SYSTEM PROCESS AIR.

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System Process Air Quality.

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:

• Maximum particle size for any remaining particles
• Maximum allowable dewpoint temperature
• Maximum remaining oil content

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.

ISO 8573:2010 Explanation

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.

Measuring the dewpoint.

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.

System Process air piping.

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.

1. Interconnecting piping from the compressor discharge to the filter/dryer and from the filter/dryer should be design to have negligible pressure loss.
2. The coalescing pre-filter should be sized to handle a flow rate of 2,000 scfm/minimum and still be effective at flows as low as 100 scfm or less. The filter should have a full load pressure loss of 1 psid or less when new and wet. Filter change is due at 3 psid or less. Filter life must exceed 1 year with no particulate filter installed ahead.
3. The dryer should be rated for 2,000 scfm flow rate at 100/100/100. It should be a full cycling “heat sink” or “thermal mass” type refrigerated compressed air dryer capable of running with the refrigeration system only in direct response to actual heat load.
4. Dryer refrigeration compressor starts should be limited to 6 starts per hour based on the size and type of heat sink. The refrigeration compressor will shut off, not just unload.

SYSTEM PROCESS AIR