GB/T 47577-2026Non-destructive testing - Optical gas imaging method for the detection of volatile organic compound (VOC) leaks (English PDF)

6.2 Detector

6.2.1 General Requirements The testing instrument should meet the following requirements. --It has the function of real-time acquisition and transmission of infrared images and infrared video data; --In infrared mode, it offers multiple display modes including incandescent and black-and-white (with various pseudo-color palettes available); --Adjust the focus manually or automatically to ensure a clear image; --The image analysis software has a VOCs gas leak location function; --Real-time adjustment of imaging contrast and brightness, manual/automatic adjustment of color mark, and switching between infrared/visible light display; --Powered by battery, the continuous and stable working time is no less than 3 hours, provided that all functions are normal.

6.2.2 Performance of the Detector

6.2.2.1 Field of View The testing instrument should specify the field of view of the lens. Different lens specifications can be selected according to the testing distance to obtain the corresponding field of view. When testing... When the measurement distance is greater than 50 m, the field of view should not exceed 10°. When the measurement distance is between 10 m and 50 m, the field of view should be greater than 10° and less than [missing value]. 38°. When the detection distance is less than 10 m, the field of view should not be less than 38°. The error between the measured value and the nominal value of the field of view should not exceed ±5%.

6.2.2.2 Spatial resolution The spatial resolution should be selected based on the size of the object being inspected and the detection distance. When the detection distance is greater than 50 m, the spatial resolution should be selected as [specific value needed]. Choose a spatial resolution of

0.2 mrad to

0.7 mrad. When the detection distance is 10 m to 50 m, a spatial resolution of

0.7 mrad to

1.0 mrad is recommended. When the distance to be measured is less than 10 m, the spatial resolution should preferably be selected from

6.2.2.3 Sampling Frame Rate The frame rate for acquiring and transmitting infrared video data should not be lower than

6.2.2.4 Minimum Detectable Leak Rate (MDLR) 6.2.2.4.1 In the laboratory environment, the temperature is 23°C±5°C, the relative humidity is 40%~80%, and the temperature difference between the standard gas and the blackbody simulator is... Under the conditions of 2 °C, a distance of 2 m, and where both the gas outflow and direction are visible to the detector. --Methane standard gas, concentration of 99.99% mol/mol, minimum detectable leakage rate less than or equal to 15 mL/min; -- n-Butane standard gas, concentration of 2.0% mol/mol, minimum detectable leakage rate less than or equal to 100 mL/min. 6.2.2.4.2 On a sunny day with sufficient sunlight, an ambient temperature of 10°C~25°C, a wind speed not exceeding 4 m/s, and a relative humidity of 5%~80%, the distance... Under the condition of 3 m, and when both the gas outflow and direction are visible to the detector. --Ethylene standard gas. concentration of 3.0% mol/mol, minimum detectable leakage rate less than or equal to

2.5 L/min; --Methane standard gas. concentration 5.0% mol/mol, minimum detectable leakage rate less than or equal to

0.5 L/min; concentration is At a concentration of 2.5% mol/mol, the minimum detectable leakage rate is less than or equal to

6.2.2.5 Detectable gas types Depending on the VOCs gaseous medium inside the equipment being tested, the detector's detection range should cover its absorption spectrum or characteristic absorption band. Infrared optical gas imaging detection can detect volatile organic compounds including, but not limited to. methane, ethane, propane, butane, pentane, hexane, and heptane. Alkanes, methanol, ethanol, benzene, xylene, propionaldehyde, acetylene, etc. Long-infrared optical gas imaging detection can detect volatile organic compounds including, but not limited to, those listed below. Limited to toluene, ethylene, ethylbenzene, propylene, etc. The spectral absorption characteristics of common VOCs in the infrared and long-infrared bands are shown in Appendix A.

6.3 Maintenance and verification of testing equipment The equipment should be maintained and checked every 12 months to ensure its functionality meets testing requirements. Minimum detectable depth in a laboratory environment. The leak rate (MDLR) test shall be performed in accordance with the provisions of

8.2 and shall meet the requirements of 6.2.2.4.1. Before on-site testing, the equipment should be verified using known emission sources (such as butane lighters or propane cylinders). If the equipment is at its minimum... If a clear real-time gas image can be generated when detecting a known emission source at the focal length, it indicates that the equipment is functioning normally.

7.Testing Procedures For optical gas imaging detection of volatile organic compound (VOC) leaks, a detection process specification should be established, which should include at least, but not limited to, the following. Limited to the following elements.

8.1 Test Preparation

8.1.1 Data Inquiry The following information should be consulted to understand the relevant details of the equipment.

a) Manufacturing documents of the equipment under inspection, such as product certificates, quality certificates, and installation layout drawings, with a focus on understanding their type. Structural features, sealing point information, and material properties, etc.

b) Operating records of the inspected equipment, such as media, process flow diagrams, piping and instrumentation diagrams, operating parameters, working environment, and data during operation. Abnormal situations that occur, etc.

c) Other information about the equipment under inspection, such as process operating procedures, maintenance, repair and modification documents.

8.1.2 Debugging of the testing instrument Before conducting testing, testing personnel should perform the following debugging procedures on the testing instrument.

a) Confirm that the testing software is loaded successfully and that no error messages are displayed upon startup.

b) After the detector is powered on, image non-uniformity correction is performed. This is done using a shielding object that generates uniform radiation (such as a lens cap). After the lens was obstructed and the obstruction was removed following correction, the infrared image observed during scene viewing was uniform and showed no significant stagnation when the detector was moved slowly. Pattern noise correction is now complete.

c) Confirm that the detector is in sharp focus within the focal length range to be imaged.

