ISO 8573-1: Compressed Air Purity Classes
The international standard that defines air quality for industrial compressor systems
Compressed air quality is not a detail. In oil and gas, offshore operations, chemical processing, and critical manufacturing, the purity of compressed air directly affects process integrity, equipment reliability, and regulatory compliance. ISO 8573-1 provides the international framework to define, measure, and verify that quality.
ISO 8573-1 is the internationally recognized standard for compressed air purity. Published by the International Organization for Standardization and last revised as ISO 8573-1:2010, it defines quality classes based on the concentration of three key contaminants in compressed air systems:
Each contaminant category is classified independently, giving engineers a precise three-part quality designation. A specification written as ISO 8573-1:2010 defines the maximum permitted level for particles, water, and oil as separate, verifiable limits.
The table below shows the full ISO 8573-1 classification matrix. Lower class numbers indicate cleaner air with stricter limits. Class 0 is a special designation defined by agreement between supplier and user, and is associated with certified oil-free compressor technology.
| Class | A | B | C | |||
|---|---|---|---|---|---|---|
| SOLID / DIRT Particle size in micron |
WATER @ 7 bar / 100 psi Pressure Dewpoint |
OIL (including vapor) |
||||
| 0.1 - 0.5 µm | 0.5 - 1.0 µm | 1.0 - 5.0 µm | ||||
| Max number of particles per m³ | ° C | ° F | mg/m³ | |||
| 0 | As specified | As specified | As specified | |||
| 1 | ≤ 20.000 | ≤ 400 | ≤ 10 | -70 | -94 | ≤ 0.01 |
| 2 | ≤ 400.000 | ≤ 6.000 | ≤ 100 | -40 | -40 | ≤ 0.1 |
| 3 | – | ≤ 90.000 | ≤ 1.000 | -20 | -4 | ≤ 1 |
| 4 | – | – | ≤ 10.000 | +3 | +38 | ≤ 5 |
| 5 | – | – | ≤ 100.000 | +7 | +45 | > 5 |
| 6 | – | – | – | +10 | +50 | – |
A - Solid Particles
Particle contamination is counted per cubic meter across three size ranges (0.1–0.5 µm, 0.5–1.0 µm, and 1.0–5.0 µm). Class 1 demands the lowest particle count, required for pharmaceutical production, precision instrumentation, and critical process control. Higher class numbers allow progressively larger particle populations. Class 1 filtration is standard in instrument air packages where control valve reliability is safety-critical.
b - Water and Pressure Dewpoint
Moisture in compressed air causes internal corrosion, freezing in outdoor lines, instrument failure, and microbiological growth in food-contact applications. The standard measures moisture as pressure dewpoint (PDP) — the temperature at which condensation forms at system operating pressure. A Class 1 dewpoint of -70°C applies to arctic and offshore instrument air where any moisture would freeze control lines. Class 4 at +3°C is sufficient for general indoor industrial use. Choosing the correct dewpoint class is directly linked to dryer selection in the compressor package.
c - Oil Content
Oil enters compressed air through compressor carry-over, system lubricants, and ambient intake. The standard measures total oil concentration in mg/m³. Class 1 at ≤0.01 mg/m³ is required wherever oil contamination would compromise product quality or process safety. Class 0 is a special designation where the exact purity limit is agreed between user and supplier — typically achieved with certified oil-free compressor technology, eliminating oil at the source rather than managing it downstream.
Air quality is expressed as three consecutive class numbers: [Particles / Water /Oil]. Common specifications in industrial and process applications include:
In offshore, petrochemical, and API-class projects, the required purity class is typically defined in the project specification or instrument air package datasheet. HADETEC designs compressor packages to meet these project-specific requirements from the outset, including full compliance with API 619 and related industry standards.
Refrigerant dryers typically achieve Class 4 (+3°C PDP) and are suitable for general industrial use. Heatless desiccant dryers reach Class 2 (-40°C PDP) or Class 1 (-70°C PDP), required for instrument air in cold-climate or offshore environments. Heated PSA and TSA dryer configurations provide energy-efficient drying for high-flow applications demanding Class 1 or Class 2 water quality.
Pre-filters and coalescing filters remove bulk water, aerosols, and solid particles down to Class 1 levels. Activated carbon adsorbers remove residual oil vapor, achieving Class 1 oil quality in combination with coalescing filtration. The filter and dryer package is a core component of every HADETEC compressor package.
Oil-injected screw compressors with correctly specified downstream filtration can meet Class 1 oil content in most applications. For applications where Class 0 is specified, or where any risk of oil contamination is unacceptable, certified oil-free screw compressors eliminate oil at the source. This is the preferred approach in pharmaceutical, food, and critical process environments.
HADETEC supplies engineered compressor packages designed to meet specified ISO 8573-1 purity classes as part of a fully integrated system. Each package combines the compressor, dryer, filtration, instrumentation, and control system on a single skid, pre-wired and pre-piped for single-point hookup. This integration ensures the complete system is designed and tested to meet the required air quality class, not just the compressor in isolation.
Relevant applications where ISO 8573-1 class compliance is a core project requirement include:
For offshore and hazardous-area installations, HADETEC's screw air compressor packages for offshore are engineered to meet Class 0 or Class 1 oil requirements, combined with the dewpoint class specified for the ambient conditions of the project location. Arctic and desert environments require specific dewpoint management that is built into the package design from the engineering stage.
For process plants requiring guaranteed instrument air quality, HADETEC's instrument air packages are configured to meet the purity class defined in the project instrument air specification, including full documentation for factory acceptance testing (FAT).
What does ISO 8573-1 Class 0 mean?
Class 0 is the highest purity designation in the standard and represents air quality beyond the defined numerical classes. The exact specification is agreed between user and supplier. In practice, Class 0 is associated with certified oil-free compressor technology, where oil contamination is eliminated at the source rather than managed by downstream filtration.
What ISO 8573-1 class is required for instrument air in oil and gas?
Instrument air in oil, gas, and petrochemical applications typically requires ISO 8573-1:2010 [1.4.1] as a minimum, with Class 1 dewpoint (-70°C) required for arctic or cold-climate offshore installations. Project specifications and applicable codes such as ISA-7.0.01 define the exact requirements for each project.
What is the difference between pressure dewpoint and atmospheric dewpoint?
Pressure dewpoint (PDP) is measured at system operating pressure and is the relevant value for compressed air applications. Atmospheric dewpoint is lower in practical terms; the two values differ by approximately 20 to 30°C depending on system pressure. ISO 8573-1 specifies pressure dewpoint for all water class definitions.
Can an oil-injected compressor meet ISO 8573-1 Class 1 oil content?
Yes. With a correctly specified coalescing filter and activated carbon adsorber downstream, oil-injected compressors can achieve Class 1 oil content (≤0.01 mg/m³). However, for applications where Class 0 is specified or where oil contamination risk cannot be accepted, a certified oil-free compressor is the preferred and most reliable solution.
How is ISO 8573-1 compliance verified?
Compliance is verified by testing each contaminant category according to the corresponding measurement methods defined in the ISO 8573 series (parts 2 through 9). For project deliverables, this is typically documented in factory acceptance test (FAT) reports and system design calculations submitted as part of the project documentation package.
HADETEC engineers fully integrated air compressor packages for offshore, ATEX, instrument air, and process applications, designed to your specified purity class from the outset.
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