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Explore the sealing technology of LPG storage tank leakage

I. preface

mainly for the leakage of liquid fossil oil and gas at the lower part of LPG storage tank, it is proposed to use water seal isolation and sealing method to prevent the leakage of LPG, and transfer LPG at the same time, so as to curb the fire and explosion accidents caused by the leakage of LPG, which can win time for the rescue work of LPG stations

with the improvement of people's living standards, cities have higher and higher requirements for environmental protection and clean energy, and liquefied petroleum gas has widely entered people's lives. Liquefied petroleum gas is flammable and explosive, and its safety management is particularly important to actively position itself as the middle and low-end. According to statistics, in the past 30 years, the oil refining industry accounted for 40% of the huge fire and explosion accidents with losses of more than 10million US dollars, while the oil storage and transportation industry accounted for 12% (mainly LPG storage and transportation). Each fire and explosion accident is mainly caused by the leakage of liquefied petroleum gas. On March 5, 1998, a 400m3 spherical tank of Xi'an gas company leaked at the flange of the blowdown valve, causing an explosion and fire, which killed 13 people and injured more than 30 people. Two 400m3 spherical tanks, four 100m3 horizontal tanks, seven tank cars and the power distribution room of the water pump room were burned. The direct economic loss reached 4.77 million yuan, bringing huge losses to the lives and property of the country and people

the problem of preventing leakage has been the focus and hot spot of long-term discussion in LPG storage and distribution stations. The development of its plugging technology and plugging tools is a hot topic in LPG storage and distribution stations. This paper puts forward a method to solve the problem of tank leakage by filling water and sealing it from the tank

II. Current situation

the above Xi'an accident makes people clearly realize the importance of the first flange at the lower part of LPG storage tanks. The first flange surface does not refer to a single flange surface, but mainly refers to the flange surface of the connecting pipe under the liquid level gauge, the flange surface of the blowdown pipe, the flange surface of the gas phase pipe and the inlet and outlet liquid phase pipe, as well as the circumferential seam of the connecting pipe or the leakage of the valve. Because once this part leaks, it is all liquid leakage. The expansion coefficient of LPG is about 10 times that of water. After vaporization, the volume of LPG expands about 250 ~ 300 times, and the explosion limit is 1.5% - 9.6% (among which the explosion limit of each component is propane 2.1% - 9.5%, butane 1.5% - 8.5%, butene 1.7% - 9.6%), which is hundreds of times different from the leakage at the upper part of the storage tank. In short, the tightness of the first flange surface of the storage tank is very important, while the first flange surface at the lower part is particularly important. Therefore, in order to prevent the leakage accident of this part, the Ministry of labor proposed that the first flange should be forged high neck flange and sealed with metal spiral wound gasket after the Xi'an accident, so as to improve its sealing and compression resistance. In the design, in order to prevent the leakage accident caused by the frequent action of the valve, a standby valve is adopted in series with the working valve. The working valve is closed and opened according to the working condition, while the standby valve is normally open to ensure that there is always a valve in good standby condition. At the same time, spray method is adopted outside the tank to reduce the temperature, so as to prevent explosion accidents due to the sharp increase of pressure in the tank due to the increase of temperature; Adopt gas leakage alarm to monitor the gas concentration in the tank farm to realize early warning; Taking fire hydrants, fire extinguishers and other measures has played an important role in preventing fire and explosion accidents. However, due to the large pressure in the storage tank, once the leakage accident occurs, it cannot be blocked in a short time, and the liquefied petroleum gas source continues to leak, vaporize, expand and diffuse, which will inevitably lead to fire or explosion accidents. In order to effectively solve this problem, I propose to adopt the internal isolation method of water seal to discuss with my colleagues

III. water seal internal isolation and plugging method

1 basic principle of water seal isolation method

the specific gravity of water is far greater than that of liquid LPG. The specific gravity of water is 1, while the specific gravity of liquid LPG is about 0.58. When water is injected into the storage tank through the water injection pipe, it can be quickly separated from LPG and deposited at the bottom of the storage tank. After continuous injection, the first flange surface at the lower part of the storage tank no longer forms the leakage of liquefied petroleum gas liquid, but becomes the leakage of water, which wins time for the inversion and emergency repair of liquefied petroleum gas, prevents the expansion of the disaster, and can effectively curb the occurrence of accidents

