With the use of nitrogen gas in fire sprinkler systems to control corrosion becoming the accepted standard, it is important that the fire protection industry understand how to properly manage the risks associated with a nitrogen enhanced environment.
Nitrogen generators are now commonly used in the place of traditional air compressors or regenerative air dryers to supply dry and preaction fire sprinkler systems with supervisory gas. Nitrogen generators designed for use in the fire protection industry produce nitrogen gas at a 98% purity by separating nitrogen molecules from a compressed air supply. By removing oxygen molecules from the interior of the pipe network and replacing them with nitrogen molecules, the corrosion reaction can be effectively stopped, preventing the development of pinhole leaks and extending the life of the sprinkler system indefinitely.
Oxygen Concentration & Health Effects
To understand the risks associated with using nitrogen gas at a purity of 98%, it is important to know the limits of the human body when it comes to the requirement of oxygen for respiration. The air that we breathe is a comprised of 78% nitrogen and 21% oxygen, with the remaining 1% being other trace gases such as carbon dioxide and argon.
At these partial pressures earth’s atmosphere will sustain human life, and all is well. When oxygen concentration drops below 19.5% the environment is considered oxygen deficient by OSHA standards. At a 15% oxygen concentration, work ability decreases; 12% brings impaired judgement and perception; 10% poor judgment and bluish lips. Exposure to an 8% oxygen environment will be 100% fatal within eight minutes.
See more information on oxygen concentration and correlated health effects in the table below:
| Health Effect of Persons at Rest | Concentration |
| Decreased ability to perform tasks. May impair coordination and may induce early symptoms in persons with head, lung, or circulatory problems. | 15-19% |
| Breathing increases, especially in exertion. Pulse up. Impaired coordination, perception, and judgment. | 12-15% |
| Breathing further increases in rate and depth, poor coordination and judgment, lips slightly blue. | 10-12% |
| Mental failure, fainting, unconsciousness, ashen face, blueness of lips, nausea (upset stomach), and vomiting. | 8-10% |
| 8 minutes, may be fatal in 50 to 100% of cases; 6 minutes, may be fatal in 25 to 50% of cases; 4-5 minutes, recovery with treatment | 6-8% |
| Coma in 40 seconds, followed by convulsions, breathing failure, death. | 4-6% |
Table 1 - Oxygen Content Effects and Symptoms of acute exposure (at Atmospheric Pressure)
Nitrogen Discharge by the Numbers
A fire sprinkler system can be described as a large, irregularly shaped vessel that can often exceed 1,000 gallons of volume. A fire sprinkler system with a volume of 1,000 gallons filled to 40 psig with 98% nitrogen contains 2,721 gallons of nitrogen gas. If that system was discharged into a 10’x10’x10’ room that was completely sealed, the oxygen concentration level would drop to 15.6%. Note that this is below the 19.5% level that OSHA defines as oxygen deficient.
The following table shows the resulting oxygen level if a fire sprinkler system with a volume of 1,000 gallons filled with 98% nitrogen was completely drained into an enclosed space:
| Square Footage | Ceiling Height | Cu Ft Air | Dry Pipe System (gal) | N2 % | Composite O2 % | Composite N2 % |
| 100 | 10 | 1000 | 1000 | 98.00% | 15.57% | 83.33% |
| 150 | 10 | 1500 | 1000 | 98.00% | 17.00% | 81.90% |
| 200 | 10 | 2000 | 1000 | 98.00% | 17.82% | 81.08% |
| 250 | 10 | 2500 | 1000 | 98.00% | 18.36% | 80.54% |
| 300 | 10 | 3000 | 1000 | 98.00% | 18.74% | 80.16% |
| 350 | 10 | 3500 | 1000 | 98.00% | 19.02% | 79.88% |
| 400 | 10 | 4000 | 1000 | 98.00% | 19.23% | 79.67% |
| 450 | 10 | 4500 | 1000 | 98.00% | 19.40% | 79.50% |
| 500 | 10 | 5000 | 1000 | 98.00% | 19.54% | 79.36% |
Take Safety Precautions
When dealing with systems that have been nitrogen inerted, it is imperative to observe environmental factors. Best practices for dealing with a nitrogen inerted dry system are:
- Always leave the door to the valve room or area where the valve is located open when working on a system.
- Perform site specific calculations to determine the volume of 98% nitrogen gas that can be discharged into a given space before the oxygen level drops below 19.5%.
- Only drain one system at a time in installations that have multiple valves located in a single room.
For further reading regarding nitrogen safety, please see our White Paper – Managing Safety in Nitrogen Inerting Fire Sprinkler Systems.
To learn more about how you can take safety precautions in your own space, contact ECS experts today.
