Does the bolt load increase or decrease as the flange moves from the pre-tensioned state to the operating state?

This post was last edited by FORREST_GUMP on 2015-12-17 09:15. From pre-tensioning to operation, the load on the bolt increases. Before being subjected to load, the bolt has a pre-tightening force F0 ； Under the action of the working load F, this preload changes. The total tensile force on the bolt, F2, is not equal to the sum of the preload F0 and the working force F; rather, it equals the sum of the residual preload F1 and the working force F. That is: F2 = F1 + F. In applications where sealing is important, in order to ensure a secure seal, it is required that the residual preload F1 be equal to (1.5–1.8)F. This is also why it is required that flange bolts be tightened not only cold but also hot ； This is because the deformation and elongation of the bolt under heat conditions result in a decrease in the residual preload F1.

Thank you, Floor 2; I’ll study it further!

For some harder gaskets, the required preload is very high； When the flange is subjected to internal pressure, the gasket rebounds; although the bolt force increases due to this internal pressure, on the other hand, the rebound of the gasket releases some of that bolt force. For example, with the same gasket and the same bolt diameter, it may be controlled by the pre-tightening condition at a design pressure of 1 MPa, while at 5 MPa it may be controlled by the operating condition.

The springback of the gasket reduces the bolt force, which is reflected in the fact that the residual preload F1 is less than the initial preload F0. I would like to ask an expert: you mentioned that \"for example, with the same gasket and the same bolt diameter, control may be based on the pre-tightening condition at a design pressure of 1 MPa, whereas at 5 MPa it may be controlled by the operating condition.\" ” I couldn’t understand it. Could you explain it to me? Thank you

This requires specific calculations. To take an extreme example: do you think the pre-tensioning force in a negative-pressure container is greater, or is it greater during operation?

From the perspective of bolt deformation, in pressure vessels under internal pressure, as they go from a pre-tightened state to an operational state, the gaskets rebound, the distance between the flanges increases, and the bolts lengthen as well; therefore, the tensile force should increase. The situation is reversed in pressure vessels under negative pressure. Is this understanding correct?

Let’s use numerical values as an example: the gasket parameters are y=69, m=3, b=6.4, with DG assumed to be 800. After simplification, Wa/Wp becomes by/c(DG/4+2bm). 1. When pc=1MPa, Wa/Wp=1.85; if the design temperature is not high, and the allowable stress of the bolt material at room temperature is 1.235 times its allowable stress at the design temperature, then the minimum required bolt area is determined by the operating conditions ； If the design temperature is not high, it is still possible that the preload condition will determine it. What has been mentioned above is considered only from the perspective of the minimum required bolt area. However, flange design is actually a systematic calculation process determined by multiple factors such as the material and size of the gasket, the material and size of the bolts, and various dimensions of the flange itself. Still using the example above, even if the minimum required bolt area is determined by the operating conditions, if the actual bolt area Am is much larger than this minimum required area, resulting in a flange torque Ma>>flange torque Mp under the pre-tensioned condition, then the design torque for the flange will be determined by the pre-tensioned conditions, and the calculated flange dimensions will be significantly larger. Flange design, whether for high or low pressure, high or low temperature, is actually not that simple. In pressure vessel design, there is HG20615 for pipe flanges, and NB47023 for equipment flanges, which saves designers a great deal of work. But is choosing standard flanges really the best option? Can the gasket material be replaced? Must the bolt material be of high strength? Can the diameter of the bolt circle be compressed any further? Wait, wait—many people actually haven’t thought about these issues.

Previously, flanges were simply selected, with the corresponding bolts chosen to match those flanges. To be honest, I had never thought of these things you mentioned before. It will take some time to digest and absorb this, and I would appreciate your guidance on any points I don’t understand during that period.