the use of CPVC pipe in the correct installations.