8.2 MDLR Test

8.2.1 Test Method In the laboratory, activate the gas leak alarm and set its alarm concentration within the safe experimental range, then test as follows:

a) Adjust the distance between the detector and the air outlet to 2 m;

b) Adjust the position of the blackbody simulator so that it serves as the background for the tested standard gas in the image, ensuring that the blackbody simulator occupies a significant portion of the frame. The proportion is more than 1/4, and it is located in the center of the image;

c) Adjust the temperature of the blackbody simulator to create a certain temperature difference (Deltat) between it and the gas being tested.

d) Adjust the position of the detector so that the air outlet is in the center of the field of view, and adjust the focus of the detector to make the image clear;

e) After the detector has stabilized, it first outputs the standard gas being tested at a relatively high rate, allowing the testing personnel to observe the gas column on the monitor. Gradually reduce the leakage rate of the standard gas being tested until the testing personnel can no longer distinguish the outflow and direction of the target gas. At this point, [the gas is then...]. The corresponding leakage rate value is the minimum detectable leakage rate of the detector under a temperature difference of Deltat.

8.2.2 Test Equipment The gas flow meter used during testing should have an accuracy of no less than 1%. The blackbody simulator should have temperature adjustment capabilities, and its parameters should also meet the following requirements. The following conditions.

a) Temperature adjustment range is -20 °C to 20 °C;

b) The effective radiation area is greater than or equal to 30 cm × 30 cm;

c) Temperature control accuracy is better than 0.1 °C;

d) The temperature difference at all points on the effective radiating surface is less than 0.03 °C.

8.3 Implementation of Testing

8.3.1 Based on the data query results and the types of volatile organic compounds that may be leaking at the site, select an infrared detector whose operating band can cover the area being inspected. OGI detector for gas infrared absorption peaks.

8.3.2 Determine the maximum detection distance between the OGI detector and the equipment under test based on the actual conditions at the testing site. The maximum detection distance is mainly related to... The type of gas being tested, background temperature, gas temperature, environmental testing conditions (such as wind speed, temperature, and humidity), focal length, and pixel size are all relevant factors. The maximum detection distance of the instrument refers to the maximum distance at which it can detect and identify a standard air mass with a diameter of 1 m under on-site environmental conditions. The following two conditions must be met.

a) Imaging resolution. The minimum horizontal pixel span in which the air mass occupies at least 7 pixels in the image;

b) Detection contrast. The average gray level difference between the air mass and the background in the image is greater than 3 times the standard deviation of the system noise.

8.3.3 Within the maximum detection distance range, perform optical gas imaging inspection on the equipment under inspection, focusing on the welded joints and connections. The connection points between safety accessories such as pipes, flanges and other fittings, valves, safety valves, pressure gauges, rupture discs, and level gauges and the equipment body. Repaired or modified parts, as well as sealing parts of pumps, compressors, agitators, etc., parts connected to external pressure pipelines, and other suspicious parts.

8.3.4 When performing optical gas imaging inspection, the inspection position is selected based on the inspection distance and field of view to image the device under inspection. The testing area should cover the entire area of the equipment under inspection, as shown in Figure

3.The testing personnel should strive to ensure a uniform temperature difference in the background image, and at each testing angle... The dwell time should not be less than 5 seconds before moving to the next detection location to continue detection. If preliminary detection reveals signs of leakage, the temperature difference should be located. The largest position is used to obtain the leaked image data (including videos and photos) with the best contrast.

8.3.5 When the equipment under inspection contains complex components, the inspector should divide the equipment into several components and ensure that each component is in good working order. The image should fill more than half of the field of view, and the dwell time for each component in the field of view should not be less than 5 seconds.

9.1 Data storage and its effectiveness evaluation

9.1.1 When the detector detects a leak, a leak photograph with the best contrast should be saved (one optical gas imaging photograph and one optical photograph). Capture video data of the leak location for at least 10 seconds, record and describe the specific location of the leak, and save the video data and OGI detection records. Immediately attach a leak warning sign or make appropriate markings at the leak point and indicate the leaking medium.

9.1.2 When the detector does not detect a leak, photos of each location tested in accordance with the requirements of

8.3.4 should be kept.

9.1.3 Before analyzing the test data, the validity of the collected data should be evaluated, and the test data should meet the following requirements.

a) The dwell time of the testing personnel at each testing location meets the requirements of 8.3.4;

b) The acquired images or videos have no obvious fixed pattern noise;

c) Simultaneously acquire optical images of the same detection location to eliminate environmental interference factors;

d) The stored photos are in accurate focus, without blurring, and have clear edge contours.

9.1.4 To reduce testing errors or highlight abnormal situations such as leakage, appropriate signal processing and image processing operations should be performed, such as threshold processing, Operations include averaging, smoothing, background removal, difference processing, frequency analysis, locking, and motion compensation.

9.2 Leakage Analysis During testing, a testing instrument that meets the performance requirements of Chapter 6 should be used. Qualitative analysis can be performed when obvious airflow and air masses are observed during the testing process. For VOCs gas leaks, the leak point is located using full-range, multi-position optical gas imaging detection. If necessary, a measuring tape or laser rangefinder can be used. When using an instrument for measurement, the measurement accuracy should not be less than 2 cm. When the testing personnel deem a device suspicious and the sampling probe can reach the device under test, photoionization detector (PID) technology or hydrogen flame ionization detector (HF) technology can be used. Ion detector (FID) technology is used to further verify whether a leak has occurred. Examples of optical gas imaging images of VOCs leaks from equipment in the oil refining and chemical industries are shown in Appendix B.