Appendix 1

CALCULATION OF OXYGEN DEPLETION DUE TO LIQUID NITROGEN LOSSES

Four cases are considered here: (a) normal evaporative losses, (b) filling losses, (c) spillage of the vessel's contents, and (d) loss of the entire contents of the vessel immediately after filling. The British Compressed Gases Association (BCGA) recommends that, for the purpose of risk assessment, the worst case possibility (d) is considered.

The oxygen concentration, C_oxygen in a room may be calculated using the formula

C_oxygen = 100 x V_oxygen / V_room

where V_oxygen is calculated as in b), c), or d) below, and V_room is the room volume in m³.

(a) Normal evaporative losses

Over a long enough period, the percentage decrease in oxygen concentration due to normal evaporation losses from a vessel is approximately

0.21 x 100 x C_t, where

C_t = L ÷ (V_room x N)

0.21 represents the normal concentration of oxygen in air (21%)

C_t is the increase in nitrogen concentration

L is the gas evaporation rate in m³ h^-1

N is the number of air changes per hour.

Manufacturers quote evaporation losses for their vessels (normally as a volume of liquid nitrogen lost per day). Allowance should be made, by doubling these figures, for the deterioration of insulation performance over the lifetime of the vessel.

L = 2 x 700 x liquid nitrogen loss in litres per day ÷ (24 x 1000), where

2 allows for the deterioration of insulation

700 represents the gas factor for nitrogen (1 litre of liquid nitrogen produces about 700 litres of gas)

For example, a basement room 4x3x3 m (36 m³) contains five 10 l dewars, whose evaporative losses are quoted by the manufacturer as 0.15 l liquid nitrogen per day per dewar. A basement room has about 0.4 air changes per hour.

L = (2 x 700 x 5 x 0.15) ÷ 24 x 1000 = 0.044 m³ h^-1

Ct = 0.044 ÷ (36 x 0.4) = 0.003 and the oxygen depletion is 0.06%.

In this case, the normal evaporation losses have an insignificant effect on the oxygen content of the room. However, where Ct = 0.07 or higher, then the oxygen depletion becomes 1.5% or more, and extra ventilation and/or oxygen monitoring will be required.

(b) Filling losses

When a vessel is filled, some loss always occurs as it is cooled to liquid nitrogen temperature. The BCGA recommends that a loss of 10 % of the vessel's capacity should be assumed in order to assess the risk from filling losses.

V_oxygen (m³) = 0.21 [V_room - (0.1 x V_vessel x 700 x 10^-3)], where

0.21 represents the normal concentration of oxygen in air (21%)

represents the loss of 10 % of the vessel's capacity

V_vessel is the vessel's capacity in litres

700 represents the gas factor for nitrogen (1 litre of liquid nitrogen produces about 700 litres of gas)

With the same room and dewars as in (a),

V _{oxygen = 0.21 [36 - (0.1 x 10 x 700 x 10^-3)] = 7.413 m³}

C_oxygen = 100 x V_oxygen / V_room = 20.6%

Once again, there is no significant effect on oxygen concentration in normal use.

(c) Spillage

For the spillage of the entire contents of a vessel

V_oxygen (m³) = 0.21 [V_room - (V_vessel x 700 x 10^-3)]

If the entire contents of a 10 l dewar were spilt in this 36 m³ room, then

V_oxygen = 0.21 [36 - (10 x 700 x 10^-3)] = 6.09 m³

C_oxygen = 100 x V_oxygen / V_room = 16.9 %

This spillage would significantly deplete the oxygen concentration.

(d) Filling of a vessel followed by the spillage of its entire contents

V_oxygen (m³) = 0.21 [V_room - (1.1 x V_vessel x 700 x 10^-3)], where

1.1 represents 10% filling loss + 100% loss of the vessel's contents by spillage

This is the worst case that should be considered in the risk assessment - both (b) and (c) are taken into account.

V_oxygen = 0.21 [36 - (1.1 x 10 x 700 x 10^-3)] = 5.943 m³

C_oxygen = 100 x V_oxygen / V_room = 16.5%

The risk assessment shows that alternative storage arrangements must be considered or oxygen monitoring must be installed. Alternative arrangements may include:

siting the vessels elsewhere

using smaller vessels

arranging for any pressure relief devices to vent to a safe place outside of the room

installing mechanical ventilation, possibly linked to the low oxygen alarm

requiring staff to wear personal oxygen monitors