Oxygen gas present in the fire sprinkler system piping is the primary cause of corrosion related failures. There can also be a small amount of dissolved oxygen that is introduced with the incoming fire supply water. The only way to effectively control corrosion is to prevent or eliminate the introduction of oxygen into the fire sprinkler system piping.
In wet pipe fire sprinkler systems it is oxygen in the trapped air at the high points that most often causes corrosion. Periodic testing and draining and re-filling wet pipe systems accelerates the corrosion with a fresh supply of oxygen gas to react with the pipe metal. Every time the piping is drained and filled the corrosion reaction is reignited with the fresh supply of oxygen in the air that is introduced. The cumulative effect of frequent exposure to fresh oxygen leads to the formation of a pit in the pipe wall which eventually leads to a leak.
In dry pipe fire sprinkler systems it is the periodic introduction of fresh oxygen from the pressure maintenance gas provided by the compressor that accelerates corrosion. Oxygen from the compressed air attacks the pipe at areas where there are trapped pools of water. Corrosion in dry pipe fire sprinkler systems is much more aggressive than in wet pipe fire sprinkler systems.
Bacteria are always present in fire sprinkler piping. If you look for bacteria in fire sprinkler systems, you will ALWAYS find them. They are initially introduced to the fire sprinkler systems during the initial installation of the piping and they flourish in the warm confines of the fire sprinkler system. Some bacteria do participate in the process of accelerating the corrosion of steel. However, in fire sprinkler systems Microbiologically Influenced Corrosion accounts for less that 10% of the leaks that occur. Oxygen related corrosion accounts for 90% of the leaks that occur. MIC is NOT the primary cause of corrosion in fire sprinkler systems.
In order for oxygen to cause corrosion in fire sprinkler piping the gas must first dissolve into liquid water. Then the oxygen accelerates the cathodic half of the corrosion reaction that causes the metal, in this case iron or zinc, to give up its electrons in the anodic half of the reaction. This forms a pit in the pipe wall where the metal has been converted into a positively charged ion in the water. The iron ion reacts with oxygen in the water to form iron oxide (rust). The chemical name for the iron oxide is hematite (red). With time the hematite oxidizes further to form another iron oxide called magnetite (black).
In wet pipe systems corrosion occurs most frequently where air is trapped in the system piping. The oxygen in the air dissolves in the water and immediately reacts with the first iron that it contacts. This generally occurs at the air/water interface.
In dry pipe systems corrosion always occurs under pools of trapped water. Water is introduced to the dry pipe system in one of three ways: during hydraulic testing of the piping, as condensate from the air compressor and during periodic testing of the fire sprinkler system to satisfy code requirements.
Dryers only reduce the amount of new water that is introduced into the dry (pre-action) piping. Unfortunately, most dry (pre-action) systems already have plenty of trapped water in the piping from the initial hydrostatic test and from periodic testing of the system to meet code requirements.
Compressors without attached dryers add warm, moist oxygen each time they turn on. Compressors with attached dryers simply add warm, dry oxygen to the system piping. With regard to corrosion, adding fresh oxygen is far more damaging to the piping than adding a little condensate water.
There are two factors that limit corrosion in coolers and freezers:
Galvanized steel piping is subject to highly localized attack by oxygen under the persistently moist, oxygen rich environments that exist within fire sprinkler piping. ECS research indicates that galvanized steel piping suffers from corrosion related leaks 3 – 4 times faster than black steel piping under identical operating conditions. We have seen failures occur in schedule 10 galvanized steel piping in 12 months. We recommend against the use of galvanized steel piping in any fire sprinkler application.
The current ASTM standards for fire sprinkler piping does not require heat annealing of the weld seam in the piping to reduce the risk of corrosion in the heat affected zone around the weld seam. As a result, the weld seam in fire sprinkler piping is highly susceptible to accelerated corrosion rates relative to the remainder of the pipe. Weld seams under trapped pools of water in dry (pre-action) fire sprinkler systems almost always experience high rates of corrosion and are frequently the point of failure.
The process of repairing a leak in a wet pipe fire sprinkler system typically involves draining the system to replace the failed piece of piping. This repair process adds enough oxygen to the system to fuel the next leak. The process of leak formation can increase quite dramatically once it starts. Fortunately, oxygen corrosion attack is highly localized in wet pipe fire sprinkler systems and typically only damages about 20-25% of the piping. The remainder of the piping is generally unaffected by oxygen corrosion.
Dry (pre-action) pipe fire sprinkler systems generally only have corrosion related leaks under pools of trapped water. Here again, the corrosion is localized to only a small section of the main lines where the water collects into pools. This is generally only about 20-25% of the main piping. The dry branch lines often show no corrosion because there is no water present in them. It is not necessary to replace the complete system.
There are generally five approaches to controlling corrosion in any industrial application:
In the fire sprinkler industry:
DPNI is a process that involves replacing the compressor as the source for pressure maintenance gas in dry (pre-action) fire sprinkler systems with a nitrogen generator. Through a process of fill and purge breathing the oxygen rich air in the piping is replaced with nitrogen gas using a vent that is attached directly to the fire sprinkler system piping. Once the piping is inerted, the breathing stops and nitrogen gas is used to maintain the pressure within the piping. DPNI completely prevents oxygen corrosion within the fire sprinkler system.
WPNI is a process that involves replacing the air in the wet pipe fire sprinkler system piping with nitrogen gas. The incoming water is sparged using nitrogen gas. A vent on the system piping is used to remove all of the oxygen. The end result is a system that is oxygen free and nitrogen inerted. WPNI completely prevents oxygen corrosion within the fire sprinkler system.