Warning Mixing contaminated or nonoil free air with 100 oxygen can result in a catastrophic fire and explosion

■ Compressed air for NITROX mixing shall meet the purity standards for "Oil-Free Air," (Table 10-2). All compressors producing air for NITROX mixing shall have a filtration system designed to produce oil-free air that has been approved by NAVSEA 00C3. In addition, all compressors producing oil-free air for NITROX charging shall have an air sample taken within 90 days prior to use.

Table 10-2. Oil-Free Air.

Constituent

Specification

Oxygen (percent by volume)

20-22%

Carbon dioxide (by volume)

500 ppm (max)

Carbon monoxide (by volume)

2 ppm (max)

Total hydrocarbons [as Methane (CH4) by volume]

25 ppm (max)

Odor

Not objectionable

Oil, mist, particulates

0.1 mg/m3 (max)

Separated Water

None

Total Water

0.02 mg/1 (max)

Halogenated Compounds (by volume):

Solvents

0.2 ppm (max)

3. Mixing Using a Membrane System. Membrane systems selectively separate gas molecules of different sizes such as nitrogen or oxygen from the air. By removing the nitrogen from the air in a NITROX membrane system the oxygen percent is increased. The resulting mixture is NITROX. Air is fed into an in-line filter canister system that removes hydrocarbons and other contaminants. It is then passed into the membrane canister containing thousands of hollow membrane fibers. Oxygen permeates across the membrane at a controlled rate. The amount of nitrogen removed is determined by a needle valve. Once the desired nitrogen-oxygen ratio is achieved, the gas is diverted through a NITROX-approved compressor and sent to the storage banks (see Figure 10-4 and Figure 10-5). Membrane systems can also concentrate CO2 and argon.

4. Mixing Using Molecular Sieves. Molecular sieves are columns of solid, highly selective chemical absorbent which perform a similar function to membrane systems, and are used in a similar fashion. Molecular sieves have the added advantage of absorbing CO2 and moisture from the feed gas.

5. Purchasing Premixed NITROX. Purchasing premixed NITROX is an acceptable way of obtaining a NITROX mixture. When purchasing premixed NITROX it is requisite that the gases used in the mixture meet the minimum purity standards for oxygen (Table 4-3) and nitrogen (Table 4-5).

10-12 NITROX MIXING, BLENDING, AND STORAGE SYSTEMS

Nitrox mixing, blending, and storage systems shall be designed for oxygen service and constructed using oxygen-compatible material following accepted military and commercial practices in accordance with either ASTM G-88, G-63, G-94, or MILSTD 438 and 777. Commands should contact NAVSEA 00C for specific guidance on developing NITROX mixing, blending, or storage systems. Commands are not authorized to build or use a NITROX system without prior NAVSEA 00C review and approval.

Figure 10-4. LP Air Supply NITROX Membrane Configuration.

CHAPTER 13

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