Because the manhole is located in the lower part of the oil tank, the manhole bears a large liquid pressure. In order to prevent leakage, the installation quality of the manhole must be strictly required. The flange and cover plate are processed with sealing water lines, and attention should be paid to protection during construction to avoid oil leakage during use. Mark each time the manhole is removed, so as to avoid misplacement during re-installation and affect the tightness. When installing the nuts on the manhole cover, apply force evenly in pairs diagonally to prevent oil leakage caused by deformation of the cover or uneven force.