Warehouses / Manufacturing Facilities
Large Scale Structures Having Potentially Large Scale Corrosion Problems
Large warehouse and manufacturing facilities are primarily affected by corrosion problems at their fire sprinkler systems. In sizes of up to 1 million square feet and greater, tens of thousands of linear feet of fire sprinkler pipe typically exist. In most examples, office space is limited, and does not contain any significant piping other than domestic water. Fire sprinkler pipe, however, holds the potential to cause not only water damage to inventory and production lines, but also threatens enormous replacement costs.
The potential for such damage is primarily dependent upon the type of fire system installed.
Dry Fire Service
“Dry” fire or preaction systems are unquestionably the greatest source of corrosion related failures for all building properties simply due to the fact that they are never actually dry. Once filled with water for testing, substantial water remains behind having abundant air and oxygen to greatly accelerate the corrosion process. Draining, even where the pipe is inclined to specifications, does not sufficiently remove the water. In most indoor fire pipe installations, little to no incline or elevation of the pipe exists.
The raised rolled groove at every single pipe section and tee or elbow, common to all fire systems, itself produces multiple internal obstructions to water flowing back to its source. And the larger the warehouse, in some examples having branch or distribution lines 300 ft. or more in length, the more impossible it is to remove the water. Each quarterly test of the fire system unavoidably introduces fresh new oxygenated water into the system to perpetuate the problem. Certainly they can be called “dry” and referenced as such, but unless no water has ever been introduced into the fire piping, a dry fire system is never actually dry.
Results From Remaininig Water
Water Lines – A common finding at “dry” fire systems due to water remaining at horizontal lines. Here, we show a water line at the 4 o’clock and 8 o’clock positions and straight line corrosion activity at its air / water interface. Water level can vary dramatically for any dry fire system, and except for vertically oriented pipe, some water almost always exists.
Bottom Failure – Water remaining in this section of dry fire pipe focused its impact directly at the bottom, and in specific areas to produce pinholes through the pipe wall. In comparison, this approximately 5% coverage of water within the rolled groove barrier has the remaining 95% of the pipe volume of air and oxygen to aggressively drive the corrosion mechanism.
Failures at dry fire sprinkler systems at warehouse locations are typically at the bottom and lower sides of long horizontal runs, regardless of how it is pitched. This is where water remains; fed toward higher corrosion and pitting activity by the abundant air and oxygen above it. Galvanized pipe suffers similar deterioration; in fact often greater than carbon steel piping due to more aggressive localized pitting.
Vertical risers and smaller vertical sections of pipe typically show little corrosion loss. Fortunately, dry fire sprinkler systems are less common to warehouse and manufacturing facilities, and are more likely to be installed at data centers, and at outdoor locations.
Wet Fire Service
Wet fire sprinkler systems at warehouse facilities, although technically wet or water filled in their design and installation, can exist dry in many areas to produce very similar failures to an actual dry or preaction system. Except for the dry or preaction valve, its discharge and activation controls, and any additional incline provided to a dry fire system, dry and wet fire sprinkler systems are constructed similarly.
A critical fault for large warehouse facilities is the requirement to place the fire sprinkler head within a short distance from the absolute roof of the structure. This in turn creates two major design flaws strictly from a corrosion perspective:
1. Sprinkler heads directly threaded into long 2 in. or 2-1/2 in. branch or distribution lines must be fed from below in order to place the sprinkler heads close to the roof. This in turn defines that air will be trapped in this area of pipe as all water is supplied from below. Without carefully placed air vents at each and every high point, and a fire piping system designed to route the air to those vents, such areas of pipe remain airbound and partially water filled. In effect, the wet fire system becomes a dry fire system in some areas, and therefore subject to the same higher corrosion activity.
2. A large warehouse or manufacturing structure will have a bowed or concave roof profile along its length in order to shed the volume of rain water its large surface area will capture. This in turn exacerbates the angle of the fire sprinkler pipe required to follow that roof bowed roof profile. For most fire sprinkler systems at warehouses, the branch or distribution fire pipe is directly attached to the roof beams, thereby establishing the same upward curvature. For a large facility, a difference of 10 ft. is possible between the side wall and center supports; producing an unavoidable sharp incline in fire piping profile.
For a dry fire piping system, such a steep incline is highly advantageous to the removal of water from its horizontal branch lines, but for a wet fire system, such design is negative, and only results in additional trapped air at the building’s peak. Without air vents to each distribution line, (not the lower main feed line), accelerated corrosion activity is guaranteed.
