Nitrogen gas is a common inert protecting gas during pharmaceutical manufacturing. It is supplied in either compressed gas cylinders or liquid nitrogen dewars. Continuous usage of nitrogen gas in confined area with limit ventilation or air exchange can pose a serious safety hazard to operators. This article illustrates a practical example of engineering assessment of using nitrogen in confined area.
Compressed gas cylinders employ Restrictive Flow Orifices (RFO) to restrict the potential danger of an uncontrolled flow. For example, a 44 liter compressed gas cylinder filled under 2,000 psig can flow as much as 20,000 liters per minute. Using an RFO at the outlet of the cylinder connection, the flow rate could be reduced by a factor of 100 to approximately 200 liters per minute.
The orifice flow rate at the various pressure is available from many engineering source or from manufacturers. For example, compressed nitrogen under 500 psig can have a flow rate of 535 slpm (standard liter per minute) if the orifice size (internal diameter) is 0.052 in (or 1.3 mm).
This calculation consider the following facts or factors:
- Standard atmospheric pressure: 14.7 psia (or 0 psig)
- Nitrogen level in air under standard atmosphere: 78.09% v/v
- Oxygen level in air under standard atmosphere: 20.95% v/v
- Unsafe Oxygen level in air under standard atmosphere: 19.5% v/v
- Nitrogen cylinder pressure: 2640 psig
- Nitrogen cylinder volume: 304 cu.ft.
- Orifice size: 0.052 in
- Operating Nitrogen pressure: 20 psig
- Volume of Confined Area: 18 ft (L) x 16 ft x 30 ft (H) = 8640 cu.ft.
The flow rate is:
535 slpm x (20 psig / 500 psig) = 21.4 slpm = 0.756 scfpm
where 1 slpm = 0.0353147 scfpm (standard cubic feet per minute).
For 6 hours of operation, Nitrogen released from cylinder:
0.756 scfpm x 60 min x 6 hours = 272.064 cu.ft.
If this volume of Nitrogen were all released to the confined area at once, a volume of 272 cu.ft. of air would be replaced by pure Nitrogen in a room with 8640 cu.ft. volume. This represents 3.15% of air being replaced:
272 cu.ft. / 8640 cu.ft. x 100% = 3.15%
Or 0.66% of Oxygen being replaced/depleted in the room:
3.15% x 20.95% = 0.66%
Thus, resulting Oxygen level drops to 20.3%, which is still within safe operating condition.
20.95% – 0.66% = 20.3%
20.3% Oxygen level is above the safety threshold of 19.5%. Therefore, it is safe under the proposed operating condition. If the ventilation and air exchange is considered for the confined area, the Nitrogen level will be even lower.
Despite the assessment presented, it is strongly recommended to implement active engineering control and monitoring during such operation. For example, deploying Oxygen sensors in various locations, especially in areas with low ventilation; and setting minimal air exchange rate per hour, etc.