Technical Tip Tuesday… nitrogen is NOT heavier than air! In fact, pure Nitrogen gas is LIGHTER (~3% lighter) than the Oxygen and Argon (with the Nitrogen) that make up our atmosphere. It is perfectly mixed in the air. Hence the air we are breathing right now (assuming you’re on the surface of the earth) is 78% Nitrogen. Nitrogen and Oxygen (21%) are perfectly mixed in or atmosphere. If N was heavier, those of us walking on the surface of the earth would be dead from lack of oxygen and the space ships we launch would never make it into space as they would explode as soon as they entered into the oxygen-enriched atmosphere above us. So when we need to inert the headspace of a vessel of flammable liquids, NEVER assume that just a little bit of nitrogen will “sit on the surface of the flammable liquid”… we MUST inert the entire headspace to below the “Limiting Oxygen Concentration (LOC)” of the flammable vapors. This requires a detailed engineering design basis – not a SWAG (seriously wild ass guess)!
When we put enough Nitrogen into the headspace, we are increasing the Nitrogen concentration above the normal 78% and thus we are decreasing the Oxygen (and Argon less than 1%) concentrations. Essentially, the practice of inerting is to add an inert gas at concentrations that will DISPLACE the 21% oxygen from the headspace. It is the Oxygen that is needed for a fire/explosion. But the engineering behind how low we have to go with the Oxygen concentration is true science and engineering. As a starting point, assume we always have to be below 10% Oxygen concentration – THAT IS MERELY A POINT TO BEGIN the discussion/design. Most flammable liquids will have a LOC less than 10% and then I always add a “safety factor” of 50%. For example, Toluene has a LOC of 9.5% when the inerting media is Nitrogen. So I will design a delivery system that will be capable of ensuring the headspace of the tank during pump-out (or other removal methods which will pull in atmospheric air) to maintain an Oxygen level of less than 4.7. when the tank is at its Safe Lower Limit (e.g. the largest possible headspace). Keep in mind that when the tank is being filled, you will lose Nitrogen via the tanks vent, but the filling is the most hazardous time for an atmospheric storage tank and the LOC is at its most critical stage!
PLEASE seek professional help in designing/assessing your inerting systems and practices BEFORE they are claimed as a safeguard in our Risk Assumptions. Too often these systems have established a false sense of security for management to think they can never have an incident inside their storage tanks. After the incident, we almost always find an undersized and poorly designed interesting system.
LASTLY – LABEL YOUR VESSELS that have any kind of inerting/purging system. The same safeguard that aids in PREVENTING fires/explosions during processing can be lethal during down times and workers interact with the vessels atmosphere. There MUST be a specific means for POSITIVE isolation of these systems when workers enter the vessel – NO SINGLE VALVE isolation!!