Nitrogen
blanketing.
What it is, why it protects olive oil and other oxygen-sensitive products, and how it works in a storage plant.
Why oxygen is a problem
Olive oil, like wine and many other food products, oxidises in contact with oxygen: over time it loses aroma and quality. In a tank, the oxygen is in the empty space above the product, the headspace, and this space changes all the time: every time product is drawn off, air enters to take the place of the product that has left.
What nitrogen is
Nitrogen is the most abundant gas in the air we breathe. It is inert: it does not react with the oil and does not change its flavour or aroma. This is why it is used instead of air to fill the empty space in tanks.
How it works in a plant
On our tanks, nitrogen enters through the manway neck: an elbow with a tee carries the pressure/vacuum relief valve on one side and, on the other, the hose tail for the flexible hose or a direct connection to the stainless steel piping. This is how it works as the product goes in and out:
- During filling the product enters and the level rises; the gas in the headspace leaves through the vent.
- During emptying the product flows out and nitrogen, not air, enters from the line at the top to take its place.
- When the tank is idle the headspace remains full of nitrogen.
Our animated drawing of the turnkey plant shows the four phases: filling, temperature control, emptying and cleaning.
How a tank is blanketed
Our advice is simple: fill the tank almost to the apex of the roof, so that the only volume left without oil is the manway neck. There is very little space to blanket, and very little nitrogen is needed.
From then on, the nitrogen protects the oil: every time oil is drawn off, nitrogen takes its place and the headspace stays blanketed. If that small volume needs active purging, with nitrogen flowing through until the oxygen level drops, the nitrogen must enter on one side and leave on the other: this is why we connect to two opposite points on the manway neck. We provide for this according to how the customer intends to use nitrogen. The result is checked with an oxygen analyser, and the oxygen level can never fall below the residual oxygen in the nitrogen itself: this is why the purity of the generator also matters.
Tanks must therefore be sized for the real volumes. If you put 15 tonnes of oil in a 30-tonne tank, the headspace is half the tank: the nitrogen enters and leaves at the top, and close to the oil there remains air that cannot be purged. Several tanks of the right capacity, kept full, are better than a few large tanks filled halfway: it is one of the first things we assess when we design a tank farm.
The relief valve
On the top of blanketed tanks we fit a pressure/vacuum relief valve for nitrogen, in food-grade stainless steel: it protects the tank both from overpressure during filling and from vacuum during emptying. Tanks without nitrogen have a vent valve. In both cases the valve is sized for the tank capacity and the filling and emptying flow rates of the pumps.
Nitrogen generator or cylinders
Nitrogen can be supplied in cylinders or produced on site by a generator, which separates it from the air. In a plant with several tanks, a generator avoids constant cylinder changes and keeps nitrogen always available. We size it according to the quantity of oil stored, the average volumes transferred and the way nitrogen is used, and supply it with its buffer tank on a stainless steel skid. The generator, the lines and the tank connections are part of our turnkey plants.
Safety
Nitrogen is not toxic, but in an enclosed space it displaces oxygen. This is why nobody should ever enter a blanketed tank without first purging it with air and following confined-space procedures. This is another reason why closed-loop cleaning with a spray ball, which does not require entering the tank, is a convenient solution.
Frequently asked questions
Nitrogen blanketing
Does nitrogen change the taste of the oil?
No. Nitrogen is an inert gas: it does not react with the oil and does not alter its flavour or aroma.
Do I need a generator, or are cylinders enough?
It depends on the number of tanks and how much oil you handle. With several tanks, a generator producing nitrogen on site is more practical than cylinders. We size it according to the storage volume and the average volumes transferred.
Why don’t you use a sparging stone at the bottom?
Because it is a point where dirt can build up. We prefer a well-sized tank, kept full and filled almost to the apex of the roof, with nitrogen introduced through the manway neck.