2 modification of process pipelines

the storage tanks in gas storage stations are generally in parallel. Generally, there are at least four interfaces at the lower part of each storage tank, one for the blowdown valve, one for the gas phase pipe, and two for the liquid phase pipe. The liquid phase pipe is divided into one liquid inlet and one liquid outlet. The inlet and outlet nozzles are plug-in type, and the outlet nozzle extends more than 10cm into the tank, mainly to block the residual liquid and prevent the sediment from entering the hydrocarbon pump and causing damage. The gas and liquid phase flanges of each storage tank are designed as two valves, of which the normally open standby valve is close to the flange surface on the side of the storage tank, and the working valve is closely connected. In case of leakage of the working valve, the standby valve can be closed and the maintenance working valve can be replaced. In order to realize the water seal isolation method, we must choose a reasonable import. Because the water temperature at the inlet pipe is lower than the set temperature and is flush with the bottom of the tank, once the water is injected, it can quickly spread to the rest of the pipe orifices. Therefore, choose to install a pipe between the standby valve and the working valve of the inlet pipe, and extend the pipe to the process operation room (because leakage may cause combustion, it is impossible for rescue personnel to get close to the process operation room), and lead out the inlet pipes of each tank in parallel in the process operation room, A tee is added between the original process pipelines to connect its interface with the outlet valve of the water pump, and the inlet of the water pump is connected with the fire pool. The pump head must be higher than the working pressure of the storage tank. No matter any tank leaks, as long as you open the working valve of the inlet pipe connected to it, close the working valve of the tank without leakage, open the outlet valve of the water pump, close the main inlet channel valve, and start the water pump, the water in the fire pool will be forcibly injected into the tank to achieve effective water seal isolation

3 transfer of liquefied petroleum gas

due to the increase of water level, liquefied petroleum gas will inevitably overflow from the gas phase pipe. If all storage tanks are equal in size and at the same horizontal position, liquefied petroleum gas will pass through the gas phase pipe (we recommend customers to use the hydraulic universal testing machine for material testing samples of more than 30 tons. Usually, only the residual liquid tank is in working state, and the valves of each gas phase pipe are opened in parallel to maintain pressure balance) and be transferred to each storage tank. If the storage tanks are of different sizes, by closing the working valve of the gas phase pipe that has not leaked, and then closing the working valve of the pipeline leading to the process room when the standard was submitted to the Ministry of public security for approval, the liquefied petroleum gas flows from the bypass pipeline to the hydrocarbon pump, starts the hydrocarbon pump, and opens the pipeline leading to the storage tank that has not leaked, the transfer of liquefied petroleum gas to other storage tanks can be realized

4 examples of leakage plugging and liquefied gas transfer

see Figure 1. It is assumed that tank B leaks: close F1, A2, C2, A10, open E3, B2, start the water pump, and the water in the fire pool will be continuously injected into tank B through b1 from the B2 valve pipeline, realizing water seal plugging. When the water level rises, the liquefied petroleum gas overflows from the gas phase pipe, and each storage tank is equal in size and at the same horizontal position, and the liquefied petroleum gas will be transferred to each storage tank through the gas phase pipe. When the size of storage tanks is inconsistent, close valves A14, C14 and D1 (because there is no LPG leakage, you can get close to the storage tank); Open C3, A3 and D3 and start the hydrocarbon pump, that is, to realize the safe transfer of liquefied petroleum gas from leaking tank B. Leakage of other tanks is the same

Figure 1 Schematic diagram of water leakage plugging and liquefied gas transfer

normally open valves: A12, A13, A14, B1, B12, B13, B14, C1, C12, C13, C14, D1, E1, F1, A10

Gongmen valve: A2, B2, C2, B21, C21, A1

transfer valve: A3, B3, C3, D3

water injection valve: E3

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