Accurate ultrasonic thickness measurement is crucial in industries ranging from oil and gas to shipping, to inspect and monitor levels of corrosion and damage. Now, usually, this non-destructive testing method is performed at normal environmental temperatures, but there are situations when the gauging needs to be done at high temperatures of even over 500°C.
This is the case for instance in the crude oil refining process where periodic thickness measurements are needed to prevent equipment failure, but also where the high temperatures can make inspecting equipment challenging. In cases like this, using conventional measuring equipment may compromise accuracy and efficiency as well as cause damage.
That’s when high-temperature ultrasonic thickness gauging equipment comes into play. This specialized equipment can overcome test hot materials to an accuracy of even 0.001 mm.
So what can you do to overcome the challenges and improve the accuracy of testing thickness in hot materials?
Ensure that a high-temperature dual element transducer is used to measure the thickness. The transducer needs to be rated for use at the temperature of inspection. If not, it will result in signal loss and potential damage to the transducer. The reason is that the gauging equipment works by measuring how long it takes for the sound pulse generated by the transducer to travel through a test piece and reflect from the inside surface.
As temperature increases, the piezoelectric transducer’s signal quality can decrease resulting in more errors in determining the peak position of the signals. This can be prevented by placing a solid type ultrasonic coupling medium to maintain the acoustic contact between the ultrasonic transducer and test pieces even at high temperatures.
Ultrasonic testing at high temperatures requires specially formulated couplants that remain in a stable liquid or paste form without burning or releasing toxic fumes. Using a special solid coupling medium like a thin gold plate can also maintain a good acoustic contact between the buffer rod and test pieces to overcome the challenge of the ultrasonic energy transfer between the transducer and test piece failing due to a broken contact.
Application of the couplant is equally important. If you apply couplant to the surface of the hot material, it is like to burn off before you can make a measurement. Instead, apply couplant to the tip of the transducer and couple it to the hot surface using firm pressure. If you don’t get a thickness reading in 5 seconds, uncouple the transducer, apply more, and repeat the process.
Variation in the ultrasonic wave velocity at high temperatures can result in erroneous readings as ultrasonic velocity is a function of temperature. A prior calibration to show the relationship between the ultrasonic velocity and temperature will prevent this.
Blue Star E&E offers a range of thickness gauge solutions from basic handheld thickness gauges to advanced ultrasonic gauges that are tested to industry standards and specifications. Give our consultants a call today to explore more.