For pipes without expansion joints, is there any unbalanced internal pressure force due to the thrust of the fixed supports?

Only bellows compensators and sleeve compensators generate internal pressure thrust; pipes with natural compensation have no internal pressure thrust.

As long as the pipeline direction, cross-sectional area, etc. change, an internal pressure thrust exists. Only expansion joints require additional consideration for support brackets, as the stiffness of expansion joints differs significantly from that of the pipes. On ordinary pipes, the internal pressure thrust has a very small effect on pipe deformation and can be ignored. However, in pipes with expansion joints, the internal pressure thrust has a significant impact.

This post was last edited by Little Worker on 2021-8-25 at 16:04. I can understand what you said earlier, but the issue is this: since pipes without expansion joints are also subject to the effect of blind plate forces, the axial thrust exerted by the supports should also take these blind plate forces into account. This has nothing to do with the stiffness of the pipe; it’s similar to the fact that friction must be considered in the axial thrust of fixed supports. Does that mean the forces acting on fixed supports in pipes without expansion joints, due to blind plate forces, are very small? Smaller than friction? The calculation formulas I saw took into account the effect of the blind plate force in the stress calculation of the fixed supports only when expansion joints were present. According to the principle of blind plate force, even in the absence of expansion joints, the blind plate force remains quite high; this should also have a significant impact on the stress experienced by the fixing brackets, right? For example, the fixing brackets at the elbows are calculated based on the force exerted by the blind flange; this force is equal to pressure multiplied by the cross-sectional area of the pipe, and it is quite large.

It is of course related to the pipe stiffness. In pipes without expansion joints, the thrust from the blind flange does not cause deformation or displacement of the pipe, nor does it exert any force on the structure. The thrust of this blind flange is equivalent only to the internal forces in the pipeline, and it has no effect on the structure. Therefore, the fixed point does not take into account the blind plate thrust

As I understand it, the blind plate force is an axial unbalanced force resulting from pressure changes due to variations in the pipe area; what is the relationship between this force and stiffness deformation? This force is not stress; it is an external force resulting from internal pressure. As long as there is a change in the pipe area, such as at elbows or tees, a blind plate force will exist. This force acts on the pipe, which in turn exerts a force on the structure. The stiffness of expansion joints is taken into account because the force exerted by blind flanges can tear them; rigid pipes have high stiffness and thus cannot be torn, but this does not mean that no forces are acting on them. Since force acts on the pipe, why doesn’t it act on the structure? Are there any recommended theoretical books?

As long as there is pressure in the pipe, an axial force exists (this is the blind plate force), and the pipe’s own axial stiffness can balance this axial force, so that it does not act on the support frame. Since the axial stiffness of the bellows compensator is very low, the force exerted by the blind plate is transferred to the fixing frame.

Take an L-shaped balloon, hold the end in your hand, and then blow air into it; you will feel the balloon expanding as well as a pushing force against your hand – that is the internal pressure force； Then use a material similar to that of an L-shaped bicycle inner tube; inflate it so as to generate the same thrust in the hand, with the air pressure being much higher than that of a balloon ； Consider another L-shaped closed metal pipe, where the internal pressure exerts a significant thrust on the fixing points. How large must this pressure be? This is a very interesting question; finite element analysis could be considered for it. You thought about changes in area and elbows, but what about the combined effect of these components? No matter how complex the component is, for a fixed point, internal pressure thrust exists only when the pipe deforms. Pipelines will deform under internal pressure, but in engineering practice, the deformation caused by internal pressure is so small that it can be ignored; the deformation resulting from a 10°C change in pipeline temperature is much greater than this value, so there is no need to take it into consideration at all.

This post was last edited by Little Worker on 2021-8-30 at 16:46. I understand what you mean – force can only have an effect when there is deformation. But from your description, isn’t your assumption that the force of the blind plate is the internal stress of the pipeline? According to you, the frictional force of the pipeline is not and should not be taken into account? In my opinion, the blind plate force is an axial outward force resulting from pressure due to changes in the pipe area; it operates in a manner similar to friction, rather than being an internal force generated by pipe deformation caused by pressure, as you might think. But this is just my personal understanding, and there is no corresponding evidence to support it. However, I reviewed relevant journals, and it was found that the force exerted by the blind plate arises as long as there is a change in the pipe area; it has no clear relationship with whether the pipe deforms or not, nor with the presence of expansion joints. The examples given in those journals were related to the force generated by blind plates at elbows, and such forces are quite substantial. Secondly, do you think the force of the blind plate is greater or the frictional force is greater, compared to each other? Finally, let me make an assumption as well: you are holding a water pipe in each hand, with the same pressure and the same diameter. One is a straight pipe, and the other is a pipe with a elbow to change the direction. Your two hands act as a kind of fixing frame; do you think your hands will experience more stress? I think it must be the one with the elbow; the change in flow direction at the elbow inevitably exerts an axial force on the elbow, and this force is then converted into an axial force acting on the fixing point.

How does axial stiffness balance axial external forces? The axial stiffness should be balanced by the axial internal stress of the pipe, whereas the force exerted by the blind plate is clearly an external force caused by pressure