The System Process Air ISO is an international standard (ISO8573.1) that specifies the quality of facility utility air system. The standard defines limits for three categories of air quality:
Each category is given a quality class number between 1 and 6 according to the reference values shown in the table below. As an example, a system that conforms to ISO8573.1 and is rated for class 1.1.1 will provide air with a dewpoint no higher than -94 °F (–70 °C). All remaining particles in the air will be 0.1 um or smaller, and the maximum oil content will be 0.01 mg/m3. There are other standards for high pressure air quality, such as ANSI/ISA-7.0.01-1996 for instrument air.
ANSI/ISA-7.0.01-1996 for System Process air.
Of the separate parts in the standard, ISO 8573-1:2001 part 1 relates to defining the quality of compressed air used in different applications. The other parts are concerned with the methods to test the air for a range of parameters used to indicate the level of contamination.
ISO 8573-1 specifies purity classes of compressed air in respect of particles, water and oil. The purity classes can be used to describe the current quality of a compressed air system or to specify the required quality for a precise application.
Some principles of dewpoint measurement apply to all types of instruments, regardless of manufacturer. However the single most important thing is to:
• Select an instrument with the correct measuring range:
A low pressure instrument connected to a high pressure system may explode and cause serious injury to anyone in the vicinity. Some instruments are suitable for measuring high dewpoints, but not low dewpoints. Similarly, some instruments are suitable for very low dewpoints but are compromised when exposed to high dewpoints.
• Understand the pressure characteristics of the dewpoint instrument:
Some instruments are not suitable for use at process
pressure. They can be installed to measure high pressure air after it is expanded
to atmospheric pressure, but the measured dewpoint value will have to be
corrected if pressure dewpoint is the desired measurement parameter.
The piping and compressed air treatment equipment should have characteristics rated at 2,000 scfm at pressures as low as 85 psig; not to exceed 20 fps pipeline velocities.