High Temperature Thickness Measurement Occasions
Towers, tanks and pipelines in petrochemical, refining, chemical, pharmaceutical and other industries.
During in-service operation, the medium (whether liquid or vapor) in the pipeline of these containers is in a high temperature state, and is conducted to the metal outer wall, and the temperature is still hundreds of degrees.
Corrosion is exacerbated by high temperatures, and the flow of media is driven by pressure - therefore, the degree of wall thickness reduction in these vessels must be monitored, especially at the bends of the pipes and the elbows of valve bodies such as tees.
The Principle of Piezoelectric Ultrasonic Thickness Measurement
Key points:
- Since the ultrasonic wave generated by piezoelectricity is difficult to propagate in the air, couplant must be applied between the probe and the measured surface to "squeeze out" the air gap, so that the ultrasonic wave can effectively propagate into the measured part and obtain the bottom surface (ie. , the reflection signal of the inner wall);
- The reflected signal must be stable, clear, and clear enough for the instrument to identify and calculate to display the thickness value.
Influencing factors for measurement at high temperature
Couplant, probe, the condition of the measured surface, the material of the measured object, the instrument
About Couplant
At high temperatures, the moisture in the couplant evaporates instantly, thus losing the effect of "squeezing out" the air gap. Therefore, the instrument can only "see" the fleeting reflection signal, and it is often too late to identify and calculate.
In order to protect the bottom surface of the probe from being "scalded", the so-called ultra-high temperature couplant is often mixed with some mineral particles to ensure that the bottom surface of the probe does not directly contact the high-temperature measured surface, but this also causes the mineral particles to be exposed to high temperature (coupling). After the agent moisture is evaporated instantly), it is easy to carbonize and stick to the bottom of the probe, which will affect the measurement (equivalent to a "pocket" on the bottom of the probe).
About the probe
The probe has a built-in "crystal oscillator", which generates and receives ultrasonic signals. The crystal oscillator piece has a physical parameter, "Curie temperature point Tc", which means that below this temperature value, the ultrasonic wave generated by the crystal oscillator piece has an appropriate acoustic impedance, that is, the difficulty of propagation is appropriate. Crystal oscillators made of different materials have different Tc values.
Among the existing crystal oscillator materials, the Tc value can reach 500℃, but another parameter, the "mechanical quality factor θm", is very high, reaching the 6th power of 10, which makes it impossible to obtain effective signal resolution—— The θ m of the crystal oscillator used by the ultrasonic thickness gauge is all below 1000, otherwise the starting point of thickness measurement must be above 500mm.
Therefore, the existing so-called "high temperature" probes cannot achieve breakthroughs in the most essential crystal oscillators. The current solution is only to protect the probe itself (for example, the protective film on the bottom of the probe is not easy to be "burned". ”, adding a protective cover to the shell, etc.), there is no way to solve the problem of effective propagation (transmission and reception) of piezoelectric ultrasonic signals in high-temperature media.
About the condition of the measured surface
When the probe is placed on the surface of the measured surface, the ultrasonic wave propagates in the direction perpendicular to the tangent of the contact point, and the bottom surface (inner wall) is also reflected in the direction perpendicular to the tangent. This effect is the most ideal (refer to the schematic diagram). ). This effect is relatively easy to achieve in plane measurement, as well as in the measurement of large diameter cylinders, tubes, and tanks. But for curves and bends (especially the 90-degree convex point), it is often difficult to get the probe right.
In addition, in the field environment, oil stains and paint (especially bulge) on the measured surface will affect the measurement.
About the material of the object to be measured
In any case, the transmission of ultrasonic waves will be affected by the material of the measured object, which will bring about changes in the scattering effect and the speed of sound. However, in most cases, the interference caused by these effects is small, and the small errors caused are in the negligible range.
v However, due to the high temperature and high pressure corrosion resistance, the influence of the material of some parts cannot be ignored. For example, stainless steel valve bodies with different specifications and grades have different degrees of signal scattering attenuation.
About Instruments - General Questions
The usual ultrasonic thickness gauges are of the reading type - after detecting a stable, clear and clear reflection signal, the calculation is performed and the result is displayed.
In the high temperature state, due to the aforementioned factors, the reflected signal is unstable, clear, and clear, and the instrument cannot detect the signal or detect the wrong signal, so "no reading value" and "reading value error" appear. etc. phenomenon.
About Instruments - Primary Solutions
- The original instrument could only detect four signals. Now, let it detect the signal 20 times, which is the scanning function; at the same time, it is best to display the maximum or minimum value detected by scanning, which is called "extreme value display".
- If the signal is too weak, increase the signal, which is called "gain adjustment", and it is better to adjust it digitally, rather than adjusting it in steps;
- Because the signal appeared very short, the instrument just finished reading the signal, he disappeared, and the instrument was not sure. Then let it display the read signal first, which is called "read value hold".
About Instruments - Advanced Solutions
We let the instrument display all the signals it can capture, instead of letting the instrument judge by itself, let the operator judge, this is "A signal scanning thickness gauge".
The improvement brought by A scanning thickness gauge: The most essential is that the signal can be observed by the operator himself, and there is no need to worry about the reading instrument missing or missing the signal. In terms of function, in addition to the same scanning and reading functions, the gain can be adjusted digitally instead of sub-level adjustment (this is more accurate). In addition, the clutter shielding function has been added to allow the instrument to read the specified signal - this Very important!
Model: 2190 with A-signal scan function
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