Dry Pipe Sprinkler Systems The Importance of Accuracy CHRISTOPHER CULP & BOB RENTON | Henderson Engineers
A variety of factors have contributed to the increased implementation of dry pipe sprinkler systems. The inclusion of exterior design elements such as exterior canopies, in addition to NFPA 13’s prohibition of the use of antifreeze in new installations, has forced designers to include dry pipe systems in buildings. When properly installed in accordance with NFPA 13, Standard for the Installation of Sprinkler Systems, these systems can provide the same level of protection as their standard water-filled counter parts; however, using tools without sufficient accuracy can lead to material failures and subsequent collateral damage to the owner’s facility. Water leaks caused by pipe corrosion and/or mechanical freeze damage are common with improperly installed systems. Henderson Engineers has performed multiple forensic investigations that identified installations that do not comply with NFPA 13 requirements as the root cause that led to pipe failures and subsequent property damage. If these installation inconsistencies are not identified during the initial inspection and commissioning processes, it’s unlikely that they will be corrected prior to a system malfunction. Installation Standards Matter NFPA 13 establishes the minimum standards for the design and installation of dry pipe systems. No two systems are exactly the same due variable factors like building construction, occupancy, contents, and physical location. One of the most challenging – and most important – aspects of installing dry pipe systems is ensuring the sprinkler piping has the proper pitch. As a consulting engineer, we review many contractor submittals that state the piping will be pitched to the riser or auxiliary drain in accordance with NFPA 13. Of course, installing pipe in the field is a more challenging task than adding a plan note. Case Study Case in point, our firm was recently retained by an owner’s insurance company to survey their facility fitted with a dry pipe sprinkler system that protected an unconditioned attic and a conditioned area below the ceiling. The sprinkler system had been plagued with issues throughout the building’s life. Pinhole leaks and oxygen corrosion developed shortly after the building opened, requiring system piping to be partially replaced. A nitrogen generator was recommended and installed by the service contractor to address the corrosion issues and reduce the number of leaks. These were all valid suggestions by well-meaning service contractors. Unfortunately, despite these attempted remedies the corrosion and leaks continued for several years because the root cause of the problem was not addressed. Our analysis identified large 44 Sprinkler Age | January/February 2019
portions of the system that were improperly pitched causing large amounts of water to be trapped in the system. Ultimately the system froze, resulting in a broken pipe and water damage loss claim costing the building owner’s insurer thousands of dollars in damages in addition to temporary loss of the damaged portion of the facility. Installation Parameters are Clear Parameters for proper dry pipe pitch are clearly established in NFPA 13. In a correct installation, the water in the pipe drains completely after system tests or activation prior to putting the system back in service. During installation, the pipe should be pitched to drain toward either the water source (such as a dry pipe valve) or another designated drain valve called an auxiliary drain. If the pipe is not pitched, or worse yet, is pitched in the wrong direction, water will be trapped in the pipe permanently. Trapped water can result in internal corrosion and subsequent pinhole leaks and/or fractured pipes (caused by expansion and contraction due to freezing and thawing). NFPA 13 8.16.2.3 provides direction on how much slope is required for dry pipe installations: ¼ in. per 10 linear feet of pipe for mains and ½ in. per 10 linear feet for branch lines. Translating into degrees of slope [defined as tan-1 Rise required:
( Run )
• Main – Degrees Slope: tan-1
in. 1 ft = 0.119° required ( 0.25 10 ft ) ( 12 in. )
• Branch Lines – Degrees Slope: tan-1
( 0.1050ft in.)( 121 ftin. ) = 0.239° required
During our case study, a forensic examination of the damaged piping confirmed our initial hypothesis: the dry pipe sprinkler system did have water trapped in the pipe due to errors in installation. The trapped water likely contributed to several of the system’s issues throughout its life. The overall finding was the system had not been installed in accordance with the designated pitch in the approved plans and NFPA 13, and the inconsistencies were not identified during acceptance inspections and commissioning. Subsequently, system components were replaced without addressing the pipe pitch errors, perpetuating the problem. Interestingly, the root cause of the issue may have been a simple spirit level. The Accuracy Conundrum During a sprinkler system’s installation, accuracy is paramount. Correctly verifying NFPA