An external inspection is no guarantee of the condition of the inside structure. A pipe can appear good on the outside but be ready to rust through on the inside. Ultrasonic thickness testing or ultrasonic thickness measurement (UTM) is a fast, reliable and nondestructive methodology used to evaluate an object’s thickness. It is especially useful in determining the thickness of pipe walls in order to identify corrosion or material deposition. Many successful Predictive Maintenance (PdM) programs use a combination of ultrasonic thickness and vibration testing analysis to provide a comprehensive analysis of rotating equipment (please refer to Predictive Maintenance: Programs That Save You Money and Headaches).
UTM is also versatile. Essentially any material— ceramics, coatings, composite materials, fiberglass, glass, laminates, liquids, metals, organic materials, and solid or extruded plastics and polymers— can be measured. Even objects with unusual geometries— concave surfaces, sharp corners or radii, as well as deep grooves, cavities and channels, can be accurately measured.
The human ear can normally detect sound vibrations in the range of 20-20,000 Hz. As the name implies, ultrasonic testing frequencies are at a higher range from 50,000 Hz to 100 MHz, although most testing is performed at a narrower bandwidth of 500 kHz to 20 MHz. UTM equipment creates a sound pulse, generated by a transducer, which travels through the test object. Transducers come in two basic types, piezoelectric for smooth, surface contact applications, and electromagnetic (EMAT) for non-contact measurements. In tests using one transducer, the pulse travels through the object and creates an echo by bouncing off the inside surface of the object’s far side. The transducer picks up the echo, the speed of return is calculated, and the object’s actual thickness is determined so that it can be compared to operating specifications.
In tests using dual element transducers, the pulse initiated by one transducer travels through the object and the pulse or echo is picked up by the other transducer. The speed of travel is between the two transducers is calculated, and the object’s thickness is determined. Regardless whether single or dual transducers are used, calculations also include the object’s approximate thickness, geometry, temperature and density to provide a precise reading. With proper calibration, measurements can attain an accuracy of ±0.001” (0.025 mm). When used to measure the wall thickness of metal pipes or tanks, a thinner than normal reading may indicate wear or corrosion, whereas a thicker reading may indicate oxidation, contamination or undesirable chemical deposition.
Some of the key advantages of UTM are listed below:
- Highly portable
- Highly accurate
- Highly automated
- Non-hazardous to humans
- Non-invasive and non-destructive to test object and surroundings
- High penetration allowing detection of deep flaws
- High sensitivity, permitting detection of extremely small flaws
- Requires limited access to the test object
- Allows the user to estimate the size, orientation, shape and nature of the defect
The Reliability Technicians from L&S Electric are experts in using a combination of visual inspection, ultrasonic thickness testing, and vibration analysis. When the job is done, we provide you with a report of our findings, along with the solutions necessary to fix any problems we find.[subscribe2]