|FPE Extra Issue 6, June 2016|
Issue 6, June 2016
Fundamental Corrosion Control Strategies for Fire Sprinkler Systems
By Lucas Kirn, P.E.
There are multiple options available to remediate corrosion-related pipe failures in existing fire sprinkler systems, but there are also several approaches that can be implemented during the design or commissioning phase of a fire sprinkler system that significantly reduce corrosion activity and extend the useful life of the system.
Any time water, oxygen and steel are combined it results in a corrosion reaction. This discussion serves to provide practical examples that follow the basic tenets of choosing appropriate materials for construction, minimizing trapped water and excess oxygen in dry pipe and pre-action systems, and minimizing trapped air in wet pipe systems. Corrosion results in life safety, structure, property and business continuity risks, and as fire sprinkler systems continue to age it is imperative that the industry take a serious and proactive approach to this growing problem.
The recommendations included in this discussion are based on the principle that the primary cause of internal corrosion in fire sprinkler systems is oxygen attack1 of the iron and zinc materials used to fabricate fire sprinkler system piping. When oxygen dissolves in any water that contacts the sprinkler system piping it becomes available to react with the metal pipe.
In dry and pre-action fire sprinkler systems, oxygen corrosion is generally limited to wet areas of the pipe. In wet fire sprinkler systems, oxygen corrosion is generally limited to areas of the pipe adjacent to trapped air. Unlike wet fire sprinkler systems, where the supply of trapped oxygen is limited, oxygen is nearly unlimited in dry and pre-action fire sprinkler systems maintained with an air compressor.
Ensure Leak Integrity for Dry Pipe and Pre-action Systems
NFPA 13: Standard for the Installation of Sprinkler Systems requires dry pipe and pre-action sprinkler systems to meet an air test to ensure system integrity. Dry pipe and pre-action sprinkler systems are equipped with air compressors because they all leak to varying degrees. A sprinkler system with a higher leak rate will operate the air compressor more frequently. Every time the air compressor pressurizes the sprinkler system it introduces more oxygen and water vapor into the system.
One of the simplest, most cost-effective approaches to reducing corrosion in dry pipe and pre-action sprinkler systems is to enforce the maximum leak requirements as directed by NFPA 13. While the system air pressure gauge can be used to monitor this test, the most accurate and secure method of recording pressure loss involves the use of a pressure transducer and data logger attached to the system. Taking the extra step of ensuring leak integrity will substantially lengthen the service life of a dry pipe or pre-action sprinkler system.
Install Supervised Control Valve on System Side of Dry Pipe or Pre-action Valve on Riser
NFPA 25: Standard for the Inspection, Testing and Maintenance of Water-Based Fire Protection Systems requires that pre-action and dry pipe valves are full-flow trip-tested every three years with partial flow trip tests required annually in intervening years. A trip test will invariably permit water to enter a dry pipe or pre-action sprinkler system. This additional water will provide more locations within the system susceptible to corrosion even if the piping is well-pitched and equipped with multiple drains. It is virtually impossible to remove all trapped water from dry pipe and pre-action sprinkler systems, particularly in systems with roll grooved mechanical fittings, so reducing the opportunity to introduce additional water will greatly reduce the areas inside the pipe exposed to corrosive conditions.
Eliminate the Use of Galvanized Pipe for Dry and Pre-action Sprinkler Systems
In theory, use of galvanized steel piping in dry pipe fire sprinkler system applications makes sense. The exterior of the tubing will not rust due to atmospheric oxygen corrosion because the protective zinc carbonate layer forms over the external surfaces and the essentially dry state of the interior piping should mean that corrosion is minimal. But as discussed previously, the interior surfaces of dry and pre-action fire sprinkler piping are rarely completely dry.
Figure 1 – Corrosion in Galvanized Water Main
If residual water is trapped inside the pipe the zinc layer will break down quickly and ultimately lead to a pinhole leak. This problem is complicated further because the nature of the attack is localized. Once the zinc coating is breached and the underlying steel is exposed to water, oxygen corrosion will be concentrated at the point of the breach. Because of the highly localized nature of corrosion attack in galvanized steel piping, through‐the‐wall penetrations occur faster in galvanized steel corrosion than in black steel exposed to the same conditions. New galvanized fire sprinkler systems can develop pinhole leaks in as little as two to three years after initial installation.1 Fortunately, the fire sprinkler industry is starting to become aware of the problems associated with galvanized pipe as evidenced by the reduction in C-values for galvanized pipe in the 2013 Edition of NFPA 13, but more awareness is necessary.
Install Supervised Isolation Valves in Areas Subject to Frequent Remodels or Tenant Improvements
It is very common to see accelerated corrosion failures in systems exposed to high-frequency draining and filling related to construction activity, particularly in retail facilities. These are typically wet pipe sprinkler systems installed in a big box store, shopping center or enclosed mall. When a wet pipe sprinkler system sits dormant, all of the oxygen in the system is consumed by the pipe within 90 days2 at this point reaching equilibrium, where no additional corrosion can occur.
Initiating the process of repeatedly draining and filling a system throughout the course of several days or weeks allows fresh oxygen to enter the system continuously and restarts the corrosion process. This also helps to explain why older systems that are rarely tested or taken out of service for remodel show minimal signs of corrosion activity.
It’s not unusual to see corrosion damage throughout large parts of a system where only a small area was renovated or under construction. By compartmentalizing larger systems that serve multiple tenants it is possible to minimize corrosion activity by allowing a smaller volume of oxygen to enter the system during draining and filling. This approach also serves to isolate the location where corrosion is likely to occur to a much smaller area. Installing an electronically supervised isolation valve on smaller portions of the system likely to be exposed to greater draining and filling frequency is an effective approach to preventing larger-scale and more severe corrosion in a larger fire sprinkler system protecting multiple tenant spaces or defined use areas.
Vent Trapped Air from a Wet Pipe System during Filling
In wet pipe fire sprinkler systems, more than 99% of the oxygen available to react with and degrade system piping is located in trapped gas pockets.2 As previously discussed, corrosion activity in wet pipe systems is generally limited to areas of trapped air that is comprised of 21% oxygen. There is a direct relationship between oxygen content and corrosion activity in a fire sprinkler system. A reduction of trapped air volume in a fire sprinkler system results in a proportional reduction in corrosion activity because every molecule of oxygen that is removed from the system can no longer react with steel piping.
Figure 2 – Automatic Air Vent on Wet Pipe Sprinkler System
There are many variables related to system design that affect the volume of trapped air in wet pipe fire sprinkler systems, and many of them (structural elevation changes, coordination with other trades, dead-end lines, etc.) are unavoidable. While it is not possible to vent all of the trapped air from a system, it should not prohibit fire sprinkler practitioners from providing a manual or automatic means of venting as much air as is practical for the individual system. It is especially important to ensure gridded systems that have been successfully vented are equipped with the required pressure relief valve to prevent the buildup of potentially damaging pressures related to temperature increases.
There is not a simpler, more cost-effective approach to corrosion control in wet pipe fire sprinkler systems than venting trapped air. The 2016 Edition of NFPA 13 now requires the installation of a manual or automatic vent on every wet pipe sprinkler system.
Lucas Kirn is with Engineered Corrosion Solutions.
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