Covered ball and mainly used the liquid level upper role. Such as: for the acid-base storage tank, to prevent the acid-base gas spillover, volatile, to reduce the pollution of the external environment of the acid-base gas, and saving the role of acid-base material; for desalting water, so that air and water separation can be reduced air, carbon dioxide, oxygen and dust impurities such as of demineralized water pollution, to ensure the quality of the water