Research On A Method For Measuring Pressure Of Vessels Based On Critically Refracted Longitudinal Wave And Surface Wave

Pressure vessels are widely used in modern industry. To guarantee that they are working properly, it is essential to measure and monitor the pressure in the vessels. Conventional methods for pressure measurement commonly require drilling a hole in the wall of the vessels, which usually cause local stress concentration, and lead to potential safety problems. Therefore, a nondestructive method without drilling holes to measure the pressure is desirable. The propagation velocity of ultrasonic wave is related to the stress and the temperature of the propagation media. Research results show that nondestructive pressure measurement is feasible using surface wave. Howerve, using only surface wave, this method is difficult to apply to spherical pressure vessels. In the thesis, a new pressure measurement method using critically refracted longitudinal wave (LCR wave) and sureface wave is proposed. A model among the change of ultrasonic wave's propagation time and the temperature and the pressure of vessels is established. According to this model, a reference method by LCR wave and surface wave is proposed. A correlative time estimation method based on the zero-phase digital filter is presented. The main contents and innovations of the thesis are summarized as follows:(1) Models among the propagation time of ultrasonic waves and the pressure of vessels and the temperature are developed respectively for LCR wave and surface wave. The influence of stress and temperature to the velocity and the propagation distance of ultrasonic waves is analysised. According to the stress distribution of cylinder pressure vessels and spherical pressure vessels, models among the propagation time of ultrasonic waves and the pressure of vessels and the temperature are developed through appropriate simplification, respectively for LCR wave and surface wave. A reference method for inducing the influence of temperature is presented.(2) A general cross correlation method based on zero-phase digital filter is applied to improve the accuracy of time delay estimation. Ultrasonic signals contain the correlative noise, therefore, pre-filtering of signal is needed. Zero-phase digital filter which can effectively eliminate the phase distortion that caused by common digital filter is proposed to process the signals. Compared to Hilbert-Huang Transform, the experimental result shows that the general cross correlation method based on zero-phase digital filter is more stable and simple.(3) A combined transducer is designed to generate or receive the LCR wave and surface wave. Comparison tests between the combined transducers and the custom-used transducers show that the combined transducers have better repeatability. Using the combined transducers in the experiments can reduce the influence of unevenness of temperature distribution to pressure measurement, with less transducers needed, also leading to a more simple and more convenient measurement.(4) A water vessel and an air vessel have been chosen as specimen. The reference pressure measurement experiments are carried out. The results show that the model established in the thesis is correct. The results also show that the reference method is effective, the influence of temperature variation could be reduced and the pressure measurement could gain high accuracy.