The flange looks like two metal plates with holes, which are used to buckle the pipe openings face to face and can be stably connected by bolts.
Because there is a metal gasket or rubber in the middle of the flange, the tighter the better when tightening the screws.
Experienced masters tighten the screws diagonally, similar to changing tires on a car. In the past, a pharmaceutical factory over-tightened the screws, causing the gasket to be crushed and the entire batch of vaccines to be contaminated by lubricating oil.