Reviewing technologies to slow the breakdown of dry and preaction sprinkler systems
Dry pipe fire sprinkler systems solve the problem of water freezing in pipes but add an issue: faster corrosion. The system’s dry valve separates the water supply from the pipe network in potentially freezing areas. However, water left over in the pipes after system activations or tests or due to condensation combines with oxygen-rich air to promote metal oxidation—much more than in water-filled wet pipes. The same issue impacts the dry pipe networks of preaction sprinkler systems.
For years, nitrogen sprinkler systems have been the preferred solution for mitigating corrosion. However, the 2025 edition of NFPA 13: Standard for the Installation of Sprinkler Systems gave a new technology the thumbs up: vapor corrosion inhibitors, also known as volatile corrosion inhibitors, vapor phase corrosion inhibitors, or by the brand name Vapor Pipe Shield.
So, which is better? In this article, we’ll compare the features and NFPA rules surrounding vapor corrosion inhibitors and nitrogen systems. Read on to learn:
- NFPA 13 allows multiple anti-corrosion technologies
- Nitrogen sprinkler systems are a tried-and-true approach but require additional setup and maintenance
- The considerations with vapor corrosion inhibitor systems
- Nitrogen and VCI systems are effective corrosion-mitigation options
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NFPA 13 allows multiple anti-corrosion technologies
Dry and preaction systems both rely on gas- instead of water-filled pipes to prevent freezing (for dry and no-interlock preaction systems) or unnecessary sprinkler activation (for single- and double-interlock preaction systems). However, the mix of some moisture, oxygen in the air, and metal pipes can accelerate corrosion in a blend known as the “corrosion triangle.”
Besides eating away at the pipe walls from the inside, leading to leaks, corrosion also creates hydraulic challenges that can impair water flow and pressure. In its advanced forms, it can cause system blockages. Fortunately, NFPA 13 approves several technologies to mitigate this issue, including filling dry systems with nitrogen gas instead of oxygen-rich air, using a vapor corrosion inhibitor, or employing vacuum systems.
Friction from pipe walls fights against the available pressure in a fire sprinkler system, and corrosion buildup creates more friction. The extra corrosion present in dry systems is factored into sprinkler system design calculations. When designers determine the pressure needs of a sprinkler system, they use a formula—often the Hazen-Williams formula—to model the pressure lost due to friction from pipe walls. Pipe smoothness is represented in the formula by the C-value, with a higher number indicating a smoother surface. For example, per NFPA 13 Table 28.2.4.8.1, both black steel and galvanized steel in wet systems have C-values of 120, but this number is reduced to 100 in dry systems to account for the effects of corrosion.
Solving the corrosion problem can also solve the added-friction problem (in addition to degradation issues), and NFPA 13 approves three technologies for this purpose. Vacuum systems are new to the 2025 edition and fight corrosion by creating negative pressure in the pipes. Nitrogen systems have had NFPA 13’s blessing for multiple editions. Unlike oxygen, nitrogen gas does not corrode iron or steel. Thus, systems filled with at least 98% nitrogen gas resist corrosion and can use a C-value of 120 for the pipes instead of 100. Vapor corrosion inhibitors, a newly approved technology for fire sprinkler usage that coats the pipe to inhibit corrosion, also allow designers to take the 120 C-value. Of course, all of these technologies also mitigate leaks and system failures. But of the latter two options, which is better? Each has unique considerations. Let’s compare.
(To learn more about the significant changes to NFPA 13 in the 2025 edition, check out Part 1 and Part 2 of our recent series. You can also read our deep-dive series on dry sprinkler systems, including an article focused on nitrogen systems. For more on how the C-factor figures into sprinkler system design, see our article on how much water pressure a fire sprinkler system needs.)
Nitrogen systems are a tried-and-true approach but require additional setup and maintenance
Nitrogen systems rely on nearly pure nitrogen gas to fill the sprinkler pipes. As NFPA 13 says, this gas is sourced from commercially available tanks of compressed nitrogen or an onsite nitrogen generator (8.2.7.6.1), with nitrogen generators preferred because they don’t require regular tank deliveries. Nitrogen generators separate nitrogen from other gases in the air, especially oxygen. The process relies on a compressor pump to generate air pressure and a dryer to remove moisture, followed by either pressure swing absorption (PSA) or membrane stages.
PSA-based nitrogen generators use carbon filters that bind to oxygen under pressure. These carbon sieves eventually become saturated with oxygen, so they switch between at least two units: one is allowed to off-gas while the other pulls oxygen out of the air. In contrast, membrane systems rely on tubular membranes to exclude gas by size and don’t require the mechanical switching that PSA generators need. PSA and membrane generators are both controlled by a pressure monitor; when the gas pressure in the sprinkler pipes drops, the compressor kicks on to replenish the system with nitrogen.
Nitrogen sprinkler systems have been around the longest, so in addition to being NFPA 13 compliant, designers have different kinds of nitrogen generators to choose from.
On the other hand, nitrogen systems require installing additional parts with specific maintenance. The inspection, testing, and maintenance (ITM) schedule for nitrogen generators and compressors is found in section 13.10 of NPFA 25: Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems (2023 edition). In short, these systems need:
- Monthly inspections of the nitrogen generator and compressor machinery
- Monthly inspections of the power wiring, piping, oil level, and anchoring
- Annual tests to verify that a pressure drop in the system causes the generator system to operate and restore pressure
- A yearly test to ensure the compressor does not overheat while running
- An annual (or more often if specified by the manufacturer) oil change for compressor pumps using oil
NFPA 25 also requires completing any additional maintenance that a manufacturer calls for in its instructions.
