How many hours is optimal for vacuum extraction of LNG cylinders?
3 Answers
There is no specific time limit for this; it should be determined based on the usage condition of the cylinder. When the vehicle leaves the factory, the vacuum level in the interlayer of an LNG heavy-duty truck's cylinder is around 3*10-3Pa, providing excellent insulation. However, as usage time increases, the vacuum level in the interlayer may decrease, causing the internal pressure of the LNG cylinder to rise, which in turn leads to frequent venting by the safety valve. Advantages of LNG trucks: 1. Safe and reliable: LNG has a higher ignition point than gasoline and diesel, and its explosion limit is nearly four times higher than that of gasoline. Even in the event of a leak, spontaneous combustion or explosion is highly unlikely, ensuring high safety. 2. Clean and environmentally friendly: Compared to gasoline and diesel, LNG's comprehensive exhaust emissions are reduced by nearly 85%, and it contains virtually no sulfides, making it extremely eco-friendly and easier to upgrade to the China VI emission standard. 3. Long driving range: Compared to CNG vehicles, a 335L LNG cylinder can provide a driving range of over 500 kilometers.
About the LNG cylinder vacuum pumping, I watched the technician operate it last time at the factory. The optimal duration depends on the cylinder size and the vacuum pump's suction capacity. Generally, small-capacity cylinders take about three to four hours, while those large 500-liter-plus cylinders for heavy trucks require at least eight to ten hours of continuous pumping. The key is monitoring the vacuum gauge reading—it must stabilize below 5Pa to qualify. Temperature significantly impacts the process; in summer heat, extend the duration by one or two hours to prevent vacuum level rebound. That time we processed a 200-liter cylinder in sweltering weather—it took a full six and a half hours to meet standards. The worst is a power outage midway, forcing a restart. This job absolutely can't be rushed.
I discussed this topic with the repair shop owner, and he mentioned that the vacuum extraction time primarily depends on three factors. First is the equipment performance: using an ordinary mechanical pump can take over ten hours, while a professional Roots pump can reduce it to five or six hours. Second is the cylinder's sealing condition: new cylinders extract faster, whereas older ones with minor leaks require continuous extraction and testing. Third is the required endpoint pressure: for instance, LNG vehicle cylinders demand excellent insulation, requiring the vacuum level to stabilize at 3-5Pa to be considered adequate. Their workshop's standard procedure is to extract for a full eight hours followed by a pressure retention test, extending to around ten hours if pressure rebound is detected. In practice, environmental humidity must also be considered, with the rainy season requiring additional time.
