The most critical piping system for any building property or plant operation is unquestionably the fire sprinkler service. Often considered trouble free, corrosion related failures at fire sprinkler lines have dramatically increased over the past decade - raising not only operating and repair costs, but the threat to building inhabitants as well.

     By definition, fire sprinkler service always exists under a corrosion threat since carbon steel black pipe is traditionally used, and its water is never chemically treated. This raises the importance of recognizing and addressing other corrosion influencing factors in order to maximize its useful service life.

     CorrView International's 10 year involvement in ultrasonically testing fire sprinkler systems has provided us with an extensive database of hard, factual wall loss and corrosion rate data. It has also allowed us to identify very clear relationships between the design and operation of fire sprinkler systems, and resulting corrosion conditions.

     Prior investigations have identified 75 year old sprinkler pipe in virtually new condition, and yet nearly new 4-5 year old installations in need of replacement. Clearly evident, a much higher incidence of corrosion related failures exist at newer fire sprinkler installations. Ultrasonic testing at hundreds of building properties has revealed wildly varying corrosion rates and remaining service life - which can be often correlated to sprinkler pipe location, material, age, thickness schedule, and most importantly, how often the pipe is drained and re-filled. Each factor impacts fire sprinkler pipe significantly.

     One major reason for such problems is the more common use of thinner schedule 10 pipe, which offers savings on both material and installation costs. Whereas extra strong schedule 80 would have been typically installed 50-60 years ago, lighter schedule 40 has been used since around the 1970's.

     Over the past 20 years, this lite wall schedule 40 pipe has been often replaced with even thinner schedule 10 - leaving very little available pipe material to corrode before reaching minimum acceptable thickness limits and the inevitable failure. For higher pressure applications, schedule 10 pipe will provide acceptable service only assuming that virtually no corrosion will take place - a known impossibility for steel pipe containing water.

     Where 8 in. schedule 80 A 53 pipe having a wall thickness of 0.500 in. would have been installed back in 1955, it is not unusual to find 0.188 in. thick schedule 10 installed today. This pipe offers less than half the wall thickness of schedule 80, as is shown in the below relative comparison, and will typically provide 10-20 years of service and little more, depending upon other factors related to the operation of the system. Smaller diameter pipe has an inherently thinner wall, and combined with any corrosion condition, can lead to a failure in as little as 5 years or less.

     A default minimum acceptable wall thickness of 0.100 in. exists for most steel fire sprinkler service, or higher depending upon pipe diameter, pressure, and construction. For 8 in. schedule 10 steel pipe, therefore, only 0.088 in. or less remains available to corrode before reaching those minimum acceptable safe limits. At a moderate corrosion rate of 5 MPY, the pipe will reach its minimum acceptable thickness limit in roughly 17 years, whereas should a high loss MIC condition develop having a 20 MPY rate, far less service life would exist. See Technical Bulletin # P-1 regarding minimum wall thickness calculations.

     In most of our ultrasonic testing investigations, we can demonstrate that the advanced failure of a fire sprinkler system is directly related to water flow. Where the pipe is filled, hydrostatically tested, and then left stationary, a certain amount of corrosion takes places, the oxygen level is depleted, the lines remain stagnant, and further corrosion virtually ceases. This is the most reasonable explanation for older piping systems of the 1930's found in virtually new condition today.

     Where fire service piping is frequently drained or the system often extended or modified, fresh new water enters to greatly increase corrosion and pitting activity. In the worst of examples, a constant flow of water may be present to create what is essentially an open and chemically untreated condenser water system. This will occur where small and unaddressed leaks exist to constantly bring in fresh oxygenated water.

     A frequently running jockey pump, or cold sweaty fire sprinkler pipe are two sure signs of a leak somewhere within the system. But while a leak within a building property is likely to be addressed immediately, underground or other outdoor water loss problems are often allowed to continue for years.

     In such cases, the greatest wall loss is typically found at the main line closest to the water inlet due to the greater flow of water, and due to turbulence from the city main, which introduces fresh oxygenated water. Lowest corrosion activity, on the other hand, is typically found at the smallest diameter sprinkler branch lines - this due to their existing in an essentially dead ended and stagnant condition.

     One sure tell tale sign of a water influx into fire sprinkler pipe is a cold temperature at the pipe surface, as a stagnant pipe should exist at ambient temperatures. The rusted exterior of unpainted steel pipe, in the absence of high area humidity and especially along the bottom, is another sure sign. Given enough cold water flow, typically un-insulated fire sprinkler pipe will actually sweat moisture condensation in the same way as an un-insulated domestic cold water, chill water, or any other cold pipe surface.

     The below photographs offer the type of evidence which often exists showing an active cold water flow. Similar such evidence should always be investigated further.

