Iron and steel sprinkler piping, particularly dry pipe sprinkler systems, are subject to blockage and degraded flow performance by internal corrosion and tuberculation of the interior of the piping. We are all familiar with common corrosion, which on ferrous metals is commonly seen as rust.
Rust is the direct oxidation and reduction of metals forming a scaly surface layer of iron oxide. Within the interior of a pipe, the scaly layer adds dimension to the interior wall of the pipe, thus restricting the pipes hydraulic capacity to carry sprinkler water.
A characteristic of the common corrosion processes is that the reduction and oxidation (Redox) steps occur at separate locations on the metal. This is possible because metals are conductive, so the electrons can flow through the metal from the anodic to the cathodic regions. The presence of water is necessary in order to transport ions to and from the metal; however, as little as a thin film of adsorbed moisture can be sufficient.
Pipe scale formed by Redox corrosion processes is generally a relatively flat flakey layer, which over time can build up to appreciable thickness. Additionally, pieces of pipe scale will flake off and may lodge in downstream piping to form a solid obstruction. Sprinkler heads that typically have an orifice much smaller that the pipe supplying them are particularly prone to becoming obstructed in this manner.
Another form of pipe scale may occur from mineral deposition on the pipe interior. Dissolved minerals in “hard water” over time will coat the interior of a pipe. This is generally a relatively slow process, with deposits occurring over extended periods. It is not often an issue with sprinkler piping.
Yet another type of corrosion is microbial induced corrosion, or MIC. Many different bacteria metabolically process impurities found in the water, and produce chemical byproducts which act electro-chemically to damage pipes. MIC processes can occur both aerobically and aerobically. MIC damage to sprinkler piping most frequently manifests as pinhole leaks, but can also result in internal pipe blockage from bacterial “sludge” which is formed during the active process.
A particular instance of corrosion, and one that can be insidiously damaging, is known as tuberculation. This is defined as “a corrosion process that produces hard knob-like mounds of corrosive products on metal surfaces.” Tuberculation products may have a very bubbly, lumpy appearance which could be described as “bubbly mud.” A badly tuberculated pipe can effectively block over 95% of waterflow.
Obviously obstructed sprinkler piping is a major concern if we anticipate that piping will flow required amounts of water for effective fire control. With the exception of pinhole leaks from MIC, internal pipe corrosion occurs unseen and often unnoticed. Performing routine flow tests and observing pressure drops is a basic method for assessing the health of larger underground piping, but is not practical on overhead piping.
Enter NFPA 25 (2014 ed.), where section 14.2 recommends an assessment of internal piper conditions be performed as frequently as every 5 years. This is a limited check to assess the condition of “typical” piping. If indications of significant corrosion are found, further investigation, flushing and/or replacement of piping may be necessary.
The following are methods which can be employed to help limit corrosion. They are generally best applied during the design phase, rather than as a curative fix after problematic corrosion is found.
Galvanized Piping – recommended for all new dry pipe sprinkler systems.
Coated Piping – cement lined piping for underground main use is a common example, and generally fares extremely well as corrosion resistant. Cement lined pipe has largely been replaced by PVC type materials.
PVC Piping – commonly utilized as underground sprinkler piping, this alternative is virtually immune to corrosion. Not generally acceptable for exposed sprinkler piping.
Trapped Air/Water – where there is an air water interface within piping, corrosion tends to occur. Dry pipe sprinkler systems are particularly prone, as they are occasionally flooded and must be fully drained prior to returning to service. Proper design and pitch of sprinkler piping will help eliminate these spots. Additional drains or air relief devices may need to be installed.
Removing Dissolved Oxygen – as corrosion typically requires oxygen, removing O2 form water sources may have an appreciable effect in reducing corrosion. Typically treated municipal water supplies will have acceptable levels of O2.
Chemical Additives/Water Treatment – altering corrosive water chemistry may limit traditional corrosion, and is a necessity for MIC treatment.
Grounding, bonding, anode/cathode – limiting stray electrical currents in piping systems can affect both internal and external corrosion.
Risk Logic Inc. can help with identifying and mitigating all types of corrosion outlined above in automatic sprinkler piping.