The concept is simple: Smoother pipe creates less friction loss, which possibly can allow smaller pipe sizes costing less in materials and labor. In the Hazen Williams equation, which is the primary equation used to determine friction loss in a fire sprinkler system, the internal smoothness of a pipe is represented by its C Value or roughness coefficient.
This value can range anywhere from 100 to 150, depending on the pipe material and system type, where higher C Values represent smoother pipe. It should be noted that the C Value used for hydraulic calculations is not the same as the actual roughness in a pipe hot off the mill run. The C Values used in calculations consider typical internal deterioration and corrosion over the life cycle of the installed pipe.
Historically, a C Value of 120 is used for wet sprinkler systems, and a C Value of 100 is used for dry and preaction systems installed with black steel piping. Prior to the 2013 edition of NFPA 13, Standard for the Installation of Sprinkler Systems, a C Value of 120 was permitted to be used in dry and preaction systems when internally galvanized steel pipe was used.
However, the technical committee acted during the revision cycle for the 2013 edition. It required all dry and preaction system piping, regardless of pipe type, to use a C Value of 100 — effectively eliminating the use of galvanized steel pipe by taking away its benefit.
On the other hand, FM Global still requires galvanized steel pipe, or an equivalent, to be used in dry and preaction systems they insure. FM Data Sheet 2-0 section 220.127.116.11 states:
“Use internally galvanized, stainless steel, or similar corrosion-resistant pipe in all new dry-pipe, preaction, refrigerated-area, deluge, and exposure-protection sprinkler systems. Do not use galvanized pipe in areas where the ambient temperature could exceed [130 F (54 C)] unless the pipe is specifically [FM-approved] for use in such conditions.”
So why the conflicting requirements between two standards-making organizations? While galvanized steel pipe can corrode, the corrosion typically occurs at imperfections in the internal coating. These imperfections lead to pinhole leaks, which are a nuisance but not an impairment of the system. Other than pinholes at imperfections, galvanized steel pipe is clean internally and has minimal debris compared to black steel pipe.
Black steel pipe can have internal corrosion that flakes off and plugs a sprinkler orifice. This plugging is a catastrophic failure that may prevent water from reaching a fire. An insurance company would rather replace a couple of pieces of pipe if a pinhole leak develops than write off a loss caused by internal obstruction to sprinklers. This is evident through FM’s allowance to install black steel pipe when an inert gas is used in the system:
“Exception: Black steel pipe can be used in dry-pipe sprinkler systems equipped with closed-type sprinklers if the piping system is filled with an inert gas.”
Although dry and preaction systems installed with steel piping have used a C Value of 100 since the 2013 edition of NFPA 13, with the advent of nitrogen generators on the market, the pendulum has begun to swing towards permitting a C Value of 120 under certain conditions. In the 2022 edition of NFPA 13, there is proposed language to allow a C Value of 120 to be used in dry and preaction systems if the steel pipe is maintained with a listed nitrogen generator at a concentration of at least 98 percent nitrogen.
“18.104.22.168* Nitrogen Supply for Increased C Value.
“A.22.214.171.124 Nitrogen systems are equipped with an air compressor capable of restoring system air pressure within 30 or 60 minutes. It is not the intent of this section to require a 98 percent concentration of nitrogen within the 30 or 60 minutes as required in 126.96.36.199.
“188.8.131.52.1 Where nitrogen is used to allow for increased C value in accordance with Table 184.108.40.206.1, the nitrogen supply shall be in accordance with 220.127.116.11.2 through 18.104.22.168.6.
“22.214.171.124.2 Nitrogen shall be from a listed nitrogen generator permanently installed.
“126.96.36.199.3 The generator shall be capable of supplying and maintaining at least 98 percent nitrogen concentration throughout the system at a minimum leakage rate of 1.5 psi (0.1 bar) per hour.
“188.8.131.52.4 A means of verifying nitrogen concentration shall be provided for each system where increased C value is used.
“184.108.40.206.5 The nitrogen generator shall be installed per the manufacturer’s instruction.
“220.127.116.11.6 The nitrogen generator shall be maintained in accordance with Chapter 32.”
Again, this is a simple concept — air (oxygen) plus iron plus water equals corrosion. Take out the air (oxygen) and corrosion cannot occur. This proposed language requires a permanently installed listed nitrogen generator (i.e., no bottles) and a means of verifying the 98 percent concentration in the system.
Questions and studies are currently in progress determining whether 98 percent nitrogen purity exists at all points throughout the system or just near the test site, but that is a discussion for another day.
Corresponding language was submitted to NFPA 25, Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems; however, the proposal was rejected during the first-draft stage because NFPA 13 was not yet finalized. By the time the second draft comes around, the 2022 edition of NFPA 13 will be an approved standard, and the periodic inspection, testing and maintenance requirements should be accepted.
If not, keep an eye out for a tentative interim amendment to revert to the 2019 edition language.
The last aspect of nitrogen generator design is the 30-minute fill required by NFPA 13. Nitrogen generators, by design, have low flow rates due to the filters or membranes air must pass through to separate the nitrogen from the air. This means that a nitrogen generator alone is often not sufficient for a single dry system.
An additional air compressor sized to fill the system within 30 minutes is necessary to meet the fill-time requirement of NFPA 13. Once the system is filled to an appropriate level, the nitrogen generator will take over and vent purified nitrogen into the system piping until 98 percent purity is reached.
A footnote is attached to Table 18.104.22.168.1 in NFPA 13’s 2019 edition stating that the authority having jurisdiction (AHJ) is permitted to allow other C Values. AHJ is a broad term in the context of NFPA requirements, and this is not permitting the owner or fire marshal to change a C Value on a whim.
Rather, the specifying engineer is permitted to use other C Values using their engineering judgment if they have the appropriate technical data to support that decision. I don’t envy someone who takes on that responsibility, risk and liability, but it is an option if the proposed revisions to NFPA 13 and NFPA 25 do not turn out as expected.
Registration is available for SFPEs Advanced Fire Detection and Alarm seminar beginning Aug. 2. This 12-session seminar is derived from our two-part course and will review the performance-based design process and its applicability to the design of fire alarm systems, and the computational methods used to implement a performance-based design approach to fire alarm design problems. Participants will earn 15 Professional Development Hours; a certificate of attendance will be provided. Learn more at www.sfpe.org/courses.