Using Nitrogen Gas to Prevent Corrosion Of Fire Sprinklers in Parking Structures
For heated parking structures in southern states where temperatures within the garage stay above freezing, the sprinkler system of choice will be a “wet” type. In this, the sprinkler piping is continuously pressurized with water at 70 to 100 psig. When a sprinkler head opens due to elevated temperature, water is discharged to control the fire.
In unheated parking structures in most states and in Canada where temperatures within the garage can be expected to fall below freezing, the sprinkler system of choice will be a “dry” type. In this, the sprinkler piping is pressurized, not with water since this could freeze, but typically with compressed air at 20 to 40 psig supplied by a small compressor.
With the dry system, the heat from a fire will open a sprinkler head in the same way as in a wet system, but then for the first few seconds, air within the piping vents and the reduction in pressure allow the main water valve to open, thus flooding the piping with pressurized water, which is discharged at the open sprinkler head onto the fire.
In both systems, the most common choice of sprinkler piping material is plain “black” steel. More expensive internally galvanized steel pipe also is used for dry systems.
Failure caused by corrosion is not often a major problem in wet systems and will not be addressed below. Surprisingly, dry systems are much more prone to internal corrosion, which can cause leakage within –two to five years. As it turns out (explained below), upgrading to galvanized steel pipe to prolong the life of the piping often results in leakage in even less time!
Why do dry systems corrode and how can this be addressed?
Let’s look at the environment within a dry pipe system. The first point to note is that the environment within the pipe is not, in fact, “dry.”
National Fire Protection Association (NFPA) codes and standards require that, at commissioning, the piping is pressure-tested with water up to 175 psig. It’s infeasible to completely drain and dry the complex matrix of piping that forms a sprinkler system. Also, when compressed air is fed into the piping to maintain pressure, it’s saturated with water vapor.
What’s called microbial-induced corrosion (MIC) has, until recently, taken most of the blame for the high failure rates in dry systems. This form of corrosion is created in localized areas as a result of the action of certain microbes that thrive under the moist static conditions found within the piping.
This is now no longer believed to be the case. Although about 10% of the failures may be attributable to MIC, the main cause is corrosion of the steel by water and oxygen in the compressed air. This is similar to the orange-colored “rusting” that you might expect to see on steel tools left outside in the rain.
In the case of galvanized steel, where low points are always wet, the zinc coating gradually dissolves, causing a deposit of material that then covers the flooded surface. Locations where the steel substrate is exposed under the deposit can then experience accelerated corrosion due to a phenomenon known as an oxygen concentration cell, resulting in localized pinhole corrosion appearing as if the pipe wall has been drilled.
The solution is straightforward and environmentally friendly. Instead of using air containing 21% oxygen, compressed nitrogen gas with a lower oxygen concentration can be used. “Oxygen-free” is ideal, but 1% oxygen is better than 2%, and much better than 5% because the corrosion rate is proportional.
Once oxygen is 1% or less, any residual water is no longer corrosive, and the piping should last the life of the structure.
Photos accompanying this article show piping after seven years in 99% nitrogen (1% oxygen) produced by a Holtec nitrogen generator. Complete evaluation and testing by a major corrosion testing laboratory confirmed that internal corrosion in the sample was negligible.
Pure nitrogen gas has always been available in cylinders, but cost and safety concerns have limited its use. Holtec pioneered the use of on-site generated nitrogen for this application, using specially designed equipment.
The company’s family of generators produces 99% nitrogen, and includes patent-pending pressure-cycling technology to purge the sprinkler piping of residual oxygen before corrosion can set in. This technique even removes oxygen from the remote sections of the many closed-end branches in the typical sprinkler system.
A photo of a fully assembled skid-mounted unit also is shown here.
For more information, contact Ken Jones, an Officer and
Partner of Holtec Gas Systems, at holtecllc.com.