Quality Standard for Instrument Air

Quality Standard for Instrument Air

Pressure dew point

The pressure dew point as measured at the dryer outlet shall be at least 10°C (18°F) below the minimum temperature to which any part of the instrument air system is exposed. The pressure dew point shall not exceed 4°C (39°F) at line pressure.

Particle size

A maximum 40 micrometer particle size in the instrument air system is acceptable for the majority of pneumatic devices. Pneumatic devices that require instrument air with less than 40 micrometer particle sizes shall have additional filtration to meet the particulate size limit for the device.

Lubricant content

The lubricant content should be as close to zero as possible, and under no circumstances shall it exceed one (1) ppm w/w or v/v. Any lubricant in the compressed air system shall be evaluated for compatibility with end-use pneumatic devices.

Contaminants

Instrument air should be free of corrosive contaminants and hazardous gases, which could be drawn into the instrument air supply.

The permissible leakage tolerance in a pneumatic system cannot be critically defined. These pneumatic systems vary in characteristics; some are more tolerant of leaks than others.

Current methods of testing vary widely in

a) Test pressures;

b) Static or cycling pressure; and

c) Time duration of holding test pressures.

Pneumatic system design should minimize the number of probable leakage sources.

Inspections and testing

1) Initial inspection

Confirm that the name plate data is consistent with the system design criteria; e.g., pressure, capacity, and temperature.

2) Verification of air path

Verify the air path from the air supply valve to the air-operated device

It is necessary to test for dew point, lubricants, particles, and other contaminates. Tests or analysis must be conducted on initial start-up and periodically thereafter. However, continuous monitoring for dew point is strongly recommended.

It is necessary to monitor performance of individual system devices because improper use or malfunction can adversely affect system performance. For example, high dew point can result in component malfunction and system degradation. Therefore, when high dew point problems occur, action should be taken to lower dew point within limits. Continuous dew point monitoring provides early detection and/or warning to help prevent high moisture content at the air-operated device

3) Pressure tests

Pressure testing should be performed after initial system or component installation, maintenance, and/or modification to verify the following:

a) Component and/or system operability and integrity at design pressure, for initial testing

b) Component and/or system operability at operating pressure, for in-service testing

c) System integrity

Technicians can use a bubble fluid, ultrasonic probe, or tracer gas-measuring device (electronic or infrared) to observe indication of leakage.