Analysis of safety technology for opening inspecti

Analysis of safety technology for opening inspection of LPG underground storage tanks

1 overview

Tongda Dafeng Natural Gas Co., Ltd., located in Dafeng City, Jiangsu Province, is a wholly-owned subsidiary of Tongda energy group. Before Dafeng compressed natural gas (CNG) decompression station was put into operation, it was mainly engaged in pipeline liquefied gas operation and bottled liquefied gas filling business. In June, 2004, Dafeng compressed natural gas (CNG) decompression station was put into operation, and the pipeline gas in the urban area of Dafeng was converted from liquefied petroleum gas to natural gas. At present, the liquefied petroleum gas station is only responsible for the filling of liquefied petroleum gas. The LPG storage capacity of the station area is 45m3. There are two 20m3 LPG storage tanks and one 5M3 residual liquid tank. This year, it has been five years since the last tank opening inspection. Therefore, the station area has stopped operation recently, and the three storage tanks and the relevant equipment, valves and process pipelines in the station area have been inspected and repaired

2 technical analysis of tank opening inspection operation

this tank opening inspection process can be roughly divided into four steps: preliminary preparation, gas station shutdown and replacement, tank opening inspection and maintenance, and gas station resumption of operation

2.1 preliminary preparation

tank opening inspection is a major operation for any LPG gas station. In view of the characteristics that three tanks will be inspected at the same time and the normal production and operation of the whole gas station will be stopped in Dafeng company's tank opening inspection, the preparatory work in the early stage is particularly important

1. Discuss and formulate the main contents of inspection and maintenance with the competent department of tank opening inspection and the person in charge of the construction unit

2. Formulate a reasonable implementation plan for tank opening inspection and safety emergency plan

3. Prepare various equipment, apparatus and accessories

4. Train relevant operators

5. Arrange LPG intake plan in advance

2.2 shutdown and replacement of the gas station

2.2.1 after filling the LPG in the storage tank, use the compressor to increase the pressure of the storage tank for filling the steel cylinder. When the filling pump is evacuated, change the tank for filling, rely on the LPG air pressure to pump the residual hydraulic pressure in the empty tank to the residual liquid tank, use the compressor to pump the gas phase of the tank to the residual liquid tank after the residual hydraulic pressure is empty, and stop the compressor when the pressure of the storage tank in the Great Pacific Garbage belt between Hawaii and California in the United States drops to 0.04MPa

2.2.2 discharge the residual liquid

when the pressure of the residual liquid tank rises to 0.5MPa, open the residual liquid discharge valve of the residual liquid tank to discharge the residual liquid to the residual liquid pool and flush with water. When Gaseous LPG is seen in the residual liquid pipe, close the valve of the residual liquid discharge pipe

2.2.3 pressure relief and release of storage tank

slowly open the vent valve of the storage tank to discharge the residual LPG gas phase at high altitude, open the valve connecting the pipeline and the storage tank, depressurize the pipeline and the storage tank at the same time, control the release speed according to the wind direction and concentration, and use the method of intermittent release to discharge the residual pressure in the storage tank to zero

2.2.4 water immersion

connect the fire water pipe from the LPG discharge pipe, introduce the fire water into the storage tank to fill the tank, and discharge the residual LPG gas driven out by the water from the vent pipe at the same time. Soak it for 24 hours after the water is filled

2.2.5 open the manhole

after soaking in water, pressurize the unloading gas phase pipe with an air compressor. After the water in the storage tank and pipeline system is removed, open the tank vent valve to vent the air in the storage tank and system, remove the safety valve and liquid level gauge, and open the manhole cover for ventilation

2.3 tank opening inspection and maintenance

2.3.1 storage tank

1. Excavate the yellow sand buried in the storage tank and transport it to the ground

2. The circumferential and longitudinal welds inside the storage tank shall be polished and derusted, and magnetic particle flaw detection shall be carried out to confirm that the welds are qualified

3. Measure the thickness of the storage tank locally and confirm that the wall thickness has not decreased

4. External corrosion protection inspection of storage tank, local recovery, sacrificial anode protection, magnesium anode and filler replacement

2.3.2 process pipelines

more than 10 process pipelines in the station area are randomly selected for thickness measurement to confirm that there is no thinning of the pipe wall

2.3.3 valves

all valves in the station area shall be disassembled for maintenance, and the packing and sealing gasket shall be replaced

2.3.4 safety valves

all safety valves shall be calibrated again and the outdoor lighting measurement method gb/t 15240 (9) 4 pressure

2.4 the gas station resumes operation

2.4.1 the equipment resumes

after all inspection and maintenance items are completed, the manhole cover of the storage tank is closed, the process pipeline valves, safety valves and liquid level meters are installed in place, and the storage tank is buried with yellow sand

2.4.2 pressure test and air tightness of storage tank and pipeline system 8 For cables repeatedly bitten by rats,

use an air compressor to press air into the storage tank and pipeline system. The system pressure rises to 1.6Mpa and the pressure is maintained for 3 hours. Use soapy water to check the leakage at the flange connections and valve seals. After confirming that there is no leakage point, open the air valve to relieve the system pressure to zero

2.4.3 system vacuum pumping

connect the vacuum pump at the unloading pipe to vacuum the storage tank and pipeline system, and maintain the vacuum degree at -0.08mpa

2.4.4 unloading LPG

lpg tanker entering the station, changing the normal unloading process, unloading LPG into the storage tank extruder originated in the 18th century

2.4.5 steel cylinder filling

change the steel cylinder filling process, release a small amount at the bottling table, measure the oxygen content of the vented air with an oxygen meter, and then carry out LPG bottling operation, and the whole gas station returns to normal operation