     Perhaps the strongest recommendation we can offer is to minimize the draining and filling of any fire sprinkler system. This is difficult for modern office buildings, however, where renovations and changes in tenants is frequent - prompting piping changes and frequent draining and filling. Regular testing of the fire sprinkler system, in itself and required by code, may also play a role in its deterioration.

     The quality of the steel pipe is an important factor in fire sprinkler system corrosion as well as for any piping application. While it is almost impossible to judge the quality of a material source without first performing a metallurgical or salt spray corrosion study (an unlikely event), accepting pipe from only known and reliable sources is always a worthwhile recommendation. Seamed pipe should always be avoided in favor of seamless stock.

     Whenever a corrosion problem is identified at one pipe size but not at another, and given the absence of some other factor such as additions to a specific fire zone or frequent drain downs, the possibility that pipe from different steel mills, and perhaps different countries, should not be excluded. The larger the installation, and the larger the pipe size, the more likely different sources of material were used. Larger size projects always favor lower cost pipe.

     Dry pipe or pre-action fire sprinkler systems are common wherever freezing is a concern, where accidental water damage may occur, and for other engineering and design reasons. With them, they bring new and typically unseen problems in the area of corrosion activity.

     While perhaps "dry" in terminology, such fire sprinkler piping typically presents one of the most corrosion susceptible environments for steel pipe due to the moisture which inevitably remains within the system. A "dry" fire sprinkler system may be drained after hydrostatic testing, but the water left behind produces a moisture and oxygen saturated environment far more aggressive against steel pipe than a completely water filled system.

     Proper pitch of the fire sprinkler system is essential, but in reality, rarely drains the pipe sufficiently to remove the moisture threat. For this reason, corrosion at the lower areas of piping often exceeds the more elevated areas.

     To combat this problem, galvanized steel pipe is often employed in dry systems, and will generally provide longer service life strictly depending upon the quality of the pipe and strength of the galvanizing finish. This benefit provided by galvanized pipe is actually double edged. Once one or more areas of the galvanized finish fail, a much greater degree of corrosive energy is focused on those select areas to produce extremely high corrosion rates which can easily exceed 20 MPY, and produce advanced failure.

     While a corrosive potential would be acting against the entire interior surface of a carbon steel pipe, much of that same corrosive energy in a "dry" galvanized system is now focused at a very limited surface area where the galvanizing finish has been compromised. The result is typically pinholes through the pipe wall as if produced by a drill bit, at the same time the overwhelming majority of pipe wall remains at or near factory specification. Under such conditions, galvanized pipe will actually fail sooner than standard carbon steel, where corrosive attack is distributed over a larger surface area.

     MIC has the potential to virtually destroy an entire fire sprinkler system in just a few years given corrosion rates which can exceed 50 MPY. Microbiologically influenced corrosion is often suggested or mistaken for the more common condition of simple under deposit corrosion, which can show similar result.

     Identifying an MIC condition requires a thorough metallurgical and microbiological examination of a current pipe sample. What causes MIC and how it can be prevented are less known. Since it is biological, a through cleaning and sterilization of the pipe when first installed is mandatory. Minimizing water flow through the system once placed into service is another easy recommendation to offer, since the greater the amount of water means the greater the potential microbiological source.

     MIC presents a greater threat to Victaulic or clamped joint piping systems due to the end to end gap which exists for microbiological growth to accumulate and flourish. The threat of MIC is so severe that once it is firmly established, most corrosion authorities consider it impossible to correct in any piping system. A cut groove further compounds the problem. See Technical Bulletin P-3 about the threat created by cutting the groove for clamped pipe construction.

     But while a leak at a fire sprinkler line presents the obvious problems, a further and often unrecognized threat exists in the form of the iron oxide deposits created as a result of the corrosion process. Such deposits can easily add up to thousands of pounds of moveable rust debris capable of being dislodged from the shock of a 150 HP fire pump kicking on, and moving downstream into the critical control and actuating valves, and ultimately - the sprinkler heads. Read a case history of a sprinkler system that was clogged sufficiently by rust to block all water flow during an actual fire emergency.

     A pipe testing report showing a 40% average loss of wall thickness over 500 ft. of a 10 in. schedule 10 fire sprinkler main would clearly explain the leaks and operating problems experienced. Yet, that same 40% loss of steel from pipe which weighed a factory new 21 lbs. per linear foot, also means that 8.4 lbs. of steel per linear foot has now been removed from the pipe and placed into its interior in the form of less dense iron oxide particulates.

     For this 500 ft. fire sprinkler run, over 4,000 lbs. of steel can be assumed to exist inside the pipe in the form of both hardened tubercular deposits and loose iron oxide mud - some of which will be easily re-suspended during a fire emergency. See Technical Bulletin # M-15 regarding this hidden but often greater threat to fire sprinkler systems.

     Overall, new concerns exist for fire sprinkler systems which seem to not have existed decades ago. This demands much greater planning in their construction and maintenance, and some form of corrosion monitoring to detect problems before they expand beyond repair.

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