Galvanized Pipe and Dry Sprinkler Systems
by Curt Brown, from the 10/2004 issue of Sprinkler Age, page 23
There are many reasons why galvanized pipe is a superior alternative to black, uncoated steel in a dry sprinkler systems. Scale buildup occurring inside a black pipe from corrosion has frequently been cited as a contributing factor in uncontrolled fires resulting from obstructed sprinklers. A dry sprinkler system is never truly dry. Zinc is a sacrificial metal - protecting the steel substrate from being corroded. Higher C factors can periodically lead to smaller pipe diameters.
The lines above are clear and concise, but a little more detail should help explain how those facts are important for designing systems with galvanized pipe.
One of the issues surrounding sprinkler systems is that an engineer designs buildings to last 50 to 100 years. Most code specifications where the Authorities Having Jurisdiction (AHJ) or the insurance carrier refers to the minimum standard of NFPA 13 allow black pipe in a dry system.
FM Global (FM) has for many years required galvanized pipe for dry systems. It is FM's belief that the reliability and performance of the sprinkler system over its expected life will be greatly increased by this simple change.
Why should a dry system be different from a wet system? First and foremost, a dry sprinkler system is never truly dry. When the system is installed it must be trip tested and then retested every few years according to the NFPA 25 guidelines. It is filled with water and drained. In the best scenario, the pipe was made perfectly straight, when the outlet was welded on there was no warping of the pipe, and the pipe was installed with the exact pitch angle required as per NFPA 13. But what about the grooved areas? How did the water get past the "dam" where the grooves indent towards the inside?
Let's look at it another way. (See Figure 1 on page 25.) If you had a perfectly straight 2 in. schedule 10 pipe, grooved both ends, 21 ft 0 in. long, and it was placed horizontal, after it was drained there would be over 1/2 cup of water still inside that pipe. A dry system is not truly dry.
While it is true that NFPA 13 has specific minimum slopes and drainage requirements, the reality is that, in an imperfect world, there are many ways in a dry system that water can and will be trapped - unable to get out. As a pipe manufacturer, I will tell you my pipe is always perfectly straight. But the reality is that there are bowing tolerances allowed. When a weld-o-let is welded onto the pipe, there is additional bowing. The contractors will intend on installing dry systems as per the requirements, but building conditions will sometimes dictate otherwise. Low point drains and drum drips are accounted for in the standards, but need to be maintained for them to be effective. In the end, there are many potential situations where water can and will stay in the pipe. It is ultimately better to plan for the potential.
What is the leading cause of corrosion? Microbiologically Influenced Corrosion (MIC) is the latest phenomenon blamed for premature failure, but it is not the leader. Oxygen cell corrosion is the leading cause of pipe failure. In order to have oxygen cell corrosion you need a few key ingredients. First would be oxygen, second would be the metal surface. Put water into the mix and you have a specific cell location for the corrosion to start. There has been any number of articles found in this magazine and in other publications dedicated to the concept of corrosion, so I will not get into the specifics here.
It is widely known that zinc holds up well to oxygen cell corrosion. A point of reference that most people are familiar with is with their cars. Thirty to 40 years ago cars would rust out in three to five years. When was the last time a new car rusted out in three to five years? Factor in that the wall thickness is almost half of what it was 30 to 40 years ago, and that is quite an improvement. Originally, when the cars used ungalvanized sheet steel, the paint covered the steel and protected it for as long as the paint was covering the base metal. Once the paint was cut or scratched, the rust cancer would start and continue below the paint. The auto industry started to use galvanized and galvannealed sheet steel below the paint. Where that same scratch might occur now, the zinc on either side of the scratch - under the paint - sacrifices itself to protect the steel substrate. Eventually it will rust through, but ultimately that thin coating of zinc is the difference between cars today and cars from yesterday.
Zinc is a sacrificial anode. All metals corrode. But metals corrode at different rates. One form of corrosion is called galvanic corrosion. The term "galvanized" is most often associated with steel coated with zinc. But the "Galvanic Series" to a metallurgist refers to the corrosion potential for different metals in seawater. Rust is a form of iron oxide. Where there is potential for oxygen to combine with a metal and make an oxide, the galvanic series to some degree dictates which metal will form an oxide first. Zinc is a sacrificial metal to Iron, when the potential for an oxide is present, the zinc will combine with the oxygen to form a zinc oxide. Zinc is the "anode." Not all metals will sacrifice themselves for other metals, but zinc absolutely will protect steel. When discussing zinc as a sacrificial metal, the terms "throw of protection" or "distance" of protection are used. On an ocean-going ship, a small anode of zinc can protect up to a 100 ft2 of steel surface area in the electrolytic salt water. The "throw" or distance of protection would be less than 6 ft in any direction (an 11 ft diameter circle). On a fence in the middle of a dry desert, the sacrificial "throw" or distance of protection may only total 1/4 in. That said, the greater the area the zinc is required to protect, the faster the zinc will be used up.