Two Corrosion Causes
Inclune – A steep incline for a dry fire system is greatly preferred, but for a wet fire system where the pipe is directly mounted to the curved roof beams, air is trapped at the top to produce an essentially dry fire system subject to the same higher corrosion and pitting activity. Without air vents, this condition will ultimately lead to premature failure at the higher elevations of the fire system.
Trapped Air – This standard design for large warehouse fire sprinkler systems, whereby the branch or distribution lines must be mounted to the top of the roof level, invites corrosion problems. With the water fed from below, as the above photo illustrates, each branch line contains some volume of trapped air. Abundant air and oxygen in that region then promotes significantly greater corrosion similar to a dry fire system
Some simple calculations illustrate the problem. For a zoned wet warehouse fire sprinkler system comprising 500 ft. of 6 in. schedule 10 main feed piping, 7,500 ft. of 2-1/2 in. schedule 10 branch lines, and 450 ft. of back feeding 4 in. schedule 10 main, we can calculate an internal volume of approximately 439 cu. ft. of air. Installed under atmospheric conditions, the piping system is then filled from below typically to 125 psi to 180 psi, which in turn forces the air upward as it is compressed.
Using the universal gas pressure formula known as “Boyles Law,” or (Pressure 1 * Volume 1) = (Pressure 2 * Volume 2), we can calculate the final volume of that 439 cubic feet or 3,282 gallons of air having now been compressed by the introduction of incompressible water at 150 psi. In this example, we have (1 * 439) = 150 * V 2. Solving for V 2 produces a result showing that the original 439 cu. ft. of air within the system is compressed down to 2.93 cu. ft. (21.7 gallons) of air remaining above the water line. A larger volume of air will exist at a lower static pressure, and double this volume of air would eist at 75 PSI. This trapped air does not simply disappear as often argued, but instead remains to accelerate the corrosion process.
With each linear foot of 2-1/2 in. schedule 10 pipe containing a volume of 0.038 cubic feet, we can then estimate that roughly 300 linear feet of pipe at its uppermost areas of the structure likely exist partially water filled (filled 25%) and therefore exist subject to the higher corrosion activity common to a true dry fire system. With this mathematical illustration, it is easy to understand how placing the dead ended branch line piping ABOVE its source of water, absent adequate and properly placed air vents, results in more advanced corrosion problems.
A further seemingly insignificant issue is temperature. Temperature slightly expands and contracts the pipe as well as the air inside it. This produces changes or movement in the water level to essentially wash the pipe back and forth at its air / water interface thereby even further accelerating corrosion. Each release of water from the system and variation in air or water pressure via its jockey pump or air compressor also changes this water line to produce the same result.
Pipe Wall Thickness
Common to both forms of fire sprinkler system is the use of thin or light wall and ultra thin wall piping in today’s installation. Ultra thin schedule 5 and schedule 7 pipe, however, is not called ultra thin wall, but typically labeled as being capable of flowing more water due to its greater inside diameter, MaxiFlow, for example. Up front savings in cost, shipping, handling, and installation often has a significant payback, however, in terms of far lower useful service life under any other than extremely low corrosion condition.
Take as an example 4 in. steel pipe having a standard outer diameter of 4.5 in. For schedule 40 pipe, commonly used for fire sprinkler systems decades ago, the original ASTM wall thickness is 0.237 in., shown at top left.
For thin wall schedule 10 pipe commonly installed today, that wall thickness is 0.120 in. or approximately half. Ultra thin wall schedule 7 pipe reduces further to a wall thickness of 0.098 in., with schedule 5 pipe offering even less. Under equal corrosion conditions, failure is guaranteed far sooner as wall thickness is reduced.
Although the NFPA code calls for the use of heavier schedule 40 pipe where threaded, thin wall schedule 10 and ultra thin wall schedule 7 pipe is acceptable in threaded applications by using special threading dies and handling procedures. Once again, various initial savings are gained in its original construction, but quickly disappear once failures occur requiring its replacement.
Threaded pipe introdues even greater threat where thin wall material is installed. For 2 in. schedule 40 pipe having a wall thickness of 0.154 in., the loss of 0.070 in. at its outer thread cut still leaves 0.085 in. remaining. For schedule 7 pipe having an initial wall thickness of 0.084 in., this same thread cut leaves only 0.014 in. at the threads – less than half the thickness of a common credit card. Long service life is rarely possible; requiring an assumption that in contrast to all fundamental laws of nature, corrosion at such ultra thin pipe will not occur