Additionally, as Mark Hopkins reported in the May/June 2024 issue of Sprinkler Age, some systems have experienced challenges with nitrogen generators struggling to reach the required concentration of 98% nitrogen. Lower nitrogen levels can create longevity issues with the pipe and affect the hydraulic properties, which is why NFPA 25 requires a system’s nitrogen concentration to be tested semiannually (13.4.3.2.14).
Considerations with vapor corrosion inhibitor systems
A vapor corrosion inhibitor works by a completely different principle than nitrogen systems. Rather than removing corrosive oxygen from the air, VCIs form a barrier between metal and air.
Though the technology is relatively new to fire protection, vapor corrosion inhibitor chemicals were known in the early 1900s and first used commercially by Shell Oil Company in the 1950s to protect military equipment. The original compound patented by Shell, dicyclohexylammonium nitrite (trade name DICHAN), was ultimately discontinued due to health concerns. However, different, safer compounds that work by the same principle are now EPA-approved, UL-Listed, and don’t meet the requirements for issuing Safety Data Sheets (SDS).
Here’s how they work: The corrosion inhibitor is a chemical that vaporizes easily. As a vapor, it can spread easily through an entire pipe system, and its electrostatic attraction to metal causes it to stick to metal pipe surfaces and form a chemical barrier that prevents corrosion. In dry or preaction fire sprinkler systems, canisters of corrosion-inhibiting compound are installed in a vaporizer device between the pressure-maintenance pump and the dry or preaction valve. The pump facilitates vaporization and spreads the vaporized compound throughout the pipe system, even penetrating any standing water to form a protective layer on the metal.
General Air Products, the manufacturer of the Vapor Pipe Shield, states that vapor-based inhibitors have a practical advantage over nitrogen systems. Being fully mechanical, they don’t require electrical connections for the vaporizer device or extra parts like pumps or dryers. In addition, they obviously avoid the challenge of keeping nitrogen concentrations at 98 percent throughout the pipe network. This cuts down on the number of ITM tasks. However, with VCI systems, system owners and the fire protection pros who work on a system are responsible for specific maintenance steps specified by the manufacturer’s instructions.
With a General Air Products VCI system, the annual steps involve changing out the canisters holding the chemical inhibitor, changing and cleaning filter units, and changing O-rings. The consumables in these systems are an added cost. However, the regular VCI ITM expense is comparable to nitrogen systems, and it is simpler than the pressure tests, concentration tests, and other ITM steps needed for nitrogen.
Besides the convenient upkeep, VCI manufacturers point to data and claim that inhibitor systems are between seven and 14 times more effective at preventing corrosion than nitrogen systems. The data they provide seem to support this claim based on a year-long study. However, this study also showed that nitrogen-based systems are still effective. The experiments are also based on monitoring corrosion in submerged metal strips, which may not always be the situation within a properly pitched and maintained dry sprinkler system.
Some strong proponents of nitrogen-based systems have raised questions about VCI effectiveness. For example, these fire protection pros (who exclusively offer nitrogen) argue that a standard test of a dry sprinkler system, which shoots high-velocity water through the pipe, can disrupt the VCI coating, requiring its reapplication. The General Air Products handbook for its VCI system does not mention this specific issue but notes that it takes 21 days for the coating to form fully. Thus, can system activations present a potential ‘gap’ in anti-corrosion protection?
We contacted General Air Products to ask about this scenario. A technical rep stated that while high-velocity water during a system test or activation can remove some of the VCI-coating molecules, it does not strip all of them away—so a good degree of protection remains. In addition, the rep notes that every time the system runs its air compressor, more vapor corrosion inhibitor is introduced into the pipes and immediately gravitates to bond with the metal. Thus, General Air Products rejects the idea that high-velocity water causes protection gaps.
Another question some individuals raise is whether VCI compounds can penetrate through sludge or other deposits that might form in a pipe. Again, we asked General Air Products, and a representative explained that a lack of penetration is not a problem. For example, its partner company, Cortec, adds a VCI that is capable of protecting rebar to wet concrete pours —”a good indication of this product’s ability to travel through mediums to seek out metal.”
Nitrogen and VCI systems are effective corrosion-mitigation options
Though they use very different technologies, VCI and nitrogen-based dry and preaction sprinkler systems are both NFPA 13-compliant technologies and effective at limiting corrosion. Nitrogen is a longstanding approach but requires more equipment, maintenance, and testing. Setting up a nitrogen system is also more expensive than a vapor corrosion inhibitor system.
VCI systems are also very effective at mitigating corrosion and offer simpler ITM but also come with some unique annual consumables and maintenance steps to be aware of.
Remember, the 2025 edition of NFPA 13 also greenlit a third option that we’ll cover in a future piece: vacuum systems that fight corrosion by achieving negative pressure in the system. In addition, another technology, a dry air generator, is specifically employed in cold storage environments like freezers. These devices are primarily concerned with preventing moisture that can form ice plugs and block a system rather than reducing corrosion.
The bottom line: building owners and system designers should carefully evaluate a specific system and these approaches to determine the best solution—and always consult fire protection pros who have experience with this equipment.
For more tech insights and deep dives into fire protection codes and standards, check out the QRFS blog. QRFS also carries the fire protection parts and tools you need to install and maintain systems. If you need sprinklers, check out our full selection. If you require dry heads, see our inventory of standard barrel lengths or contact our sales team to place an order for sprinklers with custom barrel lengths!
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