To put it in an understandable concept, let's revisit the automotive example. Cars still get scratched, and the scratch will often penetrate the zinc layer exposing bare steel. Where you expect the exposed steel to rust through, it doesn't because the zinc on either side of the scratch will sacrifice itself to keep the steel from rusting. Eventually the zinc will be used up and the steel will rust, but only as the zinc sacrifices itself further away from the base steel and the distance from the scratch is farther than the "throw" or distance of protection.
We know corrosion exists, and we know it can cause problems, but where does the corrosion material come from? All we had in the pipe was some oxygen, moisture and a steel surface. But ultimately the pipe was filled full of "corrosion." The answer is simple. According to Corrview International, "Steel, when corroded back into iron oxide, produces a significantly greater volume of less dense material by a factor of approximately 18 to 20 times. Such deposits, in turn, ultimately create constricted flow under deposit pitting and wall loss." What that means is shown graphically on page 25. (See Figure 2).
The above information is somewhat "worst case scenario." Most of the oxide will form at the bottom of the pipe rather than build up as shown, and the buildup is not completely around the circumference of the pipe. That said, the trip testing required by NFPA 25 and periodic flushing should get rid of most of it. But after the testing and flushing, the oxide corrosion starts all over again. The corrosion being flushed was once part of the wall thickness. While it is acknowledged and accounted for that black pipe will corrode through the periodic testing procedures, the continual erosion of the wall thickness is not. The irony about dry systems and requiring heavy wall is if a black pipe schedule 5 or 7 dry system fails from corrosion, the typical solution is to use a heavier wall. Why? The corrosion doesn't go away, and it is obvious because the lighter wall pipe failed from corrosion that there is a problem. The heavier wall solution means the corrosion will build internally for a longer period of time before external failure occurs. But the heavier wall solution masks the ultimate problem.
The American Galvanizers Association performed a two-year study on the corrosion/erosion rates of zinc compared to steel. It found that in 38 various locations around North America the median rate of loss for the steel vs. the zinc was about 23.1 to 1. In practical terms, that means the three thousandths of an inch of zinc on the ASTM A 53 zinc requirement (1.8 ounces per square foot of surface area ) is equivalent to almost 0.072-in. of equivalent steel. Zinc corrodes at a much slower rate than steel, and does not leave a thick disgusting product in its place.
Uses of zinc protected steel are found in every day life. Drive along the streets and highways in your zinc protected car, and you will see that signposts, guardrails, bridge railings, etc. are all galvanized. Stop at the park and look at the playground equipment, or the chain for swing sets, or the fence surrounding the park; all are galvanized. Railings, farm equipment, watering systems. The list goes on, with the common thread being a desire to minimize corrosion and maximize the life of the item.
The significance of the use of black pipe in a dry system is found where the dry pipe systems are typically used. Dry systems are often considered for locations were there is a potential for freezing conditions - parking garages, unheated warehouses, etc. In contrast, the inside of most buildings next to the parking garage are often climate controlled. It is warm, dry and clean; yet in many areas of the building, other applications of galvanized steel are required. The ducting for the heating and air conditioning is galvanized sheet steel. It will typically never see less than 60¡ F, or above 90¡ F, and never have moisture in the system, but an engineer would never allow non-galvanized sheet steel to be used. The entire electrical system consists of either galvanized or painted (inside the conduit) materials feeding galvanized electrical boxes. Would the electrical engineer allow uncoated black steel to be used? And, how would the electrical engineer feel about moisture inside his conduit or electrical boxes? The inside wall studs of today's buildings are all galvanized sheet steel - none expected to encounter moisture. The roofs of most large buildings are flat and covered with several materials - including corrugated galvanized sheet steel. If there are appliances in the building, almost all will have galvanized sheet steel covered with an enamel color coating. Now compare those items with a dry sprinkler system that will see water inside the pipe unable to get out. In the winter, it may see temperatures below 0¡ F, and in the summer it may see temperatures over 100¡ F. It will go from low humidity to high humidity. Yet it is expected to last as long as the dry, room temperature galvanized steel products within the building.
