The sealing principle of flat-welded flanges: the two sealing surfaces of the bolts squeeze the flange gaskets and form a seal, but this also leads to the destruction of the seal.
　　 In order to maintain the seal, it is necessary to maintain a huge bolt force. For this reason, the bolt must be made larger.
　　A larger bolt must match a larger nut, which means that a bolt with a larger diameter is needed to create conditions for tightening the nut.

　　 However, the larger the diameter of the flat-welded flange bolts, the applicable flange will become bent. The solution is to increase the wall thickness of the flange part.
　　 The entire device will require a huge size and weight, which becomes a special problem in the offshore environment, because in this case weight is always the main issue that people must pay attention to.
　　Furthermore, basically, the flat welded flange is an ineffective seal. It requires 50% of the bolt load to be used for extruding the gasket, while only 50% of the load used to maintain the pressure is left.