Wheel Nut:
dieseldave:
This is the only bit that they got wrong.
the result being that the nitrogen cleans the tank out
The reason they’re wrong is that the tank wouldn’t contain clean fresh air.
Instead, there’d be nitrogen at atmospheric pressure, but that’s clearly allowed by the exemption. 
But it is badly wrong Dave, as you mentioned asphyxiant gas.
The customer told the driver it was “clean,” so the driver is a nosey little beggar (not Quinny) and decides to stick his head inside to see how they work 
Composition of “air” Nitrogen 78% Oxygen 21% Argon 1%
After discharging a tank of IPA and using a nitrogen blanket, the tank is empty but still unclean. It was suggested that Bob Hurst from Castleford died in Marl Germany because of this misunderstanding.
That’s a very fair point Malc, and the dangers of asphyxiation are very commonly misunderstood.
Nitrogen is commonly used as a ‘gas blanket’ to prevent flammable atmospheres from forming.
For instance, Propane tanks are emptied of propane, then purged with nitrogen, then welding or other hot work can be safely carried out without fear of an explosion, because nitrogen contains no oxygen.
I’m keeping this one on-topic though, because Ken’s OP wasn’t speaking of a nitrogen ‘blanket.’
Nitrogen is correctly said to be an ‘inert’ gas, so it’s neither toxic, nor flammable nor reactive in a chemical sense.
I have some American figures that suggest the extent of the lack of understanding of the dangers of nitrogen asphyxiation. The figures come from OSHA (= similar to the HSE in the UK.)
Going back to the OP, Ken was asked to move a depressurised tank that had contained refridgerated/pressurised nitrogen, which is subject to a perfectly legitimate ADR exemption.
In the normal way of ADR, anybody could be forgiven for remembering that any residue of dangerous goods remaining in a tank, would therefore mean that carriage by road is subject to full ADR as a result.
However, even if the tank is fully depressurised right to the point where it’s at atmospheric pressure, most people don’t realise what’s actually in the tank at that time.
The answer is that there would still be pure nitrogen, but at atmospheric pressure.
It certainly wouldn’t mean that the atmosphere inside the tank is breathable. For the atmosphere to be breathable, the nitrogen atmosphere would first need to be purged/ventilated with fresh air and the % confirmed by a properly certificated gas analysis device.
Although the normal air we breathe contains 21% oxygen, humans can encounter breathing difficulties if the oxygen level drops (for whatever reason) to approx 19% (or less.)
To much hassle 