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An empty fuel tank is more dangerous than a full one. It seems counterintuitive, but it's true: in a full tank, the liquid occupies the space. In an empty tank, what remains is a mixture of flammable vapors and air — exactly the combination a spark needs to generate an explosion.
Inerting exists to eliminate this condition. And NFPA 69 is the standard that defines how to do it correctly.
What is inertization?
Inerting is the process of replacing the atmosphere of a piece of equipment—tank, pressure vessel, reactor, or process line—with an inert gas, reducing the oxygen concentration to a level where combustion becomes impossible.
For fire to occur, three elements must coexist: fuel, oxygen, and an ignition source. Inerting attacks the second element. Without sufficient oxygen, neither the fuel nor the spark can generate a flame.
“Inerting does not eliminate the fuel or the ignition source — it removes the oxygen. It is the only one of the three variables that we can reliably control during maintenance. That is why it is the standard method in any hot intervention on hydrocarbon tanks.”
— Reimar Cruz, Operations Director at TSL
What does NFPA 69 establish?
NFPA 69 — Standard on Explosion Prevention Systems This is the standard from the National Fire Protection Association that defines explosion prevention systems, including oxygen concentration control.
Its central concept is LOC (Limiting Oxygen Concentration), or limiting oxygen concentration: the percentage of oxygen below which combustion cannot be sustained, regardless of the fuel concentration.
This value varies depending on the fuel involved. For most hydrocarbons, the LOC is around 10% to 12% of oxygen. The standard establishes that the operation must maintain a safety margin below this limit—it is not enough to get close, it is necessary to stay comfortably below it.
Continuous monitoring
NFPA 69 does not treat inerting as a one-off action. It requires continuous monitoring of oxygen concentration throughout the operation, with calibrated instrumentation and recording of measured values.
This matters because the atmosphere inside a piece of equipment is not static: leaks, openings, and even the movement of the product itself can reintroduce oxygen into the system during operation.
When inertization is mandatory
Inerting becomes necessary whenever flammable vapors are present and there is a potential source of ignition. In industrial practice, this occurs in situations such as:
- Hot maintenance on tanks that have stored hydrocarbons — welding, cutting or grinding
- Internal cleaning of tanks containing petroleum, fuel, or flammable chemicals.
- Entry of people into confined spaces with a history of flammable vapors.
- Decommissioning of equipment and platforms
- Starting and stopping process units
- Transfer of products with a risk of creating an explosive atmosphere.
It's worth highlighting that inerting often precedes other operations. Before performing internal hydroblasting of an oil tank, for example, the atmosphere needs to be inerted. The two operations are complementary, especially when carrying out processes that require... Boiler Rental and Operation.
What gases are used?
Nitrogen
It is the most widely used gas. Being inert, dry, and relatively inexpensive, nitrogen displaces oxygen without reacting with the product or the equipment. It is the standard choice for oil tanks, process lines, and pressure vessels.
Water vapor
Used when the goal is to combine inerting and heating — for example, in tanks containing viscous products that need to be fluidized before removal. The steam displaces oxygen and simultaneously heats the residue.
Carbon dioxide
An alternative in specific situations. Denser than air, it has good mobility, but requires extra care due to the risk of suffocation and because it can react with some products.
“The choice of gas is not a matter of preference. It depends on the product being stored, the operating temperature, and what comes next—whether the tank will be opened for people to enter, whether there will be welding, or whether the residue needs to be heated. Making the wrong choice compromises the entire operation.”
— Reimar Cruz, Operations Director at TSL
How does an inerting operation work?
The process follows defined steps:
- Preliminary technical assessment — product identification, analysis of the current atmosphere, and definition of the applicable LOC (Local Operating Condition).
- Equipment isolation — blocking lines and valves to prevent product or air from re-entering.
- Purge — injection of inert gas to displace the existing atmosphere
- Monitoring — continuous measurement of oxygen concentration until the target value is reached
- Atmosphere maintenance — controlled injection throughout the procedure
- Liberation — controlled return to normal atmosphere, when applicable
Step 1 is the one most frequently underestimated. A poorly designed inerting system—with inadequate gas, insufficient flow rate, or a poorly calculated LOC—creates a false sense of security, which is more dangerous than not inerting at all.
Inertization and NR-33: the relationship
NFPA 69 is an international standard focused on explosion prevention. NR-33 is the Brazilian standard that regulates work in confined spaces.
The two meet in practice: when someone needs to enter a tank, NR-33 requires that the atmosphere be safe — which, in tanks with a history of containing flammable materials, implies inerting. But there is a critical difference that many ignore.
An inert atmosphere is NOT breathable.
Reducing oxygen to 8% eliminates the risk of explosion, but creates a risk of asphyxiation. Therefore, before people enter, the tank needs to undergo an additional step: controlled deinertization, returning oxygen to levels safe for breathing (above 19.5%), while verifying that flammable vapors have been removed.
“The most serious mistake I’ve ever seen in the field was a team entering a still-inert tank, thinking that ‘inert’ meant ‘safe.’ It means safe from explosion. It doesn’t mean safe to breathe. Those are two different things, and they both need to be checked separately.”
— Reimar Cruz, Operations Director at TSL
Frequently Asked Questions
What is NFPA 69?
It is the international standard from the National Fire Protection Association that defines explosion prevention systems, including parameters for controlling oxygen concentration through inerting.
What oxygen concentration is required for inerting?
It depends on the fuel involved. The parameter is the LOC (Limiting Oxygen Concentration) — the concentration below which combustion cannot be sustained. For most hydrocarbons, it is between 10% and 12%, and the operation must maintain a safety margin below this value.
What is the difference between inerting and degassing?
Degassing removes flammable vapors from equipment. Inerting replaces the atmosphere with inert gas, reducing oxygen. They are complementary processes: in many operations, one is applied immediately after the other.
Is an inerted atmosphere safe for people to enter?
No. Inerting eliminates the risk of explosion, but the low oxygen concentration makes the atmosphere unbreathable and potentially fatal. Before entry, controlled deinertization is necessary, with verification of oxygen levels above 19.5% and absence of flammable vapors, according to NR-33.
Is inertization mandatory by law in Brazil?
There is no specific law that explicitly requires it, but it becomes mandatory as a consequence: NR-33 requires a safe atmosphere in confined spaces, and NR-20 requires risk control in areas with flammable materials. In tanks with a history of hydrocarbons, inerting is the means of meeting these requirements.
How long does an inerting operation take?
It varies depending on the equipment volume, the product stored, and the available gas flow rate. A medium-sized tank can take anywhere from a few hours to a full shift. A prior technical assessment allows for an accurate estimate of the timeframe.
TSL Engenharia e Serviços performs inerting services in industrial and offshore environments, as well as specialized solutions in pipeline inerting, with prior technical assessment, continuous monitoring, and compliance with NFPA 69 and NR-33.
