Research On Pressure Measurement Technology For Pressure Vessels Based On Converted Waves

Pressure vessels are widely used in industry. As special equipment, pressure vessels usually carry with dangerous media, such as flammable, explosive, virulent and corrosive material. Once an accident happens, pressure vessels often cause fire, toxicsis and pollution, which seriously impedes the development of national economy. And it is essential to measure and monitor the pressure in the vessels. Traditional methods for pressure measurement commonly require drilling holes in the vessel wall, which will destruct the structural integrity and lead to explosion. Therefore, a non-intrusive method in pressure measurement is desirable. On the basis of previous research, a new method for pressure measurement with converted waves by acoustoelastic effect and the thin-shell theory is proposed. Compared with the method based on a single wave that is used previously in pressure measurement, the new method here can lower the temperature influence and improve measurement accuracy. Experimental results show that the relative-average error (RAE) is less than 7% and the root-mean-square error (RMSE) is within 0.3 MPA. The main contents and innovations of this thesis are summarized as follows:(1) The relationship between the travel-time change of ultrasonic wave (including critically refracted longitudinal wave (LcR) and reflected longitudinal waves) and the pressure is developed. Based on the acoustoelastic effect, the relationship between the ultrasonic speed and the stress is established. Meanwhile, the relationship between the pressure and the stress can be derived by the thin-shell theory. With relationships above, the pressure measurement model, which describes the relationship between the pressure and the travel-time change, can be derived. As the pressure-induced travel-time change is very small, the cross-correlation method based on the FIR digital filter is introduced to get the travel-time change. Experimental results show that the cross-correlation method is effective and reliable.(2) The frequency’s effect on the sensitivity and signal level in pressure measurement is investigated. A qualitative function is introduced to develop the relationship between the penetration depth of LcR wave and the frequency. And the distribution of stresses in the pressure vessel wall is analyzed with the ANSYS software. Experimental results validate that travel-time of LcR wave will be more sensitive with the increase of frequency. However, the received signal level of LcR wave attenuates seriously as the frequency increases. According to the actual circumstance, the probes of 5 MHz are chosen to excite and receive the ultrasonic wave.(3) A new non-intrusive method for pressure measurement based on converted waves is proposed. The method based on a single wave is overly dependent on the accuracy of travel-time, which is easily affected by the noise and temperature. Therefore, the method based on a single wave usually gains low measuring accuracy. As the LcR wave and reflected longitudinal waves are linearly changed with the pressure and temperature, the new method based on converted waves is proposed to effectively lower the temperature influence with fusion algorithm.(4) Two water vessels have been chosen as specimens. The related pressure experiments for different measuring models are carried out. Experimental results validate that the new method based on converted waves can gain higher measuring accuracy than that based on a single wave.