The pressure valves of surface facilities and equipment in Daqing Oilfield have experienced internal leakage due to wear, corrosion, or other reasons. Due to the difficulty in detecting internal leaks, some faulty valves have not been replaced in a timely manner, affecting production and posing safety hazards. Therefore, online valve detection technology is receiving increasing attention, and the current core technology is to use acoustic emission technology to online detect leaks in various industries such as petroleum, petrochemical, and power plants.

Basic principles of acoustic emission leak detection

(1) Acoustic emission signals and their characterization parameters

Acoustic emission refers to the phenomenon that any metal material or component releases strain energy in the form of elastic wave when friction, crack or plastic deformation occurs under the action of external force or internal force. When acoustic emission waves propagate in a finite medium, they undergo refraction and reflection when encountering interfaces, and are converted into surface waves that propagate along the surface at high speeds on the solid surface. These acoustic emission waves from acoustic emission sources (i.e. fault points) have active characteristic information, which can be used to reflect the faults and defects of components. The parameters characterizing acoustic emission signals include acoustic emissivity, amplitude and amplitude distribution, and energy.

(2) Characteristics of acoustic emission signals when internal leakage occurs in valves

When the tightness of the valve is poor, a small amount of fluid is always sprayed out through the gaps of the valve, generating a high-speed jet. This high-speed fluid impacts the pipe wall and triggers elastic waves, namely acoustic emission. It is a continuous acoustic emission signal, similar to white noise, and its frequency is 30~50kHz.

The characteristics of the acoustic emission signal generated when a valve leaks: 1. The acoustic emission wave excited by the leak is continuous; The acoustic emission signal generated by leakage is relatively strong, and its amplitude is proportional to the leakage rate and the root mean square value of the signal.

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

On site application of ultrasonic testing instrument

(1) Introduction to the Tester

When the acoustic emission wave generated by 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, the function of the filter is to improve the signal-to-noise ratio, and the function of the threshold circuit is to remove background noise. The microprocessor calculates, analyzes, and judges the signal. When the valve has no leakage, the display screen reads 0; If there is a leak, the displayed value changes with the degree of leakage, and the larger the leak, the greater the value. The audio components in the acoustic emission signal generated by the leak are processed and sent to the headphones, which are used to listen to the sound during the leak. When the valve leaks, different sound intensities can be heard through headphones, and the degree of leakage can also be determined based on the intensity of the sound heard.

(2) Valve tightness inspection

When using an ultrasonic detector to detect the high-pressure system of a valve, a comparison of readings is usually used to determine whether there is a leak: the first step is to contact the upstream side, reduce sensitivity, and minimize other sounds; Step 2, contact the valve seat and downstream side; The third step is to compare the differences in sound. If the valve leaks, the sound vibration intensity on the valve seat or downstream side is equal to or greater than the sound intensity on the upstream side.

When detecting a low noise system, contact the downstream of the valve and use a frequency selection method until the liquid flow sound can be clearly heard. Based on the reading size, determine whether there is a leak and the leakage situation.

When using the four point comparison method to detect downstream interference, these interferences are sometimes very large and can propagate to the area that needs to be detected, leading to erroneous indications of valve leakage. The four point method includes selecting two equidistant points A and B upstream, and then selecting two equidistant points C and D downstream. Compare the signal strength of points A and B with that of points C and D. If point C is higher than points A and B, it indicates that the valve is leaking; If the D point is higher than the C point, it indicates that the sound is coming from other downstream points.

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

3 Conclusion

(1) The application of acoustic emission technology for leak testing of valves (such as water mixing valves and gas valve group bypass valves) can ensure the accuracy of safety production and energy consumption control.

(2) Testing the valves replaced during the renovation of the old district can provide a basis for project decision-making and reduce the renovation cost.

(3) Ultrasonic testing technology can achieve online valve inspection, with simple operation, fast dynamic response, and intuitive data. At present, the testing equipment can only qualitatively determine whether there is a leak inside the valve.