There are two types of liquefaction tanks used by citizens. One is the screw connection between the cylinder shield and the bottle body, referred to as "screw bottle"; the other is the connection between the cylinder shield and the bottle body by electric welding, referred to as "welded bottle". ". According to national regulations, the safe use period of liquefied gas tanks is only 15 years, and regular inspections are required every 4 years. Since there is no permanent mark on the bottle body of the "screw bottle", since September 1996, the state has clearly stipulated that the "screw bottle" must not be reproduced. That is, the last batch of "screw bottles" produced is more than 15 years old and must be scrapped.