Several safety issues to consider at LPG storage and distribution stations

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Several safety issues that need attention at LPG storage and distribution stations. Following the LPG leakage and explosion incident that occurred in Xi’an on March 5th, in accordance with the requirements set out in Document No. 143 issued by the Quality and Technical Supervision Bureau in 1999, titled “Notice on Strengthening the Supervision and Management of LPG Safety,” we conducted investigations of dozens of LPG storage and distribution stations in Henan Province. We found that there were problems such as inadequate implementation of safety management systems, defects in equipment and pipeline accessories, weak capabilities and means for accident prevention, and a lack of experience in handling emergencies. The following safety issues require rectification at existing stations, and new storage and distribution stations should take precautions to avoid them. I. Selection of pipe flanges, fasteners, and gaskets: LPG stations put into operation before 1995 primarily used plate-type butt weld flanges and oil-resistant rubber asbestos sheets, adhering to the HG58 and JB59 pipe flange standards. Due to the insufficient strength and stiffness of plate-type face weld flanges, in order to improve sealing performance they are often used at a higher grade than recommended, yet this still does not resolve the leakage problem. The use of asbestos rubber gasket sheets is increasingly declining internationally. ? To improve the flange sealing structure, the first flange of LPG storage tanks should utilize a raised-face wafer butt-welded flange, metal wound gaskets, and bolts and nuts of special grade. For the pipe flanges connecting pipes and equipment as well as valves, necked butt-weld flanges, metal wound gaskets, and bolts and nuts of special grade should be used. The materials for flanges, gaskets, bolts, and nuts shall meet the requirements of **current relevant standards. In areas with lower temperatures, during early winter and the beginning of spring, the moisture present in liquefied petroleum gas can cause asbestos rubber gaskets to expand and contract, leading to failures in the sealing of flanges. Therefore, liquefied gas stations that use asbestos rubber gaskets should carry out enhanced safety inspections during these two periods, and gradually phase out the use of such gaskets. ? II. Stability of pipes and accessories connected to the storage tank? The process piping for liquefied petroleum gas storage tanks typically includes liquid inlet and outlet pipes, liquid return pipes, gas pipes, drain pipes, vent pipes, safety valve vent pipes, etc. There are between 10 and 20 valves installed in these process piping systems; it is necessary to pay attention to the loads, stresses, and vibrations generated during operation, carry out appropriate stress analysis, and determine the appropriate support and vibration prevention measures to avoid potential accidents caused by excessive stress or vibrations. ? The Xi’an \"3.5\" LPG explosion was caused by a leak in the oil-resistant asbestos rubber gasket located between the flange at the base of a 400m3 spherical tank and the drain valve. Due to the prolonged duration and large volume of the leak, three explosions occurred. As a result of this fire and explosion incident, 2 40m3 spherical tanks, 4 100m3 horizontal tanks, and 7 tank trucks were destroyed; buildings such as the power distribution room and pump house were also damaged. 12 people died (including 7 firefighters), more than 30 people were injured, and the direct economic loss amounted to 4.77 million yuan. The accident investigation team analyzed the cause of the leakage and concluded that there were areas where there was no contact between the upper and lower surfaces of the gasket on the discharge valve’s flange, as well as between those surfaces and the sealing surfaces of the spherical tank’s flange and the discharge valve’s flange; in addition, there was a sector with a radius of 60 mm between the sealing surfaces and the gasket where no contact existed. Due to the degradation of the physical properties of the gasketing, the valves and pipes connected to the spherical tank remain in a suspended state; furthermore, the periodic impacts and vibrations resulting from the opening and closing of the valve bodies cause changes in the stress conditions on various parts of the flange sealing surfaces as well as on the bolts, leading to seal failure and leakage. ? Therefore, the support for the process piping at the bottom of LPG storage tanks must be reliable and proper; pipelines and valves must under no circumstances be left suspended in mid-air. ? III. Appropriately increase the number of gas detection and alarm devices. By placing combustible gas concentration detectors in areas at risk of explosion, and alarms in safe locations where there are staff on duty, it is possible to detect leaks early and effectively, allowing timely action to be taken to prevent accidents. The effective coverage radius of the detector is 7.5 m indoors and 15.0 m outdoors. Currently, liquefied gas stations generally suffer from an insufficient number of combustible gas detectors. Given the current level of manufacturing for detection alarms and the management standards for liquefied gas, the number of combustible gas detection alarms installed should be increased appropriately. ?IV. Emergency Rescue and Protective Equipment? 1. Provide emergency rescue personnel with no less than 2 sets of protective equipment, which should include protective clothing, protective shoes, protective helmets, protective gloves, and air respirators. ?2. Sealing equipment and tools suitable for the first flange of the storage tank should be provided, based on the structural dimensions of that flange; stations with the necessary facilities should install water injection devices on the liquefied gas storage tanks. ?3. Develop practical accident early-warning plans and conduct regular emergency response drills to improve employees’ ability to handle emergencies.