Nitrogen is the main component of air and is present at approximately 78% by volume (oxygen is approximately 21% and argon 1%). Any alterations in the concentrations of these gases, especially oxygen, have an effect on life. In the case of liquid nitrogen, there is a risk of asphyxiation where ventilation is inadequate and the nitrogen gas evolved can build up and displace oxygen from the local atmosphere. An atmosphere containing less than 18% oxygen is potentially hazardous and entry into atmospheres containing less than 20% should be avoided.
The general effects of reduced oxygen content in the atmosphere are given in the table below:
|Oxygen content (vol. %)||Effects and symptoms|
|11 - 14||Physical and intellectual performance diminishes without the person being aware|
|8 - 11||Possibility of fainting without prior warning|
|6 - 8||Fainting within a few minutes - resuscitation possible if carried out immediately|
|0 - 6||Fainting almost immediate, death ensues, brain damage even if resuscitated|
In typical situations the concentration of nitrogen gas which may accumulate in a room over a period of time (assuming a certain evaporation rate from vessels and / or pipework) may be calculated using the following equation:
C = L / Vn , approximately
Where: C = gas concentration; L = gas release (m3 / h); V = room volume (m3); n = air changes per hour
For rooms at or above ground level, natural ventilation will typically provide 1 air change per hour. However, this is not the case with rooms which are windowless or have windows which are tightly sealed, in which case the number of air changes will be less than 1 per hour. For underground rooms with small windows, 0.4 changes per hour could be considered a typical value.
A room (H = 2.8m, W = 3.0m, D = 4.0m) houses 6 x 25 litre capacity non-pressurised liquid nitrogen vessels. The rate of evaporation from the vessels is 0.5 litres / 24 hours (this information should be obtainable from the manufacturers and is typically 1 - 2% of the liquid capacity of the vessel per 24 hours). The figure is also multiplied x 2 to allow for deterioration in the vacuum insulation with time. The room is above ground but has no windows and is estimated to have 0.5 air changes per hour by natural ventilation. The gas expansion factor for nitrogen is 683.
L = ((6 x 0.5) x 2 x 683) / (24 x 1000) = 0.171 m3 / h
V = 2.8 x 3.0 x 4.0 = 33.6 m3
n = 0.5
therefore: C = 0.171 / (33.6 x 0.5) = 0.010 (x100) = 1.0%
The nitrogen concentration of the room is increased by 1.0%. The normal oxygen content of the atmosphere is approximately 21%, therefore:
current oxygen content = (21 x 100) / (100 + 1.0) = 20.8%
Under these circumstances the evaporation from the vessels only reduces the atmospheric oxygen content from 21% to 20.8% - negligible and well within the safe working limit. It should be noted however, that the nitrogen evolution will be greater during filling operations when the lids of the vessels are open and liquid nitrogen is being transferred. In most cases, this is a relatively short term operation.
Alternatively, oxygen deficiency resulting from a large spillage of liquid nitrogen or sudden rapid release of nitrogen gas from a pressurised vessel may be calculated as follows - this is the 'worst case scenario':
Resulting oxygen concentration (%):
%O2 = (100 x Vo) / Vr
Where, for nitrogen: Vo = 0.2095 (Vr - Vg); Vr = room volume (m3); Vg = maximum gas release, which is the liquid volume capacity of the vessel V x gas expansion factor.
A pressurised liquid nitrogen vessel of 100 litre capacity located in a room 2.8 m x 5.0m x 10.0 m loses vacuum suddenly and vents its contents to atmosphere in a very short space of time:
Vr = 2.8 x 5.0 x 10.0 = 140 m3
Vg = 100 x 683 = 68300 litres = 68.3m3
Vo = 0.2095 (140 - 68.3) = 15.02
%O2 = (100 x 15.02) / 140 = 10.7%
The oxygen content of the room is halved to 10.7%.
If the calculation suggests an oxygen content of less than 18% then the following should be considered: