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    Physical Examination of Spherical Gas Storage Tank

    There are four giant spherical gas storage tanks near the TV Tower in the southern suburbs of Xi'an. Recently, two of them have undergone a six-year "general physical examination" to eliminate all possible safety hazards. So what is the "physical examination" of the gas tank? Will it affect residents'gas consumption? Chinese Business Journal reporter reveals the secret for you.
    Basic information of gas tank
    Four tanks are 33 meters high and each can store 100,000 cubic meters of gas.
    There are four spherical gas storage tanks in the Southern Suburb Storage and Distribution Plant of Xi'an Qinhua Natural Gas Company located in the southern section of Zhuqu Road, which guarantees the natural gas supply of residents and gas enterprises in Xi'an.
    Four giant gas tanks, each 33 meters high, are equivalent to 11 stories high. Two of them are being tested, and scaffolding is all around them. The other two tanks are still in normal operation and are scheduled to be tested next year.
    The four tanks are all 10,000 cubic metres in volume, and each can store 100,000 cubic metres of compressed natural gas. The inner diameter of the gas tank is 26.73 meters. It is designed and manufactured by French company. The thickness of the spherical shell plate is about 38 mm. It is the largest spherical gas tank in China. It was accepted and delivered to use in November 2000.
    Because it is a special high-pressure equipment, it must be inspected regularly and compulsorily. The gas tank was inspected for the first time from August to September 2002, the second time from August to September 2008, and the third time was started on July 30 this year. It is expected to be completed in two months. It will be inspected by Xi'an Special Equipment Inspection Institute (hereinafter referred to as "Special Inspection Institute").
    Up to now, the inspection of the outside of the gas tank has been basically completed, and some cracks have been found. After all the tests are completed, the Special Inspection Institute will issue an inspection report. After professional repair by the Natural Gas Company, the Special Inspection Institute will review and verify the inspection, and the qualified inspection can be started again. The gas loading operation is planned before National Day.
    Whether there is a crack in the weld seam to measure the hardness of spherical shell
    Jing Qiangzheng, Vice President of the Special Inspection Institute, said that the key to the "physical examination" of gas tanks was the weld. If there were cracks in the weld, the high-pressure gas would leak and the consequences would be unimaginable. Not only the surface but also the interior of the spherical shell plate should be inspected.
    Ultrasound and X-ray are used in the detection, just like human body examination, but because the spherical shell plate is very thick, the dosage needed is much larger than that for physical examination.
    It is understood that a gas tank consists of 92 steel plates, and the weld is more than 3000 meters long. The magnetic particle testing method or penetration testing method can be used to detect the surface cracks of welds.
    In magnetic particle detection, magnetic suspension and magnetic particle flaw detector formed by adding water to black magnetic powder are used. The magnetic particle flaw detector can be divided into North and South poles. When the magnetic powder sprayed on the weld is magnetized, where the gap appears, the black magnetic powder will run to where, and the shape of the gap can be observed. The penetration detection method is to clean the surface of the spherical shell plate and spray the permeate containing Sudan red for industrial use. If there is a crack, the permeate will infiltrate, erase the permeate and spray the imaging agent again, showing the appearance of the crack.
    When inspecting the defect inside the spherical shell plate, we need to use the ultrasonic flaw detector. With the oscillation of the probe, the image on the instrument shows the defect. X-ray inspection is also used, and the film will be taken, and the defect pattern can be seen on the film.
    In addition to the key detection of weld seam, the hardness of spherical shell should also be measured to check whether the material of gas tank has changed after operation. At this time, the hardness tester should be used. The hardness tester will report the data after the professional equipment hits, showing the change of material.