FPEeXTRA Issue 61

Initial Non-Destructive Testing of a Long-Term Corrosion Experiment

By Virginia R. Charter, PhD, PE and Ahmed H. Aljawad

The internal corrosion of fire sprinkler systems is a major concern in the reliability and life of the system(s). Corrosion in sprinkler systems can affect the system’s hydraulics and cause system leakage, which may result in property damage, as well as significant replacement costs. Fire sprinkler systems are subject to deterioration through different forms of corrosion including microbial, pitting, stress-cracking, crevice, erosion, galvanic, intergranular, and selective leaching corrosion. Corrosion can affect the fire sprinkler systems’ components, such as pipes, fittings, valves, and sprinkler heads. This article discusses a long-term corrosion experiment and some of the initial findings that is currently underway at the Oklahoma State University Department Fire Protection & Safety Engineering Technology. The topic of corrosion of fire sprinkler systems is identified in the SFPE Research Roadmap as an important research need in the profession of fire protection engineering and SFPE has a strategic goal of promoting research that is identified in the roadmap.

Corrosion protection for fire sprinkler systems is needed in order to maintain optimal performance and save money. One method utilized to extend the lifecycle of fire sprinkler systems is using nitrogen gas instead of compressed air for dry pipe systems and nitrogen inerting for wet pipe systems. Nitrogen gas is an inert diatomic molecule, and due to its unique properties—such as its inability to react with metals—is used in many different applications. Furthermore, generalized corrosion is caused by at least one oxidation and reduction reaction. This means that corrosion needs oxygen to propagate (Tihen, n.d.). Therefore, the use of nitrogen gas in both wet and dry fire sprinkler systems can control corrosion initiation by: a) limiting the amount of oxygen that dissolves in water; and b) preventing sprinkler systems failures, such as corrosion-related leaks.

Understanding corrosion in fire sprinkler systems is a marathon, not a sprint, particularly in understanding the impact corrosion has on both system components and the life of the system through inspection, testing, and maintanance. This experiment consists of four automatic fire sprinkler systems, three types of pipes, three types of sprinklers and two types of connections. The different sprinkler systems that were designed include a dry system with a nitrogen generator, a dry system utilizing compressed air, a wet system with nitrogen inerting, and a wet system without nitrogen. The different systems, all of which were designed and installed in accordance with the 2016 edition of NFPA 13, Standard on Installation of Sprinkler Systems, include separate branchlines of Schedule 40 galvanized steel, Schedule 40 steel, and Schedule 10 steel pipes. Figure 1 shows the systems layouts. The blue color represents the Schedule 40 steel pipe, the red color represents the Schedule 10 steel pipe, and the gray color represents the Schedule 40 galvanized steel pipe. Additionally, all of the systems incorporate three different sprinkler orientations: upright, pendant, and sidewall. This will allow for future analysis to determine whether or not the sprinkler head orientation affects the corrosion process in the pipes. Roll grooved and threaded connections are used throughout the system(s), with the exception of Schedule 10 pipes, which only used roll grooved. An inspectors test connection is then provided at the far end of the system(s).

The initial phase of the project included the installation and commissioning of the systems. Additionally, preliminary data was collected utilizing non-destructive testing (NDT). The primary purpose of collecting this data was to provide a baseline of pipe thickness as well as to be able to compare between the observed values of pipe wall thicknesses from NDT and the minimum thickness requirements per ASTM standards.

Two different techniques will be used to complete the non-destructive testing, including a Lixi Profiler and an Ultrasonic device. The Lixi Profiler (Low-Intensity X-ray Instrument) uses radiometric profiling consisting of a beam source and a detector. Therefore, it works by placing the beam source on one side of the pipe and the detector on the other side to measure the pipe’s wall thickness. Also, when using the Lixi Profiler, the water inside the pipe is considered because the equipment measures the pipe’s wall thickness along with the thickness of the water (Lixi, Inc., n.d.).

The Ultrasonic device uses high-frequency sound energy to measure the wall thickness of the pipe being tested. This technology operates when the receiver produces high voltage electrical pulses and the transducer uses these pulses to generate high-frequency ultrasonic energy. The sound energy is introduced into the pipes and spread in the form of waves, where the energy reflects back from the surface when there is a discontinuity in the wave path. The transducer converts the reflected sound energy into an electrical signal which provides information regarding the pipe’s wall thickness (NDT Resource Center, n.d.).

Table 1. Required and average pipe wall thickness

The fire sprinkler systems used in the long-term corrosion experiment are inspected, tested, and maintained in accordance with NFPA 25, Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems, the same as traditionally installed systems. The systems will also be periodically inspected utilizing NDT, to allow for monitoring of potential corrosion within the piping. The experiment is anticipated to continue until the failure of the system(s). At that time, destructive testing will occur for the failed system(s). The authors intend on updating the fire safety community of these research results as the experiment progresses and once this experiment is completed.

A special thanks to James Whitcomb and GFS Texas in assisting with the design of the project.  And thank you to our donors of the equipment and labor that made this project possible including GFS Texas, Wheatland Tube, Ferguson Fire & Fabrication, Viking, Potter, General Air Products, Anvil International, and Engineered Corrosion Solutions.

Virginia R. Charter, PhD, PE, is with OKLAHOMA STATE UNIVERSITY and Ahmed H. Aljawad, is with SAUDI ARAMCO

References

Lixi, Inc. (n.d). The lixi profiler. Retrieved from Lixi: http://lixi.com/lixi-profiler/

NDT Resource Center. (n.d.). Basic principles of ultrasonic testing. Retrieved from Nde-Ed: https://www.nde-ed.org/EducationResources/CommunityCollege/Ultrasonics/ Introduction/description.htm

NFPA 13: Standard for the installation of sprinkler systems (2016 Edition ed.). (2016). National Fire Protection Association.

NFPA 25: Standard for the inspection, testing, and maintenance of water-based fire protection systems (2014 Edition ed.). (2014). National Fire Protection Association.

Tihen, J. (n.d.). Corrosion inhibition of dry and pre-action fire suppression systems using nitrogen gas. St. Louis: Potter Electric Signal Company, LLC.