Black (bare) pipe has a C=120 rating for wet systems, and a C=100 rating for dry systems. This implies corrosion rates of the dry systems are substantially higher than those of wet systems. This is true, and the phenomenon is entirely due to the effects of dissolved oxygen. Dissolved oxygen in wet systems is consumed in a relatively short period of time (a few days to a month). At that point the corrosion rate drops to low levels that are still dependent on water chemistry. In contrast, dry black steel systems exhibit high corrosion rates. This is because - as already discussed - "dry" systems are never really dry. They experience atmospheric corrosion in which water condenses inside the pipe. This water is nearly saturated in oxygen, and compressed air is continually being added by the system to keep pressure in the system. Again, it is the presence of oxygen in the water that directly causes the high corrosion rates. The NFPA 13 C factor table takes this into consideration by lowering the "C" Value of black pipe to 100 for the dry systems. For this article, I am renaming that value as the Rate of Corrosion - The increased potential that corrosive materials will build up inside a pipe.
Galvanized pipe has a C=120 rating regardless of whether it is in a wet or dry system. This is consistent with the fact that galvanizing is very effective in controlling corrosion. In environments where black pipe fails rapidly in the presence of oxygen-rich water (condensed water in dry systems), the galvanized pipe performs reliably for many years. Similarly, the galvanized pipe can be expected to control corrosion of wet systems during periods when oxygen is present or when aggressive water is in the system.
Using galvanized pipe does not always mean higher costs. In discussions with engineering design firms, it was approximated that 40 percent of systems designed with galvanized pipe - and a C value of 120 - can be hydraulically downsized vs. black pipe - and a C value of 100. The 40 percent will vary from system to system, but recalculating systems to the higher C value is worth looking into just on the downsizing potential alone.
According to the NFPA 25 guidelines, scheduled trip testing is required. If the testing finds that there are potential corrosion problems, flushing the system is recommended. Many AHJ and fire protection associations recommend visual inspections of the systems by breaking a joint or two and looking into the system for corrosion buildup. If the system requires flushing, the new oxygen and water in a sense restarts the corrosion clock. This requires a consistent program for the building. This does not always occur in the timely manner as required. The amount of corrosion to be flushed is tied to the "Rate of Corrosion." If a building is designed to last 50 to 100 years, and the rate of corrosion continues to develop, and if the corrosive products are significant enough in terms of volume and size, the potential to plug the system is an issue. Minimizing the potential for corrosive products was and is ultimately the goal when specifying corrosive resistant piping for dry systems.
Now that we have established that moisture, air, and metal surfaces are an issue, let's discuss the worst case internal corrosion issue. Corrosion means the interior of the surface is slowly disintegrating. The rust, the scale, and the tubercles formed are not firmly attached. They sit where they form. The issue is that the corrosion buildups break away in the event of water flow (trip test or actual fire) and can end up clogging the sprinkler heads first activated. Depending on the occupancy of the area where the fire starts and the number and location of sprinklers that are obstructed, the results can be anything from increased fire and water damage to an uncontrolled fire
In summary, sprinkler systems are better seen and not used. Most buildings don't have fires in their lifetimes, so the emergency service of the sprinkler system is never an issue. Black pipe in a dry system will corrode. A dry pipe system is never truly dry. Where corrosion can and should be flushed from a system to keep it fire ready, the corrosive materials flushed were once part of the steel wall system. The wall has been weakened and the pitting affects water flow. Eventually, those pitted areas will result in a rupture. But when a fire does occur, a plugged sprinkler head can be the difference between life and death or minimal damage and total loss. Dry sprinkler systems should be designed with corrosion potential in mind. Potential hydraulic downsizing with the C value of 120 for a galvanized pipe system can mean that the cost of the better system does not need to be significantly higher than a black pipe system. Galvanized pipe is a significant deterrent to corrosion, and in the end, the small additional cost becomes insignificant.
ABOUT THE AUTHOR: Curt Brown is vice president of IDOD Systems, LC. He holds a bachelor of science degree from Illinois State University and is a member of AFSA. Previously he was the engineering manager at Allied Tube and Conduit, Harvey, Ill. IDOD is the manufacturer of GAL-7 and GAL-5 Sprinkler pipe. For more information visit www.idodsystems.com. Brown can be reached via email at firstname.lastname@example.org.