What Does Nitrogen Purity Mean?
Nitrogen purity refers to the percentage of nitrogen in the gas stream by volume. The remainder is primarily oxygen, with trace amounts of argon, CO₂, and water vapor. When a nitrogen generator is rated at “99% purity,” it means the output gas contains 99% nitrogen and 1% other gases — predominantly residual oxygen that did not separate from the air stream during generation.
Purity is typically expressed as a percentage (e.g., 98%, 99%, 99.9%) or in units of parts per million (ppm) for very high purity applications (e.g., 10 ppm O₂ = 99.999% purity).
How Nitrogen Purity Is Produced in the Field
Three main methods produce nitrogen at different purity levels:
- Membrane nitrogen generators: Hollow-fiber membrane separation produces 95–99.5% nitrogen. Higher purity requires slower flow (longer membrane contact time) or a larger membrane area. Most practical field range: 97–99%.
- PSA (Pressure Swing Adsorption) generators: Carbon molecular sieve PSA produces 99–99.999% nitrogen. Higher capital cost, more complex operation, but enables higher purity at higher flow rates than membrane alone.
- Liquid nitrogen (LN2) vaporizers: Liquid nitrogen is produced at industrial gas plants to 99.999%+ purity. Transported as a cryogenic liquid in Dewar vessels or bulk trailers and vaporized at the point of use. Highest purity, highest cost per SCF, limited by trailer capacity rather than onsite generation rate.
Nitrogen Purity Requirements by Application
Pipeline Purging — Air to Inert (Gas Commissioning)
Required purity: 95–98%
For purging air from a pipeline before gas introduction, the goal is simply to displace the oxygen-containing air and replace it with a non-combustible atmosphere. The nitrogen itself does not need to be extremely pure — even 95% nitrogen (5% residual O₂) flowing through the pipe will eventually reduce the outlet O₂ below 1% by dilution and displacement. In practice, most operators use 98–99% purity nitrogen for gas commissioning purges, which provides comfortable margin over the 1% O₂ outlet specification.
Post-Hydrotest Nitrogen Drying
Required purity: 97–99%
The primary variable in nitrogen drying is the dew point of the nitrogen being introduced — not purity. As long as the generator is producing nitrogen at an outlet dew point well below the target pipeline dew point (-40°F to -60°F at the unit), purity in the 97–99% range is adequate. The oxygen content of the drying nitrogen is low enough that it does not prevent achieving the target dew point at the pipeline outlet.
Nitrogen Pressure Testing
Required purity: 97–99%
For inert nitrogen pressure testing (where nitrogen is used as the pressurization medium instead of water or air), purity in the 97–99% range is standard. The oxygen content at this purity level is low enough to be non-combustible with any residual hydrocarbon vapors in the system, which is the safety requirement for nitrogen pressure testing in potentially hydrocarbon-containing systems.
Storage Tank Blanket Nitrogen
Required purity: 98–99%
Tank blanketing nitrogen maintains an oxygen concentration in the vapor space below the limiting oxygen concentration (LOC) for the specific hydrocarbon product. For most crude oil and condensate applications, the target vapor space O₂ concentration is below 2%, which is achieved easily with 98–99% nitrogen blanket supply. Some operators specify the blanket nitrogen itself at 99% to provide maximum margin.
Catalyst Bed Protection
Required purity: 99.5–99.999%
Many refinery and chemical plant catalysts are oxygen-sensitive — exposure to oxygen above trace levels causes irreversible deactivation. Catalyst bed inerting and preservation nitrogen typically requires 99.9%+ purity to maintain the oxygen in the catalyst environment at trace levels.
LNG Facility Nitrogen
Required purity: 99.9%+, often moisture specification more critical
LNG liquefaction processes require oxygen-free environments throughout the cold end of the facility. Nitrogen for LNG applications is typically specified at 99.9%+ purity, with moisture content below 1 ppm (corresponding to a dew point below -100°F). This level of dryness and purity requires PSA generation with molecular sieve drying, or liquid nitrogen vaporization.
Analytical Instrument Carrier and Purge Gas
Required purity: 99.999%+ (ultra-high purity, UHP)
Gas chromatographs, mass spectrometers, and other analytical instruments used in laboratory settings require ultra-high purity nitrogen (99.999% or better). This level of purity is produced by liquid nitrogen vaporization with additional molecular sieve purification, or by industrial gas suppliers’ specialty UHP products. Field-generated membrane or PSA nitrogen does not meet these specifications.
Practical Guidance: What to Specify on Your Job
For most pipeline and facilities nitrogen applications — purging, drying, pressure testing, inerting — specifying 98–99% purity covers your requirement with margin. This is what standard membrane or PSA nitrogen trailers deliver, and it matches the specifications in most North American pipeline commissioning procedures.
If your application requires higher purity (LNG, catalyst protection, specialty chemical), work with your nitrogen service contractor to verify their equipment can meet your specification and request verification data. For these applications, PSA units or liquid nitrogen supply are typically required.
If your application requires extremely high volume at modest purity (large pipeline purge, facility-wide inerting), specify the minimum acceptable purity and let the contractor optimize their equipment selection — running at 97% purity instead of 99% may double the available flow rate from a membrane unit, shortening the job significantly.
NitroTech can provide nitrogen at the purity your application requires — from 95% high-volume membrane output to 99.9%+ PSA generation. Request a quote and include your purity specification in the request.