Discussion on the roundness deviation of the outer pressure cylinder
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As stated in the title, in GB150.4, Figure 10 in Section 6.5.11 specifies the maximum allowable deviation for the roundness of pressure vessels. According to this figure, when the outer diameter and effective thickness are constant, the longer the length L, the greater the allowable deviation in roundness. The shorter the length L, the smaller the allowable out-of-roundness deviation. As we all know, when the outer diameter and effective thickness are the same, the shorter the length, the better the stiffness of the cylinder and the less likely it is to become unstable. Then why do the standards impose stricter requirements on the roundness tolerance for shorter cylinders? Everyone is welcome to discuss.
This post was last edited by zjq1962 on 2019-1-23 at 12:48. From this graph, it seems that when D0/δe remains constant, the larger L is, the smaller the value of e becomes, right? The original poster got it backwards. The curve on this graph is downward itself.
As L gets larger, shouldn’t e get smaller? I’m really impressed by the picture you showed
Thank you for the reply. If D0/δe remains constant, the larger L is, the greater the value of e becomes; please take another close look at the graph. The figure shows a family of e-value curves. The curve shows that, for a fixed value of e, the larger L is, the smaller the required ratio of D0/δe becomes.
Thank you for the reminder. I took another close look at this table, and indeed, when the diameter remains constant, the larger L is, the greater the value of e becomes. My understanding is that when calculating external pressure, the larger L is, the greater the thickness of the cylinder required; in other words, the factor of L is already taken into account in the strength calculations. In the manufacturing process, the factor that has a significant impact on the roundness deviation of the cylinder is its diameter.
What is considered here is the allowable error in roll forming, not the strength of the load-bearing capacity; they are completely different things
This post was last edited by wanlirn on 2019-2-3 at 18:08. I didn’t look carefully enough; I searched for the definition of standard 89, and this table is entirely copied from ASME’s version. The definition states that the values in this table are related to the value of L as well as the design safety factor for pressure vessels. I think the phenomenon you mentioned results from a comprehensive consideration of factors such as the design safety factor and manufacturing costs, and it can’t be simply handled according to conventional methods. I don’t have an electronic version of this definition; you might want to look for it
This takes into account the influence of processing conditions, rather than the stiffness of the cylinder itself