Internal pressure induces axial force
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In the chapter on flanges of GB/T150.3, FD denotes the axial force exerted on the inner diameter cross-section of the flange due to internal pressure; FD = 0.785(Di^2)*P, where Di is the inner diameter of the flange or the end structure of the cylinder. I have a question: where does this force come from? The cylinder has openings at both ends, so there is no surface on which the fluid can act axially; then how is FD determined?
Try to understand on your own what internal pressure is. It’s too basic.
FD represents the axial force exerted on the inner diameter cross-section of the flange due to internal pressure; it has already been specified that this force is caused by internal pressure. If both ends are open, then there is no such thing as internal pressure.
This is a model for force analysis. Unless it is vented, it is also closed at one or both ends at some distant location. Furthermore, if the space is not sealed, pressure cannot be maintained. Of course, this axial force might be larger than the actual value.
Axial force caused by internal pressure between the two flanges
Open at both ends? Are you still a pressure vessel?
Since there is internal pressure, it means that something must be closed somewhere; otherwise, the condition of internal pressure would not hold.