The large and small cover bodies are made of cast aluminum alloy with a hard anodized surface treatment. The small cover has a diameter of Ф250 and is equipped with an emergency pressure relief function. It can be fitted with a single or dual built-in breather valve. The large cover has 2 spare holes, which can be used to install an anti-overflow probe, an oil measurement port, or an external breather valve, and is installed on the top of the tank truck.