Measuring the Thickness of The Inner Wall Oxide Layer of Boiler Tubes with an Ultrasonic Thickness Gauge.

In power station boilers, superheater tubes operate under long-term high-temperature and high-pressure conditions, causing the microstructure and properties to degrade over time, which eventually leads to creep failure and tube rupture.

The causes of boiler tube rupture are numerous and complex. One major factor is that, at extremely high temperatures, steam reacts with the iron in the steel material to form magnetite (Fe₃O₄) and hydrogen. This results in the formation of a hard yet brittle layer of magnetite scale—a special type of iron oxide—on both the inner and outer walls of the steel boiler tubes.

For example, phenomena such as spheroidization of the tube metal microstructure, oxidation on the inner and outer walls of the tube, and thinning of the tube wall are all related to the thickness of the oxide layer. The thickening of the magnetite scale (Fe₃O₄) and the associated metal damage are critical factors that affect the service life of boiler tubes.

By using ultrasonic technology to measure the oxide film thickness on the inner wall of tubes, it is possible to perform non-destructive testing of boiler tubes. Selecting appropriate parameters can improve measurement accuracy. The ultrasonic thickness gauge is a portable testing instrument designed for measuring oxide layer thickness, offering fast and non-destructive measurement.

By using an ultrasonic thickness gauge to measure the oxide layer thickness on the inner wall of tubes, the remaining service life of the tubes can be calculated based on parameters such as wall thickness and tube stress. This enables power plants to carry out targeted tube replacement, effectively preventing tube rupture caused by overheating. On-site measurements in power plants allow non-destructive testing, thereby reducing the risk of boiler tube failures and improving the operational efficiency of components.

Another issue with boiler tubes is oxide spalling. During boiler start-up or shutdown, thermal stress often causes the oxide layer on the tube walls to crack and break into fragments. These hard fragments can then be carried by the steam flow into the turbine, and over time, they eventually cause erosion damage to the equipment.

When ultrasound propagates from one medium to another, reflections occur at the interface between the two media. The time interval between the reflected pulses is proportional to the material thickness. By converting this time into a thickness value, the thickness of the tested material can be determined.

Our PM-5 series thickness gauges, when used with a dedicated high-frequency probe and specialized couplant, are capable of measuring inner-wall oxide film thicknesses as low as approximately 0.2 mm. At the same time, they can display the steel tube wall thickness, the inner-wall oxide film thickness, and the overall thickness.

As shown in Figure 2 (in the figure, all represents the delay block interface echo, bll represents the reflection wave from the inner wall of the steel tube, and B11 is the reflection signal generated when the ultrasonic wave passes through the oxide scale and encounters air).