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Acoustic online detection of valve internal leakage

  • Mar 21, 2019
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The pressure-bearing valves of ground facilities and equipment in an oil field have experienced internal leakage due to wear, corrosion or other reasons. Because the internal leakage is difficult to find, some of the faulty valves have not been replaced in time, which has affected production and has potential safety hazards. Therefore, the valve online detection technology has attracted more and more people's attention. The current core technology is to use the acoustic emission technology to detect the leakage of various valves in the petroleum, petrochemical, power plant and other industries.

Basic principle of acoustic emission leak detection

(1) Acoustic emission signal and its characterization parameters
When any metal material or member is subjected to friction, crack or plastic deformation under external force or internal force, the strain energy is released in the form of elastic wave, which is called acoustic emission. When the acoustic emission waves propagate in a finite medium, they are refracted and reflected at the interface, and are converted to surface waves at a solid surface and propagate along the surface at a high speed. These acoustic emission waves from the acoustic emission source (ie, the fault point) have characteristic information of the source, and the information can be used to reflect the failure and defect of the component. The parameters characterizing the acoustic emission signal are acoustic emissivity, amplitude and amplitude distribution, and energy.

(2) Characteristics of acoustic emission signals when the valve is leaky
When the tightness of the valve is poor, a small amount of fluid is always ejected through the slit of the valve to generate a high-speed jet that impacts the wall of the tube to excite elastic waves, that is, acoustic emission. It is a continuous type of acoustic emission signal, similar to white noise, with a frequency of 30 to 50 kHz. 

Characteristics of the acoustic emission signal generated when the valve leaks:
The acoustic emission wave excited by the leakage is continuous;
The acoustic emission signal generated by the leakage is relatively strong, and its amplitude is proportional to the leakage rate and proportional to the rms value of the signal.

According to the characteristics of the acoustic emission signal generated by the leakage, the characterization parameters adopt acoustic emission rate and energy, which can obtain more abundant acoustic emission signal data and provide a more sufficient basis for fault identification.

Field application of ultrasonic detector

(1) Introduction to the detector
When the acoustic emission wave generated by the leakage is transmitted to the sensor, the sensor converts the acoustic emission wave into a voltage signal and transmits it along the wire to the detector. The function of the preamplifier is to amplify the signal sent by the sensor, and the function of the filter is to improve the SNR (signal to noise ratio). The role of the threshold circuit is to eliminate background noise. The microprocessor calculates, analyzes, and judges the signal. When the valve has no leakage, the display reads 0; if there is a leak, the displayed value changes with the degree of leakage, the larger the leakage, the larger the value. The audio component of the acoustic emission signal generated by the leakage is processed and sent to the earphone, and the earphone is used to listen to the sound of the leak. When the valve leaks, the sound of different sound intensity can be heard by the earphone, and the degree of the leak can be judged according to the strength of the sound heard.

(2) The tightness test of the valve
When using an ultrasonic detector to detect a high-pressure system of a valve, a comparative reading is usually used to determine whether there is leakage: the first step, contact the upstream side, reduce the sensitivity, and minimize other sounds; the second step, contact the valve seat and the downstream side; the third step, compare the difference in sound, if the valve leaks, the sound intensity of the valve seat or the downstream side is equal to or greater than the sound intensity of the upstream side.

When detecting a low-noise system, contact the downstream of the valve, use the frequency selection method to clearly hear the sound of the liquid flow, and judge whether there is leakage or leakage condition according to the reading value.

When using the four-point comparison method to detect downstream interference, these disturbances are sometimes large and can propagate to the area that needs to be detected, resulting in an incorrect indication of valve leakage. The four-point method includes: selecting two equidistant points A and B at the upstream, and then selecting two equidistant points C and D at the downstream. Compare the signal strengths of points A and B with the signal strengths of points C and D. If the signal strength of point C is higher than those of points A and B, the valve has a leak; if the signal strength of point D is higher than that of point C, the sound is coming from other points downstream.

When gas or liquid leaks in the pipeline, ultrasonic waves are generated due to friction. SONAPHONEE can receive the ultrasonic signal and convert the ultrasonic signal into an electrical signal displayed on the display screen. At the same time, the acoustic emission signal is processed into an audible sound and transmitted to the headset, and can be transmitted to the PC through the infrared interface.