How does the pre-tightening force of container flange bolts change after temperature increase?
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This post was last edited by mikech on 2009-10-13 at 14:28. The flange bolts of the container (made of chromium-molybdenum steel) are pre-tightened at room temperature; the stress resulting from this pre-tightening exceeds the allowable stress, but it is less than 40% of the yield strength. Does the pre-tightening force increase or decrease when the temperature of the container rises to 500°C?
Personal opinion: During the heating process, the flange rises in temperature along with the shell; generally, the temperature increase of the flange is greater than that of the studs. As a result, the thermal deformation of the flange in its thickness direction (i.e., the increase in the flange’s thickness) will be greater than the thermal elongation of the studs. Since the stiffness of the flange in its thickness direction is much greater than that of the studs, it can be assumed that the studs cannot restrict the flange from thickening during container operation. On the contrary, it is the flange that forces the studs to undergo a certain amount of elastic deformation in addition to their thermal expansion. Therefore, to ensure sealing, the bolts must provide both the sealing preload and the axial force resulting from temperature increases (or pressure increases); in other words, the force acting on the bolts is the sum of these two forces, which leads to secondary preloading.
1. Pre-tightening force refers to the force exerted on the bolt during pre-tightening; the force on the bolt when the temperature rises represents the force under specific operating conditions, and it should not be referred to as a change in pre-tightening force； 2. If temperature plays a dominant role, as mentioned above, it is likely due to expansion in the thickness direction of the flange, which causes the bolts to be stretched further; if there are no issues with strength, the sealing effect will be better (ignoring the strength of the gaskets).
Hehe... It seems that even if the elongation of the studs is less than that of the flanges, the bolts still bear more stress. So if there’s no issue with strength, then sealing is better. Then why is secondary pre-tightening necessary?
As the temperature rises, the expansion amount of the bolts should be greater than that of the flanges; a decrease in the bolt preload can easily lead to leaks at high temperatures, which is why thermal tightening is necessary. I’m not sure if it’s right or wrong; please advise.
Hmm, is what was said upstairs correct?···
As the temperature rises, the expansion of the studs is greater than that of the flange, so the preload on the studs decreases, requiring them to be tightened again
When heated, the bolt stretches along its axis while the flange expands radially; as a result, the bolt preload decreases. Therefore, most maintenance procedures require thermal tightening
This post was last edited by mikech on 2009-10-14 at 13:43. The reply from the user above is exactly what I wanted to know, even though our opinions differ! The individual raised this question mainly to examine the impact of temperature rise on the stress and strength changes in bolts, and to conduct an analysis from the perspective of the thermal expansion coefficients of the bolt and flange materials: if the thermal expansion coefficient of the flange is lower than that of the bolt, the preload will decrease ； If the thermal expansion coefficient of the flange is greater than that of the bolt, the preload will increase. I wonder if Haiyou has the thermal expansion coefficient of the material? Thank you!
Push on your own. I hope fellow travelers will not hesitate to offer their advice!
I have benefited greatly from the guidance of the teachers upstairs; thank you very much