ISO 29821-1:2011 pdf free download – Condition monitoring and diagnostics of machines一Ultrasound – Part 1: General guidelines.
8.5 Assessment criteria
The most common form of severity criterion is the use of a signal level differential (delta dB).
To apply ultrasound inspection to the condition monitoring of machines and their components, severity crfteria should be established by the practitioner, their company or client, It is common practice that these criteria are established through experience and the accumulation of data on specific machines or components. Since there are many applications of ultrasound that differ in the way data are evaluated, there are currently no universally accepted severity criteria in industry. Therefore, each category of equipment and its application should have severity criteria established based on its design, operating conditions, baseline condition, maintenance, and criticality.
These severity criteria are usually based on historical data showing an increase in decibel value (quantitative baseline method). This can help establish a rate of deterioration and help establish when corrective action is taken to prolong component life. This can apply to applications that would use a decibel value increase as a criterion, such as on bearing lubrication levels where the corrective action would be to lubricate the bearing to reduce the ultrasonic decibel value. Other examples of severity criteria are: elevated ultrasonic signals along with information from spectral analysis of electrical components which could cause machinery shutdown and decibel value increases in compressed air system leaks, which could cause machine performance degradation, or high energy costs.
As multiple measurements are taken over time on the same piece of equipment under the same operating conditions, the recorded data can be used to set parameters for trending and aid in the prediction of the failure of these components.
An example of severity criteria based on increased decibel value above an established reference or baseline for rotating equipment bearings is as follows:
— Pre-failure stage — 8 dB: This is the earliest stage of failure. The bearings may have developed hairline cracks or microscopic spots that are not visible to the human eye. This may also signal a need to lubricate the bearing.
— Failure stage — 16 dB: At this stage, visible flaws develop along with a marked rise in acoustic energy and the temperature of the bearing begins to rise. It is at this stage that the bearings should be replaced or more frequent monitoring should occur.
— Catastrophic stage — catastrophic failure — 35 dB to 50 dB: Here, rapid failure is imminent. The acoustic sound level is so intense as to be audible and the temperature of the bearings has risen enough to cause overheating. This is a highly dangerous stage since changes in the bearing clearances and tolerances can cause additional friction and rubbing within the machine, potentially damaging other components.
9 Sensitivity validation guidelines
Ultrasound inspectors shall verify the sensitivity of their instruments on a regular basis and shall have them calibrated to the manufacturer’s specifications. One common sensitivity validation method is described in ASTM E1002[7] where a calibrated reference ultrasonic source is used. It is recommended that a sensitivity validation procedure be performed prior to each inspection or survey. A quick sensitivity check can be made by using the detection of a human eye blink as a low level signal for airborne sensors and the detection of the signal from a quartz watch crystal in a wristwatch for structure-